Chilled Water Set-Chiller: Description
A liquid chiller is the main component of a cooling system that removes heat from water or brine via vapor-compression or absorption refrigeration cycle ( CSET, 2019 ; ETB, 2019 ). The chiller is mainly used in large and medium-sized companies, industries, and even in institutions. The chill liquid is used to cool equipment in plants by centralizing three heat exchange processes: the refrigeration cycle, the chilled water process, and the cooling water cycle (CSET, 2019). The liquid chiller is preferred to air-cooled systems since it is more environmental friendly and energy efficient.
Chiller Operation
Refrigeration loop
The chiller is a comprehensive system that relies on the vapor compression cycle. The vapor compression cycle uses four main components in the chiller: a compressor, a condenser, an expansion valve, and an evaporator ( CSET, 2019 ). The refrigerant enters the compressor whose main function is to increase the temperature and pressure of the refrigerant before it enters the pipes where it is superheated. The refrigerant then goes through the condenser, which is a heat exchanger that rejects heat from the refrigerant to heat a different water flow. This latter water flow is then directed to the cooling tower to be cooled.
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The refrigerant then maintains its pressure through the heat exchanger although it leaves at a lower temperature and flows into the expansion valve. The expansion valve allows it to evaporate due to the reduced pressure. Additionally, this also serves to cool the warmer water from the evaporator, drawing all the heat from the water. The refrigerant loses its pressure when it evaporates and returns to the compressor where it regains the heat before beginning the cycle.
Alternatively, some designs of the chiller seek to increase the efficiency of the machine and so they involve several expansion and compression stages. These models include an economizer, which is a flash chamber between the condenser and the evaporator at intermediate pressure (ASHRAE , 2000 ; CSET, 2019 ). The refrigerant is partially expanded and sent to the economizer instead of controlling the refrigerant completely into the evaporator like the standard models do. The more efficient two phase model ensures that the compressor has reduced workload since the refrigerant from the economizer does not need to be compressed fully (because the vapor from the economizer still has intermediate pressure).
Chilled water cycle
The chilled water is transported via the distribution network to the components that need cooling. It first enters the chiller’s evaporator, which is another variation of a heat exchanger. The type of evaporator may vary according to manufacturer although the most common model would be a spray shell and tube heat exchanger. The tube exchanger contains a fluid-filled outer pressure vessel which holds several tubes that contain a different fluid. The refrigerant is then sprayed over the surfaces of the tubes with the chilled water ( CSET, 2019 ). The energy is passed from the water to the refrigerant through a heat transfer process which chills the water as it moves through the evaporator.
Cooling water cycle
The cooling water continues the cooling process to serve a purpose separate from the chilled water loop. This is because the loop transfers energy from the refrigerant to the atmosphere. Due to the refrigerant’s heat in the condenser, the cooling water has an increase in temperature. The water then has to be sent to the cooling towers where it is sent down a wet deck to lower its temperature ( CSET, 2019 ). The cooling tower blows air through the deck so that some of the water evaporates thus reducing the temperature. The remaining amount flows down the structure into the cold water basin where the amount is replenished to match the initial quantity. The fluid is sent back to the central utility plant where the loop begins.
Inter-relationships
The chilled water system requires expansion tanks to allow for the thermal expansion of water. The type of tank that is selected, either open, or closed (with the air-water interface or closed with a diaphragm) depends on its location ( Daikin, 2019 ). Open tanks are normally placed at the highest point in the system while closed type tanks may be placed anywhere in the system. The chilled water system may also rely on centrifugal pumps to maintain the system dynamic and the necessary flow rate (by increasing redundancy). The pumps may discharge into the chiller and the heat exchangers.
Cooling towers are essential for rejecting the heat collected from the plant and from the compressor. The main types of cooling towers are induced draft and forced draft towers. The two may be differentiated by the position of the propeller fan, since induced have fans at the top while forced have fans on the air inlet ( Daikin, 2019 ). In a cooling tower, water from the condenser is distributed through the tower via nozzles or trays to increase the air-to-water surface contact area. A portion of the water evaporates and provides latent cooling. The cooling towers rely on sensible cooling when the temperatures are lower before feeding back the water to the chiller.
