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DaBit
14-08-2003, 01:22 PM
We overclockers often use a DX evaporator on the CPU to chill the chip (example: my own DX evaporator to be placed on the CPU (http://www.icecoldcomputing.com/text/show_page.php?id=63)).

Besides getting the evaporator to do it's job, providing good expansion is still a major headache. The reasons why providing good expansion is so difficult:

- Huge load variations. The heat output of a CPU depends greatly on the amount of work it is doing.

- Low output power. I usually calculate with 150W maximum, and this includes heat ingress from the surrounding. Minimum power can be as low as 30W, including heat ingress.

The two most common expansion devices used and their disadvantages for us:

The TEV
These valves have two major disadvantages: their nominal capacity is too large for us, even the capacity of a 1/8 ton valve. The second disadvantage is reaction speed. When the TEV decides that more refrigerant is needed, the evaporator is already starving. When the TEV decides less refrigerant is needed, the suction line is already filled up with liquid.
This is because the heat capacity of most of our evaporators is insufficient to maintain a stable regulation loop. Temp just rises/drops too fast.
Also, TEV superheat setting seem to skyrocket below -40C/-40F.

The capillary tube / fixed orifice / manual expansion valve
These can be adjusted for our loads, and since they do not sense anything, there is no control problem. But fixed restrictions are hard to finetune, and their ability to cope with load variations is mediocre at best.

Now, I see one solution for our problems: the EEV, and then especially the stepper motor operated EEV such as the Sporlan SEI-.5 (1/2 ton EEV stepper type, 1596 nominal steps). This valve has a nominal capacity of 1.7kW / 0.5 tons (R22 @ 8.5 bar/115psi PD, SST = -7C/20F).

We need only about 8% of this capacity. Will such a valve with custom electronics be able to control these light loads? This would leave about 127 useable steps. With R404a even less.

Andy
14-08-2003, 08:53 PM
Hi Dabit
just a thought why not de-rate the valve further by the addition of a capillary after it, if capacity is the sole problem.
Regards. Andy.

DaBit
14-08-2003, 11:17 PM
That works fine to get my TEV capacity a bit lower, so it should work for EEV's too. But is it necessary? Can EEV's work fine at the ends of the scale?

herefishy
14-08-2003, 11:59 PM
Perhaps Sporlan's tools for calculating distributor tube sizes and lengths could determine such cap tube sizes for proper pressure drop for the application oa an EEV in this situation?

This is just the first thought that crossed my mindd. not a LOT of thought into it.

:)

Prof Sporlan
15-08-2003, 02:48 AM
Though there is merit is sizing a step motor EEV such that it modulates more in its middle to upper range of its total stroke, there is no reason why this valve with the proper controller can’t happily modulate in and around 8 percent of its total stroke.

The cost of this arrangement…. the SEI valve, temperature sensor(s) and/or pressure transducer, and the controller makes the Prof wonder if it is not simpler to upgrade the microprocessor to something like the fastest one the market? :confused: Ok, Ok…. the Prof has already answered this question…. DaBit would want to overclock this microprocessor also… :D

BTW, the Prof, in his humble opinion, can estimate two-phase flow pressure drop thru a tube or restriction with the best of them… :D :D

DaBit
15-08-2003, 08:52 AM
Originally posted by herefishy
Perhaps Sporlan's tools for calculating distributor tube sizes and lengths could determine such cap tube sizes for proper pressure drop for the application oa an EEV in this situation?


Does Sporlan has tools online for this? Hmm, need to check it out.
But it doesn't sound bad after all. If it is needed.


Originally posted by Prof Sporlan
[B]Though there is merit is sizing a step motor EEV such that it modulates more in its middle to upper range of its total stroke, there is no reason why this valve with the proper controller can’t happily modulate in and around 8 percent of its total stroke.

OK, that's the answer I would like to hear.


The cost of this arrangement…. the SEI valve, temperature sensor(s) and/or pressure transducer, and the controller makes the Prof wonder if it is not simpler to upgrade the microprocessor to something like the fastest one the market? :confused:

SEI valve is probably the most expensive part. If I ever decide to use a step-control valve, I will design and build the controller myself. Not exactly an impossible task for someone comfortable with microcontrollers, control loops and stepper motors.


Ok, Ok…. the Prof has already answered this question…. DaBit would want to overclock this microprocessor also… :D

Indeed Prof, that's the fun of it :D
Although currently I enjoy designing and building these quite extreme CPU coolers more than the actual overclocking of the CPU.

A simple R404a captube system works fine, but it can be done better, and finding out how is fun :)


BTW, the Prof, in his humble opinion, can estimate two-phase flow pressure drop thru a tube or restriction with the best of them… :D :D

Prof, you can do a LOT of things I would like to be able to do.
I suppose making a table or wrapping the pressure drop in a set of easy to solve equations is too much work? :D

Gary
16-08-2003, 01:03 PM
Another possibility might be an automatic expansion valve (AEV) which maintains a constant evaporator pressure, although this may require an accumulator.

DaBit
18-08-2003, 09:11 AM
Originally posted by Gary
Another possibility might be an automatic expansion valve (AEV) which maintains a constant evaporator pressure, although this may require an accumulator.

Why would someone want a constant evaporator pressure? With constant evaporator pressure, superheat is only OK for one single evaporation tremperature only. For all other evaporation temperatures, superheat is either too low or too high.

Just another crazy idea: why not convert an externally equalised TEV to an electronic expansion valve by putting a constant pressure on the equaliser port, and connecting the bulb to a TEC (Thermo-electric cooler, a.k.a. Peltier element), effectively turning the TEV into a heat motor controlled EEV? By controlling polarity and voltage of the TEC, rapid adjustments of the flow can be made.