18-04-2012, 02:40 PM
Vapor Absorption Cooling Systems
[attachment=20298]
What is Absorption?
Comparing the absorption refrigeration cycle with the more familiar vapor compression refrigeration cycle is often an easy way to introduce it.
The standard vapor compression refrigeration system is a condenser, evaporator, throttling valve, and a compressor. Figure below is a schematic of the components and flow arrangements for the vapor compression cycle
Applications of Absorption Systems
The main advantage of absorption chillers is their ability to utilize waste heat streams that would be otherwise discarded. In terms of energy performance, motor-driven vapor compression chillers will beat absorption chillers every time. Still there are specific applications where absorption chillers have a substantial advantage over motor-driven vapor compression chillers. Some of those applications include:
1. For facilities that use lot of thermal energy for their processes, a large chunk of heat is usually discarded to the surrounding as waste. This waste heat can be converted to useful refrigeration by using a VAM.
2. For facilities that have a simultaneous need for heat and power (cogeneration system), absorption chillers can utilize the thermal energy to produce chilled water.
3. For facilities that have high electrical demand charges. Absorption chillers minimize or flatten the sharp spikes in a building’s electric load profile can be used as part of a peak shaving strategy.
4. For facilities where the electrical supply is not robust, expensive, unreliable, or unavailable, it is easier to achieve heat input with a flame than with
electricity. Absorption chillers uses very little electricity compared to an electric motor driven compression cycle chiller.
5. For facilities, where the cost of electricity verses fuel oil/gas tips the scale in favor of fuel/gas. Various studies indicate that the absorption chillers provide economic benefit in most geographical areas, due to the differential in the cost between gas and electric energy.
6. For facilities wanting to use a “natural refrigerant and aspiring for LEED certification (Leadership in Energy and Environmental Design) absorption chillers are a good choice. Absorption chillers do not use CFCs or HCFCs - the compounds known for causing Ozone depletion.
7. For facilities implementing clean development mechanism (CDM) and accumulating carbon credits, the absorption use coupled to waste heat recovery and cogeneration system help reduce problems related to greenhouse effect from CO2 emission.
Vapor absorption system allows use of variable heat sources: directly using a gas burner, recovering waste heat in the form of hot water or low-pressure steam, or boiler-generated hot water or steam.
The Basic Principle of Absorption Cooling
Water boils and evaporates at 212 °F [100 °C] at standard atmospheric pressure (14.7psia [101.3kPa]). When the pressure is reduced, water boils at a lower temperature. The following table gives the total pressure in inches of mercury and the corresponding approximate water boiling temperature at different pressures:
Efficiency of Vapor Absorption Machine (VAM)
Efficiencies of absorption chillers is described in terms of Coefficient of Performance (COP), and is defined as the refrigeration effect, in Btu, divided by the net heat input, in Btu.
The COP can be thought of as a sort of index of the efficiency of the machine. The absorption systems with a COP of 1.0 will burn 12,000 BTUs of heat energy for each ton-hour of cooling. For example, a 500-ton absorption chiller operating at a COP of 0.70 would require: (500 x 12,000 Btu/h) divided by 0.70 = 8,571,429 Btu/h heat input.
*Cooling capacity is measured in tons of refrigeration. A ton of refrigeration is defined as the capacity to remove heat at a rate of 12,000 Btu/hr at the evaporator.
TYPES OF VAM
Absorption chillers are classified as:
1. Single effect absorption chiller
2. Double effect absorption chiller
Single Effect Absorption Chillers
The single-effect absorption chiller includes a single generator, condenser, evaporator, absorber, heat exchanger, and pumps. Fig. below shows a Single Effect Chiller.
AVAILABLE CAPACITIES OF VAM
The chiller ratings are typically given at 100 percent design capacity. The units are rated according to ARI standards. The ARI standard specifies the condenser water inlet temperature (85 °F) and the chilled water exit temperature (44 °F). The actual unit capacity will vary as the above conditions change. The capacity and performance will decrease with higher condenser water or lower chilled water temperatures and vice-versa.
