heat chiller

Introduction​

Heat chillers, often referred to as heat – activated chillers or absorption chillers, represent a distinct category of cooling equipment. Unlike the more common vapor – compression chillers that rely on mechanical compression of refrigerants, heat chillers operate on a heat – driven cycle. This makes them particularly suitable for applications where a reliable heat source is available, and there is a need to convert this heat energy into cooling capacity.​

Working Principles of Heat Chillers​

The Absorption Cycle​

Absorption Process: At the heart of a heat chiller is the absorption cycle. In a typical water – lithium bromide heat chiller (one of the most common types), water serves as the refrigerant, and lithium bromide acts as the absorbent. The process begins in the absorber. Here, the low – pressure water vapor (refrigerant) is absorbed by the lithium bromide solution. This absorption is an exothermic reaction, meaning it releases heat. The heat released during absorption is removed by a cooling medium, usually water, which is circulated through a cooling coil in the absorber. As the water vapor is absorbed, the lithium bromide solution becomes richer in water.​

Generation Process: The rich lithium bromide – water solution is then pumped to the generator. In the generator, heat is applied. This heat can come from various sources such as waste heat from industrial processes, solar – thermal collectors, or natural – gas – fired heaters. The heat causes the water to vaporize from the lithium bromide solution. As the water vaporizes, it becomes high – pressure steam. The lithium bromide solution, now depleted of water, returns to the absorber to repeat the absorption process.​

Condensation and Expansion: The high – pressure water vapor (refrigerant) from the generator enters the condenser. In the condenser, the water vapor releases heat to a secondary cooling medium, usually water or air. As it releases heat, the water vapor condenses back into a liquid state. The high – pressure liquid water then passes through an expansion valve. The expansion valve reduces the pressure of the liquid water, causing it to flash into a low – pressure liquid – vapor mixture.​

Evaporation: The low – pressure liquid – vapor mixture enters the evaporator. In the evaporator, the refrigerant (water) absorbs heat from the water or air that needs to be cooled. As the refrigerant absorbs heat, it evaporates and turns back into a low – pressure vapor. This cooled water or air can then be used for cooling purposes, such as in air – conditioning systems or industrial cooling processes. The low – pressure water vapor then returns to the absorber to complete the cycle.​

Ammonia – Water Heat Chillers​

Alternative Refrigerant – Absorbent Pair: Another type of heat chiller uses ammonia as the refrigerant and water as the absorbent. In this system, the working principle is similar to the water – lithium bromide system but with some differences due to the properties of ammonia and water. In the absorber, ammonia vapor is absorbed by water, releasing heat. The rich ammonia – water solution is then pumped to the generator. Heat is applied in the generator, causing ammonia to vaporize from the solution. The high – pressure ammonia vapor enters the condenser, where it condenses into a liquid. After passing through an expansion valve, the low – pressure ammonia liquid – vapor mixture enters the evaporator. In the evaporator, ammonia absorbs heat from the medium to be cooled, evaporates, and returns to the absorber as a low – pressure vapor. Ammonia – water heat chillers are often used in industrial applications where the cooling requirements are more demanding and the properties of ammonia, such as its high heat – transfer coefficients, are advantageous.​

Types of Heat Chillers​

Water – Lithium Bromide Heat Chillers​

Common Applications: Water – lithium bromide heat chillers are widely used in commercial buildings, such as hotels, large – scale office buildings, and hospitals. They are also used in some industrial applications where the heat source is relatively clean and the cooling requirements are not extremely low. For example, in a hotel, waste heat from the boiler used for heating water can be used to power a water – lithium bromide heat chiller. This chiller can then provide cooling for the hotel’s air – conditioning system, reducing the overall energy consumption of the building.​

