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Best Strategies for Optimizing Absorption Chiller Performance

Key Points
  • Absorption chillers can be a cost-effective alternative to traditional cooling systems.
  • Proper maintenance is required to achieve a long service life.
  • Major areas of maintenance include mechanical components, heat transfer components and controls.
Adsorption Chiller

Absorption chillers are a cost-effective alternative to traditional cooling systems in certain applications. Although natural gas is often used to fuel absorption units, facilities also can use waste heat as a no cost fuel source for improving overall energy efficiency. The major advantages of absorption chillers include:

  • Lower fuel costs (natural gas costs less and waste heat is free)
  • Minimize peak electric demand charges by taking advantage of off-peak electric rates
  • Reduce your environmental footprint since no hazardous refrigerants are used
  • Flexibility with dual fuel capability
  • Eliminate the need for a separate boiler (and the extra floor space) if direct-fired chiller is chosen
  • High reliability and low maintenance because there are fewer moving parts

As with all equipment, proper maintenance procedures are still required. If well-maintained, an absorption chiller has an expected service life of 20 to 25 years.

What to look for

There are three primary maintenance areas: mechanical components, heat transfer components and controls. Any unusual noises and vibrations may indicate damage to mechanical components. The following highlights what other problems to investigate:

  • Pump shaft seals are a potential source of air leakage and should be examined for wear every three years; hermetic pump designs avoid this problem.
  • Monitor the condition of the pump's internal bearings; cleaning and/or replacement can restore the pump’s performance. Adjust the belt as needed.
  • Check for refrigerant leaks. For most applications, a loss rate of less than one percent is considered normal. Test the lithium bromide for solids content and viscosity.
  • Keep heat transfer surfaces free of scale (removed chemically) and sludge (removed mechanically) by maintaining proper cooling tower water chemistry; failure to properly treat this water could void warranties.
  • Inspect the water treatment system for proper operation. Test chilled and condenser water for pH and corrosion inhibitor levels to combat corrosion and fouling.
  • Purge noncondensable gases to maintain performance using a vacuum pump. High purge unit run times (one hour per week is normal) indicate an air leak in the absorber or its piping.
  • Flood the vacuum pump with oil during seasonal shutdown to prevent internal corrosion. Check the oil level and change oil as necessary.
  • Check heat exchanger tubes for any pitting, corrosion or cracking by using an eddy current tester. Eddy currents are produced when an electromagnetic field interacts; changes in current indicate defects.
  • Calibrate controls; test chiller safeties; inspect wiring, starters and disconnects to assure a solid connection.

Buy a new one

If you notice your chiller's maintenance costs are becoming astronomical and performance is rapidly deteriorating, you may need to upgrade to a new unit. One manufacturer now offers a chiller with a coefficient of performance (COP) of 1.5 in the output range of 281 kW to 3,516 kW. COP is the ratio of cooling capacity to heat input and 1.5 is claimed to be the highest available for a double-effect chiller. The high COP is made possible by incorporating a high-performance plate heat exchanger, two-stage evaporation/absorption structure; high-efficiency heat exchanger tubes and exhaust gas heat exchanger. The installed cost of an absorption chiller typically ranges from $700/ton (single-effect or gas-fired) to $1,200/ton (double-effect).

Operational efficiency can be improved by selecting a model with excellent part-load efficiency if your system is operated under part-load for an extended period of time. For cooling loads ranging from 10 percent to 100 percent of the design capacity, chillers are available with modulating burners in order to maintain the temperature of the cold water leaving the chiller. This ensures better part-load performance.

Also make sure you select the right size. All chillers operate at reduced efficiency levels if they are oversized for the application; serious operational problems occur if they run for extended periods of time at low loads. If your current chiller is oversized, it must be replaced with a properly sized one.

Another way to optimize performance is to take advantage of multiple energy sources to drive the chiller and produce cold water: waste heat from the gas engine's jacket and exhaust, and/or direct firing using a supplemental burner. For a 500-kW gas engine, cooling capacity from the exhaust and jacket water combined ranges from 175-200 kW (50-60 tons).