Second stage Refrigerant in cascade

[Skull Knight]

Hi im sorry if this question was already answered i did some research but found nothing on it.

From the research i did the most commonly used gas for the second stage in cascade is the R1150 which evaporate around -145c but have critical temperature of 9.5c but im wondering if people have considered using CO2 as it around -130c but have critical temperature at 31c so in theory we can put more refrigerant in the system before getting pressure that go out of safe zone when the cascade is turn off as the pressure at 9.5c of co2 is around 217 psi and 1150 is 800 psi. If im wrong correct me but if we can put more refrigerant we get more heating absorbtion capacity with same compressor as from what i understand the charging in refrigerant of cascade unit is done in with the pressure when the system is off so it dont explode as pressure get too high for our common refrigeration component.
R-1150
http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/Ethylene_Vapor_Pressure.GIF

Co2
http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/Carbon_dioxide_Vapor_Pressure.GIF

[SaFrOuT]

pictures not working

[[XC] MarioMaster]

the problem with CO2 as a cascade refrigerant is that there are problems with dry ice forming and clogging up the evaporator

[Skull Knight]

never thought about that it going to go in sublimation as our objective is lower than it sublimation point i should had done more research. Thank for reply

[Brettbeck]

R1150 doesn't evaporate at -145°c. It boils at -103°c at 1.013 bar.

Also, there is no way you can get co2 (r744) to -130°c either. It boils (sublimates) at -78.5°c.

You don't get dry-ice issues if you mix the co2 in with a small bit of something else, e.g. r290.

[Clemmaster]

Anyway the static pressure depends on the quantity of gas you put into the system too, you'll never have 800PSI static pressure with ethylen because of the ambiant temperature.

[Brettbeck]

You would have to cool down the pipework of the 2nd stage to get 800psi static, but I don't think thats such a good idea. What if a brazed joint weakens? It's a disaster waiting to happen...

[sparkY1000]

Assuming you choose the right components and pressure test the system i don't see any reason why a high static charge is (on it's own) a bad thing.

A lot of people i know are building cascades are using suction lines and plate heat exchangers rated to or above 30Bar, if the system is then pressure tested to this there's no reason why it would be unsafe to opperate the system (with appropriate safety devices) slightly below this.

But the main problem i can see with such a high static charge is that the physical amount of high pressure refrigerant moved into the interstage heat exchanger on startup will be much larger from the high suction pressure - statup 'shock' overwhelming the previous interstage heat exchanger.

The other problem i can think of is that for the initial startup period of the second+ stage the load on the compressor motor from the very high suction pressure will be bad...

If you need that much refrigerant in the second+ stage use an expansion tank with a short length of bleed capilary tube and keep the static charge to something sensible such as 300PSI or the system pressure test limit, whichever is less.

The static refrigerant charge on a very high pressure refrigerant secondary+ stage won't be saturated, so the critical temperature of the refrigerant is fairly irrelevant. In a high pressure cascaded system the second+ stage static refrigerant charge should always be gas state. We want to build coolers - not bombs :D

All this is assuming we understand the original question you're asking. If you're asking about gas density at a certain pressure then a high pressure refrigerant will have a lower density than a lower pressure refrigerant.

For example an ethylene gas charge to 200PSI static might involve 200g of refrigerant whereas the same system charged with carbon dioxide to 200PSI might give you 400g of refrigerant. Those figures are made up but is this what you're asking? :)

[Skull Knight]

For example an ethylene gas charge to 200PSI static might involve 200g of refrigerant whereas the same system charged with carbon dioxide to 200PSI might give you 400g of refrigerant. Those figures are made up but is this what you're asking? :)

That exactly what i mean because from what i saw it would seem that you we have 3 way to charge system.

1. you are charging second stage when idle to get static pressure of let say 250 psi. so you either must have much bigger system for your need due to the lack of a balanced charge for load. and by using co2 by this method you can get more refrigerant in the system and having same solidity

2. charging by using entalpy chart and calculating tubing so you know the exact weight of refrigerant you will need for the second stage to get the most effiency

3.sight glass you charge it till you see no bubble.

