-------

1. Refrigeration - For what, and how *1

A. Introduction 101 Refrigerating engineering is a techn.oiogy tJ create and maintain temperatures lower than

thG([urrounding. This implies that refrig :ration is the art of "pumping heat" from one body (or fluid) of lo\\' temperature to cae with higher. From that follows also that the technology in principle is identical to that of heat pumps - a technology for heating

There are a grm,ving number of applications for refrigeration. Common for these are that they satisfies basic human needs. The employment of refrigeration for food handling and storage and of temperature control of living spaces by air conditioning are examples where refrigeration help us to improve the standard of living. More details and examples of applications \vill later on be given in the text. Increased use of refrigeration and Increase of GNP goes hand in hand.

1 The classical, and in some respect still the most important application for refrigeration is in the handling, storage and supply ojJood products.

Other large areas of applications are in climate control and air conditioning, especialiy in warmer climates than what i.e. is prevailing in Sweden. In Nordic climates, instead, there are interesting applications for heat pumps Jar energy savings in buildings

1"\1so in industrial processes there are numerous applications: in many chemical industries as well as in mechanical workshops (for instance for treatment of certain alloys and local cooling).

For special applications in industry, in laboratories and in medical sciences, one need low temperatures The technology to achieve temperatures lower than -60 or -70°C is sometimes called CJyo engineering technology. A large application is the tedmology to manufacture gases, for instance by separation from atmospheric air (liquid nitrogen, oxygen etc. and gases like argon for \velding). Other large areas (for transport or storage) are for natural gas ("LNG", liquid natural gas like methane, or "LPG", liquid petroleum gas like propane).

Technologies for temperature control In supra-conducting materials are expected to grow in importance in the future.

Extremely low temperatures, approaching the absolute zero temperature, are of basic lnterest in physics and the study of matter. Special processes are required, but this area, will not be covered in any detail in this textbook

1. The concept oj temperature is important in refrigeration. For strict definitions the reader is referred to textbooks in thermodynamics. In most parts of the ,Yorld we nommlly use the CelslU5 temperature scale) t(CC) ,

Important is also the absolute temperature scale for \vhich we use the symbol T and Kelvin The relation between the and temperatures is as

follows'

I.

*/

-

1.03a

some parts USA) the temperature is more COITh'TIon than the The betv,reen the Fahrenheit and the Celsius scales is

1.03b

The equivalent Rankine temperature scale for abso :ite temperatures is

TfR) 1.03c

Comment for curiosity:

Lord Kelvin, after which the 3.bsolute ter:1perarure scale is named. the absolute terr,perature, In some respects this more relevant thlli'1 the linear, T(K), scale, for dd'ficult !t is to reach very lov. temperatures. The relation between the 1";0 scales can be Vinnen,

.03d

where Ba hence is a new rnermod:yna;-nic Ino""thrm{' temperature T is the actual absolute temperature and To a

reference temperature and C is a constant.

The constants C and To can be chosen ~t us for an example a temperatcLre scale tor Be that would

two points: Let Ba be 0 at O°C and 100 at 100 cC

(that is for T== 273,15 a.nd T= 373,15, respectively). These values would be achIeved if we, in equation 1.03d use To

=273,15andC 320,5.

Hence: e 320,5 The temperature scale wouid be hke the fol1ov/ing~

the same values as for the Celsius scale in the

a

T, K 8a

also a logari!'lmic temperature scale for how

6000 62/3,15 1004,5 900

327},15 <ll ' ­::l

1273; 15 493,3 6001il ' ­il:)

300Ioa 373,15 100,0 Q.

E50 323)15 53,9 .s 0

-300 -20 253,15 -24.4 -600 -50 223,15 -64,8

2 -900-10C- ! 73,15 146,1 ::l

-200 73,15 -422.3 -0 -1200(f)

.0 « 1500

3,15 -1438,3 -30 o 300 600 900

-273,00 0,J5 -2406,0 0 ~2758,l I.e

eels s temperature,-32'743)

..A.5 is seen the differences 'would be ~he differences Increase for

scale we kr:O\V it, than if the

to the

way linear Keh in

1:2

=

-- ------

1. , AND HOW

B. About history ... nore sun'ey about the de\(elopment may be appropriate to give here. Refrigerating

engineering has developed the' last century, although the basis in thermod}'namics was established somewhat earlier, as well as several important im'entions. The development 3b of the science of thermodynamics and tr . .:! "art" of creating low temperatures was tied together and, as often is the case, practic ; came before theory.

3c Since very old times ice is used for chilling of foodstuff Ice was "harvested" in wintertime and 'vvas stored to be used in surr1Jl1ertime in "ice piles" with simple insulation methods. It is found that there were religious ceremonies in China in connection with ice

e for harvesting. The early Romans knew the art of storing the ice, collected from the AJps,how

for use to cool foods and drinks. In Pompeii it has been found remains of what seem to have been an ice-cabinet Such Ice-cabinets are the predecessors of the kitchen refrigerators of today. In Stockholm - as in many cities of the world there was (still in

To a the SOties!) an elaborate system for ice distribution to household ice-cabinets and to grocery stores to help keep the milk and meat \vell cooled.

mid One old method to create temperatures lower than that of melting ice is to use "freezing mixtures". Such mixtures are based on the observation that, by adding salt to ice, the temperature will drop while the ice is melting. This was known to peopJe very early in

16thhistory, already in the century. A reminder is actually also the Fahrenheit temperature scale. Fahrenheit, a Dutch-Polish physicist (living 1686 - 1736), established in 1714 a temperature scale of his ovm where the zero temperature was set to the lowest temperature that he could create. As a second fi~eq point he used the body temperature which was set to lOOcF. The lowest temperature Fahrenheit achieved by mixing ice and a salt ("sal-ammoniac", a hydro chlorate of ammonia).

