AWHP superheat & sub-cooling

[Gary]

Eventually we will need a temp control (preferably two-stage) for the fan, the center of its range being in the neighborhood of 100C.

And a multi-speed fan motor for it to control. Its physical size, shaft size, mounting and electrical characteristics should be identical to your present fan motor. Its top speed amperage should be at least equal to the present motor.

[desA]

Eventually we will need a temp control (preferably two-stage) for the fan, the center of its range being in the neighborhood of 100C.

Ok. I'll chat to my suppliers in BKK & source a suitable temp controller. I do have two spare temp controllers - although not two-stage. I'll go through the manuals to see if these could be used, alternatively look for a better option.

And a multi-speed fan motor for it to control. Its physical size, shaft size, mounting and electrical characteristics should be identical to your present fan motor. Its top speed amperage should be at least equal to the present motor.

Have you used phase-control devices for conventional single-phase motor control? These can be used to alter the fan motor speed & seem to be pretty evenly priced. I'd have to order in from the US for these.

[desA]

One of our initial assumptions was that we would not be able to exceed Te,sat=15C, but in fact we have gone considerably beyond this. I'm wondering if there is some max that we need to stop at. Possibly a max amperage?... or a max V*I? Any idea what point the internal overload trips at?

Very good point. This lower boundary of the compressor has been worrying me. I has expected that, Te,sat would have reduced a little, with the fine-tuning & do expect this, with a fan speed control strategy.

You're right that we need to check this. Give me a few hours to check my information & report back.

[Gary]

We have yet to see a load that the compressor cannot handle, but I assume this will eventually happen. Ideally we could hit the limit with Ta,in=25C, then find a much lower fan speed that would hit that limit at Ta,in=35C. We will need to do a lot of experimenting to find the ideal control points and fan speeds. A phase control device providing variable fan speed might make this process easier... and we could probably switch it through a single stage controller.

[Gary]

Very good point. This lower boundary of the compressor has been worrying me. I has expected that, Te,sat would have reduced a little, with the fine-tuning & do expect this, with a fan speed control strategy.

The finer we tune this and the higher the Ta,in goes, the higher that Te,sat is going to go... and that's a very good thing... until we overload the compressor... which is a very bad thing.

We want the compressor to work its butt off... but we don't want to give it a hernia.

Just short of overload is where we need to bring in the fan speed control... but we need to define "overload".

[desA]

We have yet to see a load that the compressor cannot handle, but I assume this will eventually happen. Ideally we could hit the limit with Ta,in=25C, then find a much lower fan speed that would hit that limit at Ta,in=35C. We will need to do a lot of experimenting to find the ideal control points and fan speeds. A phase control device providing variable fan speed might make this process easier... and we could probably switch it through a single stage controller.

Good. That would suit me just fine.

Let me research it for a few days & chat to the suppliers. If it makes sense, on the back of the single stage temp controller, as the switch, then I'll get the parts ordered in. We can discuss more along the way as the info comes in.

[desA]

Originally Posted by desA
Very good point. This lower boundary of the compressor has been worrying me. I has expected that, Te,sat would have reduced a little, with the fine-tuning & do expect this, with a fan speed control strategy.

Gary:
The finer we tune this and the higher the Ta,in goes, the higher that Te,sat is going to go... and that's a very good thing... until we overload the compressor... which is a very bad thing.

We want the compressor to work its butt off... but we don't want to give it a hernia.

Just short of overload is where we need to bring in the fan speed control.

How will we know when the critical point has been reached for the compressor? Are there clear warning signs - other than black smoke?

[Gary]

How will we know when the critical point has been reached for the compressor? Are there clear warning signs - other than black smoke?

I'm hoping the compressor manufacturer has a clear answer to that question.

[desA]

I've been researching the compressor manufacturer's technical documentation & application guidelines with regard to the Te,sat>15'C limit.

Nothing is mentioned at all regarding V*I limits in regards to the heat-pump application.

The most applicable document covers considerations for high ambient conditions, & then it concerns itself more with high condensing temps (a given for an AWHP), & issues for high Tc,sat coupled with low Te,sat, leading to high compression ratios.

