High discharge temp

[paul_h]

I went to an airwell package a while back that had fried the discharge thermistor.
I fitted a replacement today and test ran it. The discharge temp was 99C, which I thought was high for a maneurop recip mt80hp4a compressor.
But what do I know, I haven't works on a recip or maneurop for ages.
Suction was 6C, LP was 360kPa, HP was 1850kPa, liquid was 39C.
Condenser air (on/off) was 28/38C) Evap air (on/off) was 19.5/11.5C. The compressor cranckcase was cool.
Any ideas for the high discharge?
I tried adding refrigerant even though I could find no leaks, sp went up to 380kPa, st down to 5C, liquid, HP still the same. Discharge temp went up to 103C, but that could have just been because it was running for longer building up the heat.
Incondensibles perhaps? The system has been running for a long time and is a package unit so factory charged though.

[nike123]

You have slightly low suction pressure!
Evaporator TD is at the upper limit and you should check air flow at evaporator.
Suction superheat is not high and that could mean that compressor bearings are worn.
How about voltage and voltage balance at site? Measure that current is not unbalanced more than 5% from mean value.


Sorry, I taught this was at R22 refrigerant?
What refrigerant is used?

[paul_h]

You're right, I forgot to mention it, R22.

[nike123]

Than everything said stays!

[nike123]

Measure that current is not unbalanced more than 5% from mean value.

Sorry, I meant that voltage is not unbalanced more than 2% from mean value.

[stefs_cruiser]

How many circuits in the condensor?
If you have a couple of paralell circuits could it be a partially blocked condensor?

[nike123]

How many circuits in the condensor?
If you have a couple of paralell circuits could it be a partially blocked condensor?

Sorry, but how is that related to high discharge temperature?

[SteinarN]

Maneurop recips is notorious ineficcient causing relatively high discharge temps. In addition many hermetics and certainly maneurop hermetics is 100% suction gas cooled. This also lead to higher discharge gas temps.

Eyeballing a log ph chart gives roughly 80C discharge at your operating conditions and assuming a compressor isentropic efficiency of 100%. Of cource no compressor has an efficiency of 100%. Maneurop recips is probably in the 55-60% range. Assuming 60% efficiency the discharge will increase to roughly 105C.

This number fits exactly with your measured temp. I would say the compressor is good.

All numbers derived by eyeballing a log ph chart.

[paul_h]

Interesting.
Most hermetic piston comps I've worked on in large splits have had 80C normal DT.
But from what you and nike said, the problem seems to be with the compressor, age or design.
An important thing to note is that even though the discharge was high, it wasn't cutting out on safety, so maybe there isn't a problem.
I guess I just have to stock up on thermistors and change them when they inevitably fry because of the high temps.
Most expensive thermistor I've ever bought too, au$95

[SteinarN]

You have roughly 50C condensing. Air on/off is 28/38. This indicates the condenser is either to small or dirty or both.
A good condenser should ideally acheive roughly 38C/1350kPa discharge at 28C air on/ambient. If you had such HP conditions the discharge temp would be reduced to roughly 85-90C.

Edit:
The suction pressure could ideally be slightly higher as Nike sugests, say 430 to 460kPa. Higher suction pressure would also cause a decrease in the disharge temp.

[nike123]

Interesting.
Most hermetic piston comps I've worked on in large splits have had 80C normal DT.
But from what you and nike said, the problem seems to be with the compressor, age or design.
An important thing to note is that even though the discharge was high, it wasn't cutting out on safety, so maybe there isn't a problem.
I guess I just have to stock up on thermistors and change them when they inevitably fry because of the high temps.
Most expensive thermistor I've ever bought too, au$95

Yesterday I was calculated in CoolPack from your first measured data and compressor catalog data and found that isentropic efficiency is 0,6 (60%) and heat loss factor is 20% which gives your first measured value of 97°C as discharge temperature.
As Maneurop is refrigerant cooled, it could be very likely that heat loss factor is actualy 20% and that is nothing wrong vith compresor.
So, you need to work around condenser to lower that condensing pressure.

[philjd26]

my guess is excessive wearing/friction of mechanical components inside machine...increasing dt

[Gary]

The system is grossly overcharged. When the overcharge is removed, the suction pressure will no doubt drop to an unacceptable level, because the system is restricted.

