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Thread: AWHP superheat & sub-cooling
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29-07-2009, 04:07 PM #101
Re: AWHP superheat & sub-cooling
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29-07-2009, 04:24 PM #102
Re: AWHP superheat & sub-cooling
^^ Thanks for the pic.
That would be fairly easy to implement. Install valve on water ?exit? line - install bulb in discharge line prior to condenser, or at entry.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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29-07-2009, 04:28 PM #103
Re: AWHP superheat & sub-cooling
What I was aiming at is that for many hot water applications - at least for hotels, is that the hot water from the storage vessel (sat 60-65'C) is piped into a blender/mixer head in the shower, where it is blended with cold water.
So, the storage tank temperature could actually be acceptable at an upper temp of 63-65'C say, with little noticeable difference to the person taking a shower.Last edited by desA; 29-07-2009 at 04:31 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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29-07-2009, 04:31 PM #104
Re: AWHP superheat & sub-cooling
Yes, the valve would be installed at the water exit.
On the end of that cap tube in the picture there is a 1/4 inch flare nut, which would be connected to a standard 1/4 inch access fitting, preferably in the discharge line prior to the condenser. It might be a good idea to also install a small shutoff valve between the access fitting and the flare nut, in case the valve may someday need to be replaced.Last edited by Gary; 29-07-2009 at 04:36 PM.
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29-07-2009, 04:49 PM #105
Re: AWHP superheat & sub-cooling
^ Thanks, Gary. I'm very happy with using the water flow-control concept via a capillary tube. I loathe too much reliance on electronics, if at all possible.
Next on the list:
At startup temperatures, with a Tc,sat temperature of around 35'C, say, the required refrigerant mass charge could be around 1200g, for instance. At Tc,sat of around 70'C, the required mass charge would only be 1020g - per calculation.
The required mass charges for the cold & hot condition are different. How to set up a suitable refrigerant loop such that the charge difference doesn't end up swamping the condenser in the hot condition?
The mass charge in the evaporator seems to decrease as the cycle moves upwards towards Tc,sat=70/75'C, since the quality moves from around x=0.12 to around 0.4. The excess refrigerant then needs to move to the condenser, where it holds up.Last edited by desA; 29-07-2009 at 05:05 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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29-07-2009, 05:02 PM #106
Re: AWHP superheat & sub-cooling
The refrigerant charge should be suitable for the hot condition, not the cold condition... and given the water regulating valve that hot condition would be very quickly reached.
Last edited by Gary; 29-07-2009 at 05:23 PM.
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29-07-2009, 06:28 PM #107
Re: AWHP superheat & sub-cooling
Perhaps this answer is too simplistic. To elaborate:
A TXV system requires surplus refrigerant in order to respond to variations in heat load. That surplus is stored in a receiver, or takes up valuable space in the condenser if there is no receiver. In addition there is excessive liquid pressure at the TXV due to the desired high side temperature, requiring a PRV to reduce the liquid pressure.
In adding the fan control we eliminate the heat load variations and stabilize the low side pressure... and in adding the water regulating valve we stabilize the high side pressure. Thus we ride the upper limits of the compressor (Te,sat 15C, Tc,sat 75C) throughout the cycle.
Having stabilized the heat load as well as both low and high side pressures, the system is now ideal for a cap tube as the metering device, thus eliminating the TXV, PRV, receiver... and the surplus refrigerant.Last edited by Gary; 29-07-2009 at 07:33 PM.
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30-07-2009, 01:06 AM #108
Re: AWHP superheat & sub-cooling
The required mass charges for the cold & hot condition are different. How to set up a suitable refrigerant loop such that the charge difference doesn't end up swamping the condenser in the hot condition?
1. Set system charge at start-up temp mass requirements - to optimize start-up heat-performance;
2. Oversize condenser suitably such that its internal storage provides sufficient space to store the excess refrigerant for the high-temp end point.
At this point, the condenser acts as a receiver.