The zero-point of the Fahrenheit scale as we know it today is at -17.8"C This is a slightly lower temperarure than what can be achieyed by just mixing ice and ordinary salt, NaCL Furthennore lOO°F is corresponding to 37,8°C (equiyalent of a light fever.). .

The refrigerating effect that can be achieved when water is evaporating in aie evaporative cooling, was used in ancient times. The Egyptians had already long time ago a technique for storing water in clay containers or vases. These were not glazed, and this material then allows a certain diffusion of water, which is evaporated on the outer surface. This will keep the water cool and fresh, especially in a dry climate, like in a desert, where there is a large difference between the dry and wet temperatures of the ambient airo

1 For refrigeration by artifiCial means, the following is a short calendar indicating some important events in the history of the technology

or 5 Wiliiam Culien, a professor of the university of Glasgow rr:ade the obsen'ation e that he could freeze water to ice in a vacuum chamber in process of

evacuating the aiL At about the same time a German scientist, that was a m if the

was expanding.

points to two important of a gas

refugeration processes today.

Jacob Perkins, an English gentleman (who had emigrated to A..'l1erica), applies ior a patent (which receives the number 666'. in the English patent office) an Apparatus for Producing Cold and Cool ng Fluids". The main claim of

patent is, here somewhat shortened:

"All arrangement whereby I am enabled to use volatile fluids the purpose of producing the cooling or freezing fluids, and yet at the same time constantly condensing such volatile fluids and bringing them again and into operation without waste."

The description is still good for the basic principle used for vapor compression refrigerating cycles used to­day. Perkins had, as we have today, the problem to choose suitable working medium. One of his choices was ether, which was close at hand at that time How-

it led to unpleasant ether explosions, and was

'replaced. Figure 1.0Sa shows a picture of the Figure 1.05a. Perkins apparatus for Perkins apparatus. compression refrigeration

1849: John Gonie builds in England the first cold air machine

Edmond ,Carre of France builds a combined compression and absorption machine. \Vater and S02 are used in mixiure as the working fluid.

861: Thomas Mort (an EnglishInan in Australia) and Nicolle French engineer) build the first reingerated storage house known is erected m Sidney, Australia, for storage of meat which was to be shipped to Europe.

1862 1867: Ferdinand Carre (a brother of Edmond, as earlier ,,"'as mentioned) builds first absorption machine for refrigeration. He describes a periodic as vie!! as

a continuos machine, as well as such based in solid or liquid absorbents He is also working with compression machines (1 and uses as working medium (1 He was obviously working \\ith many different principles, still interest today, and is probably the inventor in history of refrigeration.

£: . .1. Carl Linde, another' in refrigerating history, builds plants relDgeratlOTI

in breweries Munich and at other places These uses as refrigerant he

14

i

HOWCHAPTER ic

compressors "Maschinenfabrik

mvn, A IS

first refrigeratetl transport of food between and \\as )plies managed. The steamship 'Jrigorifique" built by i\1r. Tellier (France) was used

an { his 1881 Several cold stores for refrigerat ;d storage were built in London and Boston,

1895' Carl Linde develops the "Linde process" for condensing air into liquid [annthe

time 1903 Abbe Audiffien designs the first fuIIy hennetic refrigeration unit. This in an

tgain interesting design where the whole unit rotates. For turning the built-in compressor when the unit is rotated the necessary torque is created by means of counter weights in the crankshaft. Figure l.OSb shows a section of the design,

7 6 5 4

Figure 1. 05b. A version ofan early hermetic refrigeration unit Aud~ffren-units were manufactured by Brown Baveri. (from Backstrom 1970)

The unit is encapsulating all components of the cycle inside a rotation shell which driven the wheel indicated to the right in the figure. The unit have two shells, 1 and 2, of it'hich 2 contains the evaporator. A pressure difference is maintained the

Ion compressor. The brine in container 10, is cooled by this rotating evaporator, inside which the vapor passes through the shaft to the shell on the right hand side the This one encloses the compressor and the condenser. The torque necessary to turn the shaft the compressor is created by a counterweight 3, as indicated in the small picture insert inleh theJigure, water isfurnished through connection 9,In

1909: The first artificially frozen ice skating rink is built, in Vienna

Ids 1911: J. A. Ot:esen, Denmark, develops methods for quick freezing foodstuff.as

~ IS The so called Ottesen-method involved the immersion of the products into a refi-jgerated salt solution. as

:f1t 1922: Baltzar von Platen and Munters to develop the

refrigerator without any moving parts (The project started as a diploma at the Roval Institute , KTH, Stockholm) heat. It was developed at Electrolux and became a mass productlon

umt IS

on with world wide distribution and sale 1 shows an

=

1

Platen-Munters principle is same media as proposed ammonia, water and hydrogen.

/ 1-:

<

Figure 1. 05c. The v Platen and A1unters unit for refrigeration without any moving parts.

1924: Clarence Birdsye of USA introduces a method (called the Birdseye-method) plate freezing of products.

1930: Midgley, Henne and McNary, USA, applies for a patent to use "Freon" as working medium in refrigeration. Primarily it is proposed to use CFiCh a fluid which later on was named R12 (or "Freon 12"). This chemical is a methane based halogenated hydrocarbon, (the chemistry of which was first described by

a Belgian scientist, already m 1893). create R (CR.:) is modified by replacing the atoms by t\\'o "_~'L,,'rn atoms. For this of chemical we, today, use the tenn from 1930 was the starting point of an enormous development,

see the The eFC-fluids are, reieased in into atmosphere, shmvn to have serious effects on the global environmental.

