Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Pressure loss calculation

  1. Jul 26, 2010 #1
    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. jcsd
  3. Jul 26, 2010 #2
    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? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook