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Thread: Something I missed?
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13-12-2004, 09:22 AM #1
Something I missed?
I designed my own heat pump for my house. It's been running now for about 2 years now, but there is one question that I have failed to find an answer to now, and thought that someone here, may have an answer.
Here's the problem:
When running in heating, the outdoor (evap) has 7 circuits for the distributor. 3 ton nominal system. R22. Balanced port TXV. According to Sporlan's software, the TXV, distributor, and orifice all work good on paper. I have an un-even frosting pattern on the coil however. I'm wondering now, do I have too many circuits?
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13-12-2004, 03:13 PM #2
Re: Something I missed?
Is the frost uneven between circuits (more frost on some circuits than others)? Or is the frost uneven across the circuits (more frost at beginning of circuits than end of circuits)?
What are the following:
Evap air in temp
Evap air out temp
SST (or low side pressure)
Suction line temp near evap outlet
Suction line temp near compressor inlet
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13-12-2004, 03:20 PM #3
Re: Something I missed?
I will go for poor soldering of the distributor, some of the tubes are partialy blocked.
On another thing, I'd go with Marc, air flow through the coil.
Chemi
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13-12-2004, 03:32 PM #4
Re: Something I missed?
We can't see it from here. The only way we can "see" a system online is by the description and the temperature measurements.
Going on the insufficient information given, my guess would be an oversized coil, in which case the evap dT will be very low and the superheat will match the evap TD.Last edited by Gary; 13-12-2004 at 03:44 PM.
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13-12-2004, 05:28 PM #5
Re: Something I missed?
Well, air flow seems to be fine, in fact too much (approx 5000 cfm) with an 825 rpm 1/2hp fan motor 26" 4wing 27° pitch. Oversized coil, yes, old 10 ton a/c condensor coil with 7 circuits, and 1 subcooling loop (not used in heating).
Since it's a heat pump, the outdoor temperature varies much, and affects all the numbers. Seem to notice it most when temperatures get cold outside, and frosting begins on coil.
Several circuits in the middle of the coil is what is frosting up. There is flow through all circuits, all the time. It's not a no-flow situation in some circuits. Solder job looked great on install, looked down inside, and no visible obstructions in the refrigerant path. Upon running, there is a band of frost the appears in the center two or three circuits. It's not at the bottom, (along the ground where ground moisture may be more abundant) or at the top, but dead in the middle.
Distributor is pointing downward to lend to proper distribution.
Moisture and dirt *should* not be present, or any other form of contaimination. Pulled an eight our vacuum on the system at 500 microns when I got it, pressurized with nitrogen. It sat for several years, assembled system purging with nitrogen during brazing process, had another 500 micron vacuum and have suction *filter* and bi-flow liquid filter/drier.
System mass flow, that's something I have not explored. Can you expound on that?
Thanks!
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13-12-2004, 09:20 PM #6
Re: Something I missed?
As I understand it, the outdoor coil is a 10 ton coil with a 3 ton TXV and a 3 ton compressor.
The coil being grossly oversized, likely the superheat at all of the suction ends as well as at the TXV bulb, will be identical to the TD. IOW, the suction line readings will all be at ambient.Last edited by Gary; 13-12-2004 at 09:31 PM.
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13-12-2004, 10:00 PM #7
Re: Something I missed?
Originally Posted by Marc O'Brien
Given a 10 ton coil and 5000 CFM of airflow, the 3 ton flow of refrigerant will be completely boiled off long before it gets to the other end of the coil, and the suction cannot rise above ambient because there is no (above ambient) heat source to cause it to do so. Therefore all coil outlet superheats should theoretically match the TD.Last edited by Gary; 13-12-2004 at 10:26 PM.
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14-12-2004, 12:06 AM #8
Re: Something I missed?
Hopefully Busman will provide us with temperature measurements so we can see what it is doing.
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14-12-2004, 03:47 AM #9
Re: Something I missed?
TXV is a Sporlan C-charge head (medium temp refrigeration.)
