Fan discharge velocity
Fan discharge velocity
(OP)
I have an issue regarding fan selection for an AHU application with relatively low volume and high external static (in SI units) 500l/s @ 320Pa (total fan pressure approx 450Pa). In order to get a workable fan speed(no higher than 2000rpm), the available fan sizes (backward curved centrif) have a low discharge velocity (in the order of 5m/s). Typical selections by choice would be 8-9m/s. What is the effect on the fan performance in the system with a low discharge velocity and hence low velocity pressure?? I have looked through the ashrae handbook and cant find any reference to issues with lower discharge velocities.
Thanks in advance
Thanks in advance





RE: Fan discharge velocity
Fan discharge velocity should not exceed a certain limit, because its related to noise. for low discharge velocity, its good that will have lesser noise.
RE: Fan discharge velocity
If your AHU is draw-through, then I can't picture any issues with low fan discharge velocity.
RE: Fan discharge velocity
RE: Fan discharge velocity
cause severe performance degradation and fan imbalance. Build-up problems are promoted by a shallow blade angle with surfaces that allow contaminants to collect. Fans with blade shapes that promote low-velocity air across the blades, such as backward inclined fans, are susceptible to
contaminant build-up. Another consequence of backward-incline blade orientation
is a low angle of impingement with the airstream. This promotes the accumulation of particulates
on the fan blades, which can create performance problems. Thin airfoil blades are more
efficient than the other blade types because of
their lower rotating mass. However, this thin walled characteristic makes this fan type highly
susceptible to erosion problems. Loss of blade wall thickness can lead to cavity formation in
the blades, which can severely interfere with fan performance
RE: Fan discharge velocity
your static is higher... seems like you are having a bag filter in AHU. if this is the case, fan would be safer for long period of time.
RE: Fan discharge velocity
2. Typically, a backward curved or airfoil fan would run at more than 2000 rpm for air quantities of upto 2000 L/s @ total static pressures of around 600 to 800 Pa, at fan outlet velocities of around 10 m/s.
3. If higher speeds are a major constraint, then for such low air quantities, forward curved fans are a solution, as they run at much lower speeds as compared to a backward curved or airfoil fan. However, the disadvantages of a forward curved fan such as lower efficiency, flat characteristics (the fan curve is overloading type) and u predictable performance in the flat portion of the curve has to be borne in mind before taking this decision.
Good luck
HVAC68
RE: Fan discharge velocity
RE: Fan discharge velocity
HVAC68
RE: Fan discharge velocity
RE: Fan discharge velocity
The practical solution then would be to select a high operational speed of 3000 rpm and take measures to mitigate the noise.