AWHP superheat & sub-cooling

[Gary]

A question regarding Te,sat drift

This Te,sat drift phenomenon whereby the evaporator sat pressure begins at a reasonable value of say 10'C at start of heating cycle, but gradually begins to drift upwards, with an overall drift of sometimes as much as 5'C during a heating cycle, seems to be a consequence of the refrigerant loop itself.

At times, if Te,sat begins at say 10'C, but drifts up to 15'C at cycle end, then compressor integrity is not threatened. On the other hand, for a different combination of Ta,in & RH%, Te,sat can begin around 14.5'C & drift towards 19.5'C by cycle end - definitely causing concern for the compressor.

My question is that I've seen countless manufacturers - many from colder climates, it must be admitted - but none of them seem to even consider the Te,sat drift phenomenon.

Did the iced drier test alter the Te,sat drift?

[Gary]

It seems very strange to me that the compressor manufacturers have not designed a non-inverter compressor with an operating envelope that is more suited to AWHP... or perhaps they have?

Is there some reason the water needs to be heated to 65C?

How hot does the water need to be?... and why?

[Gary]

Practically, I've seen some roo-based machines suffer something like 20% compressor failures within an 18 month period.

Are you referring to the machine we are working on? I would point out that the closer we look, the more problems we find, starting with a failure to limit the load and ending with a poorly designed condenser... either of which could destroy the compressor.

And who knows what else we will stumble across?

[Gary]

When this phenomenon is brought to the attention of the Asian compressor technical experts, they all turn a whiter shade of pale & admit that "it is difficult to find a compressor that can make 55'C water in Asia".

Shouldn't it be easier to make hot water in a hot climate?

Given an open compressor and a variable pulley, I'm pretty sure I could make steam.

[desA]

The price and operating cost (and repair costs) need to be evaluated over a 1 year or 2 year period. It may be more expensive kit in the first place turns out to be cheaper to run and less cost to service in the long run. You will have to do the numbers

Agreed for a Euro/US-spec machine - wholeheartedly.

but as you have pointed out the user is concerned with price per hot shower and for your market they probably wont think a week in advance.

This is exactly the crunch point.

It would be interesting to see the cost of the system using variable compressor and savings over a year on power. You will never know until you do the numbers.

Plus you get a PID controller to plug in the other stuff you have.

Chef

Very good comments & points well taken. Thank you very much.

[Chef]

DesA - This simple equation might explain why you get Te drift

Te2=Ta + (Te-Ta)*[m2(h12-h42)*u]/[m(h1-h4)*u2]

For a starting state at Tw=45C and ending at Tw2=65C and where all suffix 2 are the second state (which is the hot water at temperature state).

Assuming a start point of Ta=30C and Te=10C
The 1st iteration gives Te2 as 14C and the 2nd iteration gives it as 16c whilst convergence is finally 15.4C.

If U and U2 are included it reduces Te2 by a few percent but also increases the SH and so reduces the m2 and about balances out. However the inclusion of fluid backup into the condensor and subcooling changes also alter the final value but that gets too complex to write in a simple equation and really needs simulation.

Chef

[desA]

The drift is part of the cycle and you need to design it into your system and not try to get rid of it. If you look at the position of h4 during the heating cycle you will see it moves to the right. The quality changes and causes the drift, also the charge is repositioned in the system to compensate. One way is to reduce the volume flow rate of the system to balance the new (lesser) cooling effect. As you have said before the system is unstable and controlling from the TXV is also included in the instable part of the loop.

To get a stable loop you will need to control on an element in the loop that does not have a positive value in the feedback loop. One would be volume flow rate.

Chef

I like your logic. Let me add in a few comments.

As the heating cycle advances, & Tc,sat rises, the quality (vapour volume ratio) will increase, as a function of the narrowing of the vapour bell.

The critical destabilising effect & Te,sat drift is caused by the fact that the TXV & compressor response curves are at variance. The TXV orifice seeks to increase the mass-flowrate with increasing Tc,sat, whereas the compressor characteristics seek to decrease mass-flowrate with increasing Tc,sat.

The TXV-compressor 'sweet spot' typically occurs around Tc,sat = 50 to 55'C. To the left of this point, the TXV orifice underfeeds - with the temp bulb compensating some of the way. To the right of the 'sweet point' the TXV orifice overfeeds. This tends to force the compressor/TXV operating point to move to the right & upwards, pulling Te,sat with it. There is precious little that can be done here, unless a limiting choke is installed, or TXV with different characteristic is developed, or variable speed compressor is used as the controlling device pulling against an orifice.

[desA]

Did the iced drier test alter the Te,sat drift?

