Pump down and suction pressure switches: legacy?

[NoNickName]

I would like to hear comments on the subject line from field engineers.
As aftersales for compressor manufacturer, 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.

Additionally, when the LP pump down switch is unreasonable set to a very low setting, the compressor may be subject to work outside the operational envelope until it finally comes to a stop.

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.

[Grizzly]

I would like to hear comments on the subject line from field engineers.
As aftersales for compressor manufacturer, 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.

Additionally, when the LP pump down switch is unreasonable set to a very low setting, the compressor may be subject to work outside the operational envelope until it finally comes to a stop.

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.

Sorry nonickname.
I have never doubted your sound and Knowledgeable advise before.
But i have to say there are still plenty of situations where a controlled pump-down is necessary.
How else do you ensure long pipe runs to remote evaporators are void of liquid.
Sure there are plenty of situations where it may be possible to do without the pump-down.
But I can think of many industrial applications where it is necessary.
I am talking about recips here.
Grizzly

[NoNickName]

How else do you ensure long pipe runs to remote evaporators are void of liquid.

My guess: suction strainers?

[nike123]

How long pump down lasts? 10-20 sec?
I dont think that that is doing any damage for that short period of time.
Do you have any study to support this:

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

[al]

get the pitch forks and set the fire.............

tis a point of view i suppose but like nike is there any evidence??

i do agree that lp cut out can be way too low especially on medium temp stuff, but in fairness all the talks/discussions from comp manufacturers have never pointed out pump down as a main cause of failure or is this a late april fools

al

[NoNickName]

This is not a late april fools. This is a legitimate point of view from a second biggest european compressor manufacturer after sales service, that is me.

And for nike123: yes I have a study. Actually I have more than that: I have an empirical test in the calorimetric lab. And the time of the pump down is limited by the liquid solenoid valve tighteness. It can be well over minutes, if the diaphragm is not refrigerant-tight.

But for what it's worth, whether pump down procedure is letting the compressor out of the working envelope, whether it is 1 second or 1 minute or 1 hour, IT IS FORBIDDEN.

It is true, no manufacturer ever pointed out this as a cause of failure, but there is a first time for everything, don't you think?

[Grizzly]

Are we at cross purposes here?
A pump down does not have to be in negative pressure so where is the harm?
Grizzly

[NoNickName]

Are we at cross purposes here?
A pump down does not have to be in negative pressure so where is the harm?
Grizzly

I'm sorry if I'm not clear. Suppose compressor XY has a lower Te at -45°C corresponding to a refrigerant Rxyz at 1 bar(g). This is the lowest left end of the envelope.

Now suppose that at any given time an installing company sets a LP pump down switch at 0.5 bar(g) (remember we are talking about gauges pressure here).

This is clearly prohibited by the same operational envelope, which does now allow compressor to operate AT ANY TIME below 1 bar(g).

This is the POTENTIAL harm.

[al]

so your solution to no pump down is a suction accumulator(not strainer which does bugger all for liquid!), as a manufacturer are you willing to honour the increased claims due to liquid slugging.........i would doubt it.

Pump down is an economical answer to this problem, again pressure switches should not be set below the recommended settings.

From your own point of view maybe sending the setting information to the wholesalers and a bit of general reeducation would not go amiss??

I do see where you're coming from, though with costs being squeezed i can't see any realistic alternative?

al

[modvalve]

I would like to hear comments on the subject line from field engineers.
As aftersales for compressor manufacturer, 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

You are wrong in saying that no gas is pumped when a system goes into pump down. When a system goes into pumpdown the amps on the compressor drop as the suction pressure decreases as long as it isn't overcharged and the pumpdown never usually lasts more than a minute. I have never seen a motor overheat on a properly maintained system. If you had no pumpdown on a system and it went on defrost you would get severe floodback at the end of the defrost cycle causing more damage to a compressor than pumping it down

[US Iceman]

Come on guys: my grandpa used to stop the compressor on suction pressure...

