In this column, we will look at local cold spots or frost where the restriction occurs and issues related to bubbles in a sight glass.
Just to re-establish the situation, we have a system with a TXV valve, filter drier, and sight glass. The refrigerant is R-134a.
Here are the measured values:
• Compressor discharge temperature is 125°F.
• Condenser outlet temperature is 70°.
• Evaporator outlet temperature is 30°.
• Compressor inlet temperature is 60°.
• Ambient temperature is 70°.
• Box temperature is 30°.
• Compressor voltage is 230.
• Compressor amperage is low.
• Lowside (evaporating) pressure is 1.8 pounds per square inch gauge at –10°.
• Highside (condensing) pressure is 95 psig at 85°.
Calculated values in °F are:
• Condenser split: 15
• Condenser subcooling: 15
• Evaporator superheat: 40
• Compressor superheat: 70.
COLD SPOTNow let’s look at the symptom of a 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 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 a temperature difference across it with their hands. The truth is that humans can distinguish a temperature difference of more than 10° across something. But this is only if the temperature differences are a little higher than their body temperature of 98.6°.
A filter drier in a R-134a system with a condensing temperature of 110° would need about 20-psi pressure drop to exhibit a 10° temperature difference. Because of this, many filter driers restrictions go unchecked by technicians because of them being difficult to sense by touch and feel. The use of a sight glass after the filter drier to show any flashing will assist the technician. This same sight glass will assist in system charging. A moisture indicating sightglass will alert the technician if the system is contaminated with moisture by changing colors.
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 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. The bubbling is caused from 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 sightglass 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 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 or 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 refrigerant 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 sightglass will only be partially filled. So, do not confuse a low refrigerant 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. Standard filter driers can be purchased with Shrader 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 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 kept charging refrigerant trying to clear up the sight glass.
Systematic troubleshooting with a system checklist is the only sure way to find the actual cause of any system problem.