Heat exchangers transfer heat from one medium to another and come in various types. They consist of channels that separate the mediums but allow heat transfer between them. Common examples include coolers that use sea water to cool engine fluids, and heaters that use steam to heat fuels. Tubular heat exchangers are most widely used and can accommodate a variety of temperatures and pressures. Heat exchangers effectively transfer heat through processes like cooling, heating, condensing, and evaporating.

1. Heat ExchangersHeat Exchangers

2.  Coolers and heaters are heatCoolers and heaters are heat
exchangers and they transfer heatexchangers and they transfer heat
energy from one medium to another.energy from one medium to another.
 Consist of two channels or system ofConsist of two channels or system of
channels, one for each medium.channels, one for each medium.
 Mediums are separated by partitionsMediums are separated by partitions
through which heat is transferred fromthrough which heat is transferred from
the warmer to the cooler one. Heat canthe warmer to the cooler one. Heat can
only flow from a higher heat source to aonly flow from a higher heat source to a
lower heat region.lower heat region.

3.  Heat exchangers are called by variousHeat exchangers are called by various
names such as coolers, heaters,names such as coolers, heaters,
condensers and evaporators. The freshcondensers and evaporators. The fresh
water coolers, fuel oil heaters, lubricatingwater coolers, fuel oil heaters, lubricating
oil coolers, air coolers are just some of theoil coolers, air coolers are just some of the
heat exchangers found on board ships.heat exchangers found on board ships.

4. CoolersCoolers
 On board ships, the main engine is cooledOn board ships, the main engine is cooled
by the jacket cooling water and lubricatingby the jacket cooling water and lubricating
oil. These fluids are then passed throughoil. These fluids are then passed through
a cooler where sea water is used as thea cooler where sea water is used as the
cooling medium to cool the fresh watercooling medium to cool the fresh water
and the lubricating oil. After being cooledand the lubricating oil. After being cooled
the fluids return back to the main enginethe fluids return back to the main engine
and the sea water is dischargedand the sea water is discharged
overboard.overboard.

5. HeatersHeaters
 Heaters are used for various purposes onHeaters are used for various purposes on
board ships. The most essential one isboard ships. The most essential one is
used for the fuel oil heating. Theused for the fuel oil heating. The
temperature of the fuel oil is raised bytemperature of the fuel oil is raised by
passing steam through the heating tubespassing steam through the heating tubes
and the oil on the outside of these tubes.and the oil on the outside of these tubes.
Thus heat is transferred from the steam toThus heat is transferred from the steam to
the fuel oil.the fuel oil.

6. Types of Heat ExchangersTypes of Heat Exchangers
 In theIn the shell & tube typeshell & tube type heat exchangerheat exchanger
one medium flows through a set of tubesone medium flows through a set of tubes
and the other medium flows on theand the other medium flows on the
outside. Thus heat transfer take placeoutside. Thus heat transfer take place
across the tube walls.across the tube walls.
 In theIn the plate typeplate type heat exchanger, a setheat exchanger, a set
of corrugated plates are tightly packedof corrugated plates are tightly packed
with spaces in between. The differentwith spaces in between. The different
mediums flow on either sides of the platemediums flow on either sides of the plate
and heat transfer takes place across theand heat transfer takes place across the
plate.plate.

7.  Tubular exchangers are used in greatTubular exchangers are used in great
numbers, far more than any other type ofnumbers, far more than any other type of
exchanger. They are made in a wideexchanger. They are made in a wide
variety of sizes and styles.variety of sizes and styles.
 Tubular exchangers are so widely usedTubular exchangers are so widely used
because the technology is wellbecause the technology is well
established for making precision metalestablished for making precision metal
tubes capable of containing hightubes capable of containing high
pressures in a variety of materials. Therepressures in a variety of materials. There
is virtually no limit to the range ofis virtually no limit to the range of
pressures and temperatures that can bepressures and temperatures that can be
accommodated.accommodated.

8.  1.1. Liquid/liquidLiquid/liquid.. This is by far the mostThis is by far the most
common application of tubularcommon application of tubular
exchangers. Typically, cooling water onexchangers. Typically, cooling water on
one side is used to cool a hot effluentone side is used to cool a hot effluent
stream. Both fluids are pumped throughstream. Both fluids are pumped through
the exchanger so that the principal modethe exchanger so that the principal mode
of heat transfer is forced convective heatof heat transfer is forced convective heat
transfer.transfer.

9.  2.2. Liquid/gasLiquid/gas.. This is also very common,This is also very common,
typically for the air cooling of a hot liquidtypically for the air cooling of a hot liquid
effluent. The liquid is pumped through theeffluent. The liquid is pumped through the
tubes with very high rates of convectivetubes with very high rates of convective
heat transfer. The air, in cross-flow overheat transfer. The air, in cross-flow over
the tubes, may be in the forced or freethe tubes, may be in the forced or free
convective mode. Heat-transferconvective mode. Heat-transfer
coefficients on the air side are lowcoefficients on the air side are low
compared with those on the liquid side.compared with those on the liquid side.
Fins are usually added on the outsides (airFins are usually added on the outsides (air
side) of the tubes to compensate.side) of the tubes to compensate.

