air cooled and water cooled chiller

Working Principles​
Air Cooled Chillers​
Refrigeration Cycle Basics​
Air cooled chillers operate based on the vapor – compression refrigeration cycle. The cycle starts with the compressor. The compressor takes in low – pressure, low – temperature refrigerant vapor from the evaporator. It then compresses the refrigerant, increasing its pressure and temperature.​

After compression, the hot, high – pressure refrigerant vapor flows to the condenser. In an air cooled chiller, the condenser is cooled by ambient air. The air is forced over the condenser coils by fans. As the hot refrigerant vapor comes into contact with the cooler air, heat is transferred from the refrigerant to the air. This causes the refrigerant to condense into a high – pressure liquid.​
The high – pressure liquid refrigerant then passes through an expansion device, such as a thermostatic expansion valve or a capillary tube. The expansion device reduces the pressure of the refrigerant, causing it to expand and cool down.​
The low – pressure, low – temperature refrigerant then enters the evaporator. In the evaporator, the refrigerant absorbs heat from the process fluid (usually water) that needs to be cooled. As the refrigerant absorbs heat, it vaporizes back into a low – pressure vapor, and the cycle repeats.​
Components and Their Functions​
Compressor: The heart of the chiller, it provides the energy to compress the refrigerant and drive the refrigeration cycle. Common types of compressors used in air cooled chillers include reciprocating, scroll, and screw compressors.​
Condenser: Made of finned tubes, it is responsible for rejecting the heat absorbed by the refrigerant during the cooling process to the ambient air. The fins increase the surface area for better heat transfer.​
Expansion Device: Controls the flow of refrigerant into the evaporator and reduces its pressure, enabling the refrigerant to expand and cool.​
Evaporator: Absorbs heat from the process fluid, cooling it down. It can be a shell – and – tube type or a plate – type heat exchanger, depending on the application.​
Water Cooled Chillers​
Refrigeration Cycle with Water as a Heat Transfer Medium​
Water cooled chillers also operate on the vapor – compression refrigeration cycle, but they use water as an intermediate heat transfer medium between the refrigerant and the environment.​
Similar to air cooled chillers, the compressor compresses the low – pressure, low – temperature refrigerant vapor, raising its pressure and temperature. The hot, high – pressure refrigerant vapor then enters the condenser.​
In a water cooled chiller, the condenser is cooled by water. The condenser water is circulated through the condenser coils. As the hot refrigerant vapor passes through the condenser, heat is transferred from the refrigerant to the water, causing the refrigerant to condense into a liquid.​
The warm condenser water then leaves the condenser and is pumped to a cooling tower. In the cooling tower, the warm water is exposed to air. Heat is transferred from the water to the air through evaporation and sensible heat transfer. The cooled water then returns to the condenser to repeat the cycle.​
Meanwhile, the high – pressure liquid refrigerant passes through the expansion device, where its pressure is reduced. The low – pressure, low – temperature refrigerant then enters the evaporator, where it absorbs heat from the process fluid (chilled water), cooling it down. The refrigerant vaporizes and returns to the compressor to complete the cycle.​
Components and Their Functions​
Compressor: Similar to air cooled chillers, it compresses the refrigerant to drive the refrigeration cycle.​
Condenser: Transfers heat from the refrigerant to the condenser water. It is typically a shell – and – tube heat exchanger.​
Expansion Device: Reduces the pressure of the refrigerant before it enters the evaporator.​
Evaporator: Cools the process fluid (chilled water) by absorbing heat from it.​
Cooling Tower: Rejects heat from the condenser water to the atmosphere. It can be of different types, such as cross – flow, counter – flow, natural draft, or mechanical draft.​
Condenser Water Pump: Circulates the condenser water between the condenser and the cooling tower.​
Performance Differences​
Efficiency​
Cooling Capacity and Coefficient of Performance (COP)​
Water cooled chillers generally have a higher coefficient of performance (COP) compared to air cooled chillers. The COP is a measure of a chiller’s efficiency, defined as the ratio of the cooling capacity to the power input. Water cooled chillers can achieve higher COPs because they can reject heat more effectively. In an air cooled chiller, the condenser temperature is limited by the dry – bulb temperature of the ambient air. In a water cooled chiller, the condenser temperature is limited by the wet – bulb temperature of the ambient air, which is typically lower than the dry – bulb temperature. A lower condenser temperature results in a more efficient refrigeration cycle.​

For example, a well – designed water cooled chiller may have a COP in the range of 4 – 6, while a typical air cooled chiller may have a COP in the range of 2.5 – 4. However, it should be noted that the actual COP of both types of chillers can vary depending on factors such as operating conditions, load, and the quality of the equipment.​
Part – Load Performance​
Water cooled chillers also tend to have better part – load performance. Many industrial and commercial applications do not operate at full load all the time. As the load on a chiller decreases, the efficiency of the chiller can change. Water cooled chillers are often able to maintain a relatively high COP even at part – load conditions. This is because the cooling tower can adjust the water temperature and flow rate to match the load, allowing the chiller to operate more efficiently. In contrast, air cooled chillers may experience a more significant drop in efficiency at part – load, as the air – cooling fans may not be able to adjust as precisely to the changing load.​