16

CHAPTER 1. HOW

The commercial is The water of

A heating capacity more than river Limmat, and the purpose of heat pump was for

erected tOWI1 haH Switzeriand the operating power \vas reported (i.e, COP] in excess of 3 for river

temperatures of+10°C and with radiator temperatures of about 35°C),

1961: Per Oskar Persson demonslrate :: a method for freezing of peas by fluidizing in a stream of cold aIr. The development at the company Frigoscandia subsequently led to a whole range of freezers for individual freezing of small products (like peas), called FloFREEZE, sold \vorldwide. See Figure L05d, Also several other designs for quick freezing of other types of foodstuff was later on developed,

1, Conveyor for liE­

frozen products

2, Product

3, Discharge of fro­zen products

4, Air coolers

5. Fans

?5c. Figure 1. 05d. S'ection ofa flUidized bedfreezer. ?ten tel'S The development has continued in an accelerated pace and the calendarium may become for too large to be continued, The technology of today will be in focus for the rest of this rfl texi, However, only two more years wiH be mentioned here, and that any ts. 1974: when Row'land and Molina published an article in the Journal "Nature"

describing their findings in relation to the influence of the CFC-compounds ("Freons") and other "artificial" gases in the global atmosphere. Background

fur material was deveioped by Krutzen and the three scientists Molina, Rowland and Krutzen were awarded the Nobel prize in chemistry of 1994 for their work. It was shown that CFC-compounds (having very stable atoms and hence long­

as lived in the atmosphere, if released) can carry chlorine atoms up to the a stratosphere. Here the atoms of CI then can act as a catalyst for destruction of

me ozone atoms in the stratosphere These and other Endings were basis of the bv so called Montreal Protocol

Montreal Protocol, 1 (with several additions later on). In this document a number of industrialized countries have agreed to reduce, and on stop the production of CFC-compounds. An intense development was initiated,

we are in the midst thIS now.,

.=___.n___________________~________·~__·.---·-·~-~.,...-.---...~-.-.

are

1 A coarse

C. Examples of applications

made in the foilmving areas: ro

• Applications for transport and distribution of food products fa • Climate control, air and heat pur ps • Industrial applications

A short mtroduction will be Some of the topics are treated more thoroughly than T others so the treatment may seem unbalanced. The author's excuse is that this is the ci resull of an attempt to give at least a brief introduction to some of the important areas that not will be treated in the rest of the text. T

e:

C1. Applications in handling of food products Si

1.07 .JVmost all processes in nature runs more slowly at lower temperatures than at higher. An old powerful rule of thumb is that the time of a certain chemical process doubles if the c temperature is decreased by 10°C. This rule thumb it is qualitatively correct, even if there are large variations and deviations from it. allowable storage time of foodstuff u is one appiication. a

To have a longer useful life of perishable products it is obviously beneficial to keep the products at low temperatures. Many foodstuff contains water. At (or slightly below) 11

there will be ice crystals formed in the product it frozen. The ice crystals 1formed in this process may damage the cells or rather cell walls - of the product

Fruits, many vegetables and processed products are stored temperatures slightly above freez~ng in order to prolong their life. Other products some vegetables J

and berries are frozen and stored at low without c

noticeable change in quality when they are defrosted and cooked

Notice that there is a clear distinction made in refrigeration between chilling and freezing of products, and for chilled storage and freeze storage. One usually defines a chilled storage as one where temperatures are in the range of about aoc up to 8°e while freeze storage has temperatures considerabiy lower. Typical temperatures of refrigerated rooms for freeze storage are -18 to -25"C.

1. Equipment to create refrigeration of a space or for temperature control of foodstuff are available in many different forms. The tech110log:{ includes products and to create refrigeration, the refrigeration plant, as well as the techrlOlogy to create a

space or building, including the technique to avoid heat transfer have eqUipmentspace, insulation techniques. For the producer it is essential to

or perhaps freezing the products. Heat transfer tech...l1ology is thus'

months

distributed from the producer to large storage wholesaler, and then to the local dealers. Refrigerated storage are products to even out seasonal variations in production or

needed for storage a limited time of products \vaiting

CHAPTER 1 FOR "fVHAT HOV(

to

large stOres by airplanes, ships, trains or containers for freeze the local

roomsfor but also in di!Jplay cabinets (vertical ones or open horizontal; cts for frozen or chilled food to expose the products to the customers. Finally, the user -­

the consumer in his home or the chef in t'.e restaurant - need refrigerated cabinets for chilled or frozenfoodstujf, \vruch are staLlard in any apartment today.

Iy than There are obviously a large variety of equipment necessary in order to make the is the chain" just described, as effective as possible

t areas

The process of freezing the food may for some products change the texture to a certain exient. The reason for this is, as mentioned earlier, that cell \~'alls can be damaged due to the ice crystals that are formed in freezing. It has been shown that the ice crystals are smaller if the freezing process is very rapid than if a long time is used for the process. It

er. An is obviously desirable to have the freezing done as quickly as possible so that the ice the crystals have little time to grow. Different products have in this respect diffelcill

ven if sensitivity. Products like meat or fish are not at all as sensitive as some fruits. For

dstuff instance strawberries or raspberries are very sensitive, and the quicker the freezing is arranged the smaller is the "drip" from the frozen product when thawed and the better "texiure" when eating. Advanced techniques for quick freezing have been developed, for~p the instance in Sweden by Frigoscandia. dow)

ystals The freezing has, however, no adverse influence of the product in tenns nounsnment.duct and content of vitamins. A classical example is that the coment of C-vitamins ojfrozen[bove peas bought in't/ui grocery shop is higher than in those bought directlY from the farmer ables or at the local market! thout

TIle reason tl'..is is that the vitamins decompose relatively rapidly at ambient

?zing temperatures, while this process is quite sb.v at the temperatures of a frozen product. For industria! use peas are frozen directly after L1e harvesting the tune the illled peas are exposed to ambient temperatures is only a matter of an hour or so;-eeze harvesting is follmved directly by the freezing process. This means that the loss of}oms C-vitamins IS quite small for the frozen peas, while the fresh peas on the local market probably was harvested the evening of L~e day before it \vas sold, if not earlier, and the products has been exposed for several hours to temperatures that are

fare destructive for the vitamins.

