corrugated pipe = more surface area. pipe is cheap too home depot 5.00 two feet cost 15 for two heat exchangers.. These are basically the smae things you can buy for 30 bucks at johnstone there called suction line subcoolers.
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corrugated pipe = more surface area. pipe is cheap too home depot 5.00 two feet cost 15 for two heat exchangers.. These are basically the smae things you can buy for 30 bucks at johnstone there called suction line subcoolers.
Quote:
Originally posted by Gary Lloyd
The inside. When you pull the end bells to punch the tubes, they are water tubes.
As to the insulation argument, we have two fluids below room temp, and we want to insulate both. There is less heat transfer through the insulation if there is less temp difference. If the warmer of the two fluids is on the outside there is less temp difference. The vapor is being cooled by the liquid, therefore the liquid is the colder fluid and the vapor is the warmer fluid.
ok well how about this arguement. Ive taken(ripped) apart many cascade systems and all of those that utilized a coaxial heatexchanger, had the 2nd stage on the inside. Not to mention ive built more than a couple working cascade systems. Nevermind the water arguement, we are talking about seriously low temps here, and the heat load picked up on the outside is not worth it. trust me gary. this is what i do.
Sorry about that one aenigma, Im am very happy to listen to your advise and I think this cascade discussion is bringing up alot of points and trying to find out who is right or wrong is not as straight forward as we would like. On that note I want to ask if we can work out the problem with the high R290 evap exit temps without having to redo the piping. From what captain says, the way I have it is standard for cascade but Gary has his opinion, which should not be disregarded off hand as he is a and that would mean some tricky reworking of the pipes (not imposible but definitly a tricky one) Could it be there is too much volume in the heat exchanger and not enough R290 so the liquid isint getting near the exit and the warm R23 coming in is keeping the temps up? Just a thought.Quote:
this is what I get when I listen to aenigma and not you.
The lemming augment, its ok if it is done by another... There may be a reason why the commercial cascade coolers are done this way. Either way the surface area transferring the heat to the refirgerant is basically the samp but the surface area at a lower temp is significantly larger with the evaporating side is out.Also most of these cabinets have the exchangers in the cabinet walls ours is external to the cooled device. It maybe that this additional cooling is used for some other purpose in these cabinets such as subcooling or precooling?
Well... I could be wrong, but I don't think I am. It just makes more sense to me for reasons I have already stated. How others do it doesn't particularly impress me, since it is not accompanied by the reasoning behind their decision. It makes perfect sense to have the condenser on the outside, evaporator on the inside, and Chilly's experience seems to support the theory.
We are probably not going to find any definitive answer, short of switching them to test the theory.
In the meantime, this should not stop us from chopping pieces off the R290 cap tube until we find the right length. Once we get it all working the best it can, we can try the switch if you like, to see if it does indeed improve the performance.
On the R23 charging, let's go for lowest CPU temp instead of outlet superheat, and see if that works out better.
This is from a kelvinator training tape dated 1986 for cascade systems.
Notice the use of an additional refrigerant this is for oil return / flock prevention.
http://www.blairwing.com/images/cascade1.jpg
The condenser is counterflow and the evaporator is inside
http://www.blairwing.com/images/cascade2.jpg
This is the interior of the interstage heatexchanger notice the rough surface and the spirals, the spirals and roughsurface cause turbulance which will create better heat transferr
http://www.blairwing.com/images/cascade3.jpg
The expansion tank usually only used where lowstarting torque compressors are used or where internal volume is needed for more refrigerant and to reduce static
http://www.blairwing.com/images/cascade4.jpg
Rupture disk is used as a safty device depending on design it can be anywhere from 175 to 450 psi and and is usually a one time fail device.
http://www.blairwing.com/images/cascade5.jpg
The strainer is used to protect the capillary or metering device from plugging, the heaters are used to prevent oil flocking in these areas they heat this area up to 0F
http://www.blairwing.com/images/cascade6.jpg
I hope this clears this up thanks...
Notice the small volume in that heat exchanger?Not like a 1/2" tube around a 1/4" tube.And a whole lot of surface area on the evaporator pipe compared to 1/4" in 1/2".I believe these 2 things would be necessary if your evaporator is on the inside of the pipe.
I really dont see it as practical to change russels current piping just to try a theory when the original way works very good.
Those are some very cool heat exchanger though, I like the spiral.Although both condensers being shown upside down is quite questionable. :rolleyes:
Here's something else to think about:
We want the liquid in the evaporator and the vapor in the condenser to exchange heat. The entire heat exchanger is wound up in a spiral. Where does centrifugal force concentrate the liquid in each tube? The liquid is thrown to the outside curve in both tubes, right?
If the evaporator is the inner tube, then this is exactly where we want the liquid to be in both tubes. If the evaporator is the outer tube, this is the opposite of where we want it to be in both tubes.
Calculate centrifugal forces with the refrigerant speeds and masses encountered, and you will see that these are neglectible. Besides that, boiling and condesning change the picture. You won't see sewer flow in correctly dimensioned evaporator pipes, not even if the micture at that point is only 20% liquid. The liquid keeps wetting the pipes.
