Evaporator Performance

[mEsUsah1]

I'm trying to design my own evaporators for CPU and GPU cooling. But have no experience in how to do a cuple of things. I'm a capable CNC programmer, and know my way around a workshop. When you guys design yours, what "rules" do you follow conserning:

1) Wall thickness. I guess it's something about the area and pressure...

2) Overall flow design. What's the best way of doing it?

3) The suction flexible-pipe thingie you use. What's it called? (what do you ask for at the hardware store)

4) How do you test it? Do you insert it into a system and test how low and how fast it goes down in temp at a spesific condition or what?

Thanks in advance

[Sgrios]

I'm going to quote Soddemmfx's guide because he has said it better than I could. I can no longer find a working link of the guide, but I'll copy and paste my copy.

The evaporator is such a major component of a direct-die system that I felt it warranted its own section in this guide. As mentioned before, the purpose of the evaporator is to allow the transfer of thermal energy from the hot CPU into the cold refrigerant vapor. As I see it, these are the main performance factors which you should hope to achieve in your evaporator:

* Low resistance conduction path(s) from your heat source to the areas in contact with refrigerant vapor. The amount of power which can be transferred per unit temperature difference in a conductor depends on the thermal resistance of the material, the width of the conducting channel and the length of the conducting channel. Whilst there are complex mathematical packages available to simulate this I feel it is best just to think about it and through trial and error evolve your designs to perform more effectively.

* Large surface area which is in contact with the refrigerant vapor. This will allow more power to be transferred for a given temperature difference between the evaporator and the refrigerant.

* Turbulence in your design. Whilst refrigerant will normally boil very violently, I believe that the vapor in the centre of the flow path may remain reasonably unaffected, especially if your flow channel is large. Creating harsh angles or rough surfaces in you design will cause disruption to the normal flow pattern and mix the inner and outer refrigerant allowing more of the inner layers contact to the evaporator surface.

* A small difference between the pressure of the refrigerant entering the evaporator from the metering device (typically capillary tube) and the suction line. Higher pressures in the evaporator with respect to the suction line will lead to higher evaporator temperatures and the possibility of more liquid refrigerant passing to the suction line.

Nearly every change you will make to an evaporator design will trade off one (or more) of these factors against another; for example, higher surface area will almost always mean a more resistive conduction path and greater turbulence will cause increased pressure difference. You should aim to produce a good balance and target your design to your application.

Occasionally someone will mention evaporator "mass". In my opinion this is completely irrelevant to performance and merely a by-product of the other factors and usually comes from low resistance conduction path(s).

Only dimensions that I have of the chilly1 spiral evaporator.


3.) Most people use corrugated stainless steel tubing, however even the CSST tends to expand and or elongate with pressure. So most people use a stainless steel braid placed over top of the CSST to control the expansion. I buy pre-built suction lines from under the ice myself. http://www.under-the-ice.com/index.php?cPath=38

[SoddemFX]

Christ that was written a long time ago...

A small difference between the pressure of the refrigerant entering the evaporator from the metering device (typically capillary tube) and the suction line. Higher pressures in the evaporator with respect to the suction line will lead to higher evaporator temperatures and the possibility of more liquid refrigerant passing to the suction line.

A larger pressure drop will just increas the final temperatures as it increases the saturation pressure.

Partial evaporation before the refrigerant reaches the point where the conduction path and surface area combine to allow maximum energy transfer. Typically this is at a point closest to the heat source. The maximum capacity of the refrigerant is reached when ~30% of the liquid has turned to vapor; whilst this may have been utilized in commercial refrigeration for many years, to the best of my knowledge the first person to apply this to CPU evaporator design was Chilly1 of www.xtremesystems.org.

Ignore. The whole paragraph above is complete and utter nonsense, it just doesn't work this way.

Tom

[Sgrios]

Thanks for clearing that up Tom. Was just a copy/paste to get him started, I'll remove the parts that do not pertain.

Side note: You should write another guide Tom, the original was great when I was starting out.

