screw compressor failure

Hello everybody,
I have a big problem about our chiller units (water cooled evaporator, water cooled conderser and R22; screw compressors).
We have a chiller unit with water cooled condensers and the unit have 3 independent circuits with semi-hermetic screw compresors. At that moment two compressors were burnt and only one compressor is runing. One of them these burnt compressors was failure at start up , and it worked only 5 minutes. The other one was failure in five days after start up.
In attached file you can find the measurements for running compressor. Could you please give me answers of questions?

1- According to attached file, I think superheat is very low... Is there a flood back problem in the system?

2- If txv is adjusted again, can this problem be solved?

3- How can i understand oil migration? (These compressors have oil sperators)

4- Is there any other reasons for burnt compresors?

Thanks for your comments and helps.....

Regards

There are two major reasons for burnout.

electrical problems, check the power circuit.

Something inside the compressor brakes and caused a short.

If the system was OK before, I don't see how sudden changes can happened.
Also could be some problems with the controls.
What make is it?
How old?
What HP?
Chemi :)

This unit is new, three months ago it was start up... In start up first compressor was burnt out and second compressor was also burnt one week later from start up.

HP value (from attached file "2.point") is 261psi measured from high pressure side gauge.


Regards

Aygul
Here are somes things to consider when asking the question “what can cause a burnout”?
Overheating problems occur when oil in a compressor is heated to the point where it loses its ability to lubricate. ( is the oil cooler keeping the oil below 225*F and is it leaking water into the oil?) If the heat is high enough, the oil breaks down chemically and if there is any moisture with hi discharge temperatures , this creates acid. Major reasons for overheating due to discharge temperatures are:
· Low suction pressures
· High condensing pressures
· High compression ratios
Low suction pressure is normally the result of incorrect pressure switch settings, pressure drop in suction line, light load operating conditions or restricted evaporator coils.

High condensing pressures can be caused by inadequate waterflow through the condenser, and overcharge of refrigerant or noncondensables in the system.

High-pressure ratios are a combination of low suction pressures and high condensing pressures. If the compressor is operated within the manufacturer guidelines, this condition will not cause a problem. Then there is also the possibility that the 3 phase power is single phasing . Do you have single protection on these compressors? If and when you solve the cause you also need to know the proper procedure for cleaning up these burnouts so you don’t have any more failures.
:)

1-
These compressors have part winding start.
Winding measurements:

1. compressor:
L1-L2: 22,5 ohm
L1-L3: 0,9 ohm
L1-L4: 22,3ohm
L1-L5: 0,6ohm
L1-L6: 22,2 ohm
burnt compressor

2.compressor:
L1-L2: 3 ohm L2-L3: 0,8ohm L3-L4: 1,2ohm
L1-L3: 2,4 ohm L2-L4: 0,6ohm L3-L5: 16,2ohm
L1-L4: 3,3 ohm L2-L5: 16,3ohm L3-L6: 0,3ohm
L1-L5: 18,2 ohm L2-L6: 0,9ohm
L1-L6: 2,4 ohm
burnt compressor

2-
What do you thing about superheat and subcooling values for running compressor:
superheat: 3C
subcooling:10C

I have to increase superheat, haven't i?

3-
In addition to that, i want to ask another question...
What do you thing about oil analysis test?... Can test results help us?

Thanks & Regards
:)

Hi Aygul.
How do you check superheat and subcooling if compressors are burnout?

What make is the unit?
Why don't you get the dealer to check the unit? If its new, you are under warranty.

Chemi

Not used to working on large chiller sets or with ambients outside of the UK, but is the discharge right ? 265psig seems very high to me for R22. :confused:

265psi is not very high.
Very high is over 350psi.

You have to remember that the ambient in some places' stays at 35C >40C all summer.

If you get some temp records from last summer in the UK, all of the units that somehow managed to work without a hose in the condenser' worked at these pressures and higher.

Chemi :)

I receive these values from running compressor (third one)... This unit is using for a/c.

regards

Aygul.

What is the make of the unit? i.e. carrier or Trane or what?
What model?
Check the tension and the currant draw, is it the same as it says on the unit?

I think this unit isn't made of Carrier , Trane etc... It is made of a local company. It had been bought one or two years before but it's start up is on June 2004.
Running compressor - drawn current: 108-101-101 amp (aproximately same as compressor cataloque)
There is no any voltage unbalance...
water cooled condenser - water inlet outlet: 29/38C
water cooled evaporator - water inlet outlet :10/12C

Regards

The only way I see it if a compressor is burnout at startup is a mistake in power wiring.

Check this out.
A YY start should not have any problems but if on the second Y two cables got changed in place, the result is instant burnout.

