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Re: AWHP superheat & sub-cooling
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Originally Posted by desA
Next, what sense can we make of this in our developments?
Gary:
We can use Te,sat=15C and Tc,sat=75C.
Agreed. Boy, I'm relieved. :)
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.
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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...
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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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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
MOP valves, act similar to a CPR, at little extra cost, I do not ever remember having any problems,
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.
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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.
Many thanks for this. I do see some folks use the on/off option.
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?
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50-60Hz, 20% more capacity on 60Hz, thus more work undertaken by the motor!
Also, more motor heat. :)
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As a designer of similar products, my advice is focus on what the end goal is "heating water"
Yes, it is - of course - but safely.
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.
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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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Re: AWHP superheat & sub-cooling
my favorite saying
"engineering is easy, doing for a price is hard"
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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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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
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.
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.
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If money is not an issue, then balancing the system becomes easy, speed control all motors, (comp, fan and pump) use electronic expansion valve.
There are far simpler ways to do the same - mechanically. I hate electronics - terribly unreliable stuff, to be honest - especially in humid, hot climates.
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My machine has been tested from
-10C to 43C, and water upto 98C (pushing it a bit though) normally 75C
Either you're using a high Tc,sat compressor, or you will face problems in the field. This is not something you can do on a domestic budget, I'm afraid. I'd discount that technical claim, to be blunt. :)
What Tc,sat maximum are you working to?
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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
Also should remind you that a lot of the world requires a vented double barrier heat exchanger, and that causes its own problems
There are better ways to manage the sanitary requirement, than a vented HE. :D
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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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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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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
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
Sometimes, sometimes not...
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Re my machine, different fundementals used, to achieve desired results.
Not using special compressor!
You will not get 98'C water out of a standard, dometic-variant compressor, running within manufacturer's design limits - even if you use de-superheaters. Well, not for long, that is. You will be exceeding the safe limits of the compressor envelope. You can't beat thermodynamics.
The alternative is to inject electric heating along the way, or light a fire under it... :p
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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
regs indicate a double barrier that is vented (does not need to be a single hx)
;)
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A santitry coil in a cylinder is another method, but you always have to a delta T between to achieve required temps
True...
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
We can use Te,sat=15C and Tc,sat=75C. :)
We continue along this track, then.
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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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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?
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Re: AWHP superheat & sub-cooling
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Originally Posted by
mad fridgie
I did state that 98C was pushing it a bit. No fire, no heater, there is more than one type of refrigeration!
On a domestic budget? Ok, then, we'll let it go at that. Thanks for your kind contribution to the thread.
All the very best with your developments. :)
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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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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
mad fridgie
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!
I agree with all but the last. The CPR does not stop the control of the evap superheat.
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Re: AWHP superheat & sub-cooling
So, to partially summarise:
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MOP, reduces flow into the evap, thus increasing superheat,
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CPR restricts flow out of the evap basically flooding the evap, thus reducing superheat.
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CPR superheat at the compressor can be calculated based upon the pressure drop across the CPR.
Is this a fair reflection of the MOP, versus CPR operational comparison?
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?
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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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Re: AWHP superheat & sub-cooling
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Originally Posted by
Gary
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.
On further thought, the superheat at the compressor may actually rise a little.
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Re: AWHP superheat & sub-cooling
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Originally Posted by
desA
Question 1:
1. How does the MOP reduce flow into the evap?
It doesn't actually reduce the flow, it limits the flow, this much and no more.
Quote:
Originally Posted by
desA
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.
Okay... although again, the flow is limited, not restricted.
Quote:
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?
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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?
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
Question 3:
3. What about evap pressure controllers? Could these assist in controlling drifting Te,sat?
Gary:
I don't understand the question. What sort of controllers did you have in mind?
Ok, what I understand is that the CPR restricts suction flow into the compressor, hence we see a small pressure drop, with associated temperature change. This works to control the compressor end.
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.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Ok, what I understand is that the CPR restricts suction flow into the compressor, hence we see a small pressure drop, with associated temperature change. This works to control the compressor end.
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.
An EPR valve does not allow the Te,sat to drop below setpoint pressure. The exact opposite of what we are trying to accomplish.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^ 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?
They both limit flow to maintain a pre-selected pressure, the MOP according to evap pressure and the CPR according to compressor inlet pressure.
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Re: AWHP superheat & sub-cooling
^ Thanks for clearing that up. :)
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Re: AWHP superheat & sub-cooling
Done any test runs lately?
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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.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by Gary
We can use Te,sat=15C and Tc,sat=75C.
desA:
We continue along this track, then.
I can now confirm that if the 50Hz operating conditions are moved up to Tc,sat=75'C, that similar compressor mass-flow conditions exist as for the compressor running at 60Hz, Te,sat=10'C, Tc,sat=65'C. This calculation also allows for efficiency variation (small).
The difference now lies in the pressure ratio moving from 4.56 to 5.99 - with rpm@60Hz = 3500 & rpm@50Hz = 2900. So the compressor runs slower, but pushes to higher pressure.
I will run thermodynamic simulations on these new assumptions to better characterise the Tcomp,disch for the 60Hz & 50Hz conditions.
What would the effect on oil distribution & wear be at the 50Hz revised operating conditions?
Now, at least, the difference in compressor operating window for US & EUR is more clear. Pity it took two technicians to unravel what the best of the so-called best technical experts, for arguably the largest compressor manufacturer in the world, could not work out. Incredible stuff.
