Building Services Engineering
Building Services Engineering in IESL Part I examines the design and sizing of HVAC, water supply and drainage, lighting, fire protection, vertical transport and electrical services in buildings. Questions are usually numerical and test whether you can select the correct load, flow or illumination relationship.
Core rules and concepts
- Sensible cooling load Qs = ρ·cp·V̇·ΔT; latent load comes from moisture gain. Total coil load = sensible + latent + fan and duct gains.
- Air change rate: V̇ = N × room volume / 3600, giving m³/s when volume is in m³.
- Lumen method: N = E × A / (Φ × UF × MF), where E is illuminance, Φ lamp lumens, UF utilisation factor and MF maintenance factor.
- Water supply pressure = ρgh; pipe sizing respects velocity limits to control noise and head loss.
- Drainage relies on trap seals, venting and gradients typically between 1:60 and 1:120.
- Fire egress depends on travel distance, exit width and occupancy load.
Worked example
A 300 m³ room requires 6 air changes per hour with a supply-to-room temperature difference of 8 K. Take ρ = 1.2 kg/m³ and cp = 1.005 kJ/kg·K. V̇ = 300 × 6 / 3600 = 0.5 m³/s; mass flow = 1.2 × 0.5 = 0.6 kg/s; Qs = 0.6 × 1.005 × 8 ≈ 4.82 kW. If the latent load is 1.5 kW, the total is about 6.3 kW. The reasoning is to convert air changes to volumetric flow, then apply the sensible heat equation.
Traps that cost marks
- Basing air changes on gross volume when the occupied volume is intended.
- Ignoring latent heat, fan heat and duct gains.
- Confusing lumens (lamp output) with lux (illuminance).
- Oversizing pumps or fans by ignoring system resistance and diversity.