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Thread: Restriction and no superheat?
06-22-2008, 09:55 PM #53
Evaporater pressure is a closing force of the TXV. the other being spring pressure and opening being the bulb.
the TXV will care about distributors being 3/4 blocked, However if the distibutor being blocked will cause pressure to back against the TXV. It may thing there is plenty of evap pressure and further close more, Which may add to the problem here.
06-22-2008, 10:43 PM #54Professional Member*
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- Jun 2008
- NW Florida
Just a suggestion
Next time you suspect a restriction try running unit with indoor blower disconnected. Frost will stop at restriction. If it stops at valve then its your valve stopped up or not feeding properly.
06-23-2008, 02:04 AM #55Regular Guest
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- May 2008
- Southern California
Wow, good discussion! I didn't know they made them in sizes down to 7.5!
But really, In spite of my math error I'm sticking with my original post. If you don't include 75% of the air and fin area of the evaporator (that is inactive) you will have your symptoms. With a 1.25t evap and 500cfm of air a compressor designed for 5t and 2.5k of air will have low suction and low superheat.
I don't have the benefit of an engineering degree or years designing systems but I did stay at a Holiday Inn last night.
06-23-2008, 07:18 AM #56
06-23-2008, 07:30 AM #57
way to subcool the rhetoric dudeI don't have the benefit of an engineering degree or years designing systems but I did stay at a Holiday Inn last night.
I started out in Jorgas's camp a bit on intuition but I'm backing around to Andy's scenario that the blocked tubes are effecting a low load scenario but not significantly increasing the pressure drop. So, the way to figure this out I would think is to have a schrader in the few inches after the TEV just before the entry to the evaporator as well as on the suction. Then you are actually monitoring pressure drop associated with the evaporator alone.
Both Quixote and Andy have said that this type of scenario can produce low superheat - e.g.
The TEV will control the flow thru the one distributor tube. Hence the low evaporator/suction pressure. And operate at a low to normal superheat. That is what the TEV is designed to do.
But maybe we could give it a range. HVAC 689 was had 275 heads and 30 tails so that's a PD of 245 psi. My normal range for the chillers I work on is 190-200 vs. 55-60 so PD is about 135 psi. On startup maybe of course its 0 but early on it might be in the range of 150-60 vs. 70-80 so maybe early start open valve is a PD of 80 psi.
The latter is just a semi-educated guess while the former two scenarios are reported operating conditions.
So at the higher suction pressures the boiling point is much warmer and the system quickly boils everything in the evaporator under normal circumstances and warms the resulting gas giving much superheat. Valve responds by throttling down toward normal operating conditions and presumably if the valve is sized to the load somewhere in the center of its own operating range.
This creates modestly lower suction pressures and lowers the boiling point of the refrigerant.
Now we reach the point where I got fooled in thinking about plugged evaporator tubes. I'm thinking that if 3/4 of the air flow is not getting any cooling then the airflow will be warm relative to normal operating conditions and its going to warm the refrigerant in the remaining tube and boil it all and leave excess superheat. But you don't have the heat transfer area to do that. While it is tending in that general direction, the TEV sees the rising superheat...
(Andy maybe as design engineer you can expound on exactly where in the valve it senses or factors the pressure drop or measure of suction pressure that allows it to 'calculate' superheat, since of course that varies with pressure. D. Griff says:
Evaporater pressure is a closing force of the TXV.
...so the design of the valve to throttle on downstream pressure is augmented by the temperature bulb which throttles more decreasing suction pressure significantly, lowering the boiling point way down. Now suction temps will be significantly below normal operating. The one tube will have a higher temp difference with house air and have more capacity to boil the smaller amount of liquid being passed through to it (and of course head pressure will increase necessarily. This would be the main symptom of the oversize compressor although this will be slightly moderated by a less dense gas return, i.e. compressing a fixed volume of gas at 30 psi to a known smaller volume will result in lower head than compressing the same volume change commencing at 60 psi, but the additional restriction of the throttled valve is what is building head pressure.)
If the valve is actually able to maintain superheat at these lower temps with the modified load, then you're not flooding the compressor but will still observe iceback. I went back to the initial post and that is the symptom that HVAC689 reported. Aside from possible change in compressor noise and observing superheat are there other tricks for knowing if you're getting liquid back?