10.  3.3. Gas/gasGas/gas.. This type of exchanger isThis type of exchanger is
found in the exhaust-gas/air preheating,found in the exhaust-gas/air preheating,
steel furnaces, and etc. Insteel furnaces, and etc. In many casesmany cases
one gas is compressed so the density isone gas is compressed so the density is
high, while the other is at low pressurehigh, while the other is at low pressure
with a low density. Normally the high-with a low density. Normally the high-
density fluid flows inside the tubes.density fluid flows inside the tubes.
Internal and external fins are sometimesInternal and external fins are sometimes
provided to enhance the heat transfer.provided to enhance the heat transfer.

11.  4.4. Condensers.Condensers. These may be liquidThese may be liquid
(water) or gas (air) cooled. Alternatively,(water) or gas (air) cooled. Alternatively,
the heat from condensing steams may bethe heat from condensing steams may be
used to heat another processing fluid.used to heat another processing fluid.
Normally the condensing fluid is routed inNormally the condensing fluid is routed in
either of two ways:either of two ways:

12.  a.a. Outside the tubes with liquid cooling,Outside the tubes with liquid cooling,
i.e., a water-cooled steam condenser. Finsi.e., a water-cooled steam condenser. Fins
are not necessary either inside or outsideare not necessary either inside or outside
the tubes.the tubes.
 b.b. Inside the tubes with gas cooling, i.e.,Inside the tubes with gas cooling, i.e.,
an air-cooled Freon condenser heat pumpan air-cooled Freon condenser heat pump
or refrigerating system. Fins are normallyor refrigerating system. Fins are normally
provided on the outsides of the tubes.provided on the outsides of the tubes.
The tubes may be flattened to obtain aThe tubes may be flattened to obtain a
better ratio of surface area to flow crossbetter ratio of surface area to flow cross

13. 5.5. Evaporators and boilersEvaporators and boilers
 a. Fired systems,a. Fired systems, in which the combustionin which the combustion
of fuels at very high temperatures butof fuels at very high temperatures but
ambient pressure (and low density)ambient pressure (and low density)
generate steam under pressure. Normallygenerate steam under pressure. Normally
the water and/or steam is contain insidethe water and/or steam is contain inside
the tubes (water tube boiler), and pin fins,the tubes (water tube boiler), and pin fins,
studs, or some other form of extendedstuds, or some other form of extended
surface is applied to the tubes to enhancesurface is applied to the tubes to enhance
the heat transfer. In smaller systems thethe heat transfer. In smaller systems the
combustion products may pass throughcombustion products may pass through
the tubes (fire tube boilers).the tubes (fire tube boilers).

14.  b. Unfired systems.b. Unfired systems. These embrace aThese embrace a
great variety of different boiler andgreat variety of different boiler and
evaporator heat exchangers. Manyevaporator heat exchangers. Many
chemical and food-processing applicationschemical and food-processing applications
involve the use ofinvolve the use of steamsteam oror electricelectric
heating to evaporate solvents, concentrateheating to evaporate solvents, concentrate
solutions, distill liquors, or dehydratesolutions, distill liquors, or dehydrate
compounds.compounds.

15. Type of Flow and Flow Patterns inType of Flow and Flow Patterns in
HeatHeat
 Laminar FlowLaminar Flow

16.  TurbulentTurbulent
FlowFlow

17. Flow ArrangementsFlow Arrangements
 Parallel Flow PatternParallel Flow Pattern
 In this type, both the hot andIn this type, both the hot and
thethe
cold fluids flow in the samecold fluids flow in the same
direction.direction.
 This type is not normally usedThis type is not normally used
inin
marine practice except incasesmarine practice except incases
where a combination of thewhere a combination of the
basic flow arrangement is used.basic flow arrangement is used.
 This is due to its inherentThis is due to its inherent
poorer heat transfer rates.poorer heat transfer rates.
 Advantages –Advantages –
a) lower heat transfer surfacea) lower heat transfer surface
temperature.temperature.
b) more even heat transferb) more even heat transfer

18. Contra or Counter Flow PatternContra or Counter Flow Pattern
 This arrangement is widelyThis arrangement is widely
used but the use of baffles orused but the use of baffles or
multi-passes of coolant upsetmulti-passes of coolant upset
the basic flow arrangement sothe basic flow arrangement so
that in most cases athat in most cases a
combination of all three iscombination of all three is
used.used.
 Plate type of heat exchangerPlate type of heat exchanger
can use this arrangementcan use this arrangement
without combination with anywithout combination with any
other.other.
 Advantages –Advantages –
a) best heat transfer rates.a) best heat transfer rates.
b) closer temperatureb) closer temperature
differential.differential.
c) outlet temperature of coolantc) outlet temperature of coolant
can be higher than outletcan be higher than outlet
temperature of hot fluidtemperature of hot fluid

19. Cross Flow PatternCross Flow Pattern
 Exchangers such as airExchangers such as air
coolers for dieselcoolers for diesel
engines, super-heatersengines, super-heaters
or tubular gas heaters inor tubular gas heaters in
boilers use thisboilers use this
arrangement but again itarrangement but again it
is usual to find that theis usual to find that the
heat basic flow isheat basic flow is
complicated by multi-complicated by multi-
passes and the result ispasses and the result is
a combination of flowa combination of flow
arrangements.arrangements.