Ventilation Measurements

VENTILATION MEASUREMENTS

Determination of ventilation rate

Natural ventilation occurs because of wind and thermal forces which produce a flow of outdoor air through the various Im ni be9qe briw - openings in a building.

The ventilation flow needed to remove a given amoun heat from a building can be calculated using the following equation.

where

V - ventilation rate

Q - total heat gain in a space

C_p - average specific heat

(T_o – T_i ) temperature rise of incoming air

As a general rule, the ventilation rate of 2.8 m³/min_to 5.7 m³/min per person is adequate. When power-driven equipment and heat sources are present, a higher ventilation rate is necessary.

Wind Action

For determining the rate of ventilation based action, the wind may be assumed to come from any direction within 45° of the direction of prevailing wind. Ventilation rate due to external wind is given by the following formula.

Q=KA U

where

Q - the rate of air flow in m³/hr

K -  coefficient of effectiveness, which may be taken as 0.6 for wind perpendicular to openings and 0.3 for wind at an angle less than 45° to the openings;

A - free area of inlet openings in m and

υ  - wind speed in m/hr

The value of the coefficient of effectiveness K depends on the direction of the wind relative to the opening and on the ratio between the areas of two openings.

Figure 2.5 shows the increase in values of K by the percentage of unequal areas expressed as ratios of the two openings.

Stack Effect

Ventilation due to convection effects arising from temperature difference between inside and outside is given by:

Q = 7.0 A √h (t_r -t_o)

where

Q - the rate of air flow in m³/hr,

A - free area of inlet openings in m²,

t_r - vertical distance between inlets and outlets in m,

t_o - temperature of outdoor air in °C.

The equation is based on 0.65 effectiveness of openings. This should be reduced to 0.50 if conditions are not favorable.

Infiltration

It is the random flow of air through unintentional openings (leaks/ cracks) driven by wind and difference in pressures.

Infiltration is balanced by an equal amount of exfiltration since, there is no net storage of air in a building.

Air leakage is the sum of all parallel air flows through cracks and other openings into or out of a building.

The air leakage rates describe the relative tightness of a building. The rate can be measured under standardized conditions by connecting a suitable fan and a flow measurement device.

Infiltration can be calculated by using

(i) air change method or

(ii) crack method

Crack Method

The crack method calculates the flow produced by the pressure difference acting on each leakage path or building component. The flow is given by

V=C(∆P)ⁿ

where

V - volume flow rate of air (litre / s)

C -  flow coefficient, volumetric flow rate per unit length of crack or unit area, at a unit

∆P - pressure difference

AP-pressure difference

n is the flow exponent between 0.5 and 1, usually near 0.65 for leakage openings.