Is this a cap tube or TXV system?

[nike123]

The system is grossly overcharged. When the overcharge is removed, the suction pressure will no doubt drop to an unacceptable level, because the system is restricted.

Is this a cap tube or TXV system?

As we expected, you are right on spot.

The Symptoms Of Refrigerant Overcharge

By John Tomczyk
August 27, 2003



Many times technicians will have the tendency to overcharge refrigeration systems. It is an easy mistake to put too much refrigerant into a system because of the vast array of different systems on the market today. Also, many symptoms for other system problems look similar, but not exactly like an overcharge of refrigerant.The system described here that is overcharged is a low-temperature commercial refrigeration system incorporating a TXV metering device with a receiver. The refrigerant is R-134a.
The following system checklist shows a system with an overcharge of refrigerant (values in degrees F except where otherwise indicated).
Compressor discharge temperature: 240 degrees (115,5°C)
Condenser outlet temperature: 90 degrees (32,2°C)
Evaporator outlet temperature: 15 degrees (-9,4°C)
Compressor in temperature: 25 degrees (-3,9°C)
Ambient temperature: 70 degrees (21,1°C)
Box temperature: 10 degrees ( -12,2°C)
Compressor volts: 230 V
Compressor amps: high
Low-side (evaporating) pressure (psig): 8.8 (5 degrees) (-15°C)
High-side (condensing) pressure (psig): 172 (120 degrees) ( 48,9°C)
Here are the calculated values:
Condenser split: 50 degrees ( 27,8K)
Condenser subcooling: 30 degrees (16,7K)
Evaporator superheat: 10 degrees (5,6K)
Compressor superheat: 20 degrees (11,1K)



Symptoms

Symptoms of this overcharged system include:

High discharge temperature.
High condenser subcooling.
High condensing pressures.
Higher condenser splits.
Normal to high evaporator pressures.
Normal superheat.


High compression ratio.High discharge temperature: With an overcharged system, the high discharge temperature of 240 degrees is caused by the high compression ratio. Liquid backed up in the condenser will flood some of the condensing surface area, causing high head pressures. All of the heat being absorbed in the evaporator and suction line, along with motor heat and high heat of compression from the high compression ratio, has to be rejected into a smaller condenser because of backed-up liquid.
High condenser subcooling: Because of the overcharge of refrigerant in the system, the condenser will have too much liquid backed up at its bottom, causing high subcooling. Remember, any liquid in the condenser lower than the condensing temperature is considered subcooling. You can measure this at the condenser outlet with a thermometer or thermocouple. Subtract the condensing out temperature from the condensing temperature to get the amount of liquid subcooling. A forced-air condenser should have at least 5 degrees of liquid subcooling. However, subcooling amounts do depend on system piping configurations and liquid line static and friction pressure drops. Condenser subcooling is an excellent indicator of the system’s refrigerant charge, but it is not the only one. The lower the refrigerant charge, the lower the subcooling. The higher the charge, the higher the subcooling.
High condensing pressures: Subcooled liquid backed up in the condenser will cause a reduced condensing surface area and raise condensing pressures. Now that the condensing pressures are raised, there is more of a temperature difference between the ambient and condensing temperature, causing greater heat flow to compensate for the reduced condensing surface area. The system will still reject heat, but at higher condensing pressure and temperature.
High condenser splits: Because of the higher condensing pressures — thus higher condensing temperatures — there will be a greater temperature difference (split) between the ambient and condensing temperature.
Normal to high evaporator pressures: Since these systems have TXV metering devices, the TXV will still try to maintain its evaporator superheat, and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. If the overcharge is excessive, the evaporator’s higher pressure would be caused by the decreased mass flow rate through the compressor from the high compression ratios causing low volumetric efficiencies. The evaporator would have a harder time keeping up with the higher heat loads from the warming entering-air temperature. The TXV will have a tendency to overfeed on its opening stroke due to the high head pressures, unless it is a balance port TXV.
Normal evaporator superheats: The TXV will try to maintain superheat even at an excessive overcharge. As mentioned above, the TXV may overfeed slightly during its opening strokes, but then should catch up to itself if still within its operating ranges.



High compression ratios: The condenser flooded with liquid during the overcharge will run high condensing pressures. This causes high compression ratios and causes low volumetric efficiencies causing low refrigerant flow rates.


