When dealing with refrigerant pumps, it is important 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 handle liquids is supplied with a mixture of liquid and gas, it is said to cavitate. Most any pump can tolerate a certain amount 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 temperature with refrigerants, and by extension, sub cooling.

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 coming on or loading up (causing pressure to drop), or an evaporator being brought on the line (causing pressure to rise). The condition that tracks pressure fluctuations but never changes as quickly, is refrigerant temperature.

The supply 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 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 gas bubbles entrained, because the slightest amount of heat will create vapor; however as vapor is released from the liquid it causes an increase in pressure which un-interfered with will raise the boiling temperature and reduce the rate of vapor generation.

Even if the liquid in the LPR is at an actual temperature 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 usually be fitted with a valve, possibly a strainer, and some number of fittings, each of which will cause some amount of pressure drop.

A good pump installation incorporates the following practices to ameliorate the effect of entrained gas entering the pumps.

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

• Valves and fittings are sized to create the smallest amount of pressure drop as is practicable for the expected flow rate.

• The pumps are mounted well beneath the liquid level in the LPR, to take advantage of the effect of gravity. The pressure at the inlet of the pump 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 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 approximately 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 amount of boiling will not interfere with 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 refrigerant 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 immediately boil, because its actual temperature (-40°) is 5° warmer than its boiling temperature (-45). A rapid decrease in pressure will result in violent boiling, making it more likely that cavitation will interfere with correct operation of the pump.

Cavitation will at a minimum, decrease the amount of liquid being delivered to the evaporators as it causes the pump discharge pressure to decrease. If it is severe, the rate of flow will decrease to the point where there is little 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 common is a low differential pressure switch.

With the above in mind, it is important that the suction pressure never be allowed to drop at a rate that will result in the type of violent boiling described above. If the compressor is microprocessor controlled, it will likely have a ramp feature 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 installation will determine 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 starting a system with R-22 at 50°F would require 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 gradually and unloading occurs more quickly (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 travel from 100% back to minimum takes one minute or less. With these or similar settings, violent boiling will be less likely to occur. When dealing with a 4 hour freeze cycle or an 8 hour chill time, adding compressor capacity slowly does not appreciably affect the refrigerating time required, and the value of the positive effect on the LPR and the refrigerant pumps cannot be overstated.

High condensing pressure is one of the most often misunderstood and misdiagnosed conditions in refrigeration systems. With a good understanding of the basics of condensing, and a few simple diagnostic steps, high condensing pressure can be easily diagnosed and corrected.

If condensing pressure rises above normal levels, the cause must be determined before taking corrective action. A few simple steps can determine the cause. First, it is necessary to understand that if the supply of condensing water is maintained at a steady temperature and flow rate, condensing pressure will fluctuate in direct correspondence to compressor load. It is therefore important to keep diligent log entries, so patterns can be recognized. The study of operating logs will establish predictable condensing temperatures under a variety of operating conditions.

NON-CONDENSABLE GASES

In the event that air or any other non-condensable gas enters the system, it will be evidenced by an increase in condensing, or discharge pressure. This increase in pressure results from two separate but related causes.

(1) The effect of combining two or more different gasses within one pressure vessel, governed by the law of partial pressures (Dalton’s Law of Partial Pressures). Simply stated, the total pressure of a gas mixture equals the sum of the partial pressures that make up the mixture. If the pressure at which refrigerant is condensing is 150 PSIG, and there is a quantity of air in the condenser that would create 20 PSIG with no refrigerant present, then the resultant pressure will be 170 PSIG. In other words, if the refrigerant was removed and the air left behind, the remaining pressure would be 20 PSIG; or if the air was removed (as with purging), the resultant pressure would be 150 PSIG.

(2) As air accumulates in the condenser, it occupies space that would otherwise be available for refrigerant. This will effectively reduce the amount of surface area available upon which refrigerant vapor can condense. Any condensing surface that is in direct contact with air cannot also be in direct contact with refrigerant. If other factors remain constant, a reduction in condensing surface will always cause an increase in condensing pressure.

Air, or any other non-condensable gas, will always flow to the condenser (and sometimes the receiver as well), regardless of how or where it entered. Once a non-condensable gas enters the condenser and receiver, it will not flow out with liquid to other parts of the system since it cannot condense. It can only be removed by purging to atmosphere. Purging from any point on the low pressure or intermediate pressure sections of a system WILL NOT remove non condensable gases.