No flash gas is present in the liquid line. I have a site glass mounted in the liquid line at mid-point between the air handler, and the outdoor unit. This is outside my kitchen window, so that I can monitor system performance. The only time that bubbles are present, is during restart back to heating after a defrost. That may take 5 minutes for bubbles to clear at that mid-point site glass. There is a second site glass mounted at the inlet of the outdoor TXV. (wont Prof sporlan be happy!) That site glass remains clear almost always because it's at the lowest point in the system. No extra means of subcooling is used in heating mode. It comes directly out of the air handler (second floor) and down into the outdoor unit.
I have experimented with opening up the TXV to decrease superheat. My thinking was attempting to increase the pressure drop across the nozzel and distributor, as opposed to using the TXV pressure drop. This would also create a higher suction pressure too, which should increase the efficiency of the unit in heating mode. Regardless of TXV superheat setting, the results remained unchanged.
Side note: coil is clean. It's been washed out, cleaned out, cleaned with Calgon's Nu-Brite, so there "shoud" not be any thing restricting air flow through any part of the coil.
I will see what other hard data I can gather to see which path to go down next.
I have a second heat pump on the addition to my house. It is a factory built system. It's an ICP (Arcoaire) 12 SEER unit. The homemade system averaged about 15% higher suction pressure (in psia) than that of the factory system in heating mode.
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14-12-2004, 04:29 AM #10
Re: Something I missed?
Originally Posted by busman
Originally Posted by busman
Originally Posted by busmanLast edited by Gary; 14-12-2004 at 04:32 AM.
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14-12-2004, 05:11 AM #11
Re: Something I missed?
With that oversized coil, there is no way you can adjust the TXV superheat. Adjustment is futile.(QUOTE) Gary
With that said I believe you can get some sort of balance by eliminating 3 of the cap tubes and derating the air flow to about 3000 CFM
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14-12-2004, 05:40 AM #12
Re: Something I missed?
Originally Posted by shogun7
Actually, I had the other approach in mind: Leave the coil as is and drop the airflow down to about 1000 CFM, give or take a couple hundred.
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14-12-2004, 05:55 AM #13
Re: Something I missed?
Originally Posted by Marc O'Brien
Originally Posted by Marc O'Brien
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14-12-2004, 04:53 PM #14
Re: Something I missed?
TXV is externally equalized (a must for distributor setups.) I don't recall the exact design temperature that I used. I attempted to try numbers at many different ambients to account for all the different temperatures that the unit would be exposed to.
I don't "think" oil logging is a problem with this setup. It's the same tubes over and over, and frosting begins just after defrost, almost too soon for it too begin to log any oil.
I have a set of gauges afixed to the unit. This morning, with an ambient of 36°F, it was running 48psig (24°F SST). No superheat reading available yet. I'm home taking care of kids today (sick wife), so that reading is going to be a little be difficult.
Will post more info soon. Thanks!
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14-12-2004, 05:49 PM #15
Re: Something I missed?
Hi busman.
At 36F, frost on the outdoor coil is normal.
I dont know what control your defrost, but you can use a thermostat for controlling the amount of frost.
At these low ambients, you will have more defrost cycles per hour.
Chemi
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14-12-2004, 07:56 PM #16
Re: Something I missed?
I have to say that it is fun watching you convert temperatures for a change, Marc.
As I see it, the factory superheat setting is more like 7F. Since the saturated mixture is boiling off long before it reaches the bulb, the TXV is "seeing" 12F superheat (equal to TD), and is running wide open in a futile attempt to bring down the superheat.
I would love to do some experimenting here, but we need temperature readings so that we can do before and after comparisons.
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14-12-2004, 08:03 PM #17
Re: Something I missed?
One more thing that comes to mind, busman.
Is the outdoor fan sucking the air from the coil or pushing it through?
If it pushes the air through the coil or if it sucks it but it is close to the coil then there is almost no air movement at the centre of the coil
Chemi
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14-12-2004, 08:16 PM #18
Re: Something I missed?
I used a balanced port TXV in this application, because I thought it would do a better job of controling the refrigerant in this wild application.
I will go out and get some pics, and parts list of this setup soon.
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14-12-2004, 08:26 PM #19
Re: Something I missed?
Originally Posted by chemi-cool
The issue is the imbalance of frosting on the coil between circuits.
I also have put on a time delay relay that runs the outdoor fan for an extra 2 minutes after compressor shut down to allow warmer air in milder temps to help melt off frost, and prevent the need for more call for defrosts.
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15-12-2004, 02:14 AM #20
Re: Something I missed?
[QUOTE=busman]Well, air flow seems to be fine, in fact too much (approx 5000 cfm) with an 825 rpm 1/2hp fan motor 26" 4wing 27° pitch. Oversized coil, yes, old 10 ton a/c condensor coil with 7 circuits, and 1 subcooling loop (not used in heating).
How does one use a 3 ton valve to feed a 10 ton heat exchanger and not starve the coil even if its wide open?
Seems to me you will never get the proper mass flow to feed all the circuits properly (evenly?) thats why I suggested cutting out some circuits and air flow. He's pumping 5 to 7 tons of air through the coil with 5000 CFM in addition to starving itLast edited by shogun7; 15-12-2004 at 02:19 AM.
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15-12-2004, 03:48 AM #21
Re: Something I missed?
Well, it's a 10 ton condensor coil, but if you go by coil face area, it's about close to some of your more garden variety residential systems. I think the way that got 10 tons out of this stinker was moving a huge amount of air, and not really worried about SEER rating.
I was looking at the feeding characteristics tonight, (but was unable to spend enough time to take temps ) The distributor tubes are off of opposite sides of the distributor, so it kinda rules out blockage on one side of the distributor idea.
Oh, sorry, it was asked before and I forgot to answer. It's not the original fan motor. I replaced it with a slower RPM motor. The original was a 1 hp 1075 rpm motor. Now I have an 825 rpm 1/2hp motor, with original fan blade. I could get away with a 1/3 hp, but when it comes to the coil frosting up, the motor begins to lag down too much. So, electrical draw is much more appealing. Original 5 Amps, now just under 2.
As promised here is the stat sheet on the parts:
Copeland ZR34K1PFV Scroll Compressor
Sporlan BFBVE body with C charge power head
Sporlan ASC7 Aux Side connector
Sporlan 1113-7-1/4 distributor
Sporlan #3 nozzle for distributor
Now, here's something that I forgot all about, until I started looking at this thing again tonight. Sporlan did not offer a 3/16 connection for distributor that I needed. I needed 3/16 according to their program to gain enough pressure drop for a proper operating distributor. So, I used a 1/4 distributor. I used 1/4" tubing stubs, exactly cut, fitted the 3/16's into the 1/4" tubing, at exact insertations, (so all would be equal). The other part that I noticed, was I had to use an ASC7 (7/8) connector to fit the distributor. So, I had to sweat a reducer between the TXV and the ASC.
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15-12-2004, 10:51 AM #22
Re: Something I missed?
Does the unit ever cylce? Not sure on the temp as i am in Australia and indoor heating not used that often.
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15-12-2004, 11:31 AM #23
Re: Something I missed?
So have I dropped off my internal coil velocity too far you think?
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15-12-2004, 11:47 AM #24
Re: Something I missed?
Originally Posted by peterweston
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15-12-2004, 02:07 PM #25
Re: Something I missed?
When are you going to give us some temperature readings, so we can see what this thing is doing?
Evap air in temp
Evap air out temp
SST (or low side pressure)
Suction line temp near evap outlet
Suction line temp near compressor inlet
A measurement is worth a thousand calculations.Last edited by Gary; 15-12-2004 at 02:15 PM.
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15-12-2004, 02:32 PM #26
Re: Something I missed?
Originally Posted by busman
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15-12-2004, 10:12 PM #27
Re: Something I missed?
Okay, here are the winning numbers:
34°F Ambient Temp
46#psig Suction Pressure
(22°F) SSTemp
32°F Suction Line Temp at compressor
32°F outlet on a circuit that has known to frost (not during measurement)
Discharge air temperature 29°F.
So, looks like about a 5°TD on the coil, running 10°F Superheat.
