Hybrid systems are popular because they offer choices such as gas chiller operation during peak electrical periods and standard electrical chiller usage during off-peak hours. In some instances, standard electrical chillers have not been used because of the efficient operation of the main gas-chiller system.
The process of gas absorption uses an evaporator and condenser much like conventional vapor compression units. However, instead of using a standard electrical compressor and motor, a thermal compression system is used.
Within a simplified thermal compression system, an absorber and generator are integrated along with a pumping system. An evaporator removes heat from the circulating water system to produce cool water. From the evaporator, refrigerant vapor moves to an absorber where it is compressed and absorbed into a solution, usually lithium bromide. This solution then moves to the generator where heat - either direct-fired gas or steam - is added to remove the refrigerant from the solution.
The solution then goes through heat exchangers and is returned to the absorber. The refrigerant returns to the condenser where it is liquefied and sent back to the evaporator. The entire cycle then starts over again.
Gas-fired steam absorption chillers work in much the same manner as direct-fired gas chillers. The main difference is that the heat source for the generator is usually an external gas-fired boiler system. These systems are favorable for facilities that have a boiler system already installed on site.
In addition to being either gas-fired steam absorption or direct-fired gas, chillers are categorized into two different types: Single-effect and double-effect. There is a triple-effect being developed. Single-effect, or single-stage, absorption chillers usually require low internal pressures around 20 psig to produce chilled water. Double-effect, or two-stage, absorption chillers work at a much higher pressure, around 40 to 140 psig. These chillers also have an extra generator integrated into the absorption system that increases the efficiency by about 30 percent. Double-effect chillers are currently more popular than the single-effect types.
Many direct-fired absorption chillers are dual-fuel rated. Natural gas is commonly the primary fuel, however No. 2 fuel oil can be used as an alternate. If for any reason the natural gas supply is interrupted, having the capability to use an emergency fuel may be invaluable. This fuel redundancy may be an important purchasing factor for many facility professionals.
Maintenance requirements for gas-absorption chillers are minimal. Pump seals need inspecting periodically if the pumps are not hermetically rated. In addition, scaling and sludge build-up are areas of concern with this equipment. However, automatic chiller and purge controls, along with periodic general maintenance checks, will alleviate most of these potential problems.
Equipment size can be a concern for some facilities. Typically, gas absorption chillers are larger than standard electrical chillers of the same Btu rating. Adequate space needs to be allowed for any gas chiller retrofit or new installation.
One of the benefits of using gas-absorption chillers is that it produces enough heat to also activate certain dehumidification systems, which are becoming important components of HVAC systems. Molds, mildews and bacteria flourish in high-humidity conditions. Controlling humidity reduces risks to human health and ensures that sensitive manufacturing processes can be conducted. Dehumidification also increases cooling efficiency. In some instances, installing a dehumidification system can reduce a building's cooling load by as much as 50 percent.
Solid desiccant dehumidification systems remove moisture from the air by implementing a heat-activated material. Although a complete system contains many components, the main internal component is the desiccant wheel. Usually the desiccant wheel is divided into two halves. One half allows unconditioned air to enter where the air is then dried. When this half becomes saturated with moisture, it is rotated into a heated area that subsequently regenerates the desiccant in that half of the wheel. Within this rotating cycle, one half of the wheel regenerates while the other half absorbs moisture.
Absorption chillers are not the only means in which gas can be adapted to conserve electrical consumption. Gas-driven chillers are another viable means to reduce peak electrical costs. The chiller portion of this unit is really just a standard vapor compression system driven by an externally powered internal combustion engine. Typically, with this type of unit, the engine couples directly to the input shaft of a rotary-type chiller. The engine can be operated with many fuels, including natural gas, LP or diesel. Cogeneration systems are a possible addition when an external engine is used in this way. Hot water, steam, dehumidification, cooling and electricity production all use an internal combustion engine as a power source.
Although most gas chiller systems are not as well-known as the conventional electrically driven chiller systems, they are becoming more popular because of rising electrical costs. Many facility executives are experiencing lower electrical consumption since they have made the transition to gas cooling. When making gas chiller decisions, it may be a good idea to get advice from facility executives that have already made the transition. Their input and the help of qualified energy management engineers will help in making a wise energy choice.
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