evap chiller

Introduction​

Evap chillers, also known as evaporative chillers, play a significant role in providing cooling solutions across various industries. They operate based on the natural process of evaporation, which is an effective way to transfer heat. By harnessing this principle, evap chillers can offer efficient cooling while consuming relatively less energy compared to some other types of chillers. This makes them a popular choice in regions where energy conservation is a priority and where the climate conditions are favorable for evaporative cooling.​

Working Principles of Evap Chillers​

The Evaporative Cooling Process​

Water Evaporation and Heat Absorption: At the core of an evap chiller is the evaporative section. This section typically contains a media, such as a honeycomb – like structure made of a hydrophilic material. Water is continuously sprayed over this media. As the water comes into contact with the air flowing through the media, a portion of the water evaporates. Evaporation is an endothermic process, meaning it absorbs heat from the surrounding environment. In this case, the heat is absorbed from the air passing through the evaporative section. The air, which is now cooler, is then used to cool the refrigerant in the chiller’s refrigerant system.​

Cooling of the Refrigerant: The cooled air from the evaporative section is directed to a heat exchanger where it exchanges heat with the refrigerant. The refrigerant, usually a substance with a low boiling point, absorbs the heat from the air and evaporates. This evaporation process causes the refrigerant to change from a liquid state to a vapor state. The vaporized refrigerant then moves through the rest of the refrigerant system, where it will be compressed, condensed, and expanded in a cycle similar to that of a traditional vapor – compression chiller. However, the initial cooling of the refrigerant through the evaporative process is what sets evap chillers apart.​

The Refrigerant Cycle​

Compression: Once the refrigerant has absorbed heat and vaporized in the heat exchanger, it is drawn into a compressor. The compressor increases the pressure and temperature of the refrigerant vapor. By compressing the vapor, the refrigerant’s internal energy is increased, and its temperature rises significantly. This high – pressure, high – temperature refrigerant vapor then moves to the condenser.​

Condensation: In the condenser, the high – pressure refrigerant vapor releases heat to a secondary cooling medium. In some evap chillers, this secondary cooling medium can be air or water. As the refrigerant releases heat, it condenses back into a liquid state. The heat released during condensation is dissipated into the environment. For example, in an air – cooled condenser, fans blow air over the condenser coils, carrying away the heat from the refrigerant. In a water – cooled condenser, the heat is transferred to water, which may then be cooled in a cooling tower or a water – cooled heat exchanger.​

Expansion: The high – pressure liquid refrigerant then passes through an expansion valve. The expansion valve reduces the pressure of the refrigerant, causing it to expand rapidly. This sudden expansion leads to a drop in the refrigerant’s temperature, and it enters the evaporator as a low – pressure liquid – vapor mixture. In the evaporator, the refrigerant absorbs heat from the air or process fluid that needs to be cooled, repeating the cycle.​

Water Circulation System​

Spray and Recirculation: The water used in the evaporative section needs to be continuously circulated. A water pump is used to draw water from a sump at the bottom of the evaporative section and spray it over the media. After the water has evaporated and cooled the air, the remaining water collects in the sump. From the sump, the water is recirculated back to the spray nozzles. This continuous circulation ensures a constant supply of water for evaporation and maintains the efficiency of the evaporative cooling process.​

Water Treatment: Since the water in the evap chiller is in an open – loop system (in most cases), it is important to treat the water to prevent issues such as scale formation, corrosion, and the growth of biological organisms. Water treatment may involve adding chemicals to adjust the pH level of the water, prevent the formation of mineral deposits, and control the growth of bacteria and algae. In addition, filters may be used to remove any impurities from the water before it is recirculated.​

Components of Evap Chillers​

Evaporative Section​

Evaporative Media: The evaporative media is a key component of the evaporative section. It is designed to maximize the surface area for water – air contact, promoting efficient evaporation. Common types of evaporative media include cellulose – based materials, synthetic polymers, and ceramic materials. Cellulose – based media is popular due to its high water – holding capacity and good air – flow characteristics. Synthetic polymer media may offer better durability and resistance to biological growth. The media is usually arranged in a way that allows air to flow through it in a uniform manner, ensuring that the water sprayed over it has sufficient time to evaporate and cool the air.​

Spray Nozzles: Spray nozzles are responsible for evenly distributing the water over the evaporative media. They are designed to produce a fine mist of water, which increases the surface area of the water exposed to the air, enhancing the evaporation rate. The spray nozzles need to be carefully selected based on the flow rate of the water, the size of the evaporative section, and the type of media used. Regular maintenance of the spray nozzles is important to ensure that they do not become clogged, as clogged nozzles can lead to uneven water distribution and reduced cooling efficiency.​

