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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.
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Re: AWHP superheat & sub-cooling
Quote:
b. The compressor base reached 63.2'C - the highest so far.
The test machine compressor is housed in an enclosure with the tube-in-tube heat-exchanger. The outside panels of this housing are insulated.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
#4 = repeat of #3 on the day - insulated discharge line.
#5 = removed insulation on discharge line...
Now I'm confused. I thought we were talking about removing the insulation on the liquid line, not the discharge line.
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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... :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
The test machine compressor is housed in an enclosure with the tube-in-tube heat-exchanger. The outside panels of this housing are insulated.
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.
The condenser should definitely be insulated.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The condenser should definitely be insulated.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Some folks set the design approach at 10K for the condenser, for instance.
We can do better. In fact we already have. :)
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Re: AWHP superheat & sub-cooling
http://i29.tinypic.com/2604pee.jpg
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
We can do better. In fact we already have. :)
Yes, agreed. Much to my amazement. Frankly, I did not think it possible.
You, sir, are a master craftsman. :)
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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 also thought so, to be honest.
Now, if you were to build a new machine & had the choice of insulated, or uninsulated liquid line (no VIC in this one) - what option would you take?
Also a factor in this particular machine is that the liquid line filter-drier itself was uninsulated, & exposed directly to the evap off air stream - although the rest of the line was insulated - albeit with a rather loose (oversize) Aeroflex insulation. I wondered about the bulk of liquid sitting in the drier, being cooled.
Quote:
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.
I'd expect the same. It will also reduce the size of the evap required for future machines.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Now, if you were to build a new machine & had the choice of insulated, or uninsulated liquid line (no VIC in this one) - what option would you take?
In that case, I would leave it uninsulated.
With the addition of the VIC, I would insulate everything from the condenser outlet to the TXV inlet.
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Re: AWHP superheat & sub-cooling
I'm thinking the next test run should be with the condenser insulated to see how much difference this makes.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
As I understand - from the compressor manufacturer's tech sheets - the right line of the compressor envelope may be crossed if compressor shell cooling is implemented.
Quote:
Originally Posted by
desA
It makes no sense at all to place the compressor in an insulated compartment heated by the condenser.
This could have been avoided in this design by simply removing the partition section behind the compressor and placing that partition between the condenser and compressor. The compressor would then be in the evaporator compartment... and the condenser would be isolated and insulated.
A simple fix for a big improvement.
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Re: AWHP superheat & sub-cooling
http://i32.tinypic.com/33p3hj5.png
http://tinypic.com/r/33p3hj5/3
Summary comments across runs #3,4,5:
1. Runs 4/5 (32.9'C/31.5'C) are at slightly higher Ta,in than run #3 (30.7'C).
2. Runs 4 to 5 :
2.1 Compressor superheat decreases;
2.2 Tc,exit increases => SC reduces;
2.3 Compressor power reduces => COP increases.
3. Run 4:
3.1 Power anomaly due to V*I trading.
3.2 Causes compressor base temp to increase to 63.2'C.
In summary, removing the insulation from the liquid line is helpful (see points 2.1/2.2/2.3)
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Re: AWHP superheat & sub-cooling
Quote:
Gary
Quote:
Originally Posted by desA
Now, if you were to build a new machine & had the choice of insulated, or uninsulated liquid line (no VIC in this one) - what option would you take?
In that case, I would leave it uninsulated.
With the addition of the VIC, I would insulate everything from the condenser outlet to the TXV inlet.
Agreed. This makes sense.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
It makes no sense at all to place the compressor in an insulated compartment heated by the condenser.
This could have been avoided in this design by simply removing the partition section behind the compressor and placing that partition between the condenser and compressor. The compressor would then be in the evaporator compartment... and the condenser would be isolated and insulated.
A simple fix for a big improvement.
Absolutely agree.
Quote:
Originally Posted by
Gary
I'm thinking the next test run should be with the condenser insulated to see how much difference this makes.
As a quick fix, I'll insulate the condenser portion from the compressor with some insulation I have to hand & run a test.
I have to travel for a few days from Saturday, for around 10 days & will modify the metallic partition on the test machine, to split the compressor into the evap section, on my return.
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Re: AWHP superheat & sub-cooling
Refrigerant mass charge determination
Now that we've reached a reasonable operating mass charge for the heat-pump, by bleeding off the excess charge, (the original charge had been based on the start-up mass requirement, not hot requirement) - how do we determine the correct charge for future machines, or re-fills?
Do we:
1. Charge at hot condition mass, then add in incremental charge (bleeding off air entrapment in lines), until system at desired operating conditions,
or,
2. Charge at start-up condition, then slowly bleed off refrigerant until system at desired operating conditions.
With option (1) we know the final mass added to the system.