Cooling Capacity Range​
Typical Sizes for Each Type​
Air cooled chillers are available in a wide range of sizes, but they are more commonly used for smaller to medium – sized applications. They can have cooling capacities ranging from a few kilowatts to several hundred kilowatts. For example, in small commercial buildings such as offices or restaurants, air cooled chillers with cooling capacities of 10 – 100 kW may be used.​
Water cooled chillers, on the other hand, are often used for larger – scale applications. They can have cooling capacities ranging from hundreds of kilowatts to several megawatts. In large industrial plants, hospitals, or multi – story commercial buildings, water cooled chillers with capacities of 500 kW to 5 MW or more may be installed.​
Temperature Limitations​
Ambient Temperature Effects​
Air cooled chillers are more sensitive to ambient temperature. As the ambient air temperature increases, the temperature difference between the hot refrigerant vapor in the condenser and the ambient air decreases. This reduces the rate of heat transfer in the condenser, leading to a higher condenser temperature and a lower COP. In extreme cases, if the ambient temperature is too high, the chiller may not be able to meet the cooling demand or may even shut down to protect the equipment. For example, in a hot climate with ambient temperatures regularly exceeding 35°C, an air cooled chiller may struggle to maintain the desired cooling capacity.​
Water cooled chillers are less affected by high ambient air temperatures. The cooling tower can still effectively reject heat even in high – temperature environments because it uses evaporation to cool the water. However, water cooled chillers can be affected by low ambient temperatures. In very cold weather, the water in the cooling tower may freeze, which can damage the tower and the associated piping. Special anti – freeze measures, such as adding glycol to the water or using heating elements, may be required in cold climates.​
Installation and Maintenance Requirements​
Installation​
Space Requirements​
Air cooled chillers are relatively compact and do not require a large amount of additional space for auxiliary equipment. They can be installed on rooftops, in mechanical rooms, or even outdoors in some cases, as long as there is sufficient clearance around the unit for air circulation. The footprint of an air cooled chiller is generally smaller compared to a water cooled chiller of the same cooling capacity.​
Water cooled chillers, on the other hand, require more space. In addition to the chiller unit itself, a cooling tower needs to be installed. Cooling towers can be large and bulky, and they require a dedicated area with proper drainage and ventilation. The installation of a water cooled chiller system also involves running pipes for the condenser water, which may require additional space for routing and support.​
Piping and Ventilation Considerations​
Air cooled chillers only require piping for the refrigerant and the process fluid (chilled water). The refrigerant piping is relatively simple compared to water cooled systems. However, proper ventilation is crucial for air cooled chillers. They need to be installed in an area where there is good air circulation to ensure effective heat rejection. If the air circulation is restricted, the performance of the chiller will be severely degraded.​

Water cooled chillers require extensive piping for both the condenser water and the chilled water. The condenser water piping needs to connect the chiller to the cooling tower, and proper sizing and installation are necessary to ensure efficient water circulation. Additionally, the cooling tower requires proper ventilation to facilitate heat transfer. The installation of water cooled chiller systems also needs to consider the location of the cooling tower in relation to the chiller to minimize the length of the piping and pressure drops.​
Maintenance​
Component Maintenance for Each Type​
Air cooled chillers have relatively fewer components to maintain compared to water cooled chillers. The main components that require regular maintenance are the compressor, fans, and filters. The compressor needs to be serviced periodically to ensure proper lubrication and to check for wear and tear. The fans should be inspected for proper operation and balanced to prevent excessive vibration. The air filters need to be cleaned or replaced regularly to ensure good air circulation through the condenser.​
Water cooled chillers have more components that require maintenance. In addition to the compressor, condenser, and evaporator, the cooling tower, condenser water pumps, and make – up water system need to be maintained. The cooling tower requires regular cleaning to prevent the build – up of algae, scale, and other contaminants. The condenser water pumps need to be checked for proper operation, and the impellers may need to be cleaned or replaced over time. The make – up water system, which adds water to the cooling tower to compensate for evaporation and blowdown, needs to be calibrated and maintained to ensure proper water quality.​
Frequency of Maintenance​
Generally, air cooled chillers may require less frequent maintenance compared to water cooled chillers. However, this can vary depending on the operating conditions and the quality of the equipment. In a clean and well – maintained environment, air cooled chillers may need to be serviced every 6 – 12 months.​
Water cooled chillers, due to their more complex systems and the presence of water, which can cause issues such as corrosion and scale formation, typically require more frequent maintenance. They may need to be serviced every 3 – 6 months, and more frequent checks may be required during periods of high use or in areas with poor water quality.​
Cost Considerations​
Initial Cost​
Equipment and Installation Costs​