Refrigerant System​

Compressor: The compressor in an evap chiller is similar to those used in traditional vapor – compression chillers. There are different types of compressors, such as reciprocating, screw, and centrifugal compressors. The choice of compressor depends on the cooling capacity requirements of the chiller. Reciprocating compressors are suitable for small – to – medium – sized chillers and offer good efficiency at specific load conditions. Screw compressors are more commonly used in medium – to – large – scale evap chillers due to their higher capacity and better part – load performance. Centrifugal compressors are typically used in large – scale industrial applications where high cooling capacities are required.​

Condenser: The condenser in an evap chiller can be either air – cooled or water – cooled. Air – cooled condensers are simpler in design and do not require a separate water – cooling infrastructure. They use fans to blow air over the condenser coils, dissipating the heat from the refrigerant. Water – cooled condensers, on the other hand, are more energy – efficient as water has a higher heat – carrying capacity than air. However, they require a water – cooling system, such as a cooling tower or a water – cooled heat exchanger, and proper water treatment to prevent scale formation and corrosion.​

Evaporator: The evaporator in an evap chiller is where the refrigerant absorbs heat from the air or process fluid. It is designed to maximize the heat – transfer between the refrigerant and the cooling medium. The evaporator can be a shell – and – tube type or a plate – type heat exchanger. In a shell – and – tube evaporator, the refrigerant is typically in the tubes, and the cooling medium (air or process fluid) flows around the tubes. Plate – type evaporators consist of a series of thin, corrugated metal plates, which offer high heat – transfer coefficients and are more compact compared to shell – and – tube evaporators.​

Water Circulation System Components​

Water Pump: The water pump is responsible for circulating the water in the evaporative section. It needs to provide sufficient pressure to ensure that the water is sprayed evenly over the evaporative media. The size and capacity of the water pump are determined by the flow rate requirements of the water in the system. Variable – speed pumps are often used in evap chillers, as they can adjust the water flow rate according to the cooling load, saving energy.​

Sump: The sump is a reservoir at the bottom of the evaporative section where the water collects after evaporation. It needs to be large enough to hold a sufficient volume of water to ensure continuous operation of the chiller. The sump may also be equipped with features such as level sensors to monitor the water level and float valves to automatically add water when the level drops below a certain point. In addition, the sump may have a drain valve for easy draining and cleaning of the water.​

Applications of Evap Chillers​

Commercial Buildings​

Office Buildings: In office buildings, evap chillers can be used to provide cooling for the air – conditioning system. The chilled water or air produced by the evap chiller can be distributed to air – handling units (AHUs) or fan – coil units (FCUs) throughout the building. Evap chillers are particularly suitable for office buildings in regions with dry climates, as they can offer significant energy savings compared to traditional air – conditioning systems. The use of evap chillers can also help to reduce the peak electricity demand in the building, as they consume less electrical energy compared to chillers that rely solely on mechanical compression.​

Hotels: Hotels require efficient cooling systems to keep guest rooms, lobbies, restaurants, and other areas comfortable. Evap chillers can be integrated into the hotel’s cooling infrastructure to provide the necessary cooling capacity. The ability of evap chillers to operate quietly makes them an attractive option for hotels, as noise pollution can be a concern in guest areas. In addition, the energy – efficiency of evap chillers can help hotels reduce their operating costs, which is especially important in a competitive industry.​

Industrial Applications​

Manufacturing Plants: In manufacturing plants, there are often many heat – generating processes, such as welding, machining, and chemical reactions. Evap chillers can be used to cool the equipment and processes, preventing overheating and ensuring smooth operation. For example, in a plastic injection – molding factory, the molds need to be cooled rapidly and precisely to ensure the proper shaping of plastic products. Evap chillers can provide the necessary cooling water to cool the molds, and their energy – efficiency can contribute to cost savings in the manufacturing process.​

Data Centers: Data centers generate a large amount of heat due to the continuous operation of servers, storage devices, and other IT equipment. Evap chillers can be used to cool the air in the data center, maintaining a stable temperature and preventing the overheating of the equipment. The high heat – load density in data centers makes the use of energy – efficient cooling systems, such as evap chillers, crucial. In addition, the water – based cooling mechanism of evap chillers can be more effective in removing heat compared to traditional air – cooled systems, especially in high – heat – load environments.​

Advantages of Evap Chillers​

Energy – Efficiency​

Reduced Electrical Energy Consumption: Evap chillers consume less electrical energy compared to traditional vapor – compression chillers. This is because they rely on the natural process of evaporation for cooling, which requires less mechanical work. In regions with high electricity costs, the use of evap chillers can lead to significant cost savings. For example, in a large commercial building, replacing a traditional chiller with an evap chiller can result in a 30 – 50% reduction in electrical energy consumption for cooling, depending on the climate and operating conditions.​

Good Performance in Dry Climates: Evap chillers are most effective in dry climates where the evaporation rate is high. In such climates, the evaporative cooling process can achieve a greater temperature drop, resulting in more efficient cooling. The ability to take advantage of the natural cooling potential of the environment makes evap chillers an ideal choice for areas with arid or semi – arid climates. In a desert – region data center, an evap chiller can provide efficient cooling while consuming relatively little energy, as the dry air allows for rapid evaporation of water in the evaporative section.​

