AWHP superheat & sub-cooling

[Gary]

I'm hoping to have a better idea of just how well these CPR's operate tomorrow. I have a sneaky feeling that this may be a super solution.

The trick then is surely to push the evap a little harder & push up the Te,sat to 15'C & hold it there. Perhaps this could assist in early startup performance?

So, basically - push the evap a little harder at startup, then back off as load climbs.

Anything we do to improve the evap performance will have little effect at higher Ta,in temps as we are limiting the load with our CPR and/or fan speed control.

Where these improvements to evap performance will make a huge difference is at lower Ta,in temps, allowing us to design for lower ambients OR to downsize the evap.

[Gary]

Fan-speed control system - re-visited

So far:
1. Input signal from compressor discharge temp;
2. Fan speed curve set to lower fan speed as heating cycle climbs;
3. Curve for (2) still to be determined;
4. Fan speed modulation mechanism to be discussed.

To address:
a. What should fan do when max Tcomp,d reached? Should it drop to say 50% to limit heat-generation?
b. Can a second input signal be used to trim the fan profile;
b.1 Target Tevap,exit; or
b.2 Pressure P,evap;
c. Could a simple 2-speed motor be used, with pressure switches controlling the speeds? (Much like the option used to stop 1-fan for a 2-fan system)

I think that a few other options have been discussed along the way, & can be added (slip my mind at the moment).

The fan could have simple on/off control and would work reasonably well, but the short cycling may ultimately shorten the life of the fan motor.

Cycling between two speeds would be easier on the motor, but the lower speed must be sufficient to bring down the discharge temp within a reasonable time period.

[Gary]

You mentioned earlier that the fan is small relative to the evap. This being the case, we could get a lot more performance out of the evap at lower ambients by increasing the airflow.

The fan and the evap should be matched to each other. If anything we might want to oversize the fan a little relative to the evap.

[Gary]

The 'boys' got onto the spanners & were trying to get the suction gas temp down to 15'C, instead of Te,sat=15'C. Frantic e-mail shows that Te,sat=-0.5'C was reached & the only reason for stoppage was that the Tcomp,disc rose to around 105'C.

Did the boys notice if the evap superheat went up or down as they closed off the CPR to Te,sat=15C?

Their inability to get the suction line temp down to 15C would seem to indicate that the coil was not flooded.

Also notable is that the Tcomp,disc rose to 105C. The fan control would have brought this down. Yet another confirmation of the advantages of 'belt and braces' controls.

[Gary]

The 'boys' got onto the spanners & were trying to get the suction gas temp down to 15'C, instead of Te,sat=15'C. Frantic e-mail shows that Te,sat=-0.5'C was reached & the only reason for stoppage was that the Tcomp,disc rose to around 105'C.

For clarity, it should be noted that they are not adjusting the Te,sat to 15C, but rather they are adjusting the CPR to allow no more than 57psi at the compressor inlet.

In order to make this adjustment, the pressure in the evap must be more than 57psi at the time of adjustment.

[desA]

Anything we do to improve the evap performance will have little effect at higher Ta,in temps as we are limiting the load with our CPR and/or fan speed control.

Yes - very true.

Where these improvements to evap performance will make a huge difference is at lower Ta,in temps, allowing us to design for lower ambients OR to downsize the evap.

I would most definitely like to explore both options:
1. Designing for lower ambients (where required);
2. Downsizing the evap.

Option (1) would be useful for machines going to colder climates - I need to get a good handle on this aspect.

Option (2) would be useful under local conditions, for ultra-compact machines. The single-most determining factor in AWHP space package is the evaporator !!!size!!!. Face area is a major issue, as coil efficiency gains drop off with increasing core depth/thickness/rows.

[desA]

The fan could have simple on/off control and would work reasonably well, but the short cycling may ultimately shorten the life of the fan motor.

I agree. There will be a certain recovery time for the fan motor, I'd imagine - to prevent gradual heat build-up at each re-start. I'd have to look into the minimum recovery time on this. Very interesting point.

