Stack effect in relation to ventilation

Last updated on May 15th, 2024 at 08:10 am

Ventilation, i.e. both the supply of fresh air and convective cooling, involves the movement of air at a relatively slow rate. The motive force can be either thermal or dynamic (wind).

The stack effect ventilation relies on thermal force set up by density difference ( caused by temperature difference) between the indoor and outdoor air.

It can occur through an open window (when the air is still): the warmer and lighter indoor air will flow out at the top and the cooler, denser outdoor air will flow in at the bottom. The principle is the same as in wind generation.

Explain the stack effect in relation to ventilation
                Explain the stack effect in relation to ventilation

Special provisions can be made for it in the form of ventilating shafts. The higher the shaft, the larger the cross-sectional area, and the greater the temperature difference, the greater the motive force therefore, the more air will be moved.

The motive force is the stack pressure multiplied by the cross-sectional area (force in Newtons- area in m²). The stack pressure can be calculated from the equation.

P. = 0.042xATxh
Where Ps = stack pressure in N/m²
h = height of stack in m
T = temperature difference in deg C

(the constant is N/m³ deg C)

Such shafts are often used for the ventilation of internal, window less rooms (bathrooms and toilets) in Europe.

Figure 11.1 shows some duct arrangements for multistory buildings, with vertical or horizontal single or double duct systems.

These systems operate satisfactorily under winter conditions when the temperature difference is enough to generate an adequate air flow.

Air flow pattern through the buildings with the help of a diagram

As no satisfactory and complete theory is available, air flow patterns can only be predicated on the basis of empirical rules derived from measurements in the actual buildings or in wind tunnel studies.

Such empirical rules can give a useful guide to the designer bur in critical cases it is advisable to prepare a model of the design and test it on a wind simulator.

Wind simulators may be of the open-jet type (Figure 12.1) or the wind tunnel type. The former type is in use with the Architectural Association School of Architecture.

Explain the air flow pattern through the buildings with the help of a diagram.
     Air flow pattern through the buildings with the help of a diagram.

which has been developed with the cooperation of the Department of Fluid Mechanics, University of Liverpool.

The latter type is best represented by an economical model developed by the Building Research Station which is described in BRS Current Paper 69/1968.

For qualitative studies, a smoke generator can be used and the smoke traces can be photographed. This gives a convincing picture of flow pattern, position laminar flow, and turbulences.

With some practice, the wind tunnel operator can estimate velocity ratios from smoke traces with quite a reasonable accuracy.

For quantities analyses, air velocity or air pressure measurements must be taken with miniature instruments at predetermined grid points.

On the basis of such experimental observations the following factors can be isolated which affect the indoor air flow (both pattern and velocities):

a. orientation
b. external features
c. cross-ventilation
d. position of openings
e. size of openings
f. controls of openings.

What do mean by periodic heat flow? Explain in detail

In nature the variation of climatic conditions produces a nonsteady state, Diurnal variations produce an approximately repetitive 24-hour cycle of increasing and decreasing temperatures.

The effect of this on a building is that in the hot period heat flow from the environment into the building, where some of it is stored, and at night during the cool period the heat flow is reversed: from the building to the environment.

As the cycle is repetitive, it can be described as periodic heat flow.

Air movements inside and around buildings

The diagram given in Figure 3.1 shows the diurnal variations of external and internal temperatures in a periodically changing thermal regime in the morning, as the out door temperature increases, the heat starts entering the outer surface of the wall.

Each particle in the wall will absorb a certain amount of heat for every degree of rise in temperature; depending on the specific heat of the wall material Heat to the next particle will only be transmitted after the temperature of the first particle has increased.

Thus the corresponding increase of the internal surface temperature will be delayed, as shown by the broken line.

The out-door temperature will have reached its peak and started decreasing before the inner surface temperature has reached the same level.

From this moment the heat stored in the wall will be dissipated partly to the outside and only

partly to the inside. As the out-door air wall temperature falls below the indoor temperature the direction of the heat flow is completely reversed.

The two quantities characterizing this periodic change are the time-lag (or phase shift,) and the decrement factor (amplitude surface attenuation).

The latter is the ratio of the maximum outer and inner surface temperature amplitudes taken from the daily mean.

Explain the heat exchange process in building?

The Human body was considered as a defined unit and its heat exchange processes with the environment were analysed . The building can similarly be considered as a defined unit and its heat exchange processes with the outdoor environment can be examined (Figure 3.2)

a. Conduction of heat may occur through the walls either inwards or outwards the rate of which will be denoted as Q. (convective and radiant components in the transfer of the same heat at the surface are included in the term: transmittance)

b. The effect of solar radiation on opaque surfaces can be included in the above by using the sol-air temperature concept, but through the transparent surface (windows) the solar heat gain must be considered separately. It may be denoted as Q₁.

c. Heat exchange in either direction may take place with the movement of air, i.e. ventilation, and the rate of this will be denoted as Q₁.

d. An internal heat gain may result from the heat output of human bodies, lamps, motors, and appliances, This may be denoted as Q₁.

e. There may be a deliberate introduction or removal of heat (heating or cooling), using some form of outside energy supply. The heat flow rate of such mechanical controls may be denoted as Qm.

f. Finally, if evaporation takes place on the surface of the building (e. g .a roof pool) or within the building (human sweat or water in a fountain) and the vapors are removed, this will produce a cooling effect, the rate of which will be denoted as Qe.

The thermal balance, i.e. the existing thermal condition is maintained if:
Q₁ + Q₂±Q₂ ± Q,+Q-Q₂ = 0

If the sum of this equation is less than zero (negative), the building will be cooling and if it is more than zero, the temperature in the building will increase.

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