Cascade System Advice

[_hl4e_halflife_]

I hope a few of you guys that browse these forums are good with cascade systems.

I built a cascade system a while ago that was gonna be used to super cool my CPU but since then i've changed its purpose to chill a small box. I've been making changes and fine tuning it over the past 2 weeks trying to get a box temp of at least -80c.

Im using 2 1HP Rotarys, 1st stage gas is 290 and 2nd stage is a blend of 290/CO2(75%/25%).The HX is 3/8 inside of 7/8 and is about 20 feet long. The TXV on 1st stage is a TX2 R22, and the TXV on 2nd stage is a TY2 R502.

The condensor for 1st stage.
http://img511.imageshack.us/img511/2108/coil19mg.jpg
The Heat Exchanger
http://img511.imageshack.us/img511/1025/hx5ok.jpg
The 1st stage compressor.
http://img511.imageshack.us/img511/5058/rebuilt6iw.jpg
The 2nd stage compressor.
http://img511.imageshack.us/img511/382/2ndcompressor2xj.jpg
The loop near the top of the condensor fan is the desuperheater.
http://img511.imageshack.us/img511/153/desuperheater28bq.jpg
The oil separator is about 10" of 7/8 tubing with copper mesh inside it
http://img511.imageshack.us/img511/5615/oilsep17bp.jpg
The evaporator that coold the whole box.
http://img511.imageshack.us/img511/7687/evap3brazed2gy.jpg


[Edited by _hl4e_halflife_ on 12-11-2005 at 05:08 PM]

[_hl4e_halflife_]

More pics.

This is the temp i got off the suction line.The small thermometer is measuring 1st stage.
http://img504.imageshack.us/img504/894/test47es.jpg
This is the temp i am currently reaching in the box.
http://img504.imageshack.us/img504/595/boxtemp10sn.jpg
The TXV installed for 1st stage.
http://img504.imageshack.us/img504/5996/txvinstalled3mx.jpg
TXV on 2nd stage
http://img504.imageshack.us/img504/6596/2ndtxvinstalled1tu.jpg
The fan that circulates the air in the box
http://img504.imageshack.us/img504/4179/boxfan9ph.jpg
This helps direct the circulating air and provides a floor.
http://img504.imageshack.us/img504/131/fancover9gb.jpg
Last but not least the top cover for the box.
http://img504.imageshack.us/img504/5431/topcover0wd.jpg

After looking at the pics above if anyone can see something that should be changed or something that could be done to help lower the temps in the box please post away.

[_hl4e_halflife_]

Comon not a single reply in the last 48 hours?? im starting to think posting this was a waste of my time.

[eng2sales4mny]

I do alot of plasma design, mostly on large walk-ins though. What is the thickness of your insulation? for something that you are trying to get this cold I would recommend something at least 6". if your superheat is good on each compressor then your system is working as it should. The only ways to increase its capacity to get the box temp down is to either

1)if your compressors are not at full capacity (amps) then

a) increase airflow (CFM) over evap coil, keep in mind that face velocities over 600ft/min are not very productive.

b) increase the size of the evap coil

C) look at a bare tube multi-circuit arangement
which could be made out of 5/16 tubing and 2 pieces of aluminum. The return bend from 1 tube to the next could be difficult to get.

2) if compressors are at full capacity

a) reduce the infiltration load by increasing the thickness of the insulation and eliminate any possible leaks etc into the box.

Hoe this helps, are your intentions to place the CPU unit inside of the box?

[rimek]

What lubricant are you using? WIth that much propane, oil return shouldn't be an issue, though. Do you have any other temp readings? like:
interstage heat xchr inlet and outlet
low stage evap in and out
I can't see where the oil return line goes from the separator. The -101° suction line temp indicates you should be getting lower evap temps- how long did it take to reach these temps? Why co2 mixture instead of more common, like 508b? Any idea what your mixture of propane and co2 evaporates at? How did you engineer your heat xchr?
How did you measure the ratio of gases in your low stage charge? Is this instrument grade 290? And usp CO2?
How about some pressures? both stages? static?
How did you charge this unit? Have you tried adjusting the charge?

Most -80°C boxes use two 1hp recip compressors to cover up to 25 cu ft, with a static cold wall evaporator. The capacity of the system at setpoint with a very low suction pressure is around 900 btu's. Those units also use HFC's and not the same ones in each stage. You could have used one of several common refrigerants in the high stage.
Be patient- some of us don't live on the site.

