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Thread: AWHP superheat & sub-cooling
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28-08-2009, 05:05 AM #501
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28-08-2009, 05:14 AM #502
Re: AWHP superheat & sub-cooling
http://www.ra.danfoss.com/TechnicalI...Subcooling.pdf
An interesting article by Danfoss on the influence of subcooling on the system.
I'd be interested to hear your comments on their view.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 05:36 AM #503
Re: AWHP superheat & sub-cooling
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28-08-2009, 05:54 AM #504
Re: AWHP superheat & sub-cooling
^ You may be onto something here. Very, very interesting. Brilliant.
In your experience, what could be the influence of the SAME compressor running at 50/60=83.3% of the US designed speed?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 06:20 AM #505
Re: AWHP superheat & sub-cooling
Last edited by Gary; 28-08-2009 at 06:31 AM.
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28-08-2009, 06:47 AM #506
Re: AWHP superheat & sub-cooling
^
I'd say that the take-away values for me, from the article were:
1. The recommended evaporator inlet quality condition should be in the range 0.2 < x < 0.3 .
2. Sub-cooling should be set in order to make (1) possible.
3. "Under constant marginal conditions, the compressor capacity increases when subcooling increases".
These statements should be debated to see whether they hold water, or not, in terms of an AWHP, where 0.2 < x < 0.4 under reasonable subcooling.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 06:52 AM #507
Re: AWHP superheat & sub-cooling
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28-08-2009, 06:58 AM #508
Re: AWHP superheat & sub-cooling
Last edited by Gary; 28-08-2009 at 07:08 AM.
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28-08-2009, 10:10 AM #509
Re: AWHP superheat & sub-cooling
Originally Posted by desA
^ You may be onto something here. Very, very interesting. Brilliant.
In your experience, what could be the influence of the SAME compressor running at 50/60=83.3% of the US designed speed?
Given the same Te,sat and the same Tc,sat, less RPM gives you less mass flow, which reduces system capacity.
Now, with less refrigerant mass-flow, what would be the effect on the compressor internal heating, friction, motor heat, that may allow Te,sat & Tc,sat to be pushed up slightly higher (from the US baseline of Te,sat=13'C, Te,sat=65.6'C)?
The answer to this could provide the link to the 60Hz versus 50Hz theory.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 10:17 AM #510
Re: AWHP superheat & sub-cooling
Checked the second option:
VIC -> evap
There is a very narrow band at extremely high dT,liq where this is feasible, but, outside that, it increases the evap duty.
Scrapped...
I'm now back to trying to figure out why the first option:
evap -> VIC
is giving me such a modeling headache. Bear with me on this, I've only done around 1000 simulation runs so far... lolEngineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 01:49 PM #511
Re: AWHP superheat & sub-cooling
VIC feedback
The answer for the instability boils down to the following:
1. Cp,hp = 182036.5 J/kmol.K
2. Cp,lp,v = 84538.5 J/kmol.K
3. hfg,lp = 1.987e7 J/kmol
The huge differences between the latent heat of evaporation (hfg,lp) & the various specific heats (Cp) is causing all kinds of local numeric unhappiness in the simulator. I'll continue working on it.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 02:53 PM #512
Re: AWHP superheat & sub-cooling
I think the key is the mass flow. In order for the 50hz compressor to pump the same mass flow (same compressor load) as the 60hz compressor, the 50hz model would have to increase Te,sat and Tc,sat.
I think we have found the difference between the baselines, in which case we can comfortably raise our Te,sat to 15C.Last edited by Gary; 28-08-2009 at 04:07 PM.
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28-08-2009, 03:48 PM #513
Re: AWHP superheat & sub-cooling
Should we elect to go the direct heating route, ideally the supply of fully heated water would be sufficient to keep the tank level from dropping too low during periods of peak water usage.
But if it is not sufficient to maintain a minimum tank level, we need a means of bypassing the water regulating valve to provide full flow through the condenser. Perhaps a dual float switch and bypass solenoid.
As the tank level drops, the first float switch turns on the AWHP in feedwater heating mode. Should the level drop further, the bypass is energized by the second float switch, bringing the level back up (with partially heated water) and then de-energizing the bypass.Last edited by Gary; 28-08-2009 at 04:12 PM.
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28-08-2009, 04:05 PM #514
Re: AWHP superheat & sub-cooling
Originally Posted by desA
Ok.
