Question on steam velocity

mel rowe

I was just re-reading in Dan's book about how to calculate steam velocity and I was confused by something.  The directions for doing the calculation say to use the EDR from the boiler spec sheet, or the boiler load in BTUH.  When I stopped and thought about this I wondered why either of those should be used, since they include the pickup factor of .3.  Why wouldn't the formula be based on the boiler's D.O.E. output, since it seems that's  what determines the amount of steam leaving the boiler?  

clammy

steam velocity

Mel ,i have used the formula just as dan has it lay out in the laosh i belive if i understand your post that your thinking of using the connetive load but the formula is based upon the boilers output not just the  connetive edr ,i have been using this formula on alot of jobs where there was alot of surging yet the boiler was clean it usually came down to improper near boiler piping and under sized headers .I personally always try to size my headers for below 15 fps which usually means bringing your header size 1 size larger then the manafatures suggestes .This formuala is very handy for larger residental systems espically vapor systems ,and counter flow systems .As with anything doing the math always pays off peace and good luck clammy

mel rowe

steam velocity

Thanks Clammy, for the reply.  I'm sure that there is just something i don't understand about the formula, but it seems to be using the connected load (the EDR shown on the boiler), which has a pickup factor of .3 included.  Seems like the actual steam velocity would  be dependent on the total output of the boiler at the risers (boiler capacity times the % efficiency),  rather than EDR of the boiler (which is the the total output divided by  1.3)   What am I missing here?

clammy

steam velocity

Mel in the formula if i remenber corretly you an use either the edr of the boiler or the input of the boiler .What your really trying to do is fiqure the velocity the steam is leaving the risers depending upon the sizing of the boilers tappings (2 or 3 inch and 1 or 2 tapping on the boiler)and wheather you are using one or two riser .That is what will determine the existing velocity  and also the  fact of what size your main header is that you are tieing the riser to,also i firmly believe that your header size is also dependant upon how many existing steam mains you are tieing into and there size .i have been told and i follow the rule that your header size should always have a larger area then say the mains that are tieing into them example 2 -2 mains are better suited to come off a 3 header rather then a 2 1/2 .I usually always up size my header one size larger then the manafatures suggests and when it over say 200 btu i go to the formula  and pipes start to get quite large up to 6 inch.I have yet to run into any promblems with over sized header except that the rads get hotter then they had been cause now the coats of latex paint is slinding off the rads but that's what happenes when you make dry steam and as Dan has said more of them btu passengers an get on the train when your making dry steam .Hope this helped peace and good luck clammy

mike jones_2

measuring steam velocity`

i hope u dont mind a dumb question, but it sounds like the steam velocity is not readily measured, rather it is calculated by the formula. 



therefore, it sounds like if u have the proper above boiler pipe sizes and heights you would be assured proper steam velocity.  is this oversimplified or is there a way to check it?  thanks clammy!

clammy

yeap

Yes sir that is correct,i have used this formula in the past now i basically just do a 3 inch header for every thing up to about 175,mbtu after that 4 inch anything bigger then 300 i refree back to the formula and brush up on my math .peace and good luck clammy

mel rowe

steam velocity

Clammy gave you the good info on what works.  If you want to estimate the actual steam velocity so that you can compare to levels recommended in LAOSH, then if the riser coming out of the boiler is 2 inch, just multiply the Net rating of the boiler in BTU's by .0003.  In my case, it was 300,000 BTU capacity times 82% efficiency times .0003 = 73.8.  Since I had added a second 2 inch riser, then the velocity was cut in half, down to 37.  After downfiring, it is now more like 24.

In Dan's book, on pages 42-43,he referenced that the ideal, based on the huge old boilers, was 15 fps, and the maximum rate to avoid interference with the backward flow of water in a 2 inch pipe was 23.