Chemi

Marc,

If it is a mistake in the power cables, no motor protection in the world is fast enough to cut the power.

Chemi

aygul: If the starting tourque is very hi as Mark suggests than you may have the wrong application of starters for those compressors, please read the following and pay particular attenion to the last paragraph
Part Winding Starter: This starter is not a true reduced voltage starter. The Part Winding starter is designed for motors which have two separate sets of windings, and the starter energizes half of the motor's windings with full voltage during starting and then the other half of the windings for a run condition.
Typical starting current is 50-65% of the motor's full voltage inrush current and starting torque is approximately 50% of what would be developed at full voltage.
This starter is the least expensive of the reduced voltage starters, however, there are several limitations which restrict its use. As stated, the motor must have two separate windings, which is standard on dual voltage (230/460V) motors, but these motors can only be started part winding on the lower voltage (230V). Some single voltage motor designs can be started by part-winding method, but usually a special motor must be built (therefore not a common distributor stock motor). Having a special wound motor for part-wind starting would cause the end-user or distributor to stock a special motor, increasing inventory cost, or there would be a possibility of the equipment being down while a new motor is made or the existing motor is rebuilt.
Another restriction to the application of a Part Winding starter is that standard motors have an allowable acceleration time (only half the windings energized) of one to two seconds (maximum three seconds) based on the motor design.
Applications - Only loads which have low starting torque requirement such as low inertia fans and blowers, and some compressors designs can be started using part-winding starter method. The applications are limited by the simple fact that the load must be able to be accelerated from zero to full rated speed in only two seconds, with only half the motor capacity. If the load cannot reach full speed before the second winding energizes, the motor's torque and inrush current will jump to the full voltage values and defeat the purpose of the starter. Essentially the Part Winding starter is used for only two things; it is the least expensive of the reduced voltage starters and it meets the Utility Company's requirement of ``Motors over `X' horsepower must be started by reduced voltage. So if you are too long in start you may be drawing excessive motor current :)

[QUOTE=Marc O'Brien]Lol, Chemi, as they say in England "Bollocks!!"

Marc, is that like baloney? as we in the colonies understand it? :rolleyes:

Thanks for your comments. As i see your answers, you are thinking electrical problems... Why don't you think mechanical problems? When i look the superheat and subcoolings values, i see that superheat is low and subcooling is high. In addition to that in burbt compressors when i looked compressor sight glass, i didn't see oil. I don't know perhaps oil colour was changed so i didn't see it.

First compressor was burnt after 5 minutes from start up and second one was burnt after 5 days from star up....

What do you think of for mechanical problems?
These compressors are Bitzer compaq screw type compressors and refrigerant type R22.


Thanks and regards

Hi Aygul.
Now you are telling a different story. OK.
Have you checked with the compressor rebuilds what was wrong?
Was there oil in the compressor?

If the unit was still for a long time, I would go for mechanical or electrical problem.

Marc. You are wrong but why not try in your work shop, use a 50HP YY compressor and connect rhe second Y wrong.
Use the best motor protection and see for yourself.


Bollocks - is a well known term but will not do for this case.

Chemi :)

Hi aygul :)
the start up failure, you didn't run it in reverse by any chance :confused:

Refrigerant Charge
I would remove some until a decent discharge temperature is acheived say 60 to 70 deg C. Then investigate the sizing of the expansion valves, then their set up. If the system is common to all compressors you will have too little gas for all if you decant to allow one compressor to run, but refrigerant can be added later when the fault is fixed.

Kind Regards. Andy :)

We once had a burn-out with Bitzer screws on a Profroid pack.
REason?
We never knew it because we were not allowed to open the compressor due to warranty reasons.
We always tought of a copper debris in the tubes after cutting the tubes. Thus copper particle went a high speed in the suction and made a contact with the motor windings.

Even with a high load, motor can't burn that fast.

There must be other reasons.
Injected oil not too hot?
Oil cooler working properly?
Oil acid?
Voltage OK?
Running direction OK?
Starting at lowest stage (valve retracted completely)?
If you don't exceed max allowed LP and HP at startup, then it should be OK.
If it's Y-Y starting, then I think you know you may not set the max allowable AMP on your current protection device. Power is then divided over 6 wires instead of 3. There must be setted a much lower AMP than max allowable AMP's.

Hi Aygul,if you have a megger tester you'll be able to see if the fault is electrical.I worked in Turkey and i noticed that the voltage tends to be low during certain periods of the day.Also it would be good practice to check the contactors.We have a big customer in Turkey and i've seen that low voltage was the cause of many faults around Istanbul,Adana,Izmir.
bye arkadas
Leo

Hi to all,

I just hope to give some lights from my experiences.

