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7/25/2019 APV EVAPORATORS
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FOURT
H
EDITION
EHB-599
An Invensys company
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2
C O N T E N T S
Introduction...................................................3
Evaporators...................................................4
Evaporator Type Selection............................20
Configurations For Energy Conservation .......24
Residence Time In Film Evaporation ..............28
Designing For Energy Efficiency....................32
Physical Properties.......................................34
Mechanical VaporRecompression Evaporators..........................36
Evaporators For Industrial And
Chemical Applications.................................42
Waste Water Evaporators............................47
Tubular EvaporatorsFor Sanitary Standards................................50
Evaporator Control ......................................52
Preassembled Evaporators............................54
The Production Of
High Quality Juice Concentrates...................55
Engineering Conversions..............................60
Properties Of Saturated
Steam Temperature Tables............................61
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3
I N T R O D U C T I O NAs one of the m ost energy intensive processes used in the dairy, food and
chem ical industries, it is essential that evaporation be approached from the
view point of econom ical energy utilization as w ell as process effectiveness. This
can be done only if the equipm ent m anufacturer is able to offer a full selection of
evaporation technology and system s developed to accom m odate various product
characteristics, the percent of concentration required, and regional energy costs.
This H andbook describes the m any types of evaporators and operating options
available through the experience and m anufacturing capabilities of com panies
w ithin the APV G roup.
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E V A P O R A T O R S
TYPES AND DESIGN
In the evaporation process, concentration of a product is accom plished by boiling
out a solvent, generally w ater. The recovered end product should have an
optim um solids content consistent w ith desired product quality and operating
econom ics. It is a unit operation that is used extensively in processing foods,chem icals, pharm aceuticals, fruit juices, dairy products, paper and pulp, and both
m alt and grain beverages. Also it is a unit operation w hich, w ith the possible
exception of distillation, is the m ost energy intensive.
W hile the design criteria for evaporators are the sam e regardless of the industry
involved, tw o questions alw ays exist: is this equipm ent best suited for the duty, and
is the equipm ent arranged for the m ost efficient and econom ical use? A s a result,
m any types of evaporators and m any variations in processing techniques have
been developed to take into account different product characteristics and
operating param eters.
TYPES OF EVAPORATORS
The m ore com m on types of evaporators include:
Batch pan Forced circulation
N atural circulation W iped film
Rising film tubular Plate equivalents of
Falling film tubular tubular evaporators
Rising/falling film tubular
4
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5
BATCH PAN
N ext to natural solar evaporation, the batch pan (Figure 1) is one of the oldest
m ethods of concentration. It is som ew hat outdated in today's technology, but is still
used in a few lim ited applications, such as the concentration of jam s and jellies
w here w hole fruit is present and in processing som e pharm aceutical products.
Up until the early 1960's, batch pan also enjoyed w ide use in the concentration
of corn syrups.
W ith a batch pan evaporator, product residence tim e norm ally is m any hours.
Therefore, it is essential to boil at low tem peratures and high vacuum w hen a
heat sensitive or therm odegradable product is involved. The batch pan is either
jacketed or has internal coils or heaters. H eat transfer areas norm ally are quite
sm all due to vessel shapes, and heat transfer coefficients (H TC s) tend to be low
under natural convection conditions. Low surface areas together w ith low H TC 's
generally lim it the evaporation capacity of such a system . H eat transfer is
im proved by agitation w ithin the vessel. In m any cases, large tem perature
differences cannot be used for fear of rapid fouling of the heat transfer surface.
Relatively low evaporation capacities, therefore, lim it its use.
STEAM
PRODUCT
CONDENSER
CONDENSATE
Figure 1
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T U B U L A R
E V A P O R A T O R S
NATURAL CIRCULATION
Evaporation by natural circulation is
achieved through the use of a
short tube bundle w ithin the
batch pan or by having an external
shell and tube heater outside of the
m ain vessel(Figure 2).
The external heater has the advantage
that its size is not dependent upon the
size or shape of the vessel itself. As a
result, larger evaporation capacities
m ay be obtained. The m ost com m on
application for this type of unit is as a reboiler at the base of a distillation colum n.
RISING FILM TUBULAR
C onsidered to be the first 'm odern' evaporator used in the industry, the rising film
unit dates back to the early 1900's. The rising film principle w as developed
com m ercially by using a vertical tube w ith steam condensing on its outside surface
(Figure 3). Liquid on the inside of the tube is brought to a boil, w ith the vapor
generated form ing a core in the center of the tube. As the fluid m oves up the tube,
m ore vapor is form ed resulting in a higher central core velocity that forces the
rem aining liquid to the tube w all. H igher vapor velocities, in turn, result in thinner
and m ore rapidly m oving liquid film . This provides
higher H TC 's and shorter product residence tim e.
The developm ent of the rising
film principle w as a giant
step forw ard in theevaporation field,
particularly in product quality.
In addition, higher H TC 's
resulted in reduced heat
transfer area requirem ents
and consequently, in a low er
initial capital investm ent.
6
STEAM
TO CONDENSEROR VACUUM
FEED
PRODUCTOUT
Figure 2
TO CONDENSEROR VACUUM
FEED
CONDENSATE
STEAM
PRODUCTOUT
Figure 3
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7
FALLING FILM TUBULAR
Follow ing developm ent of the rising film principle, it took alm ost half a century for
a falling film evaporation technique to be perfected (Figure 4). The m ain problem
w as how to design an adequate system for the even distribution of liquid to each
of the tubes. For the rising film evaporator, distribution w as easy since the bottom
bonnet of the calandria w as alw ays pum ped full of liquid, thus allow ing equal
flow to each tube.
W hile each m anufacturer has its ow n technique, falling film distribution generally is
based around use of a perforated plate positioned above the top tube plate of the
calandria. Spreading of liquid to each tube is som etim es further enhanced by
generating flash vapor at this point. The falling film evaporator does have the
advantage that the film is 'going w ith gravity' instead of against it. This results in
a thinner, faster m oving film and gives rise to an even shorter product contact tim e
and a further im provem ent in the value of H TC .
To establish a w ell-developed film , the rising film unit requires a driving film force,
typically a tem perature difference of at least 25F (14C ) across the heating
surface. In contrast, the falling film evaporator does not have a driving force
lim itationperm itting a greater num ber of evaporator effects to be used w ithin the
sam e overall operating lim its. For
exam ple, if steam is available at
220F (104C ), then the last effectboiling tem perature is 120F
(49C ); the total available
T is equal to 100F (55C ).
In this scenario a rising film
evaporator w ould be lim ited
to four effects, each w ith a T
of 25F (14C ). H ow ever,using the falling film
technique, it is feasible to have
as m any as 10 or m ore effects.
VACUUM
CONDENSATE
PRODUCTOUT
FEED
STEAM
STEAM
Figure 4
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RISING/ FALLING FILM TUBULAR
The rising/falling film evaporator
(Figure 5) has the advantages
of the ease of liquid distribution
of the rising film unit coupled
w ith low er head room
requirem ents. The tube bundle
is approxim ately half the height
of either a rising or falling film
evaporator, and the
vapor/liquid separator is
positioned at the bottom of
the calandria.
FORCED CIRCULATION
The forced circulation evaporator (Figure 6) w as developed for processing liquors
w hich are susceptible to scaling or crystallizing. Liquid is circulated at a high rate
through the heat exchanger, boiling being prevented w ithin the unit by virtue of a
hydrostatic head m aintained above the top tube plate. As the liquid enters the
separator w here the absolute pressure is slightly less than in the tube bundle, theliquid flashes to form a vapor.
The m ain applications for a forced circulation evaporator are in the concentration
of inversely soluble m aterials, crystallizing duties, and in the concentration of
therm ally degradable m aterials w hich result in the deposition of solids. In all cases,
the tem perature rise across the tube bundle is kept as low as possible, often as
low as 3-5F (2-3C ). This results in a recirculation ratio as high as 220 to 330
lbs (100 to 150 Kg) of liquor per pound (kilogram ) of w ater evaporated. These
high recirculation rates result in high liquor velocities through the tube w hich help
to m inim ize the build up of deposits or crystals along the heating surface. Forced
circulation evaporators norm ally are m ore expensive than film evaporators
because of the need for large bore circulating pipew ork and large recirculating
pum ps. O perating costs of such a unit also are considerably higher.
VACUUM
PRODUCTOUT
STEAM
FEED
STEAM
Figure 5
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VAPOROUTLET
DILUTELIQUORINLET
CONCENTRATEDLIQUOROUTLET
SEPARATOR
CIRCULATION PUMP GIVINGHIGH LIQUOR VELOCITIESOVER HEATING SURFACE
LOW TEMPERATURERISE ACROSSCALANDRIA
LIQUOR HEADTO PREVENT BOILINGAT HEATING SURFACE
CALANDRIA
9
WIPED FILM
The w iped or agitated thin film evaporator has lim ited applications due to the high
cost and is confined m ainly to the concentration of very viscous m aterials and the
stripping of solvents dow n to very low levels. Feed is introduced at the top of theevaporator and is spread by w iper blades on to the vertical cylindrical surface
inside the unit. Evaporation of the solvent takes place as the thin film m oves dow n
the evaporator w all. The heating m edium norm ally is high pressure steam or oil.
