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Water chiller is an integral part of an HVAC system. It removes heat from the system by cooling and dehumidifying the air. Mainly two types of water chillers are used in an HVAC system: mechanical type and absorption type. The mechanical chiller system consists of a compressor, condenser, evaporator and other control devices. The absorption chiller system doesn’t have a compressor but has a generator and absorber instead of it. We will learn more about mechanical type centrifugal chiller system.
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A brief history of centrifugal water chillers
Willis Carrier patented the first Centrifugal water chiller in 1921. Till that time Chillers with reciprocating compressors were used to move the refrigerant through the system. The main part of a centrifugal chiller is the centrifugal compressor. The design of the first centrifugal compressor was similar to the centrifugal blades in a water pump.
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How the Centrifugal chiller works?
The centrifugal chiller system works in a cycle. The refrigerant passes through evaporator, compressor, condenser, economizer and then back to evaporator. For understanding the working of the centrifugal chiller system, we will consider the refrigerant at the evaporator and about to enter the compressor.
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Refrigerant entering compressor
Refrigerant vapors (gas) from the evaporator enter the center of the centrifugal compressor impeller. The vanes in the centre of the impeller suck the vapors into the radial passages. Now the impeller rotates and thus increases the velocity of the gas. The gas is thrown to the diffuser passage and from there, inside the housing of the compressor where it is stored. The inlet vanes adjust the gas quantity and also the angle at which the gas enters the compressor, thus maintaining the overall stability of the compressor. They also play a major part in adjusting the load of the compressor by distributing the amount of vapors that enter the compressor. The gas then goes through a series of compression stages and discharges into the condenser.
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Refrigerant entering condenser
The hot vapors enter the condenser and are cooled down by the water from the cooling tower that circulates through the condenser. The vapors move through the tubes of the condensers releasing heat to the water. The water is pumped with help of the condenser pump. The water then goes back to the cooling tower where it releases the heat to the air outside. This goes on till the whole gas is converted to hot liquid. The liquid then flows through the liquid line.
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Refrigerant entering economizer
The liquid refrigerant now enters a pressure chamber called economizer. The economizer’s aim is to pre flash the refrigerant. When the refrigerant enters the economizer, the pressure on the refrigerant is reduced. The reduction of pressure results in the reduction of boiling point of the refrigerant. Now as the temperature of the refrigerant is still above the new boiling point it starts to boil off. This boiling off of the refrigerant is called flashing. The liquid vapor that gets converted to gas is called flash gas. When the part of the liquid refrigerant is flashed it removes heat. In this way more heat is removed from the refrigerant.
Pre flashing in the economizer reduces the volume of flash gas that is required to cool down the refrigerant. This reduction in the flash gas means more amount of liquid refrigerant is available for evaporator, which increases the economizer’s efficiency and thus reduces the load on the compressor. The flash gas from the economizer is sent back to the compressor to be compressed. The liquid refrigerant which is at intermediate pressure, i.e. in between compressor pressure and evaporator pressure continues through the liquid line.
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Refrigerant enters evaporator
The condensed liquid refrigerant now enters the evaporator through a system of orifices known as the metering device. The liquid refrigerant when flows through the metering devices reduce in pressure and temperature. This causes the refrigerant to flash, reducing the temperature of the refrigerant to the required evaporator temperature. The liquid vapor mixture now enters the evaporator from where it enters a liquid distributor. The distributor assures of a more uniform heat transfer throughout the entire length of evaporator. This increases the efficiency of the evaporator and of the whole system.
The temperature of the refrigerant liquid vapor mixture is about 40 degree Fahrenheit.
The temperature of the water is about 55 degree Fahrenheit. The water when it is traveling through the tubes of the evaporator is 45 degree Fahrenheit.
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HVAC fundamentals by samuel C sugarman