TEV Operation, with low Head pressure

[MikeySq]

Am I thinking right??

when Head pressure gets stupid low (like 100 psi on R-404a)and TEV gets lower pressure, subcooled) refrigerant the valve loses efficiency by basically becoming a larger valve than it's intended size by design? assuming there's no flash gas at the valve, which would be another reason for poor performance.

just something on my mind today while i was doing mindless work :P

Mike

[A1 Burt]

Another reson for poor performes is moister-pin's sticking-bulb not making
good contact-power head getting weak-:cool:

[r404a]

mikey

i don't know about a larger size but a certain amount of pressure drop is necessary on most valves to get tonnage

404

[MikeySq]

when I say larger size i mean higher capacity, just like how subcooling makes a valve larger by increasing valves rated capacity. (tonnage)


mikey

i don't know about a larger size but a certain amount of pressure drop is necessary on most valves to get tonnage

404


So are ya sayin the lower the pressure drop would mean lower capacity? or lower efficiency?

basically what happens technically to the valve when the inlet pressure is below minimum design operating pressure...

[r404a]

I hope that andy is along shortly to get me straight. i think rated tons is reduced if proper pressure drop is not attained. Some valves ( O series sporlans, in example and balanced port valves) can handle lower presure drops. Lets say the old standard was 100 psig across valve to deliver rated "flow" to meet the load in the evap. remember, you will have another pressure drop across the distributor as well, maybe 35 to 40 pounds. so, lets say that you have a certain feed rate (ie amount of liquid/vapor) you are putting into an evaporator with a txv/distributor assembly, R22, medium temp. You are looking at approx 45 psig lowside in a system to maintain a 20 degree coil. We said we need 100 psid across the valve body. we expect about 40 across the distributor BY ITSELF. so, 45 suction, PLUS the 40 we know we lose across the distributor puts us at 85 psig. add the 100 we WANT across the valve and we arrive at about a 185 head that we probably want at the valve to meter a proper flow.

Go ahead Andy, get me straight! This is just old hand method thrown out here for discussion.
my 2 cents
r404a

[klove]

All txv's are rated at a baseline pressure drop, e.g.: air conditioning duty at 100 psid (you also have to take into account distributor dp if using one). Subcooling increases a valves efficiency, and even though at extremely low OA temps you can obviously get tremendous subcooling, this is coupled with greatly reduced dp thru the valve. When you start getting that far outside design dp on the valve, operation starts gettin' squirrely, and if it goes too far, the valve throws its hands up and says "I'm not workin' here anymore". Sporlan has some very good service lit on all this. Well, they did, if Parker hasn't screwed them up like JCI did York.

[-frozen-ocean-]

I have seen the non adjustable valves act up when the head pressure is too low and the liquid temp is way too damn cold and sure the valve will act as a bigger valve. i have seen it were flood back starts in that situation. i think it has something to do with the subcooling and low head were the valve (non adjustable) acts as a over sized valve. due to above mentioned. i forget the real fundamentals about this subject but tell you the turth i still wounder why myself.:D

[MikeySq]

I'm referring to normal TEV non Balanced port

valve ratings are based on a standard evap temp, inlet liquid temp and 100psi pressure drop,

1ton r22valve is that capacity at 100psi pressure drop with 100F liquid, with 40F evap

sub cool the liquid and capacity rises
raise pressure drop and more flow thus highrr capacity

lower the pressure drop and flow slows so a reduction in capacity

so what happens in a low pressure drop, high sub cooling situation?(assuming there's a solid column of liquid at the port)

Do they cancel each other out and let the valve operate normally, efficiently?

[-frozen-ocean-]

if you do have low delta P and high subcooling it will cancel each other out and the valve will work like it should under both those situations. to what extent of what pressures and temps to see for this to happen beats me.