Those are the 3 way we could charge system but the most proper one is the second one for effiency but it would seem the that the pressure for a proper load will skyrocket if we turn off the system. as all liquid will vaporise due to the fact we break the critical temperature

R1150 doesn't evaporate at -145°c. It boils at -103°c at 1.013 bar.

Also, there is no way you can get co2 (r744) to -130°c either. It boils (sublimates) at -78.5°c.

Hmm your right i should had checked before not to be in vacuum for the boiling point

[SoddemFX]

That exactly what i mean because from what i saw it would seem that you we have 3 way to charge system.

1. you are charging second stage when idle to get static pressure of let say 250 psi. so you either must have much bigger system for your need due to the lack of a balanced charge for load. and by using co2 by this method you can get more refrigerant in the system and having same solidity

2. charging by using entalpy chart and calculating tubing so you know the exact weight of refrigerant you will need for the second stage to get the most effiency

3.sight glass you charge it till you see no bubble.

Those are the 3 way we could charge system but the most proper one is the second one for effiency but it would seem the that the pressure for a proper load will skyrocket if we turn off the system. as all liquid will vaporise due to the fact we break the critical temperature

I don't understand entirely what you're saying but the only way to charge the second+ stage safely and propperly is static. A second+ stage very high pressure refrigerant won't be in saturation at static, the critical point of the refrigerant is irrelevant when the system is static.

You find the correct static charge over many cycles with the system equalising in between until the system contains the correct refrigerant charge. If the static pressure becomes too high and the system still needs a higher charge you add in an expansion tank, the size of this tank is fairly guestimate and the tank can always be larger than needed anyway.

I think making calculations for the systems charge would involve too much error and too many assumptions. It'd be interesting to compare a real second+ stage charge against a calculated one tho.

//Sorry for previous post, i was using Sparkys computer and forgot about user accounts etc

[Clemmaster]

Shorter captube is another way to increase the mass flow and prevent both static pressure from being very high and compressor's motor from overheating :P

[SoddemFX]

...and prevent both static pressure from being very high

:confused:

[Xeon th MG Pony]

Shorter captube is another way to increase the mass flow and prevent both static pressure from being very high and compressor's motor from overheating :P

That makes no sense what so ever, how in your mind would a cap tube effect static charge in any way? Or was this just a typo of sorts?

[Skull Knight]

I don't understand entirely what you're saying but the only way to charge the second+ stage safely and propperly is static. A second+ stage very high pressure refrigerant won't be in saturation at static, the critical point of the refrigerant is irrelevant when the system is static.

You find the correct static charge over many cycles with the system equalising in between until the system contains the correct refrigerant charge. If the static pressure becomes too high and the system still needs a higher charge you add in an expansion tank, the size of this tank is fairly guestimate and the tank can always be larger than needed anyway.

I think making calculations for the systems charge would involve too much error and too many assumptions. It'd be interesting to compare a real second+ stage charge against a calculated one tho.

//Sorry for previous post, i was using Sparkys computer and forgot about user accounts etc

Thank for the explanation on that one using expansion tank is good idea. About what i meant from the start i was afraid if the liquid line is too long all of it liquid will vaporise when we shut the system down and create high pressure that could be dangerous but if we put the cap straight out the condenser we shouldnt have that problem as only 10% of the condenser is supposed to have liquid to supercool the liquid so it dont vaporise in high pressure side. Im not really used to small system like this where components are so close, i mostly work with stuff that have long liquid line but those where never cascade system.

[SoddemFX]

I think i understand what you mean.

If the liquid line on a second stage with a very high pressure refrigerant (such as ethylene) is physically large then the static pressure of the system will be much higher than if it were physically smaller.

Because when the system is static with ambinet temperature all of the former liquid ethylene will be in gas phase - a small amount of liquid at -40C turns into a lot more gas at +20C. This is why you should try to keep the liquid lines on a high pressure cascades second+ stage as short as possible.

On a single stage it is less important, single stage systems the charge can even be in saturation when static because of the lower pressure of the refrigerant. For example if you were to build a rotary r22 single stage with a TEV / reciever and large evaporator combination it almost certainly would be because of the small free space in the compressors shell (which is part of the low side). All the little household A/C systems i've seen are anyway.

I think the way you're thinking is because normal single cycle refrigeration systems are much different to high pressure cascades and what holds for one doesn't for the other :)

Tom