The Chiller

What is a chiller? It is a machine that removes heat from a liquid via vapor-compression or absorption refrigeration cycle. The liquid can be circulated through a heat exchanger to cool air(or equipment) as required. It is maid out of 4 main components. These components include compressor, evaporator, condenser and a metering system.

 The basic theory behind the works of a chiller is that the water and refrigerant sucks up heat. Then it carries it to the condenser. From there it heads to the atmosphere by means of air evaporation or water cooled heat exchanger. It uses the vapor compression cycle to chill water and reject heat collected from the chilled water and heat from the compressor to a second water loop cooled by a cooling tower.

Lets talk about the components that make up a chiller and the basics of how they work. Firstly, we'll look at the evaporator. The heat exchanger that removes the building heat from the chilled water lowering the water temperature in the process. The heat is used to boil the refrigerant, changing it from liquid to gas. 

 

Next, lets look at the compressor. It is made up of prime mover and centrifugal compressor. The centrifugal compressor is a non-positive displacement type device. It raises the pressure and temperature of the refrigerant by converting kinetic energy to pressure energy.

Now, we shall move on to the condenser. It also works as a heat exchanger. It removes heat from the refrigerant causing it to condense from gas to liquid. The heat raises the water temperature. The condenser water then carries the heat to the cooling water then carries the heat to the cooling tower where the heat is rejected to the atmosphere.  

Finally, we can look at the expansion device. This devices comes into the picture after the refrigerant condenses to a liquid, it passes through a pressure reducing device. This is the main function of this device. It can be as simple as an orifice plate or as complicated as an electronic thermal expansion valve ( in the picture to the right ).

Now that we know how the chiller works, we can look at the Air Conditioning & Refrigeration Institute (ARI) standard conditions :

 i) chilled water temperature at 44° F

ii) chilled water flow rate is at 2.4 gpm/tan

iii) entering condenser water temperature at 85°F

iv) condenser water flow rate at 3.0 gpm/tan

v) 0.0001 evaporator fouling factor and 0.00025 condenser fouling factor.

The temperature change is documented by  this formula :  

        Q = WC ΔTF 

where:

Q = quality of heat exchange (btu/hr or kw)

W = flow rate of fluid ( USgpm or l/s)

C = specific heat of fluid ( btu/lb°F /kJ/kg°C)

Δ TF = temperature change of fluid (°F or °C)

Centrifugal compressor theory :

• Aerodynamic of turbine type
• Move gas by converting kinetic energy to pressure energy
• High flow rates capability and good efficiency characteristics