Understanding Balance port TX valves

[Mobbsy]

I was hoping there may be some willing participants to enlighten me a little more on balanced port TX valves.
I have a handle on the operation and practical application (part way) of the balanced port tx valves, but I want to get a little more in depth pertaing to how I understand the valve.
For example: I have a small 3KW coolroom operating at a design temp of 2 degrees c. Design condensing is 45 C. During winter low ambients are experienced down to say 10 C + 13K condenser TD= 23 condensing (without any form of head pressure control). Obviously if this was to happen my back pressure would follow my head pressure down and I would end up drying out meat e.t.c. Large evap TD drier air. I would also experience TX problems-hunting and starving. Hypothetically if I installed an EPR to keep my S.E.T at desired conditions- which is not text book as we all learn to install EPR valves on multiple evaps with varying boiling temps, but if I did whack an EPR on the system and installed a balanced port valve could I float the discharge down and save some energy for future generations and experience the desired removal of heat.
Or should I just forget about it and go an electronic expansion valve and float down.

Are you out there professor.

[GXMPLX]

No professor here. "I can't pretend that I can teach" (The Who).

Balanced port valves have a transmission designed to cancel out the forces due to sudden pressure changes at the valve inlet.

This gives the valve the possibility to operate stable when you use fan cycling to control condensing pressure.

It also allows the valve to keep a stable control until the system capacity drops down to about 50% of its nominal capacity.

If your condensing temperature change is slow and the applied pressure drop to the valve drops below 50% of nominal summer capacity then the balanced port will not solve your problem. They work OK for me because we have 23C daily temperature changes in summer.

To solve this problem you need to use more than one (mechanical) expansion valve each chosen at the correct working conditions, installed above any distributor and controlled by a solenoid valve.

MUCH better would be to use an electronic expansion valve.

Allowing high pressure to float is a great idea to save energy but may be a control nightmare if classical controls are used. With electronic intelligent controls, no problem except understanding your controls.

[US Iceman]

I'll give you a different viewpoint.

I started using the balanced port valves when they were first made commercially available. Before this, they were OEM (original equipment manufacturer) valves only and not available to the general public, so they have been around for quite some time.

I have used these valves down to approximately 3.5 bar differential pressure with NO problems. My feeling is; if you can keep the discharge pressure reduction relatively stable (as the ambient temperature drops) they valves react just fine. The majority of these were used on supermarket display cases, so the application is the same as yours. Another RE moderator has also done something like this, so it's not magic by any definition.

On my applications these used R-12 & R-502. On the R-502 systems I would run the condensing temperatures down to about 7-8°C! The lowest I would run the R-12 systems was about 10-12°C. And they ran like this for the years I was involved in their maintenance.

In fact, on start-up at low condensing pressures you might actually hear the compressor whistle slightly until it warms up.

Fan cycling causes rapid pressure changes. If you use an inverter (VFD) to ramp the fan speed up and down slowly you should not have any problems.

If the compression ratio (discharge absolute pressure / suction absolute pressure) does not fall below 2:1 the compressor seems to work just fine at low loads.

[GXMPLX]

US_Iceman I agree with what you posted but the problem is not the valve nor the floating condensing pressure, the problem is that the orifice you need in summer and winter may be different, that's why you need different valves.

[GXMPLX]

Another post made me remember that maybe I did not explain enough the foll.:

The problem that you might need different orifices is not just because of condensing pressure floating but a combination of this AND high and low thermal load.

US_Iceman is right you would not normally need to use a different balanced port valve IF you get constant thermal load or at least above 50% of nominal valve capacity like you would if you do capacity control of the suction pressure or turn off unneeded evaps.

The problem I was thinking of is that a combination of low condensing plus low thermal load that may need another orifice. This would be better handled using a different valve or with an EEV.

In my area I have this every day in summer! ... this makes me forget that you might not.

[US Iceman]

...the problem is that the orifice you need in summer and winter may be different, that's why you need different valves.

The only time I have ever used multiple TXV's was on interlaced air conditioning coils. This is done so the entire coil surface is active at low thermal loads.

In general though, I agree with what you are saying. If the system operates beyond the stable point of superheat control by the TXV, you might want to consoider multiple TXV's (won't that be fun? Look at much trouble we have with only one TXV!)

