Introduction to Heat Exchanger & Classification Prepared by: Nimesh Gajjar

1. Introduction to Heat Exchanger & Classification Prepared by: Nimesh Gajjar

2. HEAT EXCHANGERS • Device that facilitates the exchange of heat between two fluids that are at different temperatures without mixing each other • Heat transfer in a heat exchanger involves • Convection in each fluid • Conduction through wall separating each fluid • Overall heat transfer co efficient accounts for the above • Conduction and convection effects Conduction Hot Fluid Convection Cold Fluid Hot Fluid

3. Applications of Heat Exchangers Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer Heat exchangers are used in AC and furnaces

4. Classification of Heat Exchangers These are most common heat exchangers in which hot and cold fluid do not come into contact with each other but are separated by a tube wall or a surface which may be flat or curved . Energy exchange by hot fluid to surface by convection through the wall or plate by conduction and then by convection from the surface to the cold fluid. They are used where mixing of hot and cold fluid is objectionable. For eg. Oil coolers, intercoolers, air preheaters, economisers, condensers. 2) radiators of automobiles 3) evaporator of an ice plant and milk chiller of a pasteurizing plant

5. These are heat exchangers in which hot and cold fluids flow alternatively ( i.e. periodically) through the same space with no or little physical mixing between the streams. The heat carried away is accumulated in the walls of the equipment called solid matrix and is then transferred to the cold fluid when it passes the surface next. Mostly used in gas to gas heat exchangers such as IC engines and gas turbines. Other applications in glass melting furnace and air heaters of blast furnace. Depending Parameters-1) heat capacity of regenerating material 2)rate of absorption and release of heat

7. According to Construction : • Tubular heat exchanger (double pipe, shall and tube, coiled tube) TubularHeatEx.swfTripleTubeHe.swfshelltubehex.swf • Plate heat exchanger ( spiral, plate coil, lamella) PlateHeatEx.swf • Extended surface exchangers (tube fin, plate fin) • Regenerators (fixed matrix, rotary)

8. PLATE HEAT EXCHANGER Spiral Plate HE Lamella Plate HE

9. According to Transfer Process 1) Indirect Contact (double pipe, shall and tube, coiled tube) 2) Direct contact (cooling towers)

10. COMPACT HEAT EXCHANGERS • β >700 m2/m3 –Compact • Car radiator β =1000 m2/m3 • Gas turbine HE β =1000 m2/m3 • Human lungs β =20000 m2/m3 • Achieve high heat transfer rates between two fluids in a small Volume • The ratio of the heat transfer surface area of a heat exchanger to its volume is called the “area density β’’. • A heat exchanger with area density β is greater than 700 m2/m3 or 200 ft2/ft3 is classified as being compact.

11. According to Flow Arrangement • Concurrent – Flow in same direction • Thermodynamically poor • High thermal stresses since large temperature difference at inlet • Heat sensitive materials TubularHeatEx.swf • Counter current- flow opposite to each other • Thermodynamically superior • Minimum thermal stresses • Maximum heat recovery • Least heat transfer area TubularHeatEx.swf • Cross flow- Flow perpendicular to each other • In between the above • Design of headers require less space

12. CROSS FLOW HEAT EXCHANGER Cross-flow (mixed) Cross-flow (unmixed) Tube flow (unmixed) Tube flow (unmixed) a) Both fluids unmixed b) One fluids mixed, one fluid unmixed

13. Double pipe heat exchanger: Parallel Counter flow Cold fluid in Cold fluid out Hot fluid out Hot fluid out Hot fluid in Hot fluid in Cold fluid in Cold fluid out T T Hot Fluid Hot Fluid Cold Fluid Cold Fluid TubularHeatEx.swf

14. According to Pass Arrangement • Single Pass • Two Pass • Multi Pass

15. Shell-side fluid In Shell-side fluid In One shell two tube pass Tube-side fluid Out Out In In Out Out Tube-side fluid Two shell four tube pass

16. According to Phase of Fluid: • These classifications is made according to the phase of the fluid. • gas-gas, • liquid-liquid, • Gas-liquid etc. According to Mechanism of Heat Transfer • Single phase convection, (forced or free). • Two phase convection (condensation or evaporation) by force or free convection .

17. According to Extended Surface:

18. Overall heat transfer coefficient Ai – Inside area of the tube Ao – Outside area of the tube hi – inside heat transfer coefficient ho – Outside heat transfer coefficient k – thermal conductivity of tube Cold fluid Heat transfer Hot fluid Cold fluid Hot fluid Wall U is meaningless unless area is specified