TXVs control a set amount of evaporator superheat to maintain an active evaporator. By controlling evaporator superheat, refrigeration and air conditioning systems can operate safer and more efficiently. This month’s article will cover the causes and effects of a TXV overfeeding refrigerant, while next month’s article will discuss what happens if a TXV valve underfeeds — or starves — the evaporator.
The main function of the TXV is to meter the right amount of refrigerant into the evaporator coil under all evaporator heat loading conditions to maintain a set evaporator superheat. The TXV can do this as long as the pressure and temperature ranges of the valve have not been exceeded (always consult with the valve manufacturer’s literature for detailed information on the pressure and temperature operating ranges of any TXV, as well as operating capacities of each valve).
Functions and Factors
The TXV has three main functions:
- Keep the entire evaporator full of refrigerant under varying evaporator heat load conditions;
- Provide a constant amount of evaporator superheat under varying evaporator heat load conditions, provided the range and capacity of the valve is not exceeded; and
- Prevent flood back of liquid refrigerant to the compressor under varying evaporator heat loads.
It is important to note that TXVs do not:
- Control evaporator pressure;
- Cycle the compressor;
- Control system running time; or
- Control refrigerated box temperatures.
The pressures acting on the TXV’s moveable diaphragm are:
- Remote bulb pressure (opening force);
- Spring pressure (closing force); and
- Evaporator pressure (closing force).
Flooding and slugging are two important concepts that technicians must understand. Flooding happens when liquid refrigerant enters the compressor's crankcase while the compressor is operating and only occurs during the running cycle. Slugging is when liquid refrigerant, or liquid refrigerant and oil, enters the compressor's cylinder while the compressor is running. Both flooding and slugging can be caused by a TXV overfeeding the evaporator.
There are several reasons why this can happen. The first is an oversized TXV, which can let too much refrigerant into the evaporator during the TXV’s opening stroke and overfeed the evaporator with liquid refrigerant. This phenomenon can cause the oversized TXV to lose control of the evaporator superheat and let liquid refrigerant enter the compressor’s crankcase or cylinder, depending on the compressor design.
The second is the wrong TXV superheat setting, which can cause overfeeding of refrigerant into the evaporator. If the TXV evaporator superheat is set too low, sometimes during the run cycle, especially when the evaporator is under a low heat loading, the TXV can overfeed liquid refrigerant and may cause flooding and/or slugging depending on the compressor design.
The third reason is if a TXV’s remote bulb is too loose on the evaporator outlet. If the remote bulb is not making good thermal contact with the suction line or one of its straps has loosened or broken, the valve will overfeed liquid refrigerant. Remember, the remote bulb constantly tells the valve how full or void the evaporator is with vaporizing refrigerant.
The remote bulb is the TXV's feedback mechanism that gives the valve an indication of the evaporator superheat and is the only opening force in a TXV. It is usually connected to the suction line with brass straps with bolts and nuts for strength, which prevents any heaving and breakage if water gets between the bulb and suction line and freezes.
A remote bulb with poor thermal contact with the suction line will sense a combination of suction line temperature and ambient temperature. This will average out to be a higher temperature sensed by the remote bulb than if the remote bulb was secured to the suction line and insulated. This higher temperature will cause a greater opening force on top of the TXV’s flexible diaphragm, causing the valve to open too far. This can cause the evaporator to lose all of its superheat and may even flood the compressor with liquid refrigerant. A telltale sign of this is a cold and sweating compressor crankcase. Temperatures and pressure readings in the evaporator will also show no evaporator superheat and a higher-than-normal evaporator (suction) pressure. If the remote bulb is outside the refrigerated space, it must be insulated with a good grade of waterproof insulation.
While flooding may be caused by TXV overfeeding, there are other factors that could cause it as well:
- Evaporator fan motor out;
- Damaged evaporator fan;
- Low load on evaporator;
- Flooding after hot gas termination;
- Heat pump changeover;
- Defrost termination;
- Overcharge of refrigerant;
- End of cycle (lowest heat load);
- Defrost clock or defrost heater out (iced evaporator coil);
- Dirty or blocked evaporator coil;
- Capillary tube overfeeding; and
- Capillary tube system overcharged.
Air cooled, semi-hermetic, piston-type compressors are more prone to slugging liquid than refrigerant-cooled semi-hermetic compressors. This is because refrigerant is often drawn directly into an air-cooled semi-hermetic compressor’s cylinder without passing through the motor barrel. Slugging can result in broken valves, broken head gaskets, broken connecting rods, and other major compressor damage.
Refrigerant-cooled, semi-hermetic, piston-type compressors will often draw liquid refrigerant from the suction line through hot motor windings in the motor barrel, which will assist in vaporizing any liquid present. Even if liquid refrigerant gets past the motor windings, the check valve in the partition between the crankcase and the motor barrel will prevent any liquid refrigerant from entering the crankcase. High current draws will be noticed here from dense refrigerant vapors entering the compressor's cylinder.
Most hermetic compressors suction lines end at the shell of the compressor. If liquid refrigerant enters the compressor, liquid will fall directly into the crankcase oil and eventually be flashed. This is referred to as flooding. Flooding causes oil foaming and excessively high crankcase pressures. Refrigerant and oil droplets will then reach the compressor's cylinder and slugging will soon occur.
Since liquid refrigerants are heavier than refrigeration oils, liquid refrigerant returning to the compressor will settle underneath the oil in the bottom of the compressor's crankcase. This liquid refrigerant will gradually be boiled off from the low pressures in the crankcase. However, very small oil particles will be entrained in this vaporization process, causing the oil level in the crankcase to drop and robbing mechanical parts of vital lubrication. Air-cooled semi-hermetic compressors and hermetic compressors are often more prone to flooding. Suction accumulators can help a flooding condition, but if the situation is severe, accumulators can also flood.
Crankcase pressures can become excessively high from liquid refrigerant boiling in the crankcase, causing refrigerant and entrained oil particles to escape around the rings of the pistons during its down stroke. Once in the compressor's cylinders, they will be pumped by the compressor into the discharge line, robbing the crankcase of lubrication. Oil in the system and not in the crankcase will coat the inner walls of the tubing and valves and cause unwanted inefficiencies.
Higher-than-normal crankcase pressures caused from the higher density refrigerant and oil mixture being pumped through the compressor's cylinders will cause high compressor current draw. This may overheat and even trip the compressor. Broken valves can also occur from this phenomenon. A telltale sign that a compressor's crankcase is being flooded with refrigerant will be a cold, frosted, or sweaty crankcase. A foaming oil sight glass with a low oil level is also signs of flooding. Higher-than-normal current draws will also be present from flooding.
Making sure the TXV is metering the right amount of refrigerant into the evaporator and is not overfeeding the coil is one way technicians can help their customers’ refrigeration and air conditioning systems operate safely and more efficiently.