Air Conditioning Fundamentals and Load Estimation
Core concepts
Understand psychrometrics: dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio and enthalpy. The psychrometric chart links them; sensible heat ratio (SHR) = sensible load ÷ total load determines the apparatus dew point and the required supply-air condition. Know comfort ranges (roughly 22–26 °C dry bulb, 40–60 % RH) and why latent load matters in humid climates.
Cooling load has external components (solar gain through glass, conduction through walls and roof, ventilation and infiltration) and internal components (people, lighting, equipment). Distinguish peak load from block load, and design conditions from part-load operation. Understand that load estimation is not the same as equipment selection - oversizing causes short cycling, poor dehumidification and higher energy use.
Traps in questions
- Confusing wet-bulb and dew point: dew point is the temperature at which air reaches saturation on cooling at constant humidity ratio.
- Assuming lower setpoint always improves comfort - it raises energy use and may not fix humidity.
- Ignoring latent load: a sensible-only calculation underestimates capacity in humid conditions.
- Treating ventilation air as free cooling - outdoor air carries both sensible and latent load.
Rule of thumb
A rough sensible load check for light commercial space is 80–120 W/m², but always use a proper calculation. For supply air, a 8–12 K temperature difference between room and supply is typical for comfort systems.
Worked example
Room sensible load 12 kW, latent load 3 kW. Total = 15 kW; SHR = 12/15 = 0.80. A coil must handle 15 kW total and produce an apparatus dew point low enough to remove 3 kW latent - a unit selected only on 12 kW sensible would fail to control humidity.