Overcharged Cap Tube Systems

If we are dealing with a capillary tube metering device, the same symptoms occur with the exception of the evaporator superheat.Remember, capillary tube systems are critically charged to prevent floodback of refrigerant to the compressor during low evaporator loads. The higher head pressures of an overcharged system will have a tendency to overfeed the evaporator, thus decreasing the superheat.
If the system is overcharged more than 10 percent, liquid can enter the suction line and get to the suction valves or crankcase. This will cause compressor damage and eventually failure.





Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at tomczykj@tucker-usa.com.
Publication date: 09/01/2003

In his first measuring, suction superheat is 8,4K. Then it dropped after further overcharge.

By this symptoms and above description, it is capillary tube system.

[Gary]

I met John Tomczyk on a visit to my Alma Mater (Ferris State) some years back and sat in on one of his lectures. To my mind his methods are backwards and unnecessarily complex... but he seems to end up with the same conclusions. There is more than one right way to get there.

[Gary]

You have roughly 50C condensing. Air on/off is 28/38. This indicates the condenser is either to small or dirty or both.
A good condenser should ideally acheive roughly 38C/1350kPa discharge at 28C air on/ambient. If you had such HP conditions the discharge temp would be reduced to roughly 85-90C.

A dirty condenser would give a much higher air off temp given the high condensing temp.

Overcharge backs liquid up into the condenser, reducing its effective area... so in effect this would give the appearance of an undersized condenser.

Edit:
The suction pressure could ideally be slightly higher as Nike sugests, say 430 to 460kPa. Higher suction pressure would also cause a decrease in the disharge temp.

Once the excess refrigerant is removed, the suction pressure will drop and the superheat will increase, clearly indicating a high side restriction (plugged drier, kinked liquid line, restricted cap tube, etc.).

[nike123]

I met John Tomczyk on a visit to my Alma Mater (Ferris State) some years back and sat in on one of his lectures. To my mind his methods are backwards and unnecessarily complex... but he seems to end up with the same conclusions. There is more than one right way to get there.

I already have his book and I think it is very good. Now I need to buy your book to learn other way. Then, I will be able to go in two directions in same time.
When and if you are planing to make SI version of your book, for us on the other side of pond? Or is it already available?

[paul_h]

The system is grossly overcharged. When the overcharge is removed, the suction pressure will no doubt drop to an unacceptable level, because the system is restricted.

Is this a cap tube or TXV system?

It's TXV.
Overcharged you say?
That's a surprise to me because every system I've worked on that is short of refrigerant has a high discharge temp, I haven't come across an overcharged system with high DT before.
Of course this system can't be short of refrigerant because of how it's running, but interesting that you say overcharged and restriction when the liquid temp entering the valve is 39C, it didn't seem like a back up of liquid in the condenser to me.
I'll read what you've written later on (I just walked in from a long day and have stacks of paperwork), because it's the first time I've heard of overcharging making the DT high.

[SteinarN]

If there is an overcharge, it's not in it self responsible for an increased disharge temperature, but if the overcharge is large enough the condensing pressure/temp will increase and that increase will lead to higher discharge temperature.

But that was really not the original question. The question was whether the disharge temperature was reasonable with respect to the compressor operational conditions. The answer to that question is yes as I explained in an earlier reply.

However that said, there can still be a situation with either an overcharge or incondensibles as indicated by the large subcooling.

Paul, you said the liquid temp is/was 39C at the TEV. What is/was the liquid temp at condencer/receiver? If the liquid temp there is/was higher, more like the condensing of 50C, then one can say there is no clear indication so far of either overcharge or incondensibles.

[Gary]

I already have his book and I think it is very good. Now I need to buy your book to learn other way. Then, I will be able to go in two directions in same time.

I should probably explain what I mean by backwards teaching:

In the real world, the system presents you with its symptoms which leads you to the problem... and that's how trouble shooting should be taught. The symptoms leading to the problem.

In the article you quoted, the problem is presented and then the symptoms. That's backwards.

When and if you are planing to make SI version of your book, for us on the other side of pond? Or is it already available?

The books give both F and C temps. Use whatever pressure measurements you prefer and convert them to temps with a P/T chart... because temps are what matters. The books deal with temperatures, not pressures. I sell my books all over the world.