's to

te a 1.10 are investigations as to what period one can store different products without the noticeable quality changes. Methods for such investigations include so called

"organoleptic" tests: A test panel supposedly composed of people with . for developed sense of taste of food has to judge the quality of different products. The

quality includes not the taste, and texture into atSuch tests must be carefully planned and a number of different products,

and are experience has been collected over the years.

Inare but

__...z________-------'----.-~~--~---····---~-···

1

-FOR w7iAT

'vve producer to consumer, are to airplanes, srJps, trains or road lorries

designed containers for transport. In the store there are refrigerated rooms for but also in display cabinets (vertical closed ones or open horizontal) for frozen or chilled food to expose the products to the customers. Finally, the user the consumer in his home or the chef in the restaurant - need refrigerated cabinets for chilled or frozen foodstuff, \vhich are standard in any apartment today

n r 1! than There are obviously a lar~e variety of equipment necessary in order to make the COla

is the chain" just described, as effective as possible. areas

The process of freezing the food may for some products change the texture to a certain extenL The reason for this is, as mentioned earlier, that cell walls can be damaged due to the ice crystals that are fanned in freezing. It has been shown that the ice crystals are smaller if the freezing process is very rapid than if a long time is used for the process. It

r. An is obviously desirable to have the freezing done as quickly as possible so that the ice

f crystals have little time to grow. Different products have in this respect different

en if sensitivity. Products like meat or fish are not at all as sensitive as some fruits. For

:stuff instance strawberries or raspberries are very sensitive, and the quicker the freezing is arranged the smaller is the "drip" from the frozen product when thawed and the better

) the "texture" when eating. Advanced techniques for quick freezing have been developed, for

low) instance in Sweden by Frigoscandia.

staIs The has, however, no adverse influence of the product in terrns of nourishmentluct. and content of vitamins. A classical example is that the content oj C-vitamins offrozenJove peas bought in the grocery shop is higher than in those bought directly from the farmer,bles or at the local market' 10ut

Thc rcason for this is that the vitamins decompose relatively rapidly at alnbient rIng . temperatures, while this process is quite slow at the temperatures a frozen

lied product. For industrial usc peas are frozen directly after the harvesting the time the peas are exposed to ambient temperatures is only a matter of an hour or so;:eze harvestL'1g IS followed directly by the freezing process This means that the 10s5 of)ms C-vitamins is quite small for the frozen peas, while the fresh peas on the local market probably was harvested the evening of the day before it was sold, if not earlier, and the products has been exposed for severai hours to temperatures are

are destructive for the \1.tarmns.

to ~ a 1.l0 There are many mvestigations as to what period one can store different products the

noticeable quality changes. Methods for such investigations include so for

n organoleptic tJ tests: A test pa.'1el - supposedly composed people with for

developed sense of taste of has to judge the quality of different products. quality· includes not the taste, also the color texture

Such tests must carefully planned and a number of different products, stored at and time periods temperatures, areire

material has coHected over the years. JUt be

1 9

-

500

1.10 compiled of for a number products from

(Time-Temperature-Tolerance). 1970; Stoecker 1988, and IlFi1IR

Storage life, days

1COO

Temperature, °C

Fif::,rure i.1O. Storage life ojd~fferent products. i: Chicken (a: packed in a good wrapping, and b: in a less effiCient wrapping; c: cut up and roasted); 2.' rat fish; 3: Lean fish; 4: Lean meat (bee/); 5: Oranges; 6: Apples (a: regular storage; b: storage in CO2 atmmphere); 7: Eggs; 8: Bananas,' 9:Peas, 10 Raspberries; Ii: Strawberries.

The "storage life" shown in Figure 1.10 represent what is called "High Quality Life J),

for the products This is the time of storage that can be accepted before the can detected of any deterioration in taste or quality

The diagram use a logarithmic scale on the "time" -axis and linear temperature axis this representation ofHQL versus temperature most of the products exhibit reasonably straight For storage at a temperature t we can write an equation for the acceptable storage

100~· ..······-···-·········-·-,·~~·······~O~'~;·····~~'2-···········T····-··········~~···~·~~~···············, .............................. ,

50

10~···········-·········--·······,-···············

5 f·· .. ········· .. ············ .. ·····,·············

1~~~~____~~~~~__L-~~-LL-~______····~__ ··_~____~

-30 -20 -10 o 10 20 30

HQL= 1.1

to is a reference temperature - at which the HQL is equal to IS

constant the of the lines hgLlre.l The constant estimated if two coordinates of the curve are knmvn. Assume at temperatures t

to the periods are HQL HQLc. Then we get

11

1.11

FOR URA

1 1.

