PC cooling project status update + TEV questions.

Well, as some of you still remember, I am busy developing a liquid chiller which cools a water/methanol mixture, which in turn cools various parts of my PC. More information can be found in my previous topics, but I will summarize the most important data here:

- Refrigerant: R134a
- Compressor: Danfoss NL11F
- Evaporator: coaxial heat exchanger
- Qmax=200W @ -20 °C
- Qnom=150W @ -20 °C
- Coolant used: 30/70 vol.% methanol/water mixture.
- Coolant flow: 600-1000L/hour, netto.

Currently I am using capillary expansion which is not very suitable for this application due to the large variations in system load. Superheat at the compressor inlet varies between 5K at low load conditions, going up to 22K at high load conditions. The average liquid temperature is about -12 °C.

Progress since my previous topic is not so much. I had troubles with the methanol leaking around seals, which I solved (a PVC hose<->metal junction is an ideal methanol filter, which passes methanol and keeps water inside). I also finished most of the system elektronics, and I did some minor adjustments to the coolant loop. Also, the CPU, chipset and RAM is cooled now, which allows me to run at about 2GHz CPU speed (using a 1533MHz processor). No condensation or ice forming takes place anymore on the elektronics since all elektronics are mounted into a airtight box which is equipped with silicagel to get the moisture out of the air. This works very well, even better than expected.

For the liquid chiller itself: it has not yet changed, but the current setup runs flawless. I am still trying to find parts to upgrade it.
The person who used to be able to obtain parts for me seems to be dead. He does not react on E-mail or phonecalls.

The crucial parts I still need are:
- dryer with 1/4" ODF solder connections.
- receiver.
- starting relay and capacitor to use the NL11F as HST compressor.

I can build the receiver and starting relay myself, so that's not crucial. The filer/dryer is more critical since I have not yet been able to find a company which is willing to sell me one.

One part I have been able to obtain is a Danfoss TEV type TEN2, a solder->flare adapter for 1/4" ODF, and a orifice size 00. This TEV+adapter has an external pressure equalisation port (1/4" line), 1/4" liquid line connection, and a 1/2" connection to the evaporator.

Now, orifice 00 is not the smallest orifice available for this valve (that's orifice 0X). It has a rated capacity of about 700 Watts @ -20 &deg;C (this from my memory since I do not have datasheets ready over here. The actual number might differ) when mounted in a TN2/TEN2 valve. I know I am better off using a 0X size orifice which has a rated capacity of about 300W @ -20 &deg;C, but things are just the way they are.

Questions:
- Is it safe to mount this TEV+orifice in the mentioned system, or is it a ticket to a lot of trouble? I read that TEV's can work flawless down to 30% of their rated capacity. 30% of 700W is still a lot more than the load I am applying.

- What will actually happen when the orifice is too large? I understand that a thing called 'valve hunting' starts to happen. I understand that this is the rapidly opening and closing of the valve due to control loop instabilities. But what is the effect in practice?

- Does anyone of you know an address of a company in The Netherlands which is willing to sell me at least a simple dryer with 1/4" solder connections? Please keep in mind that I do not posess the required (STEK) license.

(Due to the extreme heat here at work my English might be even worse than normal. My excuses for that.)

Using your current cap tube (which as I recall is oversized):

Install a hand shutoff valve just before the cap tube. Slowly add refrigerant while closing off the valve, until the line temperature entering the valve is about 2-3C above the surrounding air temperature AND the frost on the suction line ends about 6 inches from the compressor inlet (under low load conditions). This will take some experimenting to get both just right.

Using the TXV:

With that same shutoff valve just before the TXV, add refrigerant until the line temperature before the valve is 2-3C above the surrounding air temperature AND adjust the shutoff valve until the low side pressure is not hunting (under low load conditions).

If you have access to both high and low side pressures, we can be more precise, but these procedures should get you very close.

Using the shutoff valve to stop TXV hunting is not an ideal solution. It will eventually wear out the TXV orifice. The better solution would be to use the smaller orifice.

I want to switch to a TEV instead of a captube due to load variations, which a captube cannot handle well. I see this problem occurring in almost every CPU fridge using a captube, homebuilt or not.

As you and some others here at the forum told me, a TEV should perform better under these conditions (and another reason to try a TEV is that I want to see how such a device behaves. I am just curious). Also, a TEV should work with less tuning than a captube

Can you explain me why the shutoff valve idea works? I understand that, per unit of time, less refrigerant enters the evaporator, but at the same time this will generate a pressure drop at the TEV inlet as soon as it opens. Thus, the liquid refrigerant starts boiling at the refrigerant inlet. To me, this seems to be an undesirable situation, since the TEV cannot do it's job anymore under those conditions. Are these thoughts correct?

Unfortunately I do not have access to high/low side pressures. I know that current low side pressure is between 1150mbar and 1300mbar absolute. I have no idea about high side pressures. I can only translate temps into pressures.

I would love to use the smaller orifice, if only I knew where to get one. I asked a few AC engineers, but they don't have them in their service cars since they never use them.

As Gary notes, placing a shutoff valve ahead of the TEV to restrict refrigerant flow is not an ideal situation. You are correct, the shutoff valve will create a pressure drop which will cause refrigerant to flash, and the flashing will restrict flow across the TEV. So one has to "fine tune" the shutoff valve to get the desired effect.... :(

The Prof is not too concerned with the flash gas wearing the valve out. The problem with using the shutoff valve in this manner is it effectively overrides the function of the TEV, and the TEV ends up performing no better than a cap tube or an orifice.

Well, using a shutoff valve is not such a good idea then, since I want to use a TEV to cope with load variations.

Based on your experience in practice, what is the effect of using a one step too large orifice (which is what I am doing when I mount the 00 orifice)? Will this seriously impact performance? I still have no idea what exactly goes wrong, and what the impact is.

Of course I will try to get a 0X orifice.

Quote:

---

The problem with using the shutoff valve in this manner is it effectively overrides the function of the TEV, and the TEV ends up performing no better than a cap tube or an orifice.

---

I'm thinking the restriction needed to stop hunting would be minimal, and the performance somewhat better than a cap tube, but the Prof is far more knowledgeable in this area than I. :)

The function of the TEV is, at times, overrided on purpose. One example would be a system which employs electronic controls to cycle the liquid line solenoid valve to control air temperature. The TEV is allowed to control refrigerant flow during pulldown. But once the desired air temperature is reached, the electronics take control of the flow rate via the solenoid valve, and the TEV is left bouncing around, and not doing any real flow control.

This approach can be made to work quite well, but the Prof would prefer an electronic evaporator pressure regulator control air temperature, which would allow the TEV to function as it is designed to do.

Using a hand valve ahead of an oversized TEV to restrict flow will invariably cause the TEV to go wide open and act like an orifice. But perhaps some careful tweaking of the hand valve may allow the TEV to do some regulation.

For the curious: I have updated my website. It is still a bit behind, but some of you might like to see how the PC is chilled.

The URL of my site is:
http://dabit.trybit.com

The phase-change section:
http://www.vanderhoeven.nu/dabit/pha.../pc_index.html

I know the damned thing is ugly and hard to navigate, but making a nice site is far at the bottom of my priority list. I still prefer to fire up the torch or soldering iron over writing boring HTML.

Update about the system with TEV: still no filter/dryer and orifice 0X :(

.... Someone, please, put a dryer in the mail to DaBit!!!!!!!

Don't forget the orifice 0X >:)