Now, with less refrigerant mass-flow, what would be the effect on the compressor internal heating, friction, motor heat, that may allow Te,sat & Tc,sat to be pushed up slightly higher (from the US baseline of Te,sat=13'C, Te,sat=65.6'C)?
The answer to this could provide the link to the 60Hz versus 50Hz theory.
I think the key is the mass flow. In order for the 50hz compressor to pump the same mass flow (same compressor load) as the 60hz compressor, the 50hz model would have to increase Te,sat and Tc,sat.
I think we have found the difference between the baselines.
Now we're cooking with gas...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 04:31 PM #515
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 04:47 PM #516
Re: AWHP superheat & sub-cooling
No... it would be warm water mixing with hot, not cold water mixing with hot... and only enough to maintain a minimum level. After the peak usage the tank would gradually be filled, then if needed the system would go from feedwater heating mode to maintaining tank temp mode.
The circulating loop shown in the Grunfos diagram is a side issue here. Its sole purpose is to maintain hot water in the lines so that when you turn on the tap you almost immediately get hot water as opposed to waiting for the hot water to come down from the roof.
On the plus side, the hot water mains become part of the hot water storage system. In effect it is like having a larger storage tank.
It should also be kept in mind that while the user is waiting for hot water to arrive from the roof he is wasting water down the drain, water which must be replaced and heated.
If the circulating pump were used, I would recommend that it be integrated into our system, perhaps becoming the tank temp maintenance pump.Last edited by Gary; 28-08-2009 at 06:40 PM.
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28-08-2009, 05:01 PM #517
Re: AWHP superheat & sub-cooling
But if it is not sufficient to maintain a minimum tank level, we need a means of bypassing the water regulating valve to provide full flow through the condenser. Perhaps a dual float switch and bypass solenoid.
As the tank level drops, the first float switch turns on the AWHP in feedwater heating mode. Should the level drop further, the bypass is energized by the second float switch, bringing the level back up (with partially heated water) and then de-energizing the bypass.
So, basically, there would be two water speeds through the condenser - the slower one being that which maintains Tc,sat=75'C, as heating mode.
The second allowing the water flow to speed up through the condenser, as water requirements call for it (dropping tank level). This would then pull down Tc,sat a little, off peak.
Good point.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 08:27 PM #518
Re: AWHP superheat & sub-cooling
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28-08-2009, 11:05 PM #519
Re: AWHP superheat & sub-cooling
Originally Posted by desA
Ok.
Now, with less refrigerant mass-flow, what would be the effect on the compressor internal heating, friction, motor heat, that may allow Te,sat & Tc,sat to be pushed up slightly higher (from the US baseline of Te,sat=13'C, Te,sat=65.6'C)?
The answer to this could provide the link to the 60Hz versus 50Hz theory.
I think the key is the mass flow. In order for the 50hz compressor to pump the same mass flow (same compressor load) as the 60hz compressor, the 50hz model would have to increase Te,sat and Tc,sat.
I think we have found the difference between the baselines.
Now this is making sense. I'm going to have to get my head around how to model this, to let the simulator determine these values. Out come the thermo books...
Now we're cooking with gas...
Now, if this is the case, & these systems are indeed running at 50 Hz, whilst tests were done at 60 Hz - why on earth have the manufacturer's technical staff not picked up on this? Is this a case where the technical staff are glorified salesmen who actually don't have a clue about what they are spouting?
Next, what sense can we make of this in our developments?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-08-2009, 11:31 PM #520
Re: AWHP superheat & sub-cooling
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28-08-2009, 11:56 PM #521
Re: AWHP superheat & sub-cooling
Originally Posted by desA
Next, what sense can we make of this in our developments?
We can use Te,sat=15C and Tc,sat=75C.
Now - how on earth would a warranty claim be handled in the event of a commercial application? I'll bet when it's submitted for checking on warranty claim, the compressor technical staff will NEVER understand the frequency difference 50Hz versus 60 Hz, rpm at 2900 instead of 3500, power nominal 6700W versus 8250W etc. They would most likely always fall back on the safest option for them.
In cases like this, does the OEM heat-pump supplier, for instance, state that the unit was operating as per recommended operating values, but let the operator actually have the option to move the settings up to take advantage of the Te,sat=15'C, Tc,sat=75'C relaxation?
