5. Capillary Tube Blocking (Low Temperature Stage)
If the evaporation temperature of the low temperature stage is very low, a risk of clogging/freezing of oil becomes more and more realistic. The first position inside the refrigeration system where cold enough temperatures for oil to clog can occur is the capillary tube. Unfortunately, at this position frozen oil can stop the whole refrigeration process. Capillary tube blocking will cause the evaporation pressure in the low temperature stage to decrease. Cooling capacity will decrease to zero when the remaining liquid from the evaporator turns into gas.
5.1 Oil Separator Needed
The freezing of oil is a process which takes some time depending on temperatures, oil properties, and the amount of circulating oil. To keep the risk of the capillary tube from blocking due to frozen oil to a minimum and to allow long intervals between defrosting, it is highly recommended to install an appropriate oil separator between compressor discharge and condenser inlet of the low temperature stage. Analyze capillary tube blocking behavior of the refrigeration system very carefully to define defrosting algorithms with the lowest possible impact to temperatures of the cooling compartment.
5.2 What Happens During Blockage
Direct analysis of blocked capillary tubes which makes it possible to isolate the blocking material is very difficult since the residues are melted/evaporated again when temperatures have reached an adequate level to be handled manually, and the position of the blockage can't be identified from the outside.
Based on measurement of temperatures, pressures, and time, some assumptions can be made:
- Blockages are less frequent when an oil separator is used: Either the oil itself or “something” which is carried by the oil or dissolved inside the oil solidifies.
- The cycle time from one to the next capillary tube blockage is long and nearly constant: Build-up of the blocking material happens slowly and steadily.
- Before total blockage, significant pressure loss inside evaporator occurs: Only when a big fraction of the capillary tube cross section is clogged will the evaporating pressure drop significantly. Feeding refrigerant into the evaporator will almost stop, and superheating of the evaporator will increase rapidly. The consequence is that cooling power decreases to zero while the compressor is operating almost at normal capacity (power consumption will decrease). The decreasing evaporation pressure will cause a further temperature decrease inside the capillary tube which will speed up final blockage of the capillary tube.
- The inner diameter of the capillary tube has influence on the cycle time of the capillary tube blockage. Using a comparably “small” inner diameter (with adjusted length) for the capillary tube requires less “material” for clogging because temperature and the mass flow of refrigerant should be equal if a narrow or a wide capillary tube is used. However, the flow speed of refrigerant inside a narrow and wide capillary will be different. This will most likely have a compensating effect.
- Using a thermostatic expansion valve instead of a capillary tube seems to be a good idea to prevent the loss of cooling capacity due to a blocked capillary tube. Currently, there are no (small size) expansion valves available on the market released for temperatures down to -100°C.
5.3 Low Pressure Switch
When the capillary tube is blocked or even when it is almost blocked, all further operation of the compressor will increase the effort (time, needed heating energy) of resetting to normal operation (defrosting of the capillary tube) while cooling performance inside the evaporator is already near to zero.
A very good indicator for a blocked capillary tube is unusually low pressure inside the evaporator. A low-pressure switch (with open pressure approx. 30% below the lowest normal evaporation pressure) can give a fast and reliable signal to stop cooling and start defrosting.
If reactions to a blocked capillary tube are fast, cooling loss, temperature peak, and temperature recovery time inside the cooling compartment can be kept low.
Long operation with blocked capillary tube will lead to compressor operation without a mass flow of refrigerant at very low evaporation pressure (outside of released pressure limits) > The compressor may be damaged by running in this condition (late protector cut-off due to extreme low load > motor overheats).
5.4 Capillary Tube Defrosting
Failing to initiate active measures (by cabinet controlling) when a capillary tube is blocked, means the return to normal operation will take a long time (… compressor operation until breakdown by motor protector, natural defrosting when the compressor is stopped). To keep temperature increases inside the cooling compartment to the lowest possible level, immediate compressor stopping and defrosting is needed when the capillary tube becomes blocked.
If a blocked capillary tube is identified, the compressor must be stopped to prevent further cooling and freezing of the capillary tube and environment. To allow normal operation again, the clogged section of the capillary tube needs to be heated up to melt the blockage. An accurate prediction of the location of the clogged section is not possible (very likely in the last third of the capillary tube) and can be different from instance to instance.
- Natural defrosting
Switching off the compressor without actively heating the capillary tube (only using heat conduction) is sufficient for defrosting in many cases. The compressor-off time needed for defrosting is specific for each cabinet design and may also depend on ambient temperature (losses in capillary tube insulation).
- Defrosting with capillary tube heater
Melting of a blocked capillary tube can be sped up by heating up the capillary tube with an external heater when the compressor is switched off. Heating time and heating power must be adjusted carefully. Heating up of the last third of the capillary tube should be enough to achieve the needed effect.
- Defrosting should be started immediately if the capillary tube becomes blocked.
Preventive defrosting with a fixed cycle time helps much to avoid long recovery time and high temperature increase in the cooling compartment.
- Having an oil separator installed reduces the need for defrosting significantly.