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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Thus far, assuming everything works as envisioned, we have a system which absorbs a very stable amount of heat in the low side.
In the high side, a high percentage of that heat is transferred to the water in the condenser, while a small percentage is transferred back to the waste air stream via the pressure reduction coil.
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Perhaps we can control the variable speed fan to sense/minimize this waste heat, dropping the SST to pump only the heat that can be currently utilized by the condenser.
This is very cunning. I've sourced a range of fan speed controllers that can allow fan speed optimisation. I'll work further on this - very interesting.
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Then look for ways to improve the condenser heat transfer.
The current condensers are already pretty good in terms of their heat-transfer capacity. Very, very compact. This, plus a few other tricks, has allowed the current prototype model to reduce down to 54% of the previous box volume. Practically, this can go down a fair bit further still, judging from the piping layout.
I'm absolutely loving the challenge so far. It always gets interesting when you're spending your own money on such development projects. :D
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
It occurs to me that the pressures and loads being stable, the system is now ideal for a cap tube. A cap tube is not only less expensive, but it uses less refrigerant.
This is a very useful observation, indeed. A change of heating philosophy leads to a more compact, simpler system. Thanks for that.
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And the fan can be controlled off discharge line temp using a thermistor, rather than using a more expensive transducer to control off low side pressure.
Agreed. I do prefer this method. My current prototype has allowance for two active thermistors. The second can be put to good use here.
This is fun... :D
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Where there is both liquid and vapor in an enclosed container an increase in temperature will cause an increase in pressure.
This relationship continues until all of the liquid becomes vapor, at which point the pressure becomes fixed regardless of any further increase in temperature.
By precisely manipulating the amount of refrigerant in the TXV bulb, a fixed bulb pressure limit can be set.
The TXV judges superheat by comparing the pressure in the coil to the pressure in the bulb.
An increase in coil pressure, when compared to a fixed bulb pressure is interpreted as a decrease in superheat, which tends to reduce refrigerant flow, which in turn reduces the coil pressure.
Thus equilibrium is reached at a predetermined coil pressure. The coil is at its maximum operating pressure.
When the evaporator load decreases, the bulb temperature decreases, liquid droplets form in the bulb and everything goes back to normal.
Thanks so much for the detailed explanation. That would be quite some juggling act the bulb-manufacturer has to take care of. I'd imagine that the bulb location would also be critical here.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I should add that for your purposes, given the non-conventional pressures you need in order to maximize coil heat absorption, you would probably need to special order your MOP charge.
Ok, that will probably make for an interesting supply discussion.
Which manufacturers of TXV's would be open to supply optimised MOP options? In my view, this would actually be a useful service component in terms of service support. Perhaps also give a bit of competitive edge as well.
Some very interesting thoughts coming out of this discussion.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
You may have noticed that refrigeration coils are sized for TD's of 10-15F/5.5-8.5K, while A/C coils are sized for 35-40F/20-22K TD's. There is a very good reason for this: If an A/C coil were sized for 10-15F/5.5-8.5K TD it would be incapable of achieving acceptable humidity levels. Your home would be a cold swamp.
You have no such dehumidification needs in this system, therefore you can achieve much higher COP by sizing your coil in accordance with refrigeration practices as opposed to A/C practices.
Can you walk me through this, step-by-step? I'm assuming by refrigeration coils & A/C coils, you're referring to the evaporator coils?
If we are to size for a smaller TD (e.g. 10-15F/5.5-8.5K TD), would it not end up being a lot larger - volume, or area, for the same heat-transfer rate?
This is no problem, in practice, as the fin heat-transfer coefficient can be improved dramatically with a new design concept currently on the drawing board.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
On the other hand, conventional sizing calculations assume a portion of the coil is used for flashing off the liquid. Since we are taking steps to eliminate flashing this changes everything.
I'm thinking you are going to have to size and adjust every component through a step-by-step trial and error process.
On the bright side, at the end of this process you may be in a position to devise your own set of unique formulas for this particular industry niche.
Gary, thanks so much for the incredible input & insights you have provided on this thread. Between you & Magoo you have given me a huge amount to think about & experiment with. I really do see that innovations need to be developed in this market niche. The end result will hopefully be passed on in terms of smaller size & lower cost to the end user.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Ok, that will probably make for an interesting supply discussion.
