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Thread: AWHP superheat & sub-cooling
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03-08-2009, 04:38 PM #201
Re: AWHP superheat & sub-cooling
Fair comment. My reason for keeping things as simple & robust as possible, is that these machines are destined for 3rd world & developing nations, in the main. Getting technicians skilled in electronics will be a rarity.
I'll think about offering a an electronic option to the more developed folks.
Hmmm... When the "Cap and Trade" legislation goes through, energy prices will go right through the roof here in the ObamaNation. Maybe I should build one of these systems and put it up in my attic.
These are fun machines to develop & operate. Have fun.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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03-08-2009, 05:57 PM #202
Re: AWHP superheat & sub-cooling
Unfortunately I've grown quite lazy in my retirement, but I might talk myself into building a AWHP yet.
I would start off with my all time favorite experimental platform, i.e. the smallest cheapest window shaker I can find. Unfortunately these come with a rotary compressor, which is the least suitable for this application, but I can always switch to a scroll after I have destroyed the rotary by taking it beyond its limits.
I once chilled denatured alcohol down to -30C with one of these little beasts (heavily modified). Then I stuck it in a window in the middle of winter (Michigan) and dropped the alcohol down to -60C. Cascading off mother nature.Last edited by Gary; 03-08-2009 at 06:17 PM.
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04-08-2009, 01:31 AM #203
Re: AWHP superheat & sub-cooling
Last edited by desA; 04-08-2009 at 01:34 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 02:14 AM #204
Re: AWHP superheat & sub-cooling
Hi desA.
After you have finalized the system can you publish the design criteria for a hot water heat pump system.
As Gary stated, us old people will need to have one given the nanny state situation with global warming taxes, and Obamanation stuff.
I congratulate you for a very interesting and informative post. Keep up with the good work.
Cheers magoo.
ps, I interprute you are now in South East Asia, and going back to South Africa. A bit like frying pan into the fire.[joke ]
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04-08-2009, 02:32 AM #205
Re: AWHP superheat & sub-cooling
With pleasure - it's the least I can do, after all the wonderful input we've had on this thread.
I congratulate you for a very interesting and informative post. Keep up with the good work.
Cheers magoo.
ps, I interprute you are now in South East Asia, and going back to South Africa. A bit like frying pan into the fire.[joke ]Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 03:10 AM #206
Re: AWHP superheat & sub-cooling
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04-08-2009, 03:19 AM #207
Re: AWHP superheat & sub-cooling
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04-08-2009, 03:46 AM #208
Re: AWHP superheat & sub-cooling
Now, this is a perfectly valid question. I'll go into a little of the Designer's Philosophy & then back out again.
Designer's philosophy
- Designing something new with no rules in place, no firm standards, since there are an infinite number of design variables to consider.
1. Select nearest fit logical set of rules/standards;
2. Modify said rules from preliminary experience;
3. Build prototype unit;
4. Test/experiment;
5. Based on performance, modify original design rules
6. Iterate items (3) - (5) until technology stable.
7. Define & develop industry standards.
The ARI rules fit into step (1) above. In addition, the compressors used are often used in the air-conditioning industry.
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Backing out, to reality
- In step (6) now;
- Question:
What amount sub-cooling should be considered appropriate, throughout the heating range, that will be sufficient, to provide optimum performance & why?Last edited by desA; 04-08-2009 at 03:49 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 03:51 AM #209
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 03:54 AM #210
Re: AWHP superheat & sub-cooling
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04-08-2009, 04:00 AM #211
Re: AWHP superheat & sub-cooling
Last edited by Gary; 04-08-2009 at 04:18 AM.
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04-08-2009, 04:08 AM #212
Re: AWHP superheat & sub-cooling
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04-08-2009, 04:12 AM #213
Re: AWHP superheat & sub-cooling
From what I've been able to determine so far, the answer is not that clear.
Different rules are followed by different compressor designers:
1. Brand name - air conditioning rating conditions:
11.1K superheat / 8.3K subcooling / 35'C ambient air over
This will vary depending on compressor application, some will have 0K subcooling.
2. Bitzer - rating conditions:
*According to EN12900 (20°C suction gas temp., 0K liquid subcooling)
No mention is made of where the subcooling is measured.
Now that you mention it, & thinking through this aspect further, where is the most logical place that an air-conditioning standard would define the subcooling to be measured?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 04:23 AM #214
Re: AWHP superheat & sub-cooling
To fully feed the coil a TXV needs solid liquid at its entrance. This occurs at 5.5-8.5K SC (at the TXV). Since 5.5-8.5K SC is the point at which there is solid liquid, we would not want more than 8.5K SC at the condenser outlet in order to avoid backing liquid up into the coil.
