After Leaving The Compressor Where Does The Refrigerant Flow Refrigerant Pump Cavitation

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Refrigerant Pump Cavitation

When dealing with refrigerant pumps, it is important to understand that while pumps in other types of systems pump liquids at a steady state such as water or oil, refrigerant pumps pump boiling liquids. When a pump designed to handle liquids is supplied with a mixture of liquid and gas, it is said to be cavitation. Most any pump can tolerate a certain amount of cavitation, but extreme cavitation is detrimental.

To understand the complexities involved in refrigerant pumping, a solid understanding of the relationship between pressure and temperature with the refrigerant and, by extension, subcooling is essential.

Simply put; There is a direct correspondence between an increase in the boiling temperature or a decrease in pressure of any liquid. An often overlooked dynamic in refrigeration systems is that, generally speaking, pressures can fluctuate very rapidly as a result of the compressor starting or loading (causing a pressure drop) or evaporating on the evaporator line (causing a pressure increase). . The condition that tracks the pressure fluctuations but never changes as quickly, is the coolant temperature.

The liquid supply for the refrigerant pumps is the pump separator, also known as the low pressure receiver (LPR). Under most ideal conditions the liquid in the LPR will be saturated. This means that its actual temperature is equal to its boiling point; However this will almost never happen in a working refrigeration system. Even in a saturated liquid, some gas bubbles are trapped, because a small amount of heat produces steam; However, the escape of vapor from the liquid causes a pressure increase which, if not interfered with, increases the boiling temperature and reduces the rate of vapor formation.

Although the liquid in the LPR is at an actual temperature lower than its boiling point and therefore does not boil, the possibility of cavitation still exists. The liquid refrigerant must flow through the pipe to reach the pump suction. That pipe will typically be fitted with a valve, possibly a strainer, and some fittings, each of which will provide some degree of pressure relief.

A good pump installation includes the following practices to minimize the effect of gas entering the pump.

• The LPR and associated piping are well insulated to limit the amount of ambient heat transmitted to the refrigerant.

• Valves and fittings are sized to provide the lowest usable pressure drop for the desired flow rate.

• Pumps are mounted well below the liquid level in 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 .3 PSI per vertical foot, and the column is -40°. F R-22 weighs approx.66 PSI per vertical foot. For comparison, water weighs approximately .5 PSI per vertical foot. If the pump centerline is 6 feet below the liquid level in the LPR and the refrigerant is R-22 at -40°F, the pressure at the pump inlet will be approximately 4 PSI with no pump. On, because there is no current. As soon as the pump is turned on, the flow is started. Flow cannot occur without a pressure drop. If the piping is well insulated and the fittings and valves are sized to minimize restriction, the pressure will drop, resulting in boiling. This minor 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 accordingly. For example; If the boiling point of the refrigerant is -40° and the actual temperature is also -40°, no boiling will occur. A liquid is said to be saturated. If the pressure is then reduced to a value corresponding to the boiling temperature of -45°, the refrigerant will immediately boil, since its actual temperature (-40°) is 5° higher than its boiling temperature (-45). A rapid drop in pressure will cause violent boiling, causing cavitation to interfere with proper operation of the pump.

Cavitation will, at a minimum, reduce the amount of liquid delivered to the evaporator because it lowers the pump discharge pressure. If it is severe, the flow rate will decrease to the point where there is little or no liquid coming out of the pump. If the pump is hermetic, with a canned motor (refrigerant cooled) and refrigerant lubricated bearings, a lack of refrigerant liquid will result in damage or failure of the pump if it 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 common is the low differential pressure switch.

With the above in mind, it is important that the suction pressure is never allowed to drop at such a rate as to result in the type of violent boiling described above. If the compressor is microprocessor controlled, it may have a ramp feature that can limit the rate at which the compressor can load in terms of pressure drop per unit time. The characteristics of any given installation will determine the rate at which the pressure can be reduced without creating harmful cavities. Start at a conservative rate, such as 1 PSI every minute. This may seem slow, but it means that starting a system at 50°F with R-22 will take about 1 1⁄2 hours to bring it down to -40°F, which is very reasonable. It is also useful to set the controls so that compressor loading is slower and unloading is faster (regardless of ramp settings). For example, set the capacity control so that the compressor goes to at least 100% in less than 2 minutes. Set the unloading so that it takes at least a minute or less to go back from 100%. With these or similar settings, violent boiling will be less likely. When faced with a 4-hour freeze cycle or 8-hour cooling time, gradually adding compressor capacity does not significantly affect the required refrigerating time, and the value of the positive effect on LPR and refrigerant pumps cannot be overstated.

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