Way to calculate flow rate fed by drain? 3

[Zonkytonkman]

Hello, we have just designed/installed a new system to recover hot water. Previously we generated (through a cooling process) large quantities of hot water that had to be dumped to drain for lack of enough space to store all of it, only to have to heat similar quantities of water mere hours later for another process. We devised a system where the overflow on the existing hot water tank would be redirected to fill a new hot water tank. The over flow line on the existing tank is just a straight 3" sch 40 line coming from the bottom of the tank straight up. From the overflow line (when this pipe starts to fill) to the top of the tank there is only 6" of height. On its initial run the old tank (which should have filled the new tank as it overflowed) overflowed from its roof, leaving me with quite a mess and a water recovery system that is useless. I walked in on the middle of this project and the engineer that handed it off to me assured that he calculated the flow velocities and they checked out fine.
Anyway, i'm going back throught the calculations now and I realized that I don't know how to account for the line filling as a drain, I'm wondering if the vortex (I'm assuming it will act similarly to a drain on a sink) cause a slowdown in the flow? Will the pipes flow be turbulent, and will it fill the entire cross section of the pipe?
I havn't done much work with piping before, so i'd appreciate any help you could offer. The liquid is plain water, approximately 60-80 degrees celsius.

 

[Zonkytonkman]

Monte, I'm sorry, could you try joseph@engr.mun.ca

While I appreciate everyone's input very much, I'm getting a couple of different answers here regarding flow rate. Could some of you perhaps post your calcs or at least a guideline? That would help me to learn how to do this for myself.

 

[Montemayor]

Zonk:

No problem. Your zipped Excel workbook has been dispatched again. As I mention to you, I could easily do the calcs within the spreadsheet, but the result will be the same as the website I recommended you utilize:

http:/

http://www.freecalc.com/selffram.htm

The equivalent equation form for this application is:

D = 0.92 (gpm)^0.4

where D = pipe internal diameter, in.

P. D. Hills himself furnishes an example calc and the answer for your application, as I mentioned before, comes out to be approximately a 6" pipe - which explains you getting the results you got with your existing 3". By the way, how did the guy that did your calculations come to his conclusion? Did he assume that the vertical overflow line is liquid-full 100% of the time? I hope not, because this is not what is happening with a vertical, gravity drain line. Anyhow, have fun with the article. I even regressed the Hills semi-log curves into useful equations so that you can use a spreadsheet to resolve the problem, rather than having to go through the graphical curves. Good luck.

Art Montemayor
Spring, TX