And so did mine.

I tend to agree with you. If the compressor is allowed to do repeated pump downs on a low-pressure switch you could see motor heating if the liquid line solenoid valve leaks (or some other reason). During this, the motor would have very little mass flow past it for cooling. So... the motor starts and pumps right back down. Repeated start/stop cycles could lead to motor heating as it never really has a chance to cool off. I call this a recycling pump down cycle. This only operates off of a low-pressure switch.

Some will lower the low -pressure switch so that the suction pressure pulls down to a much lower pressure to reduce the short cycling of the recycling pump down. In this case you can be operating outside of the compressor application range and potentially cause increased wear over the same time frame.

On the other hand, if you use a control relay to lockout the compressor until the thermostat calls for cooling and energizes the solenoid valve, the compressor stays off until cooling is again required. I call this a non-recycling pump down, which is what I have used with good success.

Very good discussion going on here....

[NoNickName]

I have never seen a motor overheat on a properly maintained system.

And I have never seen a properly maintained system (or very few in honesty).
And yes, I have also seen a lot of motor overheats, especially by field engineers who start the compressor just after having evacuated the system, and before charging the refrigerant.
Q: Why are you doing this?
A: I don't have a service valve on the liquid line and don't want to flood the compressor with liquid. So I run it as I start charging. Pooof....

If you had no pumpdown on a system and it went on defrost you would get severe floodback at the end of the defrost cycle causing more damage to a compressor than pumping it down

This is not true. Defrosting, in a properly designed and maintained system, would not cause floodback, if suction accumulator is installed and the coils are fit for purpose.

From your own point of view maybe sending the setting information to the wholesalers and a bit of general reeducation would not go amiss??

That is what I'm going to do, but I wanted to feel the sentiment of the field engineers public on the subject. I'm not going to yell to windmills for nothing.

I tend to agree more with US Iceman, who also introduces some sort of "cultural experience" in the engineering practice, and I appreciate that.

[US Iceman]

And I have never seen a properly maintained system (or very few in honesty).

This may start a big argument.

I laughed when I read this and think there is a lot of truth to this statement. Almost any refrigeration system will operate trouble-free for many years if a minor amount of maintenance is performed on a regular basis, if the system is properly engineered and installed.

For any other system not properly engineered and installed your mileage may vary! In this instance the systems last as long as the equipment is able to tolerate the operating conditions or maintenance practices used. Sometimes this is still within the warranty. This is why manufacturers are concerned...

[Sandro Baptista]

Hi guys,

I agree in some point of with NoNickName but also with Grizzly.

Briefly I would say that:

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.

Severals examples I can get to justify the pumpdown (of course if we have a good liquid separator...maybe is not so necessary):

1) When you made an electrical defrost, and if your end defrost termostat is not working well or the probe has a problem it happens that the pressure inside the coil could rise so much (20 bar(a) » 45ºC for R404A) that is bad for the compressor LP limits.

2) It may have condensation of refrigerant in the suction cold pipe far from the evaporator » Result: liquid to the compressor.

3) There is some amount of liquid inside the evaporator coil. The coil can be at temperature of cold store or near after a good while. Then when the compressors starts it will pull gas but maybe it will carry droplets of liquid that are not evaporated untill have a sufficiente temperature difference between the coil and the air of the cold store.

This is my opinion about this subject.

Regards

[al]

Nonickname

I'm not going to rise to the bait of poorly maintained systems, if this is the case in your country so be it

This is not true. Defrosting, in a properly designed and maintained system, would not cause floodback, if suction accumulator is installed and the coils are fit for purpose.

again come back to cost and extra installation time, i'm thinking particularly of smaller systems.

If a compromise were needed then i would definitely be going with icemans suggestion, we do this on most of our serve over installs and also include suction solenoids.

al

[US Iceman]

One of the things we need to be careful of in this discussion is the issue of context. Different principles and methods are used in industrial refrigeration systems from those that should be commonplace in commercial systems.

Likewise, what should be done in a single compressor/evaporator system may differ from a system where multiple compressors are used in conjunction with many evaporators.

[NoNickName]

Good points from everybody. Let me further discuss:

I agree in some point of with NoNickName but also with Grizzly.

Ok, flooded, MT and LT. What about HT? I've seen a on-site-built brine chiller for treating sour cream (-5°C approx) with extremely deep pump down. LP switch set to 0.2 bar(g) with R404a. Two 8-cylinders 80HP compressors knocked out to trash, until I decided to steer everybody clear and cut the wires of the pump down switch open. No problem since.

I'm not going to rise to the bait of poorly maintained systems, if this is the case in your country so be it

I'm not going to rise to the bait of chauvinism. Good and bad are widely spread, maybe not homogeneously, but they certainly are.

pressure inside the coil could rise so much that is bad for the compressor LP limits.

And how pump down has anything to do with that? I can't get the point here.

It may have condensation of refrigerant in the suction cold pipe far from the evaporator

Ok, and how pump down will solve the problem? If you have liquid in the suction pipe, pumping down will just lengthen the floodback and liquid ingestion time.
This specific issue is correctly addressed by the simple installation of a suction P syphon and riser with a suction accumulator.

There is some amount of liquid inside the evaporator coil.

Why? Don't you have a liquid solenoid? Isn't your TXV gas tight?

Then when the compressors starts it will pull gas but maybe it will carry droplets of liquid

Have you ever heard of suction superheating? This scenario is impossible, in a "properly maintained system"

[al]

Good and bad are widely spread, maybe not homogeneously, but they certainly are.

Yes i agree, but your previous statement of never seeing a well maintained system is guaranteed to rise the BP a little!!

The tool that set this should be shot(tool=idiot politely)

I've seen a on-site-built brine chiller for treating sour cream (-5°C approx) with extremely deep pump down. LP switch set to 0.2 bar(g) with R404a. Two 8-cylinders 80HP compressors knocked out to trash, until I decided to steer everybody clear and cut the wires of the pump down switch open. No problem since.

if this had of been shutting off at say 2.5B would there have been a problem, assuming all valves seating
correctly?

al

[old gas bottle]

my one and only comment, dont lisen to this arrengent asshole, asking for comments then trying to shoot them down with numbers.

pump down works fine.

i,am off on hols for three weeks.

cheers chaps.

[al]

Happy holidays you lucky beggar


is the girl on the bike going too?? if she's not could i have her phone number?

al

[US Iceman]

I can understand where this line of questioning came from.

As a manufacturer you are exposed to many warranty issues presented from field service people. A single person or service firm may never see the correlation between one or two compressor failures, however, the compressor manufacturer sees all of them.

From a warranty perspective you try to do the analysis for the failure reasons to see if the defect is due to manufacturing issues or if it is related to field service factors, power supply, or other.

I think what we are seeing here is an attempt to understand how much impact pump down cycles may have on compressors (and the motors). I have seen similar issues with a normal pump down cycle, which is why I used the non-recycling method with the control relay lockout.

[old gas bottle]

ice man, hello old friend i just dont like the invitation of comments from the "service guys" for ther input ,just to be shot down with a load of technical waffle.

pump down has been a tried and tested means to an end for a long time now, and as the saying goes "if it aint broke,dont fix it", i,am not big on the keboard but in short, untill the compressor "working enveolope" covers HP/MP/LT,all on the same gas,as well as other relative aspects to do with start pressures after defrost etc , there will be a place for pump down,

not saying it wont work without,just majority preferance.

[US Iceman]

HI OGB. I hope you know your avatar is very distracting (very distracting indeed. In a nice way of course!).

I do not have a problem with pump downs per se, if they are setup properly. That's the reason behind my non-recycling pump down concept. On the other hand, if the system short cycles quite bit then I can foresee some potential problems with motors or contactors/starters.

I understand what you are saying. One of the things with technical discussions is; we need to find some common ground on why the questions are being asked and how the responses are replied to.

As you might guess, manufacturers and service people face different problems. I have been lucky enough (?) to have worked in several different fields within this industry and try to have an appreciation for what the other guys are thinking. What I have seen is; we almost always find similar problems or issues, but many different ways to deal with them. Most often though, it is possible to find the root cause and try to find a simple solution using the KISS principle.

[Temprite]

Dont forget there is this apparent issue regarding pumpdown.

http://www.refrigeration-engineer.co...ad.php?t=10949

[Goober]

In response to Temprites thread "to pump down or not to pump down" and to hijack this thread somewhat. I'm in the pump down camp as that's what i was taught right from the start. However, I totally accept and understand Buffola Trident being adamant not to have a pump down on there evaps during defrost. Any mass of refrigerant liquid "under pressure" will most definatley assist in a defrost of any make of coil, I think we're all missing the obvious here in that during defrost the compressor will not run (in non pump down mode) sooooooo.......where is the liquid going to boil off too? Yeah ok some might, but..........Any increase in pressure results in an increase in temperature. (Boyles Law if I'm not mistaken) Ip so facto, or something like that, better defrost is achieved.

Back to compressor issues with pump down. Not an issue as far as I'm concerned. Of course I've had most of my experience in supermarket type work, light commercial, solenoids pumping down all over the place. Comments regarding LP's cutting the compressor off at too low a pressure and leaking solenoids ARE NOT NORMAL. These are fault conditions and of course they will over heat the compressor, etc. In a perfect world everthing would be set correctly, but alas this is not the case. To use this as a reason not to have pump down just does not wash.

[US Iceman]

Comments regarding LP's cutting the compressor off at too low a pressure and leaking solenoids ARE NOT NORMAL. These are fault conditions and of course they will over heat the compressor, etc. In a perfect world everthing would be set correctly, but alas this is not the case.

And there is the basic premise of the point I was trying to make. By trying to understand the nature of the fault a person can develop a good plan of action for rectifying the issue. Obviously, if the leaking solenoids cause the LP's to short cycle the compressor the answer is to NOT adjust the LP to a lower pressure setting. Find the problem and fix it, not adjust something that reacts to the issue.

This is one of the reason why TXV's get adjusted more than they should be.

[Grizzly]

NONICKNAME.
May I say I appreciate your dilemma.
When working in the Cold Stores. I and many other Engineers used to despair at the number of idiots (sorry fork truck drivers).
That would hit the chamber doors off or even drive the forks through the doors!
The truth is it's a select few and not everyone should be tarred with the same brush.
Or the ambulance driver that regularly attends fatal accidents.
He is more aware and pays more relevance to it
Than you or I would!
Everyone is refering to motors not cooling.
So I assume an open drive is exempt?
I had never made the connection before.
But i guess that is why most chiller manufacturers have a three alarms and "lock out" Control strategy?
I am beginning to understand what you are telling us.
Based on your experiences, assisted very well by Iceman.
I would just like to add that once informed I try to use that knowledge to better my working practises.
But your original post makes me angry, because as an engineer.
I have never knowingly carried out adverse engineering practises.
Having said that, it is a very good topic.
But I can't see how it is easily resolved?
Grizzly

[NoNickName]

Thanks everybody for their participation in the subject, even those who reacted instinctively with offense. That's perfectly normal and human, and exactly what happened to tolemaic believers when galileans told them the sun was not revolving around earth.

I think what we are seeing here is an attempt to understand how much impact pump down cycles may have on compressors (and the motors).

Perfect. I think this is much more honest than just write in a small font at the end of the user manual "pump downs not allowed" and put the blame on the installing company.
Chances are that pumpdown harms the compressor, and if prohibited, warranty is void, or no pumpdown is performed, and compressor breaks for liquid floodback which is also not covered by warranty.
Instead than just promoting a stance pro or against, I wanted to feel the wrist of the users. I'm sorry that not being English my mothertongue, I may have been misunderstood.

