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Chiller knowledge

The Nuts and Bolts – How it all works
There are several major components associated with the UVM chilled water system. They include the chillers (the producer of chiller water), the cooling towers (that reject the heat), and the pumps (which “push” the system water through the piping network). The chiller (fig.1) is the heart of the system. It is a component in the air conditioning system that, as its name suggests, provides chilled water used to cool the air inside buildings, creating a more comfortable, healthy and productive environment. The principle behind the operation of a chiller is similar to conventional air conditioning unit; both provide cooling by the process of a refrigerant evaporating. There are significant differences; chillers use water as the primary cooling medium where air conditioners simply blow cool air.
The ability of a chiller to "chill" the water is derived from the properties of the refrigerants. Refrigerants are liquids that are capable of absorbing heat at comparatively low temperatures of evaporation. Since these liquids absorb heat, they produce a "cooling" effect on the surrounding area. There are many types of refrigerants used in systems, including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). As noted above, the chillers installed at UVM utilize a hydrofluorocarbon, HFC-134a, which is a relatively new refrigerant, and has an ozone depletion potential of zero, so it is very environmentally friendly.
Fig.1 – York Steam Turbine Driven Centrifugal Chiller
The University’s central chillers utilize the vapor compression cycle shown in figure 2.
The steam-driven turbine is the mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. In this case it rotates the compressor.
The compressor assembly is the prime mover of the refrigerant. The centrifugal compressor is a non-positive displacement type. It raises the pressure and temperature of the refrigerant by converting kinetic energy into pressure.
Fig.2 – Chilled Water Production process
The evaporator is a 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 a liquid to a gas. Large chillers can have over five miles of tubing in their heat exchangers.
As with the evaporator, the condenser is a heat exchanger. In this case, it removes heat from the refrigerant causing it to condense from a gas to a liquid. The heat raises the water temperature. The condenser water then carries the heat to the cooling tower where the heat is rejected to atmosphere. The same condenser water is then passed through the steam or surface condenser where the required heat is absorbed to condense the exhaust steam exiting the turbine.
Where does the heat go?
A cooling tower is a heat rejection device, which extracts “waste-heat” to the atmosphere. The condenser water loop transports the warm water from the chillers to the tower. The type of heat rejection in a cooling tower is termed "evaporative" in that it allows a small portion of the water being cooled to evaporate into a moving air to provide significant cooling to the rest of that water. The capacity of a cooling tower is typically controlled by means of a modulating the airflow through the tower. Airflow is modulated by a variable speed drive on the fan motors to control airflow through a large range of fan speeds. The type of cooling tower used at the UVM Central Chilled Water Plant is a direct induced draft, cross-flow type (Fig. 3). Fig.3 – Typical Cooling Tower
The condenser water is circulated through the cooling towers at a continuous rate via condenser water pumps. The tower is an enclosed structure that distributes the warm water vertically. The tower air travels horizontally through the fill as the water being cooled moves downward by gravity. The fan that draws the outside air through the tower is located at the top of the tower.