Cycling between two speeds would be easier on the motor, but the lower speed must be sufficient to bring down the discharge temp within a reasonable time period.

Now this is definitely something I can test on the current rig.

A proposal:
1. Run the system up to Te,sat=70-75'C, on full fan speed;
2. Cut fan speed to proposed 'hot run' condition via dimmer switch;
3. Observe & time Tcomp,d response.
4. Return fan to 100% condition - let system stabilise;
5. Cut fan speed more aggressively;
6. Observe & time Tcomp,d response.

This will give us some very useful information.

[desA]

You mentioned earlier that the fan is small relative to the evap. This being the case, we could get a lot more performance out of the evap at lower ambients by increasing the airflow.

On the test rig, the fan is overly-large for the evap design.
The design velocity is 2.03 m/S, whereas the current fan causes a face entry velocity in the region of 3.3-3.6 m/s - i.e. is over-sized - fixed speed fan, with dimmer now attached.

The fan and the evap should be matched to each other. If anything we might want to oversize the fan a little relative to the evap.

Already in place on the lab test machine.

[desA]

Originally Posted by desA
The 'boys' got onto the spanners & were trying to get the suction gas temp down to 15'C, instead of Te,sat=15'C. Frantic e-mail shows that Te,sat=-0.5'C was reached & the only reason for stoppage was that the Tcomp,disc rose to around 105'C.

Gary:
Did the boys notice if the evap superheat went up or down as they closed off the CPR to Te,sat=15C?

According to the info received:
Tevap, out : CPR open @ 22'C : CPR closed @ 22'C
Tbefore,CPR : CPR open @ 20'C : CPR closed @ 19'C
Tafter,CPR : CPR open @ 19.5'C : CPR closed @ 17.7'C

The difference between Tevap,out & Tbefore,CPR is attributed to heat losses from the suction line off evap exit. Insulation still to be wrapped after final proving runs.

The difference between Tbefore,CPR & Tafter,CPR is predicted to occur - the temperature should drop in the order of around 2-3K. This can be seen if a calculation is performed across an adiabatic throttle valve, where a pressure drop occurs. The gas will cool slightly, not heat up.

I wanted to wait until the tests had run before discussing the temp drop across the CPR - I'd simulated this once, & used hand-calcs to prove the same.

Gary:
Their inability to get the suction line temp down to 15C would seem to indicate that the coil was not flooded.

Can you explain this a little more, please?

Also notable is that the Tcomp,disc rose to 105C. The fan control would have brought this down. Yet another confirmation of the advantages of 'belt and braces' controls.

Agreed. 'Belt & braces', or 'grab & tickle'?

[desA]

Originally Posted by desA
The 'boys' got onto the spanners & were trying to get the suction gas temp down to 15'C, instead of Te,sat=15'C. Frantic e-mail shows that Te,sat=-0.5'C was reached & the only reason for stoppage was that the Tcomp,disc rose to around 105'C.

desA:
For clarity, it should be noted that they are not adjusting the Te,sat to 15C, but rather they are adjusting the CPR to allow no more than 57psi at the compressor inlet.

In order to make this adjustment, the pressure in the evap must be more than 57psi at the time of adjustment.

You are correct. It's an odd thing running a test by remote across cultural & language barriers - concepts get twisted in the translation. We could have settled this in 30 minutes tops, if I'd been on site during the test. Anyway, the boys can now learn something - it's good for the young engineers in that group -they're smart & have enquiring minds.

What actually happened is that with the CPR in its original factory setting, it was already affecting the evap Te,sat drift, in that it was settling at around 11.25'C & so actually needed to be backed off.

What had been communicated to me was a Te,sat=19'C & so I instructed to drop the CPR setpoint down to a lower value. Actually, the incorrect information had been provided, as Te,sat=11.25'C in the evap, with no upward drift.

So, the correct remedy, is to back it off slightly, wait for the Te,sat drift to set in & pull Te,sat above 15'C (look at LP pressure gauge). then gradually adjust setting in 1/4 turn intervals, 10 minute settling time, until Te,sat=15'C (0.387MPa(g)) is held.