[_hl4e_halflife_]

Originally posted by eng2sales4mny
I do alot of plasma design, mostly on large walk-ins though. What is the thickness of your insulation? for something that you are trying to get this cold I would recommend something at least 6". if your superheat is good on each compressor then your system is working as it should. The only ways to increase its capacity to get the box temp down is to either

Ok lets see, the styrofoam walls are 2" thick the floor is 3" thick and i had just added some of that yellow wall insulation around the whole box so i don't think much of any heat is getting in.


[i]1)if your compressors are not at full capacity (amps) then

a) increase airflow (CFM) over evap coil, keep in mind that face velocities over 600ft/min are not very productive.

b) increase the size of the evap coil

C) look at a bare tube multi-circuit arangement
which could be made out of 5/16 tubing and 2 pieces of aluminum. The return bend from 1 tube to the next could be difficult to get.[/B]

Both compressors are not drawing there full amps which is about 10.5 to 11 amps each. When the system is up and running the total amps is between 9 and 13.

Theres lots of airflow in the box for it's size so i don't believe that would help much.

Can you explain this "bare tube multi-circuit arangement" further.


[i]2) if compressors are at full capacity

a) reduce the infiltration load by increasing the thickness of the insulation and eliminate any possible leaks etc into the box.

Hoe this helps, are your intentions to place the CPU unit inside of the box? [/B]

Not at this moment i plan to run tests on how hardware and different types of motors are effected by extreme cold.

[Edited by _hl4e_halflife_ on 12-15-2005 at 12:34 AM]

[_hl4e_halflife_]

Originally posted by rimek
What lubricant are you using? WIth that much propane, oil return shouldn't be an issue, though.

First stage compressor is using POE oil and 2nd stage is using MO, In 2nd stage a large amount of 290 is needed to prevent the co2 from turning into dryice and blocking the evap.


[i]Do you have any other temp readings? like:
interstage heat xchr inlet and outlet
low stage evap in and out.[/B]

I have taken some temp reading right after the 1st stage TXV (-42c) also taken readings at the 1st stage suction (-34c). I did get a temp reading the other day of the 2nd stage before the evap coil of -88c, I had reached -100c using a different expansion device since than it was replaced with the TXV to see how it would preform under very low temps.



[i]I can't see where the oil return line goes from the separator. The -101° suction line temp indicates you should be getting lower evap temps- how long did it take to reach these temps? Why co2 mixture instead of more common, like 508b? Any idea what your mixture of propane and co2 evaporates at? How did you engineer your heat xchr?.[/B]

If you look at the pic with the 1st stage TXV theres a manual valve at the end of the oil separator and a line of captub that runs along the floor of the base,under the 2nd stage compressor, up the other side and into the suction line. Getting to -100c was dam hard work using co2 and took quite a while.

I don't have access to stuff like 508b,503,R23,R13,R14 and i can't afford 1150 so i had to go with co2.

I dono exactly what temp the blend evaporates at, as for the HX i just put a long straight peice 3/4 inside of 7/8 and than rolled it up its not pretty but it does the job heh.



[i]How did you measure the ratio of gases in your low stage charge? Is this instrument grade 290? And usp CO2?
How about some pressures? both stages? static?
How did you charge this unit? Have you tried adjusting the charge?B]

To be honest i dono the exact ratio of 290/co2 i adjust the blend based on the suction pressure. If the low side drops to 29-30"hg i know i got dryice forming so i add a small amount of 290,I keep doing this until the low side stays out of a deep vacuum. The 290 im using is the type u can buy in 16.4oz tanks to be used for tiny propane stoves and torches,etc

Pressures for first stage low side when 2nd stage is off are around 3 to 5"hg when 2nd stage is turned on it goes up to 10 to 15psi until 2nd stage brings the box temp down.I currently can't take a high side reading for 1st stage because when i made some changes to 1st stage i didn't install a processing stub.Static pressure for 1st stage is about 125psi.

2nd stage static pressure is about 225psi. I wait till 1st stage brings the pressures in 2nd stage down to 100psi before i start it up, apon startup the high side shoots way up to 300psi. At the same time the low side drops down to 15psi and continues to drop over the next few min to about 4"hg. At that time the high side has droped way down to about 125-150psi where it holds in that area all the time the system is on.


