Low pressure trip !!

[oilzee]

Hello everybody , yesterday i troubleshooted 1 unit Blast freezer 20Hp compressor because it has low pressure trip . i realized the system has no leak and sight glass looks clear . Room temperature can cold down to -25°c ,but once the temperature reach to -27°c low pressure will trip . The system has suction filter core and pressure regurator .Is it suction filter core block ?

[nike123]

What is cut-out setting of low pressure switch? Did you checked correct functioning of low pressure switch?
What is refrigerant, and what is temperature difference at suction filter in and out?
What is suction line temperature and suction pressure?

[Grizzly]

Oilzee
You mention a suction filter.
Or do you actually mean a strainer.
The reason being it is not normal to run with a suction filter. (Or at least most of the systems I work on are this way!)
Unless the system has suffered from a hermetic / semi-hermetic motor burn out.
Or is a new install?
Even then most manufacturers will state a recommended installation time prior to removal.
So your closing question would seem logical.
If this is irrelevant then answers to Nike's questions would be helpful!
Grizzly

Yours may well be an installation that requires it?

[oilzee]

What is cut-out setting of low pressure switch? Did you checked correct functioning of low pressure switch?
What is refrigerant, and what is temperature difference at suction filter in and out?
What is suction line temperature and suction pressure?

low pressure cut out setting is 2psi
the system is using R507
i haven't check the temperature difference at suction filter in and out .
when room temperature is -25°c ,low pressure before pressure regurator is 10 psi
This system has been running for 3 years but has not create any problem , only this few weeks . thank for reply .

[oilzee]

Oilzee
You mention a suction filter.
Or do you actually mean a strainer.
The reason being it is not normal to run with a suction filter. (Or at least most of the systems I work on are this way!)
Unless the system has suffered from a hermetic / semi-hermetic motor burn out.
Or is a new install?
Even then most manufacturers will state a recommended installation time prior to removal.
So your closing question would seem logical.
If this is irrelevant then answers to Nike's questions would be helpful!
Grizzly

Yours may well be an installation that requires it?

Yes ,the system using danfoss filter core DA48 , there is no body tell me that we need to remove the strainer after period of time .

[oilzee]

Thanks for help . i m going to check the suction filter strainer by tomorrow .

[nike123]

Did you checked correct functioning of low pressure switch?

Answer to this is...?

[Grizzly]

Yes ,the system using danfoss filter core DA48 , there is no body tell me that we need to remove the strainer after period of time .

oilzee.
As far as I can tell from the Danfoss site these DA48 are std filtercore driers.
Of the type that are used in the liquid line after the condensor and before the expansion valve or reciever etc.
As I mentioned before Suction filter cores are temporary.
Therfore either replace them if you are not sure or if you can confirm this.
Remove them completely. (I have seen suction core driers left in a system for very long periods(accidentaly) before they affect a systems performance.)
But please also consider what Nike is asking. Ie is the lp switch working correctly?
Grizzly

[oilzee]

Answer to this is...?

Yes , i have tested the high low pressure switch , its trip when pressure drop to 2 psi .

[oilzee]

i m so sorry , i gave the wrong infomation . the suction filter core is DF -48

[oilzee]

can i say , when the pressure lower and lower it has not enough pressure to past through and back to compressor ?

[nike123]

What is suction pressure when low pressure trips.
Is it 2 psi? What is suction pressure at compressor when you have -25°C room temperature. Is that temperature also at evaporator inlet? What is TD of evaporator?

[nike123]

You say that you have pressure regulator. What is his purpose?

[oilzee]

What is suction pressure when low pressure trips.
Is it 2 psi? What is suction pressure at compressor when you have -25°C room temperature. Is that temperature also at evaporator inlet? What is TD of evaporator?

i only measure the low pressure before pressure regulator ( 2 psi trip)not pressure at compressor . i have not measure the temperature at evap.inlet . No idea for TD select ,because my boss ask the supplier to design .

[oilzee]

You say that you have pressure regulator. What is his purpose?

purpose is Hot gas by pass.

[oilzee]

when the room temperature is -25°c pressure before the pressure regulator is 10psi

[oilzee]

thanks Nike and grizzly i got to go now . tomorrow is a new day , new advanture .good 9 !!

[nh3wizard]

Someday there might be an answer to what the pressure is at the compressor!!!!!!!!

[Grizzly]

thanks Nike and grizzly i got to go now . tomorrow is a new day , new advanture .good 9 !!