So finally if not briefly, Andy, on the lower throttling limits of the TEV. If the valve is designed to be able to close literally to virtually nothing then it controls for even the lowest loads and at some point you might get a suction pressure so low it trips the low limit. I have only observed this phenomenon in the case of a TEV failure where the bulb lost its charge and closed completely but I'm unfamiliar with what it might look like if the valve was just closed most of the way. If the bulb still has its charge, is the valve designed not to allow closure beyond a certain pressure drop such that low superheat would result, as you and Quioxte pointed out?
Or does low superheat result from an inability to adjust in infinitely small increments towards closed at the low end?
My observed low load dilemma, not quite as deep a trough as HVAC689's was:
suction 48 psi 27 deg.
that gives me maybe 3 deg. superheat on a valve I thought was designed to show 10.
In another thread I asked if I should adjust the valve and I got the "put down the wrench and get away from the valve" mantra. I'm waiting for the next heat spell to test under load before I do anything, but can you explain why low load might show low superheat with an fully operational TEV?
Seeing as you have Sporlan experience I can stroke that because this was a trane chiller with the XVE 5 valve fitted OEM (on a 7.5 ton unit that seems out of spec according to your charts but since it worked fine for 18 years I'm thinking that the Trane design engineers knew more or less what they were about, feel free to expound on the benefits of using as small a valve as possible).
The capillary bit the dust. I went down to the local refrigerator store and they did not have the XCP-100 head for it so I subbed an XGA head. Just to give you the full parameters.
When I started up everything looked great at first but then we stabilized at low suction and temp as detailed above. I thought maybe there was a slight mismatch between the valve and the head and was going to adjust - also got worried by the frost back and I was closer to no superheat although not all the way there. Then an astute technician pointed out that my numbers were low load numbers, not necessarily indicating the necessity for valve adjustment. He was right in that the tower had been down and many folks turned off the blowers on their fan coils, so despite good circulation through chiller I might very well have been experiencing loads as low as 25%.
So the question is, if the valve is chasing superheat, does it run out of room to adjust at these lower suctions or why would I get a lower superheat in this circumstance?
There is no excuse for me...
... and there is no such thing as an EXTRA beer
PS, Quixote, I was wrong about the avatar. It shows up in my profile but the board doesn't like its size or something. It's under 150 by 150 but I guess I haven't made it a small enough file even thought it said it uploaded. Some other day I'll try and wring some more KB out of it with photoshop.
06-23-2008, 02:19 PM #58
A nice bulletin on this subject: http://sporlan.jandrewschoen.com/10-9.htm
06-23-2008, 09:17 PM #59
06-23-2008, 11:40 PM #60Professional Member
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- May 2007
Originally Posted by jogas
I don't know about any books, but I'm relying on 30 years of service technician experience at and including residential/commercial/industrial systems.
To predict what a TEV will do in this out of design condition is impossible. System configuration, load balance, TEV type, TEV bulb location make it a guess at best.
My 30 years in this industry primarily as an engineer at Sporlan, and having done service work myself suggests you might want to rethink this.
Great Argument Guys!
Just need some popcorn!
06-25-2008, 09:12 AM #61
...and a movie
Originally Posted by archibaldtuttle View Post
...so the design of the valve to throttle on downstream pressure is augmented by the temperature bulb which throttles more decreasing suction pressure significantly, lowering the boiling point way down.
Well, I've been rightly accused of worse. So to correct this misapprehension I should have said that lower suction pressure accompanied by a significant fall in temps associated with the resultant lower boiling point reduces superheat and the valve throttles to restore superheat.
That said, i was tyring to divide the physical throttling mechnisms which appear to be some transmission of the suction pressure to needle valve regulation and the capillary action upon the valve or seat itself. I understand they work in combination, but in the abstract if temperature were held constant and pressure dropped, I get it, your superheat is high so valve stays open. Likewise if pressure stayed the same and temp dropped then the valve throttles.
Also hoping that you can clarify if, in this context, when you talk about "TD", you mean temperature difference in the refrigerant itself or in the air or water medium carrying house warmth to and chill away from the tower. While this is another flip side of a pretty similar coin or a place at which you might say you almost can't talk about one without the other, or that one implies the other, since you use the acronym repeatedly, I'd like to make sure of which you are specifying.
Finally to my current problem with a Sporlan XVE 5 on a 7.5 ton trane chiller circuit.
Ran it again yesterday to check conditions before making any valve or refrigerant adjustment.
