Air conditioning systems: window, split, cassette, ductable, package and central plants
What this domain tests
Selection and layout of window, split, cassette, ductable, package and central chilled-water systems: components, airflow, psychrometric processes and control arrangements.
Core rules
- Window AC: self-contained, room air cools the condenser, usually fixed speed. Split AC: indoor unit with evaporator, PCB, fan and filter; outdoor unit with compressor and condenser, joined by flare or brazed interconnecting pipes. Cassette: four-way air throw for ceiling grids. Ductable: high static pressure with ducted supply and return and a filter. Package: factory-assembled DX unit for a hall. Central plant: chiller, pumps, AHU/FCU, cooling tower and ducting.
- Typical airflow is about 400 cfm per TR, roughly 0.19 m³/s per TR of total capacity.
- Psychrometric processes: cooling and dehumidification below the dew point, sensible cooling, humidification; the coil has an apparatus dew point and a bypass factor.
- Sensible heat factor = sensible heat / total heat. Airflow and supply-to-room temperature difference must satisfy both sensible and latent loads.
- Duct insulation with a vapour barrier prevents sweating; drain lines slope about 1:100 and need a trap on negative-pressure coils.
Worked example
A hall has 3.5 kW sensible and 1.0 kW latent load, total 4.5 kW (1.28 TR). For a supply-to-room difference of 10 K, mass flow = Qs / (cp x delta T) = 3.5 / (1.02 x 10) = 0.343 kg/s. Volume flow = 0.343 / 1.2 = 0.286 m³/s, about 1030 m³/h or 605 cfm. Check: 605 / 1.28 = 473 cfm per TR, near the normal 400 cfm per TR, so the selection is reasonable.
Traps
Selecting on sensible load only; mixing kW with TR; sizing ducts without calculating static pressure; omitting the drain trap; ignoring a blocked outdoor coil on a window unit; confusing cassette air throw with ducted high static pressure.