Which manufacturers of TXV's would be open to supply optimised MOP options? In my view, this would actually be a useful service component in terms of service support. Perhaps also give a bit of competitive edge as well.
This would be a moot point if a cap tube is used. And a cap tube will be much better for this application. The cap tube can be used for both pressure reduction and HX and this will reduce weight, expense and footprint. It is ideal.
The key to using a cap tube for this application is the evap heat load stabilization provided by the fan control.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Can you walk me through this, step-by-step? I'm assuming by refrigeration coils & A/C coils, you're referring to the evaporator coils?
If we are to size for a smaller TD (e.g. 10-15F/5.5-8.5K TD), would it not end up being a lot larger - volume, or area, for the same heat-transfer rate?
We already have a target TD. At 25C incoming air temp, we want the SST to be 15C, therefore 25-15=10K.
Our ideal TD, assuming a minimum incoming air temp of 25C, is 10K.
If we were designing for cooler ambients, say 20C incoming air, we would want to size our coil for 20-15=5K TD. In trying to get our TD lower than 5K we would hit a point of diminishing returns.
The machine would work well in cooler ambients than we are designing for, but the SST would drop and we would no longer be riding the upper limits of the compressor, thus the COP would be reduced as the ambient temp reduces.
In other words, if we were designing for all ambients, we would want the evap TD to be 5K.
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Re: AWHP superheat & sub-cooling
^ What happens when incoming air temp rises to 35'C, as is common in Asia & Africa?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^ What happens when incoming air temp rises to 35'C, as is common in Asia & Africa?
The fan slows down to compensate for the rise in heat load. Less evap airflow = less evap heat load.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The fan slows down to compensate for the rise in heat load. Less evap airflow = less evap heat load.
Agreed.
This fan control concept is also applicable to the unsteady heat-pump cycle situation.
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Re: AWHP superheat & sub-cooling
What are your thoughts on COP,hp optimisation?
How to force the heat-pump system to operate on maximum COP,hp throughout the heating cycle?
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Re: AWHP superheat & sub-cooling
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Originally Posted by
Magoo
Hi desA
The primary consideration is compressor operational conditions, for longevity of system integraty.
Did the TEV superheat check I suggested stabalize performance, it has worked for me for decades.
I did the following today, on the lab machine:
1. Weighed in the calculated refrigerant charge sufficient for all internal components at selected operating conditions;
2. Let the system settle;
3. Tuned the TXV using the 0.6-0.7 times TD rule;
4. Let system settle between TXV adjustments;
5. Ramped system up from ambient water temp to 60'C.
The system ran as sweet as a bird. Smooth. The TXV tuning rule seems to be bang on, Magoo. I'm a very happy camper.
Tomorrow, I plan to run the machine up to my standard test point, hold it & then further fine-tune the TXV a 1/4 turn at most from its current position.
Thanks Magoo - I owe you a few cold ones.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
What are your thoughts on COP,hp optimisation?
How to force the heat-pump system to operate on maximum COP,hp throughout the heating cycle?
Now that we are maximizing the heat absorption on the low side, what remains is to transfer that heat to the water flowing through the condenser.
As heat is transferred from the refrigerant to the water, the refrigerant temp drops, having lost heat, and the leaving water temp rises, having gained heat.
The two temperatures "approach" each other. Thus the difference between the SCT temp and the leaving water temp is called the approach temp.
By installing a water regulating valve at the condenser outlet (which senses and controls SCT) we can regulate the flow to give us a steady SCT of 75C, which is the upper limit of the compressor.
Since the SCT is thereby fixed at 75C, the variable in our approach becomes the water leaving temp. The better the heat transfer, the higher the leaving water temp.
We need to have a condenser large enough to give us 65C leaving water temp, giving us an approach temp of 75-65=10K approach.
By increasing the condenser size beyond this minimum we can increase water temp and reduce the approach, but this again involves a point of diminishing returns.
I'm thinking a reasonable approach target would be about 5K (75-70=5K).