[desA]

For clarity, it should be noted that they are not adjusting the Te,sat to 15C, but rather they are adjusting the CPR to allow no more than 57psi at the compressor inlet.

In order to make this adjustment, the pressure in the evap must be more than 57psi at the time of adjustment.

We have a pressure service port between CPR & compressor suction. I've instructed to watch the LP gauge & set the CPR to begin constricting at Te,sat=15'C (0.387 MPag = 56.1 psig).

Had the boys on the phone this morning. A new paradigm of distance learning has dawned.

See me getting on a plane & flying down there to get this settled. Seems such a waste of money & time to do a 30 minute job...

[Gary]

Now this is definitely something I can test on the current rig.

A proposal:
1. Run the system up to Te,sat=70-75'C, on full fan speed;
2. Cut fan speed to proposed 'hot run' condition via dimmer switch;
3. Observe & time Tcomp,d response.
4. Return fan to 100% condition - let system stabilise;
5. Cut fan speed more aggressively;
6. Observe & time Tcomp,d response.

This will give us some very useful information.

I would like to see a full set of measurements at each hot run condition.

You might base the hot run settings on progressively lower increments of face velocity.

[mad fridgie]

Have you informed you guys to set the CPR to the new port pressure (I thought most CPRs already have a port on the outlet) The pressure that is entering the compressor, They are not trying to control the pressure in the evap with valve? Could Te before the valve and Te after the valve be causing a misunderstanding

[Gary]

The larger coil I have will just need to be checked for optimum water flow circuiting as I seem to remember that all passes are in parallel. I'll check the 3x parallel refrigerant, 3x series water side as an option to 3x parallel refrigerant, 3x parallel water side.

One concern with the series water arrangement may be a slight condensing imbalance between the 3x parallel refrigerant coils... not overly sure about the inter-coil dynamics, for the moment.

An advantaqe to parallel circuits is less pressure drop.

[desA]

^^No port on a CPR, but have a service port between CPR & compressor.

They have been instructed - simple version - to run up the unit, back off CPR until the desired pressure set point of 0.387 MPag has been achieved.

The thing with these tweeks is that the original setting of the CPR, as received, held the evap Te,sat lower than the set-point limit for the CPR. In other words, the CPR was already operating in its P-band range. The system was overly-conservative.

To allow the CPR to be set properly, the setting needs to be backed off, to allow the natural Te,sat drift upwards (the test bay is > 32'C). When the Te,sat hits 15'C (per LP service gauge), then !!!slowly!!! lower CPR set-point.

This thing is a beauty & only needs a tickle (feminine tendencies) to get to where we need to go.

My problem here is that I'm currently around 700km from my build site, with a language barrier in between, plus 'engineers' who are in name, but aren't really.

[mad fridgie]

That makes it a little more difficult!!!!!

[desA]

An advantaqe to parallel circuits is less pressure drop.

Very true. Thanks for that.

The downside is that the water velocity per tube, will now be reduced due to the third // pass. I'll need to check flow Reynolds number & make sure it does not drop into the laminar, or transition range. This will place a lower operating band constraint on the pumping flow-rate. Not a train-smash, but does need to be checked.

No problem, that's on my checklist for the re-build project.

[desA]

That makes it a little more difficult!!!!!

Where's the hair-pulling emoticon...

[desA]

Originally Posted by desA
Now this is definitely something I can test on the current rig.

A proposal:
1. Run the system up to Te,sat=70-75'C, on full fan speed;
2. Cut fan speed to proposed 'hot run' condition via dimmer switch;
3. Observe & time Tcomp,d response.
4. Return fan to 100% condition - let system stabilise;
5. Cut fan speed more aggressively;
6. Observe & time Tcomp,d response.

Gary:
This will give us some very useful information.
I would like to see a full set of measurements at each hot run condition.

You might base the hot run settings on progressively lower increments of face velocity.

It shall be done - excellent suggestion.

