Low superheat and sucooling

[jayaeros]

Hi All,

Recently we bought a small water cool chiller, 15.5 kW. We modified the chiller refrigerant pipes in order to install thermal wells and ref flow meter so that the refrigerant temperature and pressure profile can be studied.

Before the modification, the superheat and sub cooling was 5 deg K. I measured the wall temp at the suction pipe and then subtracted it with the saturation temperature corresponding to the suction pressure. The cooling capacity back then was 15.5 kW the suction was 60 psig and discharge was 220 psig.

But after the minor modification, the cooling capacity = 15.2 kW, suction = 60 psig and discharge= 220 psig)
but the superheat and sub cooling diminished to less than 1K.

I have tried to increase the superheat by closing the TXV. As a result, the was a fluctuation in all the refrigerant temperature readings and the flow meter reading fluctuates about 20%. However, the readings are quite stable, if i open the TXV but the super heat and subcooling diminished.

As for the measurement on the water side, the chill water leaving temperature (6.7 C) and cooling water entering water (29.4) were pretty stable although the obvious fluctuation is noticable in the refrigerant temp profiles when the TXV is closed to increase the superheat.


What could be the problem? Please help. Thanking you all in advance,

Regards,
jayaeros

[desA]

A few pictures of before & after the modification would help the distance-challenged to understand exactly what happened.

What kind of refrigerant flow-meter do you have? Pics & specs would assist.

[Gary]

To begin with, adjust the TXV back to it's original setting. The TXV is NOT the problem.

[jayaeros]

i'm using Krohne flow meter for R22 and all the temp sensors are 1/3 DIN 4 wire RTD.

I will try to get the photos uploaded soon. All the thermowells are installed at the elbows with a bigger T section so that the flows are disrupted. The initial charge advised by the manufacturer was 4 kg but after modification we charged 1 kg more as the we cant get the cooling capacity at 4kg.

[jayaeros]

To begin with, adjust the TXV back to it's original setting. The TXV is NOT the problem.

How to determine what is the original setting? We have been adjusting the expansion valve couple of time and now i could not recall the original setting. I have called up the supplier to give me the catalogue of the txv but till no avail.

[D.D.KORANNE]

Check tex vallve bulb location & if it wrapped reasonably tight on sction line

[The Viking]

If your subcooling dropped from 5K to less than 1K before you adjusted the TEV, then you lost refrigerant.

[Yuri B.]

If your subcooling dropped from 5K to less than 1K before you adjusted the TEV, then you lost refrigerant

In such case the capacity would have decreased dramatically.

[Yuri B.]

charge advised by the manufacturer was 4 kg but after modification we charged 1 kg more as the we cant get the cooling capacity

May be wotth trying to subtract some refrigerant ?

[Gary]

I'm thinking you are confusing superheat and subcooling. They are not the same thing.

The superheat is the suction line temperature minus the saturated suction temperature.

The subcooling is the saturated condensing temperature minus the liquid line temperature.

[Gary]

As for the measurement on the water side, the chill water leaving temperature (6.7 C) and cooling water entering water (29.4) were pretty stable although the obvious fluctuation is noticable in the refrigerant temp profiles when the TXV is closed to increase the superheat.

Telling us the chill water leaving temp doesn't do us any good unless we also know the chill water entering temp.

Telling us the cooling water entering temp doesn't do us any good unless we also know the cooling water leaving temp.

[jayaeros]

Telling us the chill water leaving temp doesn't do us any good unless we also know the chill water entering temp.

Telling us the cooling water entering temp doesn't do us any good unless we also know the cooling water leaving temp.

Hi all,

Following is the data (R22) that i gathered after the 20 minutes steady state time:

REFRIGERANT TEMPERATURE deg C Uncertainty T SUCTION 1.6 0.7 T DISCHARGE 77.1 0.2 T CONDENSER INLET 75.8 0.2 T CONDENSER OUTLET 40.7 0.1 T EXP. VALVE INLET 40.4 0.1 T EVAPORATOR INLET 0.7 0.1 T EVAPORATOR OUTLET 0.1 0.1


REFRIGERANT PRESSURE barg Uncertainty P SUCTION 3.80 0.01 P DISCHARGE 15.20 0.02 P CONDENSER INLET 15.00 0.01 P CONDENSER OUTLET 14.96 0.05 P EXP. VALVE INLET 14.90 0.02 P EVAPORATOR INLET 4.10 0.01 P EVAPORATOR OUTLET 3.90 0.01


REFRIGERANT flow
L/hr Uncertainty Refrigerant Flow rate,L/hr 326 2.8





WATER TEMPERATURE deg C Uncertainty TW EVA IN 12.1 0.1 TW EVA OUT 6.7 0.1 TW COND IN 29.4 0.1 TW COND OUT 35.0 0.1


WATER FLOW RATE Lpm Uncertainty EVA WATER FLOW RATE 40.3 0.1 COND WATER FLOW RATE 50.4 0.1


COMPRESSOR POWER INPUT,kW 4.6 0.04

It can be seen from the data the suction temperature seem to be fluctuate as well.

Thank you. I really appreciate all the advice.

Regards,
jaya

[jayaeros]

hi all,

sorry for the scattered data in the previous post.
I have uploaded a pdf file on the data again.

Please comment. I am using R22 and the data recorded over a 20 minute steady state period. The suction temperature seem to fluctuate more than the others.

Regards,
jaya

[US Iceman]

We have been adjusting the expansion valve couple of time and now i could not recall the original setting.