Last edited by Gary; 04-08-2009 at 04:27 AM.
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04-08-2009, 04:35 AM #215
Re: AWHP superheat & sub-cooling
For A/C design purposes:
Probably at the metering device inlet since it has a profound effect on the capacity of the coil.
For our purposes:
It can be assumed that the liquid will gain considerable subcooling between the condenser outlet and the TXV inlet. Thus far, our ideal SC seems to be 7K at the condenser outlet at 75C Tc,sat. I assume that this will rise well above the 8.5K minimum long before it reaches the TXV inlet.Last edited by Gary; 04-08-2009 at 04:51 AM.
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04-08-2009, 05:31 AM #216
Re: AWHP superheat & sub-cooling
Ok, let's assume that the TXV inlet SC is 8.5K
40-8.5=31.5C liquid at TXV inlet
50-8.5=41.5C liquid at TXV inlet
75-8.5=66.5C liquid at TXV inlet
I would assume that at 75C Tc,sat, with 7K SC at the condenser outlet, if we further cool the liquid along the way such that the liquid line temp at the TXV is 31.5C for a subcooling of 75-31.5=43.5K SC, then our capacity would be in the neighborhood of:
Q'evap = 6.7kW
Q'cond = 7.3kW
I bet my numbers are closer than theirs.
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04-08-2009, 05:33 AM #217
Re: AWHP superheat & sub-cooling
Good points.
Now, is the liquid line between condenser outlet & TXV inlet insulated, or not?
If not insulated, then allowance will have to be made for some heat-exchange between pipe & surroundings - in, or out - depending on the local ambient air conditions & the point in the heat-up cycle.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 05:40 AM #218
Re: AWHP superheat & sub-cooling
Last edited by Gary; 04-08-2009 at 05:59 AM.
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04-08-2009, 09:43 AM #219
Re: AWHP superheat & sub-cooling
Good - that settles that one, then.
As a first pass, let's work on the following:
Start-up condition : SC = 5.5K
Hot condition : SC = 8.5K
I'll do further experimental runs & remove gas until we get to the 8.5K condition at Tc,sat = 75'C, then see how the start-up SC settles.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 09:45 AM #220
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 09:52 AM #221
Re: AWHP superheat & sub-cooling
The sub-cooled refrigerant only contributes to the heat-transfer, if it is used to heat the incoming water stream either before entry into the condenser, or within the condenser itself.
The additional effect of sub-cooling/water interchange is to raise the COP,hp.
COP,hp = (Q'desup + Q'cond + Q'subcool) / (W'comp + W'fan)
By making use of the additional sub-cooling, we effectively raise Q'subcool & COP,hp.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 09:58 AM #222
Re: AWHP superheat & sub-cooling
For this application, I'd rather see either a dedicated sub-cooler, or have this sub-cooler built into the condenser.
At this point, to blow, what could be useful pre-heat, into the air stream, would be a waste. If the economics of a sub-cooler were to prove uneconomical, then the question I'd like to ask is this:
"Does sub-cooling of the refrigerant liquid beyond the 8.5K liquid-only temp benefit the process - say by lowering Te,sat to under 15'C?"
If the answer is "Yes", then it will be useful to have an uninsulated line & force additional cooling.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 02:53 PM #223
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/20r66a1/3
Latest experimental runs.
Run #2 - re-set TXV, SH=0.6*TD, original charge
Run #3 - TXV @ SH=0.6*TD, blew off charge for 60 sec from LP side, to lower mass charge.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 03:21 PM #224
Re: AWHP superheat & sub-cooling
Cooling the liquid before the TXV reduces flashing in the evap.
This should be easy enough to test. Do a run with the liquid line insulated and a run without the liquid line insulated and compare the results.
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04-08-2009, 03:30 PM #225
Re: AWHP superheat & sub-cooling
Will we always have a situation where the liquid line is being cooled by the air-stream?
For instance, at the start-up condition, with a warm air-stream, is it not possible that the liquid line actually gets heated for a period until Tc,sat lifts sufficiently above the Ta,out temp?
Practically, I'm trying to make sense of why this particular machine has its discharge line insulated.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 03:40 PM #226
Re: AWHP superheat & sub-cooling
Can you expand a little further on this, please.
This should be easy enough to test. Do a run with the liquid line insulated and a run without the liquid line insulated and compare the results.
