Evaporator coil

Hello guys, I have a question about the design of a system which uses an evaporator made out a coil of copper tubing as opposed to a normal evaporator made with copper plates and fins. I've made some measurements with my R290 blast chiller and it's evaporator consists in 3 meters of 10mm tubing in the shape of a normal evaporator, and a fan which blows air through the fins for improved thermal exchange. However, let's say that I replace just the evaporator with a copper tubing wound around a metal pot to cool it down, I reckon that the length should be way higher because the coild would only contact with the external steel of the pot, and steel is not an extremely efficient heat conductor and also the surface contact between the copper tubing and the steel would have gaps in between, so I think I should compensate for this by using a longer tube, maybe even 20 meters, so I ensure that all of the liquid refrigerant has the chance to change state and prevent compressor flooding. However, my question is: should the refrigerant charge be increased to accomodate for a "bigger" evaporator, not "bigger" as in greater heat capacity but as in more volume of gas that can fit inside?
Thanks!

You'd probably be able to calculate the refrigerant charge knowing the volumes of the sections in the system that hold liquid or vapour at the different pressures. Or atleast give you a rough idea.
Then when charging the refrigerant, keep an eye on pressures and temperatures.

For improving heat exchange.
- Could you place the steel pot with its copper tube wrapped around it, into a slightly larger pot. So that you could then fill the gap between the two pots with a fluid or a solid, maybe a brine? maybe even have a small agitating pump.
- Or could you get rid of the copper tube and just make a flooded evaporator around the steel pot, a pot within a pot where the refrigerant sits within the gap and is in direct contact with the steel?

Quote:

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Originally Posted by seanf

You'd probably be able to calculate the refrigerant charge knowing the volumes of the sections in the system that hold liquid or vapour at the different pressures. Or atleast give you a rough idea.
Then when charging the refrigerant, keep an eye on pressures and temperatures.

For improving heat exchange.
- Could you place the steel pot with its copper tube wrapped around it, into a slightly larger pot. So that you could then fill the gap between the two pots with a fluid or a solid, maybe a brine? maybe even have a small agitating pump.
- Or could you get rid of the copper tube and just make a flooded evaporator around the steel pot, a pot within a pot where the refrigerant sits within the gap and is in direct contact with the steel?

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Hey Sean, thanks for answering.
Yes, I plan on estimating the additional charge required and then checking the subcooling.
As for the improved heat conduction, I plan on wrapping aluminum foil between the coils so I don't leave much air gaps. Unfortunately I cannot mount the pot normally withe the opening on the top, so the first solution you proposed is not feasible, and also for the second suggestion, I don't have enough skill to build something like this, but it does sound really nice ;)

Kay86,
If you do not have contact it will not work.
Heat transfer paste is one option.
What are trying to do exactly?
Cool what?
Temperature you want to cool to?
What’s wrong with existing conventional evaporator with aluminium fins?
Normal evaporator with fins has a very large surface area.
Wrapping a bit of copper around something to same capacity as finned evaporator would require a lot of copper coils.

Quote:

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Originally Posted by RANGER1

Kay86,
If you do not have contact it will not work.
Heat transfer paste is one option.
What are trying to do exactly?
Cool what?
Temperature you want to cool to?
What’s wrong with existing conventional evaporator with aluminium fins?
Normal evaporator with fins has a very large surface area.
Wrapping a bit of copper around something to same capacity as finned evaporator would require a lot of copper coils.

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Hey Ranger, I'm trying to build a cold trap for a vacuum pump. I need to reach temperatures between -25 to -40 °C inside the pot. This is why I cannot use a normal evaporator. I've tryed sticking a normal pan inside the chamber of a blast freezer and the process did work, even if there was a lot of inefficiency, because the external surface of the pot was cooled down by convection from the evaporator of the freezer. Nonetheless, it did work, so I'm confident that if I provide enough surface area for the coils it will work in my case aswell.

I've had a look on a commercial solution and I've counted 12 turns of copper tubing separated from eachother with no conductive material among them, with a total length of approx 20 meters. That unit used R410A while I'll be using R290. Also mind you, the heat load on the evaporator will actually be very low. My main goal is to make sure I have enough superheat to avoid having liquid going back to the compressor. Unfortunately I will need to set up a test rig and to learn things the hard way because for sure I'll screw up with something, either the dimension of the condenser or the sizing of the capillary tube, so I guess I'll need to do some trial and error, while monitoring charge, subcooling , supeheat and power usage of the compressor.

Quote:

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Originally Posted by kay86

Unfortunately I cannot mount the pot normally withe the opening on the top, so the first solution you proposed is not feasible,

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Could you drill a hole in the side of the exturnal pot, and once youve got the internal pot and its coil inside the exturnal pot, seal the gap. The gap between the pots wouldnt be pressurised so shouldnt be to tricky to seal. Then turn the completed assembly and fill it with a fluid?

Quote:

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Originally Posted by kay86

also for the second suggestion, I don't have enough skill to build something like this, but it does sound really nice ;)

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Could you use a local engineering company to make the pot? have them fit stubs onto the outside so that you can braze copper tube onto them.