Troubleshooting Filter Driers — Part 2
How to identify and repair restrictions and damage in the liquid line
In last month’s article, “Inside Filter Driers,” which appeared on Page 18 of the Feb. 6 issue of The NEWS, we covered the internal construction, filtering, and drying materials within a refrigerant filter drier. This month, we’ll deal with troubleshooting, service, and replacement of the filter drier.
FILTER DRIER REPLACEMENT
The king valve (the three-way service valve at the liquid receiver’s outlet that controls liquid refrigerant leaving the receiver) is a good starting point when it’s time to replace a filter drier. The king valve is front-seated while the system is running, which causes all of the refrigerant from the king valve downstream through the compressor to be pumped into the condenser and receiver, so the entire filter drier or drier core can be replaced. This process is often referred to as a pump down.
After pump down and after shutting the refrigeration system off, apply a 500-micron vacuum to the pumped-down part of the system before putting the new filter drier into service. Never use a torch when removing a filter drier from a system, always cut the old filter drier out with tube cutters or the like. Torching the sweat fittings of a moisture-contaminated filter drier will only heat up the moisture-laden desiccant core, driving the moisture from the drier’s core back into the refrigeration system. When installing a new filter drier, an arrow on the filter drier’s housing will indicate which direction refrigerant is designed to flow. If installed backwards, the filter drier will become useless.
Take care not to overheat the newly installed filter drier when using a torch for brazing a sweat-type drier to the liquid line. The brazed connections must be thoroughly cleaned and fluxed when called for. The direction of the flame must be directed away from the filter drier’s body so as to not overheat it. Overheating may scorch the internal filters, drying agents, and other important parts of the dryer. A properly adjusted oxygen/acetylene flame is perfect for this kind of work.
Also, leave one part of the pumped-down section of the refrigerant line open to the atmosphere while brazing the new drier into place. This will act as a vent for any built-up pressures that occur during brazing, while, at the same time, preventing this pressure from bubbling the brazed joint full of tiny, microscopic vent holes. To prevent overheating, it is strongly suggested to wrap the filter drier’s body with a wet rag or apply a heat shield paste to the drier’s body when brazing.
Any restriction or damage to the liquid line from the receiver outlet to the metering device inlet will have symptoms similar to a restricted filter drier. Because the filter drier is located in the liquid line, a restricted filter drier is often referred to as a liquid line restriction.
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. Filter driers can be purchased with Schrader valves (pressure taps) on their inlets and outlets or just on the inlet. A pressure drop of more than 2 psi measured with the same gauge indicates the drier has started to restrict.
A sight glass right before the thermostatic expansion valve (TXV) will identify if liquid flashing is occurring there. However, just because the sight glass is bubbling doesn’t necessarily mean an undercharge, so do not automatically add refrigerant (see sidebar at left). Many systems are found with the receiver completely filled with liquid because the service technician kept adding refrigerant trying to clear up the sight glass. Also, many refrigerant blends may slightly bubble a sight glass because they contain two, three, four, or even five different refrigerants in the same blend, all having different properties and characteristics. Always consult with the refrigerant blend manufacturer for more detailed information on a refrigerant blend’s behavior.
CAUSES AND SYMPTOMS
Causes of restricted components in the liquid line include a:
- Restricted filter drier from moisture, dirt, oil, or sludge;
- Restricted TXV screen or orifice;
- Kinked liquid line;
- Kinked U-bend in liquid line;
- Restricted liquid line solder joint;
- Receiver outlet valve (king valve) partially closed off or sludged; or a
- Restricted solenoid valve.
Symptoms of a restricted liquid line include:
- A higher than normal discharge temperature;
- High superheats;
- Low evaporator pressures;
- Low condensing pressures;
- Normal to slightly high condenser subcooling;
- Low condenser splits;
- In some cases, a local cool spot or frost after the restriction;
- A low amp draw; or
- Short cycling on the low-pressure control.
Let’s look at each of these symptoms.
High discharge temperatures are caused from high compressor superheats and high compression ratios. A starved evaporator from the liquid line (filter drier) restriction will cause high superheats. High compression ratios from the low evaporator pressure will cause high heat of compression, thus high discharge temperatures. This is assuming there’s still some mass flow rate of refrigerant through the system. The severity of the restriction in the filter drier will determine how high the discharge temperature will be. If the system becomes completely restricted, the compressor will pump down the system and stay off or short-cycle at times on the low-pressure control.
Both the 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 while some will be in the condenser.
The low evaporator pressure is caused from 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. 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. Most of the refrigerant will be in the receiver.
As the condenser has a low heat rejection load, 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 in the filter drier. The receiver also will have a reduced flow in and out. 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.
Because the condenser is being somewhat starved, there is not much heat to reject from the evaporator being starved. This will cause low condenser splits. Remember, the split is the difference between the condensing temperature and the ambient.
Many technicians believe when any part of the system’s high side is restricted or plugged, head pressures will elevate. This is not true, especially on a TXV/receiver system. A restricted liquid line will starve the evaporator of refrigerant and cause low evaporator pressures. With a starved evaporator, the compressor also will be starved, and there will be little heat for the condenser to reject. This small amount of heat will cause a low condensing pressure and temperature. Most of the refrigerant will now be stored in the receiver, simulating a pumped-down refrigeration system. Because the compressor is being starved of refrigerant from the restriction in the liquid line, it will not have to work as hard to compress the remaining vapors. The low density of the vapors from the low evaporator pressure will require less work from the compressor and a low amp draw.
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.
Bubbling Versus Low-flow Sight Glass
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 bubbling in the sight glass. Otherwise, the bubbling sight glass could indicate a restricted liquid line, restricted filter drier, loss of receiver or liquid line subcooling from a hot ambient, or high static and friction losses in the liquid line.
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’s flow rate through the system will be the lowest. The sight glass may be only quarter to half 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 low evaporator pressures, the compression ratio will be high and will cause low volumetric efficiencies and 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 bubbly sight glass that has bubbles entrained in the liquid.
Publication date: 3/6/2017