Problem about EEV modulation

[ernestlin]

Hi, every professional guy! Has any body been good at usage of electronic expansion valve? We fitted the refrigerated system with 2 parallel EEVs, and it's difficult to get a steady evaporating temperature after modulating the EEVs, especially in lower temperature range (below -15 F). Additionally, the refrigerant is R404A. If there're some faults with the parallel EEVs design? Or other problem? Any suggestion is appreciated. Thanks.

[Brian_UK]

I may be wrong here, and someone will tell me I hope, but when your temperature comes down what happens if you close one EEV and just control on one EEV only?

Are the two EEVs controlled together or does each EEV have it's own sensor and controller?

[Pooh]

Good point Brian are the two EEV's on the same evaporater or on two seperate one's. If they are on two seperate one's there should not be a problem as each should have a seperate transducer, temp sensor and controller. if they are on one which controller are you using??

Ian

[US Iceman]

How is the coil circuited?

If you only have one coil with (2) EEV's you can have some other problems which some of the members have suggested.

Another concern is the coil surface temperature.

If you have one EEV per coil, and have two coils installed, one coil could be inactive. If any air flows through the inactive coil you can loose temperature control.

On the other side of this, you could have once large coil with the circuits interlaced so that the entire coil surface is active at part load. Then if one EEV shuts down, you still have control over the coil surface temperature and leaving air temperature.

[setrad7791]

Just a quick note to add on the previous answers, just check the piping design as you my be reiceiving inaccurate superheats due to piping design, ensure that their is no possibiltie of gas from one coil influencing probe temps of another ie..

[DeB]

What brand of EEV?

[ernestlin]

Thanks for your kindly reply! I try to answer your question as following, hope can help me slove this problem quickly.
1.Two EEVs are controlled by one integrated circuit board, and without sensors. we set the thermal couples on the lines before and after the evaporator, and manually modulate the EEVs depending on the temperature of lines.
2. These EEVs are fitted with one evaporator.
3. They're ALCO's EEVs.
Welcome any advanced suggestion.

[Brian_UK]

If you are manually modulating the EEVs then how do you expect them to control automatically? I am amazed and almost speechless.

If you are doing it manually then I don't think anyone here will be able to provide an answer to your stability question.

[chillin out]

What do you mean when you say that you manually modulate them?

I take it that the evaporator is split into two circuits?
It must be a hell of a big evap, or has a lot of duty on it?

Why don't you just fit a EPR valve?
This would hold your coils at an even temp/pressure, although I can't really see it working with 2 EEVs. I think you would struggle with just using 2 TEVs.

Chillin

[ernestlin]

Thanks. Brian and chilin, I will have a try to close a valve, just use one valve. Because they are applicated in a test bed system, not a product or others, and I'm not the designer, so there may be some errors in my presentation. The valves should be fitted with sensors, but I cann't find the connection lead..it's just connecting with a integrated circuit board, so I think there's not any sensor..Is that a design fault?? The designer may want to controll the EEVs by the PC depending on the thermal couples' data. And I have a doubt with Ian's suggestion, why the EEVs should be fitted with transducers? Do you mean the impulse transmission instrument or others?? There just have been driven by one this instrument.
Hope the further discussion. Thanks, guys!

[Peter_1]

You need in my opinion a pressure and a temperature reading. Sh is determined by temperature and pressure or a converted temperature/pressure signal

[Brian_UK]

I think that you need, most urgently, to get hold of the manufacturers instructions for the EEV and controllers.

Without those you are not going to acheive much.

[ernestlin]

Hello, Peter! The temperature and pressure before and after evaporator can be displayed in the screen, so we control the SH according to those data.
Brian, tks for your suggestion, I have collected the relative information.

[Peter_1]

You post 10 stated that there were no sensors connected at all

[ernestlin]

Pete, I have said we set the thermal couples along the evaporating line. : ) please find it in my previous posts.

[wkd]

To control the superheat the EEV controller needs to know what the saturated condition is the transducer should measure the suction pressure which then gets converted to sat temp.With regards to instability you are going to get nowhere by manually operating the valves so the best thing to do is get it running in auto.With regards to your modulating problem if it still exists when in auto there may be a heat lag on the sensors causing the sensors to operate out of phase,this will generate slight pressure deviations between the valve inlets manifesting itself as hunting.Another question is the valve sizing it may be more stable to use both valves for pull down then swith one off to maintain load at design temperature that way the operation will be much more stable.

[DeB]

What brand of EEV are you using?

[ernestlin]

Thanks, gus. Problem is solved. There exists foul gas in the system, after reflush the tubes, everything become well. Anyway, I still appreciate your kindly posts.