Cost – Effectiveness​

Lower Initial Cost: Evap chillers generally have a lower initial cost compared to some other types of chillers, especially those with complex refrigeration systems. The simpler design of evap chillers, with fewer mechanical components in the cooling process, contributes to their lower cost. In addition, the reduced energy consumption of evap chillers can lead to lower operating costs over the long term, making them a cost – effective choice for both commercial and industrial applications.​

Reduced Maintenance Costs: The maintenance requirements of evap chillers are relatively straightforward. Since they have fewer moving parts compared to traditional vapor – compression chillers, there is less wear and tear on components. Regular maintenance mainly involves checking the water quality, inspecting the evaporative media and spray nozzles, and monitoring the refrigerant system. This simplicity in maintenance can result in lower maintenance costs over the lifespan of the chiller.​

Disadvantages of Evap Chillers​

Water Consumption​

Higher Water Usage: Evap chillers consume a significant amount of water due to the continuous evaporation process. In regions where water resources are scarce or expensive, this can be a major drawback. The water consumption of an evap chiller can be several times higher than that of a traditional water – cooled chiller. For example, in a large – scale commercial building, an evap chiller may consume several hundred gallons of water per day, depending on the cooling load and the climate conditions.​

Water Treatment Requirements: Since the water in an evap chiller is in an open – loop system, it is exposed to the environment and is more likely to develop issues such as scale formation, corrosion, and the growth of biological organisms. This requires regular water treatment, which adds to the operating cost of the chiller. The cost of water treatment chemicals, as well as the energy required to operate water treatment equipment, can be a significant expense.​

Climate – Dependent Performance​

Ineffectiveness in Humid Climates: Evap chillers are less effective in humid climates. In humid conditions, the evaporation rate of water is reduced, as the air already contains a high amount of moisture. This results in a lower temperature drop in the evaporative section and reduced cooling efficiency. In a tropical region with high humidity, an evap chiller may not be able to achieve the same level of cooling as it would in a dry climate, and additional cooling measures may be required.​

Performance Variation with Temperature: The performance of evap chillers can also vary with changes in ambient temperature. In extremely hot weather, the cooling capacity of an evap chiller may decrease, as the temperature difference between the air and the water in the evaporative section is reduced. This can lead to a situation where the chiller may not be able to meet the full cooling load of the building or process, especially in peak – cooling periods.​

Maintenance of Evap Chillers​

Water – Related Maintenance​

Water Quality Monitoring and Treatment: Regularly monitoring the water quality in an evap chiller is essential. The water should be tested for parameters such as pH, hardness, and the presence of biological organisms. Based on the test results, appropriate water treatment measures should be taken. If the water is too acidic or alkaline, chemicals can be added to adjust the pH level. To prevent scale formation, scale inhibitors can be added to the water. In addition, biocides may be used to control the growth of bacteria and algae. The water treatment process should be carefully managed to ensure the proper functioning of the chiller and to prevent damage to the components.​

Inspection of Water Circulation System: The water circulation system, including the water pump, spray nozzles, and sump, should be inspected regularly. The water pump should be checked for proper operation, and any signs of leaks or abnormal noise should be addressed. The spray nozzles should be cleaned to remove any clogs, as clogged nozzles can affect the water distribution and cooling efficiency. The sump should be cleaned regularly to remove any debris or sediment that may have accumulated. In addition, the water level in the sump should be monitored, and the float valve should be checked to ensure that it is functioning properly.​

Refrigerant System Maintenance​

Refrigerant Level and Quality Checks: The refrigerant level in the chiller should be checked regularly to ensure that it is within the recommended range. A low refrigerant level can indicate a leak in the system, and the leak should be located and repaired promptly. In addition, the quality of the refrigerant should be inspected. Contaminated refrigerant can affect the performance of the chiller and may require purification or replacement. Refrigerant testing kits can be used to check for the presence of impurities such as moisture, non – condensable gases, and acids.​

Compressor and Condenser Maintenance: The compressor is a critical component of the refrigerant system and requires regular maintenance. The compressor should be checked for signs of wear, leaks, and abnormal vibration. The oil level in the compressor should be maintained, and the oil should be changed according to the manufacturer’s recommendations. The condenser, whether air – cooled or water – cooled, should be cleaned regularly to remove any dirt, dust, or debris that may have accumulated on the coils. This helps to improve the heat – transfer efficiency of the condenser and ensures the proper operation of the refrigerant system.​

Evaporative Section Maintenance​

Evaporative Media Cleaning and Replacement: The evaporative media should be cleaned regularly to remove any dirt, dust, or biological growth. The cleaning process can involve spraying the media with a mild detergent solution and then rinsing it thoroughly with clean water. Over time, the evaporative media may become damaged or less effective, and it may need to be replaced. The frequency of replacement depends on the operating conditions of the chiller and the quality of the media. In a high – dust environment, the evaporative media may need to be replaced more frequently.