In order to select a AC unit we need to complete an assessment of the cooling/heating capacity required, this will provide the system capacity in kilowatts (heat load). In order to provide this capacity an AC unit will need to pump in a certain amount of cool or hot air into the space to maintain a constant temperature. The calculations regarding ‘how much air’ and ‘how cold/cold to make it’ is called psychrometrics

Psychrometrics itself is the study of a mixture of air and fluid vapour. This has a wide variety of implications but in our case we want to look at atmospheric air and water vapour. By knowing the properties of air that form part of a given system we can determine what energy needs to be added or subtracted and where to ensure we can achieve a constant temperature balance i.e. occupant comfort.
Typically divided into 'dry air' (78% Nitrogen, 21% Oxygen, 1% Argon and 0.025% Carbon Dioxide) and 'water vapour'. Because we're at such low pressures, we assume the water to behave like a perfect gas (ideal gas law) Because of this we can effectively ignore the change in this water vapour.

In order to define the state of air we need at least three of the following properties:

• Dry Bulb Temperature (TDB)

• Wet Bulb Temperature (TWB)

• Moisture Content (w)

• Dew Point Temperature (TDP</)

• Percentage Saturation (µ)

• Relative Humidity (RH)

• Specific Enthalpy (h)

These properties are defined below:

Dry Bulb Temperature (TDB)

The temperature of air measure by a thermometer freely exposed to the air but shielded by radiation and moisture. This is the one you are typically quoted when asked ‘what’s the temperature’. It is measured in ^oC

Wet Bulb Temperature (TWB)

Is the lowest temperature air can be cooled by the evaporation of water i.e air at 100% relative humidity. Carrier proposed in that this can be approximated by placing a wet sock over a thermometer and blowing atmospheric air across it. In layman’s terms the wet bulb temperature represents the water content within the air. It is measured in ^oC

Moisture Content (W)

Is the ratio of the mass of water vapour to the mass of dry air in a given volume
Also known as humidity ratio, specific humidity or absolute humidity. It is measured in g/kg DA, where the ‘DA’ stands for Dry Air

Dew Point Temperature (TDP)

Is the temperature at which water will condensate out of the air. If a given sample of air with a certain moisture content is cooled at constant pressure it will eventually reach the saturation temperature T_DP corresponding to this pressure point. At this point drops of dew will start to form out of the air, this is considered our dew point. It is measured in ^oC.

Percentage Saturation (µ)

The ratio of the mass of water held per gram of dry air to the mass of saturated water vapour of dry air at the same temperature and pressure. Also known as percentage humidity or degree of saturation. It is measured as a percentage

Relative Humidity (RH)

The ratio of the density of water vapour at a given temperature to the density of saturated water vapour at the same temperature. It is measured as a percentage

Specific Enthalpy (h)

The total energy in a system due to pressure and temperature per unit of mass. It is measured in kJ/kg

Note that the below equation can be used when dealing with the change in enthalpy from 0 to 30 C.