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12-08-2009, 10:42 AM #1
Compact AWHP charge determination
This is a parallel thread to "AWHP superheat & sub-cooling", in order to allow logical progression to continue in that thread, without creating confusion with a new case.
I've started it in order to investigate the correct charge determination for a compact air-to-water heat-pump.
This system essentially follows the format as that for the previous test AWHP, with a few design optimisations, as the main modifications.
The condenser is also of a different family to the tube-in-tube condenser used in the "AWHP superheat & sub-cooling" thread.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-08-2009, 10:46 AM #2
Re: Compact AWHP charge determination
Experimental results after 2nd day's set-up. Refrigerant R-134a.
1st run - set starting charge mass - based on calculated prediction, set to 'hot charge condition' (minimum mass).
2nd run - added refrigerant - now up to mid charge between 'hot charge condition' & 'startup condition' (maximum mass).
The computed range between min & max charge is 220g.
Total compressor run time at Run 2, approximately 5 h.
TXV still at factory setting.
Discharge line insulation to be applied before next run.
I have an idea of how I'd like to proceed on this unit, for the next tuning point, but would really value the applied wisdom of forum members.Last edited by desA; 12-08-2009 at 10:52 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-08-2009, 06:24 PM #3
Re: Compact AWHP charge determination
PHE condenser?
Smaller evaporator?
Same compressor?
The condenser is working very well (5.32K approach), which is not too surprising given the light load from the evaporator.
The evaporator is working not so well (20.76K approach). (VIC to the rescue?)
The condenser outlet subcooling is 3.05K, while the TXV inlet subcooling is 5.62K. This is sufficient to feed the evap. Note that the cooling of the liquid line has added just enough subcooling to properly feed the evap. Is the liquid line insulated?
I would add a little more refrigerant to bring it up to 5.5K at the condenser outlet to see what effect this has on the condenser.
I would leave the TXV setting as is for now and focus on improving the evap appr.Last edited by Gary; 12-08-2009 at 07:50 PM.
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12-08-2009, 07:53 PM #4
Re: Compact AWHP charge determination
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13-08-2009, 01:15 AM #5
Re: Compact AWHP charge determination
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 01:31 AM #6
Re: Compact AWHP charge determination
Touche'.
Smaller evaporator?
Same compressor?
The condenser is working very well (5.32K approach), which is not too surprising given the light load from the evaporator.
The condenser outlet subcooling is 3.05K, while the TXV inlet subcooling is 5.62K. This is sufficient to feed the evap.
Note that the cooling of the liquid line has added just enough subcooling to properly feed the evap. Is the liquid line insulated?
I would add a little more refrigerant to bring it up to 5.5K at the condenser outlet to see what effect this has on the condenser.
I would leave the TXV setting as is for now
and focus on improving the evap appr.
The evaporator has an inlet filter, which will be creating some restriction as well. I can pull it off to check performance change - after initial setup.
There are also a few casing air leaks, which may be causing pirate air issues & evap bypass. I'll get these blocked up. The fan may be marginal here, as well. I'll look into this.
The evaporator is working not so well (20.76K approach). (VIC to the rescue?)
Important note:
The air outlet temp you will be using to calculate your approach, will include heating loss from the compressor, for this machine!!!
I will have a new probe inserted just behind the evap itself today, so that we can measure the true approach off the evap.
Thanks so much, Gary, for your stunning analysis. I am very grateful.Last edited by desA; 13-08-2009 at 01:35 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 02:44 AM #7
Re: Compact AWHP charge determination
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13-08-2009, 03:08 AM #8
Re: Compact AWHP charge determination
I think it comes down to evap surface area. The airflow is not at all marginal. That said, an increase in airflow will increase heat transfer. As usual, there will be a point of diminishing returns.
Should be interesting to see how much difference it makes.
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13-08-2009, 08:43 AM #9
Re: Compact AWHP charge determination
We will only be able to continue the testing tomorrow, for various logistical reasons. Various components were insulated to conserve heat within compressor/discharge-line section.
One good thing to report is that the fan speed control, via compressor discharge temp strategy, will be implemented tomorrow - if all goes to plan.
I have already determined the initial control function profile & hope to report back on initial trial either tomorrow, or Saturday.
This will give us a real feel for the system response to this strategy. We can then fine-tune the control curve during the coming weeks & I'll implement the same unit on the lab test machine as well.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 08:55 AM #10
Re: Compact AWHP charge determination
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 09:41 AM #11
Re: Compact AWHP charge determination
Originally Posted by desA
In your view, what could be the reasons for the evap problems? Under-size? Under-performance e.g. low heat-transfer (if so, I can get this licked pretty easily).
I think it comes down to evap surface area. The airflow is not at all marginal. That said, an increase in airflow will increase heat transfer. As usual, there will be a point of diminishing returns.
I also added a thermocouple directly behind the evap - to accurately measure the evap exit approach temp. We should have some experimental details tomorrow.
Originally Posted by desA
The evaporator has an inlet filter, which will be creating some restriction as well. I can pull it off to check performance change - after initial setup.
Should be interesting to see how much difference it makes.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 01:50 PM #12
Re: Compact AWHP charge determination
I have no practical experience with PHE's, but given the small micro-channels I would be wary of them possibly plugging more easily on the water side, thus requiring frequent maintenance. But I could be wrong on this.
