Higher-than-normal discharge temperature. High discharge temperatures are caused from high compressor superheats. A starved evaporator will cause the high superheats. High compression ratios from the low evaporator pressure will cause high heat of compressions, thus high discharge temperatures.

High superheats. Both evaporator and compressor superheats will be high. This is caused by the TXV, evaporator, and compressor being starved of refrigerant from the liquid line restriction. Most of the refrigerant will be in the receiver, with some in the condenser.

Low evaporator pressures. The low evaporator pressure is caused by the compressor being starved of refrigerant. The compressor is trying to draw refrigerant from the evaporator through the suction line, but the liquid line restriction is preventing refrigerant from entering the evaporator. This will cause the compressor to put the evaporator in a low pressure situation.

Low condensing pressures. Since both the evaporator and compressor are being starved of refrigerant, so will the condenser. Reduced refrigerant to the evaporator will cause a reduced heat load to be delivered to the condenser. The condenser, in turn, does not have to elevate its temperature and pressure to reject heat.

Normal condenser subcooling. Since the condenser is being starved, it is not condensing much vapor to liquid. All of the liquid in the condenser will probably sit there for a while and subcool because of the low refrigerant flow caused by the restriction. This receiver will have a reduced flow in and out of it. Most of the refrigerant will be in the receiver with some in the condenser. If the receiver is in a hot ambient, subcooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. Receiver bypasses are nothing more than a liquid line solenoid valve controlled by a thermostat which will bypass liquid around the receiver to the liquid line or liquid header.

Low condenser splits. Because the condenser is being somewhat starved, there is not much heat to reject. This will cause low condenser splits.

Local cool spot or frost after the restriction. Liquid refrigerant flashing to vapor might occur at the restriction if the restriction is severe enough. Simply running your hand along the liquid line and on the filter drier may find a local cold spot. A thermistor or thermocouple on the liquid line about 12 inches before the entrance of the TXV should not be colder than the ambient that surrounds it. If it is, there is a sure restriction somewhere upstream.

There are a lot of times when a filter-drier or line may be partially plugged and technicians cannot feel the temperature difference across it with their hands. The truth is that humans can distinguish a temperature difference of more than 10 degrees across something, but this is only if the temperature differences are a little higher than their body temperature, which is 98.6 degrees F. A filter-drier in an R-134a system with a condensing temperature of 110 degrees would need about 20-psi pressure drop to exhibit a 10-degree temperature difference. R-22 would need about a 30-psi pressure drop to exhibit a 10-degree temperature difference. Because of this, many filter-drier restrictions go unchecked by technicians because of difficultly in sensing by touch and feel. The use of a sight glass after the filter-drier to show the flashing will assist the technician. This same sight glass will assist in system charging. A moisture-indicating sight glass will alert the technician if the system is contaminated with moisture by changing colors.

Bubbles in sight glass if restriction is before the sight glass. With a restriction in the liquid line before the sight glass, bubbles are sure to occur in the sight glass. Many technicians believe that a bubbling sight glass means nothing but an undercharge of refrigerant. This is simply not true. On start-up on some refrigeration systems when there is a large load on the system, bubbling and flashing could occur in the sight glass downstream of the receiver. This bubbling is caused from the pressure drop at the entrance of the outlet tube of the receiver. Bubbling could also occur during rapid increases in loads. The TXV could be opened wide during an increase in load and some flashing could occur even though the receiver has sufficient liquid. Also, sudden changes in head pressure control systems, which may dump hot gas into a receiver to build up head pressure, often will bubble a sight glass even though there is sufficient liquid in the receiver to form a seal on the receiver’s dip tube outlet. A sight glass on the receiver would prevent technicians from overcharging in this case, but would cost the manufacturer a bit more money initially.

Many newer refrigerant blends such as R-410A and R-407C will flash some of their liquid when passing through the increased volume of a sight glass in a liquid line. This will appear as bubbles in a sight glass. However, once this small percentage of vapor leaves the sight glass and re-enters the liquid line again, it will form liquid again. So, the service technician should never try to charge a system using certain blends to a clear sight glass. This may only lead to overcharging the system.

A sight glass on the liquid line before the TXV would also help let the technician know if any liquid flashing is occurring before the TXV. This flashing could be from loss of subcooling or too much static and/or friction pressure drop in the liquid line before it reaches the TXV.

There is a big difference between a bubbling sight glass and a low flow rate sight glass. If bubbles are entrained in the liquid, this is a sign of a pressure drop causing liquid flashing, or an undercharge of refrigerant causing vapor and liquid to exit the receiver because of no subcooling. Remember, the condenser subcooling will be low if an undercharge is causing the bubbling of the sight glass. Otherwise, the bubbling sight glass could mean a restricted liquid line, restricted filter-drier, loss of receiver or liquid line subcooling from a hot ambient, or static and friction losses in the liquid line are too great.

On the other hand, a low flow rate sight glass is an indication that the system is about ready to cycle off because the box temperature has pulled down to a low enough temperature. It is at these times that the system is at its lowest heat loads and the refrigerant flow rate through the system will be the lowest. The sight glass may be only 1/4 to 1/2 full with no entrained bubbles. This situation is especially true with horizontal liquid lines. Do not add refrigerant in this situation because you will overcharge the system. The overcharge will be noticed at the higher heat loads. The low heat loads cause the system to be at its lowest suction pressure, thus the density of refrigerant vapors entering the compressor will be lowest. Because of the lowest evaporator pressures, the compression ratio will be high, causing low volumetric efficiencies, thus low refrigerant flow rates. There is usually plenty of subcooling in the condenser but the sight glass will only be partially filled. So, do not confuse a low flow rate sight glass with a bubbly sight glass which has bubbles entrained in the liquid.

A sight glass after the filter- drier is a good method to tell if the drier is starting to plug, because of the refrigerant flash from the added pressure drop in the restricted drier. As mentioned before, a slight glass right before the TXV will surely tell the technician if liquid flashing is occurring there. Just because the sight glass is bubbling doesn’t necessarily mean an undercharge, so do not automatically add refrigerant. A lot of systems are found with the receiver completely filled with liquid because the service technician keeps charging the refrigerant trying to clear up the sight glass.

Low amp draw. Because the compressor is being starved of refrigerant from the restrictions in the liquid line, it will not have to work as hard in compressing what vapors do pass through it. The low density of the vapors from the low evaporator pressure will require less work from the compressor, requiring a low amp draw.

Short cycle the low pressure control. The low pressure control will cycle the compressor off and on from the low evaporator (suction) pressures. Once off, refrigerant will slowly enter the evaporator and cycle the compressor back on. This on and off of the compressor will continue until the problem is fixed.

Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at tomczykj@tucker-usa.com (e-mail).

Publication date: 12/02/2002