So I assume an open drive is exempt?

Grizzly, it may be, but not being grand producers of open type compressors, I can't say.

I totally accept and understand Buffola Trident being adamant not to have a pump down on there evaps during defrost.

Goober, what I was taught, instead, is that defrosting is to be made with hot gas when the compressor is running, and by doing so, no liquid goes to the compressor.

pump down has been a tried and tested means to an end for a long time now

old gas bottle, what you are saying here is that your past is what makes you experienced, and I'm fine with that.
Besides I don't accept sentences like "I've being doing that for 20 years now, there must be a reason why" understating experience when in fact you could have possibly mistaking for the last 20 years, without even knowing. Or just a better way you didn't know it existed.
Actually, the most experienced engineers are the ones who are more reluctant in accepting corrections and improvements.

[Sandro Baptista]

Quote:

pressure inside the coil could rise so much that is bad for the compressor LP limits.

And how pump down has anything to do with that? I can't get the point here.

Before you stop to defrost you evacuate the coil as much you can. Close the EVR and mantain the ventilation running, if possible. If there is a amount of refrigerant inside the coil and due to that if during the the pressure rise above the KP1 regulation then the compressor will pumpdown during.


Quote:

It may have condensation of refrigerant in the suction cold pipe far from the evaporator

Ok, and how pump down will solve the problem? If you have liquid in the suction pipe, pumping down will just lengthen the floodback and liquid ingestion time.
This specific issue is correctly addressed by the simple installation of a suction P syphon and riser with a suction accumulator.

Nonickname, who did said that I would have liquid in the suction pipe? I do the pump down just no to have liquid.

Quote:

There is some amount of liquid inside the evaporator coil.

Why? Don't you have a liquid solenoid? Isn't your TXV gas tight?

Yes I have, but it will remain liquid in the coil and when the compressors starts it will pull gas but maybe it will carry droplets of liquid. Note that after a while the evaporator and compressor stops by cold store temperature achived the mixture of liquid and vapor will be in the two-fase region saturated liquid/saturated vapor »» without superheat.

Quote:

Then when the compressors starts it will pull gas but maybe it will carry droplets of liquid

Have you ever heard of suction superheating? This scenario is impossible, in a "properly maintained system"

Yes I have, but it will remain liquid in the coil and when the compressors starts it will pull gas but maybe it will carry droplets of liquid. Note that after a while the evaporator and compressor stops by cold store temperature achived the mixture of liquid and vapor will be in the two-fase region saturated liquid/saturated vapor »» without superheat.

Nevertheless just I have said before the subject you rise (Pump down and suction pressure switches: legacy?) is logical and correct in some points.

Regards

[modvalve]

.

And I have never seen a properly maintained system (or very few in honesty).
And yes, I have also seen a lot of motor overheats, especially by field engineers who start the compressor just after having evacuated the system, and before charging the refrigerant.
Q: Why are you doing this?
A: I don't have a service valve on the liquid line and don't want to flood the compressor with liquid. So I run it as I start charging. Pooof....

Hi Nonickname
I dont know what kind of standard of work you have in Italy but here in New Zealand we take pride in our work and generaly all systems are well maintained.

As far as overheats by engineers , who start the compressor just after having evacuated the system, and before charging the refrigerant.
I presume you are talking about little domestic compressors as all commercial /industrial systems would have a liquid line service valve

[NoNickName]

I presume you are talking about little domestic compressors as all commercial /industrial systems would have a liquid line service valve

No, not the case. You would be surprised how many. And I'm travelling the 5 continents.

here in New Zealand we take pride in our work

Who doesn't?

