oversized TXV...

[star882]

Can an oversized TXV cause excessive high side pressure during startup? A friend questioned me about it and I think it could, but I'm not sure. I'm thinking it could allow more heat in due to more flow which corresponds to a higher condensing pressure.

[powell]

Can an oversized TXV cause excessive high side pressure during startup? A friend questioned me about it and I think it could, but I'm not sure. I'm thinking it could allow more heat in due to more flow which corresponds to a higher condensing pressure.

star882,

An oversized TXV usually causes hunting, the change from little SH to high SH. This can be seen by the sudden changes in the suction pressure.

I wouldn't think you would see an excessive high DP when the valve is hunting and the SH is very high. Note the word "excessive".

In your hypothesis, high DP could be associated with the refrigeration equipment being undersized and the TXV being oversized. The load on start up would be too much for the equipment to handle and the TXV, not geting enough refrigerant to meter properly, would hunt.

Let's see what our HVACR friends on this site have to say.

[lana]

Hi there,

Does the system pump-down before compressor stops?
If you have pump-down then there will be no problem during start up.
If there is no pump down then you may get liquid flood back during off cycle; remember I said "may".

In my opinion, there will be no High pressure problem at all, either when compressor is on or off. Of course the situation depends on how much your TEV is over sized? If it is too big then you will get liquid flood back when compressor is on. If this happens then the suction pressure will decrease and subsequently the discharge pressure will decrease.
If the TEV is little bit over sized then you will get a little higher suction pressure when compressor is on.


This is worth remembering :
High discharge pressure is due to three and only three reasons (when compressor is on):
1- Excess refrigerant.
2- Presence of Non-condensible gases.
3- Lack of condenser capacity.


Cheers