A high tem perature heating m edium generally is necessary to obtain a reasonable
evaporation rate since the heat transfer surface available is relatively sm all as a
direct result of its cylindrical configuration.
The w iped film evaporator is satisfactory for its lim ited applications. H ow ever, in
addition to its sm all surface area, it also has the disadvantage of requiring m oving
parts such as the w iper blades w hich, together w ith the bearings of the rotating
shaft, need periodic m aintenance. C apital costs in term s of dollars per pound of
solvent evaporated also are very high.
Figure 6
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These APV plate evaporation system s are m ade in four arrangem ents
rising/falling film , falling film , Paravap, and Paraflash and m ay be sized
for use in new product developm ent or for production at pilot plant or full scale
operating levels.
APV plate type evaporators have been sold com m ercially for over 40 years.
Approxim ately 2000 system s have been m anufactured by APV for the
concentration of hundreds of different products.
Figure 7
PLATE TYPE EVAPORATORS
To effectively concentrate an increasing variety of products w hich differ by industry
in such characteristics as physical properties, stability, or precipitation of solid m atter,
equipm ent m anufacturers have engineered a full range of evaporation system s.
Included am ong these are a num ber of plate type evaporators (Figure 7).
Plate evaporators initially w ere developed and introduced by APV in 1957 to
provide an alternative to the tubular system s that had been in use for half a century.
The differences and advantages w ere m any. The plate evaporator, for exam ple,
offers full accessibility to the heat transfer surfaces. It also provides flexible capacity
m erely by adding m ore plate units, shorter product residence tim e resulting in a
superior quality concentrate, a m ore com pact design w ith low headroom
requirem ents, and low installation cost.
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vapor/liquidproduct
condensate
product feed
steam
Figure 8
11
APPLICATIONS
Although plate evaporators can be used on a broad range of products, the m ain
application has been w ith products that are heat sensitive and therefore benefit
from the high H TCs and low residence tim e. Products that are being processed in
this evaporator include:
Apple juice C offee Pear juice
Am ino acids Fruit purees Pectin
Beef broths G elatin Pharm aceutical products
Beet juice G rape juice Pineapple juice
Betacyclodextrin Lim e juice Skim m ilk
C aragenan Liquid egg Sugars
C heese w hey Low alcohol beer Vegetable juices
C hicken broth M ango juice W hey protein
C itrus juice O range juice W hole m ilk
RISING/ FALLING FILM PLATE
This is the original plate type evaporator. The principle of operation for the
rising/falling film plate evaporator (RFFPE) involves the use of a num ber of plate
packs or units, each consisting of tw o steam plates and tw o product plates. These
are hung in a fram e w hich resem bles that of a plate heat exchanger (Figure 8).The first product passage is a rising pass and the second, a falling pass. The steam
plates, m eanw hile, are arranged alternately betw een each product passage.
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The product to be evaporated is fed through tw o parallel feed ports and is equallydistributed to each of the rising film annuli. N orm ally, the feed liquor is introduced
at a tem perature slightly higher than the evaporation tem perature in the plate
annuli, and the ensuing flash distributes the feed liquor across the w idth of the
plate. Rising film boiling occurs as heat is transferred from the adjacent steam
passage w ith the vapors that are produced helping to generate a thin, rapidly
m oving turbulent liquid film .
During operation, the vapor and partially concentrated liquid m ixture rises to the
top of the first product pass and transfers through a 'slot' above one of the
adjacent steam passages. The m ixture enters the falling film annulus w here gravity
further assists the film m ovem ent and com pletes the evaporation process. The rapid
m ovem ent of the thin film is the key to producing low residence tim e w ithin the
evaporator as w ell as superior H TC s. At the base of the falling film annulus, a
rectangular duct connects all of the plate units and transfers the evaporated liquor
and generated vapor into a separating device. A flow schem atic for a tw o effectsystem is show n in (Figure 9).
Figure 9
12
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13
The plate evaporator is designed to operate at pressures extending from10 psig (1.7 barg) to full vacuum w hen using any num ber of effects. H ow ever,
the m axim um pressure differential norm ally experienced betw een adjacent annuli
during single effect operation is 15 psi (1 bar). This, and the fact that the pressure
differential alw ays is from the steam side to the product side, considerably reduce
design requirem ents for supporting the plates. The operating pressures are
equivalent to a w ater vapor saturation tem perature range of 245F (118C )
dow nw ards, and thus are com patible w ith the use of nitrile or butyl rubber gaskets
for sealing the plate pack.
M ost rising/falling film plate evaporators are used for duties in the food, juice and
dairy industries w here low residence tim e and a tem perature low er than 195F
(90C ) are essential for the production of quality concentrate. Also, increasing
num ber of plate evaporators are being operated successfully in both pharm aceutical
and chem ical plants on such products as antibiotics and inorganic acids. These
evaporators are available as m ulti-effect and/or m ulti-stage system s to allowrelatively high concentration ratios to be carried out in a single pass, non-
recirculatory flow .
The rising/falling film plate evaporator should be given consideration for various
applications that:
Require operating tem peratures betw een 8 0-212F (26 to 100C )
H ave a capacity range of 1000-35,000 lbs/hr
(450 to 16,000 kg/hr) w ater rem oval
H ave a need for future capacity increase since evaporator capabilities
can be extended by adding plate units or by the addition of extra effects
Require the evaporator to be installed in an area that has lim itedheadroom as low as 13 ft (4m )
W here product quality dem ands a low tim e/tem perature relationship
W here suspended solid level is low and feed can be passed
through 50 m esh screen
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14
A Juniorversion of theevaporator(Figure 1 0 ) is
available for pilot plant and test
w ork and for low capacity
production. If necessary, this
can be in m ulti-effect/m ulti-stage
arrangem ents.
FALLING FILM PLATE
Incorporating all the advantages of the original rising falling film plate evaporator
system w ith the added benefits of shorter residence tim e and larger evaporation
capabilities, the falling film plate evaporator has gained w ide acceptance for the
concentration of heat sensitive products. W ith its larger vapor ports, evaporation
capacities are typically up to 60,000 lbs/hr (27,000 kg/hr).
The falling film plate evaporator
consists of gasketed plate units
(each w ith a product and a
steam plate) com pressed w ithin
a fram e that is ducted to a
separator. The num ber of plate
units used is determ ined by the
duty to be handled.
O ne of the im portant innovations in this type of evaporator is the patented feed
distribution system (Figure 11). Feed liquor first is introduced through an orifice
(1) into a cham ber (2) above the product plate w here m ild flashing occurs.
This vapor/liquid m ixture then passes through a single product transfer hole (3)
into a flash cham ber (4) w hich extends across the top of the adjacent steam plate.
M ore flash vapor results as pressure is further reduced and the m ixture passes in
both directions into the falling film plate annulus through a row of sm all distribution
holes (5). These assure an even film flow dow n the product plate surface w here
evaporation occurs. A unique feature is the ability to operate the system either in
parallel or in series, giving a tw o-stage capability to each fram e. This is
particularly advantageous if product recirculation is not desirable.
Figure 11
Figure 10
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15
In the tw o-stage m ethod of operation, feed enters the left side of the evaporator
and passes dow n the left half of the product plate w here it is heated by steam
com ing from the steam sections. After the partially concentrated product is
discharged to the separator, it is pum ped to the right side of the product plate
w here concentration is com pleted. The final concentrate is extracted w hilevapor is discharged to a subsequent evaporator effect or to a condenser. The
falling film plate is available in an extended form w hich provides up to 4000 ft2
(370m 2) surface area in one fram e. A flow schem atic for a tw o effect system
(Figure 12) is show n above. An APV falling film plate evaporator in triple effect
m ode (Figure 1 3 ) is show n below .
Figure 12
Figure 13. Plant representation. Triple-effect Falling Film Evaporator system followedby a double-effect forced circulation tubular finisher. A distillation essence recoverysystem was provided to recover the key essence components from the juice and inparticular the methyl anthranilate.
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16
T H E A P V P A R A V A P
E V A P O R A T I O N S Y S T E M S
THE PROCESS
The APV Paravap evaporation system is designed for the evaporation of highly
viscous liquids. The system is often used as a finishing evaporator to concentrate
m aterials to high solids follow ing a low solids m ulti-effect or M VR film evaporator.
The m ain com ponents of the system are a plate heat exchanger, vapor liquid
separator, condenser and a series of pum ps (Figure 14). It is designed to operate
as a clim bing film evaporator w ith the evaporation taking place in the plate
passages. C om pared w ith forced circulation evaporators, the pum ping costs are
significantly reduced.
Under norm al operating conditions the feed is introduced at the bottom of the
plates. As the feed contacts the plate surface, w hich is heated by either steam
or hot w ater, the feed starts to evaporate. The narrow gap and corrugations in
the plate passages cause high turbulence and a resulting partial atom ization of the
fluid. This reduces the apparent liquid viscosity and generates considerably higher
H TCs than w ould occur in a shell and tube heat exchanger under sim ilar
conditions. It is particularly effective w ith non-N ew tonian viscous liquids.
