When refrigeration service technicians respond to one of their clients’ service calls, they tend to find the same issues over and over. The most common, and obvious, reason for a service call is a space temperature that is too warm. And, just as common, is suction pressure and/or head pressure being involved in the cause. Often, the refrigeration system will still run, but it’ll run inefficiently because the suction pressure is too high while the head pressure is simultaneously too low.
This poses a risk to product quality, integrity, and safety.
On the outset, this exact issue may not be easily observed because, again, the system will call for cooling as it should but it won’t be doing so in an effective or efficient manner. In such cases, products will spoil quicker if refrigerated, and not freeze as solidly if frozen. According to an article recently published in ACHR News, there are three main reasons a compressor will suffer from conflicting suction and head pressures.
LEAKY COMPRESSOR VALVE
A compressor valve may start to leak for a variety of reasons: the valve is overheated, warped, or has carbon and/or sludge deposits on them, which prevent a proper seal. There are a plethora of causes for these:
- Refrigerant migration problems;
- Refrigerant flooding problems;
- Acids and/or sludge in the system;
- An incorrectly set thermostatic expansion valve (TXV), which results in no superheat or too high superheat;
- An undercharge of refrigerant (high superheat);
- Overheating of the compressor;
- Slugging of refrigerant and/or oil; and
- Moisture and heat causing sludging problems.
In the ACHR News article, author John Tomczyk lays out an example service call with this scenario:
“Let’s consider a service call scenario in which the valves in a refrigeration system’s compressor are not sealing properly. The service technician measures the system’s temperatures and pressures and then calculates the condenser split and the condenser subcooling along with the evaporator and compressor superheat values.
Both the measured and calculated values are shown here:
Measured Values
Compressor discharge temp: 280°F
Condenser outlet temp: 75°
Evaporator outlet temp: 25°
Compressor inlet temp: 55°
Refrigerated space temp: 25°
Compressor amperes: low
Low side pressure: 11.6 psig/10°
High side pressure: 95.0 psig/85°
Ambient temp: 80°
Calculated Values
Condenser split: 5°
Condenser subcooling: 10°
Evaporator superheat: 15°
Superheat at compressor (total): 45°
In analyzing the system in the given example, it’s important to understand how higher-than-normal discharge temperatures affect lubricants. The discharge temperature is measured 2 inches from the compressor. This would mean that the actual discharge valve temperature would be approximately 355° (280° + 75°, as adding 75° to the discharge line temperature reading will give the technician an approximate discharge valve temperature).
Mineral oil lubricants will start to decompose at 350° and polyolester lubricants at 400°. Any increase in temperature above these points causes polymerization of the oil. Polymerization is where the lubricant’s molecules start to combine into larger and larger molecules. The end product is thick, dark oil; then sludge; and finally a solid powder. This is referred to as oil breakdown. Compressor bearing failure and loss of lubrication to moving parts will occur from plugged oil inlet screens and oil galleys (lubrication passages) that are manufactured within the compressor.”
There are a range of symptoms that your refrigeration system can suffer from as a result of this type of issue, including, but not limited to:
- Higher-than-normal discharge temperatures;
- Low condensing (head) pressures and temperatures;
- Normal to high condenser subcooling;
- Normal to high superheats;
- High evaporator (suction) pressures; and
- Low amp draw.
A discharge valve that isn’t sealing properly because it has been damaged or sludged will cause the head pressure to be low. The reason is that refrigerant vapor will be forced out of the cylinder and into the discharge line during the upstroke of the compressor. Another issue stemming from this, is that on the downstroke the refrigerant that has been compressed is drawn back into the cylinder and will cause a spate of short-cycling.
WORN COMPRESSOR RINGS
In the event of worn compressor rings, a system will suffer from many of the same symptoms that are caused by a leaky valve. The causes are, of course, different; with worn compressor rings, discharge gases leak through the rings during upstroke and cause the lower head pressure. Because of the leaks through the rings, the suction pressure will also be higher than normal. This pressure-change cause is slightly easier to diagnose, however, and can be observed in a thorough physical inspection of the HVACR unit.
LEAKY OIL SEPARATOR
An oil separator functions in a pretty simple manner: when the oil level is high enough to raise a float, an oil return needle is opened and the oil is returned to the crankcase. The pressure difference between the high and low sides of the refrigeration system is the driving force for the oil to travel from the separator to the crankcase. In the system’s layout, the separator is in the high side, and the crankcase in the low side. Because only a small amount of oil is needed to raise the float mechanism, it can be expected that only that small amount is ever absent from the crankcase at any given time.
“When the ball and float mechanism on an oil separator goes bad, it may bypass hot discharge gas directly into the compressor’s crankcase. The needle valve also may get stuck partially open from grit in the oil. This will cause high pressure to go directly into the compressor’s crankcase and cause high low-side pressures and low high-side pressures.”
For more in-depth analysis of the symptoms your system can experience from these causes of pressure differentials, read the full article at ACHR News.
