This article contains a detailed look at water chillers.
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A water chiller, often referred to as a chilled water system, is a refrigeration system where water acts as a secondary refrigerant. This kind of system is frequently utilized in large and intricate heating, ventilation, air conditioning, and refrigeration (HVACR) setups. Water chillers are typically used in various applications, including:
Unlike water chillers, direct-expansion (DX) refrigeration systems directly cool the air without using a secondary refrigerant. In these systems, air passes over the evaporator, where it is cooled. DX systems are generally more appropriate for smaller-scale uses, such as residential cooling systems and small refrigeration units or freezers.
There are two main loops or circuits that make up a water chiller system. These are the refrigeration loop and the chilled water loop. The refrigeration loop is the sub-system that provides cooling. This is where the thermodynamic processes occur. On the other hand, the chilled water loop is a distribution system where cold water is supplied to consumer units. The processes involved in this system are mainly heat transfer.
The refrigeration loop operates based on the vapor compression refrigeration cycle. This cycle alternates the phase of a refrigerant between liquid and gas using heat exchangers. Additionally, compressors and expansion valves are employed to pressurize and depressurize the refrigerant. The steps of a typical vapor compression cycle are detailed below.
At the start of this phase in the cycle, the refrigerant is in a low-pressure vapor state and at the same temperature as the surrounding air. It carries the heat absorbed from the evaporator.
The compressor pressurizes the vaporized refrigerant, which is then discharged to the high-pressure side of the system. At this stage, the refrigerant's temperature rises above that of the ambient air or environment.
Increasing the refrigerant's pressure requires mechanical energy. The compressor uses shaft power from a motor to elevate the pressure of the vaporized refrigerant. This step involves the system’s power input.
The condenser is the high-pressure component of the refrigeration unit. It functions as a heat exchanger, transferring heat from the refrigerant to the environment. Due to the thermal gradient between the hotter refrigerant and the cooler environment, heat transfer takes place. The condenser discharges both the heat absorbed from the evaporator and the heat generated by the compressor to the surroundings.
The environment serves as a heat sink that absorbs the heat rejected by the system. This heat sink can be outdoor air, as in air-cooled chillers, or water, as in water-cooled chillers.
As the refrigerant cools in the condenser, it condenses back into a liquid state. This occurs because, at this pressure, the saturation temperature of the refrigerant is equal to or slightly higher than its current temperature. The saturation temperature is the point at which the refrigerant starts to vaporize or condense if its temperature changes. It is a fundamental property of the refrigerant.
At the start, the liquefied refrigerant is under high pressure and has the same temperature as the surrounding environment. During the expansion phase, the refrigerant's pressure is reduced, which also causes its temperature to drop. As the pressure decreases, some of the refrigerant evaporates, further cooling the remaining liquid. This expansion process ideally occurs without additional heat or energy transfer. Following expansion, the refrigerant moves into the low-pressure side of the system.
The expansion process is facilitated by an expansion device, which reduces the refrigerant's pressure. Common types of expansion devices used in vapor compression systems include thermal expansion valves and capillary tubes.
The evaporator is the low-pressure side of the system, where heat exchange between the refrigerant and the cooling medium takes place. In water chillers, the cooling medium is typically water or brine.
During the evaporation process, the refrigerant absorbs heat from the water, raising its temperature until it vaporizes. Once evaporated, the refrigerant is in a low-pressure state and at a temperature equal to the ambient. It then flows to the compressor, and the cycle begins anew.
Water chillers with vapor compression systems are utilized by more than 90% of HVAC applications. The less popular refrigeration cycle is the absorption refrigeration cycle. Consequently, water chillers cooled by an absorption refrigeration cycle are called absorption chillers.
Absorption chillers are employed in industrial settings where a substantial supply of steam or waste heat is available. In absorption cooling, the system relies on a set of specialized heat exchangers and other components instead of a compressor. This method significantly reduces the large amount of power typically required by a compressor.
The other heat exchange circuit is the chilled water loop, where water serves as the primary working fluid. In this system, water acts as the secondary refrigerant for water chillers. It is widely used in industrial plants that require a large and continuous supply of cooling water.
For applications needing much lower temperatures, an antifreeze agent is used to lower the freezing point of the secondary refrigerant, preventing ice formation within the system. Historically, salt was used as the traditional antifreeze, which is why the term "brine" is commonly associated with this type of refrigerant. Nowadays, ethylene glycol and propylene glycol are more commonly used as antifreeze agents.