This is another reason why TXV's should not be adjusted every time something changes or the system does not react in the manner someone thinks it should! A lot of people may take issue with my statements about not adjusting the TXV's, but this simply reinforces why I say that. This is almost always the first thing people try and it's exactly the wrong thing to attempt.

If all you did was add flow meters and thermowells you should NOT have seen any difference in the way the system performs. Unless, the changes made are now affecting the system opertion in a negative manner, which is what you are describing.

Or, something else has changed. Like perceptions and data collected which may be leading you to wrong conclusions.

Thermowells tend to provide more accurate assesment of temperature data since the device is right in the flow stream (and not on the outside of a pipe). Pipe surface temperature can be different than inside flow streams.

All the thermowells are installed at the elbows with a bigger T section so that the flows are disrupted.

Thermowells located in tee's generally provide a better temperature reading since you have more area of the well (& instrument) exposed to the flow stream. On the other hand, flow through a tee can induce higher pressure losses than a long radius elbow fitting.

How did you determine the system cooling capacity decreased 0.3 kW?

[jayaeros]

Dear Iceman,

The cooling load measurement was taken from the water side (Q=m x Cp x Del T) for both cases.

It would be great help if you can take a look on the data that i posted moment ago. You words of wisdom is highly appreciated.

As far i could think, the only changes we made was on the additional 1 kg charge level. Could this be a problem of overcharging? but low subcooling is not a symptom of overcharge.

Regards,
jaya

[US Iceman]

This is what I think happened. Correct me if I'm wrong.

You measured data and calculated the performance values of the chiller.

Next, you added flow meters and thermowells and recorded data, and then calculated the performance values of the chiller.

Between the two sets of data you have 0.3 kW (~1023 Btu per hour) difference. That is about 17 Btu/minute difference.

The 40.3 liter/minute is ~ 10.5 GPM.

If you assume the flow is correct and constant, the 17 Btu/min difference is approx. 0.19413°F (~0.108°K) error in temperatures recorded to equate to the difference in capacity stated. That is within the margin of error...

I don't think anything is wrong! The difference in capacity is a result of instrument error/deviation.

[Gary]

I don't think anything is wrong!

Except...

3.8Barg = 55psi = -1C/30F
Evap out = 0.1C/32F
Evap out superheat = 1.1K/2F

I'm wondering if the TXV bulb is properly mounted and insulated.

[US Iceman]

Well....as I said before. The modifications that were made changed something or the data is being interpreted incorrectly.

If the original suction pressure of 60 psig was measured with a gauge and then the recorded data was measured with a pressure transducer who knows... A 5 psig difference between a pressure gauge and a transducer would not be surprising.

My point is, before any of the adjustments were done there was probably nothing wrong (unless the piping changes really screwed up something).

After the TXV setting has been changed back and forth you can't start to compare values of what the system did before and what it is doing now.

Too many things have changed to say the answer is XXXX.

What happened after all of the changes may be the result of those changes and not the system as it originally worked at all. This like a dog chasing his tail!

[Gary]

15.2barg = 220psi = 42C/108F
TXV inlet temp = 40.4C/105F
TXV inlet subcooling = 2.4K/3F

The low subcooling tells us there is not enough refrigerant in the high side.

The low superheat tells us that there is too much refrigerant in the low side.

Conclusion: The refrigerant that is missing from the high side is in the low side. IOW, the TXV is overfeeding.

What it does NOT tell us is to add more refrigerant.

[Gary]

Well....as I said before. The modifications that were made changed something or the data is being interpreted incorrectly.

If the original suction pressure of 60 psig was measured with a gauge and then the recorded data was measured with a pressure transducer who knows... A 5 psig difference between a pressure gauge and a transducer would not be surprising.

My point is, before any of the adjustments were done there was probably nothing wrong (unless the piping changes really screwed up something).

After the TXV setting has been changed back and forth you can't start to compare values of what the system did before and what it is doing now.

Too many things have changed to say the answer is XXXX.

What happened after all of the changes may be the result of those changes and not the system as it originally worked at all. This like a dog chasing his tail!

I agree with all of your conclusions. When they did the modifications, they did something to the TXV.

[Gary]

Since you are unable to raise the superheat by adjusting the TXV, we can assume that it is stuck in the wide open position. Most likely it was overheated and warped while installing the wells.

[US Iceman]

When they did the modifications, they did something to the TXV.

Yes, see below...

1. but the superheat and sub cooling diminished to less than 1K....
2. I have tried to increase the superheat by closing the TXV....
3. if i open the TXV but the super heat and subcooling diminished.

I'm betting the difference in superheat/subcooling readings before and after the changes resulted in the wrong conclusions being reached....

This is why I keep repeating... do not adjust the TXV's to solve cooling issues. The TXV should only be adjusted after the other issues are resolved.

In most cases the fault lies somewhere else besides the TXV.

[jayaeros]

Dear gents,

Does this mean that i have to change the TXV? Is there any other way to solve this problem? By the way, i have completed another 2 test runs at higher leaving chill water temperature, at 9.7 deg C and 13 deg C. Please refer to the attached file for more info.

At these set points, i can see that the superheat increases as the chill water rises. There is also some improvement on the sub cooling.

Im also getting some opinions to decrease the charge level back to 4 kg and test do the testing again. Will this help?

Please advice..
Regards,
jaya

[Gary]

Yes, you need to replace the TXV.

With the TXV stuck in the wide open position it acts like a fixed orifice system. If the heat load is increased, the superheat will increase. This doesn't mean the TXV is working properly.

Reducing the refrigerant charge is not going to fix it, either. Just like a fixed orifice system, this would increase the superheat, but the subcooling would disappear and the capacity would be reduced.

You need to replace the TXV and then properly charge the system.