Stepper expansion valve problem

[mrr]

Hi,

I made a thread in the refrigerants section, but I think it is worth to redirect my post here.

This post is a little bit long, but I think it is worth to read it, since it also may contribute to this forum for understanding possible causes of troubleshooting. I will provide more data if needed.

We have an experimental plant with two electronic expansion valves, a pulsed EEV from Danfoss and a stepper EEV from Carel. Only one works each time. The stepper EEV cannot control the superheat (is it small?).

The main data:
- I always get high subcooling (about 20 K) with both EEV. I think I also get high pressures. For R134a and 25 ºC of ambient temperature I get about 11 barg.
- If I raise the ambient temperature (we can control it), the condensing pressures rises and it seems the new EEV controls the superheat.
- We selected this new EEV according guidelines and according the nominal point of the evaporating temperature and frigorific power our compressor gives.

Main questions:
- What can be the reason the stepper EEV cannot control superheat. Is it undersized despite the fact it was selected according guidelines of selection?
- We have an oversized condenser. But Why I get always 20 K of subcooling? Overcharge of refrigerant? Dont really know, because superheat is well controlled with the pulsed EEV.
- Is 11 barg a normal condesing pressure for R134a? If we set the ambient temperature to 35 ºC the pressure rises up to 16 barg, which is 60ºC, the upperlimit of the discharge condensing pressure for our compressor for R134a.

The history:

Initially, we had (and have yet) a Danfoss AKV-10-6 expansion valve (electronic pulse expansion valve). This EEV was oversized since its controller is always working with a pulse witdh of no more than 30 % (with a cycle of 1 second, the EEV is 300 ms opened and 700 ms closed). In addition, we have refrigerant flowmeter in the liquid line. With the aforementioned EEV the refrigerant measure is not reliable, since, as you can guess, the refrigerant flowrate is pulsant. Thus, we decided to install a "step by step" electronic valve to have a reliable measurement of the refrigerant flowrate. Now, both EEV are installed in parallel, when one is working, the other is closed. Then we can make comparisons.

So here is the thing:

The main componentes of the plant are a bitzer compressor with an inverter. The evaporator is a B15THx20 SWEP heat exchanger, and the condenser is an air battery were a fan with a inverter is installed. The fluid to cool is a glycol solution in a small tank where we have a variable resistance up to 5.4 kW. This system was prepared for R404A and R134a. Finally we put R134a, which means that the components are oversized with respect to R404A.

- The bitzer compressor gives as much as 3.5 kW with R134A (10 ºC of evaporation). Our nominal point was about 0 ºC in the glycol solution, this means an evaporation of about -5 ºC of the refrigerant. The compressor gives about 1-2 kW of power at that point with R134a, depending on the condensing temperature (40-60 ºC).
- With the AKV EEV I use to get between 9 and 12 barg of condensing pressure. This happen when the ambient temperature is between 18-25 ºC. If I increase the ambient temperature (we can control it) to 35 ºC the condensing pressure rises at 16 barg. In all of this situations, the subcooling is always about 20 K.
- As I said, we have a B15THx20 heat exchanger. I can control the flowrate of the glycol solution. I chose about 13 l/min, it seems at that point the heat transfer is optimal.
- The superheat can be controlled. And the AKV EEV is between 20-30 % of pulse width.
- The EEV (step by step) valve was chosen for the aforementioned nominal point (-5 ºC of evaporating temperature, and between 1-2 kW). Software results are about a 30% of aperture. However, when I put in the system the aforementioned conditions, this valve is 100% opened and the superheat cannot be controlled. However, if I increase the ambient temperature about 30 ºC then the condensing pressure also increases and the EEV seems to control the superheat.

In both cases (with the AKV (pulsed) and the "step by step" EEV) the subcooling is high, about 20 K. I have a receiver after the condenser, my understanding is that a tank receiver guarantees liquid feeding of refrigerant to the EEV and compensates for possible excess of refrigerant (correct me if I am wrong). In addition, the higher subcooling, the greater capacity of the EEV. What can be the reasons the "step by step" EEV cannot control the superheat unless I rise the condensing pressure? (I selected this EEV taking into consideration a small delta P between aspiration and discharging pressures). Should I have selected the EEV for the evaporator capacity instead of the compressor capacity?

We are going to change the step by step EEV for a bigger one. However, I still do not know the exact cause of the problem and I'd like to know it. If you have any question or need any clarification please let me know and I will edit the post. Waiting for hints!

Some data:

- Compressor: Bitzer 2JES-07(Y)

- AKV 10-6: http://www.ra.danfoss.com/TechnicalI....02_AKV_VS.pdf
The driver is an OSAKA controller which works fine.

- The stepper model valve is a E2V11: http://www.carel.com/documents/10191...c4?version=1.0. The controller is a EVD Evolution http://www.carel.com/product/evd-evolution. I am sure the controller is not the problem. I have set manual operation and I set 100 % aperture and the stepper EEV does not control the superheat.

I am 100% sure that the controllers are not the problem. There must be another cause.

Looking forward to your hints
Regards,

[RANGER1]

Mrr,
Does Carel valve have inbuilt filter like in this brochure?

Can you disassemble valve & try to simulate & watch valve open.

http://www.carel.com/documents/10191...de?version=1.0

[Brian_UK]

You state that you have set valve to manual 100% open and that the valve does not control the superheat.

It cannot control with that setting can it? You have locked it fully open.

[mrr]

Hi Ranger,
Unfortunately, the valve is not like that one, is the other model: http://www.carel.com/product/e2v-b

@Brian_UK: Yes, I can set manually by using the controller display the aperture of the valve. If I set 100% aperture I see superheat does not go down. However, the pulsed valve can. Does it mean the new valve is undersized?

In addition, what can be the causes I get 20 K of subcooling?
What are normal condensing pressures for R134a and 40 ºC of ambient temperature?