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Thread: Low pressure trip !!
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29-10-2008, 02:57 PM #1
Low pressure trip !!
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 ?
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29-10-2008, 03:02 PM #2
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Re: Low pressure trip !!
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?
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29-10-2008, 03:12 PM #3
Re: Low pressure trip !!
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?
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29-10-2008, 03:22 PM #4
Re: Low pressure trip !!
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 .
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29-10-2008, 03:32 PM #5
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29-10-2008, 03:51 PM #6
Re: Low pressure trip !!
Thanks for help . i m going to check the suction filter strainer by tomorrow .
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29-10-2008, 03:54 PM #7
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29-10-2008, 04:09 PM #8
Re: Low pressure trip !!
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
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29-10-2008, 04:11 PM #9
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29-10-2008, 04:14 PM #10
Re: Low pressure trip !!
i m so sorry , i gave the wrong infomation . the suction filter core is DF -48
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29-10-2008, 04:24 PM #11
Re: Low pressure trip !!
can i say , when the pressure lower and lower it has not enough pressure to past through and back to compressor ?
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29-10-2008, 04:25 PM #12
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Re: Low pressure trip !!
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?Last edited by nike123; 29-10-2008 at 04:28 PM.
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29-10-2008, 04:36 PM #13
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Re: Low pressure trip !!
You say that you have pressure regulator. What is his purpose?
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29-10-2008, 04:50 PM #14
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29-10-2008, 04:52 PM #15
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29-10-2008, 05:02 PM #16
Re: Low pressure trip !!
when the room temperature is -25°c pressure before the pressure regulator is 10psi
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29-10-2008, 05:17 PM #17
Re: Low pressure trip !!
thanks Nike and grizzly i got to go now . tomorrow is a new day , new advanture .good 9 !!
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29-10-2008, 06:31 PM #18
Re: Low pressure trip !!
Someday there might be an answer to what the pressure is at the compressor!!!!!!!!
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29-10-2008, 09:25 PM #19
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30-10-2008, 11:52 AM #20
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Re: Low pressure trip !!
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!Last edited by nike123; 30-10-2008 at 12:03 PM.
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30-10-2008, 12:46 PM #21
Re: Low pressure trip !!
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,
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30-10-2008, 12:52 PM #22
Re: Low pressure trip !!
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 .
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30-10-2008, 01:11 PM #23
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Re: Low pressure trip !!
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.Last edited by nike123; 30-10-2008 at 02:49 PM. Reason: Some corrections and aditions
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30-10-2008, 01:12 PM #24
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30-10-2008, 01:19 PM #25
Re: Low pressure trip !!
Please tell me what should i do ? i get ready to take away the suction strainer . i hope by tomorrow if allow .
Last edited by oilzee; 30-10-2008 at 02:43 PM.
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30-10-2008, 02:55 PM #26
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Re: Low pressure trip !!
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.
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30-10-2008, 03:07 PM #27
Re: Low pressure trip !!
( 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 )Last edited by oilzee; 30-10-2008 at 03:09 PM.
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30-10-2008, 03:12 PM #28
Re: Low pressure trip !!
[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 ?
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30-10-2008, 03:34 PM #29
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Re: Low pressure trip !!
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.pdfLast edited by nike123; 30-10-2008 at 03:43 PM.
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30-10-2008, 03:48 PM #30
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Re: Low pressure trip !!
[quote=oilzee;124365]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?
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30-10-2008, 03:52 PM #31
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31-10-2008, 03:32 PM #32
Re: Low pressure trip !!
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 .
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31-10-2008, 03:36 PM #33
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31-10-2008, 03:45 PM #34
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31-10-2008, 03:53 PM #35
Re: Low pressure trip !!
can i say , it was too much compressor oil in the system and affected low pressure trip !!
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01-11-2008, 08:43 AM #36
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01-11-2008, 02:20 PM #37
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01-11-2008, 02:35 PM #38
Re: Low pressure trip !!
i m going to release some more oil by next tuesday and monitor the system .
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01-11-2008, 04:53 PM #39
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Re: Low pressure trip !!
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?
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02-11-2008, 06:00 AM #40
Re: Low pressure trip !!
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
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02-11-2008, 06:37 AM #41
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Re: Low pressure trip !!
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.
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02-11-2008, 07:11 AM #42
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02-11-2008, 10:55 AM #43
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02-11-2008, 11:27 AM #44
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02-11-2008, 11:34 AM #45
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02-11-2008, 11:51 AM #46
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02-11-2008, 09:53 PM #47
Re: Low pressure trip !!
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 AndyIf you can't fix it leave it that no one else will:rolleyes:
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03-11-2008, 03:27 PM #48
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