The refrigerant excess at hot condition is around (1200-1020)/1020*100 = +17.6%. For most condensers, the design oversurface is in excess of this value & so the condenser provides a natural mass storage receiver, at little additional on-cost.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 01:16 AM #109
Re: AWHP superheat & sub-cooling
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30-07-2009, 01:31 AM #110
Re: AWHP superheat & sub-cooling
I'm not sure if there was a question in your last post.
TXV's excell at handling variable loads... and for this they need surplus refrigerant.
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30-07-2009, 01:36 AM #111
Re: AWHP superheat & sub-cooling
^ I'd imagine that a TXV would have to be in that kind of circuit, due to the mass charge migration issues.
What I was thinking through is the case of some of the typical lowish-cost systems I've come across, where no liquid receiver, or suction accumulators are present in the system.
In academic literature, the mass migration effect is known & some have even talked about the idea of storing the excess refrigerant outside the main circuit, until required. All kinds of ideas. The idea of using the heat-exchanger as a storage device probably allows this to be done, at low cost - although the TXV comes at a price.
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Cap-tube system (open to review):
So, for a cap-tube system, the main idea would then be to rather size the refrigerant mass charge for hot-load condition (not start-up) & run a little low on start-up heat-performance, knowing that most of the run time will be spent at the hot condition anyway.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 01:54 AM #112
Re: AWHP superheat & sub-cooling
More of thinking out aloud.
TXV's excell at handling variable loads... and for this they need surplus refrigerant.
To extend the condenser storage idea & TXV a little further, on such systems. Would it be feasible to further over-size the condenser so that it sub-cools beyond the typical ~8K amount?
For example, to try & force a larger sub-cooling in the condenser through a combination of design over-surface & liquid-line design.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 03:13 AM #113
Re: AWHP superheat & sub-cooling
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30-07-2009, 03:45 AM #114
Re: AWHP superheat & sub-cooling
The basic idea is as follows:
Increasing the sub-cooling in the condenser will allow additional heat to the water stream. This will increase COP,hp slightly for very little on-cost.
Essentially, this is integrating the sub-cooler into the condenser through designed over-surface. As long as the amount of over-surface doesn't wreak havoc with the de-superheating & condensing operation, then it should, in principle, be possible.
I'm not sure just how far this can be pushed - I'd estimate that the 20-30% range would be within normal design limits. For ultra-compact condenser designs, the increase in condenser volume is marginal, although for concentric tube condensers, the impact could be much larger.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 04:23 AM #115
Re: AWHP superheat & sub-cooling
I am getting a tad confused here, generall parallel flow has a heat transfer lose, but are you using that to maintain higher condensing/ SDT. I would tend to go counter flow with water and control water outlet temp., with a variable water flow to acheive same result.
Years ago I was involved with the developemnt of a HW heat pump as at the experimental stage on R12 [ OK years ago ]. tube and tube counter flow. Had to be tube soldered to tube, due to health regs of double wall separation. Fractional HP compressor as a pre-heater for general electric hot water heating system [ domestic ]. ran like a dream cut hot water heating costs by around 50%.
Carrier Inc.. also then marketed a system called a "hot shot "as and add on to existing air con system. Acted as a de-superheater on discharge.
Both systems probably way ahead of there time as and energy saver. Now everyone wants to save costs. Strange how things turn out.
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30-07-2009, 04:38 AM #116
Re: AWHP superheat & sub-cooling
Last edited by Gary; 30-07-2009 at 04:47 AM.
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30-07-2009, 04:51 AM #117
Re: AWHP superheat & sub-cooling
^^ For heat-pumps, the condenser is typically either a tube-in-tube, or plate. The piping is usually run in a counterflow direction.
Sidebar:
Practically though, if the condensing part carries most of the load, then even a parallel flow condition (via water connections), will not make a huge difference - theorectically, at least. Reason is that the de-superheating & sub-cooling parts of the condenser typically take up around 15-20% of the heat-load. If all is well, then the volume used by de-superheating & sub-cooling will be close to the heat-load fraction, although this is not actually always the case (needs to be checked carefully).
