Ultra low-temperature cascade refrigeration system repair----make money online

hey everyone I recently picked up this

refrigeration unit at a surplus sale and

just got it working last night so I

thought I'd talk about what I learned

about the system and how I got it

working

this unit is called a cold trap and the

idea is that this chamber here gets down

to about negative 100 degrees C and you

connect your vacuum line to this glass

chamber here and submerge this in

alcohol to help conduct the cold

temperatures it to the glass vessel and

then as you're pumping down a system

with your vacuum pump it's so cold on

the exterior of this chamber that all

the impurities in your vacuum line will

get condensed solidified into here and

won't go into your vacuum pump in a

basic phase change refrigeration system

we have a high-pressure side and a

low-pressure side and the two sides are

divided by the compressor itself and a

restriction here and this is very often

a capillary tube so if you ever look

into a refrigerator and see a coil of

small diameter copper tubing it's most

likely serving as this resistance

between the high pressure side and the

low pressure side so the way the system

works is the compressor raises the

pressure here and when we extract heat

from that compressed gas it condenses to

a liquid which gets sent over to the

cold side of the system and since the

pressure is lower over here that liquid

will now evaporate turn back to a gas

and complete the cycle so we get a large

amount of heat moved from here to here

and we're making use of that phase

change point in the refrigerant so what

if we want to make a system that gets

very cold what if we want negative 100

degrees C here how come the system

doesn't just get colder and colder as

it's moving heat from here to here the

answer is that it depends on the fluid

properties of the refrigerant so if the

refrigerant itself boils at negative 40

degrees C at a very very low pressure it

almost doesn't matter how good the

compressor is once this cold side gets

down to negative 40 or a little bit

below that liquid is not going to want

to evaporate and we're not going to get

much more heat transferred now you could

say well I just want you know a bigger

and bigger compressor

I'll just make the pressure lower and

lower here and eventually that liquid

will boil because we can always get the

print of the pressure arbitrarily low

enough but it becomes a hugely

diminishing return and the bigger the

ratio of compression across your

compressor the lower the efficiency of

the whole system so if we want a

negative 100 degrees C refrigerator it's

very impractical to do it with a single

refrigerant like this just because for a

given material our compression ratio

would have to be enormous lehigh to make

it work like for example if we had a

substance that was that boiled at

negative 150 degrees C so that we can

still have a phase change here the

pressure ratio might have to be 50 or a

hundred to one and that makes other

parts of the system difficult to since

now the copper tubing here has to be

thick enough to handle the really high

pressure over here and the compressor

has to be extremely over spec to have

such a really high compression ratio

across it so one solution to this

problem but not the only one is to break

our job into two pieces so this is

called a two-stage refrigeration system

or a cascade refrigeration system and

the idea is that the first stage will

take us down from room temperature to an

intermediate low temperature and we can

select a refrigerant that boils at an

appropriate point for that and then

we'll have a second stage refrigeration

system that takes us from that

intermediate pressure down to the very

low pressures and we can choose a

different refrigerant for that which is

better tuned for that really low

temperature operation so essentially

we've taken that massive compression

ratio and spread it across two different

compressors in fact two different

systems with different refrigerants in

this case the condenser in this cold

trap unit operates at you know above

ambient probably 40 or 50 degrees C the

heat exchanger where the two systems

joined together is maintained at about

negative 15 degrees C or even negative

20 or 25 and then the cold side is about

negative 100 degrees C so the manual

Specht these two refrigerants that

should be used in the system

there's a decal on the side of the unit

that actually says it's charged with our

1150 and I'll show you in a minute that

I opened the system up and found

evidence that it looks like it has the

refrigerant had been changed in it I

should also note that in most

refrigeration systems the amount of

refrigerant in the system is usually

stated in terms of mass so if you go to

charge your car with r134a it'll say it

needs you know one and a half kilograms

or something like that and the reason is

that when you add it to your car at the

pressure is required to have it in the

system at ambient pressures there will

be a gas liquid mix inside there so you

can't really just measure it you can't

measure the amount of stuff in the

system just by checking the pressure

because the pressure will always be the

same until you completely fill the

system with liquid because there's this

equilibrium between the gas and a liquid

in there in these low temperature

systems the refrigerant is always a gas

