A chiller is a device that removes heat from a liquid coolant using vapor compression, adsorption refrigeration, or absorption refrigeration cycles. The treated coolant liquid is then circulated through a heat exchanger to cool equipment or other process streams, such as air or process water. The refrigeration process inevitably generates waste heat as a byproduct, which can be discharged to the environment or recovered for heating purposes to improve overall system efficiency.
Vapor compression chillers can use a variety of compressor types, with hermetic scroll, semi-hermetic screw, or centrifugal compressors being the most common today. The condensing side of a chiller can be air-cooled or water-cooled. Even when liquid-cooled, chillers often utilize induction or forced-draft cooling towers to achieve cooling. Furthermore, absorption and adsorption chillers require a heat source to operate properly.
Children's water is primarily used for cooling and dehumidifying air in medium- and large-scale commercial, industrial, and institutional facilities. Water-cooled chillers offer a variety of cooling methods, including liquid cooling (via cooling towers), air cooling, and evaporative cooling. Compared to air-cooled systems, water- or liquid-cooled systems offer significant advantages in terms of increased efficiency and reduced environmental impact.
For air conditioning applications
In air conditioning systems, refrigerant (typically chilled water mixed with glycol) from the chiller of an air conditioner or chiller is distributed to heat exchangers or coils in air handlers or other types of terminal units. These terminal units cool the air within their respective spaces, after which the water is recirculated to the chiller for further cooling. The cooling coils transfer sensible and latent heat from the air to the chilled water, thereby cooling and dehumidifying the airflow.
Typical chiller ratings for air conditioning applications range from 50 kW (170.000 BTU/h) to 7 MW (24 million BTU/h). At least two manufacturers (York International and LG) produce chillers with cooling capacities up to 21 MW (72 million BTU/h). Chilled water temperature (at discharge from the chiller) typically ranges from 1 to 7°C (34 to 45°F), depending on the application requirements. Typically, the chiller receives water at 12°C (entry temperature) and cools it to 7°C (exit temperature). Emergency chillers are used to supply chilled water when the air conditioning system's chiller is inoperable, requires repair, or requires replacement. Rental chillers are mounted on trailers for quick deployment to the site, and large chilled water hoses are used to connect the rental chiller to the air conditioning system.
Industrial Application Overview
In industrial applications, chilled water or other coolant from a chiller is pumped through process or laboratory equipment. Industrial chillers are widely used across various industries for controlled cooling of products, mechanisms, and factory machinery. Applications include the plastics industry (injection and blow molding), metalworking (cutting oils, welding equipment, die casting, and machine tools), chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power and gas turbine power plants (such as turbine inlet air cooling), analytical equipment, semiconductors, and compressed air and gas cooling. They are also used to cool high-temperature, specialized items such as MRI machines and lasers in hospitals, hotels, and campuses.
Types and Advantages of Industrial Chillers
Chillers for industrial applications can be categorized as centralized or decentralized. Centralized chillers, in which a single chiller can meet multiple cooling needs, while decentralized chillers, in which each application or machine is equipped with its own chiller, each approach offers unique advantages. A combination of centralized and decentralized chillers is also possible, particularly when cooling requirements are common to some applications or points of use, but not all.
Characteristics of Different Chiller Types
Chilled water is used to cool and dehumidify air in medium- and large-scale commercial, industrial, and institutional (CII) facilities. Liquid chillers can be liquid-cooled, air-cooled, or evaporatively cooled. Compared to air-cooled chillers, water- or liquid-cooled chillers utilize a cooling tower, resulting in improved thermodynamic efficiency. This is because the air's wet-bulb temperature is at or near thermal resistance, while the dry-bulb temperature is higher (sometimes significantly higher). Evaporatively cooled chillers are more efficient than air-cooled chillers, but less efficient than liquid-cooled chillers.
