Equivalent Pipe Length of Water to Air Heat Exchanger

[Dan Jorgensen]

I am trying to calculate the circulating pump requirements for my home heating system. I've an outdoor wood stove feeding a water to air heat exchanger in a forced air furnace plenum. I have calculated the equivalent lengths of 1" and 3/4" pipe and fittings in the system, but am at a bit of a loss when it comes to establishing the equivalent length of the heat exchanger. The feed and return lines are 3/4" copper, each ending in a 4 branch manifold, feeding and returning 4 parallel circuits of 1/2" copper pipe. Each circuit consists of 22 feet of 1/2" copper and 11x180 degree elbows. Total piping for all four circuits is therefore 88 feet of copper pipe and 44x180 degree elbows.

I cannot find the equivalent length of 1/2"x180 elbows anywhere and am not certain what effect the parallel circuits have on the calculations as opposed to calculating the equivalent length of a continuous circuit 88 feet long with 44x180 degree elbows. I have no idea who manufactured the heat exchanger so I can't make an inquiry there.

Should I consider the 180 degree elbows to have the same equivalent length as two 90 degree elbows? What is the effect of four parallel circuits on the calculations versus one continuous circuit?

Thanks.

[PipeFlowCalcs]

You should use resistance factor calculator.

Also you should know R/D ratio as it influence resistance very much. I suggest you to use top left where n is the total number of bends which is in your case 2x44=88 pcs of 90 deg, K=25.87 if R/D = 1.
Once you have K, equivalent length is calculated as K = fL/D.

It is OK to calculate the rest of pipes as one long 88 feet pipeline.

[Dan Jorgensen]

Thank you Zoran.

My only remaining question is what effect having 4 circuits in parallel has on the calculations. Do I calculate for the total number of bends and pipe length in all four circuits or do I only need to calculate for one circuit to arrive at the resistance factor in the system?

My understanding is that for 4 circuits in series I would calculate the total length (ie. 88x90 degree bends + 88' of pipe), but that if the four circuits are arranged in parallel, as they are in this case, I only need to calculate for the circuit with the highest resistance (ie. 22x90 degree bends + 22' of pipe). Is that correct?

Thanks.

Dan

[PipeFlowCalcs]

You can calculate like it is one straight pipe and sum all bends in one.