Pump down and suction pressure switches: legacy?

[NoNickName]

Yes, it's not the idea, but how it is abused.
And I think I would still disconnect the pump down switch as soon as I find one.

[Peter_1]

Then the change is great you will find a lot of compressors broken if you don't first examine the schematic and the logic behind it.
If a control is wrong set, then every system is a dangerous system, like said before , it stands or falls with the proper settings of the controls as with any system.
A system without pump down and a LP set in vacuum is more dangerous than a pumpdown with a correct set LP.

[Gary]

So the question is: how come so many engineers are so fond of pump down procedures? Isn't it time for the industry to realise we are in 21st century and abandon the legacy of old-fashioned habits?

And for you in refrigeration: isn't it time to replace the suction pressure switch with a real thermostat in your cells or freezers. Come on guys: my grandpa used to stop the compressor on suction pressure... there is no place for this in 2009.

So you are saying that if something has been done in a certain way for a very long period of time, then it must be the wrong way?

Sorry, but that does not compute. Ridicule is not proof, and change is not always a good thing. Winning the lottery is change. A sharp stick in the eye is change.

If you want change, then you need to prove that it is a change for the better. As the person who is advocating the change, the burden of proof is on you.

[US Iceman]

So you are saying that if something has been done in a certain way for a very long period of time, then it must be the wrong way?

In some respects, doing something for a long time is no indication it has always been done correctly. This is not too much different than offering the response; That's the way we have always done it, so it must be right!

Sometimes you have to read between the lines. I'm not advocating change for the sake of changing something, only if it is done for the proper reasons and based on sound logic.

[Gary]

In some respects, doing something for a long time is no indication it has always been done correctly.

... or that it has always been done incorrectly. And that is my point. Ridicule is not evidence... one way or the other.

Show us the evidence... and spare us the meaningless ridicule. Such remarks add nothing to the discussion.

[Peter_1]

Originally Posted by NoNickName
And for you in refrigeration: isn't it time to replace the suction pressure switch with a real thermostat in your cells or freezers. Come on guys: my grandpa used to stop the compressor on suction pressure... there is no place for this in 2009.
This isn't pumpdown for me: a pump down has always a thermostat which cuts off the SV and the LP finally stops the compressor.
What you're describing is regulating the temperature with the LP but this isn't pumpdown for me.

[US Iceman]

... or that it has always been done incorrectly. And that is my point. Ridicule is not evidence... one way or the other.

Show us the evidence... and spare us the meaningless ridicule. Such remarks add nothing to the discussion.

My purpose in those comments is NOT to ridicule or insight a riot. And I do think they add to the conversation.

What I was trying to get across is that people tend to do the same thing as those that have trained them do (or what they have a seen repeated by others). Some times they may not understand the ramifications of what they do.

Right, wrong, or indifferent, if we think about the basics of what we do and understand the impact of those decisions we can effect a positive outcome.

I believe this intent of the original post is to get some input on the effects of the compressor due to service or installation techniques commonly used. From a manufacturers viewpoint you always want to understand why the warranty failures arise. If you can pinpoint those issues, then you can find a way to minimize them.

That, I think is what NoNickName was trying to find.

[Gary]

I have spent most of my adult life trying to change the way people trouble shoot systems, without once demeaning them with a comparison to "Ptolemaic believers". Am I taking the wrong approach? Should I be insulting them rather than persuading them?

[NoNickName]

There are people who only listen when they are insulted at. And I also take that way quite often, in recent times. Sorry for that, but it is exactly for refrigeration engineers, thinking they are right, just because they did it for 20 years time.
One can be right or wrong irrespective of how much time he/she's been doing it.

That's the point behind pump down. It's not good just because our grandpa's did it until retirement and we are pumping down for legacy that makes pump down a good thing.
And it's not even because it's written on a manual from Bitzer!
Look at their manuals: they say they are compliant with low voltage directive 73/23/CEE which was obsolete and replaced by 93/68/EC which in turn is already been replaced by 2006/95/EC!! But please, go ahead, make my day and take Bitzer's word as gospel.

