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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
As an experimental system, I would go mechanical as much as possible, then when it is perfected consider going electronic.
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.
Quote:
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.
First time I'd heard the term ObamaNation... :)
These are fun machines to develop & operate. Have fun.
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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. :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The upper limit for the suction pressure at the compressor inlet (CPR valve outlet) should be the saturation pressure corresponding to 15C.
The fan should be controlled by a temperature sensor on the discharge line (6 inches from the compressor). The ideal setting is yet to be determined. That setting would be the discharge temp at which Tc,sat is 75C, Te,sat is 15C, and superheat is 7K.
The water regulating valve should control the flow such that the Tc,sat is 75C.
Further to this specification:
The refrigerant mass charge should be sufficient for the condenser to provide a liquid sub-cooling of 8.33-8.5K, at a Tc,sat = 75'C.
(The 8.33K comes from the ARI specification).
Is there anything else that should be added to this?
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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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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Magoo
Hi desA.
After you have finalized the system can you publish the design criteria for a hot water heat pump system.
With pleasure - it's the least I can do, after all the wonderful input we've had on this thread.
Quote:
I congratulate you for a very interesting and informative post. Keep up with the good work.
Cheers magoo.
Thank you for your very wise input on the TXV super-heat setting - this rule has been extremely helpful.
Quote:
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 ]
:D It is a little worrying, I must say. Have ailing parents & am the only child. What to do?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Further to this specification:
The refrigerant mass charge should be sufficient for the condenser to provide a liquid sub-cooling of 8.33-8.5K, at a Tc,sat = 75'C.
(The 8.33K comes from the ARI specification).
I'm confused. ARI has specs for an AWHP? And if so, why do we care?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
The system thermodynamic balance calls for the following, at say Te,sat=12.5'C:
1. At Tc,sat=40'C :
Q'evap = 6.7kW
Q'cond = 7.3kW
2. At Tc,sat=50'C :
Q'evap = 6.1kW
Q'cond = 6.9kW
3. At Tc,sat=75'C :
Q'evap = 4.2kW
Q'cond = 6.1kW
Here is another set of specs that has me confused. How is any of this relevant to what we are doing?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I'm confused. ARI has specs for an AWHP? And if so, why do we care?
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.
-------------
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?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Here is another set of specs that has me confused. How is any of this relevant to what we are doing?
See the previous post, regarding appropriate initial rules/specifications (item 1).
In this particular case, you will see that the quoted data is pegged on Te,sat & spans the Tc,sat working range.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
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.
[/B]
Okay. The next question would be, at what point is the subcooling measured? Are they referring to SC at the condenser outlet, receiver outlet or TXV inlet? It makes a huge difference.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
See the previous post, regarding appropriate initial rules/specifications (item 1).
In this particular case, you will see that the quoted data is pegged on Te,sat & spans the Tc,sat working range.
The quoted data depends upon a great many more variables than Te,sat and Tc,sat. First and foremost on the list would be the temperature of the liquid entering the coil.
I'm thinking the air coil and vertical HX would do wonders for this coil.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
-------------
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?
Whatever SC fully feeds the coil (maintains SH), without interferring with the condenser, under all conditions.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Okay. The next question would be, at what point is the subcooling measured? Are they referring to SC at the condenser outlet, receiver outlet or TXV inlet? It makes a huge difference.
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?
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
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?
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
The system thermodynamic balance calls for the following, at say Te,sat=12.5'C:
Ok, let's assume that the TXV inlet SC is 8.5K
Quote:
Originally Posted by
desA
1. At Tc,sat=40'C :
Q'evap = 6.7kW
Q'cond = 7.3kW
40-8.5=31.5C liquid at TXV inlet
Quote:
Originally Posted by
desA
2. At Tc,sat=50'C :
Q'evap = 6.1kW
Q'cond = 6.9kW
50-8.5=41.5C liquid at TXV inlet
Quote:
Originally Posted by
desA
3. At Tc,sat=75'C :
Q'evap = 4.2kW
Q'cond = 6.1kW
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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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Good points.
