Chilled water cooling systems, also known simply as “chillers”, are a popular choice for commercial air conditioning repair and institutions such as schools and hospitals. A chiller is a compressor based cooling system that is similar to an air conditioner except it cools and controls the temperature of a liquid instead of air. Large commercial buildings that require a substantial amount of cooling often use water chillers because they are cost effective and there is a reduced hazard by not having refrigerant piped all over the building. The chiller will provide a stable temperature, flow and pressure once it has been programmed by a user for their individual needs. Harmful particles are kept out of the system by an internal strainer. Air-cooled chillers, meanwhile, can be located in open spaces like car parks, roofs or ground level areas and have the advantages of a relatively low installation cost and low maintenance. They can also do without a plant room, cooling tower or condenser pumps. The advantages are no heat buildup in a room and no need for ventilation. Noise level is greatly reduced since there is no fan operation.
Both a chiller and a cooling tower are used to remove heat from a liquid, which is used as a coolant in large devices like power stations. A cooling tower removes heat from the water that is discharged from a condenser. The discharged water is then recycled back into the plant to be used to cool the system again, or discharged into the environment. Chillers absorb heat from a coolant, which is fully contained in a cooling system. The chiller then transfers heat to the air around the chiller unit. Though chillers and cooling towers perform similar functions, they vary according to their types and components used, and the nature of the equipment they cool and power. A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream (such as air or process water). As a necessary by product, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. The industrial chiller is a cooling system that removes heat from one element (water) and transfers it into another (ambient air or water). The other main components to a chiller are a temperature controller, a recirculating pump and a reservoir. Operation and setup is simple. Fill the reservoir with fluid to be recirculated, typically water or an ethylene glycol/water mix. Install plumbing between the chiller and the application and provide power to the chiller. The controller regulates the chiller’s functions. A portable chiller is a liquid cooling system on casters that can be relocated from one application to another with relative ease. It can be used to cool one or more heat generating devices. Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional facilities.
Need for Chillers
The most compelling reason for a chiller is the protection it provides to our valuable processing equipment such as spot welders, injection molding equipment and other applications. A chiller commonly represents a small fraction of the cost of the processing equipment, yet it provides solid protection of our investment, 24-hours-a-day, seven-days-a-week for years and years to come.
The speed and accuracy of production will increase as we maintain a constant and proper cooling temperature in the equipment. A chiller will reduce the number of rejected parts while increasing the number of parts produced per hour.
These chillers absorb heat from process water and can be transferred to the surrounding air. Air-cooled chillers are generally used in applications where the additional heat they discharge is not a factor. They require less maintenance than water-cooled units and eliminate the need for a cooling tower and condense water pump. They generally consume approximately 10% more power than a water-cooled unit as a wet surface transfer’s heat better than a dry surface.
These chillers absorb heat from process water and transfer it to a separate water source such as a cooling tower, river, pond, etc. They are generally used for large capacity applications, where the heat generated by an air-cooled chiller creates a problem. They are also considered when a cooling tower is already in place, or where the customer requires optimum efficiency of power consumption. Water-cooled chillers require condenser water treatment to eliminate mineral buildup. Mineral deposits create poor heat transfer situations that reduce the efficiency of the unit. Water chillers can be water-cooled, air-cooled, or evaporative cooled. Water-cooled systems can provide efficiency and environmental impact advantages over air-cooled systems.
One chiller cannot control every heat load. Some chillers are designed to cool to very low temperatures while others are designed for only mid-range applications. Some designs can support very high flow rates of fluid while other may be designed for just a trickle of fluid. The same issues apply with ambient temperatures. Some chillers use refrigerant suited for a high ambient temperature environment while other refrigerants are formulated for cooler conditions. The customer must also consider the fluid being cooled. Distilled water or di-ionized water requires different conditions than tap water. Di-ionized and distilled water can cause the breakdown of metal they come in contact with. In cases like this the chiller is designed with no brass, copper or mild steel components that would come in contact with the water, instead, plastic or stainless steel are used. This eliminates the corrosive effects of the fluid.
- Refrigeration Compressors – are essentially a pump for refrigerant gas. The capacity of the compressor, and hence the chiller cooling capacity is measured in kilowatts input (kW), Horse Power input (HP), or volumetric flow (m3/h, ft3/h). The mechanism for compressing refrigerant gas differs between compressors, and each has its own application. Common refrigeration compressors include Reciprocating, Scroll, Screw, or Centrifugal. These can be powered by electric motors, steam turbines or gas turbines. Compressors can have an integrated motor from a specific manufacturer, or be open drive – allowing the connection to another type of mechanical connection. Compressors can also be either Hermetic (welded closed) or semi-hermetic (bolted together).
- The condenser is a heat exchanger which allows heat to migrate from the refrigerant gas to either water or air. Condensers can be air-cooled, water-cooled, or evaporative. Air cooled condenser are manufactured from copper tubes (for the refrigerant flow) and aluminum fins (for the air flow). Each condenser has a different material cost and they vary in terms of efficiency. With evaporative cooling condensers, their coefficients-of-performance (COPs) are very high; typically 4.0 or more.
- Evaporators can be plate type or shell and tube type. The evaporator is a heat exchanger which allows the heat energy to migrate from the water stream into the refrigerant gas. During the state change of the remaining liquid to gas, the refrigerant can absorb large amounts of heat without changing temperature.