For Peter1: no, that's not pump down. It was a way of showing how obsolete some state-of-industry solutions are. Still controlling refrigeration by suction pressure, instead than measuring the primary physical quantity, temperature.

[Gary]

Somebody wake me when the actual discussion of the issue begins (yawn).

[US Iceman]

I think this discussion if nothing else has shown what happens when you challenge conventional wisdom. I am not advocating for one or other here, just the principle.

Now before anyone tries to hang me, think about this; when a question was posed which challenged conventional wisdom what happened? Most of us jumped in with how we do things. Nothing wrong with that.

This is not meant to be self-righteous or condescending on my part, but this was an interesting thread to follow.

[Gary]

Really? Conventional wisdom was challenged and I missed it? Did I sleep though the part where evidence was presented and logical discussion ensued?

Nope. Looking back through the posts I see a few unfounded assertions and a lot of "I am right and you are wrong and the only reason you don't believe this is that you are stuck in the stone age".

Note that I haven't come down on either side of the issue yet. I am waiting to see the evidence.

[US Iceman]

What's more important? Trying to understand the nature of the supposition or proposition presented or using sarcasm to respond to posts which might be opinionated?

We all have opinions on something. How we respond to those is another matter.

I'm not necessarily agreeing that pumpdowns and low pressure switches should be avoided, but the underlying problems and how they affect compressor warranties is something I would think we would all interested in.

I would like to see evidence also. But as this is a public forum privately held specifics may not be available.

[Gary]

I would like to see evidence also. But as this is a public forum privately held specifics may not be available.

I would settle for logical discussion.

[US Iceman]

OK, let's try a different approach.

Here is the original premise...

... I must say that pump down is more harmful than not, because then that procedure is in action the motor is not cooled and no gas is pumped.
The motor overheats, and the rod-shaft assembly is subject to extreme differentials.

To be fair, we could start with the known facts.

What are the damages to the compressor? - overheated motor & potentially higher operating pressure ratios

The tricky part is trying to correlate those issues to specific failure modes.

So....my question is: How was this determined to be pumpdown related and not high suction/discharge superheat? Either one could have caused similar results for motor burnouts or valve damage.

However, rod-shaft assembly issues would either be lubrication induced or high pressure ratio issues.

Perhaps NoNickName can address these, if we can all be objective.

[Peter_1]

Still controlling refrigeration by suction pressure, instead than measuring the primary physical quantity, temperature.

If you regulate very precise your LP then you're measuring very precise your temperature.
I have some small compressors this way running which were installed around 1970 -1980. The end-users are very satisfied and this long time regulating this way with a satisfied customer is a prove that the installation was set right at the first moment. But if the customer wants it a little bit colder - which isn't needed once regulated- then he need the technician for this.

[Peter_1]

And truck freezers (likd bofrost, eismann's and the like) are built with eutectic plates without the suction accumulator. You know how they start the compressor? Pulsing it until all liquid is digested through the compressor, 30 starts a minute.

We happen to service some Bofrost trucks (9) because our shop is three shops from me.
They don't have a suction accumulator but they have an SV closing together with the compressor like I mentioned in another post.
But what happens with a normal evaporator when shutting down without an SV? There's also gas migration and liquid can also collect then. So every evaporator can collect some liquid and every compressor can start with some liquid.

[Peter_1]

I because then that procedure is in action the motor is not cooled and no gas is pumped.

There's still gas pumped, otherwise it should cut out on LP but load is also reduced, so it doesn't need to be cooled that much as in full load.

[Peter_1]

That's the point behind pump down. It's not good just because our grandpa's did it until retirement and we are pumping down for legacy that makes pump down a good thing.
And it's not even because it's written on a manual from Bitzer!
Look at their manuals: they say they are compliant with low voltage directive 73/23/CEE which was obsolete and replaced by 93/68/EC which in turn is already been replaced by 2006/95/EC!!

It's not because it's not mentioned in their latest manuals they're not compliant with 2006/95/EC.
If I don't hand over a pressure certificate to my client doesn't mean it hasn't been pressurized.
These are just documents - for most techs non saying - and doesn't say anything about the qualities of a compressor.