(Sidebar: I'll need to set up my video cam to record the frantic dances performed during such tests. I currently do everything manually, swapping instruments in & out. It would look hilarious on a video... )

[Gary]

I'm trying to sort out the water flow through the condenser in this picture:

http://i29.tinypic.com/2604pee.jpg

In each coil, the water should flow into the bottom, spiral upwards, and then flow out the top. The entrance manifold should have the water entering the bottom and the exit manifold should have the water exiting the top.

Is it just me, or is this piping all wrong?

Does anyone else see a problem with this?

[desA]

I'll pull out my spare unit of that type, to get a better look at it.

The way I seem to remember it is configured with inlet & exit manifolds on both ends, but that the connections were placed on the top ends of the manifolds.

I'll go out to the lab & take some pics of the bare coil & an installed unit.

To be honest, I don't particularly like these condensers - too bulky, heavy, pretty inefficient, but, they are able to use a good level of condensate sub-cooling, leading to reasonable COP's.

[Gary]

The way I seem to remember it is configured with inlet & exit manifolds on both ends, but that the connections were placed on the top ends of the manifolds.

Draw a simple ladder diagram, consisting of two parallel vertical lines with three horizontal lines between them. In other words, a ladder with three rungs.

Measure the distances from the top of one vertical line, through each of the horizontal lines, to the top of the other vertical line. The path through each horizontal will give you a different length, a different resistance, a different flow.

Now measure the distances from the bottom of one vertical to the top of the other vertical. No matter which path through the horizontals is taken, there is equal distance, equal resistance, equal flow.

[desA]

There it is.

Can I ask you to explain your 'ladder concept' a little more? I actually have manufacturer's sketches for these, with dimensions - if these will help.

[mad fridgie]

looks right counter flow, also looks like a double barrier, (if so I would reley on trail and error, therory does not seem to work)

[Gary]

There it is.

Can I ask you to explain your 'ladder concept' a little more? I actually have manufacturer's sketches for these, with dimensions - if these will help.

The shortest distance between water in and water out would seem to be through the top coil. Seems like there would be far less water flow through the other two coils.

It looks wrong on the refrigerant side, too.

Maybe those sketches would help.

[desA]

looks right counter flow, also looks like a double barrier, (if so I would reley on trail and error, therory does not seem to work)

Theory always works... if you use the right theory, that is.

I had the turbulation part right - missed the air-gap effect due to the double wall. I'll add that in & re-check.

My estimate was ~ 7 kW max. Practically, the unit operated at 6.7kW... (Heat-exchanger design was a past life...

[desA]

The shortest distance between water in and water out would seem to be through the top coil. Seems like there would be far less water flow through the other two coils.
It looks wrong on the refrigerant side, too.
Maybe those sketches would help.

I'll sketch those out in the morning.

Manifolds are funny things - some combinations work better than others.

I feel that the refrigerant should be flowing downwards after exit, with the water flowing upwards.

[desA]

Had an interesting day, all told.

Made a minor modification (not fan) to the current test rig. The thing now runs as sweet as a whistle, making hot water at Tw,o=70.4'C.

Tsat,equivalant to compressor suction was contained at under 14'C, Tcomp,d was allowed to max out at 107'C, Tc,sat~74'C.

The evaporator Te,sat was ~ 17.5'C.

In my view, refrigerant circuits are inherently unstable beasts & really do need to be taken in hand for heat-pump duty. Today's exercise merely reinforced that.

I'll see if the mod still holds during the day tomorrow, where the temps are sure to be hotter.

[Gary]

I'll sketch those out in the morning.

Manifolds are funny things - some combinations work better than others.

I feel that the refrigerant should be flowing downwards after exit, with the water flowing upwards.

It seems unlikely that a condenser manufacturer would make such a basic mistake in design. It is entirely possible that the water input/refrigerant output may be shifted within the header manifolds where we can't see it.

[Gary]

Had an interesting day, all told.

Made a minor modification (not fan) to the current test rig. The thing now runs as sweet as a whistle, making hot water at Tw,o=70.4'C.

