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subzero*psia
24-02-2001, 07:25 PM
How about some cap tube stories?

How do you think cap tubes are chosen by manufacturers? Do you think it could have been sized wrong? Why?

Dan
24-04-2001, 01:25 AM
I think cap tubes are a guess and bygosh arrangement. But the manufacturers supposedly do all the guessing and bygoshing before the unit is approved and made available.

Same goes for the critical charge. I once called a manufacturer and asked why I am having these problems after repeatedly weighing in a charge per the data plate.

The engineer told me that really I should add 2 ounces more than the data plate states.

I wondered to myself, why do I work for such a company?:)

Prof Sporlan
24-04-2001, 02:14 AM
<i>I think cap tubes are a guess and bygosh arrangement.</i>

Lol. To some extent, Dan, you are correct. But there has been decent tables set up over the years which make it relatively simple for the design engineer to zero in on a proper capillary tube sizing. Also, the Prof has noted some interesting papers published in recent years describing refrigerant flow thru capillary tubes, so there are still some people studying the subject.

Capillary tubes have largely vanished from unitary residential a/c and heat pump units, being replaced by short tube restrictors, aka "plug orifices". Capillary tubes are still used with domestic refrigerators, window shakers, and dehumidifiers. High efficiency (12 SEER plus) residential a/c will often use TEVs instead of short tube restrictors, which obviously the Prof is in favor. TEV operation and its affect on SEER rating is an interesting topic in itself..... :)

Dan
24-04-2001, 03:05 AM
Any time the Professor suggests an interesting topic, I gather around the campfire!

Dan

subzero*psia
24-04-2001, 02:08 PM
Hey Dan,

The only time I have ever heard or suggested that a critically charged system be increased is if the LL drier is changed to a larger drier and most driers state capacity of the drier.

I agree with the Prof... TXV's the way to go. I have often wondered about plug orifices in some small refrigeration applications though?

Dan
24-04-2001, 11:42 PM
Originally posted by subzero*psia
Hey Dan,

The only time I have ever heard or suggested that a critically charged system be increased is if the LL drier is changed to a larger drier and most driers state capacity of the drier.

I agree with the Prof... TXV's the way to go. I have often wondered about plug orifices in some small refrigeration applications though?




This was a dipping cabinet application that the factory hastily re-engineered to operate with R22. I think that they were learning that the data plate was wrong.

Interesting thought, Dean, why don't they use orifices in small equipment instead of cap tubes. If anybody would, I would bet it would be the Japanese.

subzero*psia
24-04-2001, 11:51 PM
I would sure like to try it... what the heck I have some old used equipment that I am remanufacturing, maybe if I can find a variety of different sized orifices inexpensively enough I may try it. I may try having a few bonnets drilled out for an experiment.

Any thoughts or suggestions on this from anybody?

Tony
07-05-2001, 10:42 AM
What is SEER?

subzero*psia
07-05-2001, 12:01 PM
System Energy Efficiency Rating

Jack Lester
28-05-2001, 03:03 AM
Cap tubes are an interesting creature. They are a Metering device used mostly for a fixed load. They do have what is called Capillry effect. This cam occur when the heat load is increased. They will allow more refrigerant to flow much like a TXV but not to the extent of a TXV. Therefore not used as much for varrying loads.TXV'S are better. I have found that just because a unit says it requires just so many ounces or lbs. it is not always so. There are many factors that can account for the charge not being accurate. Cap Tube length, Piping, Dryer size and even miss print. I use there information as a guide and then use my preasure temperture chart to adjust from there. Secret is to know what is happening in the refrigeration system True for any system. I work mostly on cascade stuff and most Bio storage cabinets use cap tubes. What is written on the cabinet is rarely correct. We have even been knowen to change refrigerants to get the desired effect. Know your Super Heat!

subzero*psia
29-05-2001, 12:28 PM
Hello Jack, welcome aboard!

As you work with cascade systems... I would like to get your opinion on reading superheat correctly with capillary tube fed systems. We have gone over this before but I think your situation may put a lid on it and seal it.

Please explain how and when you obtain your superheat reading on cap tube systems?

