What is Heat Rejection loop ? What is Heat Rejection Loop in HVAC system ? ~ HVACR DICTIONARY

Heat rejection loop, Heat rejection in HVAC system

The fourth loop is the heat rejection loop. In the refrigeration loop, the condenser transfers heat from the recent refrigerant to air, water, or another fluid. In a water cooled condenser, water flows through the tubes while the recent refrigerant vapor enters the shell space surrounding the tubes. Heat is transferred from the refrigerant to the water, warming the water.

Water flowing through the condenser must be colder than the hot refrigerant vapor. A heat exchanger is required to cool the water that returns from the condenser back to the desired temperature before it is pumped back to the condenser. When a water cooled condenser is used, this heat exchanger is typically either a cooling tower or a fluid cooler(known as dry cooler).

Cooling tower

In a cooling system , the nice and cozy water coming back from the condenser is sprayed over the fill inside the tower while a propeller fan draws outdoor air upward through the fill. One common sort of fill consists of several thin, closely spaced layers of plastic or wood. The water spreads over the surface of the fill to extend the contact with the passing air. The movement of air through the fill allows heat to transfer from the water to the air. This causes some of the water to evaporate, a process that cools the remaining water. The remaining cooled water then falls to the tower sump and is returned to the condenser.

A fluid cooler is similar to an aircooled condenser. Water flows through the tubes of a finned tube device and fans draw outdoor air over the surfaces of the tubes and fins. Heat is transferred from the hotter water to the cooler air.

Pump and control valve

The third component of the heat-rejection loop moves the condensing media (water, in the example) around the loop. In the case of a water-cooled condenser, a pump is needed to move the water through the tubes of the condenser, the piping, the cooling system , and any other accessories installed in the heat-rejection loop.

The heat-rejection capacity of this loop are often varied in response to changing heat-rejection requirements. In the case of a water-cooled condenser, this is commonly accomplished by varying the temperature of water delivered to the condenser. Varying the temperature of the entering condenser water could also be accomplished by using variable-speed fans within the cooling system or by cycling the fans on and off.

One method of varying the number of water flowing through the water-cooled condenser is to use a modulating control valve. As the heat-rejection requirement decreases, the modulating control valve directs less water through the condenser. If a three-way valve is employed , the surplus water bypasses the condenser and mixes downstream with the water that flows through the condenser.

Water chiller and cooling tower

The water-cooled condenser on this chiller transfers heat from the refrigerant to the water in the loop. This water passes through a cooling system and warmth is rejected to outdoor air passing through the tower. A pump is employed to circulate water through the condenser, the piping, the cooling tower, and therefore the control valve.

Finally, a modulating, three-way control valve is employed to vary the water flow through the condenser in response to a changing heat-rejection requirement. This valve modulates the water flow through the condenser by diverting a number of the water round the condenser through the bypass pipe, directly back to the cooling system .

Packaged air-cooled chiller

A second example of the heat rejection loop is a packaged, air-cooled chiller. It combines all the components of the refrigeration and heat-rejection loops.

This example air-cooled chiller contains an evaporator, two or more compressors, an air-cooled condenser coil, and expansion devices. Propeller-type condenser fans draw outdoor air across the condenser coil.

In the case of an air-cooled condenser, heat is transferred from the hot refrigerant vapor directly to the outdoor air without the need for a separate condenser-water loop.

As the heat-rejection requirement decreases, the number of air passing through the condenser coil(s) is reduced. This is accomplished by cycling the condenser fans on and off, or by modulating a damper or variable-speed drive on one or more of the fans.

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