r410a Scroll Chiller Cooling Performance Problem-Expert advise please

Dear All,

I have the weirdest situation in my test bench for a 4x scroll compressor (Bitzer GSD80385) 2 circuit air cooled water chiller.

The design dT water in out is 5C. both circuits are equipped with identical elements, all piping guides are the same- same diameter etc nothing is different.

Both circuits are charged with 18kG of r410a. One of the circuit is giving excelent cooling capacity 2,5C. The other circuit is giving 0,7C of cooling. The superheat values are normal, EXV is modulating. I write the conditions of both circuits below:

circuit 1:
Water inlet:10C
Water Outlet: 7,5C
Evaporating Pressure: 7,1Barg
Discharge Pressure:23Barg
Superheat:7K
EXV opening state:%58
Subcooling:1K


circuit 2
Water inlet:10C
Water Outlet: 9,3C
Evaporating Pressure: 7,7Barg
Discharge Pressure:24Barg
Superheat:7K
EXV opening state:%95
Subcooling:1K

I have made also comparison for discharge - liquid line equipments pressure drop values;

Circuit 1
microchannel condenser inlet-outlet pressure drop: 1,0Bar
Dryer cartridge inlet-outlet pressure drop: 0,07Bar

Circuit 2
microchannel condenser inlet-outlet pressure drop: 1,5Bar
Dryer cartridge inlet-outlet pressure drop: 0,20Bar

First i replaced the compressors with the new ones as it was the only thing that made sense to me; nothing changed after replacement. After finding that i have excessive pressure drops on discharge line i thought about restricted condenser and dryer so i replaced both the condenser and the dryer cartridge with new ones.

Unfortunately the result is the same. 0,2Bar , 1,5 bar pressure drops, 0,7C cooling(almost 1/4 of required capacity).

I appreciate any advise and thoughts- really feel helpless

What type of evaporator do you have?
What is the water flowrates?

Dear Frank,

The evaporator is Shell&Tube type, and the chilled water flow rate is 50m3/h.

Regards

Quote:

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Originally Posted by AliCanGonc

Dear Frank,

The evaporator is Shell&Tube type, and the chilled water flow rate is 50m3/h.

Regards

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I think Frank is interested in balanced water flow rate through each evaporator.

In a lot of cases valves are installed so water flows can be balanced.
From what we read no2 cold have higher water flow rate, so evaporating pressure higher, condensing pressure higher, mass flow rate through machine & filter drier could be higher giving more pressure drop, TX valve open more.
Maybe No2 is doing more work making No1 look better.

Compressor amps on No2 would expect also higher.

Superheat can also be a bit a bit misleading in some cases as it can only reach highest value of water outlet temperature.In your case both are pretty close.
Can you confirm things with a touch probe, compare each, also where pressures on suction, are they read in identical position, no valves restricting anything etc.

If you can restrict water flow through No2 to match No1 conditions & see if any difference.
Also if units run individually with off unit isolated water flow, compare results.
Confirm TX valve has same orifice installed.

Dear Ranger,

Thank you for your detailed answer on the matter, it is an interesting point of view. Yes the No2 amp drawing is slightly higher than the no1.

i see that i did not share the information of the evaporator side pressure drops for each circuit. Please find below:
Circuit 1: 0,52Bar
Circuit 2: 0,38Bar

If your thesis is the case here, and the refrigerant flow through the evaporator tubes are higher in circuit 2, the pressure drop should also should be higher than circuit 1. Am i right, or i am not thinking this through correctly.

The suction pressure readings are available at the same mirrored position no change, only valves are available in liquid line dryer inlet and outlet.(both are fully open) We use EXV instead of thermostatic expansion valves, which are of same size/model. The shell and tube evaporator has 1 water inlet and 1 water outlet. the tube bundle inside has 2 seperate circuits. the water first encounters with circuit 1 and than passes to circuit 2. there are internal baffles to redirect the water flow inside the shell.

The results i share above is given when each circuit is individually working and the other circuit is off.

What temperatures do you get for condensor air on and off, and where are you measuring subcooling and superheat from?
Are all the test instruments calibrated?
Is it just a basic fridge circuit? Any pressure regulators or things like liquid injection or hot gas.

Agree with seanf,
Also filter drier no 2 with higher pressure drop, has it, or can you change to eliminate possibility of flash gas ahead of Expansion valve.
1 degC of subcooling seems a little low, do you have a sightglass in liquid line?
can you hear anything different with each system, turbulence, hissing sounds at expansion valve.

Dear Sean, Ranger,

This is a basic tandem compressor application. 2 compressors for each circuit. circuit is constructed from shell and tube evaporator, comressor(2 x scroll tandem) checkvalves on each compressor outlet(we checked the checkvalves nothing wrong on them, no restrictions) - condenser, liquid line valve-dryer -liquid line valve and electronical expansion vavlve. The subcooling is read on dryer inlet, the pressure is read for subcooling on dryer inlet. All instruments are calibrated(from testo). the anomaly is the pressure losses on the dryer and the condenser thats the thing i cannot understand the reason. We cannot increase the subcooling even charging extra refrigerant. We observe that the discharge pressure rises, subcooling does not improve even so.