[i]Most -80°C boxes use two 1hp recip compressors to cover up to 25 cu ft, with a static cold wall evaporator. The capacity of the system at setpoint with a very low suction pressure is around 900 btu's. Those units also use HFC's and not the same ones in each stage. You could have used one of several common refrigerants in the high stage.
Be patient- some of us don't live on the site. [/B]

True but its also well known that rotarys just love to run in a vacuum when needed.


[/B][/QUOTE]

[eng2sales4mny]

The Bare tube desing is simple you would take two rectangles or squares of aluminum and drill a symmetric pattern of holes in the sheets at your prefered distance and insert copper tubing into these and arrange it into essentially 2 or more complete refrigeration circuits with a common outlet or suction header. this is usually accomplished with a distributor but can be done without one easily on a system this size. Imagine an evap coil or condenser coil with no fins.

Now your suction is running around -100 C and your box is reaching -76.6 c which means that there is a 24 deg TD on the evap coil. There may be plenty of air flow in side the box but I would say that your evap coil is not as efficient as it needs to be. If you can increase the efficiency of the evap it will transfer more heat and the box temp will get closer to actual suction temp. To determine how effective your insulation is get it down to temp and see how long it takes the box to warm up say 10 degrees. the difference in btu capacity required from a 4 inch to a 2 inch polyeurathane box 2' x 3' x 3' is about 600 btu. This is equivalent to about 175 watts. Wouldn't take long to warm up with that type of infiltration, and that is with no cracks or crevices.

[_hl4e_halflife_]

Originally posted by eng2sales4mny
The Bare tube desing is simple you would take two rectangles or squares of aluminum and drill a symmetric pattern of holes in the sheets at your prefered distance and insert copper tubing into these and arrange it into essentially 2 or more complete refrigeration circuits with a common outlet or suction header. this is usually accomplished with a distributor but can be done without one easily on a system this size. Imagine an evap coil or condenser coil with no fins.

Now your suction is running around -100 C and your box is reaching -76.6 c which means that there is a 24 deg TD on the evap coil. There may be plenty of air flow in side the box but I would say that your evap coil is not as efficient as it needs to be. If you can increase the efficiency of the evap it will transfer more heat and the box temp will get closer to actual suction temp. To determine how effective your insulation is get it down to temp and see how long it takes the box to warm up say 10 degrees. the difference in btu capacity required from a 4 inch to a 2 inch polyeurathane box 2' x 3' x 3' is about 600 btu. This is equivalent to about 175 watts. Wouldn't take long to warm up with that type of infiltration, and that is with no cracks or crevices.


No my suction line was -88c, I had gotten -100c with a service valve believe it or not on 2nd stage instead of a TXV.

[rimek]

Well I think you should check out possible floc or wax problems with mineral oil in low stage(2nd stage?). Most of the minerals have a floc point of -40°, that could be affecting valve operation. Even with the coalescing separators, there will still be some oil out in the system. Because the HFC's we use for the low stage aren't very miscible with synthetic lubes, we add 290 to help return that oil which can restrict and displace refrigerant in an already critical application. Most(90%) of the cascades I work with are cap tubes, but I also only use HFC'S, CFC's and 290 or pentane. I did do a single stage with butane, but it was export and not UL at all. What kind of TXV are you using?
You say "a long time" to reach the -101° on the suction line and -76° on an unloaded evap, how long in hours, days?

I don't know if your interstage is that efficient, I want a drop to -50°C before the low stage starts, then a stasis point of below -35°, where the low stage will cycle on a control(ala d62) to keep the interstage from getting too warm. Again, this is based upon various combinations of 12-13 different HFC's and CFC's. But as to whether this is applicable to your particular blend of compounds, I don't know. Your propane could also have additives or impurities, I only use instrument grade, I have never tried OTC.
Did you use a nitrogen sweep during assembly?
Dry Ice? that shouldn't be happening in the system- are you sure it wasn't wax dropping out of the MO?

It is an interesting project, you have an unloaded system trying to cool another one, using the same refrigerant(except for some CO2-25%, but I figure total system charge for both units is what, about 10-13 ozs?), have you tried 290 all by itself in a single stage yet? If so, what was box temp then?
Can you valve off the low stage compressor to determine what the lowest achievable vacuum is for the pump?(or either compressor for that matter)
Without any scale, I'm guessing that the evaporator liquid line is the smaller diameter- how long is that after outlet connection of TXV? Are you using the element of the TXV?
If we're looking down from the top here, aren't you taking evap outlet above the lowest point in the evap? That's not good for oil return or performance.

