Building Mechanical Systems and HVAC Design
This domain tests whether you can select, size, lay out and document HVAC systems that satisfy the Iranian National Building Code (chiefly Mabhas 14) and accepted practice. Questions are usually quantitative or judgement-based: which system suits an occupancy, what size a duct or pipe should be, and why a design fails at part load.
Selection and zoning
Choose the system from load profile, occupancy schedule, ceiling space, first cost and energy. Water-based systems (fan coils, radiators) suit cellular hotel and office layouts; all-air systems suit large open areas that need humidity, filtration or smoke control; VRF suits part-load-heavy buildings with individual metering, but needs careful refrigerant-volume and dedicated outdoor-air design. Zone by orientation, internal gain and schedule, never by floor alone.
Air-side sizing
At sea level and about 20 °C, sensible cooling is Qs (kW) = 1.21 × V̇ (L/s) × ΔT (°C). Total coil load adds latent heat, so never size a coil from sensible alone. Size ducts by equal friction (roughly 0.8–1.2 Pa/m for low-velocity systems), then check velocity limits for noise: about 4–6 m/s in main ducts, lower in branches and near terminals. Add fan heat and duct gain (typically 3–10% for runs in unconditioned space) to the room load.
Worked example
A hall has 12 kW sensible gain, room 24 °C and supply 14 °C. V̇ = 12 / (1.21 × 10) = 0.99 m³/s ≈ 990 L/s. If fan speed is later raised 10%, fan laws give +10% flow, +21% static pressure and +33% shaft power - the reason over-speeding a fan is never a cheap fix.
Traps
Confusing kW total with kW sensible; applying 1.21 at altitude without correction; ignoring fan and duct heat; sizing ducts on total flow while checking only cooling; forgetting that VRF and fan-coil systems still need a designed outdoor-air system and condensate drainage.