cap tubing sizing with multiple evaporators

[kubae]

Hi All,

I understand there are standard recommendations for cap tub IDs and lengths (and even number of tubes) for different compressor capacities but these seem to infer the use of a single evaporator. What happens when two or three evaporators are used in the same system? Does one use the same lengths as if it is one evaporator or can this cause compressor over-loading.:confused: If no, how does one size such cap tubing?

[Brian_UK]

Hi and welcome to the forum.

The cap tube is sized for the evaporator with the compressor being sized as the last thing.

[kubae]

Thanks Brian_UK, Any ideas on methods/computations for sizing cap tubing for evaporators as they increase or decrease in size and/or varying evaporating temperature? Approximates or suggestions are most welcome :D

[Peter_1]

Download once Dancap from Danfoss.
Why captubes and no TEV?
A captube performs only the best at one specific condition which can never be met with multiple evaporators.

As soon you switch off one evaporator, suction pressure will decrease and your cap tube on the remaining evaporator will have not the right length.

It's not a good idea and it will perform very bad.

[US Iceman]

I think Peter is correct. Cap tubes only seem to work well at one condition. If the system goes through repeated high temperature pull downs or if the evaporator circuit lengths are different this will not work.

Also, all of the evaporators have to be ON at the same time otherwise you change the suction pressure.

[jan willem]

Hello and welkom,

It is possible to make a cooling system with multiple evaporators with cap-tube. Whe make them a lot with static evaporators and go with a max of 8 evaporators on one machine.Whe make them for cooling tables for dead people and in laboratory low temp.Only the school books say it is not possible;) but in practice it is.


jan willem

[The MG Pony]

yes but morgues are rather static and thus equilibrium is easy to make. How ever for a more dynamic system I do not not see it performing well at all.

[Peter_1]

Jan Willem,

I saw it as a multiple evaporator system where one or two can shut down while the others keeps working.
In this case, I persist in my view that the evaporators will not fed correctly and the setup is certainly not energy friendly.
It can work but a 4 cilinder engine can also run on 3 pistons ;)

On the other hand, if all shut down at the same time,
then I don't see a problem.
But what if one evparator has a frost build up? I will see once that compressor.

This is the same as the multiple capillary injection you often see in Italian or Spain counters

As long as load and ambient stays within some narrow margins, a capillary is a good expansion device.

[kubae]

Thanks Gentlemen. Actually the evaporators are static, none of the evaporators can or will be switched off and they are same circuit length. My issue is: using 0.070" ID cap tubing for a home built evaporator of length 900 inches ID 1/2", what length of cap tubing would be ideal? Approximations/Suggestions most welcome. And thanks

[Sledge]

I am a cap tube hater. Why would one attempt this? Is cost the primary consideration here?

I have been heading down the path of solving persistent unsolvable problems in cap tube systems, by selling the customer on the idea that the cap tube is inferior, and the tx is a better approach.

Are there apllications where the cap tube will perform better?

[Peter_1]

Perform better? Never!
Perform cheaper? never!
Initial cost lower? Allways.

[kubae]

yes, cost is plus its static evaporators doing the same thing over and over