A plate-tin heat exchanger is used to condense a satu-rated refrigerant vapor in an air-conditioning system. The vapor has a saturation temperature of 45°C, and a condensation rate of 0.015 kg/s is dictated by system performance requirements. The frontal area of the condenser is fixed at Afr, = 0.25 m2 by installation requirements, and a value of hfg = 135 kJ/kg may be assumed for the refrigerant.
(a) The condenser design is to be based on a nominal air inlet temperature of Tc,i = 30°C and nominal air inlet velocity of V = 2 mis for which the manufacturer of the heat exchanger core indicates an overall coefficient of U = 50 W/m2. K. What is the corresponding value of the heat transfer surface area required to achieve the prescribed condensa¬tion rate? What is the air outlet temperature?
b) From the manufacturer of the heat exchanger core, it is also known that U ∞ V0.7. During daily operation the air inlet temperature is not controllable and may vary from 27 to 38°C. If the heat exchanger area is fixed by the result of part (a), what is the range of air velocities needed to maintain the prescribed condensation rate? Plot the velocity as a function of the air inlet temperature.
