How Do Chillers Work By Nathan Schumacher

How Do Chillers Work? By Nathan Schumacher

Chillers are a key component of air conditioning systems. They produce cold water to remove heat from

the air in a building.

1

A chiller is an air conditioning system that often uses a centrifugal compressor

with a shell and tube heat exchanger that chills a water solution which is circulated throughout the areas to be conditioned. It is most often used in large commercial and industrial applications with multiple zones that are controlled by balancing the water flow with actuators controlled by thermostats or electronic sensing equipment. As with other types of air conditioning systems, most chillers extract heat from water by mechanically compressing a refrigerant. There are several different types of chillers. The most frequently installed systems in North America use the vapor compression cycle to achieve the cooling of a medium, which is usually water or brine. There is also a refrigerant cycle used in chillers called the absorption cycle when there is steam or natural gas available. Heat is actually used instead of a compressor to cycle the refrigerant or circulate it. This cycle can be efficient when steam is a byproduct or when it would be wasted otherwise. It is normally not efficient if the steam is generated only for the chiller. I will concentrate mainly on the components of water chillers that use the mechanical compression cycle to chill water in buildings such as hospitals and schools. The first important part of a chiller that I will talk about is the evaporator. This is the component that performs the cooling effect. Evaporators are made in a variety of sizes and designs.2 The commonly used evaporators are: shell and tube evaporators and brazed plate evaporators. Shell and tube evaporators are extremely efficient, very compact, and do not require as much space or height. There are two types 1 http://www.betterbricks.com/DetailPage.aspx?Id=539#TypesOfChillers 2 Francis, Ed (1997). "Evaporators". Refrigeration and Air Conditioning Third Edition (Pg. 153). Arlington, VA: Air-Conditioning and Refrigeration Institute.

of shell and tube evaporators commonly used. Both types use a tank or shell. Shell construction can be cast iron, brass, cupronickel, or stainless steel. Tube construction can be copper, cupronickel, or stainless steel.3 The direct expansion type uses tubes which carry refrigerant and are run through the tank in a bundle. There will be a two separate inlets and outlets for both the water and the refrigerant. The water fills the tank and the coils that run through the tank pick up the heat from the water. Another type of shell and tube evaporator is used in larger capacity systems and is called a flooded evaporator. This type fills the shell or tank with refrigerant and circulates the water through the tubes instead of water filling the tank and the refrigerant passing through the tubes. When a shell and tube construction evaporator is used it is essentially pool boiling and the tubes are immersed in a pool of refrigerant which boils when heat is applied or the pressure is reduced by the compressor. The reason they are called flooded is that liquid is heavier and flows into the evaporator by gravity. The heat transfer surfaces are essentially flooded with liquid refrigerant. As the refrigerant boils, the resulting vapor formation rises up and out of the heat exchanger. If operating properly this process occurs at the saturation temperature of the refrigerant for the pressure maintained in the evaporator by the compressor. There should be no superheat. This type of evaporator is normally used in industry where larger amounts of cooling need to be done or several hundred ton units are needed. After the process of the water going through the evaporator you will have chilled water. This chilled water is circulated by a circulation pump to air handling units. The term “air handling unit” is a generalization, used to describe everything from single zone packaged rooftop units and fan coils to full blown VAV air handlers. Technically, anything that “blows air” can be called an air handler.4 There are two different kinds of air handling units generally used in chilled water systems: Fan-coil units and central-station air-handling units. Often these units are used in both heating and cooling. Fan-coil units

3 http://www.coolingtechnology.com/about_process_cooling/industrialchillers/default.html#evap 4 Calabrese, Steven R. (2003) Practical Controls: A Guide to Mechanical Systems (Pg. 161). The Fairmont Press, Inc. Lilburn, GA

use a fan to move the air to be conditioned through coils in the unit to the space being cooled. These units have a filter, a fresh air supply or outside air supply, and a control system. One very interesting application of fan coil units is in small church air conditioning. Units are placed around the perimeter of the sanctuary, heating or cooling the exterior exposure of the building and gently supplying conditioned air to the occupants.5 Central-station air-handling units are similar to fan coil units, but they are used for larger installations. They are called central-station air handling units because they are larger units that supply the air to the building through one central location instead of having several separate fan coils throughout the building. These are used in situations where running water lines throughout the building are impractical. This type will have a very large coil and use duct work to supply and return the air from the space being cooled back to the unit.