[Peter_1]

But please, go ahead, make my day and take Bitzer's word as gospel.

I certainly will do because that the machine we mostly uses because they're for me the Rolls Royce amongst compressors.

And Bitzer affirms what I'm doing already so long.

Reason why so many OEM's uses this machine (packs, icemachines, production machines, oil coolers...)
I suppose if price isn't important but quality is needed.

You have to face it NNN, few agree with your opinion which doesn't say your can not be right.
You're right if controls are set not the correct way but then this is valid for every compressor.

I will anyhow continue pump down my compressors the right way till someone can prove me that what I'm doing can harm my compressor.

[Peter_1]

The motor overheats, and the rod-shaft assembly is subject to extreme differentials..

Differentials of pressure or temperature?

[old gas bottle]

i ,am thinking its time for this one to end i got peeded off when a certain no all starts telling me i,am stuck in the stone age and need to change , that things and people in the industry dont know as much as him even if they have been sucsessfull bussiness men for donkeys years and respected in the trade. most guys on here welcome change if its for the good and i,am in that camp,but insulting someone into change just does not cut it with me people skills could have made this a interesting debate instead of a slanging match.so carry on,i,am done with this one.

[NoNickName]

Ok, let's address one issue at a time:

Note that I haven't come down on either side of the issue yet. I am waiting to see the evidence.

The evidence was there for whom wanting to read it since the first post. Operating the compressor out of the envelope is harmful. Full stop. Operating the compressor out of the envelope AND pumping down is as harmful as or even more than.... I think this is crystal clear.

I would like to see evidence also. But as this is a public forum privately held specifics may not be available.

Pump down is not harmful PER SE. Operating the compressor out of the envelope is.

Perhaps NoNickName can address these, if we can all be objective.

Well, I'm not a designer, but it's evident that all machinery operated in conditions not foreseen at design time, may lead to failures, or at least to a shortening of MTBF.

If you regulate very precise your LP then you're measuring very precise your temperature.

No, you aren't. Pressure and temperature are not directly related in a system where more variables are involved and affecting the thermodynamical equilibrium. Eg.: refrigerant charge, pressure drops, etc.

I have some small compressors this way running which were installed around 1970 -1980.

I have too. Is this all about who spits further?

So every evaporator can collect some liquid and every compressor can start with some liquid.

So, it looks like you are following bitzer's manual as soon as it agrees with your pre-concept. Is there anywhere on their literature that the compressor is able to start with "some liquid"?
Truth is that a refrigeration engineer, who shall bear this title, would not let "some liquid" return to the compressor, even when relying on the quality of the compressor itself.
Bad design and bad maintenance, sorry. Warranty unrecognised.

If I don't hand over a pressure certificate to my client doesn't mean it hasn't been pressurized.

It just means you are unlawfully handing over your assembly.
It is irrelevant whether it was pressurised or not. Declaring compliancy to an obsolete directive is like driving in compliance with expired speed limits.

So....my question is: How was this determined to be pumpdown related and not high suction/discharge superheat? Either one could have caused similar results for motor burnouts or valve damage.
However, rod-shaft assembly issues would either be lubrication induced or high pressure ratio issues.

Exactly. The big end of the rod was consumed by excessive differential pressure between crankcase pressure and discharge pressure. It was lubricated, but still it failed after repeated attempts (hundreds or thousands tries) to evacuate the suction. Note: this was a HBP compressor, not a LBP one, therefore the envelope is limited on the left.

[Gary]

We have all seen pump down systems lasting for many years, so we are not talking about immediate damage, but rather long term damage.

Everything causes long term damage. Simply running the compressor will eventually lead to it's demise.

So the issue then is which strategy leads to the longest/shortest compressor lifespan.

[Gary]

Demonstrating any of this would require a long term statistical study, preferably under tightly controlled conditions, plotting strategy versus compressor lifespan, and the margin of error would be inversely related to the number of systems involved in the study.

[NoNickName]

Demonstrating any of this

any of this... what?

