High superheats: Both evaporator and compressor superheat will be high. This is due to 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 TXV and 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: With both the evaporator and compressor being starved of refrigerant, the condenser will be too. 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 to a bit high 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 from the restriction. The 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 location, subcooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. A receiver bypass is nothing more than a liquid line solenoid valve controlled by a thermostat, which will bypass liquid around the receiver to the liquid line.

Low condenser splits: Because the condenser is being starved, there is not much heat to reject. This will cause low condenser splits. Remember, the split is the temperature difference between the condensing temperature and the ambient.

Local cold spot or frost where restriction occurs: 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 on the liquid line about 12 inches before the entrance of the TXV should not be colder than the ambient air surrounding it. If it is, there is a restriction somewhere upstream.

There are a lot of times when a filter-drier or line may be partially plugged and technicians cannot feel a temperature difference across it with their hands. The truth is that humans can distinguish a temperature difference of more than 10° across something only if the temperature differences are a little higher than their body temperature (98.6°). A filter-drier in an R-134a system with a condensing temperature of 110° would need about a 20-psi pressure drop to exhibit a 10° temperature difference. Because of this, many filter-drier restrictions go unchecked by technicians.

Figure 1.

Bubbles in 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 startup of 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 by a 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 the 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.

A sight glass on the liquid line before the TXV would also let the technician know if any liquid flashing is occurring before the valve. 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 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 (Figure 1).

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 that static and friction losses in the liquid line are too great.

On the other hand, a low-refrigerant-flow rate sight glass indicates that the system is about ready to cycle off because the box temperature has pulled down to a low-enough temperature. At these times 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 one-fourth to one-half full with no entrained bubbles (Figure 2). This situation is especially true with horizontal liquid lines.

Figure 2.

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-refrigerant-flow-rate sight glass with a sight glass that has bubbles entrained in the liquid.

A sight glass after the filter-drier is a good way to tell if the drier is starting to plug, because refrigerant will flash from the added pressure drop in the restricted drier. Standard filter-driers can be purchased with Schrader valves (pressure taps) on their inlets and outlets. A pressure drop of more than 2 psi measured with the same gauge means that the drier has started to restrict.

Also, as mentioned before, a sight glass right before the TXV will surely tell the technician if liquid flashing is occurring there.

Just because the sight glass is bubbling, that doesn't necessarily mean there is an undercharge, so don't automatically add refrigerant. A lot of systems are found with the receiver completely filled with liquid because the service technician kept charging refrigerant, trying to clear up the sight glass.

Low amp draw: Because the compressor is being starved of refrigerant from the restriction 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 of the low-pressure control: The LPC 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.

Systematic troubleshooting with a system checklist is the only sure way to find the actual cause of any system problem.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at tomczykj@tucker-usa.com.

Publication date: 06/05/2006