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Pressure loss calculation 
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#1
Jul2610, 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 


#2
Jul2610, 05:41 AM

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 energypersecond multiplied by metrespersecond. That doesn't give a result that means anything to me physically. But if it were divided instead of multiplied, you would have energypermeter  that is the total energy in a length of pipe . That sounds like a useful thing to know? (roughly 339 kilowattseconds per meter) Am I helping or hindering? 


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