So the issue then is which strategy leads to the longest/shortest compressor lifespan.

And what you think in terms of envelope of compressor? Running it outside of the envelope lenghten or shorten its life?

[Gary]

any of this... what?

If you read the rest of the post you will realize that "any of this" refers to strategy versus compressor lifespan.

[Gary]

And what you think in terms of envelope of compressor? Running it outside of the envelope lenghten or shorten its life?

What is the envelope based upon?

[NoNickName]

What is the envelope based upon?

Extensive calorimetric lab testing. For companies adopting ASERCOM scheme, it also encompasses random competitor counter-check.

[Gary]

Does the envelope describe the conditions for optimum compressor longevity?... or optimum energy usage?... or the optimization of some other criteria? What is the intended usage of the envelope?

[US Iceman]

The operating envelopes typically specify the extent of the allowable operating conditions the manufacturers have run or considered normal. Part of the purpose of these envelopes is to define the maximum and minimum conditions like maximum and minimum pressure ratios for any given refrigerant/compressor combination.

[NoNickName]

Does the envelope describe the conditions for optimum compressor longevity?... or optimum energy usage?... or the optimization of some other criteria? What is the intended usage of the envelope?

Sorry to read that you don't know what the envelope of a compressor is for. It doesn't sound quite right for a wannabe refrigeration engineer.
But since you asked... the envelope surrounds the area of combinations of evaporating and condensing pressures for both the compressor current input, discharge temperature and winding temperature to be within specs.
In our case the current shall be lower than MRA (or FLA) and discharge + winding temperature lower than 120°C.
In other words the envelope is to ensure safety and long lasting operation to the compressor.
The testing conditions of superheating and subcooling are determined by EN12900 norm, as well as performance testing conditions.
All parameters are "tightly controlled" in our independently certified lab and run based a precise schedule, after which a metrological verification of tolerances, dimensions and shapes of compressor components are performed on a measuring 3D robot hub (Johansson and Zeiss).

EDIT: during pump down, the compression ratio increases, as HP is governed by condensing control, while suction pressure is governed by the PD switch. As the ratio increases, the discharge temperature increases two fold: lower vapour mass flow rate and increased end-compression temperature.

[Gary]

I'm thinking it would be fairly safe to say that there are times when every system runs outside the envelope. For example during pulldown... or heavy load... or each time the compressor starts.

[NoNickName]

I'm thinking it would be fairly safe to say that there are times when every system runs outside the envelope. For example during pulldown... or heavy load... or each time the compressor starts.

Got to the point. Although pulldowns are occasional (and tolerated), pump downs are SYSTEMATIC.
And I can't accept or tolerate a systematic exception. It is no longer an exception, is it?

[Gary]

You may find this surprising, but I don't much care what you can or cannot accept or tolerate.

[US Iceman]

Before this degenerates into a discussion of name-calling I suggest we put this on hold for awhile guys. We've had this happen before and it sours everyone's opinion on what could have been a really interesting discussion.

[Peter_1]

No, you aren't. Pressure and temperature are not directly related in a system where more variables are involved and affecting the thermodynamical equilibrium. Eg.: refrigerant charge, pressure drops, etc. .

I perfectly can regulate the temeprature in a coldroom with an accuracy smaller than 1°C with a LP.
I will even say more: this is an exercise students have to make in classroom to better understand the relation between temperature and pressure.

Once a certain system installed with a given length of lines, the pressure drop remains the same as long as system is running. Variation is not that big to see any diffrence.
Refrigerant charge has nothing to do with this when regulating your temperature with the LP cutout.

Refrigerant is fed to the evaporator and there is a precise equilibrium between temperature and pressure.
You once have to follow this in a real life situation and you will see that real life is different than you have in your mind now.
The LP then even acts as adefrost control.

[Peter_1]

So, it looks like you are following bitzer's manual as soon as it agrees with your pre-concept. Is there anywhere on their literature that the compressor is able to start with "some liquid"?
Truth is that a refrigeration engineer, who shall bear this title, would not let "some liquid" return to the compressor, even when relying on the quality of the compressor itself.
Bad design and bad maintenance, sorry. Warranty unrecognised.

