What will happen if the thermal expansion valve is configured too large

What will happen if the thermal expansion valve is configured too large

A 10P air-conditioning unit is equipped with a 12-ton (1 cooling ton=3.517KW) expansion valve. What will happen?

　　 If you follow the DANFOSS selection software, this configuration will not be recommended (all think this type of selection is too large)

　　 But according to their recommended expansion valve of 8 tons (2 5P machines, with 2 expansion valves of 4 tons), when used in the actual system, the expansion valve is fully opened and the superheat is still relatively large.

　　 But 12 cold tons are too big, and I am afraid of instability when used on the system, so I dare not use it.

　　 Then, for a 10P commercial air conditioning unit, the condenser, evaporator, and air volume are all relatively large. How large is the appropriate expansion valve?

　　 We all know that the minimum load of the system needs to be greater than 25% of the nominal load of the expansion valve. If the system load needs to be greater than 25% of the nominal load of the expansion valve, the phenomenon you said will appear! This is the most fundamental principle for choosing an expansion valve.

A 10P unit generally cannot be equipped with a 30RT expansion valve. The cooling capacity of a 10P unit is only 10TR under air-conditioning conditions, and only 30% of the nominal value of the expansion valve. If it is fully compliant with the operation, the system is okay, but the metropolitan part of the system Echo operation, such as 70%, then the system cooling capacity is 7TR, which is 23% of the valve load, then it is unstable.

　　The better adjustment range of the expansion valve is between 25% and 100%. Considering the margin and partial load, it is recommended that when selecting the expansion valve, the nominal load of the system is 70-80% of the nominal load of the valve under the same working condition.

It is definitely not good to choose a too large expansion valve, because the price will be higher, and secondly, the system will be low and the stable load cannot be too low (for the reason of 25% mentioned above). So I do not recommend choosing a too large expansion valve, and It is measured according to the maximum load and the minimum load of the system, and a suitable expansion valve is selected. Sometimes the maximum load and the minimum load of the system are very different, it is recommended to use two expansion valves.

　　 But if the expansion valve is larger, the system will not necessarily cause problems, it depends on how big it is. This is explained in my post above. The 50KW compressor mentioned in the attachment selects an 80KW expansion valve. If the compressor is unloaded not exceeding 50%, that is to say, if the system load is not lower than 25KW, then there will be no problem. However, if the system load is lower than 20KW, there may be liquid. (Of course, if the selected expansion valve is 60KW, then the system can operate stably above 15KW.

　　 Of course, if the expansion valve is selected larger, the static superheat must be adjusted, that is, the superheat is turned on. If the expansion is selected properly, the static superheat may be smaller, such as 1-3K (because the operating superheat is large, the total superheat is still large, and it is not easy to carry liquid), but the large expansion valve means the operating superheat It will be smaller, so if the static superheat is too small at this time, the total superheat may be too small, and it is easy to carry liquid. I think the superheat mentioned in the accessories is too small, the compressor with liquid may be related to the static superheat is too small, of course, another possibility is that the system attachment is lower than the z*small attachment load of the expansion valve.

　　 In addition, it should be said that it is unlikely that the condensing pressure will drop, because to cause this phenomenon, the opening must be large, and the large opening will cause high and low pressure gas flow. However, the expansion valve port is generally not large, and in this case it must be closed, with only a little leakage.

　　 As for the evaporation pressure, it will not. Except for high and low pressure gas, the evaporation pressure is produced by the balance between the compressor load and the external load. Of course, if the expansion valve is working in an unstable zone and switching adjustment occurs, it will lead to unstable evaporation pressure.

　　The expansion valve is smaller. The key is that when the load is low, the adjustment accuracy is not enough, which will cause a wet stroke. However, when the connecting pipeline is relatively long, it is feasible to choose a larger expansion valve, because the long pipeline itself has a certain pressure drop and throttling. This problem must be considered comprehensively, the size of the load, the length of the pipeline.

　　 In general, 10P compressor with 11RT expansion valve is too large, 8-9RT is enough.

First, briefly introduce the configuration of the test machine: the area of ​​the evaporator is more than 1.6 times that of the general household air conditioner; the air flow of the indoor unit is 1.6 times that of the general household air conditioner; the condensation temperature of the condenser under the standard condition (35C) is 47C; The expansion valve uses 2 SPORLAN FBVE-4 expansion valves (2 4RT).