Capillary tube metering devices are found mainly in domestic and small commercial applications that experience somewhat constant heat loads on their evaporators. These systems also have small refrigerant flow rates and usually employ fully hermetic compressors. Manufacturers use capillary tubes due to their simplicity and low cost. In addition, most systems employing capillary tubes as metering devices do not require high-side receivers, which add to another cost savings.
A capillary tube is nothing but a long, fixed-length tube with a very small diameter that is installed between the condenser and the evaporator. The capillary tube actually meters the refrigerant from the condenser to the evaporator. Because of its long length and small diameter, there is associated fluid friction and pressure drop as refrigerant flows through it. In fact, as subcooled liquid travels from the condenser’s bottom through the capillary tube, a portion of the liquid may flash as it experiences these pressure drops. These pressure drops bring the liquid lower than the saturation pressure for its temperature at several points along the capillary tube. This flashing is caused from the expansion of the liquid as it experiences pressure drop.
Liquid flashing amounts, if any, will depend on the amounts of liquid subcooling coming from the condenser and in the capillary tube itself. If liquid flashing does occur, it is desirable to keep the flashing as close to the evaporator as possible to ensure better system performance. The more subcooling of liquid coming from the condenser’s bottom, the less flashing of liquid through the capillary tube. Capillary tubes are usually twisted around, run inside, or soldered to suction lines to add to the subcooling effect in order to help prevent liquid flashing in the capillary tube. Because the capillary tube restricts and meters the flow of liquid to the evaporator, it helps maintain the needed pressure difference for proper system operation.
The capillary tube and compressor are the two components that separate the high side from the low side of the refrigeration system.
SIZING IS CRUCIAL
The capillary tube differs from a thermostatic expansion valve (TXV) metering device in that it has no moving parts and does not control evaporator superheat at all heat loading conditions. Even with no moving parts, the capillary tube will vary flow rate as system pressures change in the evaporator and condenser or both. In fact, it can only reach its best efficiency at one set of high- and low-side pressures. This is because the capillary tube works off of the pressure difference between the high and low sides of the refrigeration system. As the pressure difference between the high and low sides of the system becomes greater, the flow rate of refrigerant will increase. The capillary tube will operate satisfactorily over quite a large range of pressure differences, but often not very efficiently.
Since the capillary tube, evaporator, compressor, and condenser are in series, the flow rate of the capillary tube must be equal to the compressor’s pumping rate. This is why the designed length and diameter of the capillary tube at designed vaporizing and condensing pressures is critical and must be equal to the compressor’s pumping capacity at these same design conditions. Too many turns in the capillary tube will affect the resistance of its flow and, in turn, the system balance.
If the resistance of the capillary tube is too great because it is too long, a partial restriction will exist. If the diameter is too small or there are too many turns as it is coiled, the capacity of the tube will be less than that of the compressor. This will result in the evaporator being starved, which will cause low suction pressure and high superheats. At the same time, subcooled liquid will be backed up in the condenser, causing higher head pressure because of no receiver in the system to hold that refrigerant. With the higher head pressure and lower evaporator pressure, the refrigerant flow rate will be increased because of a greater pressure difference across the capillary tube. At the same time, the compressor capacity will decrease because of higher compression ratios and lower volumetric efficiencies. This will cause the system to establish a balance, but at higher head pressures and lower evaporating pressures, causing unwanted inefficiencies.
If the resistance of the capillary tube is less than called for because it is too short or its diameter is too large, the refrigerant flow will be greater than the pumping capacity of the compressor. This will cause high evaporator pressures, low superheats, and possible flooding of the compressor due to overfeeding of the evaporator. Subcooling will drop in the condenser, causing low head pressures and even a loss of liquid seal at the condenser’s bottom. This low head pressure and higher-than-normal evaporator pressures will decrease the compression ratio of the compressor, causing high volumetric efficiencies. This will increase the pumping rate of the compressor, and a balance may be reached if the compressor can keep up with the high refrigerant flow in the evaporator. Usually, refrigerant flooding the compressor will occur, and the compressor will fail to keep up.
Because of the above listed reasons, it is important that capillary tube systems have an exact (critical) charge of refrigerant in their systems. Too much or two little refrigerant will cause a severe imbalance to occur and seriously damage compressors from slugging or flooding. To size the capillary tube properly, either consult the manufacturer or use the manufacturer’s sizing charts. The system’s name plate or data plate will specify exactly how much refrigerant, usually in tenths or even hundredths of an ounce, the system requires.
Under high evaporator heat loadings, capillary tube systems will normally run high superheats; in fact, 40° or 50°F evaporator superheats are not uncommon with high evaporator heat loadings. This is because the refrigerant in the evaporator will vaporize rapidly and drive the 100 percent saturated vapor point back up in the evaporator to give the system a high superheat reading. The capillary tube simply does not have a feedback mechanism like a thermostatic expansion valve (TXV) remote bulb to tell the metering device that it is running high superheat and automatically correct for it. Because of this, the system will run very inefficiently while at higher evaporator loads with high evaporator superheat conditions.
This is perhaps one of the main disadvantages of a capillary tube system. Many technicians will have the urge to add more refrigerant to the system due to the high superheat readings, but that will only overcharge the system. Before adding refrigerant, check the superheat reading at the low evaporator heat loading and see if it is normal. Normal evaporator superheats usually range between 5° to 10°F when the system is pulled down to a desired refrigeration space temperature and the evaporator is under a low heat loading. If unsure, recover the refrigerant, evacuate the system, and add the specified nameplate critical charge of refrigerant.
Once the high heat load on the evaporator drops off and the system is under a low evaporator heat load, the 100 percent saturated vapor point in the evaporator will climb down the last passes of the evaporator. This is caused from the decreased rate of vaporization of refrigerant in the evaporator from the low heat loading. The system will now have a normal evaporator superheat of about 5° to 10°F. These normal evaporator superheat readings will only occur with low heat loads on the evaporator.
If a capillary tube system is overcharged, it will back up the excess liquid in the condenser, causing high head pressures because of no receiver in the system. The pressure difference between the low and high side of the system will then increase, causing the flow rate to the evaporator to increase and overfeed the evaporator, causing low superheats. It may even flood or slug the compressor, which is another reason why a capillary tube system must be critically or exactly charged with a specified amount of refrigerant.
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