A process stream is cooled from 80C to 40C in a shell-and-tube heat exchanger using cooling water in the tube side that enters the cooler at 30C and leaves at 38C under nominal condition. It has been determined that the process side resistance to heat transfer is dominant. It is desired that the process fluid flowrate through the cooler be decreased to 60% of its nominal value. For this condition, answer the following:a) By how much the cooling water flowrate be changed to maintain the process side exit temperature at 40C?   (b)   What is the exit temperature of the cooling water in this case?

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Asked Sep 24, 2019
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A process stream is cooled from 80C to 40C in a shell-and-tube heat exchanger using cooling water in the tube side that enters the cooler at 30C and leaves at 38C under nominal condition. It has been determined that the process side resistance to heat transfer is dominant. It is desired that the process fluid flowrate through the cooler be decreased to 60% of its nominal value. For this condition, answer the following:

a) By how much the cooling water flowrate be changed to maintain the process side exit temperature at 40C?   

(b)   What is the exit temperature of the cooling water in this case?

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Expert Answer

Step 1

Part (a)

 

For the given shell and tube heat exchanger, the amount of heat released from the hot stream is equal to the amount of heat absorbed by the cold water. For nominal conditions, subscript 1 is used to write the energy balance equation. Thus,

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= mcCpcATc1 . (1)

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Step 2

For same temperature change, if the flowrate of the process stream is decreased to 60% of its nominal value, then the flowrate of the process stream will be mH2 = 0.6 mH1.
Using subscript 2 for this case, write the energy balance equation as:

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mCATmCcATe (2)

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Step 3

Divide equation (1) by (2) and substitute the val...

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mc:9 AP mczA mci 0.6 ma mc2 mc = 0.6m

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