Psychrometric Processes: Sensible Cooling and Sensible Heating of Air

In the domestic and industrial air conditioning applications some psychrometric processes have to be performed on the air to change the psychrometric properties of air so as to obtain certain values of temperature and humidity of air within the enclosed space. Some of the common psychrometric processes carried out on air are: sensible heating and cooling of air, humidification and dehumidification of air, mixing of various streams of air, or there may be combinations of the various processes.

Illustrating and analyzing the psychrometric properties and psychrometric processes by using the psychrometric chart is very easy, convenient and time saving. In the next few paragraphs we shall see some of the most commonly employed psychrometric processes in the field of HVAC and how they are represented on the psychrometric chart.

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.

click to enlarge

Sensible heating process is opposite to sensible cooling process. In sensible heating process the temperature of air is increased without changing its moisture content. During this process the sensible heat, DB and WB temperature of the air increases while latent of air, and the DP point temperature of the air remains constant.

Sensible heating of the air is important when the air conditioner is used as the heat pump to heat the air. In the heat pump the air is heated by passing it over the condenser coil or the heating coil that carry the high temperature refrigerant. In some cases the heating of air is also done to suit different industrial and comfort air-conditioning applications where large air conditioning systems are used.

In general the sensible heating process is carried out by passing the air over the heating coil. This coil may be heated by passing the refrigerant, the hot water, the steam or by electric resistance heating coil. The hot water and steam are used for the industrial applications.

Like the sensible cooling, the sensible heating process is also represented by a straight horizontal line on the psychrometric chart. The line starts from the initial DB temperature of air and ends at the final temperature extending towards the right (see the figure). The sensible heating line is also the constant DP temperature line.

click to enlarge