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Indy2010
07-02-2011, 07:47 AM
Cap tube length recommendations provided by these applications vary to some astronomical extent despite being given identical conditions. I realize they are not intended to be exact but they should at least get you in the ballpark with cap tube length. for something so critical how is it that the apps aren't even close on their recommendations??

First DanCap. In the notes they describe no allowances being made for sub-cooling in the length of cap tube, but fail to disclose if any allowance has been made for normal sub-cooling up to the filter
http://db.tt/Q0QlYx0


next up is the Tecumseh version, which does allow entry of Sub-Cooling into the formula. But check out the more than double cap tube length compare to the DanCap recommendation
http://db.tt/0MKBjVo


Removal of the Sub-Cooling from the equation resulted in a closer match of recommendations but still we're talking a cap tube where a few inches can have a marked effect on the load capability (at least that's my understanding).
http://db.tt/2lMdlJM


Given that a colder fluid flows faster, it makes sense that the cap tube/restriction would need to be longer as shown in the Tecumseh version which also appears closer to physical cap tubes I've compared too.

but which one is correct/closest to the mark? Please

Indy2010
07-02-2011, 07:53 AM
Surely the cap tube thing can't just be trial and error can it??

Chef
08-02-2011, 10:32 PM
Indy
Use the tecumseh program as it is very close if you use reasonable inputs. The Dancap program is wildly erratic and has been discussed many times on this forum.
You are using a superheat of 52C is that correct.

Normally when you design the cap length the SC should be around 1 or 2C if you have 6C it means the condenser is backed up and Tecumseh do not account for this so you may get slightly wrong answers.

Be reasonable in your inputs and the answer will be close.

Chef

Indy2010
09-02-2011, 07:24 PM
Ok thanks for the info.

In my model the temps were taken from a running system, however I couldn't get a true reading of the suction temp (have ordered a better temp clamp and will update the data asap) the sub-cooling appears close to actual, but as with the superheat it too could be off.

The tecumseh app is it :)