Capillary dispensers are primarily used in domestic and small commercial applications where the heat load on the evaporator is somewhat constant. These systems also have lower refrigerant flow rates and typically use hermetic compressors. Manufacturers use capillaries because of their simplicity and low cost. In addition, most systems that use capillaries as the measurement device do not require a high-side receiver, further reducing costs.
Capillary tubes are nothing more than long tubes of small diameter and fixed length installed between the condenser and evaporator. The capillary actually measures the refrigerant from the condenser to the evaporator. Due to the large length and small diameter, when the refrigerant flows through it, fluid friction and pressure drop occur. In fact, as the subcooled liquid flows from the bottom of the condenser through the capillaries, some of the liquid may boil while experiencing these pressure drops. These pressure drops bring the liquid below its saturation pressure at its temperature at several points along the capillary. This blinking is caused by the expansion of the liquid when the pressure drops.
The magnitude of the liquid flash (if any) will depend on the degree of supercooling of the liquid from the condenser and the capillary itself. If liquid flashing occurs, it is desirable that the flash be as close to the evaporator as possible to ensure the best performance of the system. The colder the liquid from the bottom of the condenser, the less liquid seeps through the capillary. The capillary is usually coiled, passed through or welded to the suction line for additional subcooling to prevent the liquid in the capillary from boiling. Because the capillary restricts and measures the flow of liquid to the evaporator, it helps maintain the pressure drop required for the system to function properly.
The capillary tube and compressor are the two components that separate the high pressure side from the low pressure side of a refrigeration system.
A capillary tube differs from an expansion valve metering device in that it has no moving parts and does not control the superheat of the evaporator under any heat load condition. Even in the absence of moving parts, the capillary tubes change the flow rate as the evaporator and/or condenser system pressure changes. In fact, it only achieves optimal efficiency when the pressures on the high and low side are combined. This is because the capillary works by exploiting the pressure difference between the high and low pressure sides of the refrigeration system. As the pressure difference between the high and low sides of the system increases, the refrigerant flow will increase. Capillary tubes operate satisfactorily over a wide range of pressure drops, but are generally not very efficient.
Since the capillary, evaporator, compressor and condenser are connected in series, the flow rate in the capillary must be equal to the pump down speed of the compressor. This is why the calculated length and diameter of the capillary at the calculated evaporation and condensation pressures are critical and must be equal to the pump capacity under the same design conditions. Too many turns in the capillary will affect its resistance to flow and then affect the balance of the system.
If the capillary is too long and resists too much, there will be local flow restriction. If the diameter is too small or there are too many turns when winding, the capacity of the tube will be less than that of the compressor. This will result in a lack of oil in the evaporator, resulting in low suction pressure and severe overheating. At the same time, the subcooled liquid will flow back to the condenser, creating a higher head because there is no receiver in the system to hold the refrigerant. With higher head and lower pressure in the evaporator, the refrigerant flow rate will increase due to the higher pressure drop across the capillary tube. At the same time, compressor performance will decrease due to the higher compression ratio and lower volumetric efficiency. This will force the system to equilibrate, but at higher head and lower evaporation pressure can lead to unnecessary inefficiency.
If the capillary resistance is less than required due to a too short or too large diameter, the refrigerant flow rate will be greater than the capacity of the compressor pump. This will result in high evaporator pressure, low superheat and possible compressor flooding due to oversupply of the evaporator. Subcooling can drop in the condenser causing low head pressure and even loss of the liquid seal at the bottom of the condenser. This low head and higher than normal evaporator pressure will reduce the compression ratio of the compressor resulting in high volumetric efficiency. This will increase the capacity of the compressor, which can be balanced if the compressor can handle the high refrigerant flow in the evaporator. It often happens that the refrigerant overflows the compressor, causing the compressor to fail.
For the reasons listed above, it is important that capillary systems have an accurate (critical) refrigerant charge in their system. Too much or too little refrigerant can lead to serious imbalance and serious damage to the compressor due to fluid flow or flooding. For proper capillary sizing, consult the manufacturer or refer to the manufacturer’s size chart. The system’s nameplate or nameplate will indicate exactly how much refrigerant the system requires, usually in tenths or even hundredths of an ounce.
At high evaporator heat loads, capillary systems typically operate with high superheat; in fact, an evaporator superheat of 40° or 50°F is not uncommon at high evaporator heat loads. This is because the refrigerant in the evaporator evaporates quickly and raises the 100% vapor saturation point in the evaporator, giving the system a high superheat reading. Capillary tubes simply do not have a feedback mechanism, such as a thermostatic expansion valve (TRV) remote light, to tell the measuring device that it is operating at high superheat and automatically correct it. Therefore, when the evaporator load is high and the evaporator superheat is high, the system will operate very inefficiently.
This can be one of the main disadvantages of the capillary system. Many technicians want to add more refrigerant to the system due to high superheat readings, but this will only overload the system. Before adding refrigerant, check for normal superheat readings at low evaporator heat loads. When the temperature in the refrigerated space is reduced to the desired temperature and the evaporator is under low heat load, normal evaporator superheat is typically 5° to 10°F. When in doubt, collect the refrigerant, drain the system and add the critical refrigerant charge indicated on the nameplate.
Once the high evaporator heat load is reduced and the system switches to low evaporator heat load, the evaporator vapor 100% saturation point will decrease over the last few passes of the evaporator. This is due to a decrease in the evaporation rate of the refrigerant in the evaporator due to the low heat load. The system will now have a normal evaporator superheat of approximately 5° to 10°F. These normal evaporator superheat readings will only occur when the evaporator heat load is low.
If the capillary system is overfilled, it will accumulate excess liquid in the condenser, causing high head due to the lack of a receiver in the system. The pressure drop between the low and high pressure sides of the system will increase, causing the flow rate to the evaporator to increase and the evaporator to be overloaded, resulting in low superheat. It can even flood or clog the compressor, which is another reason why capillary systems must be strictly or precisely charged with the specified amount of refrigerant.
John Tomczyk is Professor Emeritus of HVACR at Ferris State University in Grand Rapids, Michigan and co-author of Refrigeration and Air Conditioning Technologies published by Cengage Learning. Contact him at tomczykjohn@gmail.com.
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