at ambient temperatures and pressures

within the system that the baseline

pressure so if we know the ambient

temperature we can use a gauge just to

figure out how much substance is in the

system just because it's linearly

related just because it's a gas at all

times so here we can see the two

compressors this is the first stage and

this is the second stage compressor here

you can see the capillary tube the

restriction in the first stage system

which is here this is actually another

capillary tube but that's not the

restriction in the second stage system

it's actually this tiny oil return line

and so the trick is that there's an oil

separator and there needs to be a

restriction to keep all the pressure

from going back through this oil return

line to the compressor so that's what

this small line is here going back to

here that's just to get the oil out of

the system the heat exchanger itself is

buried inside the styrofoam shield unit

and the the cold side is actually

wrapped around the stainless chamber

here so when I first turned the unit on

the front panel indicated that the heat

exchanger temperature was correctly down

to about negative 15 degrees C and both

compressors were running but the cold

side of the unit was not

getting cold at all I mean it wasn't

even going even a few degrees below

ambient and both compressors were

running and so all the electrics seemed

fine so that really narrows it down to

just two problems there's either a

blockage in the second-stage system or

there's a leak after I took the unit

apart I noticed that there was an

aftermarket clamp on sort of vampire tap

on one of the fill ports for the second

state a second stage system this is

almost certainly not a factory piece and

since the manual specs a chlorinated

fluorocarbon for the second stage

refrigerant but there's a decal on the

side of the unit that says r11 50 which

is ethylene gas I'm guessing that at

some point in this units history someone

replaced the second stage refrigerant

now it's understandable to use one of

those vampire tap things to remove

refrigerant because there's no other way

of sort of breaking into the system

there's no ports on it see the only way

that we can remove gas is by using one

of those vampire taps the problem is

that they they leak and so you can't

really use that as a permanent solution

so my plan of action here was to cut off

the vampire tap and replace it with a

more standard connector so this still

has a straighter valve inside there so

that we can fill and then you know

remove gas later and fill it again but

there's no seal to the outside of the

copper pipe so basically it's a solder

on type of system and then once the

valve is installed there's a much more

controlled rubber seal there or as with

the vampire tap it's it's not as easy to

seal to the outside of the copper pipe

first I wanted to check if there was any

remaining gas in there and so I put a

gauge on the existing tap there and it

couldn't even move the needle off the

peg but I could feel a little bit of gas

coming out when just putting my finger

over the valve so I filled up a small

bit of rubber glove with it and tested

to see if it was combustible just to see

if it was our 1150 or a non flammable

gas and yes it actually did light up so

that was slightly encouraging there

second since I wanted to do hot work on

the system and basically solder on a new

fitting

I wanted to get off of that flammable

gas out of there and replace it with

argon so I hooked the system up to my

manifold and pumped it down with a

vacuum pump and then filled the system

back up with argon and let it sit at

about 40 or 50 psi for a day or two just

to see if there are still any remaining

major leaks everything looked good so I

set up the manifold again and this time

connected a tank of ethylene to it now

this tank is actually the most difficult

thing that I think I've ever purchased

in my entire shop I've spent tens of

hours tracking down this cylinder of gas

and had I known where to go in the first

place it would have been so bad but

needless to say this was really really a

challenging thing to find there's very

little information available online

regarding these ultra low temperature

refrigeration units and since the manual

Specht an entirely different refrigerant

for this unit I really had no way of

knowing exactly what pressure to you

know add the ethylene so I basically

just chose a hundred psi since that

seemed like a good round number and it

was close to most of the bits of data

that I could actually pull out of the

Internet the system's been running for

about an hour as you can see the front

panel indicates negative 102 degrees C

and when it puts the decimal points up

that's indicating the heat exchange your

temperature so negative 18 degrees C if

the heat exchanger and as you can see

there's a bunch of ice crystals and

perhaps condensation here just putting

my hand is enough to cause a lot of

convection current and if you can see

that moving the the fog around

it's quite cold it's colder than dry ice

so if we pump co2 in there it would

actually form dry ice I plan to use this

thing as a cold trap but also to play

with the cold temperatures in general so

wherever you have to use dry ice you

could use this instead and not not need

to make the trip to the store ok see you

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