Tsat,equivalant to compressor suction was contained at under 14'C, Tcomp,d was allowed to max out at 107'C, Tc,sat~74'C.

The evaporator Te,sat was ~ 17.5'C.

In my view, refrigerant circuits are inherently unstable beasts & really do need to be taken in hand for heat-pump duty. Today's exercise merely reinforced that.

I'll see if the mod still holds during the day tomorrow, where the temps are sure to be hotter.

Hmmm... apparently you have restricted the suction line to simulate the CPR, but I wouldn't think that would account for the major difference in condenser approach.

I'll take a wild guess and say you reversed the water flow through the condenser?

[Gary]

In my view, refrigerant circuits are inherently unstable beasts & really do need to be taken in hand for heat-pump duty.

... or any other kind of duty. Refrigeration is a game of "What's wrong with this picture?"... and the smallest details can bite us in the butt.

[mad fridgie]

Theory always works... if you use the right theory, that is.

I had the turbulation part right - missed the air-gap effect due to the double wall. I'll add that in & re-check.

My estimate was ~ 7 kW max. Practically, the unit operated at 6.7kW... (Heat-exchanger design was a past life...

The air gap is where the problems lies, my testing has indicated that it is not a constant within the heat exchanger!
As you have noticed in refrigeration you are always in state of flux, this is flux level in massive in hot water heat pumps, trying to balance a small system within a budget is the difficult part. I have been working on this for about 6 years.
As Gary has mentioned there is a propblem with the heat exchanger, the header configuration could be better. (unless there is internal piping within the header)

[mad fridgie]

Had an interesting day, all told.

Made a minor modification (not fan) to the current test rig. The thing now runs as sweet as a whistle, making hot water at Tw,o=70.4'C.

Tsat,equivalant to compressor suction was contained at under 14'C, Tcomp,d was allowed to max out at 107'C, Tc,sat~74'C.

The evaporator Te,sat was ~ 17.5'C.

In my view, refrigerant circuits are inherently unstable beasts & really do need to be taken in hand for heat-pump duty. Today's exercise merely reinforced that.

I'll see if the mod still holds during the day tomorrow, where the temps are sure to be hotter.

What was your water inlet temp at these conditions

[desA]

It seems unlikely that a condenser manufacturer would make such a basic mistake in design. It is entirely possible that the water input/refrigerant output may be shifted within the header manifolds where we can't see it.

In this case, I wouldn't put too much thought into internal tricks in the coil design - it is as you see it. The manufacturer is a NZ outfit - mad_fridgie will know who they are.

I just think that it is poor design practice. If I were to tell you some of the stuff I've come across in the HE game, & what designers get up to, your hair would stand on end.

[desA]

Hmmm... apparently you have restricted the suction line to simulate the CPR, but I wouldn't think that would account for the major difference in condenser approach.

I installed a service valve between the evaporator & compressor, in a position to self-clear any evap droplet carryover that could possible occur. The evap pressure was then sensed off the inlet to this valve & SH adjusted to suit evap conditions.

A matching gauge set was used to measure near suction into compressor.

The trick in the game is to balance SH to around 7K, in order to limit Tcomp,disc from running up, & to correctly throttle on the service valve.

Tuning was set for hot condition.

I'll take a wild guess and say you reversed the water flow through the condenser?

Never thought to try this, as 'theoretically', it shouldn't matter - for the condenser section, but will reduce de-sup & subcool performances. But, I'm happy to give it a go...

[mad fridgie]

In this case, I wouldn't put too much thought into internal tricks in the coil design - it is as you see it. The manufacturer is a NZ outfit - mad_fridgie will know who they are.

I just think that it is poor design practice. If I were to tell you some of the stuff I've come across in the HE game, & what designers get up to, your hair would stand on end.

I have used plenty, build quality is good, but in my paticular case their design figures are well out.
The internal is piping is roped

[desA]

Originally Posted by desA
In my view, refrigerant circuits are inherently unstable beasts & really do need to be taken in hand for heat-pump duty.