Perhaps others have some hands on experience to share.
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13-08-2009, 01:53 PM #13
Re: Compact AWHP charge determination
We may be able to gain a little more performance from this evap, but I'm thinking the gains will be limited. It is a smaller evap coupled with a larger compressor. I'm hoping the VIC will close the gap.
Last edited by Gary; 13-08-2009 at 02:00 PM.
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13-08-2009, 02:02 PM #14
Re: Compact AWHP charge determination
Any progress on the water regulating valve?
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13-08-2009, 02:44 PM #15
Re: Compact AWHP charge determination
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 02:50 PM #16
Re: Compact AWHP charge determination
Good logic. Let's see just how far we can push this particular machine in terms of evap. We have a little more room to play, once the pirate air-leaks are closed & dust filter modified.
For the next machine (production level), I will watch this evaporator like a hawk & get involved with the designer in pushing the evap design limit. I have a few tricks up my sleeve here that will boost the performance limit up some 15-20% odd. This is my core expertise...
I'll begin specifying the VIC for this unit, next week, on my return to my lab. We can then chat through the specification & refine it until it makes sense.
Will this be a PHE, or tube-in-tube type, as you envisage it?Last edited by desA; 13-08-2009 at 03:00 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 02:54 PM #17
Re: Compact AWHP charge determination
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 03:05 PM #18
Re: Compact AWHP charge determination
Last edited by Gary; 13-08-2009 at 03:10 PM.
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13-08-2009, 03:10 PM #19
Re: Compact AWHP charge determination
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13-08-2009, 03:10 PM #20
Re: Compact AWHP charge determination
A further point to factor in on this evap:
The inlet air is now ~ 35'C upwards. If I take the previous (well oversize) evap figures from my lab machine & offset these upwards by the additional air temp difference - with the same dT,air across the evap, the exit approach works out to be in the region of around 12-15K.
In the current design, I have no need for cold off-air temps, as this will just dump excessive moisture into the drip tray. Our relative humidity over here is around 64% at 35'C. The system will fill up a 25L bucket in a few hours of run-time.
The designers watch very carefully the moisture blinding in the fins, as well as inlet air velocity - it's a real issue. So for our system, with a Te,sat of in the region of 12-13'C, I'd not be overly unhappy seeing an exit approach of 21-22K, for instance. We are under this.
I'd value your thoughts on this.Last edited by desA; 13-08-2009 at 03:14 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 04:25 PM #21
Re: Compact AWHP charge determination
To minimize/avoid condensation you will need a delta-T of about 8K or less. This is primarily a matter of airflow, not approach. We can deal with this after we have maximized performance in other areas.
That said, when we reduce airflow to reduce load (fan control) this will increase the possibility of condensation.Last edited by Gary; 13-08-2009 at 04:29 PM.
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13-08-2009, 04:33 PM #22
Re: Compact AWHP charge determination
Was there much condensation during these test runs?
In the second run the wetbulb was about 25C with the leaving air well above this. Unless the coil has a very high bypass factor, there should have been no condensation.Last edited by Gary; 13-08-2009 at 04:39 PM.
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13-08-2009, 04:35 PM #23
Re: Compact AWHP charge determination
Very fair comment. So, the upper dT,air limit is around 8K. Thanks for this.
The airflow also has an upper velocity limit in terms of noise & moisture droplet entrainment.
That said, when we reduce airflow to reduce load (fan control) this will increase the possibility of condensation.
Large drip-trays are essential.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 04:47 PM #24
Re: Compact AWHP charge determination
Originally Posted by desA
I looked into this item, but, so far, my main component supplier has come up short.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 04:49 PM #25
Re: Compact AWHP charge determination
Originally Posted by desA
Will this be a PHE, or tube-in-tube type, as you envisage it?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 04:51 PM #26
Re: Compact AWHP charge determination
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-08-2009, 05:04 PM #27
Re: Compact AWHP charge determination
Given the low dT on the lab unit, I am surprised that it has condensate problems. Possibly the coil has a high bypass factor... or perhaps a higher dT in the early part of the run... or maybe just an unusually humid day.
Seems I recall there being a formula for calculating mean coil surface temp and this may be more definitive. If the surface temp is above dewpoint, then there should be no condensation.Last edited by Gary; 13-08-2009 at 05:43 PM.
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13-08-2009, 11:17 PM #28
Re: Compact AWHP charge determination
The lab is situated around 2km from one of the largest rivers in the region - the Mee Nam Khong (MeeKong). You can imagine the RH% up there. I'll buy in a RH meter & test local humidity, for future runs.
Seems I recall there being a formula for calculating mean coil surface temp and this may be more definitive. If the surface temp is above dewpoint, then there should be no condensation.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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14-08-2009, 04:05 PM #29
Re: Compact AWHP charge determination
I'll feed back on the latest charge settings, when I've worked through today's information.
The rest of the ideas, that essentially flowed from the previous lab-test machine, will be continued over there - so that we can keep everything in its place.
I've told my builder to box up the current machine & ship it up to my lab so that I can run my own tests in peace & quiet.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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