minimum pressure diff. on TXV

[hiitsme]

Can you help me to find out the required minimal pressure drop on thermostatic valves?
There's not a word about it in Danfoss catalogues... but we have some sites where it is possible to maintain not-so-high high pressures.
Thanx!!!

[borley]

Hi
What exactly are you trying to achieve. Are you trying to save money on the electrical bill on an existing system or are you designing a new system with a low pd across the TXV?
Or maybe you have some other reason. Danfoss and other TXV manufacturers do have charts that show the TXV sizes and duties with the relevant pressure drops.
Cheer
Shaun

[1torr]

Usually about 100psi

[Josip]

Hi, hiitsme

Can you help me to find out the required minimal pressure drop on thermostatic valves?
There's not a word about it in Danfoss catalogues... but we have some sites where it is possible to maintain not-so-high high pressures.
Thanx!!!

.....are you sure

what about this...of help or not...


Best regards, Josip

[US Iceman]

The minimum required differential pressure for a TXV is determined by the system application and operating conditions and respective valve capacity to meet the evaporator duty.

You need to take the evaporating pressure plus the pressure losses of the liquid line (and accessories) plus the distributor pressure losses.

Add all these together to find the lowest condensing pressure you will be able to operate with.

Then select a TXV that has the required capacity at the minimum differential pressure.

Depending on the refrigerant the nominal valve capacity is usually based on something like the equivalent of 100 or 150 psig. Somewhere between 6.5 to 10 bar differential (if I did the conversion right ).

[hiitsme]

Hi guys,

first of all thanks for the replies!!!

Borley:
our aim is to keep the headpressure as low as possible (savings) and try to find the optimal "equilibrium". (I mean oil reclaim - capacity - electrical consumption - reliability)

1torr: I'm afraid that we need a little bit more specified value for the valve types

Josip:
even this doc does not say that the value of the minimal acceptable pressure drop (across the valve). It only says that under the specified conditions how much capacity we have. I need that value which tells me the treshold of the stable working. Table B. shows pressure drops between 2 and 16 bars but .... ..... ..... :-) :-) there's not a word :-) :-) ... .. about the acceptable minimum.

US Iceman:
yes, we were aware that we will have some capacity loss on the valve if we decrease the head pressure too much, but first we would like understand that where is the theoretical borderline for the safe (but not efficient) working.

In the case we have the factory requirement we can increase the pressure difference and play with oil reclaim and optimal capacity but we need a "guaranteed" starting value. Without that it does not smell "scientific" :-).

Thanks again!

[US Iceman]

The acceptability of a TXV for low pressure differentials can be separated into two classes: balanced port or unbalanced port construction.

A balanced port TXV will be able to control superheat at fairly low differentials.

An unbalanced port TXV is one where you really need to be careful of maintaining a minimum differential, otherwise the valve will hunt.

It's not TXV efficiency you are looking for, it's valve cabability. A balanced port TXV will provide very good performance and I have used some of these with differentials as low as 50 psi (which is about half of that needed for a unbalanced TXV I think).

[borley]

Hi
I think all your questions have been answered. But what you must consider is that the system you want to alter would have been designed with a specific pd across the valve or valve's, by altering this you will lower the evap. duty. my personal opinion is that you may cause more problems than you solve. If it is a large system, you may consider installing liquid booster pumps to maintain the correct designed pd, and then lower the system head pressure to reduce electrical consumption.
Cheers
Shaun

[hiitsme]

Thanks for the answers these are really useful things!
However it is still not clear for me what happens if I apply lower pressure difference than the "usual".
The attachment says that 2 bars pressure difference is not applicable (at least there is no capacity data...) but for every orifice in the leaflet above minus 10 evap temp there is data for 2 bars pressure diff. What happens if I go lower???

[TXiceman]

You may want to look at Sporlan. They have a wider range of valves than does Danfoss.

Ken

[Sergei]

Thanks for the answers these are really useful things!
However it is still not clear for me what happens if I apply lower pressure difference than the "usual".
The attachment says that 2 bars pressure difference is not applicable (at least there is no capacity data...) but for every orifice in the leaflet above minus 10 evap temp there is data for 2 bars pressure diff. What happens if I go lower???

Pressure difference supply liquid to evaporator coil. At lower difference less liquid will go into the coil, capacity of the coil will be reduced, suction pressure can be lowered, superheat will increase, oil return should be monitored.