Liquid chiller is the specific term for water chillers that use a secondary refrigerant composed of water and antifreeze components.
The chilled water loop involves two heat exchangers: the evaporator and the cooling coil. The evaporator, as described in the vapor compression cycle, features one side with the primary refrigerant and the other side with the secondary refrigerant or water.
The cooling coil is the heat exchanger that transfers heat from air or other process fluids to the chilled water. Water entering the coil is termed the chilled water supply, while water leaving the coil is called the chilled water return. In most chiller units, the heat exchange process creates a temperature difference of approximately 8 to 16°F (4 to 9°C). This temperature difference, known as the cooling range, helps estimate the required chilled water flow rate based on the cooling load and the temperature range.
The previous chapter covered the two refrigeration cycles used in water chillers: the vapor compression cycle and the absorption cycle. These cycles are one way to classify water chillers. Additional classifications are based on the type of condenser, compressor, and drive unit used in the system.
Water chillers can be categorized as air-cooled or water-cooled, depending on how they reject heat into the environment.
Air-cooled water chillers have condensers designed to transfer heat from the refrigerant to the ambient air, using air as the cooling medium.
An air-cooled condenser unit typically features finned coils to increase the surface area in contact with the air. One or more fans blow air over these finned coils to enhance heat transfer. The efficiency of an air-cooled condenser depends on the airflow rate over the coils and the dry-bulb temperature of the air.
The main advantages of air-cooled water chillers are their simplicity and cost-effectiveness. They can be installed as standalone units without the need for additional infrastructure such as cooling water supply lines or cooling towers.
Water-cooled water chillers use water as the condensing medium. As these chillers also utilize water for cooling, the system features two separate water loops.
Water-cooled condensers typically work in conjunction with a cooling tower. Unlike conventional heat exchangers that rely on conduction and convection, cooling towers generate cooling by exposing water to air. They provide the condenser unit with cooling water, which is then used to cool the refrigerant.
Water-cooled chillers are ideal for large industrial plants where a reliable supply of cooling water is available. They offer much higher cooling efficiency for the condenser compared to air-cooled chillers.
Water chiller compressors come in various types, including centrifugal, screw, scroll, and reciprocating. Each type has its own set of advantages and disadvantages. The choice of compressor typically depends on the required cooling capacity.
A centrifugal water chiller employs a centrifugal-type compressor, which raises the pressure of the gas by boosting its kinetic energy. The fluid's kinetic energy is then converted into potential energy as static pressure by slowing it down. This operating principle classifies centrifugal compressors as dynamic-type compressors.
Due to their high capacity, centrifugal compressors are ideal for applications with large cooling loads. They also offer higher operating efficiencies, or coefficient of performance (COP), at peak loads compared to other compressor types.
Often referred to as helical-rotary water chillers, this type utilizes a screw compressor to drive the vapor compression cycle. The screw compressor, a positive-displacement rotary compressor, typically features two interlocking helical screws. As the refrigerant is trapped in the cavities between these screws, the volume of the cavities decreases, which raises the pressure of the refrigerant.
Screw water chillers are suited for small to medium-sized applications due to their efficiency at partial loads. They are ideal for systems with varying cooling demands because they maintain high efficiency across different load conditions. Additionally, screw chillers do not experience surges at low loads, unlike centrifugal and reciprocating compressors.
Scroll water chillers use a positive-displacement, rotary compressor with two interleaved spirals, or scrolls. One scroll acts as the rotor while the other serves as the stator. Instead of rotating, the rotor moves eccentrically relative to the stator. As the refrigerant is trapped between the scrolls, it is compressed and transported towards the center, where the volume decreases progressively.
Scroll chillers are generally employed for small to moderate cooling loads. To boost their capacity, multiple scroll compressors can be integrated into a single chiller package. They offer a coefficient of performance (COP) comparable to screw compressors. For applications with fluctuating cooling demands, scroll chillers can utilize various refrigerant control methods, including speed control and variable displacement control, to enhance efficiency.
Reciprocating water chillers use a piston or plunger to draw in and compress the refrigerant, making them a type of positive-displacement compressor. This mechanism allows for efficient compression of the refrigerant, but it also introduces some challenges in terms of noise and maintenance.
Reciprocating water chillers are becoming less common due to the limitations of their compressors. Reciprocating compressors are known for their noisy operation, lower reliability, and shorter service life. However, they are relatively affordable, which can be their main advantage in certain applications.
