Cascade 508b freezer

[vango1]

Am trying to build a low temp. chest freezer utilizing a cascade compressor system. The first stage is an industrial condenser unit which gets my intercooler to a -45°C. The intercooler consists of a 50 foot coil of ½” dia. Tubing. This sits on a 50 foot coil of 3/8” coil which serves as the low stage condenser. I have packed all this away in the lower end of the freezer chamber and insulated it well. Through out my tests I have used varying sizes of cap. Tube to try to get better pressures and or temps.

I begin the test by running the high stage until the intercooler is between -40 to -45°C. Once this temp has stabilized I start the low stage compressor which pushes the 508B refrigerant thought the condenser/ intercooler loop this gas initially exits cold and flows through the .044 dia cap ( 10 foot length) tube into the freezer evaporator 5-16” diameter. This evaporator is wrap around the walls of the freezer chest which I am trying to chill.

The results are not what I would expect as the 508 discharge gets too hot and eventually heats the process and does not provide much cooling I have achieved -20°C at best. I believe I have enough cooling power to get to ultra low temps but the 508 gets too hot. Any thoughts????

[Bones74]

Hi, It sounds like the first thing is that the discharge line that leaves the low stage compressor is too hot after it leaves the heat exchanger. The Low stage line is a 3/8" to the heat x-changer? Why? above you said that you are getting high temps out of the 508 right? I think that you answered your own question. You need better heat transfer at the heat exchanger. A better contact area is critical. Look at your design and maybe change it. Then move to the low stage after that.

[vango1]

Thanks for the response. Better heat transfer is always a plus , but in this case my intercooler is getting too hot due to the hot 508b. I let the high stage run until it builds to -45C. Then turn on the low stage where the 508b excess temp. eventually eats all the -45 until the intercooler is no longer cool. So I guess the heat transfer is sufficient, just not advantageous.

[Bones74]

Thanks for the response. Better heat transfer is always a plus , but in this case my intercooler is getting too hot due to the hot 508b. I let the high stage run until it builds to -45C. Then turn on the low stage where the 508b excess temp. eventually eats all the -45 until the intercooler is no longer cool. So I guess the heat transfer is sufficient, just not advantageous.

Confused? I re read the post, You say that you are using a inter cooler " heat exchanger" that has a 50ft coil of 1/2" copper with the 3/8" low side condenser on it? is that correct? The cap tube is the wrong size I believe. I just yesterday finished a -35 cold tray and used a .035 cap tube at 30" .10ft that sounds like a medium temp application. The exchanger on the high side should be a open design, meaning that the high side enters a area and chills the low side gas. This is done inside of a " box" style exchanger. no pipe to pipe contact.

[NoL]

Try running 50 ft of 1/4" THRU( AS IN INSIDE! ) the 1/2".

[vango1]

Yes my heat exchnger is 2 separate coils of copper. The larger one sits atop the smaller(low conderser) in an insulted box. I am tryiing to chil the low stage condenser.

What does this box type heat exchanger lokk like? are they commercially available, or do I need to build??

Sounds like a coaxial tube arrangement might be better?

[Bones74]

Think of it as a box, The high side cap tube feeds this box. Inside the box is the low stage piping as say condensor routing configutation. The high stage refrigerant is in direct contact with the pipe. It directly chills the piping. The coaxial and other designs aren't as efficient due to less contact patch

[vango1]

Thanks, I see your box idea very clearly now. The only thing different from your box and the box I have is the separation in the tubing. The air in the box is transferring the energy pretty good. I do think your box would work better as the refrigerant is better able to transfer the energy. I will give this some thought.

I am running this freezer as we speak. I am having a difficult time rationalizing the stochiometry of this crazy gas (508b). This week’s test I lowered the 508 charge to try to lower the high pressure thus lowering the discharge temp. This did have some positive results as I got to -20C quicker. In the end the heat of the discharge gas is still warming the heat exchanger too quickly. I start with the heat exchanger at -36C. Within 30 minutes this temp. rises to -11C. This, I think is the bottle-neck. If the return gas is coming into the low stage compressor at -15C and is exiting at +25C, I have accomplished only in heating my basement.

At this point I feel if the crazy gas would not get so hot my box would generate some better results. I also think your box would overheat as I am doing too much heating and not enough cooling. So if I did build a new box it may be bigger than the freezer I am trying to cool

[Bones74]

How large of a ULT are you trying to build? A basic reach in size the heat exchanger is about 16" x 16" x 6".

[norseman]

Hi.
To make a good heat transfer you have to either use a tube in a tube which was common on earlier ULT units. To fix them as pair onto each other gives you that bad heat transfer. It seems like most of your trouble is there. The units I normally am into, use a packless HX, but it several choices. The cap.tube for
the high stage on a 17cft chest use a short high as
just 6ft of 0.044 and the low takes 36ft of 0.050.
The HX should stay as low as -30c or so by full load.
We have a low kick in at -42c and it rise to -32 to -34c in the first partially hot gas start and later stay at -38 to -40 and even lower. Else, I would try a static 508B at 120 to 140PSI as a test charge. The high as I mostly have hold 600-700 gram of 404A.
With the same size of compressors like two 1/2hp or even some smaller low stage a cabinet should run nice. 508B is a real good friend and does no harm to you. Low discharge temperature and good capacity.
Good luck from Pedro

[vango1]

I have tried all of your suggestions except the packless HX. These seem very small compared to my 50ft of ½” tube??? I am starting small 3 cubic feet freezer volume. With 2 -1/4 HP compressors. What size HX would you suggest??

[Bones74]

1/4 hp? How many BTU's, Sounds like a undersized first stage. Even if the design wasn't perfect with a comp that could handle the heat load, you should have marginal results. A 1/4 HP unit at -30c you would be lucky to get a unit that would do over 700btu's. Sounds undersized. A 1/3 hp unit could probably get 1000 or so BTU's, The short time it takes to not perform when you start the second stage is the tell all.