Hmv

Er:ample: Beef carl according to the diagram in 1 1 be up to 90 at

and 400 days at -18°C, If we choose to = - I8°e then obviously HQLo = 400 days. With these data inserted in eq.l lOb we get D;c (-18+1 O)/Jn(90!400) equation for estimating HQL as a function of temperatures is hence

18 - t 400·

An

For instance at -20oe we have HQL = 400exp(-18+20)/5,36) 580 days; At lower temperatures beef can be stored even longer, Le. at -26 0 the HQL ,,,ill be about 1780 days!

It is obvious that the temperature has a very strong influence on the allowable time. An analogy can be made to theories in chemical reaction kinetics, in which an important rule is the one by Arrhenius, from which the speed of reactions is proportional to exp(a/T) where a is a constant and T is the absolute temperature of the reaction. From this point of view the x-axis of figure 1.10 should not have been linear in temperature but instead linear in 1/(273..L-t). The differences are, however, almost negligible within temperature interval we are interested in.

30 1.11 During the transport of products from the producer to the consumer the products will be exposed to different temperatures. Each day of fhjs so calied "cold chain ", will take a certain portion of the allowable storage time. The loss in remaining HQL for each day at temperature 1 \vill be equivalent to l/HQLt where HQLt i~ the practical storage life at that

;ood temperature (for instance from Figure 1.10).

Fat ular

For frozen foodstuff one can regard the"loss ofHQL at the different temperatures to be . 10 additive. This means that it is easy to estimate the remaining HQL of a product if we know the "history" in tenus of temperatures and time periods it has been exposed. A storage r1 days at temperature t1 (at which the high quality life is HQLrl) will cause a loss

+.,e " , equivalent to rliHQLtl. The first detectable change in quality will occur when

first L(rrlHQLr) has reached a value of 1. Hence the for a quality of product when it is to be prepared for eating is that the follo\\'ing relation is fulfilled:

{ I['r !l-IQLJ < 1 1.11

de

Example: Assume that beef \ve used for the example m 1.10 on itS way from the slaughterhouse to the consumer is ,~vr"'\",'rl to "cold chain"

md Total sum

0,11 0,17

III

= z

"consumed" 0,375 of the """'-'ltv.> the consumer. Still he can have the beef stored in pis

the is about 400 days) another period of 0,375) = days before he would notice any change in quaiity.

12 A commorJy used reference temperature is -18cC (or OaF). Table L 12 show examples 'practical storage life" in months for a number of different products at -12"C, -18 and ­24°C (from IIF/IIR, 1986).

Table 1.12. Practical storage life in months (Source: II}iIJR, 1986).

Products Storage temperatures:

Fruits: Peaches, Apricots, Cherries (raw) 4 Raspberries or strawberries, (raw) 5 Raspberries or strawberries, in sugar 3

Fruit juices, concentrates Vegetables:

Asparagus (with green spears) 3 Broccoli Peas, green 6 French fried potatoes 9

;'Jeat andpoultry products: Beef, ground meat 6

Pork, steaks, cuts chops 6 Bacon, sliced, vacuum packed 12 Chicken (whole, or pans/cuts) 9 Turkey, whole 8

Seafood: Fatty fish, glazed 3

Lean fish 4 Shrimps (cooked/peeled)

Eggs \Vhole egg magma

A·filk products: Butter, Lactic, unsalted, pH 4,7 Butter, Lactic, alted, pH 4,7 8

Cream leeCream

Bakery and confectionery: Cakes cheese; sponge; Breads Raw

- FOR WHAT

lples of 8 and

freezing is called1. 3

It is that temperatures which cause freezing in the products are avoided in products that are sensitive to freezing. Many can be stored at temperatures close to O°C, or even somewhat below, without damage due to freezmg. Exposure to lower temperatures, however, will start a process of freezing, which for some products spoils the quality this is the case with instance lettuce, tomatoes, apples and pears.

Different products are hence influenced differently at temperatures near OCC. Table 1.13 give some indications ab.out temperatures that can be recommended for best storage conditions (Stoecker, 1988). Further information is found in IIFi1IR 1979 .•

Table 1.13 Recommended storage temperatures jar chilled products:

Product Apples Avocados Bananas Cabbage Cheese Lettuce Pears Poultry Strawberries Tomatoes

Temperature, °C -1 to 0

to +13 + 13 to ~14 o°to +1 oto -2 to 0 -1 to+2 -0,5 to 0 +3 to -t-4

1. Chi1Iing, or freezing, of a product containing water will give a certain loss of '.veight if the process is done in air. Also, storage over long periods will cause a certain dry up. This is usually not desirable and wiIi make quality inferior compared to a fresh product. The weight loss represent also a loss of sales value - hence there are strong incentives to minimize the weight losses due to dry up.

Products can be frozen by special methods like immersion and contact freezing in order to minimize the weight loss in freezing. However the majority of products are frozen in air and to minimize weight loss for such cases the most effective method is to quickly cool the product swface dovin to low temperatures. This is achieved if the freezing process is done with a high heat transfer coefficient and a low temperature on the surface. It is also desirable to have as high air humidity as possible in the cold ail. A high air humidity helps also to minimize the losses in storage. !viany factors are importance; one obvious factor is the ratio of product surface area to weight thin slices loose more weight than round shapes for given total weight. Another factor is whether or not product has a giving an extra resistance for to pass.

an 1. 4 is a piece of meat without wrapping when stored at different room temperatures.

% weight loss Room temperature

6 Days

Figure 1.14 Example ofweight loss in storing a piece ofmeat without wrapping (from Bdckstom, 1970)

1.15 There are also other applications than chilled or frozen storage for refrigeration in food industry A few examples are:

Dairy industry, milk products of different types are prepared. Iv1ilk is pasteurized, which means a heat treatment process in order to kill bacteria that can be present the raw milk. Typically this involves heating the milk to +73 ° for a short period of time seconds) and then chilling it very rapidly to temperatures of to for further treatment or for distribution. The refrigerating technolog)' is essential.

