Louvre sizing

[Conganh]

As per ASHRAE Fundamentals 2009 Chapter 21 Table 8, the design "face" velocities for determining the louvre size is 2m/s for Intake and 2.5m/s for exhaust. Can anyone clarify how to use these velocity values in sizing the louvres?

For example, if 5000 CMH (1.39 m3/s) of air has to be exhausted out, which one of the following is correct by considering, 2.5 m/s as the face velocity?

1) Face area of the louvre with 50% free area is 1.39/2.5 = 0.556 m2 (which means, free area of the louvre is 0.258 m2 and the velocity of exhaust air in this free area is 2.5 * 2 = 5 m/s)

OR

2) Face area of the lourve with 50% free area is 1.39/(2.5*50%/100%) = 1.112 m2 (which means, free area of the louvre is 0.556 m2 and the velocity of exhaust air in the free area is 2.5 m/s.

So, the ASHRAE guideline velocity is the face area velocity or free area velocity?

Thanks in advance.

 

[EnergyProfessional]

face velocity is useless, each louver has different % of free area.
Take manufacturer data that shows pressuredrop and rain entrance (for intake louver). you don't want to have rain and snow be drawn in.

ASHRAE books have a lot of good stuff, but also a lot of outdated garbage. they go by the least cheapest manufacturer so no one is offended.

 

[Drazen]

there is no such thing as "face area velocity". air flows through effective area, that is why it's called effective area.