When dealing with refrigerant pumps, it is leading to understand that unlike pumps in other types of systems that are pumping steady state liquids like water or oil, refrigerant pumps are pumping boiling liquid. When a pump that is designed to deal with liquids is supplied with a combination of liquid and gas, it is said to cavitate. Most any pump can tolerate a sure whole of cavitation but it is detrimental if at all extreme.
To understand the complexities involved in pumping refrigerant, one must have a firm grasp of the relationship between pressure and climatic characteristic with refrigerants, and by extension, sub cooling.
"Differential Pressure"
Simply stated; the boiling climatic characteristic of any liquid rises and falls in direct correspondence with any growth or decrease in pressure. The often overlooked dynamic in a refrigeration law is that ordinarily speaking, pressures can fluctuate very rapidly as a supervene of a compressor coming on or loading up (causing pressure to drop), or an evaporator being brought on the line (causing pressure to rise). The health that tracks pressure fluctuations but never changes as quickly, is refrigerant temperature.
The contribute of liquid for the refrigerant pumps is the pump separator, also referred to as the low pressure receiver (Lpr). Under the most ideal conditions the liquid in the Lpr would be saturated. This means that its actual climatic characteristic is equal to its boiling temperature; however in a working refrigeration law this would almost never be the case. Even a saturated liquid will have some gas bubbles entrained, because the slightest whole of heat will originate vapor; however as vapor is released from the liquid it causes an growth in pressure which un-interfered with will raise the boiling climatic characteristic and reduce the rate of vapor generation.
Even if the liquid in the Lpr is at an actual climatic characteristic lower than its boiling point, and therefore not boiling, the possibility of cavitation still exists. The liquid refrigerant must flow through a pipe to get to the pump suction. That pipe will ordinarily be fitted with a valve, possibly a strainer, and some whole of fittings, each of which will cause some whole of pressure drop.
A good pump factory incorporates the following practices to ameliorate the supervene of entrained gas entering the pumps.
• The Lpr and associated piping are well insulated, to limit the whole of ambient heat transmitted into the refrigerant.
• Valves and fittings are sized to originate the smallest whole of pressure drop as is practicable for the improbable flow rate.
• The pumps are mounted well below the liquid level in the Lpr, to take advantage of the supervene of gravity. The pressure at the inlet of the pump will growth in direct proportion to the height of the "column" of liquid above it.A column of -40°F ammonia weighs approximately.3 Psi per vertical foot, and a column of -40°F R-22 weighs approximately.66 Psi per vertical foot. For comparison, water weighs approximately.5 Psi per vertical foot. If the centerline of the pump is 6 ft. Below the liquid level in the Lpr, and the refrigerant is R-22 at -40°F, then the pressure at the inlet of the pump will be almost 4 Psi when the pump is not running, because there is no flow. As soon as the pump is turned on, flow is initiated. There cannot be flow without pressure drop. If the piping is well insulated, and the fittings and valves are sized correctly for minimum restriction, the pressure drop will be slight, as will the resultant boiling. This minor whole of boiling will not interfere with permissible performance of the pump.
When the pressure of the refrigerant decreases, the boiling climatic characteristic (not the actual temperature) will decrease correspondingly. For example; if the boiling climatic characteristic of the refrigerant is -40°, and the actual climatic characteristic is also -40°, there will be no boiling. The liquid is said to be saturated. If the pressure is then lowered to a value that corresponds to a boiling climatic characteristic of -45°, the refrigerant will immediately boil, because its actual climatic characteristic (-40°) is 5° warmer than its boiling climatic characteristic (-45). A rapid decrease in pressure will supervene in violent boiling, making it more likely that cavitation will interfere with exact performance of the pump.
Cavitation will at a minimum, decrease the whole of liquid being delivered to the evaporators as it causes the pump dismissal pressure to decrease. If it is severe, the rate of flow will decrease to the point where there is exiguous or no flow of liquid through the pump. If the pump is hermetic, with a canned motor (refrigerant cooled) and refrigerant lubricated bearings, the lack of refrigerant liquid will cause damage or failure if the pump continues to operate. Most refrigerant pumps will be protected by one or more devices that will automatically stop the pump in the event of severe cavitation. The most tasteless is a low differential pressure switch.
With the above in mind, it is leading that the suction pressure never be allowed to drop at a rate that will supervene in the type of violent boiling described above. If the compressor is microprocessor controlled, it will likely have a ramp highlight that can limit the rate at which the compressor can load in terms of pressure decrease per unit of time. The specifics of any given factory will resolve the rate at which the pressure can be decreased without detrimental cavitation. Start at a conservative rate, such as 1 Psi every minute. This may sound slow, but it means that beginning a law with R-22 at 50°F would want about 1 1⁄2 hours to bring to -40°F, which is quite reasonable. It is also helpful to set controls so that compressor loading occurs moderately and unloading occurs more swiftly (regardless of ramp settings). For example, set the capacity control so the compressor goes from minimum to 100% over a period of not less than 2 minutes. Set the unloading so the tour from 100% back to minimum takes one exiguous or less. With these or similar settings, violent boiling will be less likely to occur. When dealing with a 4 hour frost cycle or an 8 hour chill time, adding compressor capacity moderately does not appreciably sway the refrigerating time required, and the value of the sure supervene on the Lpr and the refrigerant pumps cannot be overstated.
Refrigerant Pump Cavitation
