HVAC and Building Mechanical Systems Design
HVAC questions ask you to size air and water systems from a load, and to justify the choice with psychrometrics. In the Iranian context the governing documents are Mabhas 14 (mechanical installations) and Mabhas 19 (energy efficiency), with ASHRAE methods used for load and duct calculation.
Core rules
- Sensible airflow: Q_s (W) = 1.21 × V̇ (L/s) × ΔT (K), or 1.08 × cfm × ΔT (°F) in imperial units.
- Latent load: Q_l (W) ≈ 3010 × V̇ (L/s) × Δw (kg/kg). Total = sensible + latent; SHR = Q_s/Q_total.
- Room condition lies on a straight line between the supply-air condition and the room; the supply point is set by the ADP and the coil bypass factor.
- Duct sizing: equal-friction method keeps roughly 0.8–1.2 Pa/m; velocity method respects noise limits (about 5–7 m/s in main ducts).
- Chilled water: Q (kW) = 4.19 × ṁ (kg/s) × ΔT (K); typical ΔT is 5–7 K.
Worked example
A room has 5 kW sensible load and a design room-to-supply temperature difference of 10 K. V̇ = Q_s/(1.21 × ΔT) = 5000/(1.21 × 10) = 413 L/s ≈ 0.41 m³/s. If the latent load is 1.2 kW, SHR = 5/(6.2) = 0.81, telling you the coil must handle meaningful dehumidification and that the ADP is well below the room dew point.
Traps
- Applying the 1.21 factor to total load rather than sensible load only.
- Ignoring altitude correction on air density, which shifts the constant by several percent.
- Sizing fresh air from comfort alone and forgetting the minimum outdoor-air requirement per person.