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Diesel Engineering Handbook
Citation preview
Diesel
Engineering
Handbook
KARL W. STINSON, M.E.
Professor of Mechanical Engineering
Ohio State University
completely rewritten
10th EDITION, 1959
22 ► COOLING SYSTEMS
SUCCESSFULoperation of any engine is dependent
upon the removal of excess heat from the cylin
ders, pistons, valves, etc., so as to keep the tempera
tures of these parts within allowable limits. Most diesel
and gas engines are water-cooled, although a few of
the small and high-speed engines are air-cooled. As a
result, most of this chapter will be devoted to water-
cooling systems.
Water Cooling
Two chief factors involved in design of a water-
cooling system are (a) a supply of water which will
not form scale in the engine jackets and heat ex
changers and (b) an adequate water supply. Consider
ation of these factors, along with the size of engine and
type of installation, will determine the most desirable
cooling system.
Quantity of Water The actual amount of water
required to remove heat passing through cylinder walls
depends upon horsepower capacity of the engine, de
sign of the engine, and allowable rise in water tem
perature. As a rule, it is advisable to install sufficient
water capacity to handle 2500 to 3000 Btu per hr per
bhp for normally-aspirated engines and 2000 to 2500
Btu for turbocharged engines.
If values of heat rejection to jacket water and oil
cooler are available from the engine builder, they
should be used to determine the water capacity re
quired.The pounds of water to be circulated can be calcu
lated from —
Table 22-1ApproximateRate of Heat Rejectionto Cooling Systems
Btu per brake-horsepower-hour
wbhp (Btu per bhp per hr)
where
W = water circulated per hour, lb
bhp = brake horsepower ratingt, = outlet jacket-water temperature, °F
U_ = inlet jacket-water temperature, °F
Required quantity of water, expressed in gallons per
minute, would be—
bhp (Btu per bhp per hr)gpm= (*,-»,) 500
'
Table 22-1 gives approximate rates of heat rejection
to the cooling system for various types of engines. Inthe case of supercharged two-cycle engines, the total
heat to raw water will be reduced from the normally-
aspirated values given by 300 to 700 Btu, depending
upon the increased cooling obtained from the greater
air-flow rate.
The chart in Fig. 22-1, prepared by G. J. Bischof,
enables the engineer to determine the approximate rate
of water circulation needed in the jacket system per
EngineType
Four-cycle—Normally-aspirated
Uncooled pistonsOil-cooled pistons
TurbochargedOil-cooled pistons
Two-cycle—Normally-aspiratedOil-cooled pistons
Loop-scavengedUniflow-scavengedOpposed-piston
Water-cooledexhaustmanifold
JacketWater
OilCooler
Total toRaw Water
1900230017502150
200-350300-600
2100-2(502050-2750
12001400 250-100 1450-2000
1300-20001700-20001200-1700
400-600400-700800-1000
additional-
1700-26002100-27002000-2700300-500
engine horsepower per minute, with various unit quan
tities of heat entering the jackets.
For example, a 1000-hp engine rejects 2590 Btu to
the cooling system per horsepower hour, and it is
planned to have a cooling range, or water temperature
rise, of 15° F. How much water must be circulated
per minute?
Entering the chart, Fig. 22-1, from the 15 point at
the left side, pass horizontally until the diagonal line
marked "2500 Btu per hour" is intercepted. From this
intersection, move perpendicularly downward to read
approximately 0.34 on the bottom scale. The 1000-hp
engine will, then, require 1000 X 0-34 = 340 gal of
water to be circulated per minute.
Jacket- Water Temperatures Engine builders are
not in agreement as to the correct inlet and discharge
water temperatures. Certain builders, notably those
who produce high-speed units, recommend a discharge
temperature of as high as 180° F. Others take the
position that 160° F is the desirable maximum.
.is
Mb
.1 2% J * * .0 T I
CRM. of Wbtcr Circulated per HP22-1 Chart to determine the amount of cooling water needed
by a plant.
237
DIESEL ENGINEERING HANDBOOK
General practice is to take 165 to 180° F as the
maximum discharge and hold the rise to from 10 to
20° F, with the lower value the preferred one. When
a high-teinperature system of cooling is employed, a
method described later in this chapter, jacket tem
peratures are usually maintained above 212° F.
In the past, engines were operated with a water
temperature rise of 40° F, or even more. Such a largetemperature rise causes an increase in the thermalstresses, particularly in the cylinder due to a greatertemperature difference between the top and bottom.Also, rate of water flow is low increasing the dangerof local overheating.
Open Cooling System In this system, water iscirculated under pressure through the water jackets.A bypass may recirculate part of the water to the
jackets, but most of the water either (a) is wasted to
some outflow or (b) is recirculated through a coolingpond or tower where much water is lost by evaporation. Continued evaporative cooling and addition of
make-up water increases concentration of both hardness and impurities in the water. Unless the engine outlet temperature is kept down to near 120° F, scale-
forming materials will be deposited in the engine water
jackets, reducing the heat transfer rate to the coolingwater.
Unless an ample supply of sediment-free, chemically-inert water is available, the open cooling system is
generally not recommended.
Closed Cooling System This system provides forrecirculation of purified water in a closed circuit which
basically consists of a circulating pump which forcesthe water through the cylinder water jackets, on to a
heat exchanger, and then back to the pump for re
circulation.