In Philippines, I came across Frick screws on York chillers. They are open drive water chillers and they don't breaK easily, although the microprocessor controller is quite troublesome.

Here in KSA, we encountered a lot of Bitzer screw breakdown e.g. burn-out, noisy, high amps, lubrication failure and overheating.

This compressor oftenly encounters trouble when used with R-22 systems. But with R-134a, they seems to be fine.

I have observe that this compressor is sensitive to high pressures, heat, flood back and migration. Maybe worse than reciprocating.

On particular example was a worn-out ballbearings after some events of floodback. The bearing causes a misalignment of the screw profiles that leads to repeated trip on thermal overload and eventually burn-out.

This compressor does'nt have an OPS for oil dilution or oil return failure protection.

In Ayub's case, there could be a failure on oil return as he mentioned of not seeing the oil level on the sight-glass.

He also mentioned of a low superheat. It could be a symptom of ungoing floodback and the liquid pushes the oil up above the sightglass unable to be determined.

This compressor if used with R-22 requires, liquid injection and adaptive discharge pressure contol to work well.

In addition, we've repaired some of this compressors with some cases as ruptured internal oil filter, scratch screw profile, burn motors and bearings.

The recommended oil is Ester oils BSE 170 for R-22.
Discharge gas temp. is from 60 to max. 120 deg C.

mabuhay!!!!! :)

Hello,

Burnt compressors "phase to phase recistances,ohm"

Comp.1

L1-L2: 30,8
L2-L3: 30,6
L1-L3: 1,1
L4-L5: 30,6
L5-L6: 30,5
L4-L6: 0,6
there is a short circuit from wiring to the housing of compressor.
Both first and second windings were burnt.

Comp.2
L1-L2: 2
L2-L3: 1,4
L1-L3: 1,4
L4-L5: 1,4
L5-L6: 17,9
L4-L6: 18,4
Second winding was burnt.

Running compressor datas

Comp.3.
pf
wind1:0,83
wind2:0,825
kw
wind1:29,29
wind2:30,98
KVAR
wind1:19,63
wind2:21,2
KVA
wind1:35,25
wind2:37,53
currents
L1:55A L2:49A L3:55A L4:53A L5:48A L6:54A

There is no supply voltage imbalance in the system...

What do you think of electrical datas?
When i look the discharge temperature, it is not too high. If the discharge temp is high, i can say this high temperature cause changing of oil properties but i thing temperatures are ok.
We will oil analyse to understand oil situation.
How can i understand "fload back" with or without open compressors?

These compressors have a ptc in motor winding and discharge gas temperature sensor as standard and in addition to that direction of rotation protection. these compressors have these kind of protections but they didn't prevent them against burnt out.

In these compressors there is a suction side filter (filter mesh 25 microns). I think it's protect the compressor against the metal scraps.

System refrigerant is R22. Cooltrain, Could you give me some information about "adaptive discharge pressure contol"? These compressors have a discharge temperature protection, and this system has a high and low pressure pressostate on each compressors. Are these protections not enough?

Thanks for your helps... bye...

Hi Aygul,

It seems that the compressor has "spot-burn" condition. Only part of the motor is affected right?

Mostly, this problem is a result of metal debris that physically cut the winding. The debris may comes from worn metal parts and carried by oil to the motor side. These debris cannot be trap by the mesh filters at the suction side of the comp'r.

Liquid refrigerant floodback could be the culprit. The running conditions of low load and low ambient as in your case, can create this problem if the superheat was not controlled properly or the EEV or TXV loses its capability to control the superheat.

Although screw profiles are tolerant to liquid slugging and floadback, the oil can still be diluted by the liquid R-22 and fail to lubricate properly.

It is still safe to apply 15 to 20 degrees F of superheat at compressor suction to prevent floodback.

The oil separator can be opened for inspection and the internal oil filter can be check for metal particles.

One more thing is to check the frequency rating of the burn compressor. It could be different than the supply. We have applications for 60HZ. & 50HZ. The supply and rated voltage must be compatible too.

"DISCHARGE ADAPTIVE CONTROL" is a method done by the controller to unload the screws when the preset temperature which is deemed critical is reach. Basically, the controller will throttle the compressor capacity within safe discharge pressures. It is plainly opposite to the function of the FCS, fan-cycling-switch.

Hope this help.

Mabuhay!!!!

Quote:

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265psi is not very high.

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We consider it high for a chiller with watercooled condensor even in summers with ambient tempreture of 42d C in fact we set HP setpoint at 260PSIG.
Regards
Tejbir Singh