I had recovered the factory charge still in the circuit into an empty recovery can. Replaced the valve head which had lost its charge. Used an XGA head because the local distributor did not have an XCP-100 in stock.
Replaced the charge fully (obviously filtered in both directions).
turned on most of the fan coils yesterday and good circulation of water through the evaporator.
Here are the results.
Ambient outdoor temps in the low 70s, but this brick building was 10 degrees warmer because the tower had been off and it was hot an humid in the 80s up until a few hours before I got back to this job. Actually went in the morning to turn on the crankcase heater and then went back in the afternoon after some severe thundershowers that cleared the humidity and lower the temperature.
First stable plateau at
170 psi 76.8 F discharge, 54 psi 29.8 F suction, 57.4 F in wat., 53.4 F out wat.
then dropped towards these readings before satisfying my evaporator temp. sensor which is set fairly warm (50 D. F cutout and there is about a 9-10 deg. non addressable differential till cut-in) at the moment because of my low superheat condition and concern about liquid return.
165 psi 74 F discharge, 49 psi 27 F suction, 54 F in wat., 50 F out wat.
So my reading on this is that I have 14 deg. subcooling at the first plateau and finish out with 15 deg. subcooling
But my superheat is barely 2 deg. , not a very comforting place to be sitting.
Unfortunately, for all my careful metering, I forgot to take a careful ambient measure, but I was pretty damn chilly in shorts and T shirt and it might even have been in the upper 60s at that point, but my liquid line temps are getting with 4 to 8 degs of ambient. According to Jim wheeler I could charge this system a little bit more to chase ambient temps. But given that the subcooling is pretty normal and in desired range and I'm still at factory charge, less maybe some bit of an ounce of multiple hose connecting and the like, do these results indicate the possibility that I might have to violate the prohibition on adjusting the TEV? Should I charge a tiny bit and see if I can bring the subcooling down even more and suction pressure up a bit before I consider adjusting the TEV?
Thanks, I'll post the latter part of this post on the TEV adjusting thread we also have running. apologize for redundance, but for those using topics to guide them to information, this question should probably be over there as well.
There is no excuse for me...
... and there is no such thing as an EXTRA beer
06-25-2008, 07:52 PM #62
06-25-2008, 08:42 PM #63
After quite a bit of thought, I'm starting to understand what you're saying.
My problem was letting go of those other plugged tubes and their effect on the system.
When you say normal to slightly lower superheat, is this because of the thermal effect of the evaporator outlet (at the bulb location) that also has the inactive circuits connected to it that are warmer, causing a slightly higher bulb temp?
BTW, I'll take that serving of humble pie now.
06-25-2008, 09:54 PM #64
The additional superheat to stroke the TEV open is called opening superheat. The ARI Standard for rating TEVs allows TEV manufacturers to rate their valve at a maximum 7°F opening superheat.
Say the TEV's setting requires 4°F of superheat just to open. And also let's say we have a 4 ton rating for a TEV at a 6°F opening superheat. If we have 4 tons flowing thru this TEV at the valve's rating conditions, we would expect the TEV to control at 4 + 6 = 10°F superheat.
If this TEV only had to regulate 2 tons, as might be in the case if 50 percent of the distributor tubes were blocked, we would need only about one half of the 6°F opening superheat, or 3°F. Therefore, we would expect operating superheat to be 4 + 3 = 7°F superheat. This is where the superheat reduction is coming from.
In the case of 75 percent blocked distributor tubes: 4 + 1.5 = 5.5°F superheat.
As a practical matter TEV control will eventually become marginal well below the valve's rating. Having the TEV operate at say 25 percent of its rating will likely make its control goosey. Increasing its static superheat (adjusting the valve in) can help prevent floodback when the TEV is unable to control low load condition well.
06-26-2008, 06:36 AM #65
sludge and TDs
I could slow it down actually by dropping out one of the pumps. That more or less halves the pressure differential. I run at 35 psi static pressure so it cirulates well to upper floors and I think I could probably get away with running just one pump. I'm going to cycle it with one on this morning and note difference in TD (which turns out to be to be a synthesis of refrigerant exit and water entry temps).
More to the point, Andy...anyone, what is best way to clean a sludged chiller in situ?
This is drained annually. Runs out reasonably clean, but of course the drain only empties at one end of the chiller. I can flush it. Access to plenty of hot water. I have a couple outlets that I can run hot water into, including reversing and sending water back in the drain and out the top of the chiller. I could inject cleaning or descaling treatment anyone recommends...