[MikeySq]

to what extent of what pressures and temps to see for this to happen beats me.

low ambient temperature with fans on the condenser running and the condenser drop leg right to liquid header, no holdback of refrigerant in condenser, but of course this is a dynamic system so there's all sorts of crap goin on

i'm not talking specific numbers or specific problems here, just thinking outloud on valve operation with less than perfect head pressures, so i've changed my point of view from my 1st post to my 3rd by reading a bit and thinking about it, sometimes i tend to think too much and i confuse myself hahaha:D:p

[mikesands]

Am I thinking right??

when Head pressure gets stupid low (like 100 psi on R-404a)and TEV gets lower pressure, subcooled) refrigerant the valve loses efficiency by basically becoming a larger valve than it's intended size by design? assuming there's no flash gas at the valve, which would be another reason for poor performance.

just something on my mind today while i was doing mindless work :P

Mike

You say lower pressure and put parenthesis around subcooled, they work inversely to each other. If your head dropped your subcooling went away so I'm not totally understanding which one we are dealiing with.

You need a pressure drop across the valve in order for it to feed the tonnage of refrigerant. The higher the subcooling ( or just the lower the temp of the refer really ) of that liquid the more capacity you will gain.

If your high pressure is falling then it is more than likely bringing your subcooling down with it also, so you aren't really getting both at the same time.

[shogun8]

Sub- cooling if any always follows condensing temperature so if condensing temperature drops then sub-cooling will follow or said another way. Liquid temperatures will also follow the ambient from the design temperature of 100° at 90° ambient, to approximately 50° at a 40° ambient.

[Phase Loss]

if head pressure gets stupid low your liquid will not want to move out of the condenser or receiver and your TEV's will suffer not only the low delta P but also be starving for the liquid sitting in the condenser/receiver. this low delta P would equate to a starving valve, a starving valve would act much like a under sized valve.

good liquid subcooling only increases the valves capacity if it is still able to maintain a good delta p. this is why they use mechanical subcoolers.

if you just drop the subcooling by dumping the head in the toilet you will lose.

[dangpgt97]

Is it possible for a system to have so much subcooling in winter time that you need to adjust the txv to compensate for the increased capacity? Then in summer time lose enough subcooling that the txv needs to be adjusted again?

[mikesands]

I would have to wonder how you got all the subcooling , cold weather doesn't equate to more subcooling. I'm thinking you are referring to just a lower temperature of refrigerant that came with a reduction in head pressure. That does not equal a higher subcooled refrigerant and if the head pressure dropped you will be feeding less refrigerant into the coil so even though it is cooler it will be a lower volume so how much more capacity you get out of it would need to be measured but it probably won't be significant.

Now if you can actually get more subcooling which means the head had to maintain a higher working pressure then you will gain some capacity but do you need it ? If you do you have a sizing issue to start with.

[wannafreeze]

if head pressure gets stupid low your liquid will not want to move out of the condenser or receiver and your TEV's will suffer not only the low delta P but also be starving for the liquid sitting in the condenser/receiver. this low delta P would equate to a starving valve, a starving valve would act much like a under sized valve.

good liquid subcooling only increases the valves capacity if it is still able to maintain a good delta p. this is why they use mechanical subcoolers.

if you just drop the subcooling by dumping the head in the toilet you will lose.

So far the best answer..:)

[Andy Schoen]

if head pressure gets stupid low your liquid will not want to move out of the condenser or receiver and your TEV's will suffer not only the low delta P but also be starving for the liquid sitting in the condenser/receiver. this low delta P would equate to a starving valve, a starving valve would act much like a under sized valve.

good liquid subcooling only increases the valves capacity if it is still able to maintain a good delta p. this is why they use mechanical subcoolers.

if you just drop the subcooling by dumping the head in the toilet you will lose.

Good explanation. ;)

Lowering head pressure will have greater influence reducing TEV capacity than the resulting lower liquid temperatures will have increasing TEV capacity. And that is assuming we maintain a liquid seal at the TEV. If we don't, the TEV will substantially lose even more capacity.

Worst case scenario: condensing and evaporating pressures and temperature are equal. The TEV has zero capacity in this case. :)

[davidd_danfoss]

I am hearing a lot of right answers, but not all in one message.

