Electronic Expansion Valve Benefits

[Dr. Downflow]

Now then chaps, I know a little bit about this refrigeration stuff but I just can't seem to "Get" electronic expansion valves!

We squirt liquid through a hole, and the reduction in pressure allows droplets to readily evaporate, providing our refrigeration effect. If the evaporator loading increases, we make the hole bigger and if it falls we close the hole up a bit, in order to regulate refrigerant flow and maintain our superheat.

How can it make our system more energy efficient opening or closing the valve (making the hole bigger or smaller) using an electronic stepper motor instead of just the usual pressure variation across a diaphragm?

The general perception seems to be that EEV will somehow work with reduced high side pressure, thereby achieving savings in compressor power input. I don't believe this to be correct, because after all an EEV is still just a hole as far as the refrigerant is concerned!

I am aware that EEV are much more "One size fits all" in terms of differing capacities and refrigerants, and can also obviate the requirement for a seperate solenoid valve, but can't really see these factors alone justifying the increased complexity.

Hopefully I am missing something profound, which will no doubt be revealed shortly?

[Segei]

Lowering condensing pressure is major energy saving measure. At lower condensing pressure compressor energy use will be reduced. However, other components of the refrigeration plant should be suitable to operate at low condensing pressure. Typical TEV have narrow rage of operating condensing pressures and it will not be able to do his duty at low cond. pressure. EEV have wide rage of operating cond. pressures and it will give opportunity to lower condensing pressure and energy will be saved.

[Sandro Baptista]

Also the superheat can be narrowed with the EEV which implies better feeding of evaporator » higher suction pressure » more capacity and COP.

Also you can avoid the "hydraulic shock" when you open or closes the EEV.

[Kalmar]

Hopefully I am missing something profound, which will no doubt be revealed shortly?

1. Every expansion device (TEV, EEV...) if controlling the flow ONLY, with a task to inject exact amount of liquid which can evaporate and be superheated for requested superheat. Thats all!

2. Each valve is changing flow by changing geometry (size of hole, achieved by moving parts) or by changing open/close times, on PWM (Pulse Width Modulated) EEV's.

3. As lower as possible superheat means better use of evaporator. Evaporator surface needed for superheat is "waste". EEV's are better than TEV's in that respect (faster reaction in load change, can work with lower superheats...).

4. TEV need higher pressure ratio (Tcond/Tevap) to work properly, therefore (if compressor can work in such conditins) EEV gives more opportunities to save energy. Each Kelvin (lower Tcond or higher Tevap) gives 3-4% better COP.

5. EEV has wider operating envelope.

6. EEV's are more expensive (and need electronic device to control it).

7. TEV's are having different fillings for different refrigerants (different products to be keept on stock), EEV's are universal, only setup change in software is needed (at least for "standard" refrigerants, s.a R22, R404a, R407c/507, R-134a..). Difference is with R744 (maybe R410a).

Any expansion device can NOT influence evaporation or condensation pressure directly.

Enough?

[Dr. Downflow]

Sorry chaps - I have heard most of those marketing arguments before and understand them. However I still don't understand WHY a hole that is made bigger or smaller by a stepper motor is so much better than one which relies upon a diaphragm to do exactly the same thing.
I can get a TEV to control nicely at 5K superheat and I'm sure it would probably go lower if I wanted to risk it. I don't find EEV are regularly set any lower than 5K superheat on sites I visit anyway.
If you look in the small print from EEV manufacturers you will find recommended design pressure differential of 10 Bar as well. Hence my dilemma.

[DTLarca]

Sorry chaps - I have heard most of those marketing arguments before and understand them. However I still don't understand WHY a hole that is made bigger or smaller by a stepper motor is so much better than one which relies upon a diaphragm to do exactly the same thing.
I can get a TEV to control nicely at 5K superheat and I'm sure it would probably go lower if I wanted to risk it. I don't find EEV are regularly set any lower than 5K superheat on sites I visit anyway.
If you look in the small print from EEV manufacturers you will find recommended design pressure differential of 10 Bar as well. Hence my dilemma.

I wrote several articles on the matter for one of the magazines. After one of my articles BSRIA did some tests to determine whether LPA was need on close coupled EEV systems. They discovered it was not - which we knew anyway - this is why I proposed TEV bypass Capillaries for low ambient reduced head pressure full load operation.

Findings from my experiments using EEV's for reduced head pressure energy savings.

EEV's have better low load stability all through the year. As with TEV's their control is more stable when inverters are used for head pressure control.

A TEV will ride its superheat curve to lower superheat at lower loads which means greater evaporator refrigerant mass which means less liquid line mass which means a hunting TEV - EEV's do not - they maintain much steadier liquid subcool values and superheats at low loads - steadier subcool values because of the steadier superheat values.

If the head pressure drops from about 45°C in a 30°C ambient to 23°C in a 10°C ambient the system power consumption can drop by as much as 60% especially since the condenser fans might constitute ordinarily only 10% of the systems full power consumption. Under these conditions a TEV's capacity would have dropped to just 75% of the required capacity and so any time the compressor(s) load above 75% the suction will crash with TEV's but not with EEV's or a TEV bypass Cap-tube. The Cap-Tube should be sized so as to maintain at least 5K superheat in winter full load duties to avoid losing too much liquid refrigerant mass to the evaporator which can also then cause hunting of the TEV for both normal reasons (low superheat) and vapour in the liquid line.

Here are some of the articles - I have not read them for perhaps 8 years so forgive me if they are not relevant to the question.

http://fridgetech.com/articles/acrnews/tevpresseffects/

http://fridgetech.com/articles/acrne...setevcapacity/

[Sandro Baptista]

For big capacities at a low suction pressure the EEV could cost almost the same price.
Notice that at the TEV loss capability of flow passage as far as the regimen is lower.