When only one water system is used, the hot wateris passed through a radiator where heat is dissipateddirectly to the air. Other closed systems use two watercircuits — jacket water and raw water. Each water
circuit has its own circulating pump. Two types of installation are in general use—
( 1 ) A shell-and-tube heat exchanger with raw waterfrom the heat exchanger being cooled in (a) a coolingpond, (b) an atmospheric cooling tower, or (c) an induced-draft or forced-draft cooling tower.
(2) A heat exchanger built into a cooling towerwhere (a) the raw w:ater is sprayed into the top of the
tower, is cooled by the air, and then passes down overthe heat exchanger tubes to cool the jacket water; or
(b) the raw water is sprayed over heat exchangertubes as air is pulled through the tower by an induced-draft fan, thus creating evaporative cooling.
A shell-and-tube heat exchanger installation is shownin Fig. 22-2. Pump C circulates hot jacket water
through the heat exchanger B, or the bypass line, and
on to the cylinder jackets. Water leaving the engineenters the standpipe A and down to the pump again.Raw water is shown passing from pump D through an
oil cooler / and then through heat exchanger B. from
which it passes through line F to spray nozzles in (a)
2 cnroiro
(Courtcsv'of Diesel Engine Manufacturers' Association)Fig. 22-2 Closed Cooling system with shell and tube heat ex
changer.
A—expansion Tank-Open F—Piping to Cooling Tower,(Discharge Line Submerged) Spray Pond or Waste
B—Heat exchange C— Piping from Haw Wafer
C-Jacket Wat* Pump H-Brato Vo/ve,D— Haw Water Pump j—Lubricating Oil Cooler
K—Thermo
Surge or Expansion TankWell May be used insteaSurge Tank)
B—Open VentC—Soft Water Make-up ConnD— Automatic Air Vent
(Hotd of
(Courtesy/of Diesel Engine Manufacturers'Fig. 22-3 Closed System with Cooling Tower.£—Heat Exchanger L— Jacket Wafer PumpF—Raw Water Basin M—Row Water PumpG—By-pass Valrlng N—ThermometersH—Tower By-pass ValveI—Lubricating Oil CoolerK—Raw Water Make-up Conn.
Association)
6—Water Discharge From"ooling To*train Valvt
238
DIESEL ENGINEERING HANDBOOK
Fig. 22-12 Trans horizontal-core cooler on a pipe-line pumpingstation.
Two-speed motor drives are frequently used. Controlof air flow is another method of modulating the cool
ing effect. It is also a means of saving fan power whenmaximum cooling effect is not required, since powerrequirements decrease faster than the degree of cool
ing. At half-speed, fans will produce 50% or more of
total cooling capacity but will require only 20% of the
power needed for full-speed operation.
Radiator Size Size of radiator needed for a specific engine depends not only on the Btu per hour go
ing to the jacket water, but also on the difference be
tween the outlet jacket- water temperature and the tem
perature of the cooling air. The less the difference, the
larger size radiator required for a given Btu per hour
heat-exchange rate.
Take for example two diesels, both operating underair temperatures of 100° F. If one engine is operatedwith a water temperature of 180° and the other at
140°, the first engine permits a temperature differentialbetween the water at 180° and the air at 100° of 80°whereas the second engine permits a temperature differential between the water at 140° and air at 100° of40", or one half. The size of radiator required variesinversely with the temperature differential permitted,therefore, with the second engine requiring the watertemperature to be maintained at 140°, a radiator twiceas large as the one required for the first engine is
necessary.
Am Velocities An average air velocity of 1500
ft per min through the core as measured by means ofan anemometer in front of the core is recommended.This air velocity causes a slight hum or noise, but the
noise is not objectionable. To obtain this average airvelocity, the fan should be operated at the required
Table 22-2.
Approiimate Radiator Fan Power Required for Engines.ENGINE HP FAN HP IN % OF ENGINE HP
100 or lets 5%100 to 500 4%500 to 1000 3%
1000 to 1500 2%1500 to 2000 1.5%2000 to 3000 1.25%3000 or more 1%
speed but should never be operated at more than 12,-
500 ft per min tip speed when using the conventional
type. The lower the average air velocity the larger the
radiator must be and, therefore, the more expensivethe installation. Following the above rule it will befound that for larger fan sizes the fan rpm will fallbelow 1150 rpm or the lowest recommended speed fordirect connecting the fan motor. This means that provision must be made for reducing fan speed where anelectric motor is used. For small installations usingconventional type fans, the fan may be directly connected to the motor. There are, however, some specially designed airplane propeller type fans which may be
operated at higher speeds and direct-connected toelectric motors running at 1750 or 1150 rpm. Wherebelts are used for driving fans, they should be of the
vee-type to prevent slippage. A fan of the conventional
type operating at a peripheral speed of 10,000 ft permin is somewhat noisy and where noise is a factor, thefan should be operated at a lower speed. It is alwayspossible to obtain an average air velocity of 1500 ftper min using relatively large fans, running at lowerthan 10,000 peripheral ft per min, and, in general, itis best to use a large fan running at a low speed thana small fan running at a high speed. For one thing,less power is required.
Propeller-type fans require less power than conventional types. For best results they should be used as a
blower-type fan and should be located from 6 inchesto 10 inches back of the core with a shroud. Theircost, including installation, is generally greater.