The two main criteria they focus on are:
1. Compressor discharge temp - measured 150mm from discharge port:
1.1 Extreme temp condition : Tcomp,disch' = 135'C (275'F);
1.2 Danger level : Tcomp,disch' = 121.11'C (250'F);
1.3 Reasonable life expectancy : Tcomp,disch' < 107.2'C (225'F) <=== fair

2. Oil sump temp:
2.1 Conservative : Tcomp,sump < 93.3'C (200'F); <=== reasonable
2.2 Acceptable (welded comp) : Tcomp,sump < 115.6'C (240'F).

Remember, this type of scroll compressor uses the incoming suction vapour to cool the motor windings.

I would suggest that as long as we control the evap SH to be reasonable, & control the discharge temp & sump oil temp, then we should be safe to exceed the envelope right boundary a small margin.

How small is safe? The compressor manufacturer's idea is for the OEM to test the arrangement under the more strenuous conditions & satisfy themselves & the supplier, if needs be, that the duty is safe for the compressor.

[Gary]

I've been researching the compressor manufacturer's technical documentation & application guidelines with regard to the Te,sat>15'C limit.

Nothing is mentioned at all regarding V*I limits in regards to the heat-pump application.

The most applicable document covers considerations for high ambient conditions, & then it concerns itself more with high condensing temps (a given for an AWHP), & issues for high Tc,sat coupled with low Te,sat, leading to high compression ratios.

The two main criteria they focus on are:
1. Compressor discharge temp - measured 150mm from discharge port:
1.1 Extreme temp condition : Tcomp,disch' = 135'C (275'F);
1.2 Danger level : Tcomp,disch' = 121.11'C (250'F);
1.3 Reasonable life expectancy : Tcomp,disch' < 107.2'C (225'F) <=== fair

2. Oil sump temp:
2.1 Conservative : Tcomp,sump < 93.3'C (200'F); <=== reasonable
2.2 Acceptable (welded comp) : Tcomp,sump < 115.6'C (240'F).

Remember, this type of scroll compressor uses the incoming suction vapour to cool the motor windings.

I would suggest that as long as we control the evap SH to be reasonable, & control the discharge temp & sump oil temp, then we should be safe to exceed the envelope right boundary a small margin.

How small is safe? The compressor manufacturer's idea is for the OEM to test the arrangement under the more strenuous conditions & satisfy themselves & the supplier, if needs be, that the duty is safe for the compressor.

We have kept the compressor well within these boundaries so far, and by insulating the condenser and partitioning the compressor into the evap compartment, we should keep the compressor even cooler.

All of the criteria mentioned (compression ratio, discharge temp, oil sump temp) plus the V*I will show itself as high discharge temp, as the heat from the motor transfers to the refrigerant being pumped through.

So... discharge temp seems to be the best control criteria for our fan speed control and we need to keep it below the 107C limit.

I'm thinking that as long as we do not exceed 107C discharge, we can continue to load up the compressor. On the other hand, if the compressor kicks out on internal overload, then we have gone too far and need to adjust our fan speed control to reduce the load and drop the discharge temp.

[Gary]

So far this compressor has shown no signs of weakness. If compressors could speak I think it would be taunting us, saying things like,

"Is that all you got, Sissy?"
"You hit like a girl."
"Where's that 35C air?... Bring it on."

We need to teach this compressor a lesson; Make an example of it. Kick its T,comp,b. It doesn't know who it's messin' with... lol

[desA]

We have kept the compressor well within these boundaries so far, and by insulating the condenser and partitioning the compressor into the evap compartment, we should keep the compressor even cooler.

True. I began stripping the compartment wall this afternoon - will cut out a section & re-work upon my return in approx 1 week's time.

All of the criteria mentioned (compression ratio, discharge temp, oil sump temp) plus the V*I will show itself as high discharge temp, as the heat from the motor transfers to the refrigerant being pumped through.

This is certainly what the literature seems to be showing. It makes good sense.

So... discharge temp seems to be the best control criteria for our fan speed control and we need to keep it below the 107C limit.