The design of water chiller systems is influenced by several factors. For equipment and process units with pre-defined chilled water parameters, the process is relatively straightforward as the cooling capacity is already established. However, for HVAC applications, the design process is more intricate, requiring detailed calculations of cooling loads and air parameters. Additionally, other crucial design aspects, such as controls, configuration, and piping, must be carefully considered.
Below are some of the key characteristics to specify when designing water chiller systems:
The cooling load refers to the rate at which energy or heat must be removed from a space to maintain a desired temperature and humidity level. It is commonly measured in tons of refrigeration (TR or TOR) or in BTU per hour. One ton of refrigeration is equivalent to 12,000 BTU/hr or approximately 3.5 kW.
In air-conditioning and ventilation applications, the cooling load is affected by various factors, including solar radiation, heat transfer through the building envelope, infiltration of outdoor air, and internal heat generated by occupants, equipment, lighting, and other sources. Several methods can be used to calculate the cooling load, such as the transfer function method (TFM), cooling load temperature differential (CLTD), heat balance method, and time-averaging (TA) method. These methods are detailed in ASHRAE Handbooks and standards from international organizations like ISO and EN.
For refrigeration equipment and process cooling applications, the cooling load is determined by the specific needs of the downstream system. While heat generation methods can vary across industries, the calculations for heat load are generally more straightforward than those for air-conditioning and ventilation systems. Manufacturers typically provide equipment cooling specifications along with other design parameters such as the chilled water flow rate and temperature.
Once the cooling load is calculated, the cooling capacity of the chiller unit can be established. Cooling capacity refers to the rate at which a chiller can provide cooling, and it is typically set slightly higher than the calculated cooling load to ensure adequate performance.
Determining the chilled water temperature and flow rate begins with defining the cooling coil specifications. In HVAC systems, the cooling coil facilitates heat exchange between the chilled water and the returning air. The chilled water supply temperature and flow rate are influenced by air parameters, which are assessed alongside the cooling load. Standards from organizations such as ASHRAE and psychrometric calculations guide the determination of these air parameters.
In refrigeration equipment and process cooling applications, cooling coils are often implemented as cooling jackets and coils within the system. Unlike HVAC systems, psychrometric calculations are typically not required. Instead, heat exchanger calculations are used to determine the chilled water supply temperature and flow rate. Other methods may also be applied depending on the specific application.
Along with determining the cooling capacity, it is crucial to assess the frequency and duration of peak loads. In many applications, conditions can vary, leading the chiller unit to operate frequently at partial loads. Therefore, it's important to consider methods for adjusting the cooling capacity to accommodate these variations.
Different types of water chillers use various methods for capacity control. For instance, scroll water chillers manage capacity through motor speed adjustments using variable frequency drives (VFDs) or inverters, or by varying displacement with solenoids that open or close compression chambers.
In contrast, centrifugal and screw water chillers control capacity by regulating the refrigerant flow into the compressor. This is achieved using inlet guide valves or inlet valves to adjust the flow. Additionally, centrifugal compressors can also utilize VFDs for capacity control.
For large-scale applications, deploying multiple water chillers can be more advantageous than using a single, large chiller. This approach offers several benefits.
Higher Operating Flexibility: Chillers often operate at partial loads. By using multiple chillers, you can shut down one unit to reduce capacity while allowing the others to operate at their full capacity. This setup helps maintain the system's optimum efficiency.
Reliability: A single chiller failure can result in the complete shutdown of the cooling system, leaving the entire facility without cooling. With multiple chillers, some cooling capacity remains even if one unit fails, and downtime can be minimized by having a spare chiller on hand.
The compressor driver provides mechanical power to the refrigeration unit’s compressor. It comes in two main types: electric-driven and engine-driven.
Electric-driven chillers utilize an electric motor to supply power to the compressor. These are the most common type, particularly in HVAC applications. Electric-driven water chillers can be further categorized based on their construction.
An open-type chiller features a separate motor and compressor connected by a coupling. The key advantage of this design is ease of repair; the motor can be accessed and serviced without disassembling other components. Additionally, in the event of motor failure, there is no risk of contaminating the refrigerant.
The downside of open-type chillers is the risk of refrigerant leakage. To prevent this, shaft seals are required, which can complicate the assembly. Despite this, open-type compressors are typically used for large industrial chillers due to their ease of repair and maintenance.
