Pressure losses in electric air heater

[kyle pender]

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.

 

[Chestermiller]

It's the static pressure.

 

[kyle pender]

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

 

[russ_watters]

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.