Agreed. This is an elegant way to control the system - simple & direct.

I'm thinking that as long as we do not exceed 107C discharge, we can continue to load up the compressor. On the other hand, if the compressor kicks out on internal overload, then we have gone too far and need to adjust our fan speed control to reduce the load and drop the discharge temp.

This scroll does have an internal temperature cut-out/cut-in switch which is set to nominal 290 'F (143.3'C) / 140'F (60'C).

The system also has a LP/HP cut-out switch, to limit under & over pressures.

[desA]

So far this compressor has shown no signs of weakness. If compressors could speak I think it would be taunting us, saying things like,

"Is that all you got, Sissy?"
"You hit like a girl."
"Where's that 35C air?... Bring it on."

We need to teach this compressor a lesson; Make an example of it. Kick its T,comp,b. It doesn't know who it's messin' with... lol

It's a tough compressor, that's for absolute sure. If you see what a hammering these things take up in Laos, you'd feel sorry for it.

Let's got for the limits...

[desA]

Water heat-up trial (brief feedback):

Performed a water heat-up trial from 28.7'C - 65.4'C. The duration was timed, & T,tank , Tw,out , current values taken at each time step.

The full analysis will follow (hopefully tomorrow morning).

General points of interest:
1. System began heating straight off the mark - no slouching, or temp delays.
2. It felt 'smooth' all the way through the range.
3. The heat-up performance improved by 10.2% over that for the machine condition before tuning & partial insulation began. !!!

We are now making progress & I'm extremely happy with the result. Pretty incredible... Thanks, Gary, so much for your kind mentoring along this path - it is very much appreciated.

[Gary]

This scroll does have an internal temperature cut-out/cut-in switch which is set to nominal 290 'F (143.3'C) / 140'F (60'C).

Internal temperature runs about 50F/28K higher than discharge temp, so this would translate to about 240F/115C on the discharge line, making 107C an entirely reasonable target temp.

[desA]

Originally Posted by desA
This scroll does have an internal temperature cut-out/cut-in switch which is set to nominal 290 'F (143.3'C) / 140'F (60'C).

Gary:
Internal temperature runs about 50F/28K higher than discharge temp, so this would translate to about 240F/115C on the discharge line, making 107C an entirely reasonable target temp.

Agreed.

107'C seems to be a safe upper working limit, for long-term operation & a reasonable compressor lifespan. We can fix that as our target.

[Gary]

Note on run#4 the drop in voltage caused a rise in discharge temp (Tc,sup). The fan control strategy will reduce the load to help protect the compressor from the region's voltage drop problems as well.

[desA]

Note on run#4 the drop in voltage caused a rise in discharge temp (Tc,sup).

Well spotted. I'd originally attributed the Tcomp,disch rise to the hotter incoming air onto the evap coil, but I see that you are correct.

With voltage drop, comes an associated current climb, until a threshold voltage of around 185V is reached. Below this, the amperage seems to run away & climb rapidly. After this, the compressor seems to cut out, & re-start This could actually be caused by the reduced pump flow forcing up the internal temp & pressure - resulting in a HP trip on the LP/HP trip.

With the associated current climb, on low voltage, comes increased base temp Tcomp,b.

I'd imagine then, that this increased power must be feeding through to the compressor discharge temperature, & so causing it to rise.

Well spotted.

The fan control strategy will reduce the load to help protect the compressor from the region's voltage drop problems as well.

I like this strategy more & more as it develops. This is an elegant way to smoothly manage the compressor.

[Gary]

With voltage drop, comes an associated current climb, until a threshold voltage of around 185V is reached. Below this, the amperage seems to run away & climb rapidly. After this, the compressor seems to cut out, & re-start This could actually be caused by the reduced pump flow forcing up the internal temp & pressure - resulting in a HP trip on the LP/HP trip.

As a precaution and preliminary test of the fan control strategy, you may want to attach a temp controller to the discharge line to shut off the fan at 107C, turning it back on a few degrees lower.

What saturation temp/pressure do you have the HP control set at?

