Chapter 11, Problem 11.36P

Consider a coupled shell-in-tube heat exchange deviceconsisting of two identical heat exchangers A and B.Air flows on the shell side of heat exchanger A. entering at ${\text{T}}_{h,i,A}\text{=520 K}$ and ${\stackrel{\dot{}}{m}}_{h,A}=10\text{ kg/s}$ . Ammoniaflows in the shell of heat exchanger B, entering at $T_{c,i,B}=280\text{ K}$ , ${\stackrel{\dot{}}{m}}_{c,B}=5\text{ kg/s}$ . The tube-side flow iscommon to both heat exchangers and consists of water at a flow rate ${\stackrel{\dot{}}{m}}_{c,A}={\stackrel{\dot{}}{m}}_{c,B}$ with two tube passes. The $UA$ product increases with water flow rate for heatexchanger A as expressed by the relation $U{A}_A=a+b{\stackrel{\dot{}}{m}}_{c,A}$ where $a=6000\text{ W/K}$ and $b=100\text{ J/kg}\cdot \text{K}$ . Forheat exchanger $\text{B}$, $U{A}_B=1.2U{A}_A$ .

(a) For ${\stackrel{\dot{}}{m}}_{c,A}={\stackrel{\dot{}}{m}}_{h,B}=1\text{ kg/s}$ , determine the outlet airand ammonia temperatures, as well as the heattransfer rate.
(b) The plant engineer wishes to line-tune the heatexchanger performance by installing a variable-speed pump to allow adjustment of the water flowrate. Plot the outlet air and outlet ammonia temperatures versus the water flow rate over the range $0\text{ kg/s}\le {\stackrel{\dot{}}{m}}_{c,A}={\stackrel{\dot{}}{m}}_{h,B}\le 2\text{ kg/s}$ .