With option (2), how will we determine the mass added to the system?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Refrigerant mass charge determination
Now that we've reached a reasonable operating mass charge for the heat-pump, by bleeding off the excess charge, (the original charge had been based on the start-up mass requirement, not hot requirement) - how do we determine the correct charge for future machines, or re-fills?
Do we:
1. Charge at hot condition mass, then add in incremental charge (bleeding off air entrapment in lines), until system at desired operating conditions,
or,
2. Charge at start-up condition, then slowly bleed off refrigerant until system at desired operating conditions.
With option (1) we know the final mass added to the system.
With option (2), how will we determine the mass added to the system?
A minimum charge is that which fully feeds the evaporator. We have seen no rise in superheat at all so we are definitely above the minimum charge.
The maximum charge is that which backs liquid up into the condenser.
At this point we are riding the upper limits on the charge and if anything we could remove a little refrigerant.
Under cold conditions (cold water and cold air) I would not be concerned if the SC drops so long as the SH does not rise substantially.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
A minimum charge is that which fully feeds the evaporator. We have seen no rise in superheat at all so we are definitely above the minimum charge.
True. Ok.
Quote:
The maximum charge is that which backs liquid up into the condenser.
We have been slowly moving from over-charge to the maximum charge point. Ok.
Quote:
At this point we are riding the upper limits on the charge and if anything we could remove a little refrigerant.
Makes good sense.
Quote:
Under cold conditions (cold water and cold air) I would not be concerned if the SC drops so long as the SH does not rise substantially.
Ok, got that. I would expect to see an SC drop-off under cold conditions, based on the previous unit performance.
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Re: AWHP superheat & sub-cooling
Hmmm... you mentioned the liquid line filter/drier. If you mount this vertically with the outlet at the bottom, this puts the liquid at the outlet, which could make a big difference if the charge becomes critical. In effect the filter/drier becomes a small receiver if there is vapor present (low SC).
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Re: AWHP superheat & sub-cooling
I assume we are still talking about a system without the VIC.
The VIC is going to give this system massive subcooling at the TXV inlet. The charge would have to be extremely low to have any effect at all on the evaporator SH, making the charge very non-critical.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Hmmm... you mentioned the liquid line filter/drier. If you mount this vertically with the outlet at the bottom, this puts the liquid at the outlet, which could make a big difference if the charge becomes critical. In effect the filter/drier becomes a small receiver if there is vapor present (low SC).
Can you talk me through this a little further?
The current test-machine has the filter-drier mounted almost horizontally.
The current FD is an Alco EK 16 4S - with expected internal refrigerant holding at around 70'C, of 228.88g. This is a lot of refrigerant. I do trust my conversion from their Imperial fluid-based figure is correct.
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Re: AWHP superheat & sub-cooling
The TXV will fully feed the coil so long as there is solid liquid at its inlet.
When the SC drops there is a liquid/vapor mixture moving through the line. As this mixture enters the drier it will separate with the liquid dropping to the bottom.
If the drier is horizontal, when the liquid level is below the halfway point, vapor will be fed to the TXV inlet and the SH will rise dramatically.
If the drier is vertical with the outlet at the bottom then all available liquid will be sent to the TXV inlet.
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Re: AWHP superheat & sub-cooling
This is very useful information - very.
What happens if the TXV inlet is mounted above the liquid discharge point from the filter-drier? Would a vertical orientation still be effective in this case?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
This is very useful information - very.
What happens if the TXV inlet is mounted above the liquid discharge point from the filter-drier? Would a vertical orientation still be effective in this case?
The relative location of the TXV doesn't matter.
Small details can make a big difference.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The relative location of the TXV doesn't matter.
Small details can make a big difference.
Very interesting. So the dynamics of how the liquid gets out of the filter-drier, with liquid flood at the bottom - vapour to the top & self-bleeding back to the condenser, are very important.
A very useful point, indeed. Thanks so much for that.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I assume we are still talking about a system without the VIC.
I'd like to try & tune the current lab machine to its limit, without having to pull it apart. We can then extract all useful information, before installing the VIC.
Quote:
The VIC is going to give this system massive subcooling at the TXV inlet. The charge would have to be extremely low to have any effect at all on the evaporator SH, making the charge very non-critical.
This is an extremely useful feature, especially with the pressure on to minimise refrigerant charge mass as far as possible.
I plan to size the VIC & have a unit brought up, for installation into the lab machine. We can then fine-tune the concept further.