I'll do a first run, as is - to benchmark against (on the day) & then do a second run without insulation.
Where would you think best to operate the machine, to see the full effect of this change? Tc,sat=70/75'C?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 04:08 PM #227
Re: AWHP superheat & sub-cooling
I think the far superior alternative is the vertical HX. This transfers the heat from the liquid to the suction, neither gaining nor losing heat. However, the resulting cold liquid at the TXV inlet minimizes flashing in the coil, in effect increasing active coil surface area.
Last edited by Gary; 04-08-2009 at 04:11 PM.
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04-08-2009, 04:15 PM #228
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 04:32 PM #229
Re: AWHP superheat & sub-cooling
Ok, I've been working through the log(p),h diagram, & it's fairly obvious that additional sub-cooling, whether used in a HX, or not, will move the liquid to the left of the liquid saturation line. This will, in turn, reduce the 2-phase quality entering the evaporator (x smaller).
This should then use up some of the evap over-capacity, in having to evaporate the entering low quality liquid.
Good - that's sorted in my head now. I get to it tomorrow. Thanks so much for that applied wisdom.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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04-08-2009, 04:41 PM #230
Re: AWHP superheat & sub-cooling
Normally a SGHX has the TXV bulb mounted upstream, increasing the compressor inlet superheat. We want to mount the TXV bulb downstream, maintaining the compressor inlet superheat while decreasing flashing in the coil. Let's call it suction/liquid heat exchanger SLHX. I suspect you won't find this strategy in your design books.
And yes... this would allow us to reduce the coil size. Both liquid cooling to prevent flashing and superheating takes place in the SLHX.
On second thought let's call this a VSLHX as the vertical aspect adds important coil trapping advantages.Last edited by Gary; 04-08-2009 at 05:17 PM.
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04-08-2009, 06:23 PM #231
Re: AWHP superheat & sub-cooling
The improvement from decreasing the coil outlet superheat from run#1 to run #2 is somewhat obscured by the increase in Ta,in, but I think it is pretty much a given that reducing coil outlet superheat improves evap capacity. And clearly the compressor is running cooler in spite of the increases in Ta,in and Te,sat. Also, the V*A decreased from 2511 to 2374 with higher Te,sat, which means the COP increased.
Run #3 clearly demonstrates that reducing the condenser outlet subcooling (by reducing mass charge) improves the performance of the condenser, reducing the approach (at Tc,sat=75C) from 9.15K to 8K, while not causing an increase in superheat.
Also worth noting is that we have increased Te,sat to 19C (well beyond the Manufacturers 15C limit) while the compressor is in fact drawing less current (V*A compared to run#1) and running cooler as well. This is a happy compressor that is nowhere near overload conditions.Last edited by Gary; 04-08-2009 at 07:08 PM.
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05-08-2009, 03:22 AM #232
Re: AWHP superheat & sub-cooling
I like this strategy very much. It should allow evap coil size reduction & allow us to control the compressor suction inlet temp well.
Would this impact the overall evap fan control strategy, with input of compressor discharge?
You've mentioned 'vertical' HX as being important. Can you perhaps explain this a little more?
Would oil migration be affected by this strategy?Last edited by desA; 05-08-2009 at 03:25 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 04:28 AM #233
Re: AWHP superheat & sub-cooling
My thinking is that we should try to get the approach down to about 5K for both evaporator and condenser and see if the compressor can handle it. If it can then we can downsize or upsize the entire system proportionately.
At this point, the evaporator approach is 7.5K and the condenser approach is 8.0K, so we are getting closer.
The fan strategy is yet to be determined. Let's follow the trail and see where it leads us.
The vertical heat exchanger forms a U shape with the evaporator which traps any liquid refrigerant in the coil on the off cycle.
The suction side of the VSLHX needs to be sized to maintain sufficient velocity to move the oil upwards.
Hmmmm... this is way too many initials. How about we call our gadget a vertical intercooler (VIC)?Last edited by Gary; 05-08-2009 at 05:02 AM.
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05-08-2009, 05:29 AM #234
Re: AWHP superheat & sub-cooling
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05-08-2009, 08:27 AM #235
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05-08-2009, 11:35 AM #236
Re: AWHP superheat & sub-cooling
Excellent analysis. Thank you.
Agreed.
"clearly the compressor is running cooler in spite of the increases in Ta,in and Te,sat." - As shown by the lower Tcomp,disch values.
"V*A decreased from 2511 to 2374 with higher Te,sat, which means the COP increased." - This is a big bonus.
