Air Conditioning Systems, Air Handling and Refrigeration
This domain covers the plant-to-room chain: how the system type is chosen, how coil and fan are matched, how the refrigeration cycle behaves at part load, and how air is distributed.
System selection
- Central chilled water with fan coils plus dedicated outdoor air suits multi-room buildings; VAV suits large open floors with varying loads; DX split or multi-split suits small zones; radiant panels need separate ventilation because they handle sensible load only and risk condensation.
- Chilled water 7/12 °C is standard; a lower supply (5/10 °C) may be needed for dehumidification but costs chiller efficiency.
- Coil selection needs entering and leaving air states, water temperatures, and a face velocity low enough (2.0–2.5 m/s) to avoid carry-over.
Ductwork and terminals
Equal-friction or velocity-reduction methods; main ducts 6–8 m/s, branches 3–5 m/s, terminal take-offs 2–3 m/s. Every fitting adds to the index run, so the critical path must be identified before sizes are fixed.
Worked example
A plant serves 1200 kW. Chiller COP is 5.6, so compressor power = 1200/5.6 = 214 kW. Chilled water pumps 30 kW, cooling water pumps 25 kW, tower fans 18 kW: total 287 kW. Plant COP = 1200/287 = 4.18, not 5.6. Energy comparisons must use the plant figure; the equipment value flatters the design by about 25% here. If load falls to 40%, a single large chiller may run poorly, so multiple chillers or variable-speed machines are needed to hold IPLV.
Traps
Reporting equipment COP as system efficiency; ignoring fan heat, which appears as a temperature rise across the AHU and must be added to the supply temperature; assuming a coil reaches a leaving moisture content without checking the apparatus dew point; sizing ducts on total pressure without identifying the index run; forgetting condensate drainage and trap height, a leading cause of commissioning failure.