AutoCascade with Plate Heat-Exchangers

[Kevin Hotton]

I mentioned that I have built AutoCascade refrigeration units using plate heat-exchangers. These are a few pictures of them. The unit is for liquefaction of CO2 at craft breweries. The refrigeration system is a morph'd AC unit (compressor is rotary of 24 cc/rev displacement).

Kevin

[Jack]

It took me a good few minutes to figure out what is what, but I think I got it now:

Bottom plate HX is evap
Top front HX is cascade HX
Top back HX is auxillary HX
The long copper line going to the left is the liquid line.
Bigger, capped off, copper tube in the back is suction.

This might save others some headache

Are the horizontally mounted plate heat exchangers working well?
They're typically mounted vertically? I thought they had to be vertical to work properly...

[Kevin Hotton]

Hi Jack,

Yes, you have figured out the plumbing correctly. I mount the heat-exchangers horizontally with the ports down for a reason. The flows in an autocascade are predominately two-phase (vapor and liquid). It is important to let these two-phase flows move through all heat-exchangers without trapping liquid (thereby disrupting the precise mixed refrigerant composition). I have found that this heat-exchanger orientation works well.

Plate heat-exchangers are not without fault. They, by their design, have parallel flow circuits of near equal size (7-channels / 8-channels in a 16-plate unit). This is not optimal for an autocascade system. The low-pressure flow volume through the heat-exchangers is about four times greater than on the high-pressure side. This results in too low a flow velocity on the high-pressure side – hurting performance. Tube-in-tube coiled heat-exchangers made of copper are better in this regard (with the low pressure flow cross-sectional area usually over three times larger than the high-pressure area). The big limitation with tube-in-tube construction is the difficulty in creating large heat exchange area. Most hobby builders cannot bend tube beyond 1/2-inch OD. A 1/2-inch with two 3/16-inch OD tubes within will not allow refrigeration duty’s greater than perhaps 300W. Further, the insulation around tube-in-tube heat-exchangers is much more important than it is with plate heat-exchangers (tube-in-tube have much greater heat loss area relative to the internal heat transfer area than do plate heat-exchangers).

I have made a small autocascade unit (I discussed in an earlier thread) that had coiled 5/8-inch outer with three 3/16-inch inner tubes. This required the purchase of an expensive bender ($300). So in summary, plate heat-exchangers are a compromise taken to easily construct larger autocascade systems (whose performance will be slightly degraded by there use).

Kevin

[Jack]

Thanks for the explanation. You know your stuff.

A plate HX does make a nice compact unit.

Can you give us some numbers? Cooling, hx capacity etc.?

[Kevin Hotton]

The plate HX’s are all 16-plate models from Duda Diesel (model B3-36A). I special ordered them with female sweat ports for 3/4 inch copper. I estimate the cooling duty at 750W at -60C evaporator inlet temperature. There is considerable temperature glide across the evaporator with the discharge temperature -45C. Suction pressure is 3.5-bar and discharge is 17-bar (both gauge pressures). Based on my earlier autocascade builds using coiled tube-in-tube HX’s, if they had been used in this system, that duty may have been increased to 850W. The charge uses R-123, R-22, R-23 and small amount of R-14 (if I didn't need to use non-flammable refrigerants, n-butane, propene, and ethylene would have likely increased performance).

Kevin

[n00b 0f l337]

Any update? And the plate hx's are nice, and hopefully they work well in that position.
I actually just scored a "used" but epicly huge plate hx on ebay and as always I'm reconsidering finding some time to build something...
It would appear tho that the goal is to a large plate hx with not that many plates.

[samsarulz]

It looks strange. Never seen something like that before. Hope the project is going well. Looking forward to numbers and maybe something else

[Buckeye]

NOL that is a beast of an exchanger !!!

What did you it weighed ... 55 lbs ?

[n00b 0f l337]

Haha yeap Buckeye.
34 plates, 13" x 5".
And for now, it's a great paperweight.
But the size, not the plates, is what Kevin's trying to point out for this application.

[duck]

Woo!! great

[4Qman]

Very interesting, nice build.

Noob, that HX

[chispy]

amazing job on the autocascade !

[sdumper]

Looks amazing not to mention confusing as heck. One of these days im going to try to build one myself but I still have a lot to figure out first.