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15-12-2004, 10:41 PM #28
Re: Something I missed?
Originally Posted by busman
34 - 22 = 12F TD
29 - 22 = 7F approach
32 - 22 = 10F superheat
So, looks like about a 12F TD on the coil, running 10F superheat.
I'm surprised that the superheat is 2F below the TD. I would question the instrumentation. But I'm thinking Marc may not agree... LOLLast edited by Gary; 15-12-2004 at 10:59 PM.
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15-12-2004, 10:46 PM #29
Re: Something I missed?
Here's an experiment you may want to try:
Remove the fan blade and flip it over, so that it "backhands" the air. This will greatly reduce the airflow. Then take a new set of readings.
With a 5F dT, you are not getting 5000 CFM, or anywhere in that neighborhood. By downsizing the motor and RPM you have brought the CFM down almost into the acceptable range.Last edited by Gary; 15-12-2004 at 11:02 PM.
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15-12-2004, 10:51 PM #30
Re: Something I missed?
Just to clarify:
When you measure the change in temperature of a single thing, such as a stream of air, the change is a delta-T or dT.
When you compare the temperatures of two different things, such as the ambient temperature and the refrigerant temperature, the difference is a TD.
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15-12-2004, 11:13 PM #31
Re: Something I missed?
Originally Posted by Marc O'Brien
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15-12-2004, 11:29 PM #32
Re: Something I missed?
Given a substantial reduction in airflow (flipping the blade), this should bring the TXV within its full control range, and circuit outlet superheat differences would then be much more pronounced.
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16-12-2004, 02:05 AM #33
Re: Something I missed?
Originally Posted by Marc O'BrienLast edited by shogun7; 16-12-2004 at 02:09 AM.
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16-12-2004, 02:17 AM #34
Re: Something I missed?
Originally Posted by Marc O'BrienLast edited by shogun7; 16-12-2004 at 03:07 AM.
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16-12-2004, 02:34 AM #35
Re: Something I missed?
One other thing I noticed. I compared the two running systems tonight when I took the readings. The 12 SEER factory system was running at 40# of suction pressure. I like the higher suction pressure of the homemade one better.
Counterflow - okay, I know what counterflow means, but counterflow what? Counterflow of air, counterflowing refrigerant, or counterflowing of what?
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16-12-2004, 03:04 AM #36
Re: Something I missed?
Originally Posted by busman
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16-12-2004, 05:11 AM #37
Re: Something I missed?
Originally Posted by busman
I would expect little if any difference in suction pressure. What I expect to change is the superheat.Last edited by Gary; 16-12-2004 at 05:13 AM.
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16-12-2004, 03:38 PM #38
Re: Something I missed?
Originally Posted by Marc O'Brien
Given less airflow, I would expect the saturated mixture to then reach the bulb, allowing for 7F superheat. IOW, it would allow the valve to close some more, not open some more.
BTW, I'm thinking the suction line is in the (lower temp) discharge air stream, which would account for lower than ambient suction line temp. This would explain why the superheat does not currently match the TD.
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17-12-2004, 02:02 AM #39
Re: Something I missed?
I think that the TXV is throttling as it should be. Last year, I do remember experiementing opening up the TXV more, to reduce superheat.
Now, if I could figure out how to post the pics from the morning. I took some pics of the frosting pattern. I was not as pronouced this morning, as I have seen it before, but you get a general idea of what's happening. Maybe this something I'm just going to have to live with, or not, I don't know. I was unable to take the specific circuit readings this morning, because I had a relay die on it this morning. My switchover relay to allow my hydronic heat to take over Aux heat failed, and would not allow any Aux heat to come on (hydronic or electric backup) Needless to say, the temp was plumiting when I got up this morning when it went into defrost.
Hopefully, conditions will come around again here pretty soon, for good dewpoint & temp's for making another good batch of frost, so that I can get some good readings.
I'm a little leary of some of my readings with this thermocouple thermometer. Whenever the instrument gets cold itself, the temperature likes to drift. So I must keep the instrument warm while taking readings in my coat pocket.