Now, is the liquid line between condenser outlet & TXV inlet insulated, or not?
No... in fact just the opposite. This is where we want to add the spiral air coil, using the waste cool air stream to further subcool the liquid as much as possible.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
For the test machine, the liquid line from condenser outlet to TXV inlet, is insulated.
So, we can probably assume that we've maintained SC ~ 8.5K from condenser exit, to TXV inlet.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
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.
:)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
No... in fact just the opposite. This is where we want to add the spiral air coil, using the waste cool air stream to further subcool the liquid as much as possible.
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.
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Re: AWHP superheat & sub-cooling
http://i31.tinypic.com/20r66a1.png
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.
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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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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Cooling the liquid before the TXV reduces flashing in the evap.
Can you expand a little further on this, please.
Quote:
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.
Easy to do - I'll sort it out tomorrow.
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?
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
Is this what is referred to as the SGHX - Suction Gas Heat Exchanger?
Does this then mean that the evaporator size can be further reduced (super-heating section removal) & most of the super-heating take place in the SGHX?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Cooling the liquid before the TXV reduces flashing in the evap.
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. :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Is this what is referred to as the SGHX - Suction Gas Heat Exchanger?
Does this then mean that the evaporator size can be further reduced (super-heating section removal) & most of the super-heating take place in the SGHX?
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
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?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
I like this strategy very much. It should allow evap coil size reduction & allow us to control the compressor suction inlet temp well.
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.
Quote:
Originally Posted by
desA
Would this impact the overall evap fan control strategy, with input of compressor discharge?
The fan strategy is yet to be determined. Let's follow the trail and see where it leads us.
Quote:
Originally Posted by
desA
You've mentioned 'vertical' HX as being important. Can you perhaps explain this a little more?
The vertical heat exchanger forms a U shape with the evaporator which traps any liquid refrigerant in the coil on the off cycle.
Quote:
Originally Posted by
desA
Would oil migration be affected by this strategy?
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)?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Practically, I'm trying to make sense of why this particular machine has its discharge line insulated.
Our intention is to absorb as much heat as possible in the evaporator and then transport it to the condenser. We don't want to lose any of that heat along the way. That's why the discharge line should be insulated.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Our intention is to absorb as much heat as possible in the evaporator and then transport it to the condenser. We don't want to lose any of that heat along the way. That's why the discharge line should be insulated.
Agreed... :)
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Re: AWHP superheat & sub-cooling
Excellent analysis. Thank you.
Quote:
Originally Posted by
Gary
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.
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.
Quote:
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.
The increase in T,tank temperature from Run#1 - #2 - #3 is also clear - as well as Tw,out 60.9'C -> 65.85'C -> 67.0'C. The condenser is definitely performing better.
Quote:
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.
This exercise has certainly taught me something.
Get the system in balance first, then measure the overall performance of that system.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
This is a useful strategy.
I do wonder, though, if the evap & condenser designs could restrict a little. Some folks set the design approach at 10K for the condenser, for instance.
Let's keep going... :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The fan strategy is yet to be determined. Let's follow the trail and see where it leads us.
Fine. Makes sense... Good - we'll put that one on hold.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The vertical heat exchanger forms a U shape with the evaporator which traps any liquid refrigerant in the coil on the off cycle.
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.
Quote:
The suction side of the VSLHX needs to be sized to maintain sufficient velocity to move the oil upwards.
Fair-enough. The suction side calculation is straightforward @ greater than 5 m/s to cope with vertical uplift. The internal flow velocity in VIC will need to be carefully monitored.
Quote:
Hmmmm... this is way too many initials. How about we call our gadget a vertical intercooler (VIC)?
VIC has a certain 'ring' to it... :D
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Re: AWHP superheat & sub-cooling
http://i27.tinypic.com/14wrv5g.png
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'... :confused: