Heat Rejection Techniques

In order for a chiller to cool the water used in a cooling system, it must first extract heat from the water and then discharge it to an available cooling medium. This process is the same as the operation of a domestic fridge, which takes heat from the food inside and then rejects it at the back of the unit via the black serpentine coil which warms the surrounding air. In the same way, a commercial chiller must reject the heat it picks up.

Heat rejection can be achieved in several ways. The simplest approach is to combine the heat rejection system and chiller into a single unit called a packaged chiller. This is located outside and incorporates one or more fans which draw fresh air through the unit to carry away the heat. Large chillers often have a separate heat rejection system linked by pipework, enabling the chiller to be located in a plant room.

A heat rejection system can take several forms. The most efficient is the evaporative cooling tower which uses the cooling effect of evaporating water to boost the cooling provided by fresh air. This approach has become less popular during the last 10 to 15 years as a result of the risk of Legionnaires’ disease associated with poor maintenance. However, for some building applications, properly maintained cooling towers remain the favoured method of heat rejection due to their high efficiency (which also enables a small footprint).

A more widely used system for providing separate heat rejection is the dry cooler. This consists of a low profile unit containing one or more fans that drive fresh air across a serpentine coil. The coil contains hot water from the chiller which is cooled and pumped back to the chiller. Alternatively, the coil can contain hot refrigerant directly from the refrigeration process, which is cooled in the same way and then travels back to the chiller.

The main types of heat rejection equipment commonly used in buildings are:

Air-cooled condenser

Fans induce air flow over finned tubing in which refrigerant condenses.

Convenient and common for chillers up to a few 100 kW. Free of hygiene risks and does not require water piping. Can be adapted to provide free cooling with thermosyphon systems.

Dry-air cooler

Similar to an air-cooled condenser but aqueous glycol solution or water is passed through the tubes instead of refrigerant.

Less efficient than an air-cooled condensor because an additional heat transfer process, and pumps, are required to reject heat from refrigeration plant. May cool water sufficiently in winter to avoid the need to operate a refrigeration plant (free cooling). Requires a larger plant area than other options. Adiabatic sprays can be added to improve their performance.

Cooling tower

Water is sprayed over a packing material. Airflow over the packing evaporates some of the water, causing the water to be cooled.

More efficient than air-cooled condenser or dry air cooler because less air is required and water is cooled to a few degrees above the wet bulb temperature. May cool water sufficiently to avoid the need to operate a refrigeration plant, known as free cooling. High maintenance requirement.

Evaporative condenser

Water is sprayed over tubing in which refrigerant condenses. Airflow across the tubing evaporates some of the water, causing the water and the tubes to be cooled.

The most efficient method of rejecting heat from a refrigeration plant. Has similar maintenance requirements as cooling tower. Can be adapted to provide free cooling with thermosyphon systems.