Sometimes I do wonder!!!!!
Grizzly

[nike123]

when room temperature is -25°c ,low pressure before pressure regurator is 10 psi

This is not logical!
If your pressure regulating valve is for hot gas bypass regulation than pressure before valve should be simmilar to discharge pressure.
Pressure after valve should be at adjusted valve pressure or higher!

But, as we newer get information on compressor suction pressure, we cannot go further with deduction!
Maybe, if we have some magic ball, so popular here!

[boy]

oilzee,

please get high and low press reading.
since you get 10 psi on the suction line
and you have a neg 25 centigrade temp if that blast have a thermostat control better check, it may de energizing the solenoid valve.

regards,

[oilzee]

This is not logical!
If your pressure regulating valve is for hot gas bypass regulation than pressure before valve should be simmilar to discharge pressure.
Pressure after valve should be at adjusted valve pressure or higher!

But, as we newer get information on compressor suction pressure, we cannot go further with deduction!
Maybe, if we have some magic ball, so popular here!

i just check out the parts name call Evaporating Pressure Regulator ( Danfoss parts no : 034L0032)
. Its fix just after suction accumulator. Suction accumulator outlet is 2-1/8" ,unfortunally Danfoss didn't have 2-1/8 Regulator,what we did was select 3x1-3/8" press. regulator and fix it before compressor suction shut off valve .when hot gas defrosting low side pressure at COMPRESSOR show 40 psi .

[nike123]

That is KVP (Evaporation pressure regulator NOT HGB VALVE) and I don't see practical reason to be in your system, but I may be wrong.

Sorry, in fact, there is practical reason to that valve and it is to keep suction pressure low, in order to not overload compressor at start-up or after Hot gas defrost. Therefore, it should be crankcase pressure regulator KVL and not evaporator pressure regulator KVP even their function is "simmilar".

Its Open on Rise of Inlet pressure function (therefore Close on Fall) instead Close on Rise of Outlet pressure, is probable reason for your low pressure switch action!

Check this fitter notes and this article.

[oilzee]

oilzee,

please get high and low press reading.
since you get 10 psi on the suction line
and you have a neg 25 centigrade temp if that blast have a thermostat control better check, it may de energizing the solenoid valve.

regards,

Cooling Solenoid valve checked ok . its open when suction pressure below 30psi, we use low pressure switch to control cooling solenoid valve )

[oilzee]

Please tell me what should i do ? i get ready to take away the suction strainer . i hope by tomorrow if allow .

[nike123]

Please tell me what should i do ? i get ready to take away the suction strainer . i hope by tomorrow if allow .

I think that you should change that pressure regulators with one crankcase pressure regulator from some other manufacturer who has bigger capacity regulator. I am not aware of such. Maybe my colleagues from industrial refrigeration knows what to recommend and confirm my recommendation or suggest another method of regulating crankcase pressure.

[oilzee]

( post by NIKE ) : it should be crankcase pressure regulator KVL and not evaporator pressure regulator KVP even their function is "simmilar".

Thanks again , i will come back to u by tomorrow ( i hope client allow , this blast freezer room has 4x20hp bitzer and 2x10hp bitzer ,production running 24hours )

[oilzee]

[quote=nike123;124362]I think that you should change that pressure regulators with one crankcase pressure regulator from some other manufacturer who has bigger capacity regulator.

What happen if not change ?

[nike123]

For 20T of refrigeration capacity you will need 8-9 x KVL28-35 if your suction pressure is set to 40 psig. That is impractical and impossible to adjust.
You need one of pilot operated pressure regulator like PM from Danfoss.
http://www.ra.danfoss.com/TechnicalI....HL0.A3.02.pdf

[nike123]

[quote=oilzee;124365]

I think that you should change that pressure regulators with one crankcase pressure regulator from some other manufacturer who has bigger capacity regulator.

What happen if not change ?

It will keep tripping on low pressure when ever evaporator pressure/temperature drops beyond that set at wrongly selected pressure regulating valve.
Who has installed that valves and for what reason? Are they are on other correctly working identical systems?

[nike123]

what we did was select 3x1-3/8" press. regulator and fix it before compressor suction shut off valve .

Ok, I see now! What is reasoning behind this? What was original solution?

[oilzee]

today early morning i open suction filter shell , surprisingly there was no strainer in it . then i changed liquid line filter drier and released approximate 3 litres of compressor oil .

when the compressor running i got the following data ,( low pressure took at compressor )

when Low press. = 10psi, air temperature at evaporator outlet -23°c and suction temperature at evaporator outlet = -20°c sight glass clear .

