Introduction to Heat Exchanger Classification Prepared by Nimesh

Introduction to Heat Exchanger & Classification Prepared by: Nimesh Gajjar

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 Ø Convection in each fluid Ø Conduction through wall separating each fluid Overall heat transfer co efficient accounts for the above Conduction and convection effects Hot Fluid Cold Fluid Hot Fluid Conduction Convection

Applications of 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

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 4

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 5

According to Construction : Ø Tubular heat exchanger (double pipe, shall and tube, coiled tube) Tubular. Heat. Ex. swf Triple. Tube. He. swf shelltubehex. swf Ø Plate heat exchanger ( spiral, plate coil, lamella) Plate. Heat. Ex. swf Ø Extended surface exchangers (tube fin, plate fin) Ø Regenerators (fixed matrix, rotary) 7

PLATE HEAT EXCHANGER Spiral Plate HE Lamella Plate HE

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 m 2/m 3 –Compact Car radiator β =1000 m 2/m 3 Gas turbine HE β =1000 m 2/m 3 Human lungs β =20000 m 2/m 3 Achieve high heat transfer rates between two fluids in a small Volume Ø The ratio of the heat transfer surface area of a heat exchanger to its volume is called the “area density β’’. Ø A heat exchanger with area density β is greater than 700 m 2/m 3 or 200 ft 2/ft 3 is classified as being compact.

According to Flow Arrangement Ø Concurrent – Flow in same direction Ø Thermodynamically poor Ø High thermal stresses since large temperature difference at inlet Ø Heat sensitive materials Tubular. Heat. Ex. swf Ø Counter current- flow opposite to each other Ø Thermodynamically superior Ø Minimum thermal stresses Ø Maximum heat recovery Ø Least heat transfer area Tubular. Heat. Ex. swf Ø Cross flow- Flow perpendicular to each other Ø In between the above Ø Design of headers require less space 12

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

Double pipe heat exchanger: T Parallel Counter flow T Hot Fluid Col d uid Fl d l o C Flu id Cold fluid in Cold fluid out Hot fluid in Cold fluid in Hot fluid out Hot fluid in Cold fluid out Tubular. Heat. Ex. swf

According to Pass Arrangement • Single Pass • Two Pass • Multi Pass 15

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

According to Phase of Fluid: • These classifications is made according to the phase of the fluid. 1. gas-gas, 2. liquid-liquid, 3. Gas-liquid etc. According to Mechanism of Heat Transfer 1. Single phase convection, (forced or free). 2. Two phase convection (condensation or evaporation) by force or free convection. 17

According to Extended Surface:

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