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.
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.
Hermetic, refrigerant cooled and lubricated pump assemblies. The hermetically sealed motor drives the solution and refrigerant pump impellers. In this multiple pump arrangement, motor coolant and lubrication is by the fluid being pumped. Hermetic pump designs eliminate the need for external shaft seals – a maintenance item and potential source of air leakage.
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.
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.
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.