[desA]

As a precaution and preliminary test of the fan control strategy, you may want to attach a temp controller to the discharge line to shut off the fan at 107C, turning it back on a few degrees lower.

That makes sense. I'll be meeting with my controller supplier tomorrow & will be training on the new controllers I'm now using. I'll have a chat to him about using multiple temp inputs & fan control. I'll post something one this once it all makes sense.

What saturation temp/pressure do you have the HP control set at?

The current settings (I'll firm up when I return to my lab) as follows - in accordance with compressor manufacturer's guidelines (slightly conservative settings, below their limit recommendations):
HP = 400 psi(g) = 2.757 MPa(g)
LP ~ 30 psi(g) = 0.207 MPa(g)

[Gary]

HP = 400 psi(g) = 2.757 MPa(g)

My P/T chart doesn't go that high for R134a... lol

[desA]

^ Do you need the Tsat value at the HP?

2.757 MPa(g) = 2.858 MPa(a)
T,sat=83.66'C

[Gary]

Those settings sound just about right.

[desA]

Thanks, Gary.

They are just inside the compressor manufacturer's specification on the HP side, & just sufficient to prevent start-up kick-out on the LP side - well within manufacturer's specification.

[desA]

The compressor safe performance envelope dilemma

I am facing a real dilemma in regards to selection of the maximum safe operating point for an air-to-water heat-pump compressor. This item comes from one of the foremost name-brand compressor manufacturers, with technology centres in both USA & Europe. The USA operation provides a performance table, while the European operation supplies a detailed perfromance-prediction program, which prints out a detailed performance envelope.

To add to the complexity, the current manufacturing arm of the compressor is in SE Asia. On enquiry, they contacted the US technology & were provided information from experimental results. Refrigerant R-134a.

US technology centre
At rating conditions:
11.1K superheat / 8.3K sub-cooling / 35'C ambient air over
-23'C<Te,sat<10'C
30'C<Tc,sat<65'C
Te,sat/Tc,sat roll-off condition at upper left of table (no explanation)

Maximum continuous rating (experimental results via SEA technical expert/USA):
Safe long-term operation : Te,sat=13'C, Tc,sat=65'C
~ 1000h test data : Te,sat=13'C, Tc,sat=67'C
Danger points : Te,sat=19'C, Tc,sat=71'C & Te,sat=15'C, Tc,sat=73'C

European technology centre - manufacturer's computer performance rating software
At rating conditions:
11.1K superheat / 8.3K sub-cooling / ??'C ambient air over (not mentioned)
-20'C<Te,sat<15'C
30'C<Tc,sat<75'C
Te,sat/Tc,sat upper left roll-off conditions at 25'C suction gas temp & 10K suction superheat
Te,sat/Tc,sat lower right roll-up condition at Te,sat=10-15'C / Tc,sat=30-35'C

-------------
The questions -
Who to believe?
What practical experience do others in this field have to shed light on the dilemma?
Is there any experience database in the field that could assist?
For a heat-pump, the operation is not continuous, but periodic over a range of water temp - hi/low temp limits.

[desA]

Who's right?

[Gary]

The compression ratios are well within acceptable limits, thus mechanical failure is not the limit in question.

That brings it down to motor failure limit caused by heat, as reflected by high discharge temp and/or high T,comp,b.

[desA]

The compression ratios are well within acceptable limits, thus mechanical failure is not the limit in question.

That brings it down to motor failure limit caused by heat, as reflected by high discharge temp and/or high T,comp,b.

I agree with this logic, completely.

I have asked the compressor supplier, in question, to rule on the discrepancy in their operational recommendations.

The real issue at hand is what safe operating window to specify for such a heat-pump, in order to prevent excessive compressor failures in the field, over the long term. It appears that these heat-pumps operate in a very marginal application range, where the manufacturer has little, or no real solid experimental evidence. The US technical operation seems to be taking an ultra-conservative route, possibly to avoid large future warranty claims.

How, as a heat-pump developer, to react wisely, & conservatively in such a situation?

