Heat Exchanger & Classification Prepared by: Nimesh Gajjar 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 Convection in each fluid Conduction through wall separating each fluid Overall heat transfer co efficient accounts for the above Conduction and convection effects Hot Fluid Cold Fluid Convection Conduction

Applications of Heat Exchangers Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer Car—prevent overheating with “high performance cooling applications” Maintain fluids at optimal temperature such that engine efficiency is increased and component wear minimized Boilers and condensers are opposite processes used in industry -Boilers used to create steam—sometimes to drive steam driven turbines -condenseros bring steam back to liquid for reuse Distillation set-ups typically use condensers to condense distillate vapors back into liquid. Ultimately, condensers-cool refrigerant vapor back to liquid-- sub cooled liquid flows out of the condenser and into the evaporator where it absorbs heat from air in the surrounding pipe and generates cold air. Air conditioners utilize a closed heat exchange system that separates the hot and the cold fluids. Energy is transported via convection between the fluid and inner pipe surface, conduction through the pipe and then convection from the outer pipe surface into the second fluid. This closed system is very similar to the one examined in this lab experiment, and its widespread applicability is why this lab was designed. 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

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

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

PLATE HEAT EXCHANGER Spiral Plate HE Lamella Plate HE

SHELL AND TUBE HEAT EXCHANGER Rear-end header Shell outlet Tube inlet Tube outlet Shell inlet Shell Baffles Tubes Front-end Compact heat exchanger Gas to gas HE Gas to liquid or liquid to gas HE Low heat transfer coefficient on gas side – Increase surface area using fin

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

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.

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 Cross flow- Flow perpendicular to each other In between the above Design of headers require less space

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

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

According to Pass Arrangement Single Pass Two Pass Multi Pass

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

According to Phase of Fluid: According to Mechanism of Heat Transfer 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 .

According to Extended Surface:

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 Hot fluid Wall Heat transfer U is meaningless unless area is specified