What Happens To The Thermal Expansion Valve Configuration?

- Apr 14, 2018-

A 10P air conditioning unit, equipped with a 12 tons (1 ton = 3.517KW) expansion valve, what will happen?

If you follow the selection software of DANFOSS, this configuration will not be recommended (they are considered to be too large)

However, in accordance with their recommended 8 cold ton (2 5P machine, with two 4 tons expansion valve) expansion valve, used in the actual system is to fully open the expansion valve superheat is still relatively large.

However, the 12 tons of cold are too scary to use, and they are afraid of instability on the system.

So, a 10P commercial air conditioning unit, condenser, evaporator, air volume is relatively large, the expansion valve with how much right?

We all know that the minimum load of the system needs to be more than 25% of the nominal load of the expansion valve. If the system load needs to be more than 25% of the nominal load of the expansion valve, you will see what you said! This is the fundamental principle of choosing an expansion valve.

The 10P unit is generally not equipped with a 30RT expansion valve. The 10P unit's cooling capacity is only 10TR under the air conditioning condition, and it is only 30% of the nominal expansion of the expansion valve. If the full operation is satisfied, the system can also be used, but the system will be partially Congruent operation, such as 70%, then the system cold capacity of 7TR, 23% of the valve load, then it is unstable.

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

If the expansion valve is too large, it is definitely not good, because the price will be higher, and then the minimum stable load of the system should not be too low (25% mentioned above). So I do not recommend choosing too much expansion valve, but based on Measured by the system's maximum load and minimum load, select a suitable expansion valve. Sometimes the maximum system load and minimum load difference are large, it is recommended to use two expansion valves.

However, if the expansion valve is selected, the system will not necessarily have a problem. This has been explained in my posting above. The 80KW expansion valve of the 50KW compressor mentioned in the attachment will not cause problems if the compressor is not unloaded by more than 50%, that is, if the system load is not less than 25KW. However, if the system load is less than 20 KW, it 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 large, it is necessary to adjust the static superheat, which is to open the superheat. If the expansion of the election is appropriate, the static superheat may be smaller, such as 1-3K (because the operation is relatively high degree of superheat, so the total degree of superheat is still relatively large, not easy to bring liquid), but the large expansion valve means that the operation of superheat It will be smaller, so if the static superheat is too small, the total superheat may be too small, it is easy to bring liquid. I think that the degree of superheat mentioned in the annex is too small, the compressor liquid may have too little static superheat, and of course the other possibility is that the system equilibration is already below the minimum load of the expansion valve.

In addition, the so-called condensing pressure drop, it should be said that the possibility is not, because to cause this phenomenon, we must open a large amount of Caixing, large openings lead to high and low pressure string gas. However, the expansion valve is generally not large, and in this case it must be closed, with only a little leakage.

As for the increase in evaporation pressure, it will not be. Unless high pressure and low pressure, the evaporating pressure is generated by the balance between the compressor load and the external load. Of course, if the expansion valve operates in an unstable zone, switch regulation occurs, which will cause the evaporation pressure to be unstable.

The expansion valve is small, the key is that when the load is low, the adjustment accuracy is not enough and it will cause wet stroke. However, when the connecting pipe is relatively long, it is feasible to select a large expansion valve, because the long pipe itself has a certain pressure drop and throttling. This issue still needs to be considered comprehensively, the size of the load and the length of the pipeline.

In general, the 10P press is equipped with 11RT expansion valve is too large, 8 ~ 9RT enough.

First, the configuration of the test machine is briefly introduced: the evaporator area is more than 1.6 times that of general household air conditioners; the indoor unit air volume is 1.6 times that of general household air conditioners; the condenser has a condensation temperature of 47C under nominal conditions (35C); Two SPORLAN FBVE-4 expansion valves (2 4RT) are used for the expansion valve.