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Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 1:01 PM

I have a chiller with comp. Working in tandem on R407c. When one comp. Cuts off the suction pressure rises from 40psi to 55 then coming down to 38. Discharge is 230psi when suction is 40. Normally I observe in other chillers with R410a when one comp. Cuts off suction pressure rises and stay at that pressure. Why it is other way round in R407c.

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#1

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 2:16 PM

You don't say what kind of compressor or system controls. Maybe the compressors have unloaders set at 40psi, or maybe ..., maybe ...

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#2
In reply to #1

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 3:04 PM

It was rising before. The problem started after a system service

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#3
In reply to #2

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 4:40 PM

Who did the system service????

After they fix whatever they damaged, find a new service company.

If it was done by an employee of your company, FIRE THEM!

Still no useable information. Call the service tech back.

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#12
In reply to #2

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 10:29 AM

Read to work order, see what was done, compare to mfg specs. -- JHF

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#4

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 4:41 PM

Why is that a problem? You haven't explained.

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#5
In reply to #4

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 5:04 PM

You still haven't given any more information. What kind of compressor, reciprocating, screw, centrifugal? Is this a freezer, water chiller, food cooler, etc? What suction pressure do you need? 40 PSIG gives an evaporator temp. of 20-25degF, I think. If one compressor meets your need, why run 2?

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#6
In reply to #5

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 6:04 PM

Thanks all for your prompt response. I will explain more for better understanding of the problem. The chiller is 15TR (7.5 + 7.5 in tandem). The compressors are scroll danfos performer with R407c. Before, when chiller was working fine, with 1 comp. running suction pr.was higher than with 2 comp. running. (Corresponding to temp.).Later evap developed crack and water entered system. I flushed the system with N2 removed water, replaced dryer and TXV.and ran vacuum pump before charge. Now suction pr. is 40psi only, at temp. 49F (super heat 25F) while discharge pr. is 230 psi(air cooled condenser using 2 fans)sub cooling +5F higher than saturation temp. Is this suction pressure enough for efficient operation? Secondly suction pr. is not rising when 1 compressor cut off running. is it normal ?

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#8
In reply to #6

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 7:21 PM

This is beginning to look like the basis for a telephone call to the compressor manufacturer's technical helpline, and not one for a CR4 thread.

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#9
In reply to #6

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 12:23 AM

There is probably pressure relief in place that does not begin to open until a greater differential pressure than is developed by 1 compressor, but once open does not fully close again until a differential pressure corresponding to the 40 psi suction you observe.

.

...basically, the single compressor discharge pressure compared to the 38 psi suction is insufficient to open the valve....but two compressors open the valve which regulates the pressure difference corresponding to the 40 psi suction you observe.

.

At least, that is what I think is most likely. It makes sense with the scenario you describe for turning off one compressor:

the valve is open with two compressors on, one compressor is shut off, the difference in pressure across the pump drops resulting in higher suction pressure. As the valve closes, the suction pressure drops until reaching a suction pressure below the 40psi but not quite sufficient to reopen the valve.

.

So, how does the pressure react when going from one compressor on, switching to two on? It should see an initial drop in suction pressure followed by an increase in suction pressure up to 40 psi if the above scenario is correct.

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#10
In reply to #6

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 10:11 AM

Read up on how TXVs work and this may answer all of your questions. The TXV is the refirigerant metering device that throttles the amount of refrigerant entering the evaporator. To loosely describe its function, the TXV will control the amount of refrigerant based upon the demand of the evaporator. If the air across the evaporator is relatively warm, the TXV may open and release more refrigerant into the evaporator. For your system you may see the suction pressure go up. It the air across the evaporator is close to the design temp of the system the TXV may reduce the amount of refrigerant entering the evaporator. The TXV subsequently controls your suction pressure, not your compressors as you may have thought. The TXV will keep your suction pressure constant based upon the demand of the evaporator. Whether your suction pressure is enough. No one can answer that without knowing the the operational parameters, I.e. ambient temp outside, temp of air across the evaporator, having the unit charging table/chart for the unit, etc... A GOOD technician can answer that best.

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#16
In reply to #10

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/13/2013 10:10 AM

ESweet - I think that's a GA.

Also, from everything I read so far, the answer may be "Working as intended". It may not have been working as intended prior to the service.

It is possible that the unit is now operating exactly as it should be.

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#17
In reply to #6

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/13/2013 5:13 PM

You replaced the dryer and the TXV after the leak was repaired so now everything is ok? Why?

As has been said below TXV's regulate the flow to match the load. That means you should not see much change in pressure with changing load.

Your old TXV may have been faulty or the temperature sensor might not have been seeing the true temperature (poor thermal contact - loose/lost some charge??).

Also your dryer might have been obstructing the flow somewhat, hence two compressors drop the suction pressure because the gas cannot get through the dryer fast enough - even with one compressor running (if it is in the suction line and the pressure gauge is connected after the dryer).

As you replaced both we'll never know if only one was at fault but never mind because it is better to replace two and cure at one go than replace one and find it hasn't done the trick.

I have a different type of system but it still illustrates the value of a good TXV.

My systems use two-stage reciprocating compressors with gauges monitoring suction, intermediate and head pressures. My larger systems have two compressors instead of the one on the smaller systems (all 20hp).

I could see the intermediate pressure increasing with the load on all systems (which is correct) but, like you, I wondered why I didn't see the suction pressure decrease and the intermediate pressure increase when the second compressor cut in on the large systems.

Eventually it dawned on me that I shouldn't because if the TXV is properly sized and working properly the TXV will open to let twice as much refrigerant round the system when two compressors are running. Each compressor is then handling the same flow as the one did on its own and there is therefore no change in the pressures. The only change is a doubling of the flow, which is why I suggest your system is now working properly.

If you know how long the dryer was in use before, then I suggest you change it sooner because it might have been clogging even before the water got in. I cannot say the same for the TXV because I don't know this one was faulty and no two TXV's fail at the same time. However they don't last for ever so a routine change is no bad thing.

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#7

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/11/2013 6:19 PM

I hear my mother calling.

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#11

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 10:16 AM

No 9 suggests where to look.

Assuming that the compressor is working properly and the gas regulating valves are operating properly, then it sounds like a temperature and/or pressure adjustment to the control valves.

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#13

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 11:49 AM

if one compressor is going off at the same time one condenser fan should be cut off then the suction pressure will rise

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#14

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 1:27 PM

Two things might be contributing to the system behaviour,maybe your solenoid is not closing proprly when your machine is pumping down for cutoff.Secondly the temperatures and characteristics of the refrigerant you are comparing are different ,plus as you should be aware of ,pressure goes proprtionally with temperature. When temperature is low ,pressure will be also low.

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#15

Re: Why Suction Pressure Is Not Rising When One Compressor Out Of Two Running In Tan

03/12/2013 2:15 PM

I'd take a long lunch break and see if it fixes itself, if not I'd call your service ppl back and find out what changes they made at the last maint

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alex varghese (1); bigg (2); Circuit Breaker (1); ESweet (1); Fredski (1); georgerin72 (2); horace40 (1); JCase (1); lyn (2); Ptrend (1); PWSlack (1); SIR G (1); Tornado (1); truth is not a compromise (1)

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