Figure 14
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A clear advantage w hen processing tem perature sensitive products is gained w itha Paravap because m ost duties do not require liquid recirculation.
For m ost duties the conventional gasketed plate heat exchanger is specified.
H ow ever, for duties w here the process fluid could attack the gasket, APV can offer
the w elded plate pair exchanger w hich elim inates elastom er gaskets on the
process side.
The Paravap is usually operated in single effect m ode although som e system s are
operating w ith double effect.
Since m ost system s are not physically large, the equipm ent can often be fully
preassem bled on a skid prior to shipm ent. Preassem bly reduces installation tim e
and, in m ost cases, significantly low ers the overall project cost.
The Paravap evaporation system is particularly effective in processing the m ore
viscous products. O ften the Paravap can be used in place of a w iped film or thin
film evaporator w ith a substantial reduction in cost. For duties w here severe fouling
can occur on boiling heat transfer surfaces, the process should be perform ed in a
APV Paraflash system .
Som e typical duties that are perform ed in a Paravap include:
C oncentration of sugar solutions to extrem ely high solids content.
In one case a solids concentration of 98% w as achieved.
Rem oval of w ater from soaps.
Finishing concentrator on certain fruit purees such as banana and apple.
C oncentration of high solids corn syrups.
Rem oval of solvents from vegetable oils.
C oncentration of fabric softeners.
Lignin solutions.
H igh concentration gelatin.
H igh concentration chicken broth.
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18
Figure 15
T H E A P V P A R A F L A S H
E V A P O R A T I O N S Y S T E M S
THE PROCESS
The APV Paraflash evaporation system is designed for the evaporation of liquids
containing high concentrations of solids. In particular, the system is used as a
finishing evaporator to concentrate m aterials to high solids follow ing a low solids
m ulti-effect or M VR film evaporator.
The m ain com ponents of the system are a plate heat exchanger, vapor liquid
separator, condenser and a series of pum ps (Figure 15). It is designed to operate
as a forced circulation evaporator w ith the evaporation being suppressed in the
heating section by back pressure. This back pressure can be generated by a
liquid head above the exchanger or by using an orifice piece or valve in thedischarge from the evaporator. The evaporation then occurs as the liquid flashes
in the entrance area to the separator.
The suppression of boiling, together w ith the high circulation rate in the plate
heat exchanger, result in less fouling than w ould occur in other types of
evaporators. This increases the length of production runs betw een cleanings.
In addition, the narrow gap and corrugations in the plate passages result in far
higher heat transfer rates than w ould be obtained in shell and tube system s.
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For m ost duties the conventional gasketed plate heat exchanger is specified.H ow ever for duties w here the process fluid could attack the gasket, APV can offer
the w elded plate pair exchanger w hich elim inates elastom er gaskets on the
process side.
The Paraflash can be used either as a single or m ultiple effect evaporator.
Since m any system s are not physically large, the equipm ent can often be fully
preassem bled on a skid prior to shipm ent. Preassem bly reduces installation tim e
and, in m ost cases, significantly low ers the overall project cost.
Because of the large range of viscosities that can be handled in a Paraflash, this
form of evaporator can econom ically handle a w ider range of duties than any
other evaporator. In particular, due to the high turbulence and corresponding high
shear rates, the Paraflash system is excellent at handling non-N ew tonian fluids w ith
high suspended solids content.
Som e typical duties that are perform ed in a Paraflash include:
C oncentration of w ash w ater from w ater based paint plants to recover
the paint and clean the w ater.
Rem oval of w ater from dyestuffs prior to drying.
Finishing concentrator on w aste products from brew eries and distilleries.
C oncentration of brew ers yeast.
C oncentration of kaolin slurries prior to drying.
Recovery of solvents in w astes from cleaning operations.
Evaporation of solvents from pharm aceutical products.
C rystallization of inorganic salts.
C heese w hey.
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E V A P O R A T O R
T Y P E S E L E C T I O N
The choice of an evaporator best suited to the duty on hand requires a num ber
of steps. Typical rules of thum b for the initial selection are detailed below .
A selection guide (Figure 16), based on viscosity and the fouling tendency
of the product is show n below on next page.
MODE OF EVAPORATION
The user needs to select one or m ore of the various types of evaporator m odes
that w ere described in the previous section. To perform this selection, there are a
num ber of rules of thum bw hich can be applied.
Falling film evaporation:
either plate or tubular, provides the highest heat transfer coefficients.
is usually the m ode chosen if the product perm its.
w ill usually be the m ost econom ic.
is not suitable for the evaporation of products w ith viscosities over 300cp.
is not suitable for products that foul heavily on heat transfer surfaces
during boiling.
Forced circulation evaporators:
can be operated up to viscosities of over 5,000cp.
w ill significantly reduce fouling.
are expensive; both capital and operating costs are high.
Paravap evaporators:
are suitable for viscosities up to 10,000cp for low fouling duties.
are suitable for very high viscosities, i.e., over 20,000cp, usually the only
suitable evaporation m odes are the w iped film and thin film system s.
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FILM EVAPORATORSPLATE OR TUBULAR
Plate evaporators:
provide a gentle type of evaporation w ith low residence tim es and are
often the choice for duties w here therm al degradation of product can occur.
often provide enhanced quality of food products.
require low headroom and less expensive building and installation costs.
are easily accessed for cleaning.
provide added flexibility, since surface area can easily be added or rem oved.
Tubular evaporators:
are usually the choice for very large evaporators.
are usually the choice for evaporators operating above 25 psia (1.7 bar).
are better at handling large suspended solids.
require less floor space than plate evaporators.
have few er gasket lim itations.
FORCED CIRCULATION EVAPORATORSPLATE OR TUBULAR
Plate system s w ill provide m uch higher H TC s for all duties. W ith viscous products,
the plate exhibits vastly im proved perform ance com pared w ith a tubular.
Tubular system s m ust be selected w hen there are particulates over 2m m diam eter.
THE APV PARAVAP
For low fouling viscous products such as high brix sugar, the Paravap
system is alw ays the preferred solution.
Forced Circulation Plateor Tubular
Recircu-latedFilm
Paravap
Film
Fouling
Viscos
ity
Wiped Film
HighLow
High
This diagram shows aselection guide based onthe viscosity and foulingtendency of the product.
Figure 16
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MATERIALS OF CONSTRUCTION
The tw o param eters w hich control the selection of the m aterial of construction are
corrosion and ease of cleaning.
All evaporators for hygienic duties m ust be capable of being frequently cleaned in
place. In m ost cases, this m eans rinsing the equipm ent w ith w ater, follow ed by
w ashing w ith caustic and then acid cleaning agents, and finally, a further rinsing
w ith w ater. It is im portant, particularly w ith dairy and m eat products, that the
evaporator is com pletely cleaned of all deposits. The cleaning processes elim inate
the use of carbon steel as the m aterial of construction. M ost hygienic evaporators
are m anufactured in either 304 or 316 stainless steel.
C orrosion is often a m ajor problem w ith chem ical duties and som e hygienic
applications. A particular problem w ith evaporators is the range of concentration
of solids in the process fluid, since the corrosive com ponent w ill be concentrated
as it passes through the evaporator. In som e evaporators, the concentration rangecan be as high as 50 to 1. For exam ple, w aste w ater w ith a chloride content of
40ppm in the feed w ould have 2000ppm in the product. W hile stainless steel
w ould be acceptable for the initial stages of evaporation, a m ore corrosion
resistant m aterial w ould be required for the last one or tw o stages.
C orrosion is also a m ajor consideration in the selection of gasket m aterials. This
is particularly im portant w ith plate evaporators w ith elastom eric gaskets sealing
each plate. M any solvents such as chlorinated and arom atic com pounds w ill
severely attack the gaskets. A less obvious form of attack is by nitric acid. This is
im portant since nitric acid can be present in som e cleaning m aterials. W hile
concentrations of about 1% up to 140F (60C ) can be accepted, it is best to
elim inate nitric acid from cleaning m aterials. Phosphoric and sulfam ic acids are
less aggressive to gaskets.
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It is not the purpose of this handbook to provide guidelines for the selection ofm aterials of construction. The reader is referred to the APV C orrosion H andbook,
as w ell as the m any publications issued by the m aterial m anufacturers.
Typical m aterials of construction for a num ber of evaporator applications are
show n below :
In som e cases, the type of evaporator is controlled by the m aterials of construction.
For exam ple a sulfuric acid evaporator, w here the acid concentration can reach
50%, w ould utilize graphite tubular heat exchangers and non-m etallic separators
and piping.
PRODUCT MATERIAL OF CONSTRUCTION
Most dairy and food products 304/316 stainless steel
Most fruit juices 316 stainless steel
Sugar products Carbon steel /304/316
Foods containing high salt (NaCl) Titanium/Monel
High alloy stainless steels
Duplex stainless steels
Caustic soda < 40% Stress relieved carbon steel
Caustic soda high concentration Nickel
Hydrochloric acid Graphite/Rubber lined carbon steel
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E V A P O R A T O RC O N F I G U R A T I O N S F O R
E N E R G Y C O N S E R V A T I O N
C onservation of energy is one m ajor param eter in the design of an evaporator
system . The larger the evaporation duty, the m ore im portant it is to conserve energy.