.

I already quoted also the Copeland manuals.
I'm not following Bitzer nor Copeland's manual. It's only showing you a written evidence that other manufacturers claims just the opposite what you're trying to say. They deliberately write it in their manuals for some reason.
They recommend it.

I do it because I'm convinced of the usefulness of this.

No compressor may start with liquid but if you don't pump them down like you propose, then they often will start with some liquid.
So what's then the best or the worst for a compressor: no pumpdown and start with some liquid or pumpdown and having no liquid?

It's not a pre-concept of me, it's a since long proven good practice used by almost everyone.

[Peter_1]

All parameters are "tightly controlled" in our independently certified lab and run based a precise schedule, after which a metrological verification of tolerances, dimensions and shapes of compressor components are performed on a measuring 3D robot hub (Johansson and Zeiss).

EDIT: during pump down, the compression ratio increases, as HP is governed by condensing control, while suction pressure is governed by the PD switch. As the ratio increases, the discharge temperature increases two fold: lower vapour mass flow rate and increased end-compression temperature.

With a 3D robot, you're measuring distances. How then you mesure differences on a compressor which ran with and one without pumpdown?
And after how many cycles do you measure then?
I'm sure this was never examined in your lab extensively.

This could have been a firm prove of your theory which remains only a certain theory, a personal vision.

NNN, while pumping down, your HP decreases a lot - era when HP was maintained on a stable HP is long time ago - because load decreases while pumping down.

AMP's are also going down very fast while pumping down.

And what do you do then when it's becoming hot outside and HP increase and DP then also increases?
DP then increases the same as when you pump down.

[NoNickName]

I perfectly can regulate the temeprature in a coldroom with an accuracy smaller than 1°C with a LP.

You can't be serious.

I will even say more: this is an exercise students have to make in classroom to better understand the relation between temperature and pressure.

My students would be rejected as any of them will try to control a cold room temperature with a LP switch.

Refrigerant charge has nothing to do with this when regulating your temperature with the LP cutout.

Refrigerant charge affects subcooling. Subcooling affects cooling capacity. Cooling capacity affects evaporating pressure. Enough said.

Refrigerant is fed to the evaporator and there is a precise equilibrium between temperature and pressure.

There is no precise equilibrium between suction pressure and cold room temperature.
Sorry. As I said, charge affects it, as well as coil clogging, frosting, air circulation... and anyone trying to set a precise temperature in a room with a pressure switch he'd be better turn back to the drawing board.

The LP then even acts as adefrost control.

Oh, yes, I've seen a lot of system defrosting a feather-clogged but otherwise frost-free coil.

With a 3D robot, you're measuring distances. How then you mesure differences on a compressor which ran with and one without pumpdown?

That is not the point. The measurement is for analysing the expected life time and repeatability of tests carried on inside of the envelope.

I'm not following Bitzer nor Copeland's manual. It's only showing you a written evidence that other manufacturers claims just the opposite what you're trying to say. They deliberately write it in their manuals for some reason.
They recommend it.

None of them recommends to start their compressors with "some liquid". Please stay on topic. And please, I am a man, not a manufacturer.

era when HP was maintained on a stable HP is long time ago

About the same age when pump down was introduced or was it later...

Again, compressor manufacturer recommendation must be complied with, disregarding color and brand. When compressor is operated outside of the limits, the compressor may be affected and eventually break. If the compressor is pumped down repeatedly at a lower-than-allowed pressure, it is a direct responsible of a possible developing fault.
I think the above is agreeable in principle by anyone, advocating pump down or not.

[al]

NNN

Would it be true to say that so long as the compressor remains within its "envelope" you have no problem with pump down per se?

al

[Gary]

Again, compressor manufacturer recommendation must be complied with, disregarding color and brand. When compressor is operated outside of the limits, the compressor may be affected and eventually break. If the compressor is pumped down repeatedly at a lower-than-allowed pressure, it is a direct responsible of a possible developing fault.
I think the above is agreeable in principle by anyone, advocating pump down or not.

