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Pressure loss calculation

by vespak
Tags: calculation, loss, pressure
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vespak
#1
Jul26-10, 01:50 AM
P: 7
My sincere apologies if this is not the right place however I will pose the problem: Being a humble plumber rather than a physicist I am used to calculating pressure loss in copper pipework for hot water heating systems by determining the required flow rate kW/h / specific heat x delta T. Looking on my resistance chart measured in m/head and by selecting a pipe size that is suitable for the index circuit multiplying the total equivalent length by the resistance figure given in the chart (which I have always assumed to be in kPa/m) to give me the m/head calculation to select the right size pump. Sorry I know thats all very basic. I now have to deal with a new type of German pipework that has completely different values in the pressure loss chart and I just want to know if I am making the right assumptions. I need a pipe size that will give me a flow rate of 2.38 l/s over an equivalent pipe length of 250m (district heating main flow/return for a 200kW biomass wood pellet boiler. The value line I am looking at for 76mm carbon steel pipe reads as follows:
Q(w) (kg/h) v (m/s) DeltaP (Pa/m)
200000 8598.5 0.59 42

I have made the following assumptions:
Q=energy and so given the value v 0.59 m/s equals a transfer rate of 20kW x 0.59m/s
My 2.38 l/s x 3600 = 8568 l/h which is the same as kg/h and is the nearest equivalent I can find on the chart.
v is just above the acceptable level for the slowest movement of heating water even though it is a closed pressurized system
DeltaP in Pa/m is converted to kPa/m to give me the total resistance to calculate my pump size: 250m*0.042=10.5 m/head
If this is all completely wrong or just in the wrong place please feel free to express your opinions to that effect however I would be grateful for some helpful direction.
Many thanks
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AJ Bentley
#2
Jul26-10, 05:41 AM
AJ Bentley's Avatar
P: 665
This is totally unfamiliar - but unless someone with specific knowledge can step in, I'll give it a go.
flow rate in kW/h / specific heat x delta T makes sense - that would be the volume of water required to transfer that amount of heat energy per second.

However - I baulk at
'given the value v 0.59 m/s equals a transfer rate of 20kW x 0.59m/s' (I think you meant 200kW)

I'm not clear on what transfer rate means in this context - you have a power multiplied by a speed - that is energy-per-second multiplied by metres-per-second. That doesn't give a result that means anything to me physically. But if it were divided instead of multiplied, you would have energy-per-meter - that is the total energy in a length of pipe . That sounds like a useful thing to know? (roughly 339 kilowatt-seconds per meter)

Am I helping or hindering?


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