The follow ing techniques are available:
MULTI-EFFECT EVAPORATION
M ulti-effect evaporation uses the steam produced from evaporation in one effect
to provide the heat to evaporate product in a second effect w hich is m aintained
at a low er pressure (Figure 17). In a tw o effect evaporator, it is possible to
evaporate approxim ately 2 kgs of steam from the product for each kg of steam
supply. As the num ber of effects is increased, the steam econom y increases. O n
som e large duties it is econom ically feasible to utilize as m any as seven effects.
Increasing the num ber of effects, for any particular duty, does increase the
capital cost significantly and therefore each system m ust be carefully evaluated.
In general, w hen the evaporation rate is above 3,000 lbs/h (1,350 kg/h),
m ulti-effect evaporation should be considered.
Figure 17
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THERMO VAPOR RECO MPRESSION (TVR)
W hen steam is available at pressures in excess of 45 psig (3 barg) and
preferably over 100 psig (7 barg), it w ill often be possible to use therm o vapor
recom pression. In this operation, a portion of the steam evaporated from the
product is recom pressed by a steam jet venturi and returned to the steam chest of
the evaporator. A system of this type can provide a 2 to 1 econom y or higher
depending on the product the steam pressure and the num ber of effects over
w hich TVR is applied.
TVR is a relatively inexpensive technique for im proving the econom y of evaporation.
TVR can also be used in conjunction w ith m ulti-effect to provide even larger
econom ies (Figure 18). Show n in (Figure 19) are the econom ies that can
be achieved.
Therm ocom pressors are som ew hat inflexible and do not operate w ell outside the
design conditions. Therefore if the product is know n to foul severely, so that the
heat transfer coefficient is significantly reduced, it is best not to use TVR. The
num ber of degrees of com pression is too sm all for m aterials that have high
boiling point elevation.
Figure 18
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MECHANICAL VAPOR RECOMPRESSION (MVR)
Therm odynam ically, the m ost efficient technique to evaporate w ater is to use
m echanical vapor therm ocom pression. This process takes the vapor that has been
evaporated from the product, com presses the vapor m echanically and then uses
the higher pressure vapor in the steam chest(Figure 20).
The vapor com pression is carried out by a radial type fan or a com pressor. The
fan provides a relatively low com pression ratio of 1:30 w hich results in high heat
transfer surface area but an extrem ely energy efficient system . Although higher
com pression ratios can be achieved w ith a centrifugal com pressor, the fan has
becom e the standard for this type of equipm ent due to its high reliability, low
m aintenance cost and generally low er RPM operation.
Figure 20
Figure 19
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This technique requires only enough energy to com press the vapor because thelatent heat energy is alw ays re-used. Therefore, an M VR evaporator is equivalent
to an evaporator of over 100 effects. In practice, due to inefficiencies in the
com pression process, the equivalent num ber of effects is in the range 30 to 55
depending on the com pression ratio.
The energy supplied to the com pressor can be derived from an electrical m otor,
steam turbine, gas turbine and internal com bustion engine. In any of the cases the
operating econom ics are extrem ely good.
Since the costs of the com pressors are high, the capital cost of the equipm ent w ill
be significantly higher than w ith m ulti-effect. H ow ever in m ost cases, for m edium
size to large evaporators, the pay back tim e for the addition capital w ill only
be 1 to 2 years.
Like the one TVR, the tw o M VR system is not appropriate for high fouling duties or
w here boiling point elevation is high.
COMBINATION OF FILM AND
FORCED CIRCULATION EVAPORATORS.
The m ost econom ic evaporators utilize falling film tubulars or plates, w ith either TVR
or M VR. H ow ever w ith m any duties, the required concentration of the final product
requires a viscosity that is too high for a film evaporator. The solution is to use film
evaporation for the pre-concentration and then a forced circulation finisher
evaporator to achieve the ultim ate concentration; e.g., a stillage or spent distillery
w ash evaporator. The m aterial w ould typically be concentrated from 4% to 40% in
a falling film evaporator and then from 40% to 50% in a forced circulation
evaporator. Usually the finisher w ould be a com pletely separate evaporator since
the finisher duty is usually relatively low . In the duty specified above, alm ost 98%
of the evaporation w ould take place in the high efficiency
film evaporator.
For cases w here the finisher load is relatively high, it is possible to incorporate
the forced circulation finisher as one of the effects in a m ulti-effect evaporator.
H ow ever this is an expensive proposition due to the low coefficients at the
high concentration.
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R E S I D E N C E T I M E I N
F I L M E V A P O R A T I O N
Since m any pharm aceutical, food and dairy products are extrem ely heat
sensitive, optim um quality is obtained w hen processing tim es and tem peratures
are kept as low as possible during concentration of the products. The m ost critical
portion in the process occurs during the brief tim e that the product is in contact
w ith a heat transfer surface w hich is hotter than the product itself. To protectagainst possible therm al degradation, the tim e/tem perature relationship therefore
m ust be considered in selecting the type and operating principle of the evaporator
to be used.
For this heat sensitive type of application, film evaporators have been found to
be ideal for tw o reasons. First, the product form s a thin film only on the heat
transfer surface rather than occupying the entire volum e, greatly reducing
residence tim e w ithin the heat exchanger. Second, a film evaporator can operate
w ith as low as 6F (3.5C ) steam -to-product tem perature difference. W ith both the
product and heating surfaces close to the sam e tem perature, localized hot spots
are m inim ized.
As previously described, there are rising film and falling film evaporators as w ell
as com bination rising/falling film designs. Both tubular and plate configurations
are available.
COMPARISON OF RISING FILM
AND FALLING FILM EVAPORATORS
In a rising film design, liquid feed enters the bottom of the heat exchanger and
w hen evaporation begins, vapor bubbles are form ed. As the product continues up
either the tubular or plate channels and the evaporation process continues, vapor
occupies an increasing am ount of the channel. Eventually, the entire center of the
is filled w ith vapor w hile the liquid form s a film on the heat transfer surface.
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The effect of gravity on a rising film evaporator is tw ofold. It acts to keep theliquid from rising in the channel. Further, the w eight of the liquid and vapor in
the channel pressurizes the fluid at the bottom and w ith the increased pressure
com es an increase in the boiling point. A rising film evaporator therefore requires
a larger m inim um T than a falling film unit.
The m ajority of the liquid residence tim e occurs in the low er portion of the channel
before there is sufficient vapor to form a film . If the liquid is not preheated above
the boiling point, there w ill be no vapor. And since a liquid pool w ill fill the entire
area, the residence tim e w ill increase.
As liquid enters the top of a falling film evaporator, a liquid film form ed by gravity
flow s dow n the heat transfer surface. During evaporation, vapor fills the center of
the channel and as the m om entum of the vapor accelerates the dow nw ard
m ovem ent, the film becom es thinner. Since the vapor is w orking w ith gravity, a
falling film evaporator produces thinner film s and shorter residence tim es than a
rising film evaporator for any given set of conditions.
TUBULAR AND PLATE FILM EVAPORATORS
W hen com pared to tubular designs, plate evaporators offer im proved residence
tim e since they carry less volum e w ithin the heat exchanger. In addition, the height
of a plate evaporator is less than that of a tubular system .
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(1)
(2)
(3)
(4)
(5)
1RL= 1
1 + ( 1 y ) ( 2 V ).5
y L
ESTIMATING RESIDENCE TIME
It is difficult to estim ate the residence tim e in film evaporators, especially rising film
units. C orrelations, how ever, are available to estim ate the volum e of the channel
occupied by liquid.Formula (1) is recom m ended for vacuum system s.
For falling film evaporators, the film thickness w ithout vapor shearing can be
calculated by Formula (2).
Since the film is thin, this can be converted to liquid volum e fraction in a tubular
evaporator by Formula (3).
For a falling film plate evaporator,Formula (4) is used. As liquid travels dow n the plate
and evaporation starts, vapors w ill accelerate the liquid. To account for this action, the
rising film correlation is used w hen the film thickness falls below that of a falling film
evaporator. In practice, the film thickness m ay be less than estim ated by either m ethod
because gravity and vapor m om entum w ill act on the fluid at the sam e tim e.
O nce the volum e fraction is know n, the liquid residence tim e is calculated by
form ula (5). In order to account for changing liquid and vapor rates, the volum e
fraction is calculated at several intervals along the channel length. Evaporation is
assum ed to be constant along w ith channel length except for flash due to high
feed tem perature.
