The air is heated by hot water flowing in the same exchanger at a rate of 11200 pounds per hour with a decrease in temp of 10F. At what temp does the air leave the heat exchanger?
Heat capacity of water in English units is 1 Btu/lbm per degree F, and 1 hour = 60 minutes, so in 1 minute, the water gives up (186.67 lbm)(10 F)(1 Btu/lbmF) = 1866.7 Btu. The heat capacity of air at constant pressure, at these conditions, is about 0.24 Btu/(lbm degF), so we just need to figure out how many pounds' mass are in the 5000 ft^3: sorry, I have to fall back on the ideal gas law -- The 5000 ft^3 of air is 142 m^3, and the specific gas constant for air is 287 J/(kg K), so at 10C = 283.15 K, the mass is found from (101325 Pa)(142 m^3) = m (287 J/kg K)(283.15 K) and m = 177 kg = 390 lbm. Hence, the temperature change of the air should be (1866.7 Btu)/[ (0.24 Btu/(lbm degF)) (390 lbm) ] = 19.9 degrees Fahrenheit. Do double-check the calculations, even though the answer seems reasonable.