[nike123]

Now, when we knew that we have TXV in game, how is fall of superheat after topping up refrigerant fit in overcharge scenario? Isn't superheat should stay same as before? Or is that only temporary until TXV catch up.

[nike123]

I should probably explain what I mean by backwards teaching:

In the real world, the system presents you with its symptoms which leads you to the problem... and that's how trouble shooting should be taught. The symptoms leading to the problem.

In the article you quoted, the problem is presented and then the symptoms. That's backwards.

Probably because of purpose of article. Article is to explain already known ( supposed) condition and therefore such approach. If condition is unknown, then your approach is more suitable.

The books give both F and C temps. Use whatever pressure measurements you prefer and convert them to temps with a P/T chart... because temps are what matters. The books deal with temperatures, not pressures. I sell my books all over the world.

Good, only what is left now, is to negotiate price.

[Gary]

Now, when we knew that we have TXV in game, how is fall of superheat after topping up refrigerant fit in overcharge scenario? Isn't superheat should stay same as before? Or is that only temporary until TXV catch up.

With all of that high side pressure behind it, each time the TXV opens to let in a little more refrigerant a lot of refrigerant is pushed through. It is probably hunting a little.

[Gary]

I've been assuming this is an A/C system. Is this assumption correct?

[Gary]

It's TXV.
Overcharged you say?
That's a surprise to me because every system I've worked on that is short of refrigerant has a high discharge temp, I haven't come across an overcharged system with high DT before.
Of course this system can't be short of refrigerant because of how it's running, but interesting that you say overcharged and restriction when the liquid temp entering the valve is 39C, it didn't seem like a back up of liquid in the condenser to me.
I'll read what you've written later on (I just walked in from a long day and have stacks of paperwork), because it's the first time I've heard of overcharging making the DT high.

Discharge temp is the result of a great many variables, thus it is not a reliable indicator of anything. I don't even consider it to be a valid symptom and I never measure it when working on a system.

Remove refrigerant until the subcooling at the receiver outlet is about 15F/8.5K and then take a full set of measurements. If, as you say, the liquid at the TXV inlet is 39C, then that narrows the restriction down to plugged inlet screen (or mis-adjusted TXV).

[chillyblue]

Why in an overcharged system does the liquid backup in the condenser, i would have thought it would backup in the reciever first until it was full then backup into the condenser???

CB

[Gary]

Why in an overcharged system does the liquid backup in the condenser, i would have thought it would backup in the reciever first until it was full then backup into the condenser???

CB

This is a common assumption, but incorrect, the exception being an equalized receiver.

[chillyblue]

This is a common assumption, but incorrect, the exception being an equalized receiver.

Thanks Gary, if you could explain it for me it would be much appreciated??

CB

[wilsoncheung]

As we expected, you are right on spot.

In his first measuring, suction superheat is 8,4K. Then it dropped after further overcharge.

By this symptoms and above description, it is capillary tube system.

In this case, the suct pressure is quite low, so I don't think it's overcharged.
I suggest charge more refrigerant to the system and check out the suct pressure change.
Or you should check the evap air flow is adequate or not.

[wilsoncheung]

Why in an overcharged system does the liquid backup in the condenser, i would have thought it would backup in the reciever first until it was full then backup into the condenser???

CB

________________

In a overcharged system, the refrigerant is too much. There would not be enough space to store the refrigerant and the the refri flow is small. If there's liquid receiver and big enough, things will be better. But if there's not,little space will be left to store refrigerant. What's more, as the THX will control the opening, the evaporator will not store too much refrigerant. So here is the problem, where is the refreigerant. It seems that they have no where to go but back to the condenser. In a sense, the condenser acts as the liquid receiver.

[wilsoncheung]

Discharge temp is the result of a great many variables, thus it is not a reliable indicator of anything. I don't even consider it to be a valid symptom and I never measure it when working on a system.

Remove refrigerant until the subcooling at the receiver outlet is about 15F/8.5K and then take a full set of measurements. If, as you say, the liquid at the TXV inlet is 39C, then that narrows the restriction down to plugged inlet screen (or mis-adjusted TXV).

Hello,Gary. I don't mean to offense, but I don't think this system is overcharged. One of the symptom of the overcharged system is that they will simultaneously have high discharge pressure and suct pressure.

I have come across this kind of problem and it turned out to be inadequate refrigerant.

