Autocascade Discussion and build attempt

[Russell_hq]

Ok, I plan to build an autocascade to cool my chip, i've got a brand new Electrolux ML80FB Low Temp R404a 1/4HP compressor that I bought for a single stage propane system, seeing how I paid good money for it I want to try and use it in the autocascade. Here is my schematic for the system:


I will add a drier or 2 and sight glasses in the usual places to keep things dry and so i can see whats going on. For the heat exchanger im thinking of using a liquid line heat exchanger like the ones from doucette industries (suction line HX ) and for the evaporative condensor I might make a coaxial coil. The phase separator will be based on the design in the US patent No. 6457325 (Search Here ). I plan on using propane and R23, I was going to use CO2 but didnt realise that the triple point temp was as high as it was, -56ºC, so that kinda defeats the purpose of going for the lower temps. I read that some of you guys have built these, captin cascade and aenigma, so some thoughs and experiences would be greatly appreciated.

[captaincascade]

have you built a single stage system yet?

[aenigma]

I used propane and nitrous oxide for my autocascade.I used a puny little 1/3hp and it cant quite handle the mass flow of an autocascade.My 1st stage txv is a weird sucker, it is for R13B1, so it is tempermental to say the least.Not only that but the 2nd stage cap is just used capillary all spliced together and I hate having to do that, I need to start out with one solid length.

Even with the small compressor and metering oddities it gets about -105f/-77c unloaded, which isnt bad at all for a little 1/3hp compressor and a high compression ratio.I never seem to end up using what I build to cool anything, so no loaded temps.
The n2o and propane seem to form a blend which lowers the interstage temp but increases the 2nd stage evap temp and makes it a nightmare when using a vapor pressure diagram.
Soon I am going to be replacing the txv, capillary and the compressor.Then I will see what she can do.

For an autocascade I would want to stick with something over 1/2hp, but even with a small compressor it does alright.

Go ahead and pm me if you want to talk about it.

[Russell_hq]

ok thanks for the info. I will see about getting some nitrous, it might be easier and cheaper than refrigerant for me to get my hands on. Captain, i've built 2 or 3 direct die systems and played around with them alot, designed and made my own evaps aswell. I've come to the point where I cant get much colder with the propane that i am using, so I want to take the next step and get alot colder I've got most the tools and equipment now, and i've always been a hands on, DIY kinda guy always building things. Its fun. So this will be my next challenge. Aenigma and captain, what did you use/how did you build your evaporating condensor? Also, would you suggest using a TXV to meter the propane as apposed to a cap tube. I can get one for £20 with a 00 oriface (Rated for 0.5kW with R22). I'm looking at a catalogue and there are various types and im not sure what to go for. The internal equaliser version is £10 cheaper than the external equaliser type, I dont know what I would want but the one thats £10 cheaper would be nice if it was the right one Also they have different MOP ºC values, 0, -10, -13 and no setting. I have no idea what a MOP value is and what it does. So you guys in the know think you could explain this to me, thanks.

Here is the catalogue link. TXV

Manufacturers datasheet TXV

[berkut]

Are these ALCO txv's ?

If they are, i own a ALCO TI-HW, ive testes both 0X and 00X orifices.

If you need some info feel free to ask

[berkut]

Do you want better datasheets ? I have a Alco catalogue and theres some more usefull data over there

[Russell_hq]

Yup, they are alco. They are supplied with flare inlets and solder outlets. So im going to get a flare to tube adapter for it. I dont have any flaring tools. The 00X oriface would appear to be enough for the job, 0.5kW nominal. Would you go for this oriface? I dont know what else to ask as i dont know what I need to know to ask it. Got any info you think is worth sharing. Also note that I have never used one of these before and have had no experience with them at all.

Data sheets are always good. Just tell me the link

[berkut]

here you go:

http://paranoya.org/~berkut/datasheet.JPG

You can check yourself now how does it suit ;]

[Russell_hq]

From the book, if im condensing between 25-30ºC (resonable for 18ºC room temp) and evaporating between -30 to -40ºC then the 00X oriface will give me between 0.2kW and 0.27kW and the 00 oriface will give 0.51kW to 0.70KW.