Advantages and Limitations: One of the main advantages of water – lithium bromide heat chillers is their use of a non – toxic and non – flammable refrigerant (water). They are also relatively quiet in operation as they do not have a mechanical compressor. However, they are sensitive to the concentration of the lithium bromide solution and the operating temperature. If the temperature in the generator is too high or the concentration of lithium bromide is not properly maintained, crystallization of the lithium bromide can occur, which can disrupt the operation of the chiller.​

Ammonia – Water Heat Chillers​

Industrial Applications: Ammonia – water heat chillers are more commonly used in industrial settings. They are suitable for applications where lower temperatures are required, such as in the food and beverage industry for freezing and cold storage, and in some chemical processes. In a food – freezing plant, an ammonia – water heat chiller can be used to provide the low – temperature cooling needed to freeze food products quickly and efficiently.​

Unique Characteristics: Ammonia has a higher heat – transfer coefficient compared to water, which allows for more efficient heat transfer in the evaporator and condenser. However, ammonia is toxic and flammable, so proper safety measures need to be in place when using ammonia – water heat chillers. The system also requires careful control of the ammonia – water concentration to ensure optimal performance.​

Applications of Heat Chillers​

Industrial Plants​

Waste Heat Recovery: In industrial plants, there is often a significant amount of waste heat generated from processes such as power generation, manufacturing, and chemical reactions. Heat chillers can be used to recover this waste heat and convert it into useful cooling. For example, in a power plant, the exhaust gases from the boilers or turbines can be used as a heat source for a heat chiller. The chiller can then provide cooling for the plant’s equipment, such as cooling the lubricating oil for machinery or providing air – conditioning for the control rooms. This waste – heat recovery not only reduces the energy consumption of the plant but also helps to improve the overall efficiency of the industrial process.​

Process Cooling: In many industrial processes, precise temperature control is crucial. Heat chillers can be used to provide the necessary cooling for these processes. In a pharmaceutical manufacturing plant, heat chillers can be used to cool the reaction vessels during the production of drugs. The ability to use waste heat or other heat sources to power the chiller makes it an attractive option for industrial applications where energy costs are a significant factor.​

Commercial Buildings​

Large – Scale Air – Conditioning: In large commercial buildings, such as shopping malls and convention centers, heat chillers can be used to provide central air – conditioning. The heat source for these chillers can be a natural – gas – fired boiler or solar – thermal collectors. For example, in a shopping mall, a natural – gas – fired heat chiller can be used to cool the air – handling units that supply conditioned air to the different areas of the mall. This can reduce the reliance on traditional electrically – driven chillers, resulting in lower electricity bills and potentially a more sustainable operation.​

Combined Heat and Power (CHP) Systems: Heat chillers are often integrated into combined heat and power systems. In a CHP system, electricity is generated, and the waste heat from the power – generation process is used for heating and cooling. The heat chiller can use the waste heat to provide cooling during the summer months, while the same heat can be used for heating in the winter. This integrated approach maximizes the use of energy resources and can improve the overall energy efficiency of the building.​

Green – Energy – Focused Facilities​

Solar – Powered Cooling: In facilities that are focused on using renewable energy, heat chillers can be powered by solar energy. Solar – thermal collectors can capture solar radiation and convert it into heat. This heat can then be used to drive a heat chiller, providing cooling without relying on fossil fuels. For example, in a solar – powered office building, a solar – thermal – driven heat chiller can be used to cool the building during the day when solar radiation is available. This helps to reduce the carbon footprint of the building and promotes the use of clean energy.​

Waste – Heat – Driven Cooling in Sustainable Communities: In sustainable communities or eco – industrial parks, heat chillers can play a role in waste – heat sharing. Different facilities within the community may generate waste heat, which can be collected and used to power heat chillers. These chillers can then provide cooling for the community’s buildings or industrial processes, creating a more sustainable and energy – efficient ecosystem.​

