A majority of these problems are found on small to medium Roof Top Units. There are other considerations that need to be checked before the restrictor can be blamed for the problem. The list of diagnosing procedures of the unit is as follows;
• Low on refrigerant charge. This will create the same symptoms as low suction pressure, high superheat and as the unit continues to operate, the evaporator coil will freeze.
• Plugged liquid line filter driers. Same symptoms as above with a temperature drop across the filter drier. Measuring the line temperature should not be greater than 2 degrees lower from enter to the leaving side of the drier.
• Low air flow from plugged filters, loose or broken belt or plugged coil. These will create low suction pressures, but low superheats as well. The evaporator and suction line will start to ice up immediately at the start up of the compressor.
• Low return air temperatures. This will cause the same results as the above conditions. The air entering the evaporator coil is too low and may be from supply air recirculating or the thermostat set too low. It is important to check these temperatures when troubleshooting.
• Low condensing temperatures. If outside temperatures drops below 60 and there is no condenser fan controls, the condensing pressures drop below 200 psig. The pressure drop across the restriction devise entering the evaporator coil is low and this will lead to the same symptoms as the unit is low on refrigerant.
There is another problem to consider when troubleshooting these units. The evaporator coil and restrictors can become oil logged. In other words the area needed for the evaporation of the refrigerant to take place is plugged or insulating the surface inside the tubing with oil from the compressor. This can be result from the unit operating at low loads for long periods of time and the velocities of the refrigerant and oil drop to a point where the two separate. The refrigerant will then flow over the top of the oil and the surface area of the coil is reduced. The difficultly is to get the two to mix again and the oil to return to the compressor. It requires the tech to operate the unit with the condenser coil blocked off and discharge pressure close to the high pressure cutoff set point for a period of time. This will increase the velocities inside the evaporator to help move the oil back to the compressor.
After all of these checks have been performed, attention needs to be focused on the restrictors of the evaporator coil. The symptoms are alike those of the unit being low on charge with low suction and high superheat. The restrictors become plugged with trash and restrict the flow of refrigerant to the evaporator coil. The coil operates under low suction pressure and ices over.
Many of the small and medium tonnage Trane and Carrier Roof Top Units are equipped with these restrictors. There is one restrictor for each refrigeration circuit in the evaporator coil and the larger the tonnage the more restrictors will be found. The restrictors are found between the liquid line header tubing and the entering side of the evaporator. Each restrictor looks like a piece of tubing and has a crimp in the tubing to hold the restrictor in place. Inside the tube is a brass or steel orifice and is set by the factory to flow the correct amount of refrigerant according to the tonnage of the equipment. These restrictors can become plugged with debris from poor service techniques at the time of manufacturing and part replacement. It is important to purge the lines with nitrogen before soldering refrigerant lines. This will stop oxidation from forming inside the tubing and later blocking the necessary flow of refrigerant through the orifice.
With the unit in this condition it is impossible to clean the orifices to get the unit back to the factory specs. The options will depend on the manufacture of the equipment and what they can offer. In most cases the evaporator coil needs to be replaced which is built with the restrictor assembly. Trane does offer replacement restrictor/liquid line header assemblies which require good soldering techniques with the evaporator coils and should be carefully consider as a repair because of the thin wall thickness of the tubing on the evaporator coil inlets. Trane has other options only with specific size units. That is to remove the liquid line header assembly and carefully cut the tubing after the restrictors before the coil. Replace it with a distributor and expansion valve assembly to the evaporator coil. This replaces the need for the restrictors and in cases where the unit operate with lower temperatures the unit will operate better.
In closing of this article, it is important to remind all of good service techniques. Cleanliness is Important. Keeping gages, hoses and manifolds clean. Using nitrogen when soldering and keep tubing free of dirt and moisture at the time of installation. By following these rules it will help the equipment to operate correctly for long time.
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