HVAC & PSYCHROMETRICS

Moist-air properties depend on dry-bulb T, wet-bulb T_wb, humidity ratio W, relative humidity φ, dew point, and enthalpy h. Sensible heat changes T (q_s = ṁ·cp·ΔT); latent heat changes W (q_L = ṁ·h_fg·ΔW). Cooling coils typically operate "wet" with simultaneous sensible cooling and dehumidification. Evaporative cooling drops dry-bulb toward wet-bulb at constant enthalpy. Mixing two airstreams gives an outlet on the line connecting their state points, weighted by mass flow.

• W = 0.622 · Pw / (P - Pw)
• h = cp·T + W·(2501 + 1.86·T) (kJ/kg)
• q_s = ṁ_a · cp · ΔT
• q_L = ṁ_a · h_fg · ΔW
• SHR = q_s / (q_s + q_L)

References & StandardsASHRAE Fundamentals Ch. 1, ARI 410, AHRI 410

Use for HVAC system load calculations, cooling-coil sizing, humidifier/dehumidifier design, energy-recovery wheels, and outdoor-air mixing. Compute sensible heat ratio SHR = q_s/q_total to pick coil bypass factor. For arid climates, evaporative cooling delivers significant capacity at a fraction of mechanical-cooling power.