Cascade: Refrigerant blending and Oil Management

[sdumper]

Guys, I normally use r-1150 in 2 stage cascades because frankly its usually pointless to use anything else.

That said I have a client that wishes to use r-23 and also would like it to hold on the contact point -80c under loads ranging from 100 watts to 250 watts.

Given the following properties of both I dont think im going to be able to obtain a guaranteed temp of -80c am I correct?

r-23
Appearance: Clear, colourless liquid and vapour.
Physical State: Gas at ambient pressure and temperature.
Chemical Formula: CHF3
Odour: Faint ethereal odour.
pH Neutral
Boiling Point: -82.1oC (-115.7oF)
Freezing Point: -155oC (-247oF)
Vapour Pressure: 42.4 Bar at 21oC (70oF)
Vapour Density (Air =1.0) 2.4


r-1150
APPEARANCE, ODOR AND STATE: Colorless gas at normal temperature and pressure; faint, sweet, musty odor.
MOLECULAR WEIGHT: 28.05
SPECIFIC GRAVITY (air=1) at 32?F (0?C) and 1 atm: 0.978
GAS DENSITY at 32?F (0?C) and 1 atm: 0.0787 lb/ft3 (1.261 kg/m3)
SOLUBILITY IN WATER, vol/vol at 32?F (0?C) and 1 atm: 0.26
BOILING POINT at 1 atm: -154.62?F (-103.68?C)
MELTING POINT at 1 atm: -272.9?F (-169.4?C)
% VOLATILES BY VOLUME: 100

[mytekcontrols]

Hi Scott,

Do you have access to R14? Adding a small amount of this to R23 wil allow you to better achieve your stated goal. Since you are only looking for -80 under load, it won't take much R14 to do this.

[sdumper]

I do have access to r-14 but I thought its condensing temp was -74c?

Also any ideas what might be happening here with these charts? Do you think the TXV isnt adjusted for a steady temp?

Not sure why I cant upload a picture but here is a graph of whats happening.



It basically holds at -80c for 4 hours or so and then it starts to spike and recover.

Ideas:
Oil Return causing spike
TXV adjusted incorrectly
Water vapor in system
excess ambient heat (room constant 68f)

[DFI pit bull]

Hi Scott,

Do you have access to R14? Adding a small amount of this to R23 wil allow you to better achieve your stated goal. Since you are only looking for -80 under load, it won't take much R14 to do this.

That's very usefull information, had no idea R14 could be used like that. Thanks.



Sorry sdumper can't offer you advice, still learning myself, good luck, hope you achieve your goal.

[mytekcontrols]

That's very usefull information, had no idea R14 could be used like that. Thanks.

Basically it's how blends such as R507 are made. Most refrigerants, especially ones within the same family, have some degree of solubility with each other. However when combining ones that have a wide spread of boiling points, such as R23 and R14 in a single-stage it is important to take condensing temperatures/pressures into consideration as Scott pointed out. So that is why I recommended using only a small amount of R14, and the best way to do this would be to add it to pure R23 in a running fully loaded system just a little at a time until the target temperature is achieved. Keep in mind that the more R14 added, the higher will be you're compressor discharge pressure and temperature.

[sdumper]

I had them adjust the txv and it ran for a little over 12 hours at -82c on the heating element and then this morning it started to spike. Im wondering if the shop temperature is cycling up since its a holiday weekend and the office is empty?

Details: Cased unit
Stage 1: 3/4 hp low temp 230 volt Embraco
Stage gas: r402a
Stage2: 1/2 hp low temp 230 volt Embraco
Stage 2 gas: r1150




Pictures at my place while I was working on it.

[sdumper]

Hmm im wondering if maybe it might have some moisture build up in the line...
It ran for a solid 12 hours before it fluctuated and recovered.

[sdumper]

Bump any thoughts? Does it seem like maybe its moisture? Strange thing is the original design had no filter/drier on the second stage and I didnt add one.

[n00b 0f l337]

it seems more like frozen oil then moisture but those are very possible explanations to the problem. What were the running pressures

[sdumper]

200psi under load 145 no load...

[n00b 0f l337]

I mean the pressure during the spike

[sdumper]

Id like to know too but its in California and Im not sure if they have a way to log the pressure.
Ill ask...

[Patrickclouds]

maybe it is the oil seperator that opens at the moment for oil return.
that causes higher suction pressure and temperature increase.
or didn't you use an oil sep?

[bazx]

maybe it is the oil seperator that opens at the moment for oil return.
that causes higher suction pressure and temperature increase.
or didn't you use an oil sep?

i would tend to agree with patrick the oil sep would need to open at some point if fitted

[sdumper]

Yes its using a floated oil sep.

Maybe a larger 2nd stage compressor to help compensate?

[Patrickclouds]

you can use a long captube from oil sep to suction line.

and maybe a small filter to collect the oil after the oil sep.

[sdumper]

Thank you sir!!

[Patrickclouds]

but a very long and thin one.
maybe 4-5m 0,7mm captube

[Sk_rmouche]

Basically it's how blends such as R507 are made. Most refrigerants, especially ones within the same family, have some degree of solubility with each other. However when combining ones that have a wide spread of boiling points

With this idea, is there something to expect with a blend of R1150/R50 ? Regardless of higher discharge pressure obviously !

[sdumper]

Depends on the condensing temp/pressure of methane.
What temp does methane condense at?

[Expat GriZ]

-161 degrees C ?

[sdumper]

I think that must be boiling point what would be useful is to know the condensing temps at varies pressures.
Methane is going to be very tough to condense considering that r1150 condenses at around -40c at a 100 psi...

[n00b 0f l337]

100psi? More like 195 psi/14.5 bar.

[sdumper]

Thanks Adam.


Expat Griz that makes it even worse ...
I dont think r50 and r1150 is going to give you much but MyTekControls is probably the best one to answer

[Sk_rmouche]

That question was already treated on my thread, but I didn't recall if we discussed about ethylene/methane together lowering the evap temp @ given pressure ...