Cooling – Absorption Chillers
Natural gas absorption chillers operate in much the same way as vapor compression chillers. That is, they rely on temperature and pressure changes in a refrigerant to absorb and transfer heat from one area to another.
A significant difference between vapor compression and absorption systems is the use of heat in the absorption chiller’s “thermal compressor” rather than a mechanically driven compressor. Heat for the absorption process can be supplied either directly by a gas burner or indirectly from recovered waste heat.
The number of heat exchangers in the system determines whether it is a single-effect or double-effect absorption chiller. Double-effect chillers capture some of the internal heat to provide energy to the generator and are more efficient than single-effect units. Although double-effect absorption chillers are more expensive than electric chillers, developments in controls and operating practices have led to lower maintenance requirements.
Another important difference between absorption and vapor compression chillers is the solution used in the vapor compression cycle. Water-cooled absorption systems use water as the refrigerant and a lithium bromide solution as the absorbent material. Air-cooled systems use ammonia as the refrigerant and water as the absorbent. Both options are environmentally benign and eliminate the harmful chlorofluorocarbons (CFCs) common to mechanical systems. The absorption process relies on the affinity of the two liquids for each other to achieve the temperatures and pressures required.
How It Works
The process begins in the absorption system’s thermal compressor—a generator, absorber, pump and heat exchanger. Heat from a burner separates the absorbent from the refrigerant and turns the refrigerant into a high-pressure vapor. In the condenser, the refrigerant gives up its heat to the outdoor atmosphere and returns to a liquid state. As the liquid moves through the throttling valve—similar to the expansion valve in a vapor compression system—into the evaporator, its temperature and pressure are reduced, enabling it to extract heat from the chilled water stream which circulates through indoor cooling coils. The refrigerant passes into the absorber where it is pulled into the absorbent solution (the absorption process) and pumped back to the generator where the process repeats.
Advantages of Absorption Chillers
Absorption chillers offer several noteworthy advantages. Like gas engine-drive chillers, absorption units can prove highly beneficial for peak shaving in areas with high demand charges, especially when teamed with electric chillers in a hybrid system. The ability for absorption chillers to use waste heat from incinerators, industrial furnaces or manufacturing equipment also greatly improves their cost effectiveness. Indirect fired chillers can operate using steam or hot water or integrate with on-site power generation. Single-effect absorption chillers require 167° to 270°F hot water or steam; double-effect units operate on 370°F, 115psig steam.