Air Cooled Water Chillers – How They Work
Air cooled water chillers are vapour compression refrigeration systems. The main components of a vapour compression refrigeration system are the compressor, condenser, expansion valve & evaporator. Vapour compression refrigeration systems cool via a refrigeration cycle. The cycle starts with a cool low pressure mixture of liquid & vapour refrigerant entering the chiller evaporator. Once inside the chiller evaporator it absorbs the heat from the warm fluid the chiller is cooling. This transfer of heat boils the liquid refrigerant in the chillers evaporator and the super-heated vapour is pulled into the chillers compressor.
The chillers compressor then compresses the refrigerant to a high temperature & pressure, high enough to allow the chillers condenser to give up its heat to the cooler ambient air. Within the chillers condenser heat is transferred from the hot refrigerant to the relatively cool ambient air. This reduction in the chillers refrigerant causes it to de-superheat and condense into a liquid, it then further sub-cools before leaving the chiller condenser. The high pressure liquid refrigerant then enters the chiller expansion valve causing a large pressure drop across the chillers refrigerant circuit. The pressure reduction causes a small portion of the refrigerant to boil off, or flash. This would be seen in the chillers site glass. The site glass also indicates if the chiller is short of gas, if this is the case the flashing inside the chillers site glass will increase. The boiled off refrigerant helps cool the remaining refrigerant to the desired temperature before the mixture enters the chiller evaporator to start the cycle again.
Air Cooled Chiller Condensers – How They Work
The condenser is a major component of a water chiller. It is used as a heat exchanger that rejects heat from the chillers refrigerant to air. A Water cooled chiller gives up its heat into relatively cool water from a cooling tower, air-blast cooler or adiabatic cooler. A typical air cooled condenser fitted to an air cooled water chiller will use axial fans (propeller type) to draw outdoor air over a finned tubed heat transfer surface (heat exchanger).
The temperature difference or “delta T” between the hot refrigerant gas that is flowing through the condenser and the cooler outdoor air induces heat transfer. The heat reduction of the chillers refrigerant vapour causes it to condense into liquid. The last part of the chiller condenser is called a sub-cooler. The chiller sub-cooler reduces the liquid gas temperature still further, until it is under its condensing temperature. Air cooled chillers are best suited to chiller hire applications due to the fact chiller hire projects require systems to be delivered and installed quickly with the minimum of fuss.
Chiller Evaporators – How They Work
The chillers evaporator is a heat exchanger that transfers heat from a process or air conditioning water circuit to the chillers cooler liquid refrigerant. Most air cooled chillers in the UK chiller hire market will be fitted with a shell and tube evaporator, a plate heat exchanger evaporator or a coil in tank evaporator.
A shell and tube evaporator is used primarily on chilled water applications. When a chiller is fitted with this type of evaporator the chillers cool liquid refrigerant flows through tubes encased in a shell. The process or air-conditioning circuit water fills the shell flowing around the tubes. As heat is transferred from the water to the chillers refrigerant the gas boils inside the tubes and the resulting vapour is drawn into the chillers compressor.
Hot water will enter the shell at one end, chilled water leaving at the opposite end. A plate heat exchanger evaporator can be used for chilled water or fluid cooling applications. Stainless steel evaporators are especially suited in food and beverage applications
such as batch cooling or portable water systems. A coil in tank evaporator comprises a coil block, usually copper, with aluminium fins to provide an efficient heat transfer surface. The coil block is then submerged in a chilled water tank which acts as a thermal buffer for the refrigeration system. This type of coil is suited to hire chillers used on process cooling applications, typically reactor cooling systems in chemical cooling, petrochemical cooling & pharmaceutical cooling systems.
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