From wattage calc (CPUWattCalc) if I can push my 2500+ Barton to 3GHz at 2.2v then it will put out 200Watts max and 175Watts at 88% (info in the calculator webpage explains the 88% value)

So it looks like the 00X oriface would just about do it and the 00 would be too large. What do you think?

[berkut]

I dont know how high are the loads on the heat exchanger but remember you have to condense the gas i dont think its only 200W

[Russell_hq]

If I want to evaporate at 1atm (-82ºC R23) I will have;

heat of vaporisation: 250kJ/kg
heat load: 175 Watts
mass flow = 2.52 kg/hr

now if I want to cool 2.52kg/hr of R23 at say 250psi I will have:

Heat of condensation(-10ºC) : 152kJ/kg
Desuperheat(25ºC to -10ºC): 50kJ/kg
Total: 202kJ/kg

As the heat per kg is less and the mass flow constant then i'd conclude that the heat load would be less. maybe i missed something in these simple calculations.

Info obtained from: Molier Data Sheet: R23

[Russell_hq]

Why would the heat load on the heat exchanger be significantly greater than that on the evaporator, all im doing is shifting energy to get lower temps to remove the cpu energy, or am I missing something? As long as the insulation is good then the heat load from atmosphere wont be that great.

[aenigma]

I would recomend a txv for 1st stage, or any kind of metering device you can adjust.MOP should maximum opening pressure, those temperatures are the maximum sst the valve will open I believe.

My interstage is a tube in shell I made from 2 1/8" pipe and 2 parallel coils of 3/16" pipe with a 3/4" pipe inside that they are wrapped around to take up volume to reduce 1st stage refrigerant needed.Captain cascade is using a factory cascade interstage, they are basically large sution/liquid heat exchangers.

[Gary Lloyd]

Hmmmm... For starters, "evaporative condenser" has a very specific meaning, and that ain't it. Let's call that thing an interstage condenser, unless/until CC or someone can tell us the proper name for it.

An external equalizer is needed where there is significant pressure drop through the evaporator, which is probably not the case here. If in doubt, it doesn't hurt.

MOP is maximum operating pressure. This is used where the TXV, when wide open, allows more heat load to be picked up than the compressor and/or condenser can handle. It keeps the compressor from overloading. Again, probably not needed here.

Cap tubes have an advantage in that the refrigerant charge is limited, which is a good thing when dealing with high pressure refrigerants, as the pressure when the system is off at room temp (static pressure) is not as high.

I have zero experience with autocascades, but it seems to me that using a TXV for the high stage (interstage condenser) would simplify charging. I could be wrong, though. Keep in mind that the pressure drop is higher, so we need the smallest orifice possible.

[aenigma]

Yeah thats why I used the txv for the 1st stage.Just adjust on the fly with super heat control.But remember to adjust superheat after you are running with the 2nd stage refrigerant in the system.The higher pressure affects the operation of the txv and it needs defferant superheat adjustments.

One thing I found on my autocascade is that it really could have benefitted from a cpr valve as pull down made the compressor moan and groan.Static pressure was usually only 200psi.Also start up pressure can really get high, usually about 350 or so.But I have a 2nd stage discharge dump tank solenoid that dumps it to the relief tank which returns to suction via capillary.
Definately plan on using a big relief tank and making it a dump tank would be a good idea too.

Its not like a regular cascade where the only refrigerant in it is the 2nd stage refrigerant at 100psi static.You have to deal with the static from your 1st stage refrigerant combined with what your 2nd stage refrigerant adds.Which could be up to 100psi over 1st stage static if you dont have a big relief tank.

[Russell_hq]

What high side pressures do you run at? Do you thing 250psi is reasonable for a 1/4 HP R404a compressor?

[Gary Lloyd]

One thing I found on my autocascade is that it really could have benefitted from a cpr valve as pull down made the compressor moan and groan.

Possibly a TXV with MOP is needed then, as it serves the same purpose as the CPR.

[Russell_hq]

Well I found a supply of R23 , 100g for £111 in disposable cylinder.
100g does not seem alot to play with so I have emailed to see what they can do.

From my diagram, would it suggest that the pressure the propane evaporates is the same as the pressure the R23 evaporates. And same with condensing pressures. Is this the case?

I see what you are saying with the oriface, and the MOP. How much MOP are we talking about? Also guys, whats you take on what the load on the interstage condensor would be compared to the load on the evaporator?

