Three tons of refrigeration are used to cool 1490 cfm of sta

Three tons of refrigeration are used to cool 1,490 cfm of standard barometric pressure air at 89 F dry bulb and 65 F wet bulb temperatures. The refrigeration coil surface is 55. At what relative humdity (%) does the air leave the system?

Solution

1490 cfm    = 0.7032 m^3/s of air

Air at 89 F dBT and 65 F wBT   from psychrometric chart

enthalpy   = 29.82 BTU /lb

specific volume = 14 ft^3/lb

three tons of refrigeration = 3 * 12000 BTU /hr   = 36000 BTU/hr

36000 = m * (h1 - h2)    = (1490 * 60 / 14) * (29.82 - h2)

h2 = 24.182 BTU/lb

from psychrometric chart we have

relative humidity = 109 %   at 55 F dbt and h = 24.182 BTU/lb