Cheese as part the manufacturing process, stored a given time in order it to gain proper taste Specified temperatures levels and time periods are important for the process

Ice cream is a dairy product that has to be frozen in a special way, involves freezing, while the ice cream is stirred, at about -soc. At this temperature the cream is only partially frozen and it is still soft. After this period of pre-freezing, a "hardening" process follO\vs during which the temperature of the ice-cream is cooled to about At this temperature it should "rest" a certain period of time before it is considered to ready for eating or for storage at normal freeze-storage temperatures. cream IS

sensitive to temperature variation during storage: for quality it should stored at a constant temperature, for instance at -18:-lC or lower.

breweries there are demands for strict temperature control in different parts the brewing process. In breweries it is especiaily important to the large tanks of

at given temperatures according to predetermined elaborate schemes. The bre\ving process is exothermic and the heat must be transferred form the The brewing tanks are typically to kept at a temperature between 8 and decisive for quality the beer to follow as closely as possible a temperature and time the brewing process and when it is to be as for storage bottling. There are also similar requirements for producing wine large winenes.

Fruit juices are concentrated in order to decrease the volumes to be and distributed. The process of concentration is by vaporizing perhaps 75(%

: 14

•

It is

as

HOW1 REFRlGERATION

,vater content of natural juice. This is done In a plant near the order to preserve the s taste, the evaporation at temperatures, 18 to 25°C. A predominant method to this is to

use a process where the water vapor is condensed. At the temperatures mentioned, pressures will be subatmospheric. Air is pumped away from the tanks w'ith . to be concentrated, the liquid is maintained at a given temperature (by heating) and the evaporating water vapor is condensed on surfaces, which in turn wi.ll have to be cooled A heat pump process is quite effective as a key element in this process, as illustrated in , Figure l.l5a. The heat pump condenser will furnish low temperature heat for the evaporation of \vater in the juice. The heat pump evaporator will at the same time keep

surfaces cool where the water (juice) vapor is to condense (the juice water condenser will be the heat source for the heat pump). The principJe is demonstrated in the scheme to the left in Figure 1.15 a , while a scheme of an industrial plant is shown to the right in the figure. It is possible to have relatively small temperature differences between the evaporation and condensing of the juice and hence the conditions are very favorable for heat pump operation. Thanks to small temperature lift very high COPs can be reached for the heat pump.

Concentrating solutions containing water can also be done by freeze concentration. By arranging the fluid to be concentrated to flow along a cold surface a certain portion of the water will freeze and collect as more or less clear ice on the cold surface. The method has, however not been used in practice except for some heat sensitive products such as coffee extracts and some fruit juices Potentially the method can be interesting from an energy saving point of view, since the latent heat of freezing of water is only of the latent heat of vaporization (which is applicable for instance in the methods Figure l.ISa)

Vacuum pump to remove air

Juice --{>iC.....~ r---+~ feed

Juice

1.15a. Heat pumps are elfeetive for concentration

l' 5

•

1

ENGINEERING

Freeze drying, IS a method in use products. is an expensive and it is special products or \veight

To be is to be soaked water before eating. \Vhen the product water it resumes to its original

form. Colors and taste is preserved well in the product thanks to low temperatures in the treatment.

The method involve a drying process with the product in frozen state. water is removed from frozen material by sublimation (that is the \vater leaves in the form of vapor directly ice crystals). The process is hence done with product temperatures O°C. A scheme of the is illustrated in Figure i.ISb.

A~ R (Evacuated by Pump;

\ \ ~. CONDENSER ICE

Vapor from Procu-:t ;ce to Conoenser lee.

Figure l.I5b. Schematic arrangementfor freeze drying ofproducts.

Before the treatment starts product is preferentially cut in thin slices, or in the form grains or small pieces. are then frozen, and put on product shelves in a vacuum

chamber, as indicated in 1 15b. Inside the chamber are also surfaces for condensing sublimating water vapor. These condenser surfaces are kept at quite low temperatures, typically at 5 to if the product is kept at (hence at 15 to

lower temperature than the product). The condenser can be arranged in a separate chamber but it is important to have a large area for streams between the product tray and the (since the volumes of vapor are large at the extremelv low saturation vapor pressures prevailing). Air must continually removed

the chamber - if not, air severely hamper the water it form a resistance at the condenser

, but there

keep the process the products must compensate for the

water vapor leavmg the the process is done are semi-continuos

L.

»

1

process IS. as mentioned, an pvnAnc

as as in operation Ho\vever, it can give very good product results vitamins remamscontent components in food as

undamaged in freeze-dried products, Common products are freeze dried coffee-powder, components in dry soups (meat, mushroom, shrimps) and for use in food where low weight is important, for advanced camping etc. Also in pharmaceutical industries freeze drying is commonly being used,

C2. Climate control, air conditioning and heat pumps

16 Ajr conditioning is often used as a term for cooling a space, Hm;vever, air conditiopjng does include more than merely to cool the air. ASHRAE give as a definition for comfort air conditioning that it is "the process of treating air to control simuitaneously its temperature, humidity, deanliness and distribution to meet the comfort requirements the occupants of the conditioned space". Air conditioning include hence also to arrange for the control of the humidity and temperature as well as to clean the air and to

distribute it fur good comfort in the occupant space, Temperature control of the air can involve cooling but also heating, which can be accomplished by reversing the cooling units for a heat pump function.