[gosmeyer]

Truth be told, Tom forgot more about evaporators than most will ever know.

[mytekcontrols]

This seems like as good a place as any to post this question.

Has anybody ever done a good comparison of some of the different evaporator designs on an actual refrigeration system? Or in other words; used a known reference (same unit, same exact charge, calibrated heat load) to compare evaporator designs ranging from simplest to most complex.

Theory is all well and good, but real life test results are usually the best way to weed out what works from what doesn't. For instance, it would be a shame to go through all the trouble to design and machine some extremely complex channeled design, just to find out that you only get a few or no percent advantage over something of a much simpler to construct design.

I ask this question, because I am looking down the road at having to create an evaporator of my own for a special project (non CPU related, but still a cold head of sorts). I would just like to get off to a good start.

[gosmeyer]

( same exact charge)

Not to my knowledge

[loonym]

It would be very interesting to see a head to head comparison of evaps under controlled conditions. Seems like it would also be a very expensive and time consuming venture.

[DonNiger]

Just a theoretical ... not even idea

Has anybody used or thought about distributors? Using design like Prometeia and placeing distributor on top.
Or was I supposed to be silent?

http://www.xtremesystems.org/forums/...45&postcount=4

[quintus]

Thinking of a big ass evap like 3inch diametre stepper...

[teyber]

i made a big ass 2.5" stepper it was bead blasted too. i never got around to completely finishing it because i realized it might be too big to work with a normal size mounting, and they don't make 1/16" snap rings that bigt in diameter

[mytekcontrols]

Well I am a bit disappointed at the response to my question

loonym stated:

It would be very interesting to see a head to head comparison of evaps under controlled conditions. Seems like it would also be a very expensive and time consuming venture.

Hmmm... Time consuming yes (for conducting the tests), but probably more expensive in the long run building unproven designs. Especially if something simple worked just as well as a complex machined maze in the proposed application.

I was really hoping that someone had seen proven design advantages when all other aspects other than the evaporator had remained the same. Of course this leads me to my next question. What led the charge in designing more complex evaporators in the first place? I would guess that it was something to do with thinking that there would be a heat transfer efficiency improvement, and hence colder CPU temperatures. So I still find it hard to believe that no one has any real life comparison data between evaporator design changes. Or has this just become a contest at who can design the coolest looking, most complex evaporator?

[Buckeye]

In just thinking about this a bit, would it be very hard to be exact in a comparision of Evaps ?

Given the same SS or Cascade and the only thing that would change is the Evap itself. In swaping out the Evap with a different one you have to remove the gas, unbraze the old Evap and braze in the new one. Then using the same gas as before you would then have to completly retune the system.

How can you make sure that the unit gets tuned to the exact way it was with the old Evap ?

Or is there another way to do this ?

[[XC] gomeler]

Use flare connections to attach the suction and discharge lines.

Use a refrigerant scale and charge to the same charge.

Record values from the same point on the heatload.

Repeat excessively.

[Foamy]

i made a big ass 2.5" stepper it was bead blasted too. i never got around to completely finishing it because i realized it might be too big to work with a normal size mounting, and they don't make 1/16" snap rings that bigt in diameter

you got pics?

[Buckeye]

Use flare connections to attach the suction and discharge lines.

Use a refrigerant scale and charge to the same charge.

Record values from the same point on the heatload.

Repeat excessively.

You make it sounds so easy LOL

[wdrzal]

Well DetroitAC built a calorimeter to test evap performance ,but I don't think anybody wanted to pay for his investment and time. You need to carefully reach steady state and balance all the parameters to get a accurate result. And accurate calibrated equipment
does not come cheap. He told me he would measure the calories and provide a written report to the customer. Seems no one wanted to pay the price for a engineers work so I think he tested a few of his own designs then dismantled the calorimeter.

I'm just repeating what I believe to be true. DetroitAC could set the record straight. To my knowledge of being around XS. He was the only one who did evaluate the performance of different evaps scientifically. But only a few at most.

