Something I missed?

[busman]

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?

[Gary]

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

[chemi-cool]

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

[Gary]

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.

[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).

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!

[Gary]

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.

[shogun7]

[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 it

[peterweston]

Does the unit ever cylce? Not sure on the temp as i am in Australia and indoor heating not used that often.

[busman]

So have I dropped off my internal coil velocity too far you think?

[busman]

Does the unit ever cylce? Not sure on the temp as i am in Australia and indoor heating not used that often.

Average wintertime ambient here can range from 25-35°F (sorry guys, I'm stuck on the dumb American system of measurements) over night, which is not too bad. However, last year, was a real bear for this area. Overnight lows were down to about +5°F for about 3 weeks straight. And this heat pump was still heating. Yes, the unit does cycle very well.

[Gary]

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.

[busman]

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.

[Gary]

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.

34 - 29 = 5F dT
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... LOL

[shogun7]

Effing moron.

Proximate Genus: Frosting.
Specific Difference: Uneven between circuits.

How does your question tackle the specific difference when it doesn't even satisfactorily indicate a proximate genus?

Oh stick it in your ear you sick baste88 What I find really funny Marc is youve been all over the place with guesses as with Gary has been at least consistant but hey keep guessing you might hit the right solution

[Gary]

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.

[shogun7]

Dummy, it's only a 10 Ton coil at it's nominal TD. Otherwise, it is just a coil. Until you state the index TD it is just a coil and not yet a coil with a capacity rating.

If it's a 10 Ton coil with a 15K TD then it's a 3.3 Ton coil at a 5K TD except then with poorer tube turbulence and heat transfer. Possibly even with poorer heat transfer in some circuits over other circuits.

I know that stupid! Did I say it was doing 10 tons at it's existing conditions? NO! I was referring to the fact that it's surface area was capable of doing 10 tons at std conditions. and it's not a 3ton coil at 5K it's a 10 ton coil doing 3 tons of refrigeration!!! so with all your bulsh** your looking either stupid dumb or fuffering from constipation of the cerebral And don't call me names...Dummy!