[modvalve]

Hi Nonickname

are you going to tell us what compressor manufacturer you work for? over here we mainly see Bitzer and Copeland in the commercial stuff and some NZ manufacturers use Refcomp on their water chillers

[nike123]

Hi Nonickname

are you going to tell us what compressor manufacturer you work for? over here we mainly see Bitzer and Copeland in the commercial stuff and some NZ manufacturers use Refcomp on their water chillers

http://www.refrigeration-engineer.co...29&postcount=5

[US Iceman]

So I assume an open drive is exempt?

Not entirely. An open drive motor (especially larger motors) have a limit of hot starts per hour you should heed. This is based on the ability of the motor to naturally cool by radiation, conduction, or natural convection when it is not running. Say a motor has been running for a long time at full load in a hot engine room. The compressor starts to unload and shuts off. When this occurs the motor simply cooks, which affects the life of the winding insulation.

There is a rule-of-thumb that states; for every 10°C rise in motor temperature the motor life is reduced by 50%! Therefore, from a motor perspective the motor would last longer if it was kept running so that the fan could continue to provide cooling of the motor windings.

In a hermetic or semi-hermetic motor there is very little natural cooling when the compressor is off. I suspect these might be more susceptible to shorter motor life for this reason.

[sterl]

I agree with US Iceman: Pump it down once, properly.

And I believe and on restart, manage the various flows and charges. What is involved does change from system to system, but most of it is control strategy & timing.

Our internal policy is for single-space or one-on-one applications:
-run all condenser fans/flows during pumpdown.
-NO diaphragm solenoids on liquid lines.
-low ambient duty, always manage liquid pressure & holdback from the discharge of compressor.
-Discharge piloted and checked oil return from separator or reservoir.

[NoNickName]

This may start a big argument.

http://www.refrigeration-engineer.co...95&postcount=8


Some engineers may agree, though.

[US Iceman]

http://www.refrigeration-engineer.co...95&postcount=8


Some engineers may agree, though.

I saw this developing into a contest.

I have seen the same thing with moisture in the system, improper evacuation, poor commissioning, and overall bad maintenance practice affect the durability and ultimate life of the compressor.

These are simply observations of what I have seen over the last 30-40 years and is not limited to one man businesses! As a manufacturer yourself you see a lot more of this than the person in the local business area since you have to work with the warranty issue.

[Peter_1]

http://www.refrigeration-engineer.co...29&postcount=5

It was up to NoNickName to respond to this question if he wanted to do this.

[nike123]

It was up to NoNickName to respond to this question if he wanted to do this.

I thought that if he already made statemant about that in public, that is no harm done by pointing to that.
But, if you thing that pointing affecting his privacy, or is rude, than you have all means to remove it!

[NoNickName]

I did neither complain nor comment. I simply think it is off topic to mention our companies. Anyone who is interested may just contact me privately off list.

[Peter_1]

Perhaps the Frascold envelopes are made to narrow or they should state that you can go out of it for a short time to pump down not exceeding some start/stops in 1 hour.

I installed hundreds of compressors and never one failed due to a pump down but several did due to not doing it.

Compressors connected to eutectic plates without suction accumulator and without pumpdown will brake within weeks.
It's the one or the other.

Agree, a suction accumulator can prevent a lot but why installing an additional item which can fail, drip, takes place, makes teh setup more expensive if years of practice has proven so many times that a pumpdown doesn't harm a compressor.

When pumping down, running amp's are seriously reduced, so is the heat formation of the windings also.
I'm almost sure a compressor is much more harmed when running with excessive SH instead of pumping it down 3 to 4 times/hour.

Bitzer even recommends pumping down, so just the opposite.

[NoNickName]

Peter, pump downs are by their same nature short in time. What I'm worried, and that worries all compressor manufacturer disregarding the colour, is "how long does it take to pump the system down?". It's not an easy question, it all depends on the volume of the evaporator and the tighteness of the LS.
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.
Too easy to put the blame on the compressor.


EDIT: opinions expressed in this forum pertain to me and not to my employer.