[aenigma]

I was running about 250psi with my 1/3hp compressor.The same model is listed with 2 refrigerant so I dont know if its r134a or r12, they are both low pressure refrigerants though.Nitrous oxide as a refrigerant does ok.It isnt that good, but it is pretty cheap and it seems to have low discharge pressures.

High and low side pressure should be pretty uniform through the system.As far as interstage load goes it shouldnt be that much more than evaporator load.But you should compensate due to heat ingress from imperfect insulation and the 2nd stage refrigerant being warm when it enters the interstage.

Gary:
Yeah I was thinking the same thing, but mine has a MOP of ~35psi but it still groans on pull down which I believe is due to static pressure being pretty high.

[DaBit]

High static pressures indeed give the compresor a hard time during startup. No EPR valve or MOP TEV can change that; what's in the shell is in the shell.

I would use two expansion tanks; one at the low side, one at the high. This is the easiest way to keep the high side pressure from rising to extreme pressures during startup. These tanks should be dimensioned in such a way that all high pressure refrigerant fits in at a reasonable pressure (300psi is acceptable for a short period of time)

About the different enthalpies of evaporating and condensing: I have no idea but I stumbled across this too.

[Russell_hq]

I had a thought on the different enthapies. When you draw the cycle on a P-h chart, the line representing expansion of the refrigerant is drawn vertically downwards. Thus if you have some sub cooling then the line will travel down, cross the saturated liquid line and stop when it reaches the evaporating pressure/temperature. Thus we do not have 100% saturated liquid, more a mix of saturated vapour and liquid. So the available heat of evaporation will be less as the liquid fraction will be less than 1. Get it?

I did this propane cycle:

Condensing Temp:30ºC
Evaporating Temp:-42ºC
Subcool:8k
Superheat:10k

Now if we look at the cycle now and measure the difference in enthalpies across the evaporation stage and condensation stage we see:

Evaporation:271kJ/kg
Condensation:326kJ/kg

Thats a 17% increase in load on the condensor. The more subcooling that we can get then the larger the the evaporating enthalpy we get thus reducing the load on the condensor.

Now if I apply this to R23 we get the following:

Condensing Temp:-10ºC
Evaporating Temp:-82C
Subcool:8k
Superheat:10k

Evaporation:153kJ/kg
Condensation:157kJ/kg

This represents a 2.5% increase in load on the heat exchanger.
The difference is only small in this case because the gradient of the saturated vapour line is near 1 across the pressure range interested, and since the gradient of the expansion line is 1 then the difference in enthalpies is very small. In this situation, if we can achieve more subcooling, then the evaporating enthalpy will be greater than the condensing enthalpy and the load across the heat exchanger will be less than the load on the evap

Lets look at the energy involved in superheat and subcooling for R23.

Exit Temp From Condensor: 30ºC (Same as propane condensing temp)
Condensing Temp: -10ºC
dH for desuperheat: 43kJ/kg

8K subcooling dh: 12.7kJ/kg
15k superheat dh: 9.8kJ/kg

Ok, it takes quite a bit to cool down to evaporating temps.

Im going to stop now, there is too much numbers and theories and strategies running through my head. Im going to excel this sucker and balance it up with heat loads mass flows and enthalpies and see what happens

[Russell_hq]

Thanks pc ice, thats good to know

[Russell_hq]

Does anyone know how to calculate isentropic efficiency? Im assuming a value of 0.8. Looking at my compressor tech specs:

Cooling Capacity:253Watts
Power Consumption:324Watts
Efficiency: 0.78

Does that look right?

[aenigma]

Originally posted by DaBit
I would use two expansion tanks; one at the low side, one at the high. This is the easiest way to keep the high side pressure from rising to extreme pressures during startup. These tanks should be dimensioned in such a way that all high pressure refrigerant fits in at a reasonable pressure (300psi is acceptable for a short period of time)

I only use one expansion tank connected to low side with a lenght of capillary, high side dumps to it on start up with a solenoid valve and that lowers head pressure alot on start up.I havent messed with it for awhile but I think it kept pressures below 200psi instead of over 350psi.

[Russell_hq]

Could I use one of these ball valves to isolate an expansion tank to reduce high side pressures on start-up instead of a solenoid. I dont want to use a solenoid.

http://www.hawco-refrigeration.com/c...ction2_p30.htm