SYMBOLS
A cross sectional area
d tube diameter
g gravitational constant
L tube length
m film thickness
RL liquid volume fraction
qL liquid rate
t time
liquid wetting rate
L liquid density
V vapor density
liquid viscosity
y local weight fraction of vapor
References
a) HTRI report, BT-2, pg. 7 (May
1978)
b) Perry's Chemical Engineer's
3 m = [ ]
1/3g
L(
L
v)
4mRL=
d
2mRL=
Z
t =RLAL
qL
FORMULAS
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The table above show s a com parison of contact tim es for typical four-effect
evaporators handling 40,000 lb/h (18,000 kg/h) of feed. The tubular designs
are based on 2 in. (51 m m ) O D tube, 30 feet (9m ) long. Incidentally, designs
using different tube lengths do not change the values for a rising film tubular system .
The given values represent total contact tim e on the evaporator surface, w hich is
the m ost crucial part of the processing tim e. Total residence tim e w ould include
contact in the preheater and separator, as w ell as additional residence w ithin
interconnecting piping.
W hile there is no experim ental data available to verify these num bers, experience
w ith falling film plate and tubular evaporators show s that the values are
reasonable. It has been noted thatFormula (2) predicts film thicknesses that are
too high as the product viscosity rises. Therefore, in actuality, 4th effect falling film
residence tim es probably are som ew hat shorter than charted.
SUMMARY
Film evaporators offer the dual advantages of low residence tim e and low
tem perature difference w hich help assure a high product quality w hen
concentrating heat sensitive products. In com paring the different types of film
evaporators that are available, falling film designs provide the low est possible
T, and the falling film plate evaporator provides the shortest residence tim e.
CONTACT RISING RISING FALLING FALLINGTIME FILM TUBULAR FILM PLATE (C) FILM TUBULAR FILM PLATE
1st effect 88 47 (A) 23 16 (A)
2nd effect 62 20 22 13
3rd effect 118 30 15 9
4th effect 236 (A) 78 (A) 123 (A) 62 (B)
Total
Contact 504 175 183 100
Time
(A) two stages (B) three stages (C) plate gap .3 in (7.5mms)
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D E S I G N I N G F O R
E N E R G Y E F F I C I E N C Y
Although the concentration of liquids by evaporation is an energy intensive
process, there are m any techniques available, as detailed in previous sections, to
reduce the energy costs. H ow ever, increased energy efficiency can only be
achieved by additional capital costs. As a general rule, the larger the system , the
m ore it w ill pay back to increase the therm al efficiency of the evaporator.
The problem is to select the correct technique for each application.
The m ain factors that w ill affect the selection of the technique are detailed below .
EVAPORATION RATE
The higher the capacity of the evaporator, the m ore the designer can justify
com plex and expensive evaporation system s in order to provide high
energy efficiency.
For evaporator design purposes, the capacity is defined as the evaporation rate
per hour. H ow ever, in som e applications such as seasonal fruit juice processors,
the equipm ent is only operated for part of the year. This m eans that an expensive
evaporator is idle for part of the year. The econom ic calculation has to include
annual operating hours.
For low capacities the designer is less concerned about energy efficiency. If the
evaporation rate is below 2,200 lb/h (1000 kg/h), it is difficult to justify m ulti-
effect evaporation. Usually a single-effect evaporator, often w ith therm o vapor
recom pression (TVR), is the system of choice at this capacity.
In m any cases, m echanical vapor recom pression (M VR) is the m ost efficient
evaporator. H ow ever, these system s operate at a low tem perature difference,w hich results in high heat transfer area. Also M VR requires either a centrifugal
com pressor or a high pressure fan w hich are expensive equipm ent item s.
These cannot usually be justified for low capacity evaporators.
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STEAM/ ELECTRICITY COSTS
For m edium to large duties, a selection has to be m ade betw een m ulti-effect and
M VR. A critical param eter that w ill affect this selection are steam costs relative to
electricity costs. Providing process conditions are favorable, M VR evaporation w ill
be m ore econom ic, particularly in areas w here the electricity cost is low , such as
localities around m ajor hydro generating plants. H ow ever if low cost steam is
available, even at pressures as low as atm ospheric, then m ulti-effect evaporation
w ill be usually m ore econom ic due to the low er capital cost.
STEAM PRESSURE
The availability of steam at a m edium pressure of about 100 psig (7 barg),
perm its the efficient use of TVR either on a single or m ulti-effect evaporator. TVR
can be applied across one, tw o or even three effects. This is the sim plest and
least costly technique for enhancing evaporator efficiency. The effectivenessdeclines significantly as the available steam pressure is reduced.
MATERIAL OF CONSTRUCTION
The m ajority of evaporators are m ade in 304 or 316 stainless steel. H ow ever
there are occasions that m uch m ore expensive m aterials of construction are
required, such as 904L, 2205, nickel, H astelloy C , titanium and even graphite.
These expensive m aterials skew the econom ic balance, w ith the capital cost
becom ing m ore significant in the equation. Typically M VR w ould becom e
less econom ic as the m aterial cost increases, due to the size of the heat
exchangers required.
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P H Y S I C A L P R O P E R T I E S
There are a num ber of physical properties that can severely influence the selection
of an evaporator.
BOILING POINT ELEVATION
A boiling point elevation of over 5F (3C ) essentially elim inates M VR
evaporators from consideration. This can be partially circum vented by using M VR asa pre concentrator. O nce the concentration is sufficient to produce significant boiling
point elevation, the final evaporation w ould be perform ed in a steam driven finisher.
PRODUCT VISCOSITY
H igh product viscosity of over 300 to 400cp usually elim inates falling film
evaporators in favor of forced circulation. Forced circulation requires a higher
tem perature difference, w hich elim inates M VR. TVR is used on som e duties.
PRODUCT FOULING
Both M VR and TVR are not particularly suitable for duties w here severe fouling of
heat transfer surfaces occurs over a short tim e period. The perform ance of these
evaporators w ill fall off m ore rapidly than w ith a m ulti-effect system . Forced
circulation evaporators w ith suppressed boiling usually perform better w ith highfouling than film evaporators.
TEMPERATURE SENSITIVE PRODUCTS
M any products, particularly in the food industry, are prone to degradation at
elevated tem peratures. The effect is usually m ade w orse by extended residence tim e.
This problem lim its the tem perature range for m ulti-effect system s. For exam ple on
a m ilk evaporator, the tem perature is lim ited to a m axim um of 160F (71C ).
Since a typical m inim um boiling tem perature is 120F (49C ), there is a lim ited
tem perature difference to perform the evaporation. This type of duty is suitable for
M VR since the evaporation occurs at essentially the sam e tem perature. Although a
low er operating tem perature increases the size of the m ajor equipm ent, M VR is the
m ost econom ic solution for large capacity dairy evaporators.
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In m any cases the selection of the energy conservation technique is obvious.H ow ever, for m any applications it is necessary to evaluate a num ber of techniques
in detail before a decision can be m ade.
The follow ing case study illustrates the various options to save energy using
different techniques.
The duty is to concentrate skim m ilk from 8% solids to 48% by evaporation.
The feed rate is 100,000 lb/h (45,500 kg/h). The data show n in the table
below sum m arizes the perform ance and costs for a straight 5 effect evaporator,
a 5 effect evaporator w ith TVR across 3 effects, and a m echanical vapor
recom pression evaporator.
N o pasteurizer is included in this cost com parison.
Annual operating costs are based on 7,000 h/ year of operation, w ith a steam
cost of $5.50/1000 lb (454 kg) and electricity at $0.068/kw h.
The exam ples illustrate that w ith a higher capital investm ent, it is possible to
significantly reduce the operating costs of the equipm ent. H ow ever the m ost
econom ic selection is controlled by the steam and electricity prices. For exam ple,
if the dairy is located alongside an electric co-generation plant, the steam cost
w ould be reduced considerably low er, and a steam heated evaporator w ould be
the m ost econom ic.
A less im portant, but still significant factor, is the cost of cooling w ater. An M VR
evaporator requires virtually no cooling w ater. O n a steam heated system the
cooling w ater requirem ent is about 6 US gallons per lb (.05 m 3 per kg) of
steam applied.
5 EFFECT 5 EFFECT WITH TVR MVR
Evaporation lb/h 83,000 83,000 83,000
kg/h 37,900 37,900 37,900
Steam Consumed lb/h 17,000 11,000 0
kg/h 7,700 5,000 0
Absorbed power kw 70 60 560
Annual costs
Steam $654,500 $423,500 0
Electricity $33,300 $28,560 $266,560Total $687,800 $452,060 $266,560
Capital costs
Equipment $2,200,000 $2,600,000 $2,700,000
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M E C H A N I C A L V A P O R
R E C O M P R E S S I O N
E V A P O R A T O R S
M echanical vapor recom pression (M VR) evaporation provides an extrem ely
energy efficient technique for the concentration of solids in w ater. Usually the
capital cost of an M VR system is higher than a com parable steam driven
evaporator. H ow ever, as the capacity of the system increases the relative cost
difference decreases. Although M VR evaporators are seldom chosen for sm all
duties, the concept is often used for m edium to large capacity evaporators.
MVR DEFINED
The basic principle of M VR is to rem ove the steam that is evaporated from the
product, com press it in a m echanical device, and use the higher pressure steam ,
w hich has a corresponding higher saturation tem perature, to provide the heating
m edium for the evaporation (Figure 21). N o steam input is required once the
system is operating. The sm all difference in enthalpy betw een the vapors on the
condensing and boiling sides is the theoretical energy required to perform the
evaporation. Essentially, the process re-uses the latent heat of the vapors. The
theoretical therm al efficiency of M VR can exceed that of a 100 effect evaporator,
although there are a num ber of practical lim itations, such as
com pressor and m otor efficiencies w hich low er
the achievable efficiency.
The m echanical device
can be a centrifugal
com pressor for applications
w ith high com pression ratios,
or a fan for low er com pression
ratios. For either device, the
drive can be an electric m otor,
steam turbine, internal
com bustion engine or
gas turbine.
Figure 21
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THERMODYNAMICS OF MVR
The process is best explained by reference to the M ollierenthalpy/entropy
diagram for steam (Figure 22).
The vapor evaporated from the product is represented on the M ollier diagram
at point A. The actual values in U S and m etric units are presented on
Table 1 a and 1 b. The vapor enters the com pressor at point A. The vapor is then
com pressed to the higher pressure, at constant entropy at point B. The com pressor,w hich in this case is a fan, has inefficiencies w hich results in an increase in
entropy above that of the entropy at inlet. This is represented by point C . Vapor
at point C is at the required pressure for the steam jacket of the condenser.
H ow ever, it is superheated and m ust be cooled in order to condense in the
evaporator. This cooling can be perform ed on the heat transfer surface of the
evaporator. H ow ever, since desuperheating H TC s are usually low , the
desuperheating is usually perform ed by the introduction of a spray of condensateinto the vapor duct. This condensate vaporizes as the vapor is cooled back to the
saturation tem perature, and generates m ore vapor. This condition is represented by
point D. At this point m ost of the vapor is condensed in the evaporator. H ow ever,
there is an excess of vapor, w hich is required for heat loss and/or preheat duties.
Any balance is condensed or vented.
S-ENTROPY BTU/LBF
H-E
NTROPYBTU/LB
DESUPERHEAT
-17
.2P
SI
ENTHALPY
DIFFERENCEAT CONDENSINGTEMPERATURES2.9 BTU/LB
SUPERHEAT
AVAILABLE FORGENERATINGEXCESS VAPOR11.2 BTU/LB
COMPRESSOR
ENERGY
INPUT14
.1
BTU/LB
VAPORLIQUIDMIXTURE
T=243F
T=220F
T=212F
1164.6
1153.4
1150.5
1.7442 1.7568 1.7596
A
B
D
C
SUPERHEATEDVAPOR
IDEAL
COMPRESSION
ACTUAL
COMPRESSION
14.7PSIA
SATURATEDVAPOR
Figure 22
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In this exam ple, the heat required to evaporate the w ater is 970.3 Btu
(539.0 Kcals). H ow ever the com pressor input is only 14.1 Btu (7.8 Kcals), w ith
m otor and gearbox losses increasing this to 14.7 Btu (8.16). The equivalent
econom y is 66 to 1 .
It should be noted that pressure losses through the evaporator ducting, calandria
and separator m ust be absorbed. This can be achieved by either a higher boost
from the com pressor at a higher pow er, or by accepting a low er tem perature
difference and increasing the surface area of the calandria.
PROPERTIES OF WATER VAPOR
Pressure-PSIA 14.7 17.2 17.2
State Saturated Saturated Superheated
Temperature F 212 220 243
H-Enthalpy Vapor BTU/LB 1150.5 1153.4 1164.6
Latent Heat BTU/LB 970.3 965.2
H-Enthalpy Liquid BTU/LB 180.2 188.2
S-Enthalpy BTU/LB F 1.7568 1.7442 1.7596
Ta ble 1a . Properties of Water Vapor.
PROPERTIES OF WATER VAPOR
Pressure-BAR 1 1.17 1.17
State Saturated Saturated Superheated
Temperature C 100 104 117
H-Enthalpy Vapor Kcals/Kg 639.1 640.8 647.0Latent Heat Kcals/Kg 539.0 536.2
H-Enthalpy Liquid Kcals/Kg 100.1 104.6
S-Enthalpy Kcals/Kg C 1.7568 1.7442 1.7596
Table 1b. Properties of Water Vapor.
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Table 2. Saturated T at Various Compression Ratios, F and C.
COMPRESSION RATIO SATURATED T AT BOILING TEMPERATURES
130F 55C 212F 100C
1.2 6.9 3.8 9.3 5.2
1.4 12.9 7.2 17.5 9.7
1.6 18.2 10.1 24.7 13.7
1.8 23.0 12.8 31.2 17.3
2.0 27.3 15.1 37.2 20.7
TYPES OF CO MPRESSION EQUIPMENT
To a large extent, the developm ent of this technology has been guided by the
capabilities of the various types of com pressors. The key to the design is the
tem perature difference that is available for the evaporator.Table 2 show s the
tem perature difference for various com pression ratios at tw o different boiling
tem peratures. It is this tem perature difference that is available as the driving force
for evaporator, less a sm all am ount required in the form of lost pressure around the
system . The original M VR system s used com pressors w ith com pression ratios of
about 1.4, w hich lim ited the available tem perature difference to 13 to 18F (7 to
10C ). This lim its the M VR to single effect operation. M ore advanced centrifugal
com pressors w ere developed in the 1970s, w hich provided com pression ratios of
approxim ately 2. This provided a m uch higher tem perature difference, w hich
allow ed operation w ith 2 and 3 effects. This reduces the flow to the com pressor
and increases the operating efficiency.
A num ber of evaporators w ere built w ith the higher com pression system s in m ulti-
effect m ode. Unfortunately, the reliability of the com pressors becam e a problem .
Because the com pressor operates at high speed, it has to be protected from
im pingem ent of w ater droplets. This usually requires a m ist elim inator in the
separator, follow ed by a superheater. Any solids carryover w ill have a detrim ental
effect on the com pressor. In addition, the designer m ust take care to preventunstable com pressor operation (surging). W hile the m ajority of the com pressors
functioned w ell, there w ere a few catastrophic com pressor failures. These failures
caused engineers to review alternative equipm ent.
The answ er to the com pressor problem w as to use a fan. This equipm ent operates
at a low er speed and is less vulnerable to dam age from droplets. Fans are also
far less likely to surge. W hen operated w ith a variable frequency drive, the fans
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provide far greater flexibility than com pressors. The only disadvantage to the fan isthat com pression ratios are lim ited to about 1.45. This results in a low available
tem perature difference and therefore a high heat transfer area. H ow ever, the
energy efficiency of such system s is very high w ith the equivalent of 55 effects
achievable for m any duties.
POWER REQUIREMENTS
The com pressor pow er requirem ents to evaporate 1000 lb/h (454 kg/ h) of
steam at various com pression ratios and tem peratures are show n in
Tables 3a and 3b. Sim ilar data for fans are show n in Tables 4a and 4b.
These data correspond quite w ell w ith installed M VR system s.
A m ore detailed com parison betw een three actual system s is show n in Table 5. The m ore
energy efficient system is the single effect fan w ith a low com pression ratio. H ow ever, the
low tem perature difference w ill result in high heat transfer area in the calandria. In m ost
cases the added capital cost w ill be justified by low er operating costs.
Table 3a . Power VsT for Centrifugal Compressors Based on 1000 lb/ h of Steam Evaporated.
COMPRESSORS
Boiling Temp 130F Boiling Temp 170F Boiling Temp 212F
CR T kw CR T kw CR T kw
1.3 10.00 7.0 1.3 11.65 7.5 1.3 13.53 7.9
1.4 12.91 9.1 1.4 15.03 9.7 1.4 17.47 10.3
1.6 18.21 12.9 1.6 21.23 13.8 1.6 24.69 14.7
1.8 22.98 16.5 1.8 26.81 20.0 1.8 31.20 18.7
2.0 27.31 19.8 2.0 31.89 21.1 2.0 37.16 22.5
2.2 31.30 22.9 2.2 36.57 27.2 2.2 41.31 24.9
Table 3b. Power VsT for Centrifugal Compressors Based on 454 kg/ h of Steam Evaporated.
COMPRESSORS
Boiling Temp 55C Boiling Temp 77C Boiling Temp 100C
CR T kw CR T kw CR T kw1.3 5.55 15.4 1.3 6.47 16.4 1.3 7.51 17.5
1.4 7.17 20.0 1.4 8.35 21.3 1.4 9.71 22.7
1.6 10.11 28.5 1.6 11.79 30.4 1.6 13.71 32.4
1.8 12.77 36.3 1.8 14.89 38.7 1.8 17.33 41.3
2.0 15.17 43.5 2.0 17.71 46.4 2.0 20.64 49.4
2.2 17.39 50.3 2.2 20.37 53.6 2.2 22.95 57.1
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Table 4b. Power Vs T for Fans Based on 454 kg/ h of Steam Evaporated.
FANS
Boiling Temp 55C Boiling Temp 77C Boiling Temp 100C
CR T kw CR T kw CR T kw
1.1 1.99 4.90 1.1 2.32 5.22 1.1 2.69 5.58
1.2 3.84 9.49 1.2 4.47 10.12 1.2 5.19 10.81
1.3 5.56 13.81 1.3 6.47 14.75 1.3 7.52 15.73
Ta ble 4a . Power Vs T for Fans Based on 1000 lb/ h of Steam Evaporated.