Very small real effect. It lowered the Te,sat initially, but over time the drift did still occur.

An important note:
A change at one point in time, may not reflect the long-term operation of the system. A single heating cycle test is not enough. After the first run up, when the system shuts back, heat-soak occurs. Upon system re-start, the system operation is slightly different to the first heat-up. Another reason is that refrigerant re-positions itself during the first heat-up cycle (conenser/evap), & then gradually for successive heat-ups thereafter.

The long-term 'quasi-steady state' operating point is of interest.

The system needs to go through the first heat-up cycle (say 25'C to 65'C) & around 2 additional short cycles (60-65'C) before the refrigerant balances really become repeatable. This is a refrigerant mass-transfer game...

[desA]

It seems very strange to me that the compressor manufacturers have not designed a non-inverter compressor with an operating envelope that is more suited to AWHP... or perhaps they have?

There is another compressor in the range, used predominantly in Europe - supposedly for heat-pump service. It has the same operating window, but has slightly better high temp efficiencies. It is also 3-4 times the price of the Asian built compressors.

Is there some reason the water needs to be heated to 65C?

How hot does the water need to be?... and why?

This is partly a numbers game, unfortunately. "My system can reach 70'C - Oh, yours only 62'C. Why do you build junk... blah,blah..."

The other thing is that in Asia, the users will push every last degree out of the blessed thing. I've seen electrical element units steaming at ~ 100'C, because it meant putting in less heating units. No matter that the customer burned his arm when he touched the hot-water pipe.



In reality, though, if you require a secondary heating loop, there needs to be a temp head from heating source over secondary exchanger of at least 5-10K. So, if you need water at 55'C, exit secondary loop, then the heating source should be 60-65'C.

[desA]

Originally Posted by desA
Practically, I've seen some roo-based machines suffer something like 20% compressor failures within an 18 month period.

Gary:
Are you referring to the machine we are working on? I would point out that the closer we look, the more problems we find, starting with a failure to limit the load and ending with a poorly designed condenser... either of which could destroy the compressor.

And who knows what else we will stumble across?

One group were predecessors to this lab machine we are dissecting. One common fault of their design does seem to be under-sized condensers, poor oil return, compressor over-heating, condenser/compressor in sealed chamber... as far as I am able to make out.

[desA]

Originally Posted by desA
When this phenomenon is brought to the attention of the Asian compressor technical experts, they all turn a whiter shade of pale & admit that "it is difficult to find a compressor that can make 55'C water in Asia".

Gary:
Shouldn't it be easier to make hot water in a hot climate?

The compressor seems to be the weak link in the chain, so far.

Given an open compressor and a variable pulley, I'm pretty sure I could make steam.

Well... suggest away...

[desA]

DesA - This simple equation might explain why you get Te drift

Te2=Ta + (Te-Ta)*[m2(h12-h42)*u]/[m(h1-h4)*u2]

For a starting state at Tw=45C and ending at Tw2=65C and where all suffix 2 are the second state (which is the hot water at temperature state).

Assuming a start point of Ta=30C and Te=10C
The 1st iteration gives Te2 as 14C and the 2nd iteration gives it as 16c whilst convergence is finally 15.4C.

If U and U2 are included it reduces Te2 by a few percent but also increases the SH and so reduces the m2 and about balances out. However the inclusion of fluid backup into the condensor and subcooling changes also alter the final value but that gets too complex to write in a simple equation and really needs simulation.

Chef

Thanks Chef,

Can you please give a key to all the terms used & also your base equations from which you derived the relationship? I'll go into my simulator & emulate the same, to see if we can agree answers.

Thanks for that...

[Chef]

Here start condition is when you turn on the machine with Tw=45C and the end condition is when the system trips on Tw2=65C (assuming a thermostat cut out or similar)

Te evap temp Te,sat in your parlance
Ta ambient temp (assumed constant)
Tw water temp at start of warm up
Tw2 water temp at finish of run (subscipt 2 is second or end state)
h1 and h4 are enthalpy at start condition
h12 and h42 are enthalpy at end condition
u is start condition thermal transfer
u2 is end condition thermal transfer (subscipt 2)
m is start condition mass flow rate
m2 is end condition mass flow rate

This is an iterative formulae and so you need to keep updating the parameters until they converge at a reasonable level - I chose +/-0.3 degree.

Both SC and SH are assumed constant - its not the case in reallity but the equation becomes too difficult if included so hence the requirement to simulate it. Simlation gives results close enough for the equation to be used as a guidline.

Hope it helps.

Chef

[desA]

^ Thanks very much for the information. I'll try & unravel how you arrived at the equation...

