Heat ExchangersHeat Exchangers

Coolers and heaters are heat Coolers and heaters are heat exchangers and they transfer heat exchangers and they transfer heat energy from one medium to another. energy from one medium to another.

Consist of two channels or system of Consist of two channels or system of channels, one for each medium. channels, one for each medium.

Mediums are separated by partitions Mediums are separated by partitions through which heat is transferred through which heat is transferred from the warmer to the cooler one. from the warmer to the cooler one. Heat can only flow from a higher Heat can only flow from a higher heat source to a lower heat region. heat source to a lower heat region.

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

CoolersCoolers On board ships, the main engine is On board ships, the main engine is

cooled by the jacket cooling water and cooled by the jacket cooling water and lubricating oil. These fluids are then lubricating oil. These fluids are then passed through a cooler where sea passed through a cooler where sea water is used as the cooling medium to water is used as the cooling medium to cool the fresh water and the lubricating cool the fresh water and the lubricating oil. After being cooled the fluids return oil. After being cooled the fluids return back to the main engine and the sea back to the main engine and the sea water is discharged overboard.water is discharged overboard.

HeatersHeaters Heaters are used for various purposes Heaters are used for various purposes

on board ships. The most essential on board ships. The most essential one is used for the fuel oil heating. one is used for the fuel oil heating. The temperature of the fuel oil is The temperature of the fuel oil is raised by passing steam through the raised by passing steam through the heating tubes and the oil on the heating tubes and the oil on the outside of these tubes. Thus heat is outside of these tubes. Thus heat is transferred from the steam to the fuel transferred from the steam to the fuel oil. oil.

Types of Heat Types of Heat ExchangersExchangers In the In the shell & tube typeshell & tube type heat heat

exchanger one medium flows through a exchanger one medium flows through a set of tubes and the other medium set of tubes and the other medium flows on the outside. Thus heat transfer flows on the outside. Thus heat transfer take place across the tube walls.take place across the tube walls.

In the In the plate typeplate type heat exchanger, a heat exchanger, a set of corrugated plates are tightly set of corrugated plates are tightly packed with spaces in between. The packed with spaces in between. The different mediums flow on either sides different mediums flow on either sides of the plate and heat transfer takes of the plate and heat transfer takes place across the plate.place across the plate.

Tubular exchangers are used in great Tubular exchangers are used in great numbers, far more than any other numbers, far more than any other type of exchanger. They are made in a type of exchanger. They are made in a wide variety of sizes and styles. wide variety of sizes and styles.

Tubular exchangers are so widely Tubular exchangers are so widely used because the technology is well used because the technology is well established for making precision metal established for making precision metal tubes capable of containing high tubes capable of containing high pressures in a variety of materials. pressures in a variety of materials. There is virtually no limit to the range There is virtually no limit to the range of pressures and temperatures that of pressures and temperatures that can be accommodated. can be accommodated.

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

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

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

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

a. a. Outside the tubes with liquid cooling, Outside the tubes with liquid cooling, i.e., a water-cooled steam condenser. i.e., a water-cooled steam condenser. Fins are not necessary either inside or Fins are not necessary either inside or outside the tubes. outside 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 an air-cooled Freon condenser heat pump or refrigerating system. Fins are pump or refrigerating system. Fins are normally provided on the outsides of normally provided on the outsides of the tubes. the tubes.

The tubes may be flattened to obtain a The tubes may be flattened to obtain a better ratio of surface area to flow cross better ratio of surface area to flow cross section. section.

5. 5. Evaporators and Evaporators and boilersboilers a. Fired systems, a. Fired systems, in which the in which the

combustion of fuels at very high combustion of fuels at very high temperatures but ambient pressure (and temperatures but ambient pressure (and low density) generate steam under low density) generate steam under pressure. Normally the water and/or pressure. Normally the water and/or steam is contain inside the tubes (water steam is contain inside the tubes (water tube boiler), and pin fins, studs, or some tube boiler), and pin fins, studs, or some other form of extended surface is other form of extended surface is applied to the tubes to enhance the heat applied to the tubes to enhance the heat transfer. In smaller systems the transfer. In smaller systems the combustion products may pass through combustion products may pass through the tubes (fire tube boilers). the tubes (fire tube boilers).

b. Unfired systems. b. Unfired systems. These embrace a These embrace a great variety of different boiler and great variety of different boiler and evaporator heat exchangers. Many evaporator heat exchangers. Many chemical and food-processing chemical and food-processing applications involve the use of applications involve the use of steam steam or or electricelectric heating to evaporate heating to evaporate solvents, concentrate solutions, distill solvents, concentrate solutions, distill liquors, or dehydrate compounds. liquors, or dehydrate compounds.

Type of Flow and Flow Patterns Type of Flow and Flow Patterns in Heat in Heat

Laminar Laminar FlowFlow

Turbulent Turbulent FlowFlow

Flow ArrangementsFlow Arrangements Parallel Flow PatternParallel Flow Pattern In this type, both the hot and In this type, both the hot and

the the cold fluids flow in the same cold fluids flow in the same direction.direction.

This type is not normally This type is not normally used in used in marine practice except marine practice except incases where a combination incases where a combination of the basic flow of the basic flow arrangement is used. arrangement is used.

This is due to its inherent This is due to its inherent poorer heat transfer rates.poorer heat transfer rates.

Advantages – Advantages – a) lower heat transfer a) lower heat transfer

surface surface temperature.temperature. b) more even heat transfer b) more even heat transfer surface temperature surface temperature

Contra or Counter Flow Pattern Contra or Counter Flow Pattern This arrangement is widely This arrangement is widely

used but the use of baffles used but the use of baffles or multi-passes of coolant or multi-passes of coolant upset the basic flow upset the basic flow arrangement so that in arrangement so that in most cases a combination of most cases a combination of all three is used.all three is used.

Plate type of heat Plate type of heat exchanger can use this exchanger can use this arrangement without arrangement without combination with any other.combination with any other.

Advantages –Advantages –a) best heat transfer rates.a) best heat transfer rates.b) closer temperature b) closer temperature

differential.differential.c) outlet temperature of c) outlet temperature of coolant coolant

can be higher than can be higher than outlet outlet

temperature of hot fluid temperature of hot fluid

Cross Flow PatternCross Flow Pattern Exchangers such as Exchangers such as

air coolers for diesel air coolers for diesel engines, super-engines, super-heaters or tubular gas heaters or tubular gas heaters in boilers use heaters in boilers use this arrangement but this arrangement but again it is usual to find again it is usual to find that the heat basic that the heat basic flow is complicated by flow is complicated by multi-passes and the multi-passes and the result is a combination result is a combination of flow arrangements.of flow arrangements.