Heating, Cooling and HVAC

Approach Velocities

The pattern of the airflow as it enters an Axial flow fan is different from the pattern as it leaves. Assuming no interference from the layout of the building, or louvres on the fan, the air entry pattern is hemispherical (shaped like half a globe or sphere). By contract the pattern of airflow as it leaves an Axial flow fan is an almost parallel sided jet, actually expanding slightly at an included angle of 15°.

The velocity of the approach air increases as it nears the fan. There is little likelihood of a draught problem on the approach side. As a guide, if the air passing through a 620mm diameter fan is at 10m/s. The air velocity at 2m from the fan is under 1m/s. The approach velocity can be calculated by dividing the air volume (in m³/s) by the surface area of half a sphere, multiplied by the distance from the fan in m².

Velocity =	Volume in m³/s
	surface of ½ sphere × distance m²

The formula for the surface area of a sphere is 4πr² , so for half a sphere 4πr²
2

Velocity =	Volume in m³/s
	6.284 × s² × m²

For a 300mm diameter fan moving 0.43 m³/s (1560 m³/h), the approach velocity at 1m from the fan is:

Velocity =	0.43	= 0.068 m/s
	6.284 × 1² × 1²

Against the actual velocity through the fan:

Aperture =	0.43	= 6.06 m/s
	0.071

Translated into domestic terms an average 150mm impeller diameter fan in a kitchen should have a minimum performance of 60 l/s (0.06 m³/s) to meet Building Regulations Document F1. The approach velocity of this fan at 1m is:

	0.06 or approx 0.01m/s
	6.284

The velocity of hot air rising from a domestic cooker can be as high as 0.3 m/s, thirty times faster than the air going towards the fan. The hot air would therefore, rise to ceiling level before being drawn slowly towards the fan. There could be some spillage before the hot air is entrained towards the fan. Does this mean the recommended fan performance in F1 is inadequate? If a fan is drawing air from outside into the building, (to pressurise it) the approach velocity is of importance only if there is a source of pollution nearby. For example, the outlet of a fuel burning appliance. For this reason another of the Building Regulations, (Document J) recommends at least 600mm between the flue outlet and an air inlet to the building.

As the hot boiler flue gases rise, it could be suggested that an air inlet is not sited vertically above an outlet and not less than 600mm either side of the vertical line.

By contract, the airflow on the discharge side of an axial fan follows a different pattern, expanding at an included angle of 15°. This means that its velocity is maintained over a far greater distance. Taking the same 620mm diameter fan with a through velocity of 10 m/s, this velocity in still air at 2m, is still air 6.5 m/s @ 5m and just under 4 m/s @ 7.8m from the fan. If these velocities are on the outside of an exhaust fan, they could cause problems with proximity of adjacent buildings. If the fan is pressurising the building, (blowing air into it), the possibility of draught discomfort inside the building is high. There are inherent design problems to ensure that the mechanical airflow into and out from the building do not suffer from conflicting airflow patterns. Air tries to flow in a straight line, when turned it tries to continue in the new straight line. It does not bounce. This aspect is discussed more fully in the section on ducting.

The same airflow patterns apply to grilles and louvres. The extract grilles from a room should be the simple eggcrate type. This allows the hemispherical approach with resultant low approach velocities. The same grille on the intake airflow to the room would give an uncomfortable high velocity downwards air flow. Inlet grilles should have deflector vanes to direct the airflow parallel to the ceiling, ensuring a mixing of air before circulating into the room volume.

Example 1	What is average velocity 1m from a size 12 unit moving 1560m³/h?
	1560 Volume = 3600 = 0.43 m³/s   0.43 m³/s = 0.43 = 0.43 = 0.068 m/s (=13.5 ft/min) 4Ï€r² / ²m²   6.284 × 1²   6.284 × 1

Example 2	In a domestic kitchen with a 150mm window model, what will be the average velocity at a cooker 1m away?
	216m³/h Volume = 3600 = 0.06m³/s 0.06m³/s Velocity = 3.284 × 1²m² = 0.001m/s

Now the velocity of hot air rising from a domestic cooker can be as much as 0.33 m/s. This is 30 times as fast as the air going towards the unit at this point. The hot air would therefore, rise to ceiling level before being slowly drawn towards the unit. As long as the unit is big enough to give the correct air change rate in the kitchen, it does not matter if the hot air from the cooker is not drawn directly towards the unit, but goes first up to the ceiling, but is a 150mm fan big enough, even though it conforms to Building Regulations?

Average velocity at any point ‘P’	=	volume m³/s
		surface area of ½ sphere × m²
	=	volume m³/s
		( 4πr²m² ) 2
	=	volume m³/s
		( 4πr²m² ) 2
	=	volume m³/s
		6.284 × r²m²
When ‘P’ is at	1D. ave. vel. = 12.5% of entry velocity
	2D. ave. vel. = 3% of entry velocity
	3D. ave. vel. = 1.4% of entry velocity
Actual velocities reduced on centreline to:	1D - 10%
(= dotted line area)	2D - 2.5%
	3D - 1