My immediate view is to run the sample test machine up to the European operating envelope boundaries, then manage Tcomp,disch & Tcomp,b carefully according to the operational temp guidelines already put forward by the manufacturer. I'll use this to study long-term effects of the operating limit on compressor reliability. We have made huge progress in this thread already, to addressing compressor reliability protection. Let's continue along this path.

As you'd be aware, this kind of technical decision, is not the kind of thing a designer takes lightly (many sleepless nights).

[Gary]

In the production model, you will of course need to work within the manufacturer's limit in order to qualify for their warranty. We can work with this.

The experimental model is a different story. We can drive it right up to the max.

[desA]

In the production model, you will of course need to work within the manufacturer's limit in order to qualify for their warranty. We can work with this.

Now, here's the next part of the equation. Practically, as these are, in the main, export machines, the compressor warranty is not in place outside of the country of manufacture - there's no legal framework to enforce it. As I understand things, there is no 'international warranty' as such. The warranty settlement then rests on me & my main fabricator's shoulders, with referral of the systems to the manufacturer, under failure, to establish root cause. A lot of this settlement is done 'in good faith'.

So, under this scenario, it comes down to an assessment of reasonable financial risk, & good name, in the end.

The experimental model is a different story. We can drive it right up to the max.

Of course. That's what experimental models are there for. This is intended to provide me peace of mind, down the road.

[Gary]

How, as a heat-pump developer, to react wisely, & conservatively in such a situation?

... while at the same time being competitive.

I suppose a key question would be, "What limits do your competitors adhere to?".

[desA]

... while at the same time being competitive.

I suppose a key question would be, "What limits do your competitors adhere to?".

I have seen a number of references from some of the manufacturers, with water discharge temps quoted in the range of 70-75'C, using R-134a.

Now, this would at the very best, require Tc,sat temps in excess of 70-75'C, with minor recovery from the de-superheating of the incoming hot gas stream. This operating point is right up hard against the European compressor performance envelope & exceeds the US advise.

Practically, with air approach temps in the range 35'C, it will be an ongoing struggle to maintain Te,sat < 15'C.

As we've found, the two viable options for maintaining Te,sat<15'C are:
1. Control fan speed;
2. Use an MOP setting for the TXV.

The first option gives lots of spare room to optimise further, the MOP option concerns me for a number of reasons (maintenance issues, performance issues at start-up?, price/availability).

[Gary]

I have seen a number of references from some of the manufacturers, with water discharge temps quoted in the range of 70-75'C, using R-134a.

Now, this would at the very best, require Tc,sat temps in excess of 70-75'C, with minor recovery from the de-superheating of the incoming hot gas stream. This operating point is right up hard against the European compressor performance envelope & exceeds the US advise.

Practically, with air approach temps in the range 35'C, it will be an ongoing struggle to maintain Te,sat < 15'C.

As we've found, the two viable options for maintaining Te,sat<15'C are:
1. Control fan speed;
2. Use an MOP setting for the TXV.

The first option gives lots of spare room to optimise further, the MOP option concerns me for a number of reasons (maintenance issues, performance issues at start-up?, price/availability).

And let's not forget:

3. CPR valve.

Of these three, the MOP would be the least suitable. It reduces the Te,sat and compressor load by reducing refrigerant flow through the evaporator, but this raises the superheat and thus reduces the compressor cooling. The reduced compressor load would probably compensate for the high superheat to control the compressor heat.

The CPR would do the job and in fact would be the simplest. The evaporator pressure increases with the load, but the CPR will allow no more than setpoint pressure to enter the compressor.

The fan control strategy responds directly to the compressor heat and I'm thinking will be the best in safeguarding the compressor.

[desA]

And let's not forget:

3. CPR valve.

Of these three, the MOP would be the least suitable. It reduces the Te,sat and compressor load by reducing refrigerant flow through the evaporator, but this raises the superheat and thus reduces the compressor cooling. The reduced compressor load would probably compensate for the high superheat to control the compressor heat.

This is most definitely not the path I'd like to implement.

The CPR would do the job and in fact would be the simplest. The evaporator pressure increases with the load, but the CPR will allow no more than setpoint pressure to enter the compressor.

