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Pressure losses in electric air heater

  1. Jul 12, 2016 #1
    I have an electric air heater with a pressure loss of 3kPa (according to manufacturer at the flow rate and temperature I will be using). Air is supplied to the heater using a centrifugal fan with a static pressure of 8.7kPa (gauge) and a flow rate of 88m3/hr, giving a dynamic pressure of around 95Pa.

    My question is; will this loss of 3kPa be static, dynamic or a bit of both? I would like to calculate the outlet velocity after passing through the heater (using bernoulli equation) and I having difficulty without knowing what pressure to deduct the 3kPa from.
     
  2. jcsd
  3. Jul 12, 2016 #2
    It's the static pressure.
     
  4. Jul 13, 2016 #3
    How would this effect the mass flow rate leaving the heater? I was under the impression the 3kPa loss would result in some loss in mass flow (i.e. cannot assume mass flow rate across heater is constant). I have a means of measuring the heater inlet velocity but not outlet (especially when it is at operating temperature) so would like get at at least a general idea of the outlet velocity.

    Ignoring the heating effects, i.e. running flow through heater when off:
    Pstatic in =108.7kPa, Pstatic out=105.7kPa therefore Pstatic in-Pstatic out=3kPa
    rho in = rho out =1.225kg/m3 (assumption made for simplicity)
    velocity in = 12m/s

    Using Bernoulli eqn.
    velocity out=SQRT(velocity in^2+2(Pstatic in-Pstatic out)/rho)

    After subbing in values I get, velocity out=71m/s....This seems really high, can you see any error in this logic?

    This value will also only increase when I add the effect of the temperature rise on the air density.

    Thanks
     
  5. Jul 13, 2016 #4

    russ_watters

    User Avatar

    Staff: Mentor

    The volumetric flow rate across a heater changes because the air expands when heated. The mass flow rate stays the same because there is nowhere else for the mass to go.
     
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