I'll work with it & see where it leads. SC does increase across the range as the cycle advances.

[desA]

In all of these simulations, we must consider HOW the mass flow around the circuit is being governed. What throttles, or otherwise, this circulation velocity?

Compressor curves, by themselves, have little value. Neither do TXV orifice values by themselves. The system hydraulics of TXV/compressor & internal system pressure drops - are what governs the refrigerant flow, with a bias via the temperature bulb (SH), affecting the orifice characteristic.

[Chef]

SC will increase as X increases and the condensor has more fluid in it. The only way to estimate how much is to study it with time. By time I mean every few seconds of the AWHP and not human time.

Chef

[desA]

The amount of sub-cooling that a condenser can stably achieve will depend on its design primarily, & on how much charge there is in the system.

There is a charge load at which the condenser begins to flood with condensate & its performance drops off dramatically. This is emphasized towards the high Tc,sat values.

[desA]

With the control of entry pressure into the compressor, it (the compressor lower casing) is running around a most respectable 40-45'C even towards the top end of the heating range. This should keep the motor coils more happy.

[Gary]

With the control of entry pressure into the compressor, it is running around a most respectable 40-45'C even towards the top end of the heating range. This should keep the motor coils more happy.

What is running 40-45'C?

[desA]

Originally Posted by desA
With the control of entry pressure into the compressor, it is running around a most respectable 40-45'C even towards the top end of the heating range. This should keep the motor coils more happy.

Gary:
What is running 40-45'C?

Oops... I meant to add that the compressor lower casing is running fairly comfortably around 40-45'C, even with some V*I challenges during our typical low-voltage period this evening.

[Gary]

Given an open compressor and a variable pulley, I'm pretty sure I could make steam.

Well... suggest away...

So long as the oil is cooled and the compression ratio is reasonable, I see no reason the Tc,sat could not be driven up in excess of 100'C.

[Gary]

Oops... I meant to add that the compressor lower casing is running fairly comfortably around 40-45'C, even with some V*I challenges during our typical low-voltage period this evening.

Are we still talking about the same system?... or the system with the reconfigured condenser and larger compressor?

[desA]

^^ I'm fascinated - making steam would be pretty amazing.

What type of 'open compressor' would you be thinking of? Any links?

[desA]

Are we still talking about the same system?... or the system with the reconfigured condenser and larger compressor?

No, it's still exactly the same system as previously, except with the installation & tuning of the service valve between evap & compressor suction.

I thought to push the system further, before hacking it apart.

So far, today's run got water to Tw,o=69.6'C, with Tc,sat~74.5'C, Tcomp,disc=107'C, comp suction equivalent Te,sat=13.5'C, in the middle of a VI challenge...

The difficulty with a manual SV, instead of the CPR, is that you have to set it for the 'worst case' of a very hot day - to not have too high a compressor suction pressure. It's wasteful at the times where conditions are more lenient. It's something that would have to be 'tuned' by the on-site installer, with OEM setting being set more conservatively.

[desA]

VIC re-visited.

Just to cheer you up, I've made some solid progress on the VIC.

My local plate tech specialist sent me some info on their idea of VIC. I'll scan the article & upload it to my website, for review.

[Gary]

^^ I'm fascinated - making steam would be pretty amazing.

What type of 'open compressor' would you be thinking of? Any links?

I'm thinking an automotive A/C compressor should be just about right.

The trick would be keeping the oil from cooking. It would get very hot at the discharge valves. We would need an oil that can take the temperature.

[Gary]

VIC re-visited.

Just to cheer you up, I've made some solid progress on the VIC.

My local plate tech specialist sent me some info on their idea of VIC. I'll scan the article & upload it to my website, for review.

Yep... that cheers me up. I look forward to reading the article.

[Gary]

I'm thinking an automotive A/C compressor should be just about right.

The trick would be keeping the oil from cooking. It would get very hot at the discharge valves. We would need an oil that can take the temperature.

Another sticking point would be the water pump. If it pumps water that is anywhere near 100'C and anywhere near atmospheric pressure, it is going to cavitate.

[desA]

I'm thinking an automotive A/C compressor should be just about right.

The trick would be keeping the oil from cooking. It would get very hot at the discharge valves. We would need an oil that can take the temperature.

I like the idea of using an automotive compressor, very much. I understand that there are also some close-coupled units available as well, nowadays - for the newer vehicles.

The oil could be moved through an oil-cooler (finned pipe) - not quite sure how to push it along, unless with a small gear pump, perhaps.

