CapTubes Consolidated: Putting this information to work.
To get the most out of this post, I would suggest your download and read the Supco Cap Tubing Manual posted above.
Right after this topic emerged, I was asked to look at a Victory UR-48-12A Salad/Sandwich Table. The owner had bought it used. It had quit cooling and she said several "techs" looked at it and said they did not know what was wrong with it. It had been running for days with little to no cooling. This cooler had factory installed access ports on both the high and low sides. The molecular sieve filter drier was the type with a high side port. Refrigerant was R-134. I unplugged the power for a few minutes to allow the pressures to equalize, in case the low side was running in a vacuum I didn't want any air introduced into the system as a result of connecting the gauges. When I connected my gauges, there was still a considerable differential between the high and low sides. When I first plugged it in it immediately tripped on overload. I unplugged it for several more minutes and when the pressures were nearly equalized I plugged it back in. The low side immediately plunged into a vacuum and discharge pressure was running around 140. I unplugged it again. Equalization was very slow. It wasn't difficult to determine that there was a restriction in the system. I told the owner that there was a restriction in the system, filter drier and/or cap tube. I would retrieve some service information and get some parts. I gave her an estimate and she said fix it. I got a 032 filter drier and determined that the unit used 10 ft of .049 cap tube. I called Supco and they gave me the option of either 12 ft of .052 or 38" of .042. (They do have the .049 now but my supplier didn't have it) The advisor told me that the 38" wouldn't be long enough. I thought he meant physically, to reach that is, but I read in the JB Data sheet posted above that the safe operating rule of thumb is to keep the cap tube no shorter than 5 ft and no longer than 16ft. While Henry Ehrens of Supco doesn't specifically state this, my guess is that anything shorter than 5 ft will prevent the cap from qualifying as a refrigerant control device, rather than a simple metering device, as he detailed in Theory and Field Problems of the Capillary Tube.
Edit Update: One page 1 of the JB A&E Guide, there is an illustration that states anything shorter than critical point S will act like an orifice.
As I continued to research and develop a plan of action for the refrigerated sandwich table, I accidentally found a very interesting piece of information on a Google search. Google has scanned and posted thousands if not millions of books it has for sale.
By goggling "capillary tube conversion", page 92 of Commercial Refrigeration for Air Conditioning Technicians by Dick Wertz came up: Troubleshooting Capillary tubes.http://books.google.com/books?id=S3QyPQT0Kv4C&pg=PA91&lpg=PA91&dq=capillary+tube+conversion&source=bl&ots=rjSfZLnyRB&sig=jQ_y6TMR4gNSgRXQgMEEbqGVmtA&hl=en&ei=xKGWSpHPOZiNtgf17-i-Dg&sa=X&oi=book_result&ct=result&resnum=8#v=onepage&q=capillary%20tube%20conversion&f=true
The viewer is protected so you can not copy and paste anything from it. Also the images do not show up. I was still able to get a very useful piece of information from it:"Most small driers for cap tubes use a beaded desiccant or filter material. Sometimes the beads become loose, rub against each other, and small particles flake off. The fine desiccant powder eventually stops up the cap tube. The symptoms include low suction pressure (maybe even in a vacuum) and lower-than-normal head pressure.
Fortunately, the blockage is usually just at the tubing inlet; therefore, it is not necessary to replace the entire cap tube. The recommended repair is as follows:
Recover the existing refrigerant.
Cut out the existing filter drier.
Cut off the first inch or two of cap tube.
Install a new drier.
Evacuate and weigh in the proper amount of refrigerant.
Note: The loss of a couple of inches of cap tube is not enough to adversely affect system performance."
While Dick Wertz does not completely agree with Henry Ehrens of Supco as to cause of restrictions, the results are the same.
Armed with my new found information, a new filter drier, and 12 feet of .052 cap tube, I headed over to the restaurant to attack the problem. After recovering the refrigerant, I removed the filter drier from the hot gas line and the cap tube. The filter drier was plugged to the point I could not blow through it. I hooked up nitrogen to the low side to back blow through the cap tube while holding my thumb over the discharge. There was very little flow through the cap tube. I etched a spot about 2" up the cap tube and broke it off. The inside of the cap tube was coated with a white substance. So now I know that not only is the filter restricted, so is the cap tube. Not wanting to completely disassemble the appliance so that I could change the cap tube, and the cap tube was soldered to the suction line, I decided to go up the tube another couple of inches. I cut off 2 more inches. I could still see the buildup on the cap tube, but it was not nearly as thick as at the first cut. I turned on the nitrogen again to test flow. There was more flow, and for some dum reason I looked in the end of the tube just in time for the last piece to blow out, hit my reading glasses, and ricocheted off my nose, or vice versa…. who knows? I would estimate that the flow had increased by a factor of 3. The tube looked clear. Just to be sure, I cut off another inch. Too my joy, there was nothing other than clean shinny copper on the inside of the cap. I would not have to completely disassemble this thing to change the cap tube. So I have cut a total of 5" off a 10 foot cap tube. 6" would have been 9-1/2 feet of 10 feet, or 95%. I still had more than 95% of the original cap tube, and the restriction was gone. I installed the new drier, access port, and tied in the cap tube to a piece of ¼ tube, and started the vacuum pump. I weighed in the 9 oz of R-134 and started the unit. It was running about 145 psi head with a 35 psi suction. As the box pulled down the temp, the suction and head pressure began to come down gradually. By the time I was approaching target temp of 35*F, it was running about 140 psi head and 25 psi suction. I decided to add a little more refrigerant just to see what would happen. It only took about ¼ oz to cause frost to start forming at the compressor suction inlet. I slowly removed that refrigerant and sealed it up. That goes to verify Henry Ehrens statement that charge is critical in a cap tube system.
The owner was elated that the unit was working again and said that it had never worked that good since she had it.
The linked pictures are of the restricted tube inlet, the first cut showing restriction, and the tube that was cut past the restriction. Its really amazing how you can see so much more detail with these digital pictures than you can with your eyes.
Glen Harrison posted the photos for me below when I was having trouble getting them posted. Here are the direct links to the photos, feel free to copy them and use them as you may.http://i748.photobucket.com/albums/xx130/MechanicalMike/CopyofVictoryCapTube008.jpghttp://i748.photobucket.com/albums/xx130/MechanicalMike/CopyofVictorycaptube013.jpghttp://i748.photobucket.com/albums/xx130/MechanicalMike/Victorycap015.jpg