A heat exchanger is needed to cool a process stream (N2+H2) in an ammonia plant. The stream enters at 431 K, and has a heat capacity of 3.45 J/g oC and needs to be cooled to 402 K. The flow rate of the stream is 20 kg/s. Cooling water is available at 85 oF and has a specified maximum temperature 2 of 120 oF. The overall heat transfer coefficient is approximately 570 W/m2 K.How much heat exchange area is necessary to cool the process stream with the cooling water?
You can do the math but the approach is to find the heat to be removed from the N2 H2 . You know the temp difference, the mass flow rate so find the heat by multiplying 29 oC 20 kg/s and the specific heat. Make sure you have the right conversion factors and units. Once you have found the heat to be removed reverse the process to find the water flow and area of the heat exchanger. This problem is somewhat strange in that the water flow rate will effect the heat transfer coefficient but they assume that it will not. So you can really solve for flow and for area separately. Good luck. Q = (mdot) (c sub p) (delta t) Amount of heat removed(Q) equals the mass flow rate (mdot) times the specific heat (c subp) times the change in temperture (delta t) Use this for the heat removed and flow rate of the water. then use Q = h A delta t or Q equals the heat transfer coefficient times the area times the change in temperature to find the area.