Have you tried loading the system yet? Actually cooling something besides air? Definition of a BTU, latent heat load, etc?

[hvac3901]

my question is'nt the mechanics of it all, but how the hell are you going to get to those temps without condensing and then subsequently causing damage to your equipment namely the electronics? what are you really doing with this thing, transporting viruses, like anthrax? stealing organs from unsuspecting people and then delivering them to recipients?

[jimp]

I use 3GS for minus 150c and don't have oil problems.

[_hl4e_halflife_]

Originally posted by rimek
Well I think you should check out possible floc or wax problems with mineral oil in low stage(2nd stage?). Most of the minerals have a floc point of -40°, that could be affecting valve operation. Even with the coalescing separators, there will still be some oil out in the system. Because the HFC's we use for the low stage aren't very miscible with synthetic lubes, we add 290 to help return that oil which can restrict and displace refrigerant in an already critical application. Most(90%) of the cascades I work with are cap tubes, but I also only use HFC'S, CFC's and 290 or pentane. I did do a single stage with butane, but it was export and not UL at all. What kind of TXV are you using?
You say "a long time" to reach the -101° on the suction line and -76° on an unloaded evap, how long in hours, days?

On 2nd stage im using a TY2 502 TXV. I know there not ment to be used in this way below -40c.

I think you misunderstood me,when i said it took a long time to reach -100c i ment that in many test runs making fine tunes each time i ran it. This was over the course of a few weeks i think


[i]I don't know if your interstage is that efficient, I want a drop to -50°C before the low stage starts, then a stasis point of below -35°, where the low stage will cycle on a control(ala d62) to keep the interstage from getting too warm. Again, this is based upon various combinations of 12-13 different HFC's and CFC's. But as to whether this is applicable to your particular blend of compounds, I don't know. Your propane could also have additives or impurities, I only use instrument grade, I have never tried OTC.
Did you use a nitrogen sweep during assembly?
Dry Ice? that shouldn't be happening in the system- are you sure it wasn't wax dropping out of the MO?? [/B]

I think the 290 is clean since i've used it in other systems with out any problems at all.

I'm not sure what you mean by a "nitrogen sweep" or this wax in the oil u speak of?


[i]It is an interesting project, you have an unloaded system trying to cool another one, using the same refrigerant(except for some CO2-25%, but I figure total system charge for both units is what, about 10-13 ozs?), have you tried 290 all by itself in a single stage yet? If so, what was box temp then?

Can you valve off the low stage compressor to determine what the lowest achievable vacuum is for the pump?(or either compressor for that matter)[/B]

I dono the amount the whole system holds but its around 2 lbs. I built a SS system with just 290 (unrelated to the cascade project) and i get a temp of -55c i could make that colder if i use a longer captub.

I don't have a micron gauge yet but both compressors basicly such the manafold gauges to 30"hg.


[i]Without any scale, I'm guessing that the evaporator liquid line is the smaller diameter- how long is that after outlet connection of TXV? Are you using the element of the TXV?
If we're looking down from the top here, aren't you taking evap outlet above the lowest point in the evap? That's not good for oil return or performance.

Have you tried loading the system yet? Actually cooling something besides air? Definition of a BTU, latent heat load, etc? [/B]

The peice of copper tubing right after the TXV that goes to the evap coil is 1/4 i know its small and i been wondering if it being that small might be part of the problem?

By element do you mean the sensor bulb? of so yes i am but im having problems with that as well since it was not ment for -70c temps.

The gas enters the top of the evap goes down one side and back up the other and out. The evap is only about 5" tall so very little if any performance at all is lost.

[_hl4e_halflife_]

Originally posted by hvac3901
my question is'nt the mechanics of it all, but how the hell are you going to get to those temps without condensing and then subsequently causing damage to your equipment namely the electronics? what are you really doing with this thing, transporting viruses, like anthrax? stealing organs from unsuspecting people and then delivering them to recipients?

O no hes found out my secret plan! LOL

Nah im just building it because its my hobby and because i can (ever wonder how long a bug can survive at -75c?) I plan to find out next summer lol

Also I can make vodka popsickles with this thing! Who wants one?

Ps. Heres a link to the orignal thread when i build this thing if anyone is interested in how it was designed and built. There are many pics and the thread is 19 pages long. Enjoy.

http://www.xtremesystems.org/forums/showthread.php?t=62280

[Edited by _hl4e_halflife_ on 12-15-2005 at 11:22 PM]