[oilzee]

What was original solution?

we have not do any modification , these are the system supplier designed

[oilzee]

[quote=nike123;124368]

Who has installed that valves and for what reason? Are they are on other correctly working identical systems?

we installed that valve according to supplier design so we had 1 year warranty for the compressor .
For me this system was first time i did .For them , I have no idea .

[oilzee]

can i say , it was too much compressor oil in the system and affected low pressure trip !!

[nike123]

can i say , it was too much compressor oil in the system and affected low pressure trip !!

Did you checked oil level after you have full load for some time to allow proper oil return?

[oilzee]

Did you checked oil level after you have full load for some time to allow proper oil return?

after i released the oil yesterday , its maintain at 3/4 full level ( half and hour running , full load )
but today i checked it was almost full .( no load )

[oilzee]

i m going to release some more oil by next tuesday and monitor the system .

[nike123]

What is that compressor model#?
Am I correct when I say that you actualy have compressor pack comprised of 4x20Hp and 2x10Hp which together feeds evaporators in blast freezer thru 1 common liquid line.
Or I get something wrong?

[oilzee]

What is that compressor model#?
Am I correct when I say that you actualy have compressor pack comprised of 4x20Hp and 2x10Hp which together feeds evaporators in blast freezer thru 1 common liquid line.
Or I get something wrong?

i m verry sorry because my infomation provided not clear
actually our system has one compressor for one evaporator , that mean 4 sets of 20Hp refrigeration system ( 4 x liquid line , 4 x Suction Accumulator )

So one system has 3 regulators then 4 system have 12 regulators .

Compressor Model : 4G-20.2-40P( compressoroil already changed to ester oil )

Evaporator : one unit is 29.8kw@6KTD

[nike123]

Tel me then, please, do you have more than one evaporator at one compressor circuit (two or more maybe)?
I ask that because use of KVP will be required, if later is the case.

[oilzee]

Tel me then, please, do you have more than one evaporator at one compressor circuit (two or more maybe)?

one compressor circuit has only one evaporator .

( 2 x 10Hp compressor have 2 evaporators as well , using heating element to defrost .)

[Andy]

i just check out the parts name call Evaporating Pressure Regulator ( Danfoss parts no : 034L0032)
. Its fix just after suction accumulator. Suction accumulator outlet is 2-1/8" ,unfortunally Danfoss didn't have 2-1/8 Regulator,what we did was select 3x1-3/8" press. regulator and fix it before compressor suction shut off valve .when hot gas defrosting low side pressure at COMPRESSOR show 40 psi .

Hello oilzee

Danfoss make regulators up to 5", your were only looking at the commercial range.

I would expect three valves would be a little difficult to set up.

Kind Regards Andy

[oilzee]

Hello oilzee

Danfoss make regulators up to 5", your were only looking at the commercial range.

I would expect three valves would be a little difficult to set up.

Kind Regards Andy

Hi Andy , you mean the regurator not suitable for these system ?thanks for reply

[Andy]

Hi Andy , you mean the regurator not suitable for these system ?thanks for reply

Hello oilzee,

in a word yes.

To allow you to set the system up properly I would advise fitting one regulator.

Kind Regards Andy

[oilzee]

Hello oilzee,

in a word yes.

To allow you to set the system up properly I would advise fitting one regulator.

Kind Regards Andy

if i fix one regulator , what would it be the result ? can we control the low pressure at 20psi when hot gas defrost . supplier wants us to control the low pressure below 20psi but quite difficult . its up to 40psi when defrost

[Andy]

if i fix one regulator , what would it be the result ? can we control the low pressure at 20psi when hot gas defrost . supplier wants us to control the low pressure below 20psi but quite difficult . its up to 40psi when defrost

Hello oilzee

what your are after is a crankcase pressure regulator.

This will keep the suction pressure below 20 psig when you need to.

The problem you are having is because the regulators themselves cause a pressure drop, this is necessary for them to operate, but in this case the pressure drop is excessive, because you have used multiple valves, which are possibly under sized.

Kind Regards Andy

[oilzee]

Hello oilzee




The problem you are having is because the regulators themselves cause a pressure drop, this is necessary for them to operate, but in this case the pressure drop is excessive, because you have used multiple valves, which are possibly under sized.

Kind Regards Andy

Hello Andy ,
thanks for your concern . i think i m going to seach and get more infomation for that valve at local danfoss distributor .

Thanks again who help me in this forum, especially Nike .