What is a heat exchanger? Classification of heat exchangers Regenerators Open-type heat exchangers Closed-type heat exchangers Classification of recuperators Single-pass vs multi-pass Parallel flow, counterflow, and crossflow Temperature profiles for single-pass heat

exchangers

LMTD and AMTD Sample problems Exercises

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A heat exchanger is a device whose primary purpose is the transfer of energy between two fluids.

Examples:

- Radiator - Evaporator - Condenser - boiler

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1. regenerators

2. open-type heat exchangers

3. closed-type heat exchangers

or recuperators

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Regenerators are exchangers in which hot and cold fluids flow alternately through the same space with as little physical mixing between the two streams as possible.

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Open-type heat exchangers are

devices wherein physical

mixing of the two fluid streams

actually occurs.

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Recuperators are devices wherein the hot and cold fluid streams do not come into direct contact with each other but are separated by a tube wall or a surface that may be flat or curved in some manner.

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1. single-pass heat exchanger

- parallel flow

- counterflow

- crossflow

2. multi-pass heat exchanger

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Single-pass heat exchanger is one

in which each fluid flows through

the exchanger only once.

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Parallel low heat exchangers is one in which

the fluids flow in the same direction.

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Counterflow heat exchanger is one in which the

fluids flow in opposite direction.

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Crossflow heat exchanger is one in which the fluids

flow at right angles to one another.

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In order to accomplish as much transfer of

energy in as little space as possible, it is

desirable to utilize multiple passes of one or

both fluids. A common configuration is the

shell-and-tube heat exchanger.

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Where LMTD = Log Mean Temperature Difference

1 = maximum temperature difference

2 = minimum temperature difference

NOTE: If 1 and 1 are nearly the same use the Arithmetic-Mean Temperature Difference (AMTD)

AMTD = 1+22

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q = UA (LMTD)

Where q = the heat tranfer between fluids, kJ or kW

U = the overall heat transfer coefficient, kJ (or kW)/m2K

A = surface area of the tube

LMTD = Log Mean Temperature Difference

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A hot process stream being cooled from 100oC to 60oC by a cooling water stream that is heated from 15oC to 30oC. What temperature driving force (LMTD) should be used to calculate the required area?

Solution:

1 = 100 15 = 85C

2 = 60 30 = 30C

=12

12

=8530

85

30

= 52. 81 (ans)

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1. A liquid to liquid counterflow heat exchanger is used to heat a cold fluid from 120F to 310F. Assuming that the hot fluid enters at 500F and leaves at 400F, calculate the log-mean temperature difference of the heat exchanger.

Ans. LMTD = 232F

2. A turbo-generator, 16 cylinder, V type Diesel engine has an air consumption of 3000 kg/hr per cylinder at rated load and speed. This air is drawn in through a filter by a centrifugal compressor directly connected to the exhaust gas turbine. The temperature of the air from the compressor is 145C and a counterflow air cooler reduces the air temperature to 45C before it goes to the engine suction header. Cooling water enters air cooler at 30C and leaves at 38C. Calculate the arithmetic-mean temperature difference.

Ans. 61C

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3. Exhaust steam at 7 kPa at the rate of 75 kg/s enters a single pass condenser containing 5,780 pieces of copper tubes with a total surface area of 2950 m2. The steam has a moisture content of 10% and the condensate leaves saturated liquid at a steam temperature. The cooling water flow rate is 4,413 liters per second entering at 20C. Size of tubes, 25 mm O.D. by 3 mm thick wall. Find the overall heat-transfer coefficient.

Ans. U = 4275 W/m2K

4. An oil having a specific heat of 1880 J/kg K enters a single-pass counterflow heat exchanger at a rate of 2 kg/s and a temperature of 400 K. It is to be cooled to 350 K. Water is available to cool the oil at a rate of 2 kg/s and a temperature of 280 K. Determine the surface area required if the overall heattransfer coefficient is 230W/m2 K.

[Note: specific heat of water is 4.80 J/kgK]

Ans. A = 9.85 m2

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5. Onehundred thousand pounds per hour ofwater are to pass

through a heat exchanger, which is to raise the water

temperature from 140 to 200F. Combustion products having a

specific heat of 0.24 Btu/lbmF are available at 800F. The

overall heat-transfer coefficient is 12 Btu/h ft2 F. If 100,000

lbm/h of the combustion products are available, determine

a. the exit temperature of the flue gas;

b. the required heat-transfer area for a counterflow

exchanger.

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