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Re: AWHP superheat & sub-cooling
Condenser coil insulation update (details to follow):
I wrapped some of the box insulation around the condenser coil & basically built a simple box around it. It stopped heat moving to the aluminium inner separation wall. An inner layer was also used inside the coil. Bare in mind that this stuff could hardly be described as effective insulation, by any stretch of the imagination.
Well, well, well...
At Tc,sat=75'C, SC reduces from 8.5K to 5.8K, approach reduces from 8.1K to 7.75K, Tw,out increases from 66.90'C to 67.25'C. :eek:
Lol... Test details to follow later tonight. :)
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Re: AWHP superheat & sub-cooling
http://i29.tinypic.com/96dkav.png
http://tinypic.com/r/96dkav/3
The experimental results for the condenser coil with a simple, rough insulation box around it.
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Re: AWHP superheat & sub-cooling
Comments - partially-insulated condenser:
1. Tc,exit temperature rises => SC reduces from 8.5K to 5.8K !!!
2. Tw,out temp rises slightly from 66.9'C to 67.25'C.
3. Approach drops from 8.10K to 7.75K !!!
4. I would not read too much into the current-voltage values as we had a fair 'V*I' challenge today, with supply voltage dropping as low as 185V around midday. The first data point was taken before the challenge & the test stopped. Resumed later in the day, for the latter tow data points. There seems to be power waves in the region & the effect can be heard in fans slowing down, lights dipping etc. This comes and goes & does not seem to be consistent - we're in a bad patch at the moment, it seems. (At the same time, I'm convinced that as the amperage rises to compensate that the local authorities collect on higher electrical toll fees.)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Condenser coil insulation update (details to follow):
I wrapped some of the box insulation around the condenser coil & basically built a simple box around it. It stopped heat moving to the aluminium inner separation wall. An inner layer was also used inside the coil. Bare in mind that this stuff could hardly be described as effective insulation, by any stretch of the imagination.
Well, well, well...
At Tc,sat=75'C, SC reduces from 8.5K to 5.8K, approach reduces from 8.1K to 7.75K, Tw,out increases from 66.90'C to 67.25'C. :eek:
Lol... Test details to follow later tonight. :)
T,comp,b has dropped as well... seems our compressor likes the change, too. :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
T,comp,b has dropped as well... seems our compressor likes the change, too. :)
It certainly does. Makes sense in the end. Does no good to have an uninsulated condenser heating the compressor shell... :D
I'll have to hold on further tests, as I'll be away for around 7-10 days, from Saturday. I'll get hold of some decent insulation for the condenser & wrap it better. In this way, we can stabilise that end, before moving forwards.
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Re: AWHP superheat & sub-cooling
If I could re-visit the refrigerant charging procedure for a minute.
Is it better to slightly overcharge, then cut back until operating sweet point is found, or
slightly underfill, then progressively add refrigerant until the sweet point is found?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
I'd like to try & tune the current lab machine to its limit, without having to pull it apart. We can then extract all useful information, before installing the VIC.
I agree. We should fine tune everything else first.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
If I could re-visit the refrigerant charging procedure for a minute.
Is it better to slightly overcharge, then cut back until operating sweet point is found, or
slightly underfill, then progressively add refrigerant until the sweet point is found?
I'm thinking we have already chosen the former path. Whatever gets us to the sweet point is a good procedure... and I'm hoping for a sweet range instead of a sweet point.
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Re: AWHP superheat & sub-cooling
Have you located a water regulating valve?
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
I'd like to try & tune the current lab machine to its limit, without having to pull it apart. We can then extract all useful information, before installing the VIC.
Gary
I agree. We should fine tune everything else first.
At this stage, we could look at the design requirements for the VIC, though.
Fluid 1 : Refrigerant - R-134A - liquid
Entry condition : Liquid sub-cooled xx.xx'C off Tc,sat=75'C condition
Exit condition : T,exit = T,inlet - y.y'C
Allowable pressure drop = ?
@ mass flowrate of refrigerant.
Fluid 2 : Refrigerant - R-134A - vapour
Entry condition : ?
Exit condition : SH = 7K
Allowable pressure drop = ?
@ mass flowrate of refrigerant.
Minimum/maximum nozzle velocities for oil transport.
Minimum vertical height between nozzle centres.
What else?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Have you located a water regulating valve?
I'll be in BKK next week & will source one while I'm there.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
If I could re-visit the refrigerant charging procedure for a minute.
Is it better to slightly overcharge, then cut back until operating sweet point is found, or
slightly underfill, then progressively add refrigerant until the sweet point is found?
Gary
I'm thinking we have already chosen the former path. Whatever gets us to the sweet point is a good procedure... and I'm hoping for a sweet range instead of a sweet point.
How will we know the mass charge in the system - in case of future servicing?