TXV vs fixed orfice

Just want some thoughts from this forum on the topic of using a Thermal expansion valve vs a piston or orfice in residential central a/c unit. I came across a study that was done on the amount of refrigerant charge that was in completed installs. And the results showed that the vast majority of installs were undercharged. Now from what I believe, a txv valve will be better in a unit that is not properly charged. The performance will be effected by about 5% decrease . However in a fixed orfice system, an inaccurate charge will result in a 15-20% loss of performance. So the real trick is to get the charge right in fixed orfice system and it will perform comparably to a txv equipped system. Your thoughts please. stiz

If the system is under charged, a TXV will start "hunting" this might cause liquid sludging back to the compressor, which in turn will kill the compressor valves...
That's why a system with TXVs should have a liquid receiver to ensure a good, continous supply of liquid.

In small air conditioning systems where capillary tubes or metering pistons are used, the charge is critical!
Those systems are called critically charged systems because charge accuracy on such systems should be within 1/100th of the total charge weight. Majority of systems charged by rule of thumb and other inappropriate methods are overcharged and poses danger for compressor.
That is why it is of utmost importance to accept god charging practice. This is one which I follow:
http://www.mediafire.com/?3xggwmbndxx

Nike 123:That is a very good article that you provided. I have saved it to my pc. How accuarte is the factory supplied precharge in a new condenser? As long as the line set length is the amount specified, say 25 feet(7.5 meters), and the evaporator coil is the proper match, will the factory installed precharge be accurate? What do you generally see as to the amount of refrigerant to add if any? thanks,stiz

To the best of my knowledge,
All reputable A/C manufacturers nowadays state that you have to charge by weight alone.
They will specify for what pipe-lengths their units are charged and provide formulas to calculate how much will have to be added or, rarely, removed.
If there been a leak they insist that the whole charge is reclaimed and then the correct charge is weighed back in.

And who am I to argue with Daikin San or Toshiba San (or anybody else for that matter)?

(And I do believe that they, on modern systems, has designed for a bit more "uncertainty", +/-10% on smaller a/c split systems, but I might be wrong here)

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Originally Posted by stiz

Nike 123:That is a very good article that you provided. I have saved it to my pc. How accuarte is the factory supplied precharge in a new condenser? As long as the line set length is the amount specified, say 25 feet(7.5 meters), and the evaporator coil is the proper match, will the factory installed precharge be accurate? What do you generally see as to the amount of refrigerant to add if any? thanks,stiz

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Here in Croatia, we installing Japanese, Korean and Chinese split systems for residential airconditioning who are already matched and we get charts for additional charging if lines exceeds
lenght for which is unit factory precharged. Because these unit uses rotary compressors for which suction accumulator is mandatory and it's constituent part of compressor, smaller units (9000-12000 Btu) don't need to adjust charge for lines not longer than 5 meters.
For larger units (18000-30000 Btu ) that could be up to 15 meters of line length.

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Originally Posted by The Viking

If the system is under charged, a TXV will start "hunting" this might cause liquid sludging back to the compressor, which in turn will kill the compressor valves...
That's why a system with TXVs should have a liquid receiver to ensure a good, continous supply of liquid.

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Hi, Viking.
Why do you think that TXV will start "hunting" when system undercharge?

Stage 1.
The TXV get a solid supply of liquid and maintains a good superheat.

Stage 2.
The TXV get liquid/gas mixture, not enough refrigerant passes through the TXV, it's bulb temperature rises whilst the pressure doesn't. The TXV will then respond by opening up.

Stage 3.
Solid liquid supply is reinstated but now to a fully open TXV, the evaporator coil gets flooded, the bulb temperature drops. The TXV will respond by closing itself (if you are lucky, quick enough to save the compressor).

Stage 4.
Scroll up to stage 1.


Oh, S##t.

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Originally Posted by Sergei

Hi, Viking.
Why do you think that TXV will start "hunting" when system undercharge?

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I didn't answer your question, did I.
Sorry, I have to learn to read the question before giving an answer....


-Experience.

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Originally Posted by The Viking

Stage 1.
The TXV get a solid supply of liquid and maintains a good superheat.

Stage 2.
The TXV get liquid/gas mixture, not enough refrigerant passes through the TXV, it's bulb temperature rises whilst the pressure doesn't. The TXV will then respond by opening up.

Stage 3.
Solid liquid supply is reinstated but now to a fully open TXV, the evaporator coil gets flooded, the bulb temperature drops. The TXV will respond by closing itself (if you are lucky, quick enough to save the compressor).

Stage 4.
Scroll up to stage 1.


Oh, S##t.


I didn't answer your question, did I.
Sorry, I have to learn to read the question before giving an answer....


-Experience.

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Stage 3. Suddenly, solid liquid supply is reinstated. This is so suddenly that TXV doesn't have a time to react. How did we get this liquid so suddenly?

If you look at a liquid line sight-glass in a slightly undercharged system, you will see that the liquid comes in bursts (or if you like, lots of bubbles, followed by little or no bubbles, followed by lots of bubbles and so on).
Also, varying conditions for the condenser will affect the liquid levels (for example a fan speed controller that's trying to cope with a hunting TXV)

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Originally Posted by The Viking

If you look at a liquid line sight-glass in a slightly undercharged system, you will see that the liquid comes in bursts (or if you like, lots of bubbles, followed by little or no bubbles, followed by lots of bubbles and so on).
Also, varying conditions for the condenser will affect the liquid levels (for example a fan speed controller that's trying to cope with a hunting TXV)

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Usually, slightly undercharge system has clear sight-glass or it has a few bubbles in this sight-glass. This can be explained by pressure fluctuation in condenser. If we have higher pressure in condenser(more refrigerant in it due to higher density), we will have a few bubbles in liquid line.
According to your scenario we have 2 opposite states of the system:
1. Liquid line is full of liquid, evaporator is full of liquid and liquid is in the suction line.
2. Liquid line is half full(a lot of bubbles), evaporator is less that 90% full, no liquid in suction line.
I have 2 questions.
1. Where is the refrigerant in state 2(compare to state 1)?
2. Why do these states change so quickly that TXV doesn't have time to react?

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Originally Posted by Sergei

Usually, slightly undercharge system has clear sight-glass or it has a few bubbles in this sight-glass. This can be explained by pressure fluctuation in condenser. If we have higher pressure in condenser(more refrigerant in it due to higher density), we will have a few bubbles in liquid line.
According to your scenario we have 2 opposite states of the system:
1. Liquid line is full of liquid, evaporator is full of liquid and liquid is in the suction line.
2. Liquid line is half full(a lot of bubbles), evaporator is less that 90% full, no liquid in suction line.
I have 2 questions.
1. Where is the refrigerant in state 2(compare to state 1)?
2. Why do these states change so quickly that TXV doesn't have time to react?

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Viking is 100% right, things do not all ways work the way paper says they will. Remeber the paper is how our modles think it will work, reality oft has other plans! If we work realy realy hard we can get the two to match up for the most part!

I just accept it comes in bursts and does odd things when under charged!

O.k Lets think about this.... if you have a capillary system that's short of gas then the main problem is a increase in superheat....??? i.e compressor burnt out now if you have the same system running a tx-valve then the superheat will still be to high due to the starving of liquid..... if you cant supply an adequate amount of liquid to the evap then compressor failure is likley... it doesnt mater if it's tx or capillary...! short of gas means increase in superheat no matter what...

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Originally Posted by The MG Pony

Remeber the paper is how our modles think it will work, reality oft has other plans! If we work realy realy hard we can get the two to match up for the most part!

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Thank you for your very valuable advice, but I have 30 years experience in refrigeration. How old are you?
I believe this forum about exchanging opinions and about proving your point. If you don't know how to prove your point, get additional theoretical knowledge as well as practical. Good luck.

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Originally Posted by setrad7791

O.k Lets think about this.... if you have a capillary system that's short of gas then the main problem is a increase in superheat....??? i.e compressor burnt out now if you have the same system running a tx-valve then the superheat will still be to high due to the starving of liquid..... if you cant supply an adequate amount of liquid to the evap then compressor failure is likley... it doesnt mater if it's tx or capillary...! short of gas means increase in superheat no matter what...

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Ya but a TXV is not a fixed metering device now is it! It will open up, fluid will surge through the coil and due to pressur wont have time to boil off thus a slug, probably not as simple but gives an idea.

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Originally Posted by Sergei

Thank you for your very valuable advice, but I have 30 years experience in refrigeration. How old are you?
I believe this forum about exchanging opinions and about proving your point. If you don't know how to prove your point, get additional theoretical knowledge as well as practical. Good luck.

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Oh I intend to, for the several years I been playing with it as a hobby I've loved it and plan on taking to engeenering, after all I've found my nich where natural aptitude and the love of technology mix perfectly!