Another thing to bear in mind that for R-134a, the T,critical is 101'C. We'd be pushing the top of the vapour bell, unless we used a desuperheater, or pushed into transcritical territory.

Not quite sure how well R-134a would react in above its critical point...

[desA]

Another sticking point would be the water pump. If it pumps water that is anywhere near 100'C and anywhere near atmospheric pressure, it is going to cavitate.

Need a secondary loop, to the water. Let the heat-pump side work on glycol-water & we can push this up to over 118'C without a worry (top tank temp for an Audi radiator, for instance).

[desA]

Well, with all this, why not go all the way into CO2 territory?

[Gary]

Well, with all this, why not go all the way into CO2 territory?

What are the pro's and con's of CO2? (thinking I need a new P/T chart)

[Gary]

Need a secondary loop, to the water. Let the heat-pump side work on glycol-water & we can push this up to over 118'C without a worry (top tank temp for an Audi radiator, for instance).

Or run a pressurized water system. So long as the pressure/temperature at the eye of the impellor is above saturation, cavitation can be avoided.

[desA]

Originally Posted by desA
VIC re-visited.

Just to cheer you up, I've made some solid progress on the VIC.

My local plate tech specialist sent me some info on their idea of VIC. I'll scan the article & upload it to my website, for review.

Gary:
Yep... that cheers me up. I look forward to reading the article.

Just finished scanning the pages. Will downsize them for decent download size & post them in the morning. Apologies for the delay.

[Gary]

I'm thinking you may find this interesting:

http://ec.europa.eu/environment/ozon...ters_paper.pdf

[Gary]

Well, with all this, why not go all the way into CO2 territory?

There is nothing magical about CO2.

Setting aside all of the environmental propoganda, CO2 is just another refrigerant and not a particularly good one at that... but I could be wrong.

[mad fridgie]

There is nothing magical about CO2.

Setting aside all of the environmental propoganda, CO2 is just another refrigerant and not a particularly good one at that... but I could be wrong.

How correct you are Gary, If the same technology was to be used (2 stage compressor with intercooler and high efficient motors) on for example R134a results would not look so good for CO2.
CO2 heat pumps to achieve the good COPs are totally reliant on cold water entering the heat exchanger (gas cooler) somewhat like a dedicated sub cooler. Yes they can achieve very hot water, but this is really a function of the properties of the CO2 in compression, "very high discharge temps"
As far as steam goes yes it is possible, but other issues occur primarily with heat HX design, using a simple tube in tube causes the refrigeration system to be come highly unstable (steam bubbles effecting heat transfer/flow) Yes this over come by increasing the pressure within the heat exchanger.

[Gary]

As far as steam goes yes it is possible, but other issues occur primarily with heat HX design, using a simple tube in tube causes the refrigeration system to be come highly unstable (steam bubbles effecting heat transfer/flow) Yes this over come by increasing the pressure within the heat exchanger.

I'm thinking that the lowest pressure in the water loop would be the eye of the impellor. If the pressure is high enough to prevent cavitation, then the rest of the circuit should have no problems.

I'm not sure that we would want steam anyway. My point was that it can be done and anything short of steam can be done also.

[desA]

Originally Posted by desA
VIC re-visited.

Just to cheer you up, I've made some solid progress on the VIC.

My local plate tech specialist sent me some info on their idea of VIC. I'll scan the article & upload it to my website, for review.

Gary:
Yep... that cheers me up. I look forward to reading the article.

desA:
Just finished scanning the pages. Will downsize them for decent download size & post them in the morning. Apologies for the delay.

Here are the articles:
http://adthermtech.com/adt/VIC_pg10.pdf
http://adthermtech.com/adt/VIC_pg11.pdf

Note the comment about the stability issues. This correlates to the findings from my simulation work.

[desA]

I'm thinking you may find this interesting:

http://ec.europa.eu/environment/ozon...ters_paper.pdf

Thanks very much for the article.

I must say, that, some of the cooler designs I've seen look exceptionally amateur, to say the very least. I do plan to move into CO2 at some point, in the not-too-distant future - merely to keep abreast with the current fashion, but also with a view to bringing in some new innovations I have in the wings.

Things like high-performance copper tubes that can withstand the high pressures (320 bar plus), with minimal deflection. There are a number of innovations just waiting to be introduced.

Not sure, at this point, how much real traction CO2 will get in the marketplace, outside of fashionable venues. We will have to wait & see.

[desA]

Originally Posted by Gary
There is nothing magical about CO2.