I doubt that the technical staff at these huge compressor operations will ever have the intellect, or desire, to solve this one.Last edited by desA; 28-08-2009 at 11:58 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 12:00 AM #522
Re: AWHP superheat & sub-cooling
What I may very well end up doing is developing a position paper on this aspect & having a long, long chat to the regional technical representative for the compressor outfit at the upcoming trade show we have here in BKK next month.
This could be the subject of a nice talk... Seems I've been given some technical talk time on this show, perhaps it's good time to raise some healthy debate...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 12:48 AM #523
Re: AWHP superheat & sub-cooling
MOP valves, act similar to a CPR, at little extra cost, I do not ever remember having any problems, another option with out the need for valving is to control the evap fan, cycle on and off (cheap option and proven) many methods of control.
Double port valve, made by Sporlan I believe, has to internal expansion ports, 1 small and 1 large, if super heat is higher both ports are open as the super heat drops then the large shuts first and controls on the small. Ideal for large variations in TXV pressure differential.
50-60Hz, 20% more capacity on 60Hz, thus more work undertaken by the motor!
As a designer of similar products, my advice is focus on what the end goal is "heating water"
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29-08-2009, 01:02 AM #524
Re: AWHP superheat & sub-cooling
For your MOP option, can I ask you your experiences on the following:
1. Spares availability (international);
2. Control of pressure, using a temperature signal (MOP bulb);
3. Delays due to tube wall, bulb response, TXV response.
another option with out the need for valving is to control the evap fan, cycle on and off (cheap option and proven) many methods of control.
Double port valve, made by Sporlan I believe, has to internal expansion ports, 1 small and 1 large, if super heat is higher both ports are open as the super heat drops then the large shuts first and controls on the small. Ideal for large variations in TXV pressure differential.
I must say though, the heat-pump response to fan is fairly quick - 30-60 sec. The fan is surely going to be cycling a fair bit on/off, to accurately manage the Te,sat drift towards the end of the heating cycle?
50-60Hz, 20% more capacity on 60Hz, thus more work undertaken by the motor!
As a designer of similar products, my advice is focus on what the end goal is "heating water"
Can you comment on what you observe for the Te,sat over the course of a complete heating cycle? I'd imagine you are in NZ. What ambient temps do you experience over there? Have you worked at 35-40'C ambients? I'd love to hear your experiences.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 01:27 AM #525
Re: AWHP superheat & sub-cooling
I presume you are designing a domestic unit, if so, then yoiu need to accept that within monentry constraints that ideal engineering goes out of the window! Difficult I know, but I also had to accept this.
If money is not an issue, then balancing the system becomes easy, speed control all motors, (comp, fan and pump) use electronic expansion valve.
My machine has been tested from
-10C to 43C, and water upto 98C (pushing it a bit though) normally 75C
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29-08-2009, 01:29 AM #526
Re: AWHP superheat & sub-cooling
my favorite saying
"engineering is easy, doing for a price is hard"
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29-08-2009, 01:32 AM #527
Re: AWHP superheat & sub-cooling
Also should remind you that a lot of the world requires a vented double barrier heat exchanger, and that causes its own problems
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29-08-2009, 01:37 AM #528
Re: AWHP superheat & sub-cooling
I will not prejudice system safety for cost-cutting. If system conditions force the compressor into compromise territory, then it is suicide.
I have seen this philosophy used by another colleague. Ended up in a something like +20% failure in the field. This is madness.
If money is not an issue, then balancing the system becomes easy, speed control all motors, (comp, fan and pump) use electronic expansion valve.
My machine has been tested from
-10C to 43C, and water upto 98C (pushing it a bit though) normally 75C
What Tc,sat maximum are you working to?Last edited by desA; 29-08-2009 at 01:43 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 01:39 AM #529
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29-08-2009, 01:53 AM #530
Re: AWHP superheat & sub-cooling
I can agree long of the equipment is very important, I perhaps should of made it clear, that system stability and efficiency can be comprimised to ensure longevity, without the need to make a machine uncompetative
Re my machine, different fundementals used, to achieve desired results.
Not using special compressor!
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29-08-2009, 01:56 AM #531
Re: AWHP superheat & sub-cooling
regs indicate a double barrier that is vented (does not need to be a single hx) A santitry coil in a cylinder is another method, but you always have to a delta T between to achieve required temps
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29-08-2009, 01:59 AM #532
Re: AWHP superheat & sub-cooling
Sometimes, sometimes not...