Also other methods than conventional refrigerating machinery cann be used, One example is shown in Figure 1.16.

Humidifying pad Heat exchanger wheel

e r-)'~I

Exhaust air

17

some

fIgure 1.16. Scheme ofa regenerative heat exchanger betv;een exhaust air (3-4) and incoming air (1-2) for buildings combined with a possibility oj cooling (3A 3).

The sense of comfort of people is affected by the conditions in the room. The room air temperature, humidity and movement have an influence (but also the temperatures of all

in a room intluence the comfort by low temperature radiation) To illustrate factors 1. j 7 give" comfort represented in a psychrometric

chart for atmospheric air. The zones given in the diagram (valid for air velocities 0,1 are investigation. The as for

summer and winter are explained the fact that people norrnally \vear more, and clothes in wintertime even inside. The experience of a draught and due to

1:1

•

surfaces exposed in influence, which

the HIVI-"\fPT is not

in a Nordic the one Sweden, it is necessary to have equipment cooling in buildings The reason for this is the heat load from installations such as computers, lighting and also people and the influence of solar heating through v.i.ndows, especially if facing south. Modern, well insulated houses have more often a demand for cooling than older houses, As an example, a nonnal (new) one-family house built in the 90-ties, has a surplus of heat (from people, lighting and apparatuses like refrigerators, freezers, stove in the kitchen and TV-sets) down to ambient temperatures about

higher ambient temperatures (prevailing for many hours of the year) it is necessary to remove heat from the house. In a climate like the one we have in Sweden this can often be arranged by "natural" cooling (in a private home simply by opening a window. ,).

In an office building or a department store this kind of"balance" temperature is often still lower than for a one-family house and for these buildings it is nonnaHy not possible to just open a window to get fresh cool aiL Of this reason almost all buildings for offices, hotels, hospitals etc. are equipped with means for artificial cooling for comfort. In a hotter climate than the one prevailing in Sweden this is even more important.

Systems for comfort air conditioning can be arranged decentralized, (with units in each room or a block of rooms), or centralized For the latter case one uses cool air or chilled \-vater in a distribution system similar to what is used for central heating of a building. It is not uncommon that there is a demand for cooling in one part of the building, and for heating in another part. The two systems are often designed v"ithout taking advantage of possibilities for energy saving This is a great field of work for the creative engineer.

20

u. 0>w ...c:: 15

::l a. f­ 10-< 0c: w e-e.. <:c:: '"f- lO

=:: i:

f­ a ~ ~0

:;)c.. :::

5'" ~ (:) 5

0

·5

• 10

Figure 1.1 "Comfort zones" for indoor climate (From Handbook

•

CHAPTER j TlON- FOR

On district cooling as as district heating world there are systems in operation. One of the first ones was a plant around

central parts of West Hartford, Conn. USA In TokyO there are a number upjts rJmishing the buildings in the block with chilled as well as heated water. A large company, Tokyo Gas, has pioneered energy efficient systems combined with electric load management: At night, when the load on the electricity grid is smalier, a large heat pump is operated for satisfying the demand of the district heating system \Vater in a large tank is used as a heat source, and part of the water is frozen to an ice slurry (by a process involving sub-cooling the water to temperatures belovi O°C in the heat pump evaporator 'and when the water is returned to the tank ice crystals will fonn to an ice slurry in the tank). The formation of ice will increase the storage capacity of a given volume of water by the latent heat to form the ice. Daytime the cold water from the ice-slurry in the tank is used to satisfy the cooling demand of the buildings. This appear to be an effective way of saving energy and of"energy management". The same principle of using ice for storage and load management has been applied in many (smaller) applications in Sweden. However traditionally the ice is accumulated on evaporator (heat exchanger) tubes.

1.18 For industrial activities there are many different special demands for cooling

Printeries have strict requirements for temperature and humidity control of the air - and of the paper in order to have high quality results. Conditions are similar in textile industries Computers, telephone equipment as well as control rooms have special requirements.

Special laboratories have extreme d~mands. Examples are laboratories for electronic components, as well as for biological and medical experiments. Also museums may be quite special in their demands One example is the \Vasa museum in Stockholm where a constant temperature as well as humidity in the air is specified.

1 19 A rapidly growing area is air conditioning of vehicles. It is more and more frequent to have an "AC-system" in the private car - even in Nordic climates as the one in Sweden. Figure 1.19a shows an example of components of such a system The capacity of an AC­system in a small car is in the order of 6 to 8 kW. The peak capacity is necessary in order to cool the car rapidly when starting the car after it has been parked on a hot and sunny parking The shaft power for the AC-compressor is about 3 k\V - hence nor an insignificant power demand (about the same as for a qualified heat pump for a large one­family house). The world market for air conditioning systems for cars is in the order of 10 million units annually!

Busses, large trucks, trains and ships are equipped \vith cooling systems of similar types, although for large systems different arrangements are used than what is shown in figure I. An example is given in Figure 1.19b of a "cold air cycle" for installations in an air plane It is essential to have an effective system for air treatment in a crowded space like that in cabin of a commercial air plane. in Figure 1.19b advantage is taken the air compressor in the gas turbine the Jet engine Part of the

air the jet engine compressor is tapped air stream 15

ambient air in a heat exchanger, and expanded to the pressure in the cabin in a The extracted in the turbine make the temperature to drop alI'

1,19

•

method is also an an Also

IS return air for desired temperature. system for pressurizing passenger compartment

aircraft the cooling system is important to give reasonably comfortable working conditions, but also for cooling computers

electronics

1. Compressor 2. Condenser 3. Filter-dryer 4. Expansion valve 5. Evaporator 6. Thermostat

Figure 1. 19a. Scheme ofa mobile air conditioning system.

