Design of heating pipes

[georgestair]

I need to replace the heat pipes for a small apartment building. Each unit will be on a separate circuit for zone control. I have a procedure, but it seems very approximate.
Do you know of a reference that would help me determine the flows in each segment for a particular head pressure applied to the circuit?
Thanks

 

[TBP]

The flow required for each apartment will be driven by the heat loss. 1 USGPM of water circulating will deliver 10,000 BTU/hr with a 20*F temp drop between the supply and return water temps. Try "Modern Hydronic Heating" by John Siegenthaler. It's for residential and light commercial buildings.

 

[georgestair]

Thanks for the reference. I've read about the 20 degree assumption other places. My existing system drops 10 degrees at the most, so that assumption doesn't seem realistic.

 

[TBP]

The lower the delta-T, the more GPM you need to move for the same BTU/hr. The short-cut calc is: GPM X 500 = lbs per hour of water. (One US gal of water weighs 8.33 lb times 60 minutes per hour is: 8.33 X 60 = 499.8 lb/hr of water circulating. Round it to 500.) It takes one BTU to raise one lb of water one degree, so with a 10*F delta-T: 1 GPM X 500 X 10 = 5,000 BTU/hr. A 1/2" steel pipe will handle 2 USGPM, a 1/2" copper line will handle 1-1/2 USGPM.

 

[georgestair]

I've checked out "Modern Hydronic Heating" by John Siegenthaler and it is very helpful. I still have the problem of designing a "reverse return" system of pipes. This requires a simultaneous solution of non-linear equations ( head = constant * flow to the 1.75 power ). I'm thinking of approximating the equations with successive linear equations that walk toward the non-linear solution, but it's a lot of trouble for this one problem and I'm sure someone has already done this. Does anyone know where I might look for a model that solves this problem?

 

[TBP]

Try

http://www.heatinghelp.com

and look in the "Books And More" section. There's an inexpensive soft cover book called "Pumping Away". It's a "cut to the chase" kind of publication, written for contractors. It doesn't get into heat loss from stuctures, though.

The rule of thumb for selecting the head of the circ pump is the longest run in feet (use a table to convert fittings to equivalent feet of head loss) and multipy it by 0.06.