Jack Lester
29-05-2001, 01:39 PM
Thank you for the welcome. I am enjoying this site.
Checking superheat in a cap tube system is not always easy. Some manufactures today are installiing T type thermocouples at the inlet and outlet of the evaporator of both stages. This makes it easy. if not you have 2 choices. 1. Dig out the insullation and install thermocouples at the inlet and as close to the cabinet outlet. 2. and the one that is the easiest is to wait until the unit is cold and wet paper towels and "stick" thermocouples as close to the inlet and outlet of the evaporator. Do this inside the freezer as the walls have the evaporator tubes against them. Most units are top fed but some are bottom fed. Watch your frost pattern as the unit gets cold.
The best time to check Superheat is when the freezer is as cold as it can get. Superheat in a cascade system at the coldest can range fron 0 to 5 depending on system and probe location. If the unit cycles off it can go negative.
Using thermocouples on a system is the easiest way to tell if a system is short of gas or perhaps a problem lies in the first stage(high stage)
Using gauges and a Temp/press chart is ok but not as accurate. Using gauges though is the only way in a cap tube system to know what is happening in the interstage. By comparing the first stage suction press to the second stage discharge press you can come close to the superheat of the interstage.Use t/p charts for this. Interstage = Evaporator of the first condenser of the second. Be patient when working with cap tube systems of any type. Nothing happens in an instant.
Know and use Temp/press charts. Hope this helps.

subzero*psia
29-05-2001, 02:59 PM
Great Post! Cascade systems are awesome... and hard to find many service companies willing to work on them!

On your second stage heat exchanger... do you thermocoulple the inlet and outlet only to obtain temperatures for superheat readings? I would assume you would also do this on a third heat exchanger etc?

Jack Lester
29-05-2001, 04:45 PM
[QUOTE]Originally posted by subzero*psia
[B]Great Post! Cascade systems are awesome... and hard to find many service companies willing to work on them!

On your second stage heat exchanger... do you thermocoulple the inlet and outlet only to obtain temperatures for superheat readings? Yes

Look at a refrigeration system.as a circle. bottom is the compressor, left is the condensor top is the metering device and on the lright is the evap. Put to circles (systems) together. you now have a cascade system. Evaporator of the first is the condensor of the second. The joining of the 2 is the interstage.Thermocouples can be used if you can get to the interstage. It is more accurate then using the T/P charts but not always possible. What your looking for is the desuperheat of the second or third or more stages. With out a good desuperheat of the following stage efficiency of that stage is decreased much like a plugged condenser.. Does this help?

Brian_UK
04-06-2001, 11:35 PM
Originally posted by Dan
I think cap tubes are a guess and bygosh arrangement. But the manufacturers supposedly do all the guessing and bygoshing before the unit is approved and made available.
Now I know this is going back a bit but I came across this site http://cbg.cubigel.com/english/tecinf3.htm - Electrolux Compressors regarding sizing of cap lines.

I quote - "The selection is based on the practical method of 'Trial and error'"

So you were right Dan after all ;)

Dan
05-06-2001, 12:30 AM
Hahaha, Brian. Thank you for that. I will show your post to my wife.:)

Dan

subzero*psia
26-06-2001, 03:18 AM
So then... anybody got any words of wisdom to share like some rules of thumb when you go to a site and a captube needs replacement...

Do you carry a dozen different size caps or what? Is there a rule of thumb as to which cap for a given capacity? I have heard this a few times but never quite sure it works...

Dan
26-06-2001, 05:00 AM
Dean, actually that is out there. I would have to do a couple of lookups, but I know I have seen it. The reason I don't have it at hand is because I cannot afford to apply such solutions. If you want a science project, that is fine. But if you want to make money fixing stuff, it is out of the question.

Understand also that the heat exchanger effect normally associated with cap tubes sort of queers standard sizing, so it is not uncommon that a cap tube assembly is called a heat exchanger assembly when you order it from the manufacturer.

Most machines that use cap tubes cost less than a thousand dollars. Most of them work correctly or are in warranty. Most of them, resold as used equipment cost around $400.00 or less.

Like I said, a science project, ok, but not home refrigerators or under-counter salad prep tables.

Cryogenics, perhaps.

But I have seen nice field work that re-installs a cap tube and makes the machine work. I just don't understand how the guy who did it stays in business.:)


Dan

Gary
26-06-2001, 07:02 PM
In 33+ years, I have repaired a great many cap tubes, but have never replaced one. Why would a cap tube need to be replaced?

subzero*psia
26-06-2001, 11:56 PM
With POE oil and driers with low crush strength bead type dessicants the capillary tube ends coat with the dessicant. You can only repair it once possibly... but why take the chance that there may be more inside the capillary?