Figure i.J9b. Scheme ofan aircraft air conditioning system.

1. built in many different ways. The and S\veden

during the early part of

central Stockholm. As is seen the heat pumps had a very Increase

120

t

CHAPTER 1. REFRlGEFATION­ HO~V

in use a few in the mid

The reason for use of heat pumps in district systems in Sv,,;eden is closely tied to bad management of the electric system. From a thermodynamic point of view it would seem more eftective to heat the district sy'stem by combining heat and power production however the electricity in Sweden is primarily based on hydro electric and nuciear power stations.

.... ~

7

~6

~ 5 i­.~ 4 Qi

..---Purchase

_----Oii

~ 3 ~.~.--------.--~~~--------~-----------..~~~-----2> (l)

~ 2 -+-----------~.+-.....;-.+___7'--~----:-~.~-~~-~.-----...;-_+-.---=........__Heat Pumps

i-----Waste heat ..

o ~980 1985 1990

Bio:-nas5

2000

Year

Figure 1.20. Supply ofheat'to the district heating system ofStockholm.

C3. Industrial applications

1.21 Some special industrial applications, outside food industry and alr conditioning, will finally be exemplified:

In chemical process industry there are many applications \vhere refrigerating engineering - or heat pump technology - are use. These are in most occasions tailor made and requires qualified engineering work for planning. Examples are:

• Distillation for separation of components in a Equid mi:x1ure • Condensation of "gases" (liquid air, natural gas etc)

Separation of gases (for production of nitrogen, oxygen ... ) • Storage at low temperatures • Cooling of exothermic processes in chemical reactions

Desalination is an important example of the first type of application. In some pans of the 'vVorld s"Vveet 'Hater is a scarce commodity desalination of sea water can be a solution Huge such are in operation in the world. methods are used, on solar heat and multiple stage evaporation or using heat pumps a principle as

1 1 although in this case it is not . to concentrate In

v.ater - instead to separate it into a clean water component). An alternative method is to

tr

PLFi7JGERATLVG --- ­

desalinate ice from sea water. arranged properly IS free of Carefully ice, and melting sweet water.

Condensation to produce (liquid natural gas) is anothe; huge area of applications. 1.21 shows an advanced but simple scheme to accompli sh this. A gas turbine for the operation the compressor in the refrigerating system The refrigeration scheme is utilizing mixtures several gases as working fluid (nitrogen, methane, ethane, propane, butane and pentane) in order to a "gliding" evaporation temperature with an extremely glide - from about 70 up to about OCe! Perhaps the simplicity of the system in figure 1,20 is deceiving analysis and design of the heat exchanger between three fluids as indicated in the requires advanced knowledge thermodynamic properties of gases and their mixtures In trjs heat exchanger the natural gas (essentially methane, CH3) is cooled from ambient temperature down to temperatures in the range of -160°C, Plants of this type are very large, with refrigeration capacities of several hundred iviW.

" 2SG~

~ ~. ~

2(C. ... E 220

....'" 20e

120

! eo

iDe

1" 0

[JiG 100 aco 300 '00 500 800 P t:l ty (1/.1')

J,21, Scheme for a process to condense natural gas (from Brendeng, Oby the Prico profile in the heat exchanger for cooling natural gas to

122 In life \ve are exposed to units such as dehumidifiers, water coolers, cabinets or freezers for the sale cooled drinks or Let us not fClfget other areas of use, like:

rinks. The ice IS always having a refrigerated brine circulating in the

is £i'ozen on top many interesting parameters 15 one of

such as is normally arranged

In pipes.

b __-",,''''.":;;:'';::'.:.::.:2=:,::~'----------------....-­

The diagram to the right indicate the tD>YfnDvnt'r:YD

122

CHAPTER }, PLFRIGERA FOR WHA

new on the ice), Itathletes that the condenser heat "uvu,,,, premises and to cover the

demand water - or why not for heating the water of a

In bUilding technology there is sometimes problems \Vlth stability in foundation work or in building of tunnels in earth wet clay or sand One technique which come to use is to freeze the water in the earth material to ice during the period of building Frozen material is much more stable and easy to handle in the building process, One example where this has corne to use is in the construction of the subway of Stockholm city (especially in the region the subway station of "T-Centralen"), Large concrete constructions may also have to be cooled for temperature control in the process of hardening and to give an end result of best possible quality, This cooling can be perfonned by using cold sand and cold water in the mixing of the concrete, but in large, advanced constructions it is not unusual that tubes are immersed in the concrete, in which cold water is circulated in order to keep the temperature under control during the hardening process,

References:

ASHR-AE Handbook, Fundamentals Volume, Chapter 3, Atlanta USA, 1993

Brendeng, : "Multicomponent Refrigerants A Giant Success for an Old Idea", 40thInternational symposium on the Anniversary of NTH Refrigeration

Engineering, Trondheim, 1992, p, 199~-214,

Backstrom, M.: "Kylteknikern." Svenska Kyltekniska Forerungens Handbok no 1, Stockholm, 1970.

"Den Svenska Kylteknikens Historia", Svenska Kyltekniska Foreningens Jubileumsbok, Stockholm, 1992,

UHlIR: "Recommendations for the processing and handling of frozen foods", International Institute of Refrigeration, Paris, 1986

IIF/IIR: "Recommendations for chilled storage of perishable produce", International Institute of Refrigeration, Paris, 1979

Stoecker, \V.F,: "Industrial Refrigeration", Troy, :Michigan, USA'7 1988

I

REFPJGERATING ---------------------------~ .......-.~---- ---­

•