Calculate cap tube

[RogGoetsch]

If you've serviced many cap tube systems, you've probably discovered that there are only two kinds: oversized and undersized! (Okay smartypants, maybe you happened once to look at a system that was behaving perfectly under its own particularly perfect conditions. All I can say is, what are you doing messing around with a system that ain't broke?)

If I recall correctly, fluid flow analysis doesn't easily apply to capillary tubes since pressure drop and viscosity are of less significance than the capillary action between the walls of the tube and the liquid surface.

Therefore, phase change before the end of the cap tube is a no-no, hence all the cap tubes out there soldered to suction lines. (No, that's not to reduce floodback!) Gas restricts big time, and that is very handy, if you read on.

It helps me to think about a refrigeration system as an equilibrium system. That is, under any set of operating conditions, the system will seek its equilibrium or steady-state point where everything is balanced: fluid flow, heat flow, etc. If anything changes, the whole system shifts to a new set of operating conditions.

For example, if one of several evaporator fans fails, airflow will be reduced through the evap coil, air will circulate backwards past the stalled fan, crippling the effectiveness of adjacent fans, suction temperature will drop a bit, condensing temp & motor amps will drop, etc. For any change, there is a domino effect until a new equilibrium point is reached.

Since a cap tube has a very limited range of responses (kind of like me when she asks: "Does this make me look fat, Honey?" "No, Dear, you ARE fat." Wait, that was my first marriage. I mean those three words every woman wants to hear from her man: "You're not fat." But I digress.) You have to optimize the critter for one set of conditions right about the middle of your normal expected operating range and hope for the best.

But maybe you're making it more difficult than it is. Because it's not the fluid flow through the cap tube that alone must properly restrict. The tubing must be sized to allow enough flow under design conditions, the size of the refrigerant charge determines the limits of that capacity.

So you could say the first step is to make sure your selection ALLOWS enough flow with a small extra capacity as a safety factor, and then select the volume of charge to fine-tune to the desired capacity.

That is why it is important to angle the drier down toward the cap tube. It is so that all of the liquid will be directed to the cap tube with the uncondensed refrigerant gas acting as the final flow restricter.

In the old days, when venting was normal, we charged cap tube units till we saw the frost line emerge from the box under as nearly as possible design conditions, then bled some off, watching the frost line recede back into the box.

You may have discovered the truly undersized cap tube where the addition of refrigerant has no effect on the capacity, but backs up into the condenser until the loss of condensing surface raises the condensing pressure so high that we can get a little more flow. That is a design to fear!

In any event, even the ASHRAE method will only get you in the ballpark. I use the tables published with the cap tubes and adjust for blends.

I have fine-tuned on my bench (using 6% Ag alloy so I can test, measure, recover, unsweat, lop off and try again) but for field repairs I select for extra capacity, adjust by carefully limiting and recording refrigerant charge, and get on to the next job.

I will cut a cap tube shorter if damaged but if I have to patch, for instance where removal will take more time than the case is worth, (don't get me started on unserviceable designs!) I will shorten or replace the accessible section with a larger size to compensate for the added restriction of the patch.

Rog