In this type, the motor and compressor are housed together in a sealed, welded shell. The refrigerant flowing into the compressor also cools the motor.
Hermetic sealing addresses the issue of refrigerant leakage. However, if the motor fails, it can contaminate the refrigerant, and repairing it is more challenging. As a result, hermetic chillers are typically used in small to medium-scale applications.
Semi-hermetic water chillers are similar to hermetic types but feature a different construction for the compressor shell. Instead of being permanently welded, the shell is bolted, which allows for some level of serviceability.
Engine-driven water chillers use gas or diesel engines to power the compressor instead of electric motors. They are often employed as stand-by units to enhance reliability. In the event of a power outage, these chillers can provide cooling to critical processes and equipment, ensuring continued operation.
In addition to being independent of the plant's power supply, engine-driven chillers can operate at variable speeds, unlike conventional electric motors that run at a fixed speed. Achieving variable speeds with electric motors typically requires more costly VFD (Variable Frequency Drive) systems.
Another crucial aspect of designing chilled water systems is the pump and piping system. This system is responsible for distributing chilled water to various consumers and process equipment. Proper design is essential for maintaining the correct water flow rate and ensuring that the cooling capacity is adequate for the intended applications.
The design process typically involves calculating the pump brake horsepower, which is determined by the chilled water flow rate and the total pump head. The chilled water flow rate was previously discussed. The total pump head is calculated by considering both the elevation changes and the frictional losses in the piping.
Additionally, the choice of piping material is important. Water often contains impurities such as salts and microorganisms that can cause scaling, fouling, and accelerate corrosion. Selecting the appropriate piping material helps maintain equipment reliability while managing costs. Common materials for distribution piping include carbon steel, copper, and PVC, while stainless steel and copper are typically used for the internal piping of chiller units.
Industrial water chillers are crucial for manufacturing operations, providing the precise temperatures required for various production processes. They are essential in applications where low temperatures must be maintained consistently over long periods to ensure the accurate performance of equipment. These dynamic cooling systems effectively remove heat, ensuring stable temperature, pressure, and airflow for refrigeration systems.
Industrial water chillers work by circulating a cooling fluid to equipment that requires cooling to complete production processes. Unlike simple fan systems, industrial water chillers are necessary for large-scale cooling applications that demand high efficiency. Their superior performance and reliability make them the preferred choice for meeting the specific cooling needs of complex production environments.
Industrial production processes generate significant heat from sources such as friction, equipment operation, heating components, ovens, and various heat treatments. To protect workers, equipment, and ensure a safe work environment, industrial chillers efficiently redirect heat away from these elements. Unlike HVAC systems, industrial chillers use a network of pumps to circulate cooled fluid from the chiller to multiple processes, effectively removing heat. The warmed fluid is then returned to the chiller, where heat is expelled and the fluid is cooled for reuse in another cycle.
Industrial chillers share similar components with smaller chillers but are designed to be more robust and dynamic to handle the demands of cooling large equipment and continuous operation. Like other chillers, industrial chillers are categorized based on their condensers, which can be either air-cooled or water-cooled.
Industrial water chillers are crucial in industrial processes for maintaining precise temperature control and providing cost-effective engineering solutions. They can support numerous pieces of equipment, often 100 or more, and are designed to accommodate future expansion and industry growth.
Given the friction, stress, and continuous operation of industrial mechanisms and tools, a reliable cooling system is essential. Industrial chillers are engineered to withstand harsh conditions and demanding environments. Designers and manufacturers of industrial chillers understand the challenges of industrial production and create systems that not only meet but exceed these rigorous requirements.
Despite their many advantages, selecting an industrial water chiller involves careful consideration. These high-capacity units are complex and must be tailored to specific application needs. Choosing the right chiller requires a detailed understanding of the application or range of applications it will support.
Water chiller manufacturers collaborate closely with clients to design, engineer, and build chillers that meet precise industry and temperature requirements. During the initial phase, experts visit client facilities to assess working conditions and determine the necessary temperature levels. This collaborative approach ensures that the chiller is well-suited to the specific conditions and requirements.
In the early stages of the selection process, manufacturers aim to choose a chilling system that aligns with both the environmental needs and the client’s budget. This partnership ensures that the chosen chiller is appropriate for the situation. For special temperature requirements or unique environments, manufacturers are prepared to engineer custom solutions. Additionally, water chiller manufacturers ensure that their equipment complies with the strict standards set by the Environmental Protection Agency (EPA).
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