Fan coil units and central station air handling units will also

have drain pans which need to be connected to a drain to collect condensation because they remove humidity from the air. The temperature of the air discharged into the space can be controlled in a number of ways. It can be controlled by a thermostat, but in some cases thermostat is not used. In some hotels the coil will be at a constant temperature and the fan speed will be varied by a switch. In this situation the speed of the fan will keep a constant temperature in the room. Other controls can consist of electric, electronic, and pneumatic controls. Control valves are used to throttle chilled water through the coils. Often in cooling it can get complicated to control the temperature of the coil because if the temperature is not low enough the humidity may not be removed. In situations like this more controls are used to control compressor output and coil temperature at the same time, but this type of system requires more controls and is more expensive. A simple temperature control system uses thermostats to control on-off solenoid valves. A better control system uses temperature sensors, controllers, and motorized control valves, and some will have controls for actuators to open and close dampers such as ones to an outside air source.

5 Francis, Ed (1997). “Air-Conditioning Systems ". Refrigeration and Air Conditioning Third Edition (Pg. 649). Arlington, VA: Air-Conditioning and Refrigeration Institute.

After the chilled water picks up heat at the air handling unit the water will be circulated to the evaporator once again to reject heat and become chilled. Now we have to go back to the refrigeration loop to understand what is going on because there are several different fluid loops and it can get confusing. Now that the evaporator has picked up heat in the refrigeration cycle, the refrigerant goes through a compressor. The compressor is responsible for the mechanical vapor cycle. You may find that when you install a chiller it is classified by the compressor or other components like a “water cooled reciprocating chiller”. Manufacturers often name their packaged chillers by the types of components they are using. This is because engineers know what components are going to fit their application. For instance they will look at reliability, economy, load, and maintaining ability which may require a different component. The compressors common to water chillers are the reciprocating, scroll, screw, and centrifugal.6 After the refrigerant leaves the evaporator it is a low pressure, low temperature, and in most cases a superheated vapor. In this process of going through the compressor heat is also added to the refrigerant vapor. The refrigerant leaves the compressor and becomes a high pressure, high temperature, superheated vapor. The refrigerant vapor circulates or is drawn by the compressor to the condenser where heat is removed and as heat is removed the refrigerant vapor condenses.

After leaving the

condenser the refrigerant is completely condensed and a high pressure liquid. As the liquid refrigerant makes its way to a metering device it is still high pressure. It is metered through the device to a low pressure zone and the refrigerant turns to a mix of vapor and liquid refrigerant. As the compressor draws the vaporized gas from the evaporator the metering device lets in more refrigerant and the evaporator chills the water once again. Water cooled condensers are used in large building refrigeration where a water cooling tower is used to cool the condenser water. Water cooling tower water loops are separate from the chilled water and refrigerant loop. There are three basic types of water cooled condensers: Tube in tube, Shell and coil, and shell and tube. In this case the condenser is a heat exchanger which rejects heat to the water cooling 6 Whitman, William/Johnson, William/Tomczyk, John, Refrigeration and Air Conditioning Technology (Pg. 1012) Clifton Park, N.Y.: Delmar Cengage Learning, c2009.

tower loop. In earlier applications of water-cooled condensers to refrigeration and air conditioning, it was common practice to tap the city water supply and then waste the discharge water to a drain connection.7 In Dubuque, Iowa the city hall has a similar system that pumps water from an aquifer well to the condenser and dumps the heat absorbed water to the drainage system. The reason they use this water source is because of the constant low temperature that ground water will have which makes the process more efficient. It is also impractical to use a cooling tower because the lack of space for this type of unit. There are two types of water cooling towers used to cool the condenser water: mechanical draft and atmospheric draft. The atmospheric drafts are less common but, it uses spray nozzles to break up the water so that a natural draft of air comes in contact with smaller particles of water. The mechanical draft uses many different variations and uses a fan to cool the water. After the refrigerant has rejected its heat to the condenser, the refrigerant cycle starts again. These are the components of a typical chiller system used in larger buildings. As I look further into refrigeration it seems that many different designs and variations of water chillers are available. I’m seeing more about the absorption system these days too. As a journeyman I believe it is very important for everyone to learn and understand these systems. It amazes me at how much I have seen these systems and worked on them never to understand how they work. This research has helped me understand the process. I now know what the components are, and what they do. I will continue my education here and I hope that as a union others do to.

7 Francis, Ed (1997). “Refrigeration Principles ". Refrigeration and Air Conditioning Third Edition (Pg. 649). Arlington, VA: Air-Conditioning and Refrigeration Institute.