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With a need to push up the sub-cooling portion of the condenser, it will occupy additional space, which will need to be carefully managed. Normally, liquid-liquid (sub-cooler) heat-exchange does not require much surface area, compared to say vapour-liquid (de-superheater), so the additional space required is not that large, in practice.
Some tricks may have to be played on the liquid-line pipework to hold back the liquid in the condenser a tad, to force the volume retention, for instance.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 04:55 AM #118
Re: AWHP superheat & sub-cooling
This is very true - it is. The entry temperature is critical, in how far the sub-cooling can be pushed.
I'm thinking there would be little if any gain and in fact I would bet there would be a loss as compared to using that same oversized condenser without the excess subcooling... but I could be wrong.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 04:57 AM #119
Re: AWHP superheat & sub-cooling
I've put up a link to the TXV rule Magoo mentioned earlier. (I hope this is ok).
http://pdf.directindustry.com/pdf/ge...57696-_27.htmlEngineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 05:06 AM #120
Re: AWHP superheat & sub-cooling
Last edited by Gary; 30-07-2009 at 05:11 AM.
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30-07-2009, 05:18 AM #121
Re: AWHP superheat & sub-cooling
^ Fair-enough - good thought.
What I'll do is to have a set of ratings done by my condenser supplier, for various combinations of water entry temp & Tc,sat. From that data we should have a more clear idea of the system trade-offs.
I've already done this for a fixed Tc,sat & variable water inlet temp, but will extend the study to see where the sensitivities lie. Good one...
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During operation with a pump-around system, the water flow-rate setting will determine the number of water passes (times) through the condenser, & hence the dTw across the condenser.
If the condenser is sized to cope with the hot-condition such that the condensing area is sufficient for this, any additional over-surface can then be put to good use, without disrupting the condensing section.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 04:12 PM #122
Re: AWHP superheat & sub-cooling
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30-07-2009, 04:32 PM #123
Re: AWHP superheat & sub-cooling
Experimental feedback - AWHP test system - 30.07.09
Evaporator:
Te,sat = 13'C (sat temp)
Te,sup = 20.8'C (vapour exit temp)
Ta,in = 24.9'C (air inlet temp)
va,in = 3.5 m/s (air inlet face velocity to evap face)
Ta,out = 21.5'C (air outlet temp)
va,out = 9.5 m/s (air outlet velocity - fan discharge)
Condenser:
Tc,sup = 64.3'C (superheated vapour inlet to condenser)
Tc,sat = 50'C (condenser sat temp)
Tc,sc = 43.7'C (liquid exit temp)
Tw,out = 39.8'C (water outlet temp)
Other:
Tw,tank = 38.9'C (water storage tank temp - mixed, half-height)
Tcomp,base = 41.1'C (compressor base temp)
Electrical:
Current = 6.1 A
Voltage = 223V (single phase, ~50Hz)
I'm very interested in hearing your comments on this unit.
The particular test unit is undersized on the condenser, in my view. It is an early test machine, using a tube-in-tube condenser, TXV & scroll compressor. Typically, under a dynamic heat-ramp test, the heat-up power to raise the storage tank water temp from ambient to hot temp temp is slightly lower than the compressor performance tables would predict. I have always suspected that this stemmed from a slightly under-sized condenser. The low sub-cooling value seems to bear this out.Last edited by desA; 30-07-2009 at 04:44 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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30-07-2009, 06:16 PM #124
Re: AWHP superheat & sub-cooling
Evaporator:
Te,sat = 13'C (sat temp)
Te,sup = 20.8'C (vapour exit temp)
Ta,in = 24.9'C (air inlet temp)
va,in = 3.5 m/s (air inlet face velocity to evap face)
Ta,out = 21.5'C (air outlet temp)
va,out = 9.5 m/s (air outlet velocity - fan discharge)
Condenser:
Tc,sup = 64.3'C (superheated vapour inlet to condenser)
Tc,sat = 50'C (condenser sat temp)
Tc,sc = 43.7'C (liquid exit temp)
Tw,out = 39.8'C (water outlet temp)
Other:
Tw,tank = 38.9'C (water storage tank temp - mixed, half-height)
Tcomp,base = 41.1'C (compressor base temp)
Electrical:
Current = 6.1 A
Voltage = 223V (single phase, ~50Hz)
dT = 24.9-21.5 = 3.4K/6.1F
TD = 24.9-13 = 11.9K/21.4F
SH = 20.8-13 = 7.8K/14F
Appr = 21.5-13 = 8.5K/15.3F
Condenser:
dT = 39.8-38.9 = 0.9K/1.6F
TD = 50-38.9 = 11.1K/20F
SC = 50-43.7 = 6.3K/11.3F
Appr = 50-39.8 = 10.2K/18.4F
I am assuming that the Tw,tank is the same as the cond entering water temp, although this may not be the case.
