Psychrometric Load — Sensible & Latent
HVAC cooling/heating load from air flow and process state changes. Sensible heat (dry-bulb temperature change), latent heat (humidity change), and total/SHR. Per ASHRAE Fundamentals ch. 1 simplified-air properties.
Defaults: 2000 cfm cooling — 78°F at 65 gr/lb (~50% RH at 78°F) entering, 55°F at 55 gr/lb leaving — typical commercial cooling coil. Latent component large in humid summer.
The 1.08 and 0.68 constants
1.08 = 60 min/hr × ρair × cp,air = 60 × 0.075 lb/ft³ × 0.24 Btu/(lb·°F) = 1.08 Btu·min / (cfm·°F·hr). It assumes standard air at 70°F and 14.7 psi (sea level).
0.68 = 60 × 0.075 × (1061 + 0.444×T) Btu/lb_water × (1 lb / 7000 gr) ≈ 0.68 (at typical room T). The 1061 Btu/lb is the latent heat of vaporization at 0°F reference.
Both constants are altitude-sensitive. At 5000 ft elevation, ρ_air drops 17%, so the constants drop to 0.90 and 0.56. Specify altitude correction for design at any elevation > 1000 ft.
SHR — what cooling coil shape to expect
SHR (sensible heat ratio) shapes the cooling coil. SHR ≈ 1.0 means dry cooling — the coil temperature stays above dew point. SHR ≈ 0.75 (typical commercial summer) means significant moisture removal, condensate drainage required. SHR < 0.6 (humid climate, dehumidification) requires special coil arrangement (pre-cool, post-reheat) to hit the leaving condition.
Sensible vs. latent — they aren't independent
You can't independently change sensible and latent load on a single cooling coil. The cooling process moves along a tangent to the saturation curve from entering condition to apparatus dew point (ADP). The "bypass factor" determines how close the leaving condition gets to ADP. For high-latent loads, you need either lower coil temperature (sub-cooling) or a separate dehumidification path (DOAS, desiccant).
Heating loads are usually all sensible
For heating-only HVAC (forced-air furnace, electric resistance, hot-water coil), the latent load is zero — humidity ratio doesn't change. Q_t = Q_s. Heating loads are simpler than cooling because there's no moisture removal/addition.
Tons-to-cfm rule of thumb
At typical comfort cooling conditions (78°F → 55°F, 50% RH inlet), 1 ton ≈ 400 cfm of supply air. This is a sanity-check constant: if your cfm/ton is far from 400 (say 250 or 600), recheck your supply temperature and humidity assumptions. Very dry climates (10% RH) or extreme humidity (humid coastal) can shift this 30%.
Reference: ASHRAE Handbook Fundamentals (2021), ch. 1 (Psychrometrics). McQuiston, F.C., et al. (2005). Heating, Ventilating, and Air Conditioning, 6th ed., Wiley.