I use the outer pipe as evaporator. Why? Liquid refrigerant will back up in the evaporator, wets the condenser walls thoroughly, and stabilizes the high stage during large load fluctuations, such as on low stage compressor turnon.
The other reason is minimalisation of high side volume. This lowers static pressures since at runtime the gas density at the high side is much higher than gas density at the low side, and this density must be maintained, so more refrigerant is needed.
BTY the heatexchanger is 30 dollars US and it is a Peerless HRX-75 this is good for 1/3 hp compressors. also notice that there is a desuperheating coil at the discharge of the compressor as well as a suction accumulator with heatexchanger so when the refrigerant gets to the condenser it will be down to ambient or less. And yes the heatexchangers are up side down for oil drain back. And this according to Kelvinator, the suction on this unit when properly charged was 5" vacume and the static charge was 5 psi pentane and then topped with 150 psi R503.
ok how bout this. as far as putting the evap on inside or outside. Make the dam thing large enough and insulated propperly, and it doesn't matter.
Well its 6m long and its in a box of expanded foam with 2-5in foam all around. I would say that is enough, I just dont get when the R290 evaporator exit temp is so high.
I need the current TD ..across the condenser, r23 interstage both TD's, across the evaporator and I also need the discharge line temp and suctionline temp. It sounds as though the phase seperator is not effecient enough. Do you have a sight glass in the R290 outlet of the phase seperator, and you may need to put a hot gas bypass in to bypass the R23 by the condenser...
I only have 2 thermocouples and they are secured to the 2 evaporator outlets, im waiting on some more arriving. Below you can see the phase separator, the sight glass is full.
From The Front
http://www.russellpetrie.plus.com/im...eseparator.jpg
From The Side
http://www.russellpetrie.plus.com/im...separator2.jpg
The phase separator is 11" tall and 28mm diameter. Just so I am straight on the temps. You want:
Condensor:
R23/R290 In
R23/R290 Out
Air In
Air Out
Interstage:
R23 In
R23 Out
R290 In
R290 Out
R23 Evaporator:
R23 Out
R23 Saturated temp@evaporating pressure.
Compressor:
Suction line 3" from inlet
Discharge line 3" from outlet
You might also check the temperature in and out of the drier. If the drier is restricted, the pressure drops and therefore the temperature drops. The two temps should be within a degree or two of each other.
What is the suction pressure also?
Currently my suction pressure is 2 barg and my discharge is 20.5 barg. Evap exit temps are -48.1ºC and 17.2ºC. Compressor inlet is subzero as its got a solid chunk of ice on it where I pulled back the insulation as seen below.
http://www.russellpetrie.plus.com/im...cade/frost.jpg
Did a test to see what effect no load had on the system. I took readings after using the computer for at least an hour or 2 then switched it of for 20 minutes and took new readings. These are the results:
Loaded:
Saturated Temperatures
2 barg -14ºC ; -61ºC
20.4 barg 60ºC; -6ºC
R290 Evap Exit: 16.5ºC
R23 Evap Exit: -48ºC
Air In: 20.7ºC
Air Out: 25.8ºC
Computer off for 20 minutes
Saturated Temperatures
1.6 barg -18ºC ; -64ºC
28.4 barg 56ºC; -9ºC
R290 Evap Exit: 14ºC
R23 Evap Exit: -60.9ºC
Air In: 20.7ºC
Air Out: 24.2ºC
Draw any conclusions from these figures?
My conclusion would still be, cut another foot off the R290 cap tube.
Did you check the drier for temperature drop?
Gary's right not enough R290 flow and the suction is a little high. We may need a method of controlling the suction a little better. Maybe another stage oc cooling. Or how about a desuperheater at the compressor outlet and then a suctionline desuperheater and then the phase seperator, so the r23 bypasses the condenser and only the R290 goes through it... I'll draw you a pic....back in a while...
The suction pressure will be whatever it will be, according to the pumping capacity of the compressor, given the high side pressure.
Charging the R23 according to best CPU temp under load I think will help to reduce the high side pressure, as will completely flooding the R290 evap.
The R23 at 20 psi suction would be ok but I would like to see the r290 at 5 psi. Maybe this would help...
moved pic...
Correct me if I'm wrong, but you seem to be separating before the condenser. Condensing is what causes the separation.
And why do we want to raise the evaporator pressure/temperature?
There is a small condenser and a suction desuperheater before the phase seperator The R23 will be at -87 at 20 psi if we put a pressure regulator after the r23 evap we can keep it at that leval and isolate the R290 evap, I may need to split the condenser and put the phase seperator between the 4th and 5th row. But with the discharge pressures we may be below the condensing temp of R290 at the phase seperator. This is just speculation.
If your phase separator works, the condenser becomes a liquid filled subcooler.
I think we will see much improved temps all around once the interstage is flooded. We need to shorten that R290 cap tube, and then see where we stand.
Russel_hq, have you located a spec chart for your compressor?