Re my machine, different fundementals used, to achieve desired results.
Not using special compressor!
The alternative is to inject electric heating along the way, or light a fire under it...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 02:01 AM #533
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29-08-2009, 02:09 AM #534
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29-08-2009, 02:10 AM #535
Re: AWHP superheat & sub-cooling
I did state that 98C was pushing it a bit. No fire, no heater, there is more than one type of refrigeration!
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29-08-2009, 02:13 AM #536
Re: AWHP superheat & sub-cooling
@ Gary:
CPR versus MOP
Can we explore the control methodology & merits of both schemes, as applied to a Te,sat value which gradually drifts upwards over the course of a water-heating cycle.
Te,sat can start at say 10'C on some systems & then drift upwards towards +19'C. For others, it can start at Te,sat=12.5'C, or even Te,sat=15'C then drift upwards.
With Te,sat control, we're surely controlling pressure, not temperature perse'.
How would MOP control interfere with the evap superheat control function?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 02:15 AM #537
Re: AWHP superheat & sub-cooling
Last edited by desA; 29-08-2009 at 02:20 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 03:54 AM #538
Re: AWHP superheat & sub-cooling
MOP, reduces flow into the evap, thus increasing superheat, CPR restricts flow out of the evap basically flooding the evap, thus reducing superheat.
CPR superheat at the compressor can be calculated based upon the pressure drop across the CPR. Both stop control of evap superheat!
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29-08-2009, 04:50 AM #539
Re: AWHP superheat & sub-cooling
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29-08-2009, 04:58 AM #540
Re: AWHP superheat & sub-cooling
So, to partially summarise:
MOP, reduces flow into the evap, thus increasing superheat,CPR restricts flow out of the evap basically flooding the evap, thus reducing superheat.CPR superheat at the compressor can be calculated based upon the pressure drop across the CPR.
Question 1:
1. How does the MOP reduce flow into the evap?
Question 2:
2. How does CPR restrict flow out of the evap? Based on what signal?
Answer 2 (please adjust):
Crankcase suction pressure pulls CPR valve open, against a curve - allows flow through according to valve characteristic at setpoint pressure.
Question 3:
3. What about evap pressure controllers? Could these assist in controlling drifting Te,sat?Last edited by desA; 29-08-2009 at 05:05 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 05:18 AM #541
Re: AWHP superheat & sub-cooling
The CPR will not flood the evap, although the superheat at the compressor inlet may drop a little. Essentially, there is very little, if any, change in superheat.
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29-08-2009, 05:23 AM #542
Re: AWHP superheat & sub-cooling
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29-08-2009, 05:42 AM #543
Re: AWHP superheat & sub-cooling
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29-08-2009, 05:48 AM #544
Re: AWHP superheat & sub-cooling
^ Thanks, Gary, for the above 3 posts.
So, essentially, the CPR & MOP are both restricting (not controlling) flow, not pressure, at a pre-selected cut-off point. Is this correct?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 05:55 AM #545
Re: AWHP superheat & sub-cooling
Originally Posted by desA
Question 3:
3. What about evap pressure controllers? Could these assist in controlling drifting Te,sat?
I don't understand the question. What sort of controllers did you have in mind?
Danfoss & others also make a range of devices, which work on controlling the evap end.
Example: Danfoss type KVP ; PKV/PKVS ; KVQ
The writeup for the KVP states:
" KVP evaporator pressure regulators are mounted in the suction line of refrigeration & airconditioning systems. They are used to maintain a constant pressure corresponding to a constant temperature on the evaporator."
The valve characterisitc is opposite to the KVL, in that capacity RISES from a setpoint (0%) towards 100% after P-band.Last edited by desA; 29-08-2009 at 05:58 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 06:08 AM #546
Re: AWHP superheat & sub-cooling
Last edited by Gary; 29-08-2009 at 06:14 AM.
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29-08-2009, 06:13 AM #547
Re: AWHP superheat & sub-cooling
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29-08-2009, 06:34 AM #548
Re: AWHP superheat & sub-cooling
^ Thanks for clearing that up.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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29-08-2009, 07:07 AM #549
Re: AWHP superheat & sub-cooling
Done any test runs lately?
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29-08-2009, 07:21 AM #550
Re: AWHP superheat & sub-cooling
I'll get back into the tests starting Monday. I needed to clear up an office backlog & needed to settle some technical questions. Basically there, now.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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