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Re: AWHP superheat & sub-cooling
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By install a water regulating valve at the condenser outlet (which senses and controls SCT) we can regulate the flow to give us a steady SCT of 75C, which is the upper limit of the compressor.
Since the SCT is thereby fixed at 75C, the variable in our approach becomes the water leaving temp. The better the heat transfer, the higher the leaving water temp.
This is excellent system design logic. Thanks for this.
I'd envisage a simple water flow control valve, taking its signal off the condenser. For a concentric-tube type, this should be no contest as long as the temp probe is well-secured.
For a plate-type condenser, I wonder whether there would be a decent place to pick up the condensing temp SCT @ 75'C on the body? May have to infer the SCT from another suitable temperature, or perhaps convert the SCP pressure, in the controller.
My current design approach is typically around 10K, for an economical design. The cost effect can be easily developed versus approach - I'll look into that.
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Re: AWHP superheat & sub-cooling
The only problem I can foresee at this point is in maintaining the water temp in the storage vessel.
Since the condenser incoming water temp is high, we may not be able to get enough flow through our condenser to prevent the SCT from exceeding the 75C limit.
If this is the case we may need to override the fan control, shutting down the fan to decrease the heat transfer... but let's cross that bridge if/when we come to it.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
I'd envisage a simple water flow control valve, taking its signal off the condenser. For a concentric-tube type, this should be no contest as long as the temp probe is well-secured.
I would control the flow off high side pressure, rather than temperature. This is a common strategy for water cooled condensers and the valve is readily available. Controlling off pressure rather than temp assures that we are riding the upper limit of the compressor but not exceeding it.
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Re: AWHP superheat & sub-cooling
The storage vessels are usually fairly large, for the larger heat-pumps. Some could be in the region of 4000L or so. There would be a fair amount of thermal lag there.
A sensor signal from the tank can be input into the heat-pump controller & action taken to shut back the pump, fan etc. Typically the water temp is used to shut the heat-pump off, when the pre-set temp has been reached.
Hi/low pressure trips are also installed. If the water flow drops too low & SCT rises above the critical value, the hi/low should catch it, if correctly set - to protect the compresor.
Basically, since the hot water is generally blended with cold water by the end user, the terminal tank temperature can undershoot, or even overshoot a little, without much problem - in general.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I would control the flow off high side pressure, rather than temperature. This is a common strategy for water cooled condensers and the valve is readily available. Controlling off pressure rather than temp assures that we are riding the upper limit of the compressor but not exceeding it.
Ok, good. That makes good sense.
Do you have any manufacturers, or model numbers that may be of use in this application? I'll research it.
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Re: AWHP superheat & sub-cooling
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Basically, since the hot water is generally blended with cold water by the end user, the terminal tank temperature can undershoot, or even overshoot a little, without much problem - in general.
Our strategy being to fully heat the feed water before sending it to the storage vessel, we would not want the hot water to be blended with cold water in the storage vessel.
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Our strategy being to fully heat the feed water before sending it to the storage vessel, we would not want the hot water to be blended with cold water in the storage vessel.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^^ 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.
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.
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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.
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
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.
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.
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Re: AWHP superheat & sub-cooling
Quote:
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?
A few thoughts on this - let's say for a typical, unoptimized, low-cost circuit (open to critical review):
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
A few thoughts on this - let's say for a typical, unoptimized, low-cost circuit (open to critical review):
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.
You forgot to add:
3. TXV
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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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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I'm not sure if there was a question in your last post.
More of thinking out aloud. :o
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TXV's excell at handling variable loads... and for this they need surplus refrigerant.
Fair-enough.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
More of thinking out aloud. :o
Fair-enough.
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.
How does this benefit the system?
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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.
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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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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
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.
The cooling would be limited to the temp of the entering water. 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.
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
The cooling would be limited to the temp of the entering water.
This is very true - it is. The entry temperature is critical, in how far the sub-cooling can be pushed.
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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.
Where would the loss come from, though?
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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.html
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Where would the loss come from, though?
I'm thinking there would be more gain from using the extra condenser area to lower the approach temp, therefore a loss by comparison.
There is very little heat transfer involved in subcooling a liquid.