- In recent years, application of Variable Speed Drive (VSD) technology has increased efficiencies of vapor compression chillers. The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll technology compressors.
- The expansion device or refrigerant metering device (RMD) restricts the flow of the liquid refrigerant causing a pressure drop that vaporizes some of the refrigerant; this vaporization absorbs heat from nearby liquid refrigerant. The RMD is located immediately prior to the evaporator so that the cold gas in the evaporator can absorb heat from the water in the evaporator. There is a sensor for the RMD on the evaporator outlet side which allows the RMD to regulate the refrigerant flow based on the chiller design requirement.
Use in air conditioning
In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handling units or other types of terminal devices which cool the air in their respective space(s). The water is then re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. A typical chiller for air conditioning applications is rated between 15 and 2000 tons, and at least one manufacturer can produce chillers capable of up to 5,200 tons of cooling. Chilled water temperatures can range from 35 to 45 F (2 to 7 C), depending upon application requirements. When the chillers for air conditioning systems are not operable or they are in need of repair or replacement, emergency chillers may be used to supply chilled water. Rental chillers are mounted on a trailer so that they can be quickly deployed to the site. Large chilled water hoses are used to connect between rental chillers and air conditioning systems.
Use in industry
In industrial application, chilled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries. They are often used in the plastic industries, injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers, and in hospitals, hotels and campuses. Chillers for industrial applications can be centralized, where a single chiller serves multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both centralized and decentralized chillers, especially if the cooling requirements are the same for some applications or points of use, but not all. Water-cooled chillers are typically intended for indoor installation and operation, and are cooled by a separate condenser water loop and connected to outdoor cooling towers to expel heat to the atmosphere. Air-cooled and evaporative cooled chillers are intended for outdoor installation and operation. Air-cooled machines are directly cooled by ambient air being mechanically circulated directly through the machine’s condenser coil to expel heat to the atmosphere. Evaporative cooled machines are similar, except they implement a mist of water over the condenser coil to aid in condenser cooling, making the machine more efficient than a traditional air-cooled machine. No remote cooling tower is typically required with either of these types of packaged air-cooled or evaporative cooled chillers.
Chillers are a popular choice for commercial air conditioning repair and institutions such as schools and hospitals. If you are in the process of making decisions about your future cooling needs, then chilled water cooling should be one of the systems on your list. Below are a few advantages that chillers offer over other cooling systems and a few disadvantages that you should also keep in mind. The advantages offered by chilled water cooling systems:
Safer for humans: Safety should always be a primary concern when making decisions about the environment in which people live, work and play. Chilled water systems are fundamentally safe due to the use of non-toxic, chemically-stable water as the refrigerant; chillers don’t require that potentially-hazardous refrigerants be circulated throughout a building in close proximity to occupants.
Cost effective: A chilled water cooling system can cut energy costs by up to one-half if it utilizes the latest in high-efficiency equipment. Water is better at absorbing heat than air, and this fundamental fact of physics means that it will always have an advantage in this regard. Not only that, water is plentiful and cheap; eliminating the need to use costly refrigerants can contribute greatly to the overall cost savings.
Sheltered from elements: The operational machinery for chilled water cooling systems, except for cooling towers, is typically installed in a mechanical room, basement or other interior space. This means these complex components, such as evaporators and condensers, are less exposed to the elements than systems that are mounted on rooftops or in exterior locations. Less exposure to rain, ice and heat can extend the lives of these components by several years.
Quiet Operation: Another advantage offered by chillers is they operate at much quieter levels than air cooling systems. The flow of water through the system is less susceptible to the expansion and contraction that causes air to affect mechanical components such as ducts and vents. This degree of quietness is important for building occupants, particularly in sensitive environments such as hospitals and schools where noise would otherwise be unhealthy or distracting.
Cooling Towers: Chillers utilize external cooling towers to transfer heat to the atmosphere, and these structures can be costly to build. They don’t need to be located immediately adjacent to the building that holds the operational machinery, but they do utilize valuable real estate which adds to the cost. Cooling towers are also unsightly, and the water vapor generated during operation can be uncomfortable for those who pass by these structures.
Enhanced Maintenance Needs: Since chilled water cooled systems use water for transferring heat, this exposes the water to a variety of conditions that can create scaling. Scaling is an accumulation of deposits on metal, and this can cause corrosion as well as decrease system efficiency. To control the problems associated with scale, the water used in chillers must be treated to remove impurities that can lead to scaling. In addition, periodic inspection and cleaning of the chiller’s internal machinery and components will be necessary. This necessitates downtime for scheduled maintenance and added maintenance costs.
Less Effective in Humid Environments: Chilled water cooling systems don’t work as well in climates with high prevailing humidity. Higher levels of humidity raise the wet-bulb temperature, which is an indicator of how efficiently water absorbs heat. An increase in wet-bulb temperature corresponds with increased operating costs as well as lower comfort levels due to the higher ambient humidity. Chillers can create a cold, clammy feeling for occupants if the humidity is too high. In this scenario, air cooled systems are much better at extracting moisture from the air.
AUTHORS CREDIT & PHOTOGRAPH
Dr S S Verma
Department of Physics
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