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Wind Flow Through Opening Calculator

HVAC Systems and Components Design Data

Flow Through Opening Caused by Wind Only Calculator

Aspects of wind that affect the ventilation rate include average speed, prevailing direction, seasonal and daily variation in speed and direction, terrain, and local obstructions such as nearby buildings, hills, trees, and shrubbery. Liddament (1988) reviewed the relevance of wind pressure as a driving mechanism. A multiflow path simulation model was developed and used to illustrate the effects of wind on air change rate.

The equation below shows the estimated rate of air forced through ventilation inlet openings by wind or can be used to determine the proper size of openings to produce given airflow rates:

Eq. 1
Q = 88.0 C_v A U

where

Q = airflow rate, cfm
C_v = effectiveness of openings (C_v is assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.35 for diagonal winds)
A = free area of inlet openings, ft²
U = wind speed, mph
88.0 = unit conversion factor

Air intakes should be placed in exterior high-pressure regions, and air reliefs should be placed in exterior low-pressure regions, but because of wind variations, these static locations will, at times, not be optimal. Other considerations include flow control when wind speed is high, and security. Air intakes should face directly into the prevailing wind. If they are not advantageously placed, flow will be less than that predicted by Equation (1); if intakes are unusually well placed, flow will be slightly more.

Desirable air relief locations are

1. on the leeward side of the building directly opposite the intake;
2. on the roof, in the low-pressure area caused by flow separation;
3. on a side perpendicular to the windward face, where low-pressure areas occur;
4. in a dormer on the leeward side; (5) in roof ventilators; or (6) by stacks.

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References

ASHRAE Handbook HVAC System and Equipment - 2012