FANSBoiling Temp 130F Boiling Temp 170F Boiling Temp 212F
CR T kw CR T kw CR T kw
1.1 3.59 2.22 1.1 4.17 2.38 1.1 4.85 2.54
1.2 6.91 4.31 1.2 8.04 4.60 1.2 9.34 4.92
1.3 10.00 6.28 1.3 11.65 6.71 1.3 13.53 7.15
SMALL MVR EVAPORATORS
For sm all system s, rotary blow ers w ere occasionally specified in an attem pt to
m ake M VR econom ic at evaporation capacities less than 15,000 lb/h
(7000 kg/h). W hile there w ere cost savings, there w ere also reliability problem s
w ith this equipm ent for this particular application. The conclusion rem ained that for
sm all system s, it is usually best to use steam driven evaporators.
Table 5. Comparison of Typical MVR Designs Approximate boiling temperature 130F (55C) evaporation rate60,000 lb/ hr (27,000 kg/ h).
SINGLE EFFECT DOUBLE EFFECT TRIPLE EFFECT
FAN CENTRIFUGAL CENTRIFUGAL
Compression Ratio 1.2 1.6 2.0
Total T Available 6.9F (3.8C) 18.2F (10.1C) 27.3F (15.2C)
Vapor to Compressor 60,000 lb/h 30,000 lb/h 20,000 lb/h
(27,300 kg/h) (13,650 kg/h) (9,090 kg/h)
Total Power Kw 280 390 400
Equivalent Steam 870 lb/h 1315 lb/h 1347 lb/h
(395 kg/h) (600 kg/h) (612 kg/h)Equivalent
Steam Economy 69 45.6 44.5
Average T Per Effect 6.9F 9.1F 9.1F
Before Losses (3.8C) (5.1C) (5.1C)
NOTE: In this example, the fan horsepower is lower than either of the centrifugal
designs, but the lower T required the greater the surface area.
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E V A P O R A T O R S F O R
I N D U S T R I A L A N D
C H E M I C A L A P P L I C A T I O N S
The APV range of evaporators covers m any duties in the concentration of
chem icals and industrial products, w ith both film and forced circulation system s
being available as required.
Film evaporators are used w hen there is little or no risk of fouling of the heating
surfaces. W here such a risk is present, forced circulation units are recom m ended.
All designs are suitable for m ulti-effect evaporation. At low concentrations,
m echanical vapor recom pression can be em ployed.
After selecting the type of evaporator required for a particular duty, the m ost
im portant factor is the selection of the m aterials of construction. M any duties
can be handled w ith 316 stainless steel. For som e applications w here chloride
ions are present, higher grades of stainless steel, such as 904L, can be an
econom ic selection.
C ertain products are so corrosive that they cannot be processed in
conventional m etals. As an exam ple,
concentration of a sulfuric acid solution
of up to 50% at 302F (150C )
w ould call for m ain plant item s of
filam ent w ound fiberglass reinforced
epoxy resin, and heating and cooling
surfaces of im pervious graphite.
If the sulfuric acid solution is betw een
50 and 80% w ith tem peratures up to
230F (110C ), m ain plant item s should
be lead-lined m ild steel protected w ith
refractories or carbon tiles. H eat transfer
surfaces again w ould be of im pervious
graphite. A typical system (Figure 23)
is show n here. It should be noted that APV
does not m arket non-m etallic evaporators.
Figure 23
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TITANIUM SULFATE
The production of titanium dioxide pigm ents involves reaction betw een sulfuric
acid and the ore w hich contains iron, titanium sulfate and other com pounds.
After pretreatm ent, w hich includes the crystallization of iron as ferrous sulfate,
the liquor is heated and hydrolyzed to precipitate titanium dioxide. Prior to this
operation, the concentration of liquor has to be adjusted by the evaporation of
w ater. It is essential that this process takes place in an evaporator w ith short
heat contact tim es in order to avoid the prem ature hydrolysis that occurs w ith
prolonged heating, w hich subsequently causes fouling of the heat surface and
blockage of the tubes. Although the liquor contains a high proportion of sulfuric
acid, the presence of other ions in solution m ay inhibit corrosion, so that copper
often can be used for heat transfer surfaces. Titanium is another m aterial used
for this application.
G enerally, single or m ultiple effect rising film evaporators are used for this duty, thenum ber of effects being determ ined by throughput and by assessing the cost of
operation against the increase in capital required for additional equipm ent.
In som e cases, it is econom ically attractive to operate the evaporator as a single
effect unit at atm ospheric pressure using the vapor given off for preheating. The
liquor is discharged at a tem perature in excess of 212F (100C ), reducing the
subsequent therm al load at the hydrolysis stage.
PHOSPHORIC ACID
Phosphoric acid can be produced by the digestion of phosphate rock (calcium
phosphate and fluoride am ong others) in sulfuric acid, better know n as the "w et
process" acid. Since calcium sulfate norm ally is a constituent, scaling m ust be
considered. Phosphate rock varies in com position, and in general, periodic
cleaning is required even in forced circulation evaporators.
Sulfuric acid plants often are located along coastal areas, and a further problem
in concentration stem s from the use of sea w ater in the direct contact condensers.
W ith silica present in the phosphate rock, fluorine reacts to form hydrofluorosilicic
acid (H 2SiF6) w hich in turn, form s a sodium salt from the N aC l. Sodium fluorosilicic
can block the condensers.
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AMMONIUM NITRATES
This m aterial has several significant properties:
Low viscosity w hich allow s it to be concentrated to
99+% w /w w hen it is prilled.
Above 95%, am m onium nitrate has an extrem ely high boiling point
elevation w hich requires exceedingly high steam pressures for heating.
This presents considerable m echanical problem s.
Any organic im purity has the potential for explosion, such that extra low
carbon stainless steels m ust be used for heat transfer surfaces, and the use
of m ineral oils for heating is excluded.
The type of evaporator best suited for am m onium nitrate depends upon the
initial and final concentrations. For the range below 70% and up to 80-85%,
rising film m ulti-effect evaporator units have been used successfully. For 80-96%
concentrations, conventional falling film system s have been em ployed. Above96%, how ever, falling film w ith a heated air sw eep w ould be used due to
partial pressure conditions. In areas of relatively low hum idity, 99+% w ater
to w ater can be achieved.
AMMONIUM SULFATE
Am m onium sulfate is used in battery spacer plate production and also hasbeen crystallized. In this process, sm all but regular sized crystals are m ixed
w ith a PVC type plastic and dissolved out of the final sheet w hich then is used
as spacer plate. Stainless steel has been successfully em ployed as the m aterial
of construction.
BARIUM SALTS
The production of barium salt involves the use of sodium sulfide, a m aterial
w hich closely resem bles caustic soda in both physical and corrosive properties.
It generally is concentrated in a high vacuum crystallizer for the production of
barium hydroxide w ith rubber lined m ild steel being used as the m aterial of
construction due to corrosion considerations. W ith liquid tem peratures below
72F, tw o hydrates--m ono and pentacan be produced on separate flakers.
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GLYCERINE
Sw eet w ater glycerine containing no N aC l has been handled in sim ple stainless
steel film evaporators by salt and oleo chem ical producers. For sodium chloride
bearing liquors as in spent lye for industrial detergent and soap m anufacture,
cupronickel alloys m ust be used.
W hen the glycerine is contam inated w ith salt, the special application of forced
circulation crystallizers has been em ployed for the recovery of the glycerine liquor
and separation of the N aC l salts.
CAUSTIC SODA
The m ost com m on process for the m anufacture of caustic soda is the electrolysis
of sodium chloride brine. The electrolytic processes produce a caustic soda
solution that has to be concentrated by evaporation. This evaporation process
is difficult since caustic soda solutions have a high boiling point elevation (BPE).
At 50% concentration the BPE is about 80F (45C ). This lim its the num ber of
effects usually to three, w ith the evaporator operated in reverse flow so that the
highest concentration in on the first effect. This effect w ill typically operate at
over 260F (125C ).
An additional problem is that at high tem perature, caustic soda solutions corrode
stainless steel. The first and second effect calandrias are usually fabricated innickel, w hich is resistant to corrosion. The third effect can be Avesta 254SLX,
w hich is far less expensive than nickel. The vapor side of the evaporator can be
304 stainless steel and som etim es carbon steel.
Due to the high tem peratures, tubular falling film evaporators are the standard
for this application.
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SOLVENT RECOVERY
N ot all evaporation processes are lim ited to the rem oval of w ater. Som e
applications require the concentration of a solution of solids and organic solvents.
O rganic solvents are frequently used for the extraction of products from raw
m aterials or ferm ented broths. The solvent then has to be rem oved from the
extracted product. A typical application w ould be the evaporation of acetone
from vegetable oils.
