Fan discharge velocity

[brad37]

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

 

[walz]

there is no issue for low discharge velocity, if your selection is taking care of total static, and less than 2000 rpm which seems like your demand.

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.

 

[ChrisConley]

The only issue you would have with low discharge velocity is if you are discharging directly at a gas-fired heat exchanger. Most gas fired equipment requires high discharge velocity to 'scrub' the heat exchanger.

If your AHU is draw-through, then I can't picture any issues with low fan discharge velocity.

 

[mauricestoker]

The only problem I could think of is if you have an airflow monitoring station that needs a higher velocity.

 

[imok2]

If they are Airfoil type blades, they are the most efficient of all, capable of achieving efficiencies exceeding 85 percent. Because airfoil blades rely on the lift created by each blade, this fan type is highly susceptible to unstable operation because of stall. Contaminant build-up on fan blades can
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

 

[walz]

Brad

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.

 

[HVAC68]

1. Is there a problem in running the fan in excess of 2000 rpm. Centrifugal chillers run at close to 8000 rpm.

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

 

[ChrisConley]

I think sound is the primary concern for limiting fan rpm. I agree we can run equipment faster, but we do try to limit fan rpm for sound and vibration; which can be mitigated with silencers and isolators, but it is easier to eliminate the issue right at the source.

 

[HVAC68]

ChrisConley - Point noted. However, as said in point 2 of my earlier post, there are limitations with backward curved fans, especially for the lower air quantity ranges.

HVAC68

 

[ChrisConley]

Agree.

 

[SAK9]

If you examine selections for centrifugal machines be they fans or pumps,higher speeds upto 3000 rpm are often required at low flow rates for efficient operation.

The practical solution then would be to select a high operational speed of 3000 rpm and take measures to mitigate the noise.