Ali,
If you have not stated, does it have a liquid receiver & a sight glass on it, or in liquid line?

Dear Ranger,

Yes i have a sight glass on the liquid line, i see small bubbles but %90 liquid feeded to EXV

Is the flow in the sight glass fast flowing?
Are the pressures, temperatures and sight glass conditions reasonably constant or do they vary?
What are the condenser fans doing, and what are the temperatures for the discharge, condenser inlet and outlet, condenser air on and off?
You said the instruments where calibrated by testo, but have you checked they all read the same temperature or pressure over a range when all on the same circuit or at the same temperature?
Is it the same test instruments used when taking readings from circuit 1 as circuit 2?

Quote:

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Originally Posted by AliCanGonc

Dear Ranger,

Yes i have a sight glass on the liquid line, i see small bubbles but %90 liquid feeded to EXV

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Liquid receiver?

The only things I can think of with your symptoms are,

- System 2 short of refrigerant.
- no liquid receiver or low level in it. Has to have one in my opinion.
- change filter drier to eliminate pressure drop & possible flash gas after it due to little sub cooling.
- Expansion valve open more as starving for refrigerant.
- superheat 7 degC possibly fooling us due to it only being able to get a maximum of that, unless ambient can increase it through heat gain which does not apply in your situation on test rig.
- If system short of refrigerant condenser may behave differently & condense at a higher pressure.

Does not explain higher suction pressure but running out of ideas.

Are you able to decrease the water flow rate on circuit 2. It may just be a case of adjusting different water flow rates through the circuits. Your flow rate of 50m3/hr equates to 13.89 kg/s - is this on each circuit?

So,
Circuit 1 = 13.89 x 4.19 x 2.5 (145.5kW)

Circuit 2 = 13.89 x 4.19 x 0.7 (40.74kW)

Both circuits are way off your design dt of 5K, so I would be looking at the water flow rates

What is your design refrigerating capacity?

Dear Frank,

The evaporator is a single piece shellandtube- the water inlet and outlet is 1 in - 1 out. So all the water travels both circuits. I don't think that this is the case.

Quote:

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Originally Posted by RANGER1

Liquid receiver?

The only things I can think of with your symptoms are,

- System 2 short of refrigerant.
(if so, why the suction pressure is 7,2Bar- plus it is charged with same amount of refrigerant as other identical circuit)
- no liquid receiver or low level in it. Has to have one in my opinion.
(It is not equiiped with a receiver yes, it has very small piping, i do not apply liquid receivers to small distance piping systems as the EXV is not starving)

- change filter drier to eliminate pressure drop & possible flash gas after it due to little sub cooling. (I changed the filter drier, no change on the pressure drop,)

- Expansion valve open more as starving for refrigerant.
(Applied this and gave EXV a bigger opening range- reduced superheat setpoint to 5K no change on cooling efffect)

- superheat 7 degC possibly fooling us due to it only being able to get a maximum of that, unless ambient can increase it through heat gain which does not apply in your situation on test rig.
Correct, testing rig is insulated+indoors. No effect on the superheat.

- If system short of refrigerant condenser may behave differently & condense at a higher pressure.
(Agreed, but the suction pressure is on design conditions-The discharge pressure increases when we add on refrigerant, subcooling doesnt change much.)


Does not explain higher suction pressure but running out of ideas.
(Thanks for spending your time on the issue. i am also out of ideas)

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Dear Frank, Please find my notes over your list. Thanks

Quote:

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Originally Posted by seanf

Is the flow in the sight glass fast flowing?
Are the pressures, temperatures and sight glass conditions reasonably constant or do they vary?
What are the condenser fans doing, and what are the temperatures for the discharge, condenser inlet and outlet, condenser air on and off?
You said the instruments where calibrated by testo, but have you checked they all read the same temperature or pressure over a range when all on the same circuit or at the same temperature?
Is it the same test instruments used when taking readings from circuit 1 as circuit 2?

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Dear Seanf,

The pressure and temperature readings are reasonably constant. they are not fluctuating. Condenser fans are pressure transmitter controled. for steady readings i run all condenser fans to keep the 24-25bar condensing pressure reading.

Unfortunately i did not measure the discharge temperature of the compressor. Do you think that these symptoms might be caused by a faulty compressor that generates excess discharge superheat on the discharge line?

The instruments that are used are the same for circuit 1 and circuit 2. So even if there is a calibration problem persay, it would be the same condition for both circuits.

Quote:

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Originally Posted by AliCanGonc

Dear Frank,

The evaporator is a single piece shellandtube- the water inlet and outlet is 1 in - 1 out. So all the water travels both circuits. I don't think that this is the case.

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That does'nt alter the flow rate - it is still at 50m3/hr. or 13.89kg/s over each circuit.

It would be interesting to know if your removing a large amount of heat at the condensor.
Do the condensor fans on circuit 2 run the same speed as circuit 1 while running the test? do they cycle quicker / run for longer?

I take it the compressors are just fixed speed?
Do the compressors pull down the suction pressure well? maybe see how they perform when restricting the liquid line valve.

I think Frank's idea of varying the evap load will probably give you some good information.