Selection of Cooling Equipment Points to be
considered when selecting cooling equipment, such as
dry or wet-type cooling towers, were presented byH. E. Degler of the Marley Company before the ASMEin 1951. A summarization of his discussion follows:
'The dry-cooling unit may be used in preference to
a water cooling towrer for applications of "high-levelheat removal" where temperatures of the fluid to becooled are above 140° F referred to a 100° F dry bulbair temperature and where water is scarce, expensiveand/or badly polluted, or where the portable featureof the dry unit would be desirable.
While the cost of the dry-type cooler is higher, itdecreases relative to a cooling tower as the fluid-to-be-
cooled temperature rises.
The choice of a cooling system will depend upon
(1) cost and availability of water, (2) quality ofwater, (3) geographic location, (4) space available,
and (5) desire to utilize waste heat.
The dry-type tower requires negligible make-upwater and scale deposit problems are unlikely. Provision must be made to prevent freeze-up in cold climates. Glycol solutions may be needed during freezingweather."
Cooling Tower Winter Operation The dry-surface, air-cooled heat exchanger is particularly goodfor use under severe winter operating conditions. Iteliminates hazards caused by extremely cold waterwhich does not improve performance to any great extent.
242
23 ► EXHAUST SYSTEMS
THEexhaust system of an engine consists of an ex
haust pipe which leads from the engine to a si
lencer, for damping the exhaust pressure waves andthe resulting noise, and a tail pipe or stack. This silencer is sometimes referred to as a muffler or a pulsation snubber. These names will be used interchangeably.
Design of Exhaust System Piping layout for an
exhaust system along with location of the silencer andselection of the correct size and type of silencer, are
very important factors in any engine installation, mo
bile or stationary. Problems will be discussed brieflyin a general manner, but it should be realized that
variation of any item, such as engine speed, enginesize, exhaust-pipe length, and many others, presents a
completely new problem which involves both pulsatinggas flow and acoustics.
Some of the actual answers can best be found ex
perimentally or from practical experience. It is therefore very advisable to consult with engineering spe
cialists of the silencer manufacturers and thus take
advantage of their wide experience in this field.The first essential to the successful design of an ex
haust system is a realization of the fact that the gascolumn in the exhaust pipe possesses the properties ofinertia and elasticity. While it is necessary to designfor low friction losses along the pipe walls, dynamicproperties of exhaust systems are of major importance.It is a relatively simple matter to compute the average
velocity of flow of exhaust gas through a pipe with the
assumption that the friction losses are a function of
pipe diameter or velocity of flow through the pipe. Itis necessary, however, to do more than compute the
average velocity.
Flow of gas must actually be smoothed out to attain the computed value. Otherwise, an exhaust systemdesigned for low resistance may be the cause of im
paired scavenging of the engine due to oscillation ofthe gas column in the pipe as it passes from the en
gine to atmosphere.
Rate of Flow and Pipe Diameter To determinethe cubic feet per minute of exhaust gas discharged tothe atmosphere, it is necessary to first determine thecubic feet per minute of intake air. This may be deter
mined by multiplying the total displacement of the en
gine cylinders (in the case of crankcase-scavenged en
gines or 4-cycle engines ) by their volumetric efficien
cies. With blower or pump-scavenged engines, it is
best to know the manufacturer's rating of the blower
or pump.Volume of exhaust gas may be determined by use
of a heat-rise factor, the value of which may be determined by referring to the chart, Fig. 23-1, showing exhaust temperature plotted against heat-rise factor. Theproducts of combustion, added to this quantity, account for probably a 5% further increase in the volume of exhaust gas. This is a direct and satisfactorymeans of determining the cubic feet per minute of ex
haust gas and is probably closer than may be determined by other more complicated formulas.
EXHAUST SYSTEMS
^0 ^5 &
Ht«P'P«diom«I.rii GASFLOW-CUWCFEETPERMINUTE
*U*»mHighMakSc»*dF
kCai. Stawngjd Is 3000FPM law4000-7000FFM
•fwrn^i. - • 4000.WOOFFMPo»ili».Sto»«M.d- 7000-9000FFM M»dk»nHighSp»d
•000-10.000FFM HighSp..d - - -
4000-7000FN*
*000-I0,000FFM
f/g. 23-2 Char* For determining diameter of exhaust pipe.
fig. 23-3 Straight run of pipe to long-sweep ell.
KK\\\\\\\<J/\PPPVU
flexible connection-
ji II|j
1'
II
l|f» |
1 & j
fig. 23-4 Flexible hose changes direction of line. With runs ofthis length, flexible metal hose may require support.
Fig. 23-5 Silencer inside plant. Flexible metal hose section isolatesengine vibration from silencer that is normally rigidly attachedto structure.
It cannot arbitrarily be said that the velocity
through an exhaust system should be, for example,5000 ft per min. The reason for this is comparatively
simple. In the case of a multi-cylinder, high-speed en
gine of 100 hp, flow of gas to the exhaust pipe is rela
tively smooth, as compared to a single-cylinder engine
running at 300 rpm, developing 100 hp. It is obvious,
at a glance, that an exhaust pipe for a single-cylinder
100-hp engine would be much too large in diameter foruse with an automotive engine of 100 horsepower.
This being the case, it has been necessary to es
tablish a range of velocities through exhaust pipessuitable for various types of engines, as listed in Fig.23-2.
This chart, devised by Burgess-Manning Company,
may be conveniently used to determine exhaust-pipe
diameter. Gas velocity is plotted against cfm at atmos
pheric pressure for any pipe size.
