Psychrometrics
The property set
Every psychrometric question hinges on seven linked properties: dry-bulb (DB), wet-bulb (WB), dew point (DP), humidity ratio (W, lb moisture per lb dry air), relative humidity (RH), enthalpy (h), and specific volume (v). On the chart, DB is vertical, W is horizontal, and the saturation curve is the boundary above which nothing exists. RH is the ratio of the actual vapor pressure to the saturation pressure at the same DB - so heating air raises DB and lowers RH with no moisture added.
Coil and space processes
A cooling coil follows a straight line toward the apparatus dew point (ADP); the bypass factor is the fraction of air that leaves at the entering condition. The sensible heat ratio (SHR) defines the slope of the room load line; the coil must meet or be below the room SHR or humidity drifts. A space process is horizontal (no moisture added) when only sensible gains occur.
Traps
Treating WB as constant-enthalpy far from saturation; confusing ADP with the leaving-air dew point; adding sensible and latent loads directly in Btu/h without expressing the total as ṁΔh; forgetting altitude correction (specific volume rises, so cfm per lb of dry air rises); and letting the ADP fall below freezing without accounting for frost.
Worked example
12,000 cfm of standard air is cooled from 78°F DB to 58°F DB. Sensible load: Qs = 1.08 × cfm × ΔT = 1.08 × 12,000 × 20 = 259,200 Btu/h ≈ 21.6 tons. If the humidity ratio also drops 20 grains, latent load = 0.68 × 12,000 × 20 = 163,200 Btu/h, and total ≈ 4.5 × cfm × Δh.