Can anyone interpret this fan curve? 1

[MarkESmith]

Hi,

This combustion air forced draft fan curve has two lines labelled Pt and Ps (or Pg?). Looks like it's from an Italian vendor. Is anybody familiar with these subscripts and could they please explain what they mean? Are the two lines simply two different fan sizes (similar to impeller size on a pump curve?)

Many thanks.

 

[ione]

Ps (static pressure): it’s defined as the pressure on the pipe’s or container’s wall. It acts in all the directions and it is not dependent on fluid velocity.

Pt (total pressure). It’s the sum of the static and dynamic pressure, where dynamic pressure is defined as the energy due to the fluid velocity (Pd = 0.5 *rho*v^2). It acts on the same direction as fluid flows.

For a given fan the performance curve reports both the static and the total pressure vs volumetric flow rate.

 

[MarkESmith]

Thank you very much. I should have thought of that. So a pressure reading taken perpendicular to the flow direction will provide the static pressure only, right?

Potentially a very stupid question coming up:
Why does a pump curve (or at least the ones I've seen) not have total pressure and static pressure? Is it because the dynamic pressure term is very small compared to the pump discharge pressure?
e.g. water at 10m/s (1/2)*rho*v^2 = 50kPa = 0.5bar = ~7psi

Also haven't seen this on a compressor curve?

 

[ione]

Further to your first question for any given flow rate, the line perpendicular to the x-axis will first intersect the static pressure curve, and then it will intersect the total pressure curve. Static pressure is the pressure needed to overcome the circuit losses. So as it is done for centrifugal pumps, the performance curve of a fan should be combined with the circuit curve in order to find the operating point of the fan.

Further to your second question, even if terminology is different, it could be done for pumps too to include in the performance curve what it’s called velocity head. Due to the range of pressures involved generally centrifugal fans are assumed to be elaborating an incompressible fluid as pumps do. Affinity laws which rule the behaviour of centrifugal pumps could be applied to centrifugal fans as well.