Two pics are: (1) the in the dark photo, soon after defrost, and the (2) photo is just as defrost begins.
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17-12-2004, 02:14 AM #40
Re: Something I missed?
Other pics to check out:
First, is just a general pic overhead of the unit.
Second, is the inside of the unit, and a pic of the distributor.
The unit is a TTA120 Trane.
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17-12-2004, 12:43 PM #41
Re: Something I missed?
It looks like each circuit does four complete loops, two of which are frosted. All of the refrigerant is boiled off half way through. It will be interesting to see what it looks like with the blade flipped over.
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18-12-2004, 12:05 AM #42
Re: Something I missed?
Originally Posted by Marc O'Brien
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18-12-2004, 01:09 AM #43
Re: Something I missed?
So Gary, we know that the larger the coil surface area of the evaporator, the closer the coil temperature will be to the entering air temperature. (low TD)This happens because of a higher operating coil temperature with a larger coil and the A/C unit would be running higher evaporating (suction) pressures and temperatures. So if we cut the air flow too low it seems to me that the frosting problem will be exiserbated. What say you?
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18-12-2004, 04:56 PM #44
Re: Something I missed?
Originally Posted by shogun7
With less airflow, the liquid/vapor mixture will make it further through the circuit before it is entirely vaporized. Given enough airflow reduction, liquid/vapor mixture will make it to the TXV sensing bulb and the TXV will begin to control the flow.
At that point, all four loops in each circuit will be frosted. IOW, the frost should be evenly distributed over the entire coil.
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18-12-2004, 08:35 PM #45
Re: Something I missed?
Well, something else I would like to throw in the mix here of discussion, is the amount of refrigerant charge. If I slow down the fan speed, [in my mind] it will require more refrigerant to run in colder temperatures. This thing already has a huge amount of refrigerant in it, and to require more, may in milder temps, run a sky high head pressure. It currently runs over 325 head on mild temperature days, and hate to see rise much higher than that.
When all these holiday engagements slow down, I hope to get some numbers. . .
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18-12-2004, 08:57 PM #46
Re: Something I missed?
Originally Posted by busman
Then report the following temps so we can see everything the system is doing:
Condenser air in temp
Condenser air out temp
Saturated condensing temp (from P/T chart)
Liquid line temp near condenser
Liquid line temp near metering device
Evaporator air in temp
Evaporator air out temp
Saturated suction temp (from P/T chart)
Suction line temp near evaporator
Suction line temp near compressorLast edited by Gary; 18-12-2004 at 09:00 PM.
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18-12-2004, 09:41 PM #47
Re: Something I missed?
BTW, if you are charging by sightglass you are doing it wrong. Yes, I know it's the way everyone does it. Over the past couple decades I have removed more refrigerant from systems than I have put into systems. Everyone overcharges.
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19-12-2004, 02:09 AM #48
Re: Something I missed?
Slow the airflow down enough for the saturated mixture to reach the sensing bulb. The TXV will pull back from the wide open position, and you won't need nearly as much refrigerant.
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19-12-2004, 02:13 AM #49
Re: Something I missed?
Originally Posted by Marc O'BrienLast edited by shogun7; 19-12-2004 at 02:25 AM.
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19-12-2004, 11:31 AM #50
Re: Something I missed?
An entirely plausible theory, and the frost pattern through the center lends credence to the uneven distribution part, however...
It seems unlikely that the TXV is throttling the refrigerant flow. If a saturated mixture were reaching the sensing bulb, then the (medium temp) TXV could be adjusted through its full range. As I recall, Busman was unable to open up the valve to further drop its superheat.
With only vapor reaching the sensing bulb, the superheat cannot be adjusted below the TD, although in this case the superheat seems to be slightly below TD, presumably because it is being influenced by the lower ambient surrounding the suction line.
In any case, the airflow needs to be reduced in order to achieve some rough balance.
Also, we need to look at the high side numbers, adjust the refrigerant charge, and then see what the numbers say about overall operation, as well as distribution.
An interesting project, for sure.Last edited by Gary; 19-12-2004 at 11:50 AM.