This must be causing some other imbalance in the system - especially with a constantly-varying heat-load. I will look into this carefully & try simulating the effect.

The fan control strategy responds directly to the compressor heat and I'm thinking will be the best in safeguarding the compressor.

I love this strategy for a number of reasons. Tuning the fan response - not only stepping it - at a critical value, has definite system balance/tuning opportunities. This is the preferred strategy, in my view.

[desA]

Ok, an update on the evaporator fan-control strategy. Back to the drawing board...

I had the fellow from Carel come over with one of their products - "FCP - Speed regulator with phase cutting control". The input sensor was rated to high temp up to 120'C. They only have an 8A option out here - but, he promised to look into the 4A model (not sure if still available).

Their chief technician spent all day trying to get the thing to reverse control (decrease fan speed) for a tiny fan. In the end, I was so concerned about the reliability of their offering - the first off was obviously not functional - that I called off the exercise. The price of the unit was in the region of USD 175, at wholesale prices. Frightening. This was not a good day for me, & Carel, in general.

Looked at the Alco pressure control condenser fan system - it is the exact reverse of the strategy we need to implement.

I had to spend all day fighting off unworkable solutions - I just want the discharge temp to fan speed control implemented in a simple way - minimum frills.

Now, I need to urgently locate a suitable alternative to the Carel.

[Gary]

For the time being, we can use a simple on/off fan control strategy for testing purposes.

Possibly the most cost effective alternative will end up being a multi-speed fan and multi-stage temp control.

[desA]

What are your thoughts on frequency inverters?

For instance:
SYSDRIVE 3G3JV Frequency Inverter
http://www.omron-ap.co.th/product_info/3G3JV/index.asp

SYSDRIVE 3G3JX / 3G3MX / 3G3RX Frequency Inverter
http://www.omron-ap.co.th/product_info/3G3JX/index.asp

Not sure about their feedback/input requirements.

[Gary]

Sounds expensive. I suppose it all comes down to cost effectiveness.

[Gary]

Given the steps we have taken to keep the compressor cool, it is entirely possible that we will be able to drive this compressor right up to the max without exceeding our 107C limit, in which case our fan control is a high limit safety control and simple on/off will suffice.

Or possibly we may find that 107C is only exceeded at the highest load conditions, in which case we can drop to an intermediate speed at highest load (keeping just under the 107C limit), using the high speed for anything less than highest load. This could be done with a two speed motor. To this we could add a two stage temp control. The first stage would drop to the intermediate speed and the second stage would stop the motor.

On/off, variable voltage, variable frequency. There are a great many options to meet our needs, but first we need to know just what those needs are.

[desA]

Sounds expensive. I suppose it all comes down to cost effectiveness.

Cost & system reliability.

[desA]

Given the steps we have taken to keep the compressor cool, it is entirely possible that we will be able to drive this compressor right up to the max without exceeding our 107C limit, in which case our fan control is a high limit safety control and simple on/off will suffice.

Or possibly we may find that 107C is only exceeded at the highest load conditions, in which case we can drop to an intermediate speed at highest load (keeping just under the 107C limit), using the high speed for anything less than highest load. This could be done with a two speed motor. To this we could add a two stage temp control. The first stage would drop to the intermediate speed and the second stage would stop the motor.

On/off, variable voltage, variable frequency. There are a great many options to meet our needs, but first we need to know just what those needs are.

A proposal - for lab test machine:
1. Use partial blockage of the evap coil face, or
2. Carefully allow air bypass air into the box, to reduce air velocity through evap. Measure evap face velocity, for reference.

The evap face velocity can be adjusted manually, at Tc,sat hold points, until the system stabilises. I'll record the system response & we can then discuss the effects.

[Gary]

A proposal - for lab test machine:
1. Use partial blockage of the evap coil face, or
2. Carefully allow air bypass air into the box, to reduce air velocity through evap. Measure evap face velocity, for reference.

The evap face velocity can be adjusted manually, at Tc,sat hold points, until the system stabilises. I'll record the system response & we can then discuss the effects.

What hypothesis are you testing?

