HVAC, Refrigeration and Building Mechanical Services
Core content
The vapour-compression cycle is compression, condensation, expansion, evaporation. From a pressure–enthalpy chart: evaporator duty Qe = m(h1 − h4), condenser duty Qc = m(h2 − h3), compressor work W = m(h2 − h1), COP = Qe/W. Superheat protects the compressor; subcooling improves efficiency. Psychrometrics: dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio and enthalpy. Sensible heat Qs ≈ 1.2 × V̇ × ΔT (kW with m³/s and K), latent heat depends on moisture, and SHR = Qs/(Qs + Ql). Cooling loads have external components (solar, conduction, infiltration) and internal ones (people, lights, equipment). Air-side design: duct velocity limits for noise, and fan laws - flow ∝ speed, pressure ∝ speed², power ∝ speed³.
Traps: confusing dry-bulb with wet-bulb; treating a ton of refrigeration as a mass (it is 3.517 kW); mixing gross and net capacity; applying fan laws linearly; sizing ducts by flow alone rather than velocity and pressure drop; and forgetting that superheat is measured leaving the evaporator while subcooling is measured leaving the condenser.
Worked example. A room at 24 °C needs 12 kW sensible with 1.2 m³/s of supply air. ΔT = 12/(1.2 × 1.2) ≈ 8.3 K, so supply air must be about 15.7 °C - workable. Halve the airflow to 0.6 m³/s and the required ΔT doubles to 16.7 K, an impossibly cold supply that would cause draughts and poor mixing.
Rule of thumb: a 1 K rise in chilled-water temperature typically improves chiller efficiency by roughly 2–3 %.