Advantages and Disadvantages of Heat Chillers​

Advantages​

Energy – Efficiency in the Right Conditions: When there is a readily available heat source, heat chillers can be highly energy – efficient. They can convert waste heat or low – grade heat into useful cooling, reducing the need for additional energy input. In an industrial plant with significant waste heat, using a heat chiller can lead to substantial energy savings. By utilizing heat that would otherwise be wasted, heat chillers can improve the overall energy balance of a facility.​

Reduced Reliance on Electricity: Heat chillers rely on heat rather than electricity for their operation (except for the pumps and controls, which consume a relatively small amount of electricity). This can be an advantage in areas where electricity is expensive, unreliable, or where there are restrictions on electricity consumption. In regions with high electricity tariffs, using a heat chiller can significantly reduce energy costs.​

Environmental Friendliness: Using waste heat or renewable heat sources (such as solar energy) to power heat chillers reduces the carbon footprint. It also helps to minimize the use of traditional refrigerants with high global – warming potential, as many heat chillers use environmentally friendly refrigerants like water or ammonia. In addition, by reducing the demand for electricity from power plants, heat chillers contribute to a more sustainable energy system.​

Disadvantages​

Complex Installation and Maintenance: Heat chillers require a more complex installation compared to traditional vapor – compression chillers. They need a reliable heat source, proper plumbing for the heat transfer fluid, and a cooling system for the absorber and condenser. Maintenance also involves ensuring the proper operation of the heat source, checking the refrigerant – absorbent levels and concentrations, and monitoring for potential crystallization in water – lithium bromide systems. This complexity can lead to higher installation and maintenance costs.​

Lower Efficiency at Partial Load: Heat chillers tend to have lower efficiency at partial – load conditions compared to some vapor – compression chillers. As the cooling load decreases, the performance of the heat chiller may degrade, and the heat – to – cooling conversion efficiency may drop. This means that they may not be as effective in applications where the cooling load varies significantly throughout the day.​

Maintenance of Heat Chillers​

Heat Source Maintenance​

Ensuring Reliable Heat Supply: The heat source for a heat chiller needs to be properly maintained to ensure continuous operation. If the heat source is a natural – gas – fired heater, regular inspections of the burner, gas lines, and ignition system are necessary. For waste – heat – driven heat chillers, the equipment generating the waste heat, such as boilers or industrial processes, should be maintained to ensure a consistent heat output. Any disruptions in the heat source can lead to a loss of cooling capacity in the heat chiller.​

Monitoring Heat Source Parameters: Parameters such as the temperature, pressure, and flow rate of the heat source should be monitored. In a solar – thermal – driven heat chiller, the performance of the solar collectors, including the temperature of the heat – transfer fluid leaving the collectors, should be closely monitored. Adjustments may need to be made to the heat – source system to optimize the operation of the heat chiller.​

Refrigerant – Absorbent System Maintenance​

Concentration Checks: For water – lithium bromide and ammonia – water heat chillers, regularly checking the concentration of the refrigerant – absorbent mixture is crucial. In a water – lithium bromide system, if the concentration of lithium bromide is too high, crystallization can occur, which can damage the chiller. In an ammonia – water system, the proper ammonia – water ratio is necessary for efficient operation. Specialized testing equipment can be used to measure the concentration of the mixture.​

Leak Detection: Checking for leaks in the refrigerant – absorbent system is essential. Leaks can lead to a loss of refrigerant or absorbent, reducing the performance of the chiller. In an ammonia – water system, leaks are particularly dangerous due to the toxicity and flammability of ammonia. Regular inspections of the pipes, joints, and valves in the system can help to detect and repair leaks promptly.​

Component Inspection​

Absorber, Generator, Condenser, and Evaporator: The main components of the heat chiller, including the absorber, generator, condenser, and evaporator, should be inspected regularly. The heat – transfer surfaces in these components can become fouled over time, reducing the efficiency of the chiller. Cleaning the heat – transfer surfaces, checking for corrosion, and ensuring proper insulation of the components are important maintenance tasks. In addition, the pumps and valves in the system should be inspected for proper operation and any signs of wear or damage.