HVAC System Fundamentals and Psychrometrics
Core concepts
You must be fluent in the air-side energy balance and the psychrometric chart. Key quantities: dry-bulb (DB), wet-bulb (WB), dew point (DP), relative humidity (RH), humidity ratio (W, lb water/lb dry air), enthalpy (h, Btu/lb dry air), and specific volume (v, ft³/lb). Remember that the chart is plotted per pound of dry air, so total heat equals mass flow of dry air times enthalpy difference.
Sensible heat equation: Qs = 1.08 × CFM × ΔT (sea level, standard air). Latent: QL = 0.68 × CFM × ΔW (grains). Total: Qt = 4.5 × CFM × Δh. The constants 1.08, 0.68, and 4.5 all derive from standard air density (0.075 lb/ft³) and must be corrected for altitude, temperature, and moisture when conditions depart from standard.
Traps
- Using 1.08 at high altitude or high temperature without a density correction.
- Confusing apparatus dew point with entering-air dew point; the coil ADP is where the saturation curve meets the coil line.
- Assuming WB equals DB at saturation only-at saturation they are equal, otherwise WB < DB.
- Mixing up sensible heat ratio (SHR = Qs/Qt) with RH.
- Forgetting that a cooling coil's bypass factor lowers the leaving DB below the ADP-implied value only partially.
Worked example
A coil handles 2,000 CFM of air entering at 80°F DB / 67°F WB and leaving at 58°F DB / 57°F WB. Sensible load = 1.08 × 2,000 × (80 − 58) = 1.08 × 2,000 × 22 = 47,520 Btu/h. If entering W = 78 gr and leaving W = 62 gr, latent = 0.68 × 2,000 × 16 = 21,760 Btu/h. Total ≈ 69,280 Btu/h; SHR ≈ 0.69. Sanity-check with 4.5 × CFM × Δh if enthalpies are read from the chart.
Rule of thumb
Standard-air constants assume 0.075 lb/ft³; correct by multiplying by actual density ÷ 0.075.