Question

QUESTION 2 2. A shell and tube heat exchanger with two shell passes and four tube passes cools a chemical with a specific heat of 2385 J/kg K at a rate of 1.25 kg/s from 140°C to 80°C, using water at 35°C. The outlet temperature of the water is 85°C. Determine the area of heat exchanger if U= 800 W/m2-K. If the unit is operated with flow rates doubled, determine the heat transfer. Also determine the heat transfer if the chemical flow rate is increased by 20% Attach File Browse My Computer Browse Content Collection

Answer 1

Hi

given

mass flow rate of the chemical = 1.25 kg/s

assuming one second of the operation.

mass =1.25kg

Specific heat (Cp)= 2385 kJ/kg

initial temperature = 140 o C

final temperature = 80 o C

heat absorbed = m*cp*(Tf-Ti)

=1.25*2385*(80-140)

= -178875 J

negative sign shows that the heat is released.

for water

inlet temperature =35 o C

oultet temperature = 85 o C

Let the area be A m2

heat absorbed = UA*(Tf-Ti)

=800*A*(85-35)

=40000A

we can neglect the heat loss in the ideal case thus heat absorbed= heat gained

40000A=178875

A=4.47 m2

If the unit is operated with the double flow rate.

heat transfer will be twice since = m*cp*(Tf-Ti)

if we assume the cp and T to be the same doubling mass flow rate will double heat transfer.

heat transfer = 178875*2= 357750 J

If chemical flow rate is increased by 20 %.

heat transfer = 1.2*178875= 214650 J

please note that here we are taking assumption that the values other than the flow rates are remaining same.

Hope this helps

feel free to comment in the case of any doubt.

Thank you