Liquid-cooled chillers are typically installed and operated indoors, cooled by a separate condenser water circuit connected to an outdoor cooling tower to reject heat to the atmosphere. Air- and evaporatively cooled chillers are suitable for outdoor installation and operation. Air-cooled chillers cool ambient air by circulating it directly through the machine's condenser coils, rejecting the heat to the atmosphere. Evaporative coolers are similar, but apply a water mist to the condenser coils to assist in condenser cooling, making them more efficient than traditional air-cooled chillers. These types of packaged air-cooled or evaporatively cooled chillers typically eliminate the need for remote cooling towers.
Where conditions permit, readily available cold water from nearby bodies of water can be used directly for cooling, replacing, or supplementing cooling towers. For example, the deep water source cooling system in Toronto, Ontario, Canada, utilizes cold lake water to cool chillers, which in turn provide cooling to city buildings through a district cooling system. Return water is used to heat the city's drinking water supply, a significant advantage in cold climates. Whenever the chiller's heat rejection can be used for productive purposes, it achieves very high thermal efficiency in addition to its cooling function.
Operating Principle
The thermodynamic cycle of an absorption chiller is driven by a heat source, typically delivered to the chiller via steam, hot water, or combustion. Compared to electric chillers, it has significantly lower power requirements, with the combined power consumption of the solution and refrigerant pumps rarely exceeding 15 kW. However, its heat input requirements are higher, and its coefficient of performance (COP) typically ranges from 0.5 (single-effect) to 1.0 (double-effect). For the same cooling capacity, absorption chillers require significantly larger cooling towers than vapor compression chillers. However, from an energy efficiency perspective, absorption chillers excel when cheap, low-grade heat or waste heat is readily available. In sunny climates, absorption chillers can also be powered by solar energy.
The single-effect absorption cycle uses water as the refrigerant and lithium bromide as the absorbent. The strong affinity between the two is key to the cycle's operation, and the entire process operates in a near-perfect vacuum. A solution pump collects a 60% dilute lithium bromide solution at the bottom of the absorber shell and preheats it in a shell-and-tube heat exchanger. The preheated dilute solution enters the upper shell, where it flows around tubes carrying steam or hot water, absorbing heat and boiling. The refrigerant vapor then flows upward into the condenser, leaving behind a concentrated lithium bromide solution. This concentrated solution then flows downward to the heat exchanger and is cooled by a weaker solution pumped to the generator. The refrigerant vapor passes through a demister and enters the condenser tube bundle for condensation. The heat is removed by the cooling water within the tubes, and the condensed refrigerant accumulates in a sump at the bottom of the condenser. The refrigerant liquid flows from the condenser down to the evaporator, spraying onto the evaporator tube bundle. Due to the extreme vacuum in the lower shell (6 mmHg, 0.8 kPa absolute pressure), it boils at approximately 39°F (4°C), creating a cooling effect. This vacuum is created by the strong affinity of lithium bromide for water (hygroscopic effect) in the absorber below. As the refrigerant vapor enters the absorber from the evaporator, a strong lithium bromide solution from the generator is sprayed onto the top of the absorber tube bundle, drawing the refrigerant vapor into the solution, creating an extreme vacuum in the evaporator. The heat generated by the refrigerant vapor absorbing the solution is carried away by the cooling water, and the dilute lithium bromide solution collects at the bottom of the lower shell and flows to the solution pump, completing the cooling cycle and starting the process again.
