CarbonKevin raises a valid point, so I pulled out an ASHRAE #1 pschrometric chart and did a pair of example calculations.
1) 40 deg F in, 60 deg F out, 0 RH, 100 CFM in. From the chart, 12.6 ft^3 per lb in, 9.6 BTU/lb in, and 14.6 BTU/lb out. The calculation: (14.6  9.6) X 100 CFM / 12.6 ft^3/lb X 60 min/hr = 2380 BTUH.
2) 85 deg F in, 105 deg F out, 100% RH in, 100 CFM in. From the chart, 14.3 ft^3/lb in, 49.4 BTU/lb in, and 54.1 BTU/lb out. The calculation: (54.1  49.4) X 100 CFM / 14.3 ft^3/lb X 60 min/hr = 1970 BTUH.
The simplified approximation is 0.018 BTU/ft^3/deg F X 20 deg F X 100 CFM X 60 min/hr = 2160 BTUH. That's 10% low at the lower temperature, and 10% high at the higher temperature, and very close at normal room temperature. The error from the simplified approximation is less than the typical error from measuring temperature and CFM.
