Sensible Heating

In HVAC systems, air is typically heated by passing it over a heating coil or use of electrical strip heaters. A schematic of a cooling coil is shown below.

The process of sensible heating is represented on the psychrometric chart by straight horizontal lines parallel to the abscissa.

Since the humidity ratio remains unchanged, and so we use a horizontal line on the psychrometric chart to represent this process. Heating will result in lower relative humidity. Typical sensible cooling or heating processes include space heating loads, sensible cooling loads, heating coils, space heating devices, fans that move the air, sensible cooling coils and radiant cooling or heating devices.

Observable features of a sensible heating process

•         The dry bulb temperature increases

•         the relative humidity decreases

•         the enthalpy increases

•         the wet bulb temperature increases

•         the specific volume increases

•         the humidity ratio, vapor pressure and dew point remains constant

Heating and cooling do change the relative humidity. This is because the saturation point changes with dry bulb temperature. As relative humidity equals the absolute humidity divided by the saturation point, if the temperature and the saturation points change but the absolute humidity remains the same, then the relative humidity must change.