Air-conditioning systems
Air conditioning makes indoor air comfortable by controlling four things: temperature, humidity, cleanliness and air movement.
Common system types:
- All-air (central duct) system: cooled air travels in ducts to rooms.
- Air-water system: chilled water goes to fan coil units in each room.
- Refrigerant system: split units or VRF carry refrigerant straight to indoor units.
Which type to use depends on the building size, use and cost. The next lesson shows the equipment.
Cooling and heating loads
The cooling load is the heat (in kW) that must be removed to hold the room at the set temperature. It comes from:
- Outside: heat through walls and roof, Q = U x A x ΔT; sunlight through glass.
- Inside: people (about 100 W each), lights and machines.
- Fresh air: hot, humid air brought in for ventilation.
The heating load is the heat that must be added in cold weather to replace what leaks out through walls, windows and ventilation. The size of the equipment is chosen from the biggest load.
Heat is of two kinds: sensible heat changes temperature; latent heat changes the amount of water vapour. Total load = sensible + latent. A big plant is rated in kW or tonnes of refrigeration (1 TR = 3.517 kW).
State of moist air
Air is a mixture of dry air and water vapour. Its state is described by:
- Dry-bulb temperature: the ordinary thermometer reading.
- Relative humidity (RH): RH = vapour pressure / saturation pressure x 100%. Comfort is about 40 to 60%.
- Dew point: the temperature where vapour starts to condense. If a surface is colder than this, drops form on it.
- Wet-bulb temperature: the reading of a thermometer with a wet wick; it is lower when the air is drier.
When air is cooled below its dew point on a cold coil, it is cooled and dried at the same time. All these values are shown on one graph called the psychrometric chart.
Why loads decide the design
An equipment that is too small cannot reach the set temperature. One that is too big switches on and off often, wastes energy and does not dry the air well. So the designer first estimates the loads, then selects the plant and the airflow: Q = density x flow x specific heat x temperature difference.
Key formulas and definitions
- Wall heat gain Q = U x A x deltaT
- Sensible heat of air Q = rho x V x cp x deltaT (rho = 1.2 kg/m3, cp = 1.005 kJ/kg K)
- Total load = sensible + latent
- RH = vapour pressure / saturation pressure x 100%
- 1 TR = 3.517 kW
Worked examples
1. U = 2 W/m2K, wall area 15 m2, temperature difference 10 K. Find the heat through the wall.
Q = 2 x 15 x 10 = 300 W.
2. Six people (100 W each) and five 40 W lamps are in a room. Find the internal heat.
600 + 200 = 800 W.
3. Vapour pressure is 1.6 kPa and saturation pressure is 3.2 kPa. Find the RH.
RH = 1.6 / 3.2 x 100 = 50%.
4. A supply of 0.5 m3/s air is 10 K cooler than the room. Find the sensible cooling.
Q = 1.2 x 0.5 x 1.005 x 10 = 6.03 kW.
5. Sensible load 4.2 kW and latent load 0.8 kW. Find the total in TR.
5.0 kW / 3.517 = 1.42 TR.
Common mistakes
- Thinking AC only changes temperature and ignoring humidity.
- Forgetting people, lights and fresh air when counting loads.
- Saying relative humidity stays the same when air is cooled.
- Choosing a much bigger unit "to be safe": it wastes energy and dries poorly.