Hope Paul will show us the final result.

[nike123]

In this case, the suct pressure is quite low, so I don't think it's overcharged.
I suggest charge more refrigerant to the system and check out the suct pressure change.
Or you should check the evap air flow is adequate or not.

When you dealing with TXV system than subcooling is main indication of charge, not superheat.
High superheat (and therefore lower suction pressure) could be because of partial restriction at TXV.

[nike123]

Hello,Gary. I don't mean to offense, but I don't think this system is overcharged. One of the symptom of the overcharged system is that they will simultaneously have high discharge pressure and suct pressure.

I have come across this kind of problem and it turned out to be inadequate refrigerant.

Hope Paul will show us the final result.

Normal to high evaporator pressures: Since these systems have TXV metering devices, the TXV will still try to maintain its evaporator superheat, and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. If the overcharge is excessive, the evaporator’s higher pressure would be caused by the decreased mass flow rate through the compressor from the high compression ratios causing low volumetric efficiencies. The evaporator would have a harder time keeping up with the higher heat loads from the warming entering-air temperature. The TXV will have a tendency to overfeed on its opening stroke due to the high head pressures, unless it is a balance port TXV.

If we have overcharged system and restriction at or before TXV, then superheat will be high and suction pressure will be lower, because of that high superheat. Since low side pressure depend on evaporator air temperature, amount of air and superheat, it is obvious that high superheat mean lower suction pressure, then when that superheat is normal.
TXV (if correctly adjusted and fully functional) tries to maintain adjusted superheat. If subcooling is adequate, that should mean that it has liquid at entrance, and that it should maintain superheat at adjusted level if load at evaporator is within TEV capacity . But, in this case, we have high superheat at first measuring and that indicate that TEV have problem to maintain superheat at normal level, because of restriction, high load or it is wrongly adjusted.
That is exactly what first measurement pointed out, and common mistake is, when technician see low pressure, to add more charge. Subcooling of 10K should stop it for doing that and to find problem elsewhere.

If system is undercharged, subcooling will be low or nonexistent.
Now, after additional charge, we should take full set of measurements again to judge if this is case of undercharge or overcharge and restriction at liquid side, but I highly doubt that it is undercharge because of subcooling measurements.
Of course, our conclusions are good as much as measurements are good.

[nike123]

You have slightly low suction pressure!
Evaporator TD is at the upper limit and you should check air flow at evaporator.
Suction superheat is not high and that could mean that compressor bearings are worn.

By "suction superheat is not high" I mean here that is not enough high to make compressor discharge temperature high!

However it is high as evaporator superheat. Since this is packaged unit, high suction superheat usually mean also high evaporator superheat.

[Sten]

hello!
if you make an pump down test, to check if there is leakage through the valves inside the compressor!

[chillyblue]

________________

In a overcharged system, the refrigerant is too much. There would not be enough space to store the refrigerant and the the refri flow is small. If there's liquid receiver and big enough, things will be better. But if there's not,little space will be left to store refrigerant. What's more, as the THX will control the opening, the evaporator will not store too much refrigerant. So here is the problem, where is the refreigerant. It seems that they have no where to go but back to the condenser. In a sense, the condenser acts as the liquid receiver.

I think your thinking the same as me and Gary is saying that is the wrong way to think!!

CB

[Gary]

Hello,Gary. I don't mean to offense, but I don't think this system is overcharged. One of the symptom of the overcharged system is that they will simultaneously have high discharge pressure and suct pressure.

That would be true if the system had only one problem, but the system has two problems (overcharge and restriction).

Someone added excess refrigerant in an effort to compensate for the low suction pressure/high superheat caused by the restriction.

[Gary]

Thanks Gary, if you could explain it for me it would be much appreciated??

CB

Assuming both are at the same temperature/pressure, excess refrigerant raises the liquid level in both the receiver and the condenser, but there is always a layer of vapor at the top of the receiver. Possibly this is different for undermount receivers, and it is certainly different for equalized receivers.

[Gary]

The evaporator air on/off temps tell me there is sufficient airflow through the evaporator.

The condenser air on/off temps tell me there is sufficient airflow through the condenser.

The subcooling tells me there is too much refrigerant in the high side.

The superheat tells me there is not enough refrigerant in the low side.

Too much in the high side and not enough in the low side means it isn't flowing from the high side to the low side fast enough. In other words, there is a liquid restriction.

