Heat rejection

Heat rejection is the final and crucial step in the cooling process of HVAC (Heating, Ventilation, and Air Conditioning) systems. Its purpose is to transfer the unwanted heat from a building or space to the outside environment. The following table summarizes various types of heat rejection equipment.

Types of heat rejection. (Source: IBPSA-USA BEMP Training Workshop)

Type	Primary heat transfer method	Description
Cooling Towers	Evaporation and Convection	Transfer heat from water to the outside air through evaporation. Open: exposes water directly to air, allowing heat to dissipate through evaporation Closed-Circuit: Uses a coil to prevent water from directly contacting the air, reducing contamination and water loss (convection-only)
Air-Cooled Condensers	Sensible	Use fans to blow outdoor air over a refrigerant coil, transferring heat from the refrigerant to the air.
Evaporative Condensers	Evaporation and Convection	Similar to air-cooled condensers but incorporates water spray to enhance heat rejection. Water is sprayed over the refrigerant coil, increasing the cooling effect through evaporation.
Dry Coolers	Sensible	Use air to cool a fluid (often glycol or water), transferring heat from the HVAC system to the air without any water evaporation. They operate like large radiators with fans blowing air over finned tubes carrying the fluid.
Geo-Exchange	Ground-source	Use the earth as a heat sink, transferring heat to the ground through a series of underground pipes (ground loops). Heat is either absorbed by the ground or released to it, depending on the system’s requirements.