For these types of duties it is necessary to use explosion proof electrical equipm ent
and intrinsically safe instrum entation. It is also necessary to observe environm ental
regulations particularly since som e solvents, such as m ethylene chloride are
classified as regulated com pounds w ith stringent discharge lim its to both air and
w ater. The system m ust be designed w ithout leaks and w ith conservation devices
on all possible vents. For evaporator selection, the norm al guidelines that are used
for the evaporator of w ater rem ain the sam e. The only m ajor difference is that
H TC s are significantly low er for organic solvents.
A solvent evaporator (Figure 2 4 ) is show n below .
Figure 24
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W A S T E W A T E R
E V A P O R A T O R S
As the w orld has becom e m ore concerned about the environm ent, there has been an
increase in the application of evaporator system s to w aste w ater treatm ent. These
types of evaporators essentially reduce the volum e of the w aste by rem oving and
recovering m ost of the w ater in the w aste. In som e applications, the concentrate
contains product of value, w hich can be sold or further processed in a dryer to a solidproduct. In cases w here the product has no value, the concentrate can be dried and
the resulting product buried in a landfill. Since the condensate from these evaporators
is usually quite pure, the w ater that is recovered can be used as boiler feed, as rinse
fluid for cleaning or m erely disposed into the sew er or directly into a river.
As w ith the processing of any w aste product, there are usually severe cost restraints
since, unless forced by regulation, few organizations w ish to spend valuable capital
on w aste treatm ent processes. The equipm ent therefore has to have both low
capital and operating costs.
The key points in the design of a w aste w ater evaporator are:
Flow rate. This is usually quite high for these evaporators.
Solids concentration in the feed. This is usually quite low .
Product concentration and the viscosity of the product at that concentration.
Problem s w ith possible volatile com ponents in the feed. C orrosive nature of the feed. Since m ay w aste w ater evaporators have
high concentration ratios, the effects of corrosion can be enhanced in
the final stages of evaporation.
Potential for fouling. This can be very serious in m any cases.
Boiling point elevation.
Because of the large duties, m echanical vapor recom pression (M VR) evaporation is
usually the choice. H ow ever if boiling point elevation is high, the M VR w ould
be lim ited to the pre concentration and w ould require a separate steam pow ered
single or m ulti-effect finisher to arrive at the final concentration. The presence of
volatile com ponents in the feed, such as ethanol, can also lim it the application of
M VR. In this case, it is usually necessary to rem ove the volatile com ponents in a
stripper colum n. In a m ulti-effect evaporator the stripper colum n can be placed
betw een effects, w hich allow s recovery of the heat needed to operate the stripper.
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BREWERY/ DISTILLERY EFFLUENTS
The effluents from brew ery and distillery plants are often processed w ith
evaporators to recover the w ater and produce a concentrated syrup, w hich can
be sold in liquid form , or added to the spent grains prior to drying. In this case the
solids have value as anim al food.
Both M VR and m ulti-effect evaporators have been used for these types of duty.
N orm ally falling film tubular calandrias w ould be used for the pre evaporator
w ith forced circulation plate or tubular evaporators as the finisher. As in m ost
applications the viscosity of the product at the evaporation tem perature controls
the point at w hich the m aterial has to be processed in a forced circulation system .
An M VR stillage evaporator (Figure 25) is show n below .
The problem w ith these applications is that the product, w hich can be called
stillage, spent w ash, spent grains or pot ale, is extrem ely variable. In particular the
viscosity characteristics of the concentrated liquids depend on the raw m aterialgrain. W aste w ater produced from plants using corn (m aize) as the feed stock are
relatively easy to process. H ow ever, w aste w ater from plants w hich use w heat or
barley as the feedstock w ill be far m ore viscous at elevated concentrations.
In som e cases, the viscosity characteristics w ill be so bad that even a forced
circulation evaporator w ill only be able to concentrate to 35% solids. This usually
m eans treating the feed w ith enzym es prior to evaporation so that a solids
concentration of 45% solids can be achieved.
The higher the viscosity, the m ore
frequently the evaporator w ill have to
be cleaned. The accepted run tim e in
the industry is 6 to 8 days. To achieve
this, it is usually necessary to provide
high recirculation around the calandrias
to provide a high w etting rate and
prevent burn on. O n the finisher, it is
occasionally necessary to provide
duplex heat exchangers, so that
one can be cleaned w hile the
other is in operation.
Figure 25
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BLACK LIQUOR
Black liquor is a caustic w aste w ater
generated at paper plants. The quantity
of this m aterial produced is large, and
som e of the largest evaporator applications
are w ith black liquor. An evaporator,
designed by A PV, for a feed rate of
170 ton/h (Figure 26), is show n here.
Typically black liquor contains about
3% solids, of w hich about half is caustic
soda. The presence of this caustic soda
results in quite high boiling point elevation as the concentration increases.
This places som e restrictions on the application of M VR, so that m ost black liquor
evaporators are m ulti-effect.
This w aste w ater also varies
from plant to plant, and the
concentrate properties are
extrem ely dependent on the
feedstock. The product viscosity
( Figure 2 7 ) is plotted against
% solids for different types offeedstock. In som e cases
there is a tenfold difference
in viscosity.
% SOLIDS
VISCOSITY OF BLACK LIQUORS AT 90C
BAMBOO (KRAFT) PINE (1) EUCALYPTUS
(PREHYDROLYSIS KRAFT) BAGASSE (KRAFT) BAGASSE (SODA)
STRAW (KRAFT) (3)NEWTONIANNON-NEWTONIAN
7010 20 30 40 50 600.3
100
101
102
103
CP
Figure 27
Figure 26
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T U B U L A R E V A P O R A T O R ST O S A N I T A R Y S T A N D A R D S
O ver the last 20 years the capacity for m any dairy applications has becom e
extrem ely large. The plate evaporator w hich w as prim arily designed for hygienic
duties has a m axim um capacity of about 60,000 lbs/h (27,000 kg/h). M any
duties in the dairy industry exceed this lim it, and therefore it is necessary to use
tubular evaporators.
To m eet the processing needs of these large food and dairy cooperatives, the
design and fabrication of tubular evaporators have been brought into com pliance
w ith the legal standards established by various official regulatory groups for
sanitary operations.
The tubular evaporator system consists prim arily of three com ponents:
A vertical heating tube bundle w hich is housed w ithin an insulatedsteam jacket or calandria.
A distribution device w hich allow s even liquid flow over the tube
circum ference, and therefore prevents 'dry spots' on the heat transfer surface.
A separator to efficiently extract liquid droplets from the vapor stream , and
thereby m inim ize carry-over of product.
To these are added basic feed tanks, preheaters, duct w ork, product and C IP
piping, and necessary pum ps.
3A DESIGN
W hile tubular evaporators designed for chem ical processing duties generally are
m anufactured of carbon or pickled stainless steel, the very nature of the products
handled in the dairy and food fields necessitates the use of highly polished
stainless steel for all product contact surfaces.
The basic evaporator effect, for exam ple, consists of a shell, a top and bottom
tube sheet, and a bundle of 2" diam eter seam less seal w elded heat transfer tubes
num bering anyw here from 100 to 1600 or m ore depending upon the
requirem ents of the evaporation duty. This is topped by a feed distribution system
and a cover or bonnet that clam ps to the calandria. Beneath this unit is the
product sum p or discharge area and adjacent, the vapor/liquid separator.
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Feed is charged to the system through an inlet at the top of the bonnet and floods
the surface of the distribution system (Figures 28 and 29). As the liquid flow s
through m any sm all holes, it spreads evenly across the entire face of the top tube
sheet w hich is m ounted about one inch below and eventually m oves into each of
the heat transfer tubes that extend the full height of the calandria.
Since there are m any product contact surfaces in this portion of the evaporator (the
inside of the feed inlet piping, both top and bottom of the distribution plate, the
top of the tube sheet, and the interiors of all tubes), the 304 stainless steel used is
polished to a #4/150-180-or-340 grit. APV finishes thus m atch and very often
exceed 3A standards w hich call for a surface finish equivalent to 150 grit.
Furtherm ore, the one inch riser w hich is added perpendicularly to the tube sheet to
form a product reservoir is fabricated w ith a m inim um 1/ 4" radius polished fillet
w eld. This elim inates any possibility of pockets in w hich product m ay collect.
G askets sealing the perim eter of both the distribution and tube sheet plates are
m ade of dairy grade neoprene.
W hile the evaporator shell norm ally is fabricated of 304 stainless, polishing is not
required since there are no product contact areas. At the bottom of the evaporatoreffect, all areas com e in contact w ith product and, consequently, are of polished
stainless. This includes the underside of the bottom tube sheet, the product sum p,
ductw ork leading to the adjacent separator and the interior of the separator itself.
W hen sight glasses, m anw ays, nozzles and light
glasses are added, interior w elds are ground and
polished sm ooth and flush w hile the com ponents
them selves are m ounted at a m inim um 3 angle toallow for proper drainage. Finally, dow nstream
ductw ork betw een vessels is sloped from 3 to 5
aw ay from the product contact surfaces. In effect, nothing is installed at less than a
90 angle, so that possible product traps are elim inated, and the entire system is
both cleanable and drainable (Figures 30). A further requirem ent is that the
external surface also be easily cl