And in this case, there is way too much refrigerant in the high side (overcharge).

The trouble shooting sequence is important. First evap airflow, then cond airflow, then SC, then SH.

[rookietech]

I already have his book and I think it is very good. Now I need to buy your book to learn other way. Then, I will be able to go in two directions in same time.
When and if you are planing to make SI version of your book, for us on the other side of pond? Or is it already available?

Good point, this is a Refergeration Engineering Website, all troubleshoot methods are good if they work!

[brunstar]

yep i feel it is over charged too as when systems are often over charged the pressures actually start coming down and having a non responsive effect.. which usually makes people put more gas in it.
The onyl was is to pull the gas out and weight it to rule that out.

[rookietech]

The evaporator air on/off temps tell me there is sufficient airflow through the evaporator.

The condenser air on/off temps tell me there is sufficient airflow through the condenser.

The subcooling tells me there is too much refrigerant in the high side.

The superheat tells me there is not enough refrigerant in the low side.

Too much in the high side and not enough in the low side means it isn't flowing from the high side to the low side fast enough. In other words, there is a liquid restriction.

And in this case, there is way too much refrigerant in the high side (overcharge).

The trouble shooting sequence is important. First evap airflow, then cond airflow, then SC, then SH.

Thank you for the troublshooting sequence evap airflow,condenser airflow, then check superheat and subcooling, basic but vital.

[Gary]

Thank you for the troublshooting sequence evap airflow,condenser airflow, then check superheat and subcooling, basic but vital.

That's almost right... the subcooling comes before the superheat.

[stefs_cruiser]

Did you ever get to the bottom of this one Paul??

[lexscripta]

Thanks - I have the same issue at a walkin with a TXV - I have really high Discharge - (I don't know the Sub Cooling, but its got to be high) and low Suction (R404a)

Also, the site glass is bubbling like crazy.

Condenser has been thoroughly cleaned - that's always the first thing I check when High Discharge. Evaps are nice and clean - all air flow is good.

I tried to bring the suction up by adding refrigerant, (suction pressure was around 15 - 20 degrees. and sure - it came up - but the head went a lot higher - about 380 PSI!)

There is a drier after the site glass on the liquid line - I just wonder. How do I know if that is the restriction, or if it is a problem with a restriction in the TXV?

What should the sub cooling be?

[Gary]

We can't see the system from here... we need numbers.

We need the coil in/out temps, the suction line and liquid line temps, and the high side/low side pressures.

You say the high side pressure went to 380 psi. That's about 137F. Is that high? If the ambient temp is 120F, then it is not high at all. But if the ambient is 50F, then it is very high.

We can't tell what's wrong without the numbers.

[D.D.KORANNE]

discharge valve reed could be leaky partially .

D.D.KORANNE

[kiwifridgie]

I went to an airwell package a while back that had fried the discharge thermistor.
I fitted a replacement today and test ran it. The discharge temp was 99C, which I thought was high for a maneurop recip mt80hp4a compressor.
But what do I know, I haven't works on a recip or maneurop for ages.
Suction was 6C, LP was 360kPa, HP was 1850kPa, liquid was 39C.
Condenser air (on/off) was 28/38C) Evap air (on/off) was 19.5/11.5C. The compressor cranckcase was cool.
Any ideas for the high discharge?
I tried adding refrigerant even though I could find no leaks, sp went up to 380kPa, st down to 5C, liquid, HP still the same. Discharge temp went up to 103C, but that could have just been because it was running for longer building up the heat.
Incondensibles perhaps? The system has been running for a long time and is a package unit so factory charged though.

i think the problem you have is that you are working on an airwell unit. they would have to be the most unreliable pieces of junk i have ever encountered. not the components individually, just the finished airwell product.

[just chilling]

Paul please put us out of our misery. interesting discussions. i can see how how the over charged theory stacks up as credible. i do not suspect worn valve seats. for high DT worn valve seats normally is acompanied by high suction pressures as dis pressure leaks back through reeds. but pump down/efficiency test on compressor would be useful. another idea i have is salt air corrosion to condenser. Airwell are Australian made and most cities in Aus are coastal. if condenser has fan speed contoller and liq line temp sensor is causing it to speed up. The loss of condenser surface efficiency may elevate dis temp.