Gary:
... or any other kind of duty. Refrigeration is a game of "What's wrong with this picture?"... and the smallest details can bite us in the butt.

Agreed. This is very much the case here.

I went through the circuit dynamics over the past few weeks trying to find out what was controlling the refrigerant flow. In the end, you have a TXV/compressor balance, with characteristics that run in opposite directions. They cross at a nominal design point & all seems well to the uninitiated.

Begin running the machine & then the complete story opens up.

After the nominal point, the TXV governs play & tries to push ever larger mass flows, To meet this, the compressor must oblige up to higher Te,sat & the evaporator must then follow suit. The system intersection then keeps pushing itself until Te,sat goes beyond the compressor envelope. There are no 'brakes' in such a scheme. This is inherently unsafe.

The 'brake' is the CPR (wide-range, but expensive), or a similar throttle valve (set to hot point service).

Every process has to be controlled.

The fan concept can then be used to trim off the evaporator a little, if required - i.e. if it struggles to maintain superheat towards end of cycle, especially with a tight band of SH control to limit Tcomp,disc to a reasonable number.

The rules of engagement are now very clear.

[desA]

The air gap is where the problems lies, my testing has indicated that it is not a constant within the heat exchanger!
As you have noticed in refrigeration you are always in state of flux, this is flux level in massive in hot water heat pumps, trying to balance a small system within a budget is the difficult part. I have been working on this for about 6 years.

Had any migraines yet... I have...

As Gary has mentioned there is a propblem with the heat exchanger, the header configuration could be better. (unless there is internal piping within the header)

There is no internal piping... WYSIWYG

[Gary]

... your hair would stand on end.

That would make me about 8 ft tall.

[Gary]

I installed a service valve between the evaporator & compressor, in a position to self-clear any evap droplet carryover that could possible occur. The evap pressure was then sensed off the inlet to this valve & SH adjusted to suit evap conditions.

A matching gauge set was used to measure near suction into compressor.

The trick in the game is to balance SH to around 7K, in order to limit Tcomp,disc from running up, & to correctly throttle on the service valve.

Tuning was set for hot condition.

I would very much like to see the full set of numbers on this.

Never thought to try this, as 'theoretically', it shouldn't matter - for the condenser section, but will reduce de-sup & subcool performances. But, I'm happy to give it a go...

Given the latest condenser approach of less than 4C, I would leave it as is.

[desA]

I have used plenty, build quality is good, but in my paticular case their design figures are well out.
The internal is piping is roped

Yes, it is what we call a 'corrugated spiral' formation - they call it Spirex. It typically increases tubeside performance by around 1.31. They have been around for ages e.g. Turbotec comes to mind.

Their design figures are well out because they quote them for R22 service. You need to re-model for alternative refrigerants. It's pretty straightforward.

The tube airgap effect will need to be better accounted for.

[desA]

Originally Posted by desA
... your hair would stand on end.

Gary:
That would make me about 8 ft tall.

Hahaha...

[desA]

[Quote]

Originally Posted by desA
I installed a service valve between the evaporator & compressor, in a position to self-clear any evap droplet carryover that could possible occur. The evap pressure was then sensed off the inlet to this valve & SH adjusted to suit evap conditions.

A matching gauge set was used to measure near suction into compressor.

The trick in the game is to balance SH to around 7K, in order to limit Tcomp,disc from running up, & to correctly throttle on the service valve.

Tuning was set for hot condition.]/quote]

Gary:
I would very much like to see the full set of numbers on this.

I'll do a few preliminary runs today (spent 8 hours yesterday - fine-tuning), but will have to leave the main runs until later in the week, as I have to go upcountry for a few days. I should be back in the saddle again on Thursday.

If this scheme turns out to be promising, then I'd like to investigate carefully the evaporator side of the equation & it's ability to maintain a reliable superheat across the range. So far, the SH looked to be reasonably stable, but I would like to ensure that the evap does not go wet - especially with controlling SH finely, to manage Tcomp,d to not run out of bounds.

