The amazing Mr. Cap Tube

[shogun7]

Don't ya just love technology? Take the simpilicity of the cap tube I'm always impressed with the the human brain and it's marvilous achevements
The cap tube for example acts as a pressure reducing device. If air or water is pressurized at the inlet, there will be a linear or equal reduction of pressure for each foot of tubing. For example, if water enters a cap tube at 50 psig, the pressure decreases at a constant rate as the water passes through the tube. If refrigerant responded the same way as water, the cap tube could never be used as a refrigerant control. Why not? Because when the head pressure was low, the evaporator would be starved, and when the ambient temperature was high, the evaporator would be flooded with liquid (the flow rate would be greater than the rate of vaporization).
However, if there was a way to increase the flow rate when the head pressure was low, and decrease the flow rate at high ambient temperatures, you would have a refrigerant control. This modulating effect is exactly what happens when a cap tube is installed in a well-balanced system.

[Latte]

Shogun Wrote "Dont Ya love technology" NO!!!!
" " "take the simplicity of cap tube"
" " "Cap tube fitted on a well ballanced system"


1) Technology is OK if use wisely, some manufactures seen these days to go over the top with it, all singing and dancing with nice lights and stats that can tell you to a tenth of a degree whats going on. Keep it simple then there is less to go wrong.
2) cap tube, simplicty to me is somthing that can't go wrong because there is nothing to go wrong. How many jobs have you all been to this week with blocked capillary on R134a

Guess who is not a fan of cap tube


Regards

Raymond

[chemi-cool]

Hi Raymond.

Indeed you are right. R-134a blocks everything. Its such a powerful cleaning agent, much better then good old R-11.

Just changed 15ton TEV on a R-134a system. It was completely blocked with all the dirt from installing the system but I suspect that some of the particles I've found there was from the drier core.

It is defiantly bad refrigerant for cap tube systems.

Chemi

[shogun7]

ERR. Let me guess does he go by the name of Ramon? Gee Ray, You sound like you would like to go back “to the good ole daz”, You must be older then me!
Of course it's not the cap tube it's the refrigerant; never the less it's a wonderful piece of work
“Just changed 15ton TEV on a R-134a system. It was completely blocked with all the dirt from installing the system but I suspect that some of the particles I've found there was from the drier core.” …Chemi try to be more positive and look on the other side, like “boy am I lucky now my system is REALLY clean and look at my pay check, now I can buy the kids some new shoes”

[chemi-cool]

shogun7.

You must re tighten those loose screws!

Whats wrong with you man?
Is this all it comes to? New shoes?

Get your head fixed and come back.
I'm sure you have forgotten to take all your pills today.

This is the last time I'm answering such crap.

Chemi

[shogun7]

Your breakng my heart! Grow up man and get a grip on yourslf what do you do when something really bad happens. This is nothing but just kidding with each other Hay Chemi, who knows, All things are possible until they are proved impossible - and even the
impossible may only be so, as of now. Ya we might even become tight!

[RogGoetsch]

Don't ya just love technology? Take the simpilicity of the cap tube ..... This modulating effect is exactly what happens when a cap tube is installed in a well-balanced system.

Sorry to differ with you Shogun, but I don’t consider a cap tube to be an example of technology as compared to, say, an expansion valve. Simple, yes; technologically advanced? Hmmmmm… Not! But perhaps you were speaking tongue-in-cheek?

I also have to take exception to the idea that the cap tube displays a modulating effect, even in a “well-designed system”.

So, one more time, with feeling!:
(Search earlier cap tube posts if you are not already completely bored with this topic, Dear Readers.)

The simple fact is that capillary systems do not respond well to changes in demand. They only do three things well: cool at design conditions, starve the coil at high demand, and flood the coil if the load drops.

They are used widely because they are effective for many applications and they are cheap. I, too, have found them to be troublesome with R134a, at least with the original POE formulations. Lately, I have not seen so many problems. (Fingers crossed!)

Love & Kisses,

Rog

[shogun7]

Rog:
Technology is the technical means people use to improve their surroundings.( Refrigerator) Technology is people using knowledge, tools, and systems to make their lives easier and better. In the case of the Cap Tube if you wish to get into semantics then I will call it elegant simplicity technology, that is: doing more with less. In the case of the Cap Tube as the temperature of the subcooled liquid at the inlet to the strainer varies inversely with the ambient temperature.So as the ambient temperature rises, and the temperature of the subcooled liquid is reduced.(more flash gas) This is the basic reason why the flow rate modulates in a cap tube. Remember that vapor restricts the flow of liquid. I excuse your lack of knoledge because your so young (lucky). There is another consideration you might think about and that is just because you think you know all about the Cap Tubes there may be others who may not be so informed so all things we discuss may be new to someone reading these posts. Don’t be so selfish Oh.. and I love you too

[RogGoetsch]

In the case of the Cap Tube as the temperature of the subcooled liquid at the inlet to the strainer varies inversely with the ambient temperature.So as the ambient temperature rises, and the temperature of the subcooled liquid is reduced.(more flash gas) This is the basic reason why the flow rate modulates in a cap tube.

Hmmmmmm. How exactly does the "subcooled liquid" at the strainer get cooler as the ambient (condensing temp) rises? Now that IS some impressive technology!