Pipe Layout The pipe layout with the lowest pres
sure drop through it is ordinarily the advisable one.
The layout in Fig. 23-3 illustrates how a direct linewith a minimum of bends may be obtained even
though the silencer must be placed outside the build
ing.
Similarly, in Fig. 23-4, the exhaust line is direct and
the presence of a length of flexible hose enables the
gases to enter the muffler without any abrupt right
angle turn. When it is permissible to install the silencerin the engine room, the sketch in Fig. 23-5 shows how
an arrangement with a low pressure drop may be
made. The silencer is supported by braces (not shown I
from the roof, and these should have springs or rubber
connectors. The reason for this is that the gas pulsations may set up vibrations in the silencer shell and if
there is not some kind of isolation between the silencer
and building steel, the latter may vibrate in unison
with the gas impulses.
A somewhat long exhaust system is shown in Fig.23-6. Certainly there may be a noticeable back pressure but the arrangement is by no means uncommon.By using a large diameter pipe line, this objection can
probably be corrected.
249
DIESEL ENGINEERING HANDBOOK
ir\KN\l\l/l/l/1/17-
1% 1 ft
. -r|l H 11 ll flexibleCtmeciion
m i ii irr" i
l —1
i^—g^'l J... ■.... ... L. XV... ...... 1
Fig. 23-6 Long under-floor line to silencer. Piping should be Incorered trench for accessibility.
Pipe Material Although cast-iron pipe would be
preferable due to its resistance to corrosion and rust
ing, the lighter wrought-iron or steel pipe is most com
monly used for exhaust lines. Steel pipe between en
gine and silencer may be very noisy if not of heavy-
gauge material.
The Diesel Engine Manufacturers Association does
not consider masonry desirable for exhaust ducts,
stacks, or chambers. Under no circumstances should
brickwork or tile be used.
Exhaust piping of automotive engines is frequentlymade of one of the nickel-bearing heat-resisting alloyssuch as stainless steels. Corrosion and rusting, com
mon enemies of exhaust piping, are somewhat accen
tuated in vehicular applications and the added cost
of the superior materials is justified.
Flexible Metal Hose Every effort is made to iso
late engine vibration from the building structure. This
may be done by foundation design including use ofvibration isolation materials or springs. Similarly, it is
desirable to isolate vibration from piping systems
which may be rigidly connected to the building structure. Flexible hose is commonly used for this purpose.It serves another purpose in correcting for minor piping misalignment and thus relieving strains on boththe engine and piping system. Additionally, it is use
ful in obtaining long-sweep changes in direction, thus
keeping restriction to flow at a minimum.Two general types of metal hoses are made in a wide
variety of materials and sizes. The two principal types
are: (a) hoses formed with a series of peripheral cor
rugations and (b) those formed by interlocking a continuous strip of metal. Both types are illustrated in
Fig. 23-7. Suitable types and sizes are available for all
applications.
Insulation Heat transfer from exhaust lines to
the engine room may be materially reduced by cover
ing the pipe with insulating material, frequently mag
nesia compounds, and this covered with a light metalliccover. If the remainder of the exhaust piping is covered with heat insulation, it is frequently desired to also
insulate the flexible hose. For this purpose variousmanufacturers supply duplex hose. This consists of an
inner and outer section hose, Fig. 23-8. The inner hose
is attached to the flanges while the outer hose is at
tached to a ring on the flange. Other methods of heat
isolation include magnesia and asbestos air-cell wrappers. The solid magnesia insulation may break in serv
ice unless carefully applied and held with wiring. The
asbestos air-cell wrapper is able to stand a certain
amount of pipe movement due to contraction and ex
pansion.
Exhaust Line Expansion Exhaust piping changesfrom atmospheric temperature while the engine is shut
down to exhaust gas temperature while the engineruns. Consequently, provision must be made to com
pensate for the changing length of exhaust lines re
sulting from expansion and contraction.
In some instances rigid exhaust lines can be ar
ranged to swing enough to take care of expansionwithout breaking any of the connections. In close
quarters, particularly in marine installations such ar
rangements should be avoided. Various kinds of slip
joints have been tried and found unsatisfactory. At
present universal practice is to insert a section of cor-
(Courtesy of Atlantic Metal Hose Co.)a. Metal hose formed of helically Interlocked strip material.
(Courtesy of Atlantic Metal Hose Co.)b. Corrugated hose may be bare or braid-covered tor protection and
greater strength.
(Courtesy of Allied Metal Hose Co.)c. Corrugated hose can be welded or brazed to connectors, here a
threaded nipple.