My point was make a system and try it! as theoreticle is cute and all but I prefer real world knowledge and it will be fare more use full to have a modle system on your bench that you can observe and test it against theoretical knowledge. So my staement stands, build a system and see it! I'd do that now if it wheren't for the fact I'm away.

Again practical experiance with a real world modle system will yeild far superior resualts then a paper world gustimate, thats what I meant by practical experiance FYI

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Originally Posted by The MG Pony

Oh I intend to, for the several years I been playing with it as a hobby I've loved it and plan on taking to engeenering, after all I've found my nich where natural aptitude and the love of technology mix perfectly!

My point was make a system and try it! as theoreticle is cute and all but I prefer real world knowledge and it will be fare more use full to have a modle system on your bench that you can observe and test it against theoretical knowledge. So my staement stands, build a system and see it! I'd do that now if it wheren't for the fact I'm away.

Again practical experiance with a real world modle system will yeild far superior resualts then a paper world gustimate, thats what I meant by practical experiance FYI

n!

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How on earth, do you think that somebody could do some work and understand that what he do, without theoretical knowledge.
That is like comparing some alchemist, who his hole life makes chemicals with no knowledge about their chemical laws and properties, and some chemical engineer who knows very well what to expect of chemical experiment and laws behind processes in that experiment.
That is why is necessary, that this industry, somehow, "eliminate" those who don't understand what they are doing.

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Originally Posted by The Viking

If the system is under charged, a TXV will start "hunting" this might cause liquid sludging back to the compressor, which in turn will kill the compressor valves...

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I think I would change that from "will start hunting" to "can start hunting". The conditions would have to be just right... and flooding the compressor?... well... I would have to see that one. Maybe with a fan control, short suction line and oversized TXV... and maybe a remote condenser.

Theoretical is not much good unless the person has practical too, some one who has both will be far ahead of one with out one or the other, remember, theoretical is just a model of the actual, yes those models are pretty dam excellent, but models non the less, there will all ways be a situation where only imperially acquired data will work! I've met people who can tell you every thing to know about a router, but when confronted with a real one didn't even know where to plug the cables in, yet he discribed it and its function perfectly, as in he learnt the paper side wonderously yet lacked practical. IMO One must have both, but I'd put more weight on the practical guy as he'll be better abble to figure a system out in the real dynamic world!!!

Thats what I am saying,

(What the hell is going on? Post a reply and it brought me to the welcome page! odd!)

TXVs and fixed orifices are metering devices. Orifice is just wide open TXV. People have concerns about wide open TXV. Replace TXV with orifice. Did anybody see undercharged residential A/C(metering is orifice) with flooded compressor?

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Originally Posted by Sergei

Hi, Viking.
Why do you think that TXV will start "hunting" when system undercharge?

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I was wondering about this one myself.
I never installed a suction accumulator in the 25 years I'm in this business and I never had a broken compressor due to liquid slugging provoked by a lack of gas.
I disagree this explanation.

Also, varying conditions for the condenser will affect the liquid levels (for example a fan speed controller that's trying to cope with a hunting TXV)
I don't see why the fan has something to do with a hunting TEV.

MG Pony, is it really necessary to react this way (your post of 16/01/2008 and following) You started something which we don't like here.
I suggest you edit all your posts or I will do it.

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Originally Posted by The MG Pony

Theoretical is not much good unless the person has practical too, some one who has both will be far ahead of one with out one or the other, remember, theoretical is just a model of the actual, yes those models are pretty dam excellent, but models non the less, there will all ways be a situation where only imperially acquired data will work! I've met people who can tell you every thing to know about a router, but when confronted with a real one didn't even know where to plug the cables in, yet he discribed it and its function perfectly, as in he learnt the paper side wonderously yet lacked practical. IMO One must have both, but I'd put more weight on the practical guy as he'll be better abble to figure a system out in the real dynamic world!!!

Thats what I am saying,

(What the hell is going on? Post a reply and it brought me to the welcome page! odd!)

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No one here has just gone to uni and breezed through the course and then sat a MSCE or cisco exam. The refrigeration industry is nothing like comp sci or network admin. I even left the trade in 2001 to study comp sci, so I know the kinds of "know it alls" in those fields.
Most of us have studied theory as well gone on the tools so we know what's what.
You don't have to tell me about people specialising in theory, every PC forum I post in saying something about refrigeration always has some uni student trying to start a debate with me because they know better somehow.
But leave your type of attitude to those types of forums, please don't come here and say you have more knowledge than people who have been on the tools since before you had an interest in refrigeration.