Setting aside all of the environmental propoganda, CO2 is just another refrigerant and not a particularly good one at that... but I could be wrong.

mad_fridgie:
How correct you are Gary, If the same technology was to be used (2 stage compressor with intercooler and high efficient motors) on for example R134a results would not look so good for CO2.
CO2 heat pumps to achieve the good COPs are totally reliant on cold water entering the heat exchanger (gas cooler) somewhat like a dedicated sub cooler. Yes they can achieve very hot water, but this is really a function of the properties of the CO2 in compression, "very high discharge temps"
As far as steam goes yes it is possible, but other issues occur primarily with heat HX design, using a simple tube in tube causes the refrigeration system to be come highly unstable (steam bubbles effecting heat transfer/flow) Yes this over come by increasing the pressure within the heat exchanger.

Now, let's add to this the fact that CO2 trans-critical cycles are, by nature, bi-stable, or multi-stable systems.

In other words, they can jump from a point of decent performance to one of totally inferior performance off a very small trigger. This can be seen in simulations & a few academic papers have been published in this area. The secret to stable operation seems to lie in a cunning control system.

[desA]

I'm thinking that the lowest pressure in the water loop would be the eye of the impellor. If the pressure is high enough to prevent cavitation, then the rest of the circuit should have no problems.

I'm not sure that we would want steam anyway. My point was that it can be done and anything short of steam can be done also.

Could we push the compressor through the critical point at around 101.6'C?

If so, would this be a two-stage compression operation, or a double cascade system?

If to remain in sub-critical territory, how close to the top of the vapour bell would we safely want to go?

[Gary]

Here are the articles:
http://adthermtech.com/adt/VIC_pg10.pdf
http://adthermtech.com/adt/VIC_pg11.pdf

Note the comment about the stability issues. This correlates to the findings from my simulation work.

Hmmm... it really doesn't say much.

There is an increase in MTD and K value. I'm guessing this means evap TD and capacity.

I'm not overly concerned about the stability issue.

[Gary]

Thanks very much for the article.

I must say, that, some of the cooler designs I've seen look exceptionally amateur, to say the very least. I do plan to move into CO2 at some point, in the not-too-distant future - merely to keep abreast with the current fashion, but also with a view to bringing in some new innovations I have in the wings.

Things like high-performance copper tubes that can withstand the high pressures (320 bar plus), with minimal deflection. There are a number of innovations just waiting to be introduced.

Not sure, at this point, how much real traction CO2 will get in the marketplace, outside of fashionable venues. We will have to wait & see.

What did you think of the expander?

[mad fridgie]

What did you think of the expander?

I think you will find, that a lot of these new technologies are infact quite old, some were around way before gary was a boy(I know thats hard to believe) No ofence given!
I alot of these old techs will be revisted, stirling, claude, absorbtion, adsorbtion, peltier to name a few
I personally believe the long term future is in magnetic refrigeration. (another story)

[desA]

What did you think of the expander?

A reverse Rankine cycle with expander replacing a conventional pump.

The vapour compression process uses an adiabatic throttle (constant enthalpy device), to replace the pump/expander (constant entropy process).

Adds a lot of complexity to the system... I'd estimate that the constant entropy requirement will make for some interesting evap sizing learning curves in the beginning.

[Gary]

I think you will find, that a lot of these new technologies are infact quite old, some were around way before gary was a boy(I know thats hard to believe) No ofence given!
I alot of these old techs will be revisted, stirling, claude, absorbtion, adsorbtion, peltier to name a few
I personally believe the long term future is in magnetic refrigeration. (another story)

I haven't seen that one before. A lot of the new technologies are old technologies made feasible by new materials.

[Gary]

Note the comment about the stability issues. This correlates to the findings from my simulation work.

I am fast losing faith in your simulator. I would have built a crude prototype long ago, proven or disproven the concept and moved on.

If it improves the capacity, we can find ways to deal with the stability or any other issues... and make it work.

Then simulate it.

[desA]

I am fast losing faith in your simulator. I would have built a crude prototype long ago, proven or disproven the concept and moved on.

If it improves the capacity, we can find ways to deal with the stability or any other issues... and make it work.

Then simulate it.

The simulator is actually quite good, if managed carefully. It happens to be the best process simulator on the market, with a price tag that would scare the faint-hearted.

If, during simulation, process instabilities are around, then to converge the solution is always a tough, tough call. I've been simulating for most of my trained life & know when the process non-linearities begin to bite...

If you look at the VIC article I posted, you will see what they say about this process instability - it is real & should be carefully managed, in my view. I would go so far as to install a demister pad, or filter-drier somewhere between VIC & compressor to prevent droplet fling under unstable conditions...

[desA]

This is the essential statement of importance. The system is prone to hunting. It is, by nature, an unstable process.