An inclined plate in a piece of process equipment is tilted 30? above the horizontal and is 20 cm long and 25 cm wide (in the horizontal direction). The plate is held at 280 K by a stream of liquid flowing past its bottom side; the liquid in turn is cooled by a refrigeration system capable of removing 12 watts from it. If the heat transfer from the plate to the stream exceeds 12 watts, the temperature of both the liquid and the plate will begin to rise. The upper surface of the plate is in contact with gaseous ammonia vapor at 300 K and a varying pressure. An engineer suggests that any rise in the bulk temperature of the liquid will signal that the pressure has exceeded a level of about pcrit = 551 kPa.
a. Explain why the gas's pressure will affect the heat transfer to the coolant.
b. Suppose that the pressure is 255.3 kPa. What is the heat transfer (in watts) from gas to the plate, if the plate temperature is Tw = 280 K? Will the coolant temperature rise? Data for ammonia are given in App. A.
c. Suppose that the pressure rises to 1062 kPa. What is the heat transfer to the plate if the plate is still at Tw = 280 K? Will the coolant temperature rise?