The subcooling at 6.3K is well within what I would consider to be a normal range (5.5-8.5K). This is largely a matter of refrigerant charge.
As to undersize/oversize, these are terms that are relative to accepted standards. Are there in fact any accepted standards for this type of system?
All seems to be working well at this point. It will be interesting to see how these numbers compare with (all else being equal) a much hotter incoming water temp, as will be seen at the end of the heating cycle.
An additional temp which should be monitored/recorded would be the discharge line temp near the compressor.Last edited by Gary; 30-07-2009 at 07:54 PM.
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30-07-2009, 11:59 PM #125
Re: AWHP superheat & sub-cooling
Thanks so much for your comments.
I am assuming that the Tw,tank is the same as the cond entering water temp, although this may not be the case.
The subcooling at 6.3K is well within what I would consider to be a normal range (5.5-8.5K). This is largely a matter of refrigerant charge.
As to undersize/oversize, these are terms that are relative to accepted standards. Are there in fact any accepted standards for this type of system?
All seems to be working well at this point. It will be interesting to see how these numbers compare with (all else being equal) a much hotter incoming water temp, as will be seen at the end of the heating cycle.
An additional temp which should be monitored/recorded would be the discharge line temp near the compressor.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 12:19 AM #126
Re: AWHP superheat & sub-cooling
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31-07-2009, 12:45 AM #127
Re: AWHP superheat & sub-cooling
There should also be a similar set of tests (start of cycle/end of cycle) with the incoming air at 35C, as this is the other design extreme.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 12:55 AM #128
Re: AWHP superheat & sub-cooling
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31-07-2009, 02:16 AM #129
Re: AWHP superheat & sub-cooling
You really should have some means of regulating the intake air temp to simulate the two extremes that you are designing for.
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31-07-2009, 04:43 AM #130
Re: AWHP superheat & sub-cooling
That's true.
I'm presently planning to relocate from SEA back to my homeland. Once settled, I plan to set up the lab with some level of environmental control - it has to be done. To add hot air is not a problem, it's more when you want to cool the air to just above 0'C, that it gets a little more costly.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 05:14 AM #131
Re: AWHP superheat & sub-cooling
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31-07-2009, 05:47 AM #132
Re: AWHP superheat & sub-cooling
^ Hahaha... got 4 of those already. Point taken...
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 05:57 AM #133
Re: AWHP superheat & sub-cooling
A few thoughts on using only AWHP's to create a hot/cold test capability.
Air cooling
Using a air-source AWHP, where the output air-stream is partially re-circulated back to the evap inlet would bring the air temp down. I wonder how far this could go, though - if ducted properly?
Air heating
Use the hot water storage to pre-heat the incoming air-stream, or use a bank of direct-element heaters to raise the incoming air stream. Ducted flow.
Any thoughts?Last edited by desA; 31-07-2009 at 05:59 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 06:41 AM #134
Re: AWHP superheat & sub-cooling
Aside from the obvious coil freezing problems:
As the temp drops the heat load drops, so eventually the TXV would start hunting and then uncontrolled flooding.
The orifice could be downsized, but the smaller the orifice the slower the pulldown.
The temp could theoretically keep dropping until the compressor reached its lower limit, where it just can't pull a deeper vacuum or the liquid temp coming back from the condenser engages the entire coil in flashing.
If you have a target temp you want to stop at you could do this with an evaporator pressure regulator (EPR) valve.
