- Chillers produce cold water for air conditioning or process cooling applications.
- They are often found in medium to large commercial and industrial facilities.
- Types of chillers include centrifugal, screw, reciprocating and absorption.
Chiller systems are widely used in commercial and industrial facilities for air conditioning and process cooling. Chillers use a substantial amount of energy and thus are a key target for efficiency upgrades. Understanding how chillers work and what types are available can help you choose the right system for your application.
How chillers work
More than 90 percent of all chillers operate on the principle of vapor compression, where cooling is produced when heat from the indoors is absorbed as the liquid refrigerant converts to a vapor. When the vapor reaches the compressor, its temperature and pressure are increased. It then circulates to the condenser coil, where heat is rejected and the vapor condenses to a liquid. The liquid refrigerant is metered into the evaporator through a thermal expansion valve. The refrigerant enters the evaporator as a liquid-rich, vapor/liquid mixture and the evaporation portion of the cycle is repeated.
Absorption systems have evaporators and condensers similar to conventional compression systems, but use thermal energy to boil a solution to liberate the refrigerant. Absorption systems usually have higher operating and capital costs, but can be used for applications where waste heat is readily available. An absorption chiller keeps the water at a pressure so low the water boils at the cooling temperature, eliminating the need for conventional refrigerants. Water-based lithium bromide or ammonia solutions are used as the refrigerant.
In both cases, separate cooling water circuits provide cool water to air handlers and provide warm water to cooling towers.Absorption chillers are much less efficient with Coefficient of Performance (COP) ratings of 0.6 to 1.0, compared to over 5.0 COP for vapor compression chillers.
Four types of compressors are commonly used: centrifugal, reciprocating, screw or rotary. The type of chiller chosen is dependent on the amount of cooling required, usually expressed in tons (12,000 Btu/h).
Centrifugal chillers range from 200 to 6,000 tons and dominate the market. They are the most efficient type, with efficiencies of 0.6 kW/ton and lower. They usually last for 20 to 30 years.
Chillers using reciprocating compressors are generally found in the 15 to 200 ton range, with efficiencies in the 0.8 to 1.0 kW/ton for water-cooled units and 1.0 to 1.5 kW/ton for air-cooled designs. These chillers typically last for 12 to 15 years.
Screw type or rotary compressors are typically used in systems over 70 tons and can reach capacities of up to 1,000 tons. Efficiencies for water-cooled screw compressors range from 0.55 to 0.75 kW/ton, while air-cooled units are considerably less efficient at 1.1 to 1.3 kW/ton. Scroll compressors dominate the smaller chiller sizes below 25 tons.
There are also gas-driven centrifugal compressors, although they occupy only a small percentage of the market. Engine-driven centrifugal chillers include a variable-speed power source that enables compressor capacity control down to 35 percent of load. Waste heat can be recovered to drive desiccant or absorption systems.
If you are considering a chiller replacement, identify potential cooling load reductions prior to sizing the new system. Your building cooling requirements may have changed since your last chiller installation.
Chillers typically operate less efficiently at part load, so peak times and cooling load conditions are important issues to consider when sizing a new system. For example, a facility that needs 500 tons of cooling at peak load 30 days out of the year, but only 250 tons for the remainder of the year, should choose a system that performs well in the 50 percent range.
If your system is 10 years old or older, it may be a good candidate for replacement. Newer models are significantly more efficient and there may be refrigerant replacement issues to address. Many older chiller systems use harmful refrigerants that are no longer produced due to environmental regulations. These older refrigerants will become increasingly expensive and difficult to obtain.
Image source: U.S. Department of Energy
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