1) subcooling and head pressure are completely independent of each other
2) increased subcooling increases the capacity of the valve (negative subcooling is flash gas and decreases capacity)
3) increased pressure drop (normally caused by higher head pressure) increases valve capacity
4) balanced port valve is not relavent to the discussion as it only helps keep the SH stable over several head pressure conditions

This is a way to think about it. The compressor does the work for the system, that is then used at the txv. The higher the pressure ratio at the compressor, the more work/energy is used... but also, there is more work available at the txv.

Colder refrigerant (subcooling) can absorb more heat. Also, colder refrigerant is denser. Therefore, a txv with higher subcooling will allow more refrigerant through because the refrigerant is denser, and the refrigerant that is allowed through can absorb more heat. Both of these increase the capacity of the valve.

[mikesands]

I am hearing a lot of right answers, but not all in one message.

1) subcooling and head pressure are completely independent of each other


Colder refrigerant (subcooling) can absorb more heat. Also, colder refrigerant is denser. Therefore, a txv with higher subcooling will allow more refrigerant through because the refrigerant is denser, and the refrigerant that is allowed through can absorb more heat. Both of these increase the capacity of the valve
.

Not following these statements , please explain.

[davidd_danfoss]

Mike,
Subcooling and head pressure are independent in the following way.

Head pressure is determined by the condensing temperature. Subcooling is determined by the amount of heat rejection after condensing is complete. Furthermore, Subcooling can be acheived no matter what condensing pressure the system has.

Also, the effects of head pressure and subcooling on the system are independent eventhough they both affect TXV capacity. Higher head pressure increases TXV capacity, but lowers energy efficiency. Higher subcooling increases TXV capacity, and can either increase or decrease energy efficiency.

[dangpgt97]

Also you get more efficiency with higher subcooling because the closer your liquid temp is to the evap temp the less flash gas is required to convert to the lower temperature.

[davidd_danfoss]

Also you get more efficiency with higher subcooling because the closer your liquid temp is to the evap temp the less flash gas is required to convert to the lower temperature.

That has no effect on energy efficiency. Refrigeration energy efficiency is based on compressor pressure ratio, and fan amps (aside from lights, heaters, etc that is a different topic). Subcooling can increase energy efficiency only if it helps with either of the above, like using the intermediate pressure on a compressor (two stage recip, VI scroll, or any screw). Subcooling that is obtained by extra fan usage only is energy beneficial when preventing flash gas before TXV. Subcooling past that is an energy negative.

[dangpgt97]

I pulled this off of refrigeration-engineer.com. This is what they told us in school. I thought we were talking system capacitys.

This evaporation is, in effect, simply refrigerating the remaining portion of the liquid passing through TEV until it reaches the design temperature of the evaporator. After that, the evaporating liquid is doing useful work removing heat from the product being cooled.

This is why cooling of the liquid provides energy savings; the cooler the liquid is, the less the refrigerant has to evaporate to bring the rest of the liquid down to temperature, and the more the refrigerant is available for useful work on the product being cooled.

Flash gas in the liquid line before the TEV is problematical. If you are providing a vapor/liquid refrigerant mix to the valve, it will lose capacity dramatically because so much volume has to pass through the valve compared to a full liquid feed.

One other thought: In instances where you are concerned about the state of the gas feeding the TEV, just because you have a bubbling sightglass at the compressor unit, it doesn't necessarily follow that you have the same condition at the TEV, nor does having a full sightglass at the compressor unit insure that you will have the same condition at the TEV.

[MikeySq]

Also you get more efficiency with higher subcooling because the closer your liquid temp is to the evap temp the less flash gas is required to convert to the lower temperature.


That has no effect on energy efficiency. Refrigeration energy efficiency is based on compressor pressure ratio, and fan amps

I wouldn't say it has no effect, and it would increase efficiency, just say ...

(forgetting about glide for now)
R-404a 180psi TEV inlet pressure Liquid at 70F, so there's sub cooling there so it should be a solid column of liquid at the TEV for a 20F evap temp

R-404a 180ps TEV inlet Pressure liquid at 30F, so there's a lot more sub cooling, so we know we got all liquid to the valve, also a 20F evap temp

Which evap has more flash gas after the pressure drop, there's the efficiency, more btu's needed for evaporation per pound. it's easy to see if ya plot on the Pressure enthalpy chart

Mike

[davidd_danfoss]

The issue is that it takes energy to subcool the liquid. That energy could be from fans or a pump running faster, a pump or from the compressors. So the end question is if the "energy savings of subcooling" is greater or less than the energy it takes to subcool.

