Coolant pump cavitation
When it comes to coolant pumps, it’s important to understand that, unlike pumps in other types of systems that pump steady-state liquids like water or oil, coolant pumps pump boiling liquid. When a pump designed to handle liquids receives a mixture of liquid and gas, it is said to cavitate. Most pumps can tolerate a certain amount of cavitation, but it is detrimental if it is extreme.
To understand the complexities involved in refrigerant pumping, one must have a firm understanding of the pressure and temperature relationship with refrigerants and, by extension, subcooling.
Simply stated; The boiling temperature of any liquid rises and falls in direct correspondence with any increase or decrease in pressure. The often overlooked dynamic in a refrigeration system is that, generally speaking, pressures can fluctuate very rapidly as a result of a compressor starting or loading (causing pressure to drop), or a evaporator that connects to the line (causing pressure to rise) . The condition that tracks pressure fluctuations but never changes that fast is coolant temperature.
The liquid supply for coolant pumps is the pump separator, also known as the low pressure receiver (LPR). Under the most ideal conditions, the liquid in the LPR would be saturated. This means that its actual temperature is equal to its boiling temperature; however, in a working refrigeration system, this would almost never be the case. Even a saturated liquid will have some entrained gas bubbles, because the slightest amount of heat will create steam; however, as vapor is released from the liquid, an increase in pressure occurs which, without interference, will raise the boiling temperature and reduce the rate of vapor generation.
Even if the liquid in the LPR is at an actual temperature below its boiling point and therefore does not boil, there is still the possibility of cavitation. The liquid refrigerant must flow through a pipe to reach the suction of the pump. That pipe will usually be equipped with a valve, possibly a filter, and some fittings, each of which will cause a certain amount of pressure drop.
A good pump installation incorporates the following practices to improve the effect of trained gas entering the pumps.
• The LPR and associated piping are well insulated to limit the amount of ambient heat that is transmitted to the refrigerant.
• Valves and accessories are sized to create the least amount of pressure drop possible for the expected flow rate.
• The pumps are mounted well below the liquid level in the LPR, to take advantage of the effect of gravity. The pressure at the pump inlet will increase in direct proportion to the height of the “column” of liquid above it. A column of -40°F ammonia weighs approximately 0.3 PSI per vertical foot, and a column of -40°FF R-22 weighs approximately 0.66 PSI per vertical foot. For comparison, water weighs about 0.5 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 pump inlet will be about 4 PSI when the pump is not running, because there is no flow. As soon as the pump is turned on, the flow starts. There can be no 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 resulting boiling. This small amount of boiling will not interfere with the proper operation of the pump.
When the pressure of the refrigerant decreases, the boiling temperature (not the actual temperature) will decrease correspondingly. For example; if the boiling temperature of the coolant is -40° and the actual temperature 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 temperature of -45°, the refrigerant will boil immediately, because its actual temperature (-40°) is 5° warmer than its boiling temperature (-45). A rapid drop in pressure will cause violent boiling, making it more likely that cavitation will interfere with proper pump operation.
Cavitation will, at a minimum, decrease the amount of liquid being sent to the evaporators as it causes the pump discharge pressure to decrease. If severe, the flow rate will decrease to the point where there is little or no liquid flow through the pump. If the pump is hermetic, with a canned motor (cooled by refrigerant) and bearings lubricated by refrigerant, the lack of liquid refrigerant will cause damage or failure if the pump continues to run. Most coolant pumps will be protected by one or more devices that will automatically stop the pump in the event of severe cavitation. The most common is a low differential pressure switch.
With the above in mind, it is important that the suction pressure is never allowed to drop to a rate that will result in the type of violent boiling described above. If the compressor is microprocessor controlled, it probably has a ramp function that can limit the rate at which the compressor can load in terms of pressure drop per unit of time. The details of any given installation will determine the rate at which pressure can be lowered without detrimental cavitation. Start at a conservative rate, such as 1 PSI per minute. This may seem slow, but it means that starting a system with R-22 at 50°F would require about 1 1/2 hours to get to -40°F, which is quite reasonable. It is also useful to set the controls so that the compressor loads gradually and unloads more quickly (regardless of the ramp setting). For example, set the capacity control so that the compressor goes from minimum to 100% for a period of no less than 2 minutes. Set the download so that the trip from 100% to minimum takes a minute or less. With these or similar configurations, violent boiling will be less likely to occur. When it comes to a 4 hour freeze cycle or 8 hour chill time, slowly adding compressor capacity does not appreciably affect the required chill time, and the value of the positive effect on the LPR and refrigerant pumps does not can be exaggerated.