If this is true, then you could simply refuse to honor warranties for pump down systems. Then the only remaining question would be how deeply your more tolerant competitors bite into your market share.

I'm thinking it would be more in your interests to persuade rather than dictate.

[NoNickName]

NNN
Would it be true to say that so long as the compressor remains within its "envelope" you have no problem with pump down per se?

Sure, and for that the refrigeration engineer shall have the appropriate compressor envelope with him when setting and commissioning the PD switch.

And for Gary, if pump down is within compressor envelope, no reason not to honor warranty.

At this time, no action is taken on these regards (dictation or persuasion) and in terms of tolerance the company I'm hired to is known to be the most tolerant to clients abuses.

[Peter_1]

nnn, as usual, if others don't follow your truth, your thinking, you start to play it very personally or you pointing posters back as a teacher if they're leaving the thread.

Difference between us, i did it all with my own hands since more then 25 years. I gained my experience not only behind my desk.

I'm not someone who solely relies on what 's found behind a desk and a lab.

And after all those years, I found the time right to share what I've learned over all those years with others. That's when I started school Allessandro, not sooner.

NNN, I'm very serious saying I can control a cold room within 1°C with the LP.
Fact you doubt this is for me a sign you better leave a little bit more your desk and lab to gain some real world practice and real-life experience in a much broader field then only 1 brand of compressors and one brand of heatpumps.

And I don't believe everything what others saying and what I find in books or manuals.

[NoNickName]

I come from the field and have 15 years experience hands-on. I am behind the desk since only one year. Therefore, I don't accept being taught.

If you are sharing with others the knowledge that temperature shall be controlled with a pressure switch, then I understand why this refrigeration world is going down the flush.

[FreezerGeezer]

OK gents, I think the MAIN problem here is that a lot of you are talking in a foreign language here & not understanding each other properly. As a brit myself I'm finding much of this thread difficult to follow, trying to interpret your versions of my language. I mean no offence, I haven't enough of any other language to even begin to discuss this subject in yours & applaud you all for being able to do so in mine.
Please let's all cool off a bit eh?

[Sandro Baptista]

Guys let's see if all of you (NoNickName and the others agains't the NNN ideas) agree with this and all come back to be friends

Briefly I would say that (and it's what I consider for the good operating of the refrigeration plants):

1) For flooded systems (gravity and pumped) I would never do it because it's not necessary except in some special cases.

2) For medium temperatures I would always made pumpdown (untill the minimum possible allowed by the manufacturer) and if possibly with ventilation connected to ensure that almost all refrigerant has evaporated.

3) In extremely low temperature -40ºC or less I would do it untill the compressor reaches the minimum acceptable. Many many compressors working with this temperatures can go to -45ºC or even less (also depend of the refrigerant). But I also would put a good liquid separador with capability to receive a good quantity of liquid without affect the compressor.

Case close??

[Gary]

OK gents, I think the MAIN problem here is that a lot of you are talking in a foreign language here & not understanding each other properly.

This is entirely possible. For example, perhaps when NNN says "shall" he really means "should" which would change the tone of his comments from demanding to persuading.

On the other hand I'm thinking that his insults are intentional and strategic.

[Gary]

Getting back to the issue at hand:

If we accept that the envelope describes the ideal conditions, then we can say that operation outside of the envelope (however brief), while not necessarily "wrong" is "less than ideal" and should be minimized.

[Gary]

I have zero experience in system design, albeit a pretty fair understanding of how things work. With this in mind, here is how I would design a pumpdown system:

I would install a vertical suction/liquid heat exchanger (HX) alongside the end of the coil, with the suction entering the bottom and exiting the top.

I would mount the TXV bulb on the suction line at the exit (top) of the HX.

On shutdown the liquid line solenoid (LLS) would close. A low pressure control would then shut down and lock out the compressor at a low point which is within the operating envelope of the compressor.

During the off cycle, liquid would accumulate in the coil and/or the HX.

On startup, the compressor would run but the LLS would remain closed.

When the low pressure again reaches a low point within the envelope, the LLS would open.