A hot water coil could be used easily enough.
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31-07-2009, 06:57 AM #135
Re: AWHP superheat & sub-cooling
Actually, nothing so exotic is needed. I'm thinking a cheap window A/C could be modified to provide the heating and/or cooling.
Last edited by Gary; 31-07-2009 at 06:59 AM.
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31-07-2009, 08:32 AM #136
Re: AWHP superheat & sub-cooling
Good idea. A window-rattler.
Isn't this the beauty of being in the HVAC&R game... lol...Last edited by desA; 31-07-2009 at 02:48 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 04:21 PM #137
Re: AWHP superheat & sub-cooling
The performance tests from Tc,sat = 40'C through 75'C, in steps of 5'C, are done. I need to tabulate & graph the results. Hope to have these out this weekend.
At Tc,sat=40'C, the condenser sub-cooling is 3.8K & goes up to 15.2K at Tc,sat=75'C !!!Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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31-07-2009, 04:28 PM #138
Re: AWHP superheat & sub-cooling
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31-07-2009, 04:38 PM #139
Re: AWHP superheat & sub-cooling
What was the leaving water temp at Tc,sat=75C?
Last edited by Gary; 31-07-2009 at 04:48 PM.
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01-08-2009, 02:21 AM #140
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01-08-2009, 02:49 AM #141
Re: AWHP superheat & sub-cooling
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01-08-2009, 03:23 AM #142
Re: AWHP superheat & sub-cooling
^ Can you explain a little further on this, please?
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 03:39 AM #143
Re: AWHP superheat & sub-cooling
You were speculating earlier about using part of the condenser for excess subcooling. You now have excess subcooling backing up into the condenser.
If that excess subcooling is beneficial, then removing it would increase the approach temp at Tc,sat=75C.
If that excess subcooling is not beneficial, then removing it would decrease the approach temp at Tc,sat=75C.
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01-08-2009, 03:59 AM #144
Re: AWHP superheat & sub-cooling
Ok, fair-enough.
Let me finish the full test table & put it up (wip). We can then see the progression the whole way through the heat-up cycle. I also have the dynamic ramp test from the previous day, to measure overall (average) heating rate under that mass condition.
We can then make a solid decision on how much charge to remove.
I value your input greatly - it is an extremely valuable exercise for me.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 05:05 AM #145
Re: AWHP superheat & sub-cooling
Sorry, I don't mean to rush you. You will of course, need a full spectrum of test tables for a base, in order to quantify the value of improvements.
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01-08-2009, 05:41 AM #146
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/34dn8gi/3
That's the recent raw data.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 06:11 AM #147
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/21c7ame/3
Correction... @ Tc,sat=75'C swapped Tc,exit & Tw,out data.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 06:23 AM #148
Re: AWHP superheat & sub-cooling
We can be as critical of this design as we like.
It is a machine I bartered in trade for consulting work performed for a now defunct heat-pump builder. It is not my design, I've just fine-tuned its performance a little & tested its robustness under 3rd world conditions. I use it as my test basis on which to benchmark my next machine, which is almost complete.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 07:49 AM #149
Re: AWHP superheat & sub-cooling
Note that the data speaks with a forked tongue...
You will observe that the Tw,out & T,tank values cross at some point. This should be non-physical & will definitely affect the condenser Approach calculation.
It is something I've observed in earlier trials at one of my clients. In my view the temp probe for the water outlet from the condenser, although insulated, is losing accuracy as temp increases, whereas the storage water temp is mixed & therefore more representative.
I will upload a plot showing both the conventional (Approach=Tc,sat-Tw,out) & modified (Approach'=Tc,sat-T,tank), for clarity. It makes a huge difference in interpretation.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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01-08-2009, 07:54 AM #150
Re: AWHP superheat & sub-cooling
http://tinypic.com/r/2hi3r6o/3
The value (-Cross)=Tc,exit-Tw,out. This is a critical parameter for heat-exchanger design purposes.
(-Cross)'=Tc,exit-T,tankEngineering Specialist - Cuprobraze, Nocolok, CD Technology
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