Mechanical - Heat and Mass Transfer - Radiation

1. **In a counter flow
double pipe heat exchanger, oil is cooled from 85**°**C to 55**°**C
by water** **entering at 25**°**C.
The mass flow rate of oil is 9,800 kg/h and specific heat of oil is 2000 j/kg ****K. the mass flow rate of water is 8,000
kg/h and specific heat of water is 4180 j/kg K. Determine the heat exchanger
area and heat transfer rate for an overall heat transfer coefficient of 280 W/m ^{2}
K.**

**Given: **

Hot fluid –oil Cold fluid –water

T1,T2 t1, t2

Entry temperature of oil T_{1} = 85°C

Exit temperature of oil T_{2} = 55°C

Entry temperature of water t_{1} = 25°C

Mass flow rate of oil (Hot fluid) m_{h} =
9,800 kg/h

Specific heat of water C_{pc} –4180 j/kg K

Overall heat transfer coefficient U = 280 W/m^{2}K

**To find**

1. Heat
exchanger area (A)

2. Heat
transfer rate (Q)

**Solution **

We
know that

Heat lost by oil Hot fluid = Heat gained by water
cold fluid

Qh = Q_{c}

(1) Þ Q = UA (Delta T)_{m}

Þ 162 ´10^{3} =280 A´ 35´.8

Þ A =
16.16 m^{2}

2.**An oil cooler of the form of tubular heat
exchanger cools oil from a temperature of 90**°**C**
**to 35**°**C by a large pool of
stagnant water assumed constant temperature of 28**°**C. The tube length is 32 m and diameter is 28 mm.
The specific heat and specific gravity of the oil are 2.45 kj/kg K and 0.8
respectively. The velocity of the oil is 62 cm/s. Calculate the overall heat
transfer coefficient.**

**A parallel flow heat exchanger is used
to cool. 4.2 kg/min of hot liquid of specificheat 3.5 kj/kgK at 130**°**C. A cooling water of specific heat 4.18 kj/kg K is
used for cooling purpose at a temperature of 15**°**C. The mass flow rate of cooling water is 17 kg/min
calculate the following.**

**1. ****Outlet
temperature of liquid **

**2. ****Outlet
temperature of water **

**3.
****Effectiveness of heat exchanger
Take care, **

Overall heat transfer coefficient is 1100 W/m^{2}
K.

Heat exchanger area is 0.30 m^{2}

**Given:**

Mass flow rate of hot liquid m_{h}= 4.2 kg/min

M _{h} = 0.07 kg/s

Specific heat of hot liquid C_{ph}= 3.5
kj/kg K

**5. In a cross heat exchangers both
fluids unmixed hot fluid with a specific heat of 2300 J/kg K enters at 380**°**C and leaves at 300**°**C cold fluids enters at 25**°**C and leaves at 210**°**C. Calculate the required surface of heat exchanger.
Take overall heat transfer coefficient is**

**750 W/m ^{2} K. Mass flow rate of
hot fluid is 1 kg/s. **

**Given:**

Specific heat of hot fluid C_{ph} = 23000 J/kg K

** **

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Mechanical : Heat and Mass Transfer : Phase Change Heat Transfer and Heat Exchangers : Solved Problems - Heat and Mass Transfer - Radiation |

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