Most of us have dealt with an ever changing industry, so we aren't stubborn and have had to constantly re-learn as we go. So we are not just stuck in a perfect theory world, we are continually getting feedback and learning, unlike those people you mentioned with easy one off certificates from the IT world.

edit: I'm guess what I'm trying to say is your prejudices are valid for people on an easy to get one off certificate that was 3+ years out of date when they studied it, uni curiculums being far behind as they are. But they don't apply to to refrigeration when most people learn on the tools before they even pick up a few books and study it in length in order to learn the theory to improve their knowledge.

There is difference between TXV "hunting" and modulating.
TXV "hunting", usually, happens:
1. TXV superheat setting is too low.
2. Refrigeration load is significantly less than TXV capacity.
TVX will "hunt" from 0% capacity(closed) to 20-40% capacity and back to 0%. Some liquid can go into the suction line and into the compressor. Precondition for this "hunting" is enough liquid refrigerant to fill up liquid line, evaporator(100% full) and some liquid for suction line. We don't have this liquid in undercharged system.
TXV modulating is regular operation. TVX capacity will change from 70% to 100% and back to 70% in response to fluctuating head pressure, suction pressure and refrigeration load.

If under charged with a dusty condencer the pressure can be kept high enough to allow the under charge to condence in the liquide line rather in the conder, thus as it opens to full capacity allowing the amount in the ll(Liquid Line) to surge through, after repeating this it can cool the evao suitably to make it to the suction line (Or be carried through the evap with no work don due to volocity) and thus cooling only suction line, oil build up will be attenuated by the lack of liquid refrigerant flow complicating the issue.

Then on the next hunt cycle liquid can pluasibly make it bake to the compressor

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Originally Posted by paul_h

No one here has just gone to uni and breezed through the course and then sat a MSCE or cisco exam. The refrigeration industry is nothing like comp sci or network admin. I even left the trade in 2001 to study comp sci, so I know the kinds of "know it alls" in those fields.
Most of us have studied theory as well gone on the tools so we know what's what.
You don't have to tell me about people specialising in theory, every PC forum I post in saying something about refrigeration always has some uni student trying to start a debate with me because they know better somehow.
But leave your type of attitude to those types of forums, please don't come here and say you have more knowledge than people who have been on the tools since before you had an interest in refrigeration.

Most of us have dealt with an ever changing industry, so we aren't stubborn and have had to constantly re-learn as we go. So we are not just stuck in a perfect theory world, we are continually getting feedback and learning, unlike those people you mentioned with easy one off certificates from the IT world.

edit: I'm guess what I'm trying to say is your prejudices are valid for people on an easy to get one off certificate that was 3+ years out of date when they studied it, uni curiculums being far behind as they are. But they don't apply to to refrigeration when most people learn on the tools before they even pick up a few books and study it in length in order to learn the theory to improve their knowledge.

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Ok thats good to know, it just burns me when they pull the age card to trump a statment, shoot down a statment with technical data not 10 or 30 years of this or that I've learnt that time alone is not a qualifier, and learnt it well. I am sure you have had to have met the types I'm talking about!

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Originally Posted by Peter_1

MG Pony, is it really necessary to react this way (your post of 16/01/2008 and following) You started something which we don't like here.
I suggest you edit all your posts or I will do it.

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Can you do it as I can not see what you see.

As a TXV hunts, or even modulates, liquid will surge in the liquid line, possibly causing bubbles to appear in the sight glass. Adding refrigerant will eliminate the bubbles, but if the TXV is still hunting/modulating the surge is still there... you just can't see it because it isn't bubbling.

It is the hunting/modulating that causes the surge, not the reverse. Did adding refrigerant fix it? No, it just made the liquid surge invisible.

Can a TXV hunt with a slight undercharge? Yes, but the undercharge is not the cause of the hunting. There is some other problem that is the cause.

The txv doesn't lose as much capacity as does the piston or cap at lower capacities.

according to ARI:

Coil capacity is deteriorates precipititiously below 95% and above 105% of correct charge.

KW demand increases exponentially above 105% of correct charge -- both txv & caps prefer undercharge.

Also, SEER isn't affected as much with an undercharge as with overcharge -- SEER plummets when above 105% of correct charge, not to mention possible floodback.

What is the exact location and how far from the condenser should the sight glass be located, so as to get the correct information about the system being fully charged.

A friend of mine came up with the following quote:

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Those who do not understand the operation of a thermostatic expansion valve (TXV) blame it for system problems. Those who do understand know why a TXV is nice to have.

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I actually agree with it. The ability to adjust to wide variations of load over a range of condensing pressures really helps efficiency.