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Absorption Chillers – Maintenance Considerations

Properly designed and installed absorption chillers can function without full time attendants. The machine can be started and brought on line with simple time clocks or energy management systems. Non-condensables are automatically purged and the operator can schedule normal routine maintenance. Obviously, local building codes may dictate that a full time operator is, or is not, required. This, in turn, is often a function of the size of the equipment, steam pressure, etc. Always consult local codes when considering these issues.

There are three primary maintenance areas: mechanical components, heat transfer components, and controls. The following segments discuss mechanical and heat transfer maintenance areas.

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Absorption Chiller Maintenance – Mechanical Components

One manufacturer’s absorption chillers has a single motor/multiple pump configuration for refrigerant and solution flow and a purge unit. Other manufacturers use individual hermetic solution and refrigerant pumps cooled and lubricated by the pumped solution. Another uses open motors with a shaft seal.

Absorption Chiller Maintenance – Heat Transfer Components

The life, performance, and cooling capacity of absorption equipment hinges on keeping heat transfer surfaces free of scale and sludge. Even a thin coating of scale can significantly reduce capacity. Therefore, cooling tower water chemistry is critical, and failure to properly treat this water could void manufacturer warranties.

Scale deposits are best removed chemically. Sludge is best removed mechanically, usually by removing the headers and loosening the deposits with a stiff bristle brush. The loosened material is then flushed from the tubes with clear water.

Absorption Chiller Maintenance – Pump Maintenance

When the electric motor and pump bearings fail, one design permits replacement of pump parts without removing the lithium bromide solution from the machine. The first step is closing the hand valves in the lubrication circuit, disconnecting the electrical supply, and removing the motor. The pump shaft seal maintains machine vacuum. Major pump repairs are accommodated by charging the machine with nitrogen to atmospheric pressure. Once complete, the machine is evacuated, and pump parts removed and repaired or replaced. Other designs require a more complicated replacement procedure.

Pump maintenance begins with the magnetic strainer which must be cleaned 2 weeks after the initial startup and at the mid-point in the cooling season. Shaft seals should be examined for wear at three year intervals.

Absorption Chiller Maintenance – Prolonged or Seasonal Shutdown

In the case of seasonal or prolonged shutdown, refrigerant may migrate from the evaporator to the absorption chiller causing a low refrigerant level in the evaporator pan and piping. Since refrigerant is used to lubricate pump and motor bearings, lubrication from an auxiliary source must be provided during the startup phase of operation. Once an operating charge of refrigerant has been recovered from the solution, the machine may be returned to normal operation.

This auxiliary circuit is usually established by connecting city water to the external connections of the pump lubrication piping. In all cases, follow the manufacturer’s recommended procedures.

Absorption Chiller Maintenance – Purging Non-condensable Gases

All absorption chillers must be purged of non-condensable gases to maintain performance. The three methods used are steam jet, solution jet (or “motorless purge”), or a vacuum pump, with the vacuum pump being by far the most common.

Non-condensable gases migrate to the area of lowest pressure in the absorption chiller (the evaporator) where a small portion of the vapor is extracted and condensed in the purge unit using cooling water. Non-condensable are then evacuated by the vacuum pump. In normal operation, the purge system should operate about one hour a week. The vacuum pump oil level should be observed, maintained, and changed as necessary. Oil purge pump motor bearings should be inspected and replaced, and the belt adjusted as needed. In addition, the vacuum pump should be flooded with oil during seasonal shutdown to prevent internal corrosion.

Purging of non-condensables can be accomplished using a “motorless purge” as shown here. Where motorless purging is used, an optional vacuum pump can also be used for evacuation.

In all cases, the operator should log purge operation and monitor purge operation trends. Increasing purge operation signals increasing in-leakage of air and moisture.