(Courtesy of Allied Metal Hose Co.)d. Use of one floating flange permits bolt hole alignment without
twisting hose.Fig. 23-7 Types of metal hose and typical methods of attachment
250
EXHAUST SYSTEMS
NOMinal 1. D. (Inches) 2 21/2 3 4 5 6 8 10 12 14 16 18 20
MAXimum 0. D. (inches) 3k« 3 5/8 41/8 5 7/K 6>l< 739/32 9 27/32 1113/1* U'l'» 16 18 1931/322131/32
Min. Ret Straight Lgth.* (Inches) 14 16 17 21 23 25 28 32 35 38 40 43 48
Weight, Approx. Lbs./Ft. 2.8 3.2 3.5 5.4 6.6 10.7 13.5 17.1 20.2 23.35 26.9 31.1 35.2
Min. Rec. Bend Radius for 90° Bend 22 24 26 28 30 32 38 42 46 50 56 62 68
Hose Length for Above (Inches) 37 40 42 46 49 52 62 68 74 81 92 101 110
Min. Rec. Bend Radius for 45° Bend 33 36 39 42 45 48 57 63 69 75 84 93 102
Hose Length for Above (Inches) 28 31 33 35 37 40 46 52 57 62 69 75 82
Maximum \ ' 1/2 3/8 3/8 1/4 1/8 1/8 1/16 1/16 1/32 1/32 1/32 1/32 1/32
Lateral (2 (t 2 1 3/4 1 1/2 1 1/2 1/2 1M 1/8 1/16 1/16 1/16 1/16 1/16
( 3 h-Movement I
5
9
12
4 m
7
10
4 2
3
4
1 W4 1 1M 1 3/4 1/2
3/4
1
3/8
5/8
7/8
1/4
5/8
3/4
1/4
1/2
5/8
1/4
1/2
5/8
I 4 ft.
59
2 1/2 2 1/2 13/4 1 1/4
1 1/2(Inches) / 5ft 3 1/2 3 1/2 2
. ) Max. per ft.
Expansion / 1/8 3/16 3/16 3/16 3/16 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4
or v Tot. for any Lgth. 1/4 3/8 3/8 1/2 1/2 3/4 3/4 3/4 1 1 1 1 1
Contraction ( Mjn Length Req
(Inches) J for Above Total24 24 24 36 36 36 36 36 48 48 48 48 48
Max. Press. Unrestrained Ends, (PSI) 50 47 43 35 27 23 19 15 13 11 9 7 5
"CL; H
PermanentBendUsing45° Elbows
iT
CL,
CL- Distancefromcenterline to centerline. O - Amount of offset,measuredfromcenterlineto centerline.
A - Angle fromcenterlineof straighthosesections. D - Distancefromfaceto face of
flanges.
R - Radius of
actualhosebend. C - Amount of
hosecontractionfrominstalledposition.
L - Hoselengthfromfaceto face of flanges. E - Amount of
hoseexpansionfrominstalledposition.
Reproducedby courtesy of Allied Metal Hose Co.
Recommendedradii and hose lengths for bends are intended for permanentbends only and for installations where normal vibration exists. On90-degbend installations, two 45-degelbows and a straight length of pipe is preferable where installation permits. This avoids possibility of excessive vibration due to a rapid change of direction of gas flow. When expectedlateral movementexceedsthat of normal vibration, maximumlateral movementfigures for the hose lengths given must be considered.Where a large amountof offset exists, two 45-degelbows and a straightlength of hose should be considered.Note that 24 inches is the minimum lengthof hose recommendedfor any expansionor contractioninstallation.
Table 23-1. Flexible hose— INSTALLATION RECOMMENDATIONS
rugated flexible metallic hose in the line to permit freemovement of the pipe caused by expansion.
Pipe Brackets It is necessary to support exhaust
piping on brackets or hangers. Usually the silencer is
held rigidly by braces or its own weight gives it
rigidity, so that the pipe line expands away from the
silencer. To accommodate this expansion, the pipe supports should be provided with rollers or should be
suspended from hangers supported by the roof steel.
If vibration occurs, some form of isolation, such as
rubber doughnuts or springs, should be interposed be
tween hangers and their supports. In Fig. 23-9 are
shown several typical forms of pipe supports.
Pressure Waves in Exhaust Line When the ex
haust valve, or port, of a single-cylinder engine is
opened, exhaust gases under a pressure of 50 psig, or
higher, enter the exhaust pipe, which is assumed to be
open with no silencer. The gas possesses mass and
elasticity and the slug of high-pressure gas rushes outof the pipe. This action, coupled with cooling of the
remaining gas and condensing of the vapors, creates
a partial vacuum in the pipe behind the slug. When
251
DIESEL ENGINEERING HANDBOOK
Fig. 23-8 Atlantic double-insulated exhaust hose, asbestos plusdead-air space.
the slug is out of the pipe, air is sucked in, setting upa violent oscillation condition. Pressure waves with
peak values of over 5 psi will be reflected back and
forth in the exhaust pipe at the velocity of sound.
In air, velocity of sound is 1266 feet per second at212° F and 1814 at 932° F; it is slightly higher inwater vapor and somewhat lower in CCK. Velocity ofsound in exhaust gas is often assumed to be 1400 fps.
Frequency of these pressure waves depends pri
marily on the length of the pipe and to a lesser de
gree on the pipe diameter and gas temperature.
Due to these waves, pressure existing at the exhaustvalve, or ports, at the time of closing may be above orbelow atmospheric pressure, depending upon the pipelength and the engine speed.
In a 2-cycIe engine, increased pressure at this pointwould interfere with scavenging and a low pressuremight remove some of the air charge from the cylinder.In a 4-cycle engine, these waves may interfere with
scavenging and cause an increase in the pumping loss
on the exhaust stroke. The condition in a multi-cylinder engine with a single exhaust pipe would be similar
to a single-cylinder engine running at a correspond
ingly higher speed.
The silencer, or muffler, is required to reduce the
amplitude of the exhaust pressure waves since theyaffect engine performance and to reduce the suddenness with which gas slugs leave the pipe causing ob
jectionable noise. This should be done with the least
possible exhaust back pressure.