Industrial Chiller Technology
Industrial chillers typically come as complete, packaged, closed-loop systems, including a chiller, condenser, and pump station with a circulating pump, expansion valve, no-flow shutoff, and internal chilled water control. Compressors can be of two types—scroll and screw—depending on budget and the desired performance of the chiller. An internal water tank helps maintain chilled water temperature and prevent temperature spikes. Closed-loop industrial chillers recirculate clean coolant or clean water with conditioning additives at a constant temperature and pressure to improve the stability and repeatability of water-cooled machines and instruments. Water flows from the chiller to the point of use in the application and back again. [citation needed]
If the difference in water temperature between the inlet and outlet is significant, a large external water tank is used to store the chilled water. In this case, the chilled water does not flow directly from the chiller to the application, but rather to an external water tank, which acts as a "temperature buffer." The chilled water tank is much larger than the internal water flowing from the external water tank to the application, and the hot water returning from the application returns to the external water tank rather than to the chiller. [Citation needed]
Less common, open-loop industrial chillers control the temperature of liquid in an open storage tank or sump by constantly circulating it. Liquid is drawn from the tank and pumped back through a cooler. In industrial chillers, water cooling is used instead of air. In this case, the condenser does not cool the hot refrigerant with ambient air, but instead uses water cooled by a cooling tower. Because water-based condensers have a small surface area and lack fans, this development can reduce energy requirements by over 15% and significantly reduce the size of the chiller. Furthermore, the absence of fans significantly reduces noise levels. [Citation needed]
Most industrial chillers use refrigeration as the cooling medium, but some rely on simpler technologies, such as air or water flowing through coils containing the coolant to regulate temperature. Water is the most common coolant in process chillers, but coolant mixtures (primarily water with coolant additives to enhance heat dissipation) are often used.
Industrial Chiller Selection
When searching for an industrial chiller, there are many important specifications to consider. For the chiller itself, factors include total lifecycle cost, power supply, IP rating, cooling capacity, evaporator and condenser capacity and material, evaporator type, ambient temperature, motor fan type, noise level, internal piping material, number and type of compressors, number of refrigeration circuits, refrigerant requirements, fluid discharge temperature, and COP (the ratio of cooling capacity to total chiller energy consumption in kW at RT). For medium to large chillers, this value should be between 3.5 and 7.0. with higher values indicating higher efficiency. Chiller efficiency in the US is often measured in kilowatts per ton of refrigeration (kW/RT). If the chilled water temperature is below -5°C, specialized pumps are required to pump high concentrations of ethylene glycol. Furthermore, specifications such as the process flow, process pressure, pump material, elastomer and mechanical seal material, motor voltage, electrical rating, IP rating, and pump rating are also important. Internal tank size and material, as well as full-load current, are also important.
When selecting an industrial chiller, consider control panel functionality, including local and remote control panels, fault indicators, temperature indicators, and pressure indicators. Other features such as emergency alarms, hot gas bypass, city water switches, and casters are also worth considering. Detachable chillers are suitable for deployment in remote and hot, dusty areas. If the chiller's noise level is acoustically unacceptable, noise control engineers will implement sound attenuators (often a series, sometimes called muffler banks, for larger chillers) to reduce noise. When the difference between inlet and outlet water temperatures is large, large external tanks are used to store chilled water. The chilled water enters the external tank first, acting as a "temperature buffer," with hot water returning from the application to the external tank rather than to the chiller. Less common open-loop industrial chillers control the temperature of liquid in an open storage tank or sump by continuously circulating the liquid. Liquid is drawn from the tank and pumped back through a cooler. In industrial chillers, water cooling can replace air cooling. In this case, the condenser uses water cooled by a cooling tower rather than ambient air to cool the hot refrigerant. Water-based condensers have a smaller surface area and are fanless, reducing energy requirements by over 15%, significantly reducing chiller size, and significantly lowering noise levels. Most industrial chillers use refrigeration as the cooling medium, but some rely on simpler technologies, such as passing air or water through a coolant-filled coil to regulate temperature. Water is the most common coolant in process chillers, but coolant mixtures (primarily water with coolant additives to enhance heat dissipation) are also commonly used.
In summary, chillers, as a key piece of equipment, play an irreplaceable role in both air conditioning systems and industrial applications. From their diverse refrigeration cycle principles, numerous types and characteristics, to their unique operating principles and comprehensive technical systems, to the numerous specifications that must be considered during model selection, each link is closely linked and collectively determines the performance and application scenarios of a chiller. Whether meeting the air cooling and dehumidification needs of large and medium-sized commercial, industrial, and institutional facilities, or achieving precise cooling of various processes and equipment in industrial production, water chillers, with their unique advantages and characteristics, are key factors in ensuring stable system operation and improving production efficiency. With the continuous advancement of technology, we believe that water chillers will demonstrate even more outstanding performance and broader application prospects in more fields.
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