[Sridhar1312]

Pump down is essential where you want to shut down systems for seasonal variation and in many cases the oil migrated to evaporator can be recovered by pumping down only.
For all the refrigeration system ist is important especially where you have remote indoor and otdoor unit with lengthy piping

[US Iceman]

...is "how long does it take to pump the system down?".

And I might add; how long does the compressor operate in a vacuum during pump down? I have seen some systems where the service person set the controls to operate in a vacuum to provide a complete pumpdown.

I'm sure there are some limitations of the dielectric values for winding insulation that could add to the warranty issues for a manufacturer.

From a manufacturers perspective they have no control over how their compressors are applied to system designs. When someone tries a different system design (such as the one mentioned for cold plates) the compressor takes all of the punishment and ends up looking like a warranty issue, when it's not.

[old gas bottle]

well the problem being raised over this seems to point to far to many LP,S being set inncorrectly or moving after time or whatever, well there are plenty of preset switches on the market now,in some cases they could be fitted as a back up saftey or as a single opperating switch, i,am not againt this at all,maybe a test or overide button would be nice.surley not a lot else is needed to eliminate that part of the problem.

i realy cant see the pitfalls of a pump down system,the solinoid shuts down,the loads decreasing,the amps are droping,the condenser is blowing cooler air over a hard worked compressor.[in most cases] all good stuff , soliniod opens, x ammount of pressure hits the LP,away goes the commpressor upto speed before full load is introduced.

i like it and untill i see a better system or equipement that works and lasts doing it i,ll stick as i am. also stopping with round wheels too as they work fine for me.

[Billy Ray]

To be fair to any compressor manufacturer, incorrectly set LP Pump Down switch's do have negative effect on compressor life.

On numerous occasions (example - R404A medium temperature application) we see in the field LP Pump down control switch set to 2 or 3 psi, with a 10 psi differential!!

Why or why i cry. The poor old compressor is sucking a low pressure, which is not necessary. Generally at these low pressure's the oil starts to foam & leaves the compressor.

Additionally, no sooner has the compresor stopped, it restarts because on a low differential setting.

Setting a reasonble cut out point, reasonable differential, an addition of a compressor anti-cycle timer, pump-down is a tried & tested system which works very well.

Billy Ray

[Peter_1]

...When someone tries a different system design (such as the one mentioned for cold plates) the compressor takes all of the punishment and ends up looking like a warranty issue, when it's not.

USIceman, coldplates are for me just another type of evaporator, I don't see this as a different system design.

With every system you design, you have to know what to look for and how to set safety systems.

For example, coldplates remain very cold for hours, sometimes a whole day and gaseous refrigerant can migrate with a no running compressor to this plates and condens back in a liquid form.

We also used to install an SV switched together with the compressor. So there could not be liquid collecting in the plates: as soon the compressor stopped, the SV closed. But if the system is shutting down for a longer period, you anyhow could have liquid slugging.
So pumping down is a safe way of protecting the compressor.
Or a liquid separator.

If you set your LP below atmospheric pressure, then this is against all normal used rules. If your compressor looses its gas, then you will have the same problems.
It all stands or falls with a good and safe setpoint of the LP cutout and also protected for short cycling.

Like said in a previous post, Bitzer, not a small German compressor manufacturer even recommends it.

[US Iceman]

Peter, this is what I was thinking of.

You know how they start the compressor? Pulsing it until all liquid is digested through the compressor, 30 starts a minute.

That is a system design problem or very poorly set controls. As a result the compressors absorbs all of the abuse until it breaks.

I agree with you. Cold plates are only a different type of evaporator. Unfortunately some applications are designed with problems in them. When this happens it is almost always the compressor that is impacted. It is for all intents the only rotating piece of equipment within the refrigeration system.

[Peter_1]

I just edited my previous post a little bit.

[US Iceman]

I just edited my previous post a little bit.

What you described is how I would do it also.

It is not that the pump down is creating a problem for the compressors. It is how the controls are set or the people working on the system.