Page 1 of 3PURGING

Halocarbons

Stop the compressor, isolate the condenser from the system, and leave the water running. This will allow any refrigerant to condense. Any non-condensable gas will remain at the top of the condenser, as the refrigerant vapor is heavier. Connect a service hose to the access valve on top of the condenser. Leave a pressure gauge connected, as it will be necessary for determining progress. Carefully open the service valve, allowing the non-condensables to discharge to atmosphere. As non-condensables are purged, the pressure inside the condenser will be reduced. When all non-condensables have been removed, the pressure (saturation temperature) will correspond to the water temperature provided that liquid refrigerant is present. If the condenser contained enough non-condensable gas to displace all of the refrigerant, the resulting pressure will be lower than saturated until the isolation valves are opened to expose the condenser to liquid refrigerant.

It is important to note that air can only enter the refrigeration system from leaks in the suction side when the system is operated in a vacuum, or when some part of the system is open for servicing. It is imperative that equipment or components that have been open for any reason be pressure tested and evacuated prior to the resumption of operation. If air is discovered and no part of the system has been open, it can be assumed that one or more leaks exist in the suction side of the system. If this is the case, then the leak(s) must be identified and repaired to prevent further ingress of air.

Ammonia

Removing air with system stopped

After pumping the system down to obtain a nearly full high pressure receiver, stop the compressor and leave the water running. This will allow any refrigerant to condense. Any air will remain above the liquid ammonia and below the ammonia vapor (which is lighter than air). Connect one end of a service hose to the access valve on top of the receiver and lead the other end to a barrel of water, securing it so it cannot come out of the water. Leave a pressure gauge connected, as it will be necessary for determining progress. Carefully open the service valve, allowing the air to discharge into the water. Air entering the water will form bubbles, while ammonia gas will be absorbed into the water without forming bubbles. As air is purged, the pressure inside the condenser will be reduced. When all air has been removed, the pressure (saturation temperature) will correspond to the water temperature.

Removing other gases lighter than ammonia with system stopped

See procedure for purging from halocarbon systems.

Page 2 of 3

WATER FLOW

At the first sign of abnormally high condensing pressure, check to ensure that there is adequate water flow through the condenser. There are three ways to verify this.

(1) The most obvious is a visual check.

(2) The flow rate can be gauged with fair accuracy by subtracting the water outlet pressure from the inlet pressure. Some condensers are furnished with a capacity chart that will give corresponding flow rates for various pressure drop values.

If a visual check is not practical, indeterminate, or inconclusive, a solid grasp of the following relationship will help rule water flow in or out.

Each pound of water circulating through the condenser has the ability to carry away one b.t.u. of heat per degree Fahrenheit of temperature rise.

For example, if the compressor is rejecting 1,000,000 b.t.u./hour to the condenser, and there is a flow rate of 200 gal./minute of 85°F water, the following relationship can be illustrated.

200 gal./minute X 60 = 12,000 gal./hour X 8.33 lbs./gal. = 99,960 lbs./hour

1,000,000 b.t.u./hour (rejected heat) ÷ 99,960 lbs./hour (water flow) = 10°F (10.004) water temperature rise across the condenser. Therefore 85°F entering water would exit at 95°F.

If the water flow is reduced from 200 gal./minute to 100 gal./minute, the result would be:

1,000,000 b.t.u./hour (rejected heat) ÷ 49,980 lbs./hour (water flow) = 20°F (20.008) water temperature rise across the condenser. Therefore 85°F entering water would exit at 105°F.

Using this formula, any of the three variables can be solved for if the other two are known.

If condensing pressure is higher than normal with adequate water flow, (temperature of the water exiting the condenser lower than normal) it may be assumed the condensing surface is fouled. It must next be determined whether the fouling is on the inside or outside of the tubes. Stop the compressor and maintain water flow through the condenser while observing the refrigerant pressure in the condenser. After allowing temperatures to equalize, if the refrigerant pressure corresponds to a saturation temperature that equals the water temperature, the fouling is on the water side of the tubes, and they will need to be cleaned. If the pressure remains higher than the saturation temperature that equals the water temperature, then the fouling is on the refrigerant side of the tubes, and purging of non-condensables is required. It should be noted that if the symptoms indicate that the water side is fouled, there is a possibility that the tubes are clean but water is short-circuiting. It is possible in a multi-pass configuration, for water to bypass the flow dividers and exit the condenser without flowing through all the tubes. This scenario would be unusual, but this discussion would not complete without addressing that possibility.