Natural Ventilation: Windows, Walls & Other Devices

PAUL HAY Capital Projects

www.phcjam.com

Topic:              Natural Ventilation: Windows, Walls & other influences

Author:            Paul Hay

e-mail:              paul.hay@phcjam.com

profile:             www.linkedin.com/in/phcjam

1.0       INTRODUCTION           

.1            Ventilation is the movement of air through a space, whether open or enclosed:

            1.1.1   Kinetic energy sustains ventilation through a building;

            1.1.2   Air pressure is directly proportional to the density of the air and the square of its velocity.

1.2       Natural Ventilation is the exchange of indoor air with fresh outdoor air by natural means:

            1.2.1   Temperature and pressure differences produce natural ventilation;

            1.2.2   Design of openings (i.e. windows, doors, vents, etc.) affects natural ventilation;

            1.2.3   Air exchange is necessary to maintain proper air quality;

            1.2.4   Mechanical systems should augment natural ventilation in the event of poor wind conditions.

2.0       ROOM OPENINGS

2.1       Where one opening exists, it serves as both inlet and outlet:

            2.1.1   When wind is perpendicular to the opening, natural ventilation will not be significantly influenced by increasing the size of the opening; but

            2.1.2   Ventilation will improve with larger openings when wind is oblique to the opening.

2.2       Natural ventilation is significantly improved with use of two openings if inlet and outlet sizes are increased equally.

TABLE 1:              RELATIVE AIR VELOCITIES IN NATURALLY VENTILATED ROOMS

WIND DIRECTION	OPENINGS			INTERIOR AIR SPEED
	AMT	WIDTH	LOCATION	AVERAGE	MAXIMUM
Perpendicular	1	66%	Windward	13%	18%
	1	100%	Windward	16%	20%
Oblique	1	66%	Windward	15%	33%
	1	100%	Windward	23%	36%
Oblique	1	66%	Leeward	17%	44%
	1	100%	Leeward	17%	39%
Oblique	2	66%	Leeward	22%	56%
	2	100%	Leeward	23%	50%

2.3       Cross Ventilation is the flow of air from an inlet to an outlet not located on the same wall:

            2.3.1   Cross ventilation is best when inlets and outlets are the full width of the wall;

            2.3.2   Outlets are located on walls experiencing negative wind pressure;

            2.3.3   The vertical location of outlets relative to outlets influences ventilation:

                        2.3.3.1            Ventilation is best when inlets are low and outlets high, because the air current cools occupants and extracts warm air adjacent to ceilings;

                        2.3.3.2            Occupants are also cooled when both inlets and outlets are low, but warm air adjacent to ceiling is not extracted; nevertheless,

                        2.3.3.3            Warm air adjacent to ceiling is extracted when both inlets and outlets are high, but occupants are not cooled.

TABLE 2:              RELATIVE AIR VELOCITIES IN CROSS VENTILATED ROOMS

WIND DIRECTION	OPENINGS			INTERIOR AIR VELOCITY
	AMT.	WIDTH	LOCATION	AVERAGE	MAXIMUM
Perpendicular to inlet	2	66%	Adjacent to windward inlets	45%	68%
	2	100%	Adjacent to windward inlets	51%	103%
Oblique to inlet	2	66%	Adjacent to windward inlets	37%	118%
	2	100%	Adjacent to windward inlets	40%	110%
Perpendicular to inlet	2	66%	Opposite to windward inlets	35%	65%
	2	100%	Opposite to windward inlets	37%	102%
Oblique to inlet	2	66%	Opposite to windward inlets	42%	83%
	2	100%	Opposite to windward inlets	42%	94%

3.0       OTHER INFLUENCES

3.1       Fins can improve natural ventilation:

            3.1.1   Fins increase ventilation through windward openings considerably; and

            3.1.2   Air flow can triple if wind is oblique to the opening.

3.2       The influence of internal partitioning cannot be readily generalized and should be determined by experimentation.