PROBLEM 1

Outside Air (OA) of 3000 cfm at Boston summer cooling conditions of 90°F dry bulb and 73°F wet bulb is to be conditioned to room neutral 73°F and 45% relative humidity. Using the psychrometric chart, estimate the total energy required for a traditional cooling coil and re-heat coil.

Solution

Locate the Outside Air and Leaving Air points and record the OA specific volume and LA humidity ratio and enthalpy for both points.
      OA Specific Volume = 14.156 cu.ft./lba
      OA Enthalpy = 36.642 Btu/lb
      LA Enthalpy = 26.034 Btu/lb
      LA Humidity Ratio = 54.5 gr/lb

Locate the point (CC) where the LA humidity ratio line intersects the saturation curve and record the Enthalpy.
      CC Enthalpy = 20.548 Btu/lb

The cooling coil energy for dehumidification can be estimated by the following equation:
      Q = (hOA - hCC) x (cfm x 60) / spec.vol.OA
      Q = (36.6 - 20.5) x (3000 x 60) / 14.156
      Q = 204,718 Btu/hr

The heating coil energy for air re-heating to the desired dry bulb temperature can be estimated by the following equation:
      Q = (hLA - hCC) x (cfm x 60) / spec.vol.OA
      Q = (26.0 - 20.5) x (3000 x 60) / 14.156
      Q = 69,935 Btu/hr

Estimated Total Energy
Q = 204,718 Btu/hr + 69,935 Btu/hr

Estimated Total Energy
Q = 274,653 Btu/hr