Sensible Cooling of the Air
Cooling of the air is one of the most common psychrometric processes in the air conditioning systems. The basic function of the air-conditioners is to cool the air absorbed from the room or the atmosphere, which is at higher temperatures. The sensible cooling of air is the process in which only the sensible heat of the air is removed so as to reduce its temperature, and there is no change in the moisture content (kg/kg of dry air) of the air. During sensible cooling process the dry bulb (DB) temperature and wet bulb (WB) temperature of the air reduces, while the latent heat of the air, and the dew point (DP) temperature of the air remains constant. There is overall reduction in the enthalpy of the air.
In the ordinary window or the split air conditioner the cooling of air is carried out by passing it over the evaporator coil, also called as the cooling coil. The room air or the atmospheric air passes over this coil carrying the refrigerant at extremely low temperatures, and gets cooled and passes to the space which is to be maintained at the comfort conditions.
In general the sensible cooling process is carried out by passing the air over the coil. In the unitary air conditioners these coils are cooled by the refrigerant passing through them and are called also called evaporator coils. In central air conditioners these coils are cooled by the chilled water, which is chilled by its passage through the evaporator of the large air conditioning system. In certain cases the coil is also cooled by the some gas passing inside it.
The sensible cooling process is represented by a straight horizontal line on the psychrometric chart. The line starts from the initial DB temperature of the air and ends at the final DB temperature of the air extending towards the left side from high temperature to the low temperature (see the figure below). The sensible cooling line is also the constant DP temperature line since the moisture content of the air remains constant. The initial and final points on the psychrometric chart give all the properties of the air.