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 relatively warm water or fluid that 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 relativelycool 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.

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Fig 1 - A scroll chiller compressor

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.

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Fig 2 - Axial fans on top of a chiller

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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Fig 3 - A chiller shell and tube evaporator

Summary of Chiller System Components

  • Air cooled condenser – a type of condenser where refrigerant flows through the tubes and rejects heat into a flow of ambient air, most chiller hire units are fitted with air cooled condensers.
  • Capillary Tube – A type of expansion device typically fitted on small capacity hire chillers, it comprises a long tube which reduces the pressure of the refrigerant.
  • Centrifugal Fan – A type of fan fitted to an air cooled chiller allowing the fitting of ductwork onto the hot side of an air cooled condenser. The fan is designed to work against a static pressure.
  • Compressor – The main component in a chiller system, the compressor is used to increase the pressure & temperature of the refrigerant vapour.
  • Condenser – The part of a chiller system where the refrigerant vapour is converted to liquid as it rejects heat.
  • Cooling Tower – Used for process cooling applications & water cooled chiller condenser cooling.
  • Distributor – A device used to supply uniform gas supply through a submerged coil in a tank chiller evaporator.
  • Enthalpy – The property of a refrigerant indicating its heat content per Kg of refrigerant.
  • Evaporator – The part of the chiller system where cool liquid refrigerant absorbs heat from the chilled water circuit.
  • Expansion valve – A device used to maintain the pressure difference between the high pressure & low pressure sides of the chiller system.
  • Fill – The heat transfer surface inside a cooling tower.
  • Flash – The process of liquid refrigerant being vaporised by a sudden reduction in pressure when entering the low pressure side of the chiller system.
  • Hot Gas Muffler – A device installed at the discharge side of the chiller compressor to reduce noise and vibration in reciprocating compressors.
  • Liquid Line Filter Drier – A device installed in the liquid line to remove moisture and foreign matter, designed to protect the chillers compressor.
  • Pump Down Cycle – A control sequence used in a chiller system to pump the refrigerant from the low pressure side of the system to the high pressure side.
  • Shell & Tube Evaporator – A type of evaporator where refrigerant flows through the tubes & chilled water fills the surrounding shell
  • Shut Off Valve – Used to isolate one part of the chiller system from the rest
  • Sub-Cooler – The lower portion of the chillers condenser that further cools the saturated liquid refrigerant
  • Suction Header – A section of pipe within the chiller system used to collect the refrigerant vapour when it leaves the tubes of a submerged coil evaporator
  • Suction Line Filter – A devise installed into the chillers suction line to remove foreign matter from the refrigeration system
  • Superheat – The amount of heat added to the chillers refrigerant vapour after it has vaporised within the evaporator.
  • Water Cooled Condenser – A type of chiller condenser that uses water to remove heat from the refrigerant, this is normally a shell & tube type design.

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