(Ironically, I wonder why I'm not really surprised. Saw so much BS in my previous engineering lifetimes, in various scenarios. The suits live well.)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
An EPR valve does not allow the Te,sat to drop below setpoint pressure. The exact opposite of what we are trying to accomplish.
That's what I had thought. Thanks for putting that one to rest.
I personally like the CPR concept. Wish we could find a more cost-effective producer.
The fan-control trim is starting to look better & better. I'm working on a system which we can use to better test the Tcomp,disch versus fan speed concept. :)
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Re: AWHP superheat & sub-cooling
Gary I have to disagree about a CPR not flooding the evap. Reason. CPR is limiting flow to compressor (at set point) Ct at 70C, liquid pressure sligtly less, TXVs do not close 100%. Liquid will continue to enter the evap, pressure in evap will rise along with the amount of liquid, basically flooding the evap, or should i say a wet evap outlet. so no superheat control.
Charge then becomes a limiting factor
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Re: AWHP superheat & sub-cooling
After studing more of this tread, I would suggest that you control your load (fan speed) using a point on the evap (not in the air flow) at temperature close to the limits of the compressor. As this is a slow acting systems a simple PI control loop is required, I would use a discharge temp sensor for saftey control, not for fan control (indirect) For example there is a slight loss of refrigerant, suction superheat will increase, discharge will increase, fan would slow, lowing suction, increasing compression ratio, increasing comp discharge temp and so on.
TXV if you are lucky find one which can give you enough mass flow at lower pressure ratios and can control at high pressure ratios, If you can not, add a second expansion device. TXV to control for high pressure ratios, second to allow additional mass flow. Sol valve can be controlled, simply or with levels of complexity
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
mad fridgie
... Liquid will continue to enter the evap, pressure in evap will rise along with the amount of liquid, basically flooding the evap, or should i say a wet evap outlet. so no superheat control.
Charge then becomes a limiting factor
So far, on the lab rig, the TXV has controlled well over the entire heating range, with no sign of loss of superheat.
This has been one fairly consistent factor & may have something to do with the evaporator design size.
The Te,sat drift occurs over the range of the cycle, in line with the thermodynamic imbalance (my perspective), where evap becomes increasingly more oversized, as load climbs (fixed speed fan).
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
mad fridgie
After studing more of this tread, I would suggest that you control your load (fan speed) using a point on the evap (not in the air flow) at temperature close to the limits of the compressor. As this is a slow acting systems a simple PI control loop is required, I would use a discharge temp sensor for saftey control, not for fan control (indirect)
The problem with trying to control off a small change variable, is that sensor noise will play an increasing part. The 'D' in PID can make life interesting for this scenrio, I'd have thought.
My thoughts are to use an input signal with a large range for control purposes. The smaller signal can be used to trim, or bias the output.
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For example there is a slight loss of refrigerant, suction superheat will increase, discharge will increase, fan would slow, lowing suction, increasing compression ratio, increasing comp discharge temp and so on.
Good thoughts. Thanks for that.
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TXV if you are lucky find one which can give you enough mass flow at lower pressure ratios and can control at high pressure ratios,
All good, so far. No electronics in sight, thankfully. :)
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If you can not, add a second expansion device. TXV to control for high pressure ratios, second to allow additional mass flow. Sol valve can be controlled, simply or with levels of complexity
Thanks for that. The secret seems to be in matching the TXV carefully to compressor capacity, over the range. :D
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Re: AWHP superheat & sub-cooling
Sizing the valve is the secret, In NZ we have a much wider ambient -5 to 35C dry bulb.
In your calcs have you allowed for the influcence of varying humidity levels?
We have found humidity is as important in system design
Control loop PI not PID (ambient and water temperature only change slowly?!)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
mad fridgie
Sizing the valve is the secret, In NZ we have a much wider ambient -5 to 35C dry bulb.
In SE Asia, the dry bulb can vary from say -4'C in winter, in the colder parts, to 42-45'C in the warmer regions - some, even hotter, on the odd occasion. In some parts, it can really get very much colder, in winter, however. Talk about temp spread. :)
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In your calcs have you allowed for the influcence of varying humidity levels?
We have found humidity is as important in system design
Yes, very much so. What humidity levels are you typicaly working at over your way? What influences are you observing on system design?
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Control loop PI not PID (ambient and water temperature only change slowly?!)
Water & air, no problem. The refrigerant time constants are very much shorter, however. ;)
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Re: AWHP superheat & sub-cooling
Being a temperate climate our daily changes are large, trying to design for all conditions is impossible. But primarily we are low ambient so best design evap for low refrigerant pressure drop ensuring highest LMTD, of course can cause problems elswhere, as you have noticed.
Single pass systems do tend to have greater stability (normally a constant Tc) even though i happy to allow for instability on multipass, as averaged COP is greater, and averaged compression ratios are lower
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
mad fridgie
Being a temperate climate our daily changes are large, trying to design for all conditions is impossible. But primarily we are low ambient so best design evap for low refrigerant pressure drop ensuring highest LMTD, of course can cause problems elswhere, as you have noticed.
Fair-enough.
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Single pass systems do tend to have greater stability (normally a constant Tc) even though i happy to allow for instability on multipass, as averaged COP is greater, and averaged compression ratios are lower
Agreed. May I ask what type of stability issues you see on the multipass systems?
I'd imagine things like TXV corrections, refrigerant waves (refrigerant movement from evap to condenser), would be much the same.
In terms of humidity, ever tried this test point:
Air WB in = 34.12'C
Air DB in = 32.22'C
RH% = 89.4%
:eek: :D
How well do your desuperheaters behave?