[-frozen-ocean-]

The issue is that it takes energy to subcool the liquid. That energy could be from fans or a pump running faster, a pump or from the compressors. So the end question is if the "energy savings of subcooling" is greater or less than the energy it takes to subcool.

Unless your in a cold climate and your rack is set up for ambient subcooling.

[fcs]

There is a good Article in the Dec 2008 RSES Journal Page 25. It doses a good job of addressing this subject

[icemeister]

The issue is that it takes energy to subcool the liquid. That energy could be from fans or a pump running faster, a pump or from the compressors. So the end question is if the "energy savings of subcooling" is greater or less than the energy it takes to subcool.

Or unless you're subcooling liquid for a low temp system with an EER of around 4.5 Btu/watt using a separate medium temp system with an EER of about 8 Btu/watt or higher.

[dangpgt97]

The issue is that it takes energy to subcool the liquid. That energy could be from fans or a pump running faster, a pump or from the compressors. So the end question is if the "energy savings of subcooling" is greater or less than the energy it takes to subcool.

I agree with that. Its kind of like do the ends justify the means. Got to admit though it has some effect on energy being positive or negative probably not a complete wash. Interesting though:)

[beachtech]

yeah, but how much energy does a fan motor use, compared to a compressor.

or, same motor could be used, and a larger coil could be used. makes me think of 13seer resi units :D

i was confused for a few posts there, then i realized efficiency was being used in two different ways.

one was thinking electricity and the other refrigeration :confused:

[MikeySq]

Unless your in a cold climate and your rack is set up for ambient subcooling.

yipper, how cold is it where you are today?? 5*F on the roof today the drop leg was nice and chilly on the one rack that had enough gas haha and I was tryin to get 4 Neglected racks goin good, 2nd day in a row home after 9:30pm, with a 6:30 am start!!

Mike

[dangpgt97]

We were theorizing on the effects of energy consumption and system capacitys to remove heat. The debate was about the means used to gain more subcooling to reduce the amount of flash gas needed at the exit of the txv. If the energy required to further subcool the gas is worth the increase in system capacity. icemeister brought up an interesting point. Its all debatable and probably wouldnt amount to a hill of beans.

Sometimes energy consumption with refrigeration performance is obvously like excessive superheat. High superheat is undesirable since it will result in a less than optimum filling of the evaporator and reduced evaporator efficiency. Accordingly, the power consumption of the refrigeration circuit will be higher than what is actually needed.Its related. They can go hand in hand. Sorry for the confusion beach.:)

[beachtech]

We were theorizing on the effects of energy consumption and system capacitys to remove heat. The debate was about the means used to gain more subcooling to reduce the amount of flash gas needed at the exit of the txv. If the energy required to further subcool the gas is worth the increase in system capacity. icemeister brought up an interesting point. Its all debatable and probably wouldnt amount to a hill of beans.

Sometimes energy consumption with refrigeration performance is obvously like excessive superheat. High superheat is undesirable since it will result in a less than optimum filling of the evaporator and reduced evaporator efficiency. Accordingly, the power consumption of the refrigeration circuit will be higher than what is actually needed.Its related. They can go hand in hand. Sorry for the confusion beach.:)

no, it's cool. i am still learning :) and this has been a really good read :) thanks guys :)

and thank you for helping me understand :)

[davidd_danfoss]

Or unless you're subcooling liquid for a low temp system with an EER of around 4.5 Btu/watt using a separate medium temp system with an EER of about 8 Btu/watt or higher.

The MT subcooling the LT is one of the ones that can save energy. However, the subcooling is still coming from a compressor, it is similar to an intermediate stage on a compressor.

[Phase Loss]

Some good reading in this link, some is off topic. but still good reading.

http://www.refrig.com/Characteristics%20of%20Evaporators095.pdf