[attachment=20298]
What is Absorption?
Comparing the absorption refrigeration cycle with the more familiar vapor compression refrigeration cycle is often an easy way to introduce it.
The standard vapor compression refrigeration system is a condenser, evaporator, throttling valve, and a compressor. Figure below is a schematic of the components and flow arrangements for the vapor compression cycle
Applications of Absorption Systems
The main advantage of absorption chillers is their ability to utilize waste heat streams that would be otherwise discarded. In terms of energy performance, motor-driven vapor compression chillers will beat absorption chillers every time. Still there are specific applications where absorption chillers have a substantial advantage over motor-driven vapor compression chillers. Some of those applications include:
1. For facilities that use lot of thermal energy for their processes, a large chunk of heat is usually discarded to the surrounding as waste. This waste heat can be converted to useful refrigeration by using a VAM.
2. For facilities that have a simultaneous need for heat and power (cogeneration system), absorption chillers can utilize the thermal energy to produce chilled water.
3. For facilities that have high electrical demand charges. Absorption chillers minimize or flatten the sharp spikes in a building’s electric load profile can be used as part of a peak shaving strategy.
4. For facilities where the electrical supply is not robust, expensive, unreliable, or unavailable, it is easier to achieve heat input with a flame than with
electricity. Absorption chillers uses very little electricity compared to an electric motor driven compression cycle chiller.
5. For facilities, where the cost of electricity verses fuel oil/gas tips the scale in favor of fuel/gas. Various studies indicate that the absorption chillers provide economic benefit in most geographical areas, due to the differential in the cost between gas and electric energy.
6. For facilities wanting to use a “natural refrigerant and aspiring for LEED certification (Leadership in Energy and Environmental Design) absorption chillers are a good choice. Absorption chillers do not use CFCs or HCFCs - the compounds known for causing Ozone depletion.
7. For facilities implementing clean development mechanism (CDM) and accumulating carbon credits, the absorption use coupled to waste heat recovery and cogeneration system help reduce problems related to greenhouse effect from CO2 emission.
Vapor absorption system allows use of variable heat sources: directly using a gas burner, recovering waste heat in the form of hot water or low-pressure steam, or boiler-generated hot water or steam.
The Basic Principle of Absorption Cooling
Water boils and evaporates at 212 °F [100 °C] at standard atmospheric pressure (14.7psia [101.3kPa]). When the pressure is reduced, water boils at a lower temperature. The following table gives the total pressure in inches of mercury and the corresponding approximate water boiling temperature at different pressures:
Efficiency of Vapor Absorption Machine (VAM)
Efficiencies of absorption chillers is described in terms of Coefficient of Performance (COP), and is defined as the refrigeration effect, in Btu, divided by the net heat input, in Btu.
The COP can be thought of as a sort of index of the efficiency of the machine. The absorption systems with a COP of 1.0 will burn 12,000 BTUs of heat energy for each ton-hour of cooling. For example, a 500-ton absorption chiller operating at a COP of 0.70 would require: (500 x 12,000 Btu/h) divided by 0.70 = 8,571,429 Btu/h heat input.
*Cooling capacity is measured in tons of refrigeration. A ton of refrigeration is defined as the capacity to remove heat at a rate of 12,000 Btu/hr at the evaporator.
TYPES OF VAM
Absorption chillers are classified as:
1. Single effect absorption chiller
2. Double effect absorption chiller
Single Effect Absorption Chillers
The single-effect absorption chiller includes a single generator, condenser, evaporator, absorber, heat exchanger, and pumps. Fig. below shows a Single Effect Chiller.
AVAILABLE CAPACITIES OF VAM
The chiller ratings are typically given at 100 percent design capacity. The units are rated according to ARI standards. The ARI standard specifies the condenser water inlet temperature (85 °F) and the chilled water exit temperature (44 °F). The actual unit capacity will vary as the above conditions change. The capacity and performance will decrease with higher condenser water or lower chilled water temperatures and vice-versa.