Capillary design

[kay86]

Hello everyone,
I am currently designing the capillary section of the refrigeration system for my R290 cold trap. I understand that the capillary should be connected to the suction line to enhance the cooling of the capillary and slightly increase the temperature of the suction line, reducing the likelihood of liquid refrigerant returning to the compressor. As shown in the attached image, I plan to install a small accumulator after the evaporator to ensure any residual liquid refrigerant is fully evaporated. Given that my suction line is relatively short, approximately 60 cm, I believe there isn't sufficient space to achieve optimal coupling with the capillary. Consequently, I am considering coiling the capillary around the accumulator to improve heat exchange. It's important to note that this component is not the filter drier, which is also visible in the image. I am curious about any potential drawbacks of wrapping the capillary around the accumulator, especially considering the limited length of my suction line, which falls short of the recommended 100cm for coupling.
Thank you for any advice you can provide.

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[frank]

Alternatively, you could wrap the capillary around the suction line then insulate it.

While there is a slight sub cooling advantage it is not necessary to prevent liquid floodback as you are fitting a suction accumulator

[kay86]

Thanks Frank, yes you're right, I could do that but I'm worried about coiling the capillary too tight. I should try and see how hard it is to bend the cap. Also, I was wondering if there's such a thing as "cooling the capillary too much".

Thanks!

[frank]

I've coiled small bore capillaries around 3/8th suction without any issues

[frank]

If your subcooling increases too much uou could always reduce the capillary length to adjust

[kay86]

Thank you Frank. I assume you're referring to the part where I could see the last 5-10 cm of the capillary starting to freeze up, right?

[frank]

Thank you Frank. I assume you're referring to the part where I could see the last 5-10 cm of the capillary starting to freeze up, right?

Not really. The length of the capillary will give you a fixed pressure drop, therefore a temperature difference between entering an leaving refrigerant. If your desired evaporating temperature is below what you require then shorten your capillary.

[Rob White]

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I know Frank has answered this but from a slightly different direction.

Capillary's are also threaded through the inside of the suction to aid the heat transfer.
Plus one other reason why the capillary is wrapped around or inside the suction is to protect it.

The capillary is a vulnerable pipe and needs protecting.

Regards
Rob

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