On many turbocharged engines it has been foundthat the turbine smoothes out the flow of exhaust gas
to such a degree that no additional silencing is re
quired.
Silencer Volume and Proportions Desirablevolume of a silencer for low-speed engines may be es
timated as approximately 20 times the exhaust slugvolume of one cylinder, the slug volume being equalto the piston displacement of the cylinder times itsvolumetric efficiency times the heat-rise factor fromFig. 23-1.
The common minimum ratio of length to diameter,r, or (L/D), is 4. although many are made with ratiosof 6 or even 8. A short large-diameter muffler will si
lence very well over a narrow band of frequencies. Along slim muffler will give less attenuation but will be
effective over a wider band of frequencies.L. H. Billey, of the Donaldson Company, suggests
, cu in.
Fig. 23-9 Stcndcrd exhaust-pipe supports.
the following equation to determine the approximatevolume required in a muffler for high-speed 4-cycle en
gines :
rpm N
where
V — muffler volume, cu in.D = piston displacement of engine, cu in.
rpm — revolutions per minute, maximum operatingspeed
N = number of cylindersK = constant, varying with application
— =5000 for farm tractors= 10,000 for off-highway and contractors'
equipment= 35,000 for over-the-highway trucks= 50,000 for passenger cars
The diameter and length can then be calculated:
0.7854 r
where r — L/D ratio, and
L = DrFor 2-cycle engines,
v KD V"lV " ——— , cu in.
rpm 2/V
Silencer Location Pressure waves in the exhaust
pipe, at a frequency determined chiefly by pipe length
and gas temperature, are acted upon by each succeed
ing slug of exhaust coming from the cylinders setting
up standing waves along the pipe length. Each wave
form has one or more nodes, or regions of minimum
pressure, and corresponding anti-nodes, or regions ofmaximum pressure. Location of the nodes and anti-
nodes depends upon engine speed.
When a silencer is located at an anti-node, it causesless back pressure and does more silencing than whenlocated at a node.
Effect of muffler location is illustrated by Billey inresults of tests shown in Table 23-2 for a truck installation using an exhaust system having a total lengthof 23 ft
,2 in. from manifold flange to end of tail pipe.
The engine was a 200-bhp diesel operated at full loadat 2100 rpm. A typical truck muffler was used (not a
straight-through type).
252
EXHAUST SYSTEMS
/II. J . .. I. .. J\ooo 0 0 0 0 0 0 0 0
000 Q OQOOOOM I II
.
II 1/
fig. 23-/0top: (IJ
(.Courtesy of Kittcll Muffler & Engineering, Inc.)
Different muffler designs and characteristics, fromBaffle-type muffler has advantage of low cost con
struction and is common on small engines; (2) Resonant-typemuffler causes little or no back pressure, is most effective onhigh frequencies; (3) Off-set tube-type muffler is fairly goodfor avoiding excessiveback pressure and is relatively cheap tomake; (4) Louvre-type muffler modulates or smooths outwave fronts In engine exhaust without creating excessive backpressure; (5) Three-pass tube-type muffler is excellent butfairly expensive to make.
Table 23-2.Variationof Sound Level and Back Pressurewith Muffler Location on a DieselTruck.
Muffler Exhaust Tail Muffler Sound Sound BackLocation Pipe Pipe Back Drop Coming PressureNo. Length Length Pressure Decibels Through Rise
1 4 ft lift .95"Hg.71" " 1
3
5% 135%
2 7ft 13 ft I6.S 2'A% 100%
3 10 ft 10 ft .83" " 7.5 18% 117%
4 13
ft 7ft 1.01"" I0.S »% 142%
5 16 ft 4ft .73" " 13.5 4Vj% 103%
6 l»ft 1 ft .72" " 8.0 ■*% 102%None 23 ft 2 in 0 ft Bah Base 100% Bat*
Fig. 23-11 Burgess-Manning snubber.
It can be seen that No. 2 position gives the greatest
reduction of noise level with the smallest muffler back
pressure, while No. 5 position is nearly as good. Leastreduction of noise level occurs at No. 3 position whilethe highest back pressure is developed at position No.
4.
It is often assumed that a muffler, or silencer, shouldbe located as close to the engine as possible. These re
sults show that this is not necessarily true since No.
1 position produces next to the highest back pressureand noise level is not reduced as much as in No. 2 orNo. 5 positions. These test results serve to point outthat an exhaust system presents a very complicatedproblem.
If the engine speed had been changed in these tests,
the best muffler position would probably have been
found to be some place other than No. 2 position.Silencer location for low-speed 2-cycle engines, par
ticularly crankcase scavenged, has been calculated in
the past by the following equation:2 V
L = T x IFWhen L — Length of pipe from exhaust manifold to
snubber, ft
V — Velocity in feet per second of the propaga
tion of the pressure wave through the ex
haust pipe
F = Rate of firing per second of the engine
V may be considered as 1400 ft per sec
This locates the inlet to the snubber at a distance
from the engine equal to two-thirds of a quarter wave
length having a frequency matching the rate of firing
of the engine.
Types of Silencers or Snubbers Many designs
of silencers are in use, all tending to (a) smooth out
pressure waves, and (b) quiet exhaust noise. Actual
rOUTLET
fig. 23-12 Two designs ofMaxim silencers.