[Mobbsy]

Thankyou very much for the feedback, it provides a better understanding of the valve. One point I was attempting to address however, was a scenario where we have a single system i.e one condensing unit and one evaporator. During low load or low ambients the balanced port valve would not seem to provide any benefits as the back pressure will float down with the head pressure. A number of other problems could possibly arise due to excessive condenser sub-cooling such as liquid binding e.t.c. But if I was able to keep the SET constant and float the head down would the balanced port valve be an advantageous addition to the singular system. Maybe I'm thinking too much about this, I will re-adjust my thinking and have happy thoughts of electronic expansion valves and/or fan speed control. I have just found a slight annoyance in recent manufacturers data(some) and journal articles in which the application of particular components are written and understood by many in a perfect world. As most of us are aware we do not live in a perfect world. For example the average mechanic commissioning a freezer room determines and adjusts the evap superheat under high load. Follows all the examples he has read about TX valves and assumes he/she is carrying out his/her duties in a correct manner. Unfortunalely under low load conditions 24hrs later there is no superheat: bye bye suction reeds and/or oil. My point being we all need a better understanding of the variables encountered and have a grip on the reality of what can and does actually happen.

[wambat]

From Sporlan: These factors include corrections for liquid
refrigerant density and net refrigerating
effect and are based on an evaporator
temperature of 0°F. However, they
may be used for any evaporator temperature
from -40°F to 40°F since the variation
in the actual factors across this
range is insignificant.
Thay give this example for a balanced port

TEV capacity = TEV rating x CF liquid temperature x CF pressure
drop — Example: Actual capacity of an A port in a Type
EBQ valve with R-22 at 20°F evaporator, 100 psi pressure drop
across the TEV, and 90°F liquid temperature entering the TEV
= 1.74 (from rating chart) x 1.06 (CF liquid temperature) x 0.89
(CF pressure drop) = 1.64 tons.

Now let see what happens if I change the perameters to 50psi (pressure drop)
= (1.23) and 60*F liquid temperature entering = (0.63) so 1.74 x 1.23 x 0.63 = 1.35 T
Or a drop ~ 25%

Looks like it should work!

[US Iceman]

One point I was attempting to address however, was a scenario where we have a single system i.e one condensing unit and one evaporator. During low load or low ambients the balanced port valve would not seem to provide any benefits as the back pressure will float down with the head pressure.

I don't see a problem with what you want to do. The issue of the decreasing back pressure with a reduction in head pressure is a symptom of normal TXV's. As the dP reduces the valve capacity goes down and with it, the ability to control the evaporator superheat.

The other issue you raise about starting up freezers under a high load is valid with one correction; the evaporator superheat should not be adjusted until the system operation has stabilized at normal working conditions. If you try to set the superheat during higher loads than normal, then of course you would expect to see problems after the load has been reduced.

You set the valve for it's NORMAL operating point, not a transient one that only appears at start-up.

[Mobbsy]

Once again thankyou Ice man for your feedback. You have answered what I was asking, I shall now apply theory to practice and evaluate the outcomes.

I appreciate and realise that evap superheat should not be checked or if need be adjusted until you are at your desired running conitions i.e. desired room temperature. I was merely using this as an example of what one may read within some textbooks, that is how to determine superheat, not when and under what conditions it should be determined. It should also be noted that system stabilisation will vary at different ambients for some applications, particularly if one does not exercise caution with a low head pressure, that is a mechanic may set what they deem to be a correct superheat in winter times only to return in summer to replace a compressor that has been slugging liquid because of the increased condensing pressure assisting the bulb pressure in opening the valve.

[GXMPLX]

... (won't that be fun? Look at much trouble we have with only one TXV!)

No, you have only one valve working all the time, the thing is using the right one in the right situation! You shut the other valve depending on pressure and temperature. Same ol' TXV fun, more control! If you have several evaps the control may become a bad joke!

Multiple evaporator circuits good for capacity control this problem is capacity and DP together.

... and speaking of bad jokes, plse do check your User CP once a while. TKS.

[US Iceman]

I was merely using this as an example of what one may read within some textbooks, that is how to determine superheat, not when and under what conditions it should be determined.

Absolutely. The important thing is when to check it and when to adjust it (if you have to at all).

If you follow the methodology wambat presented you won't go wrong. It's worked for me for a lot of years.

[Mobbsy]

I completely agree, it is very important at what conditions the system is running at when we check superheat. Just as important is a technicians ability to check total suction superheat and subcooling provided by the condenser and subcooling before the TX. A mechanic can never walk away from any new install or mechanical service call unless they are satisfied with these checks (personal belief).
I will defnitely adhere to the methadologies put forward by Wambat; thankyou Wambat.