My thought for the system was then to have a simple temp cut-out, or sensor to activate something - or cut compressor, when Tcomp,d reaches 107'C (max safe continuous running condition - per compressor manufacturer). Perhaps we could use this more elegantly?

A note on Tcomp,d control of fan:
On a circulating water loop, where Tw actually holds Tc,sat up, trying to tweek fan speeds & so forth off Tcomp,d helps nothing - in terms of cutting the process, that is. If the fan speed is dropped, Te,sat reduces, raising the instantaneous pressure ratio & driving Tcomp,d up further for a while, until the heat-pump performance degrades enough to begin to steady out. This is a waste of energy - better to switch the compressor off, in my view.

We could however, use Tcomp,d to control Te,at in the evap during the run (well before 107'C limit) & fine-tune it to best match the now-governing CPR/valve throttle conditions.

Actually, I'm beginning to think that CPR/valve could be replaced by the word 'orifice'... Mmmhhh...

Oh, before I forget - after (purge/vacx3) I set the mass charge to my theoretically calculated value, based on start-up condition (ie. heavy load). Seems fine.

[mad fridgie]

Hi DesA, for what purpose are you testing, are you looking at mass production yourself, are you working on behalf of another, or is this just a bit of fun

[desA]

^ Mass production - optimised for SE Asian conditions - self financed - builds by one of the region's largest OEM manufacturers - extremely cost-competitive.

Suitable collaborators of like mind, are most welcome.

My lab is my proving & debugging playground. CO2 is in the not-too-distant future...

[Gary]

I went through the circuit dynamics over the past few weeks trying to find out what was controlling the refrigerant flow. In the end, you have a TXV/compressor balance, with characteristics that run in opposite directions. They cross at a nominal design point & all seems well to the uninitiated.

Begin running the machine & then the complete story opens up.

After the nominal point, the TXV governs play & tries to push ever larger mass flows, To meet this, the compressor must oblige up to higher Te,sat & the evaporator must then follow suit. The system intersection then keeps pushing itself until Te,sat goes beyond the compressor envelope. There are no 'brakes' in such a scheme. This is inherently unsafe.

The 'brake' is the CPR (wide-range, but expensive), or a similar throttle valve (set to hot point service).

Every process has to be controlled.

The fan concept can then be used to trim off the evaporator a little, if required - i.e. if it struggles to maintain superheat towards end of cycle, especially with a tight band of SH control to limit Tcomp,disc to a reasonable number.

The rules of engagement are now very clear.

What I find extremely puzzling is how any of this could drop the condenser approach down below 4K. What changed in the condenser?

[desA]

What I find extremely puzzling is how any of this could drop the condenser approach down below 4K. What changed in the condenser?

I'll go through the detailed runs & feed this back.

One thing is that now, the system charge is bang on the calculated value.

What is new is that the evap pressure sensor (new addition) is now directly on the service valve - whereas the previous service port for LP was near the compressor. This difference then allowed me to fine tune the SH more closely to the evap itself (reset TXV adjuster progressively. The adjuster is now only 1/4 turn off factory setting).

The pressure sensor at the suction service port is the same family of pressure gauge as the new on attached to the service valve port - so I expect similar experimental uncertainties.

From initial readings, the condenser SC was in the region of around 4-5K, which is lower than before. It could be, that, once the results come out, that we may opt to add a little more refrigerant charge to meet SC, if required.

[Gary]

Judging from the three tiered condenser in the picture, I now tend to question the two tiered model you are currently using.

Can you post a picture of the current condenser?

[Gary]

^ Mass production - optimised for SE Asian conditions - self financed - builds by one of the region's largest OEM manufacturers - extremely cost-competitive.

Suitable collaborators of like mind, are most welcome.

My lab is my proving & debugging playground. CO2 is in the not-too-distant future...

Do I get a free sample when we're done?

[desA]

^ It's covered in layers of lagging... sticky stuff was all I could lay my hands on, up here.

It is exactly the same construction as the 3x above, except with only 2 layers of coils. The interconnects, manifolds & so forth are precisely the same.