Air cooled chillers are generally less expensive to purchase initially compared to water cooled chillers. The cost of an air cooled chiller is mainly determined by its cooling capacity, the type of compressor, and the quality of the components. Since air cooled chillers do not require a cooling tower or extensive water – piping systems, the overall installation cost is also lower.​
Water cooled chillers, on the other hand, are more expensive to purchase. The cost of the chiller unit itself, along with the cost of the cooling tower, condenser water pumps, and the installation of the piping systems, can make the initial investment significantly higher. For example, for a medium – sized application with a cooling capacity of 100 kW, an air cooled chiller may cost around
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20,000−30,000, while a water cooled chiller system (including the cooling tower and installation) may cost 30,000−50,000 or more.​
Operating Cost​
Energy Consumption and Water Usage​
As mentioned earlier, air cooled chillers generally have a lower COP, which means they consume more energy to produce the same amount of cooling compared to water cooled chillers. Over time, this can result in higher energy costs. However, air cooled chillers do not require water for cooling, so they do not have water – related operating costs.​
Water cooled chillers, while more energy – efficient, require water for cooling. The cost of water, as well as the cost of treating the water to prevent corrosion and scale formation, needs to be considered. In areas where water is scarce or expensive, the water – related operating costs of a water cooled chiller can be significant. Additionally, the operation of the cooling tower fans and condenser water pumps also consumes energy, although the overall energy consumption may still be lower than that of an air cooled chiller in many cases.​
Applications​
Air Cooled Chillers​
Small to Medium – Sized Commercial Buildings​
Air cooled chillers are commonly used in small to medium – sized commercial buildings such as offices, retail stores, and restaurants. These buildings typically have relatively low cooling loads, and an air cooled chiller can provide the necessary cooling in a cost – effective and space – efficient manner. For example, a small office building with a floor area of 1000 – 3000 square meters may be equipped with an air cooled chiller to provide cooling for the interior spaces.​
Locations with Limited Water Availability​
In areas where water is scarce or where there are restrictions on water usage, air cooled chillers are the preferred choice. This includes regions with arid climates or areas where water conservation is a priority. For example, in some desert – located data centers, air cooled chillers are used to cool the servers to avoid the high water consumption associated with water cooled systems.​
Portable or Mobile Applications​
Air cooled chillers are more suitable for portable or mobile applications. Their compact size and relatively simple installation requirements make them easy to move and set up in different locations. Some temporary event venues may use portable air cooled chillers to provide cooling during the event.​
Water Cooled Chillers​
Large Industrial Plants​
Water cooled chillers are widely used in large industrial plants where high cooling capacities are required. Industries such as manufacturing, chemical processing, and food and beverage production often have processes that generate a significant amount of heat. Water cooled chillers can efficiently remove this heat and maintain the required process temperatures. For example, in a large pharmaceutical manufacturing plant, water cooled chillers may be used to cool the reactors, storage tanks, and other equipment.​
Multi – Story Commercial Buildings and Hospitals​
In large multi – story commercial buildings and hospitals, water cooled chillers are often the preferred choice due to their high cooling capacity and better part – load performance. These buildings have complex HVAC systems that need to provide consistent cooling to multiple zones. Water cooled chillers can meet the varying cooling demands more effectively compared to air cooled chillers. In a large hospital, a water cooled chiller system may be used to cool the patient rooms, operating theaters, and other critical areas.​
District Cooling Systems​
Water cooled chillers are commonly used in district cooling systems, where a central chiller plant provides cooling to multiple buildings in a specific area. These systems are often found in urban areas or large campuses. The high efficiency and large cooling capacity of water cooled chillers make them suitable for such large – scale applications. For example, in a university campus with multiple academic and residential buildings, a district cooling system with water cooled chillers may be installed to provide cooling to all the buildings.​
Conclusion​
Air cooled and water cooled chillers each have their own set of characteristics, advantages, and limitations. Air cooled chillers are more suitable for small to medium – sized applications, locations with limited water availability, and portable or mobile setups. They are relatively inexpensive to purchase and install initially, but they may have higher energy consumption. Water cooled chillers, on the other hand, are better for large – scale applications, where high cooling capacities and good part – load performance are required. They are more energy – efficient but come with a higher initial cost and more complex installation and maintenance requirements, along with water – related operating costs. When choosing between air cooled and water cooled chillers, factors such as cooling capacity needs, ambient conditions, water availability, energy costs, and long – term maintenance requirements should all be carefully considered to ensure the most appropriate and cost – effective solution for a particular application.