A)
In a double pipe heat exchanger, flow rates of the hot and the cold water streams flowing through a heat exchanger are 10 and 25 kg/min, respectively. Hot and cold water stream inlet temperatures are 70 °C and 27 °C, respectively. The exit temperature of the hot stream is required to be 50°C. The specific heat of water is 4.179 kJ/ kg K. The overall heat transfer coefficient is 900 W/m2 K. Neglecting the effect of fouling, calculate the heat transfer area for a) Parallel-flow b) Counter-flow.
B) Write a brief note on the operation and application of plate heat exchangers in process industries.
Ans:
A)
Rate of heat transfer, Q (watts)
Q= m x Cp x 1000 x (T2-T1)
=(10/60)× 4.179 x 1000 ×(70 – 50) = 13930 W
Cold water exit temperature, T2
T2 = [Q/(mx Cp x 1000)]+ T1
= (13930/((25/60)* (4.179*1000)))+27
= 35°
Terminal temperature differences for parallel flow heat exchangers
= (70-27) & (50 – 35); i.e.,
43°C and 15°C respectively.
LMTD
(43 – 15)/ln(43/15) = 26.59°C
Overall heat transfer coefficient U
900 W/m2 °K
Heat transfer area required for parallel flow
A = Q/ (U*LMTD)
= [13930/ (900 × 26.59)]
= 0.582 m2
Terminal temperature differences for counter flow heat exchangers
(70–35) and (50–27) 0C i.e.,
35 °C and 23 °C respectively.
LMTD
(35–23)/ln(35/23) = 28.58
Overall heat transfer coefficient U
900 W/m2 °K
Heat transfer area required for counter flow
A = Q/ (U*LMTD)
=[13930/ (900 × 28.58)]
= 0.542 m2
(B)
Plate heat exchangers consist of a stack of parallel thin plates that lie between heavy end plates. Each fluid streampasses alternately between adjoining plates in the stack, exchanging heat through the plates. The plates are corrugated for strength and to enhance heat transfer by directing the flow and increasing turbulence. These exchangers have high heat-transfer coefficients and area, the pressure drop is also typically low, and they often provide very high effectiveness.
However, they have relatively low pressure capability. The biggest advantage of the plate and frame heat exchanger, and a situation where it is most often used, is when the heat transfer application calls for the cold side fluid to exit the exchanger at a temperature significantly higher than the hot side fluid exit temperature i.e. “temperature cross”. This would require several shell and tube exchangers in series due to the lack of purely counter-current flow.
The overall heat transfer coefficient of plate heat exchangers under favorable circumstances can be as high as 8,000 W/m2°C. With traditional shell and tube heat exchangers, the U-value will be below 2,500 W/m2°C.
Very good
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