Rog

[shogun7]

Rog you seem to misunderstand what I was trying to convey.I said "In the case of the Cap Tube the temperature of the subcooled liquid at the inlet to the strainer varies inversely with the ambient temperature.
So as the ambient temperature rises, the temperature of the subcooled liquid (IN THE CAP TUBE) is reduced.(more flash gas)
Sorry bot tat This is the basic reason why the flow rate modulates in a cap tube. Remember that vapor restricts the flow of liquid.
So as the ambient temperature rises and falls, the temperature of the subcooled liquid in the condenser and cap tube vary directly. It is this variable subcooled temperature that determines the bubble length in the cap tube—which, in turn, modulates the flow rate. This phenomenon in effect transforms the cap tube from a pressure-reducing device to a refrigerant control and that my friend is a wonderful thing

PS about your quesion "How exactly does the "subcooled liquid" at the strainer get cooler as the ambient (condensing temp) rises? Now that IS some impressive technology!

Ans: Thats what I'm talking about WOW what a great technology...and I'm goin to patent it

[Peter_1]

And then, ...we even haven't spoken about the fact that speed in a capillary tube can reach sonic speeds where the common gas laws don't match anymore.

On these moments - sonic speeds - you should need a negative length of capillary tube if U used the formulas.

They call this a choked flow. This is the same reason/phenomena why powerful jet-fighters have on the outlet of there jet-engine a nozzle they can open or close. If this nozzle blocks, then full power can not be reached.

Those who visited air-shows have probably seen already on a F16 that when they go in take-off power - especially when they use the very powerful but fuel consuming afterburners - you can see that the nozzle is going wider and smaller. The computer then calculates the opening diameter for that specific condition of gasflow and temperature.

One of the question on my ATPL exam was: we have this specific engine - it was a GE engine - these are the specs of the engine, mounted on that airplane and the outlet blocks due to a malfunction on diameter x.
To what altitude must the airplane descend to maintain straight and level flight?
I can assure your, it took more than 2 pages to calculate this and it was for me the most difficult question for the course 'Jet engines'
I know, I know,..for some it will be again peanuts, simple maths, simple equations but then... I also had exam-nerves.

This phenomena can be described with the 'Fanno Line'
Fanno line is the locus of all states for frictionless adiabatic flow in a constant-area duct plotted on a T-s diagram.

For those interested: do a Google on 'Fanno Line' and 'Fanno line capillary'.

[RogGoetsch]

Rog you seem to misunderstand what I was trying to convey.I said "In the case of the Cap Tube the temperature of the subcooled liquid at the inlet to the strainer varies inversely with the ambient temperature.

I sure can’t see your reasoning here. As liquid flows directly from the condenser to the strainer inlet, how can it become cooler as the ambient & condensing temperature rise?

So as the ambient temperature rises, the temperature of the subcooled liquid (IN THE CAP TUBE) is reduced.(more flash gas)

Higher condensing temperature = higher pressure so with the captube cooled by contact with the suction line, we have a very subcooled liquid which should only flash at the discharge of the captube.

If you are assuming that there is flash gas forming in the last few inches of the cap tube, you may be correct, but with higher condensing pressure, there should be less flash gas, not more.

Sorry bot tat This is the basic reason why the flow rate modulates in a cap tube. Remember that vapor restricts the flow of liquid.

Where we use the vapor to limit flow is at the entrance to the captube. The system is “critically charged” for this very reason, and the captube usually soldered to the suction line so that it is too cool to allow the liquid to flash. Bubbles entrained at the entrance recondense almost immediately.

So as the ambient temperature rises and falls, the temperature of the subcooled liquid in the condenser and cap tube vary directly.

In a cap tube system, the liquid exiting the condenser should be at saturation all the way to the drier since you don’t get a liquid seal until the captube entrance.

Liquid backed up into the condenser would allow subcooling, but at the cost of condensing surface. This would exact a performance penalty for no purpose since subcooling is accomplished in the captube long before it can flash. The extra liquid in the condenser would delay pressure equalization during the off cycle, requiring a start capacitor, perhaps, and contributing to the amount of liquid in the crankcase on initial start-up.

It is this variable subcooled temperature that determines the bubble length in the cap tube—which, in turn, modulates the flow rate. This phenomenon in effect transforms the cap tube from a pressure-reducing device to a refrigerant control and that my friend is a wonderful thing

DaBit, one of our esteemed posters, has made a captube system jump through hoops. He may have achieved what you believe is possible, but from my own experience, designing units for mass production and manufacturing uniformity, cap tube systems must be optimized for a given set of conditions. For variable loads and conditions, expansion valves were invented.

Shogun, I realize I probably haven’t convinced you, so please don’t take my word for any of this. Test it and let me know if your results vary.

The method I 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. 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.

Rog

[superheat]

Cap tube are pretty neat. Such a simple device that does so much. You can't blame the cap tube for POE problems. You can blame the manu. for putting out a cheap product that will cost more to maintain in the long run. I do prefer a TEV.

The cap tube can't adjust to load changes. It forces the system to adjust though. It inflates the head pressure, decreases the evap pressure and changes the compressor load to adjust to load changes. TEV would run more eff. in off loads.

[Karl Hofmann]

High tech controlers and other such gizmos do not cost hundreds of dollars or pounds, they cost pennies, electronics are far cheaper to make than quality mechanical controlers, its just that we get charged and we pay hundreds