Run #3 clearly demonstrates that reducing the condenser outlet subcooling (by reducing mass charge) improves the performance of the condenser, reducing the approach (at Tc,sat=75C) from 9.15K to 8K, while not causing an increase in superheat.
Also worth noting is that we have increased Te,sat to 19C (well beyond the Manufacturers 15C limit) while the compressor is in fact drawing less current (V*A compared to run#1) and running cooler as well. This is a happy compressor that is nowhere near overload conditions.
Get the system in balance first, then measure the overall performance of that system.Last edited by desA; 05-08-2009 at 12:46 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 01:26 PM #237
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 01:28 PM #238
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05-08-2009, 01:33 PM #239
Re: AWHP superheat & sub-cooling
I did wonder about that. This is cunning, in that it prevents migration to the compressor & saves having to use a suction accumulator. The current circuit does not use one.
The suction side of the VSLHX needs to be sized to maintain sufficient velocity to move the oil upwards.
Hmmmm... this is way too many initials. How about we call our gadget a vertical intercooler (VIC)?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 02:17 PM #240
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/14wrv5g/3
The latest runs.
#4 = repeat of #3 on the day - insulated discharge line.
#5 = removed insulation on discharge line, improved section of suction line insulation.
Corrected to read - 'liquid line' not 'discharge line'...Last edited by desA; 05-08-2009 at 03:21 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 02:25 PM #241
Re: AWHP superheat & sub-cooling
Comments
1. I expected the Te,sat to drop off slightly with removal of the insulation. This did not occur - but remained the same. Interesting.
2. Compressor superheat was reduced. Expected, as evap was forced to work harder.
3. Condenser sub-cooling reduced. As a result of increase evap load, more liquid was held back in evap, with less migrating to the condenser.
Further observations
a. Air inlet temp reached 32.9'C today.
b. The compressor base reached 63.2'C - the highest so far.
c. We had a 'power challenge' towards the end of Run #4, with some voltage-current trading. The amperage shoots up as the voltage drops toward 185V.Last edited by desA; 05-08-2009 at 02:28 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 02:31 PM #242
Re: AWHP superheat & sub-cooling
b. The compressor base reached 63.2'C - the highest so far.
This design may be fine for colder climates, but is not desirable in hot Asian climates.
Why a manufacturer would choose to house a compressor & uninsulated heat-exchanger in the same insulated chamber, defeats me.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 02:47 PM #243
Re: AWHP superheat & sub-cooling
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05-08-2009, 03:13 PM #244
Re: AWHP superheat & sub-cooling
^ Apologies - it's finger trouble. I'll correct the printout - it's been a long day of testing.
The insulation was indeed removed from the liquid line...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 03:24 PM #245
Re: AWHP superheat & sub-cooling
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05-08-2009, 03:40 PM #246
Re: AWHP superheat & sub-cooling
I agree. This test machine has a spiral tube-in-tube condenser, with the refrigerant condensing on the outside of a spiral inner tube. The water flows inside the inner tube.
I plan to use some mineral wool insulation, or alternative high-temp insulation, around this condenser. To test the effect of their configuration.
It seems that the machine designer had perhaps thought that the container outer insulation would have been sufficient. After a long day of testing, the compressor/condenser gallery outer box surfaces become fairly hot to the touch - even with the insulation.
I'll take a picture of the set-up tomorrow & post it for reference. In my view, it's something best avoided.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 04:00 PM #247
Re: AWHP superheat & sub-cooling
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05-08-2009, 04:03 PM #248
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/2604pee/3
Picture of a similar machine. The current test heat-pump in my lab, has only two condenser coils in parallel. The insulation line appears to be missing insulation in this pic - the production machines did have insulation installed, if memory serves correctly.
The other panels comprise a thin outer metal sheet, with an inner insulation layer. When the machine is assembled, this compartment is totally closed.Last edited by desA; 05-08-2009 at 04:07 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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05-08-2009, 04:05 PM #249
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05-08-2009, 04:07 PM #250
Re: AWHP superheat & sub-cooling
At best the question of liquid line insulation is a trade-off. On the one hand we are losing the liquid heat, but on the other hand we reduce flashing in the evap. Given these test results, I would have to say it is too close to call.
I would expect no such ambiguity with the VIC. Not only will it bring the liquid temp down close to Te,sat maximizing the evap capacity, but it will also recover the heat from the liquid, transferring it to the suction. We win in both directions.
I expect the VIC to make a major difference.Last edited by Gary; 05-08-2009 at 04:10 PM.
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