Another service that the chiller requires for proper functioning is piping, to connect the other external components. The chiller requires a closed-loop to be effective and the expansion tank has to be the only place the loop interacts with the atmosphere ( Daikin, 2019 ). This means that the system should have equal static pressure and the only energy needed would be to overcome the friction loss. However, the piping used in the chilled water system needs to be insulated since the water is below the dew point temperature. This would cause condensate to be formed on the pipes and heat loss would be experienced. Insulation minimizes this heat loss and maintains the temperature of the pipe’s outer surface.
Technical: Improving efficiency through maintenance
Due to the value of acquiring a chiller system and the costs that may be incurred if the system’s state detriments, owners have to ensure some maintenance requirements are met ( Senseware, 2019 ). Some of the ongoing (day-to-day) facility management practices are:
Inspect and clean the condenser coils- this is done to ensure the condenser coils are clog-free and that there is air passage despite the effect of heat transfer.
Maintain refrigerant charge- the system’s refrigerant levels determine the chiller’s cooling ability and failure to ensure this may result in great energy inefficiency which may also increase running costs.
Maintain condenser water- the condenser water loops have to maintain regular flow as they were initially designed to (Shelton, & Joyce, 1991). The presence of debris and other material may affect this flow of and subsequently the condenser water loop. This may eventually inhibit the chiller’s operational efficiency.
A newer maintenance strategy is using artificial intelligence (AI) algorithms that may detect problems before they happen. The chiller’s operational data is collected and analyzed to anticipate failures and decide on actions that should be taken to abate these.
The typical water-cooled chiller has a life expectancy 20-30 years, mainly determined by the maintenance carried out on it ( CSE, 2019 ). The chiller’s operating cost is affected by several variables and routine maintenance may greatly affect these costs ( Avery, 2001 ).
The chiller has different maintenance durations according to the components. For example, the chiller’s lubrication system is checked on a weekly bases while the system’s safety and operation controls are checked monthly. Manufacturers suggest that a regular maintenance schedule for different parts should be based on the actual chiller’s requirements and not on suggested guidelines since these could be misleading ( Hermetic, 2019 ).
References
ASHRAE, A. H. (2000). HVAC systems and equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta .
Avery, G. (2001). Improving the efficiency of chilled water plants. ASHRAE Journal , 43 (5), 14.
Berg. (2019). Refrigeration Principles and how a Refrigeration System Works | Berg Chilling Systems. Retrieved 13 November 2019, from https://berg-group.com/engineered-solutions/the-science-behind-refrigeration/
CSE. (2019). Selecting chillers, chilled water systems. Retrieved 13 November 2019, from https://www.csemag.com/articles/selecting-chillers-chilled-water-systems/
CSET. (2019). Chiller | Engaged in Thermodynamics. Retrieved 13 November 2019, from https://cset.mnsu.edu/engagethermo/components_chiller.html
Daikin. (2019). Chiller Application Guide. Retrieved 13 November 2019, from https://www.daikinapplied.com/o365/api/graphapi/GetDocument/Doc100/Daikin_AG_31-003_Chiller_Application_Guide.pdf/
ETB. (2019). Refrigerants - Environment Properties. Retrieved 13 November 2019, from https://web.archive.org/web/20130314143622/http://www.engineeringtoolbox.com/Refrigerants-Environment-Properties-d_1220.html
Hermetic. (2019). Start-Up, Operation and Maintenance Instructions. Retrieved 14 November 2019, from https://www.carrier.nl/~/media/Product_pdfs/23-serie/23XRV/03-Installatie-IOM/23XRV_IOM_09-2006_ENG.ashx
Senseware. (2019). The Ultimate Guide to Chiller Systems. Everything You Need to Know. Retrieved 13 November 2019, from https://blog.senseware.co/2017/11/16/ultimate-guide-chiller-systems
Shelton, S. V., & Joyce, C. T. (1991). Cooling tower optimization for centrifugal chillers. ASHRAE Journal (American Society of Heating, Refrigerating and Air-Conditioning Engineers);(United States) , 33 (6).
Stanford III, H. W. (2016). HVAC water chillers and cooling towers: fundamentals, application, and operation . CRC Press.