I wonder if a light dimmer switch might do the trick.

[desA]

I wonder if a light dimmer switch might do the trick.

Stunning - perfect. Now why, oh why didn't I think of that... I'll set that up early next week when I get back to the lab.

What hypothesis are you testing?

The hypothesis is very simple.

Most vapour compression cycle prediction, neglects the fact that thermodynamics need infinite time to stabilise to equilibrium. For a heat-pump, the process is dynamic & very far from thermodynamic equilibrium.

The evaporator load begins at maximum requirement at the start of the heating cycle, & ends up being around 70% of Qmax at the end. If the fan speed is not moderated, the evap ends up imbalancing the thermodynamics.

It's that simple, but means a huge, huge amount. This is why the Te,sat rises over the duration of the heating cycle - even though the TXV does its level best to compensate.

[Gary]

The hypothesis is very simple.

Most vapour compression cycle prediction, neglects the fact that thermodynamics need infinite time to stabilise to equilibrium. For a heat-pump, the process is dynamic & very far from thermodynamic equilibrium.

The evaporator load begins at maximum requirement at the start of the heating cycle, & ends up being around 70% of Qmax at the end. If the fan speed is not moderated, the evap ends up imbalancing the thermodynamics.

It's that simple, but means a huge, huge amount. This is why the Te,sat rises over the duration of the heating cycle - even though the TXV does its level best to compensate.

I'm thinking this is caused by the increasing liquid temp at the TXV inlet, but then... that's what testing is for.

[desA]

I'm thinking this is caused by the increasing liquid temp at the TXV inlet, but then... that's what testing is for.

I will go so far as to say that every heat-pump that runs a constant fan speed throughout the range, will see increased Te,sat over the range of the heating cycle.

This can actually be shown theoretically by building an equation using the evaporator heat-transfer equation & air-stream heat-balance equation.

Basically, what happens is that the equation looks something like this:

Te,sat = Ta',in - f(Q'e ; m'a ; Cpa ; UA)

As Q'e required drops off, as the cycle advances, with the other terms constant, the last term reduces. With Ta',in relatively constant, Te,sat is forced to rise...

The effect can be seen almost the instant that airflow is reduced - say through a blank, or fan speed reduction. Te,sat will drop off over the course of less than 1 minute by some 2-3'C - without even trying.

[desA]

I'm thinking this is caused by the increasing liquid temp at the TXV inlet...

An interesting thought. Why would this carry through the TXV itself? Wouldn't the flashing through the TXV tend to remain reasonably consistent, with the bulb monitoring superheat at evap exit?

It is entirely possible, that some temp drift does occur - but I wonder about the physics of this.

I'll set up some thermodynamics ideas on this & see what comes out of the numbers. Anyway, the practical tests beat the theory hands down - every time...

[Gary]

This would be easy enough to test.

Build a cardboard box around the drier.

At the end of the cycle, fill the box with ice.

If I am correct, as the liquid temp drops at the TXV inlet the Ta,out will drop. If the airflow and incoming air temp are held constant and the air out temp drops, then more heat has been extracted from the air.

[desA]

Ok. That's a simple-enough test to perform. I'll add that to my list.

Now, if the rise in incoming refrigerant temp to the TXV is responsible for some of the Te,sat upward drift, then I'd suggest that a possible solution for this would be a liquid line sub-cooler, or the VIC. Mmhhh...

Are liquid-line sub-coolers available - in a simple form? In other words, say a finned pipe, or something like that?

[Gary]

Ok. That's a simple-enough test to perform. I'll add that to my list.

Now, if the rise in incoming refrigerant temp to the TXV is responsible for some of the Te,sat upward drift, then I'd suggest that a possible solution for this would be a liquid line sub-cooler, or the VIC. Mmhhh...

Are liquid-line sub-coolers available - in a simple form? In other words, say a finned pipe, or something like that?

The air coil might do it... but I think the VIC will do it better and a lot more.

[desA]

Obtained 3 off dimmers - max 300W 220V~50Hz. Max fan power on lab machine 210W 220V~50Hz. Should be in good shape.