Calculation of natural ventilation
Natural ventilation in the room is a self-propelled air movement due to the difference in its temperature (density) from the outside and inside or (and) the wind load outside. Natural ventilation is channel and channelless, and can be conditionally still as constant and periodic. Periodic opening of swing, windows, doors and window openings is called airing. Channelless natural ventilation, organized on a permanent basis in production facilities with significant heat releases, which ensures the unnecessary multiplicity of air exchange in them, is called aeration. In residential and public buildings, channel is most commonly used natural ventilation, in which air dacts are located vertically in special blocks, mines or placed in the interior of the walls.
Calculation of aeration
Aeration of industrial premises in the long period of the year provides an inflow of air through all the lower aeration cavities in wall protections, as well as gates and entrance doors. In the cold and transition periods of the year, air in the necessary volume comes through holes in wall protections located not lower than 4 m from the floor level (down to the bottom of the hole). An extractor in any of the periods of the year is carried out through the frames of the lanterns, as well as through the mines and deflectors. In the cold and transitional periods of the year, the transom lights are opened only on areas located above or near the source of heat dissipation.
In rooms with excessive heat, the air temperature inside the room is always higher than the outside, and the density, respectively, of the mensha, which determines the difference in the pressure of the external and internal air. At a low altitude of a room, in the so-called plane of equal pressure, this difference is zero. Below the plane of equal pressure, there is a rarefaction, which causes the influx of external air, and above - some excess pressure, due to which the heated air is removed from the outside. The pressure, which causes air to move with natural ventilation, is determined by the formula:
Рn = (ρ in - ρ ex)hg,
where ρ ex - external air density, kg/m3;
ρ in- air density of the room, kg/m3;
h - vertical distance from the center of the supply air to the center of the exhaust, m;
g - acceleration of free fall, equal to 9,81 m/s2.
This pressure is spent on overcoming the resistance of the air movement in the room and giving it an unnecessary speed when emitted into the atmosphere.
In addition, in windy weather from the windward side of the building a zone of high air pressure is formed due to the retardation of air masses, moving from the leeward and over the roof of the building - rarefaction. Due to the difference in the pressure produced, the outside air enters the building through open holes on the windward side of the building and leaves through open openings from the anti-leaky, leeward side. To efficiently use the effect of wind and heat pressure, it is not necessary to properly organize the movement of air streams inside the building. This is achieved by choosing the optimal scheme for opening the lid openings and the use of unobtrusive lanterns.
The amount of inflow of air L, kg/h, which should be received in the inflow holes of the aeration building, are determined by the formula:
L = 3,6Q/(с(tre-tsp)),
where Q - heat transfer to the room, W;
с - specific mass heat capacity, kJ/(kg·oС);
tre - air temperature, which is removed, оС;
tsp - estimated temperature of supply air, оС.
Air temperature, which is removed, are determined by the formula:
where twa - temperature in the working area, which should not exceed the sanitary norms, °С;
Δτ - temperature gradient in height of the room, оС/m (is in range 0,5... 1,5 °С/m);
Н - extend from the floor to the center of the exhaust slots, m;
hwa - height of the working area, accepted as 2 m.
Natural ventilation via aeration lights is reliable and efficient. Control the opening and closing of the lantern remotely, from the camera.
The calculation of natural ventilation - aeration involves determining the area of the upper and lower exhaust ports. First, ask the area of the lower holes. The aeration scheme of the room is given. Indirectly from the opening area of the upper exhaust and lower tidal transom in the room, a level equal pressure (approximately in the middle of the height of the building) is established. The pressure in this plane is zero. Otzhe, at the level of the center of the lower holes pressure is created:
Р1 = h1(ρ ex - ρ ср),
where ρ av– average air density in the room, which corresponds to the average indoor air temperature, kg/m3;
h1– height from the plane of equal pressures to the lower cavities, m.
Average indoor air temperature
Above the plane of equal pressure there is an excess pressure, Pa, which at the level of the center of the upper openings is equal:
Р2 = h2(ρ ex - ρ av),
This pressure and prompts air extraction. The general fact of the pressure at which the air exchange in the room occurs is equal to: Рn =Р1 +Р2
Determine the air velocity in the lower slots, m/s:
V1= L / (μ1 F1)
where L – necessary air exchange, m3/h;
μ 1 – the coefficient of consumption, which consumes from the design of the lower doors and