Basic principles of natural ventilation
When the air temperature in the hot pressing workshop increases, the volume expands and the density decreases.Because the air outside the workshop temperature is low, the density is larger, and the air gravity inside and outside the workshop is different, that is, hot pressure.While the air outside the workshop flows in from the lower doors and Windows, so that the hot air inside the workshop is discharged from the upper skylight, which means natural ventilation.The greater the temperature difference between indoor and outdoor, the greater the thermal pressure, the better the natural ventilation effect.
Although the size of natural ventilation depends on the temperature difference between indoor and outdoor, it also depends on the distance between the inlet and outlet.
P=h(outside P-inside P)
Where, P is thermal pressure (Pa);H is the center distance between the inlet and outlet (m): outside p, and inside p is the density of outdoor and internal air (kg/m3).
Wind outdoor airflow when they met with the architecture, flow around, will occur after a certain distance, resumed parallel flow, air flow due to the structure of the block, the buildings around the outdoor air pressure distribution will change, windward side airflow obstruction, reduced dynamic pressure, static pressure heighten, side and leeward side due to produce local vortex, static pressure is reduced.This increase or decrease in static pressure is referred to as wind pressure in comparison to an undisturbed flow at a distance.Static pressure increases, the wind pressure is positive, called positive pressure, static pressure drops, the wind pressure is negative, called negative pressure.
The distribution of wind pressure around a building is related to the geometry of the building and the outdoor wind direction. When the wind direction is constant, the wind pressure at a certain point on the external structure of the building can be expressed as :P=KV2 /2
Where, P is wind pressure (Pa);K is the aerodynamic coefficient, the windward side is positive, and the leeward side is negative.V is wind speed (m/s);R is the outdoor air density (kg/m3).
It can be seen from the above formula, the size of the wind pressure and wind speed is proportional to the square, the windward side of the factory is positive pressure, leeward side is negative pressure, so the air from the factory into the Windows into the lee side of the doors and Windows out from the lee side of the formation of natural ventilation.
In fact, hot pressure and wind pressure work together.Generally speaking, the change of hot pressure action is small, the change of wind pressure action is large;In order to ensure the effect of natural ventilation, in the actual design and evaluation, in addition to the hot pressure, in the layout of the plant should also consider the impact of wind pressure.