DetroitAC feel free to correct me if my statement is incorrect.

Walt

[quintus]

I understand DetroitAC perfectly well. Staying around this or another forum and picking up SS builds or cascades doesn't even provide a normal income. Why should he invest in this? when with his know how he can get a verry well paid job.

Let's face it phase change cost money! And if you want to do it right it cost a lot of money.

[gosmeyer]

Of course this leads me to my next question. What led the charge in designing more complex evaporators in the first place? I would guess that it was something to do with thinking that there would be a heat transfer efficiency improvement, and hence colder CPU temperatures. ?

You answered that !!

So I still find it hard to believe that no one has any real life comparison data between evaporator design changes. Or has this just become a contest at who can design the coolest looking, most complex evaporator?

For the most part comparison data is not available and has not been available,
this has been going on for years

Designing and manufacturing are two different things, I can only speak for myself but I can assure you the bling factor was never a goal, only performance. We just need an investment casted evap that's a hybrid stepper/maze made from sterling silver at a low cost

[mytekcontrols]

Use flare connections to attach the suction and discharge lines.

Use a refrigerant scale and charge to the same charge.

Record values from the same point on the heatload.

Repeat excessively.

Thanks Chris. I would add packless isolation valves prior to the flares, in order to minimize refrigerant loss during connection and disconnection. Being sure to do a quick purge of air by slightly loosening one flared connection with only one isolation valve open at the time (or add an additional port for connection to a vacuum pump). This might eliminate the need for recharging and the scale.

gosmeyer -- Sorry about seeming to suggest that this was being done for the bling factor, but without good comparison testing and data, it is difficult to say what level of complexity is required, and what is not. I do understand the goal of trying to maximize heat transfer between the external surface and the internal refrigerant stream, and the idea of using multiple channels to try to enhance this effect. But without good real life data for comparison, some of the extra work required to do this may be a wasted effort and have unwarranted extra cost.

As you guys probably noticed with my own AutoC projects, I tend to run multiple tests with varying refrigerant charge compositions in order to zero in on what works best for a given heat load. And to help in my comparisons, I collect temperature data from multiple points across the system. The AC-2 unit went through at least 2 dozen charges before I settled on the one that took it down to -140C, as well as what would yield the best operating parameters for the compressor's longevity. The AC-3 unit went through 3 HXC designs to discover what was optimal.

And yes doing all of this does take time and money. But it often times saves time and money down the road, knowing what is really required to build an optimal design.

So back to my original question...

Has anybody ever done a good comparison of some of the different evaporator designs on an actual refrigeration system?

I know many of you are members of other forums besides this one. If any one finds the answer to this question, could you please share it with us here. I think all of us would benefit from knowing.

[sdumper]

Thanks Chris. I would add packless isolation valves prior to the flares, in order to minimize refrigerant loss during connection and disconnection. Being sure to do a quick purge of air by slightly loosening one flared connection with only one isolation valve open at the time (or add an additional port for connection to a vacuum pump). This might eliminate the need for recharging and the scale.

Great ideas....

Now we just need some evaps and a volunteer

[gosmeyer]

gosmeyer- without good comparison testing and data, it is difficult to say what level of complexity is required, and what is not. I do understand the goal of trying to maximize heat transfer between the external surface and the internal refrigerant stream, and the idea of using multiple channels to try to enhance this effect. But without good real life data for comparison, some of the extra work required to do this may be a wasted effort and have unwarranted extra cost

100% agreed

[Sweeper]

I've have a SS built for testing evaporators. I designed it so the evaporator can be removed without lossing the charge on the system. I wanted to compare my Crossflow design evaporator to other designs, but I don't have any. I made one Crossflow that is huge and really needs to be tested with a cascade, because of the mass. Later, I'm thinking of building a cascade for testing only.

[gosmeyer]

That works NICE

[sdumper]

Thats a very nice looking evap do you have any more you would like tested or to sell? Send me a PM because I need a solid cascade evap...