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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.

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Cooling Towers

All cooling towers operate on the principle of removing heat from water by evaporating a small portion of the water that is recirculated through the unit. The heat that is removed is called the latent heat of vaporization. Each one pound of water that is evaporated removes approximately 1,000 BTU's in the form of latent heat.


COOLING TOWER TERMS AND DEFINITIONS


BTU (British Thermal Unit)
A BTU is the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32° F to 212° F
Cooling Range
The difference in temperature between the hot water entering the tower and the cold water leaving the tower is the cooling range.
Approach
The difference between the temperature of the cold water leaving the tower and the wet- bulb temperature of the air is known as the approach. Establishment of the approach fixes the operating temperature of the tower and is a most important parameter in determining both tower size and cost.
Drift
The water entrained in the air flow and discharged to the atmosphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.
Heat Load
The amount of heat to be removed from the circulating water within the tower. Heat load is equal to water circulation rate (gpm) times the cooling range times 500 and is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.
Ton
An evaporative cooling ton is 15,000 BTU's per hour.
Wet-Bulb Temperature
The lowest temperature that water theoretically can reach by evaporation. Wet-Bulb temperature is an extremely important parameter in tower selection and design and should be measured by a psychrometer
Pumping Head
The pressure required to pump the water from the tower basin, through the entire system and return to the top of the tower.
Makeup
The amount of water required to replace normal losses caused by bleed off, drift, and evaporation.
Bleed Off
The circulating water in the tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit. As a result of evaporation, dissolved solids concentration will continually increase unless reduced by bleed off.

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