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

[herefishy]

Pointing the drier down, toward the cap tube inlet certainly does not hurt anything. I expect condensed liquid to be emitting from my condenser, and my condenser actually partially full of liquid, a couple of passes, perhaps.

The drier is referred to as a "liquid" drier, not a "vapor" drier, or a "saturate" drier for that matter. If you have vapor at your filter- drier, and are pointing the drier outlet down so that gravity will assure that liquid (where ever it may be coming from) drips to the bottom (outlet) of the drier, I just consider to be a misnomer.


I'd say that if you have vapor in your drier, you got other problems, Bub.

[condenseddave]

Originally posted by herefishy
I'd say that if you have vapor in your drier, you got other problems, Bub.

I generally would want to make certain that my drier was full of liquid, also.

So, what I gather from the previous post, his exwife is fat, and his current wife probably is also, but he won't tell her.

[DaBit]

A well-designed captube system has a good ability to adapt itself to various operating conditions. But designing a captube system well is quite a job.

For example: the load range to which a captube can adjust can be broadened by picking the right size condenser. The condenser should be just large enough to heat up when load increases. The resulting higher condensing temperature increases the pressure drop over the captube, and therefore the massflow, compensating for the higher load.

With a too small condenser, pressure rises too much resulting in flooding of the evaporator. A too large condenser doesn't heat up enough, resulting in a starved evaporator.

Too small hurts, and so does too large.

In my previous captube based chiller, I found that adding an accumulator helped a lot. With an accumulator it is possible to adjust the captube to the highest load that occurs instead of the lowest load. When floodback occurs, the excess refrigerant gets stored into the accumulator, resulting in an increased vapour content at the captube entrance, restricting massflow to the point where massflow matches the load.

The price paid is less efficiency at partial load since at partload we are feeding the captube with a vapour/liquid mixture. Not a big deal; full load perfromance is what counts for me.

Though, your mileage about this 'trick' may vary.

[RogGoetsch]

Originally posted by herefishy
Pointing the drier down, toward the cap tube inlet certainly does not hurt anything. I expect condensed liquid to be emitting from my condenser, and my condenser actually partially full of liquid, a couple of passes, perhaps.

The drier is referred to as a "liquid" drier, not a "vapor" drier, or a "saturate" drier for that matter. If you have vapor at your filter- drier, and are pointing the drier outlet down so that gravity will assure that liquid (where ever it may be coming from) drips to the bottom (outlet) of the drier, I just consider to be a misnomer.


I'd say that if you have vapor in your drier, you got other problems, Bub.

Interesting theories. But don't take my word for it. The Prof (Sporlan) may want to weigh in on the drier issue, but for the rest, there is no substitute for testing.

The method I originally used (I design very small refrigeration systems for bio-med applications, among other things) was to set up my test system with thermistors everywhere tied to a PC, logging temps at every critical point, including embedded at regular intervals along my heat exchanger. Combined with pressure gauges installed in several places, I was able to plot the results on a pressure-enthalpy diagram and analyze the performance under different conditions and configurations.

I know it is possible to build a working system without going into this kind of analysis, but from the posting to this thread, it seems that going back to some theory would simplify the struggle a bit. In my work, it is also important to create a system which can be manufactured with uniformity and serviced by the average technician.

Two drawbacks to liquid backed up into the condenser: subcooling the liquid in the condenser has no advantage in a CAP TUBE system if you have designed it correctly because you can accomplish it so much easier in the cap tube and if you test it, you will find that the loss of condensing surface exacts a penalty in performance. Second, excess charge lengthens time required for equalization and may require a start capacitor for the compressor.

My most important comment, which you are free to ignore if it offends you, is that your restriction is too great if excess charge backs up into the condenser instead of overfeeding the evaporator. not that you want to overfeed.

Again, don't take my word for it, test it yourself.

Rog

[Peter_1]

[QUOTE]Originally posted by herefishy
[B]Any tool (online) available to calculate cap tube size/lenght for specific application.

try the free selection software available on
http://www.tecumseh-europe.com/M5/UK/p50.htm

Perhaps this may usefull for other members.
Let me know