253
DIESEL ENGINEERING HANDBOOK
Fig. 23-13. At left:Cutaway of 51 seriesKit tell
Fig. 23-M. CCutaway of iUniversal spark
Fig. 23-15. At right:Burgess - Manningsnubber incorporatinga spark arrester.Vanes at inlet impartswirl to
design varies with engine size and installation, allow
able back pressure, and permissible noise level, as well
as with the builders. A series of different designs with
their characteristics are shown in Fig. 23-10.
The snubber principle, Fig. 23-11, retards the
velocity of flow only when it is higher than the com
puted mean velocity, thus damping or snubbing off the
oscillating gas column.
The silencer at the top in Fig. 23-12 consists of different size expansion chambers in series. The passage
between the chambers is designed to give desiredacoustic properties and still offer low resistance to gas
flow. The muffler at the bottom in Fig. 23-12 is intended for high-quality silencing.
Spark-Arrester Silencers Many diesel installations are so situated that dirt and live sparks in the
exhaust must be removed for safe and satisfactory performance. While the problem of noise is a matter ofdegree only and depends in each case upon individualcircumstances of plant location, there is no such compromise with exhaust carbon and live sparks. If these
are objectionable at all they must be eliminated 100%for complete owner satisfaction and removal of un
necessary risk.
As may be seen in the sectional view, Fig. 23-13, the
spark-arresting silencer muffles the exhaust impulsesand at the same time extracts sparks and solids en
trained in the exhaust gases.
The centrifugal principle is used to whirl the gasesaround a slotted tube to throw out the foreign matter
into the traps provided for easy periodical cleaning.
Two more types of spark-arrester silencers are shownin Figs. 23-14 and -15. In Fig. 23-14 the sparks are
collected in the cone near the top. Fig. 23-15 shows a
snubber incorporating a spark-arrester.
Air-Cooled Mufflers An induced-draft air-cooled muffler is shown in Fig. 23-16. The muffler properis surrounded by an air shell through which air is induced, partly by convection and partly by the action
of the ejector cone situated at the exhaust end of the
muffler.
Noise suppression is provided by perforated concentric baffles which dissipate the energy of exhaust pulsations in the muffling chamber by effecting a series of
rapid expansions in exhaust gas volume.
Materials for Silencers Most silencers are fabricated from black iron, low-alloy iron, or steel sheets,
although various grades of stainless steel, terne plate,and aluminized steel are sometimes used. Stainlesssteel is rather prohibitive except in the smaller sizessince increased difficulty of fabrication and cost of the
material may increase the silencer cost to over 10 timesthat for black iron. Terne plate is of little value ex
cept under moderate temperature conditions since itburns away at high temperatures.
The aluminum coating on aluminized steel is transformed into an iron-aluminum alloy which protectsthe surface from rust and corrosion, and does notcrack off when dented. Assembled silencers may be
Fig. 23-16 Maxim air-cooledsilencer. Air enters jacketing just above flangeat engine.
2S4
EXHAUST SYSTEMS
galvanized, but this coating will burn away at tem
peratures slightly higher than terne plate.
Some silencers are being given a ceramic dip coat
ing after fabrication. Ceramics of a refractory naturehave high resistance to oxidation, corrosion and ther
mal shock.
Waste Hf.\t Recovery Each pound of fuel burned in a diesel cylinder releases approximately 18,600
Btu of heat. Of this amount approximately one-third
goes to develop power at the shaft; another small
amount, around 900 Btu goes to overcome friction.About one-third is still in the exhaust gases when they
leave the cylinder. The remainder is lost in the jacketwater.
Only a portion of the exhaust-gas heat can be re
covered. This is due to the fact that to obtain heat
transfer from the gases to another medium, say, for
steam generation, hot water heating or air heating,there must be a considerable temperature difference
between the two fluids. In addition, exhaust gases contain superheated steam ( about one pound per pound ofoil burned I and if the gases are cooled to the tem
perature at which the steam will condense into water,corrosion will set in.
Waste-heat boilers on the market at the present timecan be classified in the same general manner as con
ventional boilers; that is, (1) fire-tube type, Fig. 23-
17 and; (2) water-tube type, Fig. 23-18.
In order to approximate the quantity of heat re
coverable from the exhaust of an engine, Bradford andClarkson developed the formula
H = bhp XCX-JWhere H = Total heat recovered in Btu per hr
bhp = Engine brake hpC = 12 for 4-cycle engines= 20 for 2-cycle engines
D = Temp drop of gases, °F
\\£/.8/9'
As-l8"stee/pipe
EI.8l2'-6"
Topcrone rail E/.d09'
,-Fan■1I I
Section A-A
-ft4/ / sheer metal-* -f>A;2'-4"-.^?0'i<28y ■o/uct
Ac/justable! damper
4-3"/!.J n -^L
- 9 suppor ts on ->j
Adjustable dampen ^ 4" centers £7.780-6'
f\-;"'?V<»;-"".'; ;
Fig. 23-79 Plan deviled by G. C.Bayer tor building heating.
255
DIESEL ENGINEERING HANDBOOK
Table 23-3. Steam Obtainable from Diesel Exhaust Gases
4-Cycle Diesel Engines
H. P.LB. STEAM PER HR. FULL LOAD LB. STEAM PER HR. li LOAD LB. STEAM PER HR.
10lb. perVi LOAD
SQ. FT.HT. SURF.5 lb. per 10lb. per IS lb. per 5 lb. per in lb. per 15lb. per 5 lb. per 15lb.per
sq. in. sq. in. sq. in. sq in. sq. in. sq. in. sq. in. sq. in. sq. in.
75 77~ 74 72~ 60 58~ 56 44 ~~42 40 72100 92 89 87 72.5 70 67 53 50.4 48.5 72200 154 150 147 122 118 115 90 85 81.5 72300 212 205 199 167 160 156 122 116 III 72400 327 316 309 257 248 240 188 178 171 144500 392 380 372 310 298 290 226 215 205 144600 483 466 455 380 366 354 277 265 253 192700 550 530 520 432 417 404 317 301 287 192800 638 617 600 504 481 468 368 348 334 240900
1000700780
675750
664 550612
530588
515572
414 383425
366416
240288735 450
2-CYCLE DIESEL ENGINES
75 63 60 57 40 37 34 17 14 12 72
103 78 74 71 49 45 42 21 17 14 72
200 165 157 150 102 94 88 44 36 30 144
300 229 217 208 144 133 124 60 50 42 144400 300 285 273 191 176 165 80 66 55 192500 370 351 336 234 216 202 98 80 68 240609 450 427 410 282 260 244 118 98 82 288700 525 498 477 330 305 285 139 115 96 336800 600 570 545 378 358 320 158 130 109 384900 675 640 615 425 393 368 178 147 123 432
1000 750 710 682 475 440 410 198 163 136 480
Table 23-3 gives data as computed by Foster-
Wheeler Corp. on the amount of steam generated by
exhaust gases. This tabulation is general and as so
used is reliable, but it may not apply to a specific en
gine.A more accurate value of steam recovery can be de
termined by the weight of the gases and the tempera
ture change.
Hot Water Recovery In place of steam genera
tion, the exhaust heat can be used to heat water. This
may be part of the cylinder jacket water or the raw
water of the cooling system, passed through a waste-
heat boiler.
Air Heating It is possible to heat air either in the
engine room or part of a plant by passing the air
supply across the silencer.
Building heating has been successfully accomplished
by enclosing the engine exhaust silencers and forcing
Fig. 23-20 Cut-away view ofMaxim waste heat silencer. This type can operate either wet or dry.
air through the enclosure by means of a fan. One
method for accomplishing this is shown in Fig. 23-19.
While no exact formulae are available for calculatingthe probable heat recovery from such a system, tests at
Bloomington, 111., show that with 2-cycle-engine ex
haust temperatures of 320° F, inlet heating air tem
perature of 71° F, and outlet temperature of 115° F,a heat recovery of 138,000 Btu per hour was realizedwhen circulating 3,000 cfm of heating air around thesilencer. During this test, the engine was operating at
approximately half load of 320 hp. From these data itappears that heating systems of this character willserve satisfactorily and furnish sufficient heat for a
power plant building when 5 cu ft of air per minute
per rated engine horsepower are circulated around the
silencer. This "rule-of-thumb" guide in designing a
heating system may not always prove to be correct. Itappears that the problem of designing a satisfactory
heating system as shown in Fig. 23-19 resolves itselfinto a question of passing sufficient air over the outside of the silencer in a given time to absorb up to a
maximum of 30% of the heat in the exhaust gas. Putting this into the form of an equation:
0.30 H = 0.013 V (120 — t),or
16.7//
120 — t
where
H = total heat in Btu passing through the exhaustsilencer per minute
V ~ cu ft of heating air circulated around silencer
per minute/ = inlet temperature of heating air, °F
This equation is based upon the assumption that
temperature of hot air entering the room would be120° F with the heat carried per cu ft of dry air atthis temperature being 0.018 Btu per °F.
256
EXHAUST SYSTEMS
-—m «
■ f 0Fig. 23-21 Burgess-Manning waste-heat boiler with water tubes
in path of exhaust gas How.
Heat-Recovery Silencers There are occasions
when a small amount of heat is desirable. This is pos
sible without installing a complete waste-heat recovery
system by using a combination silencer and waste-heat
boiler as shown in Figs. 23-20 and 23-21. These provide an economical means of obtaining a limited
amount of steam. The approximate rate of heat re
covery, realized in a marine installation, is reported at
1 lb of steam per bhp hr.
QUESTIONS
1. Why is exhaust back pressure at the exhaust valves or portsan important factor in engine operation?
2. Why is a section of flexible metal hose used in the engineexhaust line in many diesel plants?
3. How may exhaust-pipe heat loss to the engine room bereduced?
4. How may pressure waves in the exhaust line affect cylinderscavenging?
5. What factors determine the pressure-wave effect in theexhaust system?
6. What are the requirements for a good muffler design?
What materials are usedmufflers or silencers?
the fabrication of exhaust
8. Discuss the relative merits of stainless-steel, aluminized-steel, and galvanized-steel silencers.
9. What would be the approximate rate of heat recovery in awaste-heat boiler from the exhaust gases of a 500-bhp 4-cycle diesel engine if the temperature drop of the gases is300eF?
10. Engine-room air is heated by circulation around an enclosedexhaust silencer of a 500-bhp engine which delivers 25,000Btu per minute to the silencer. If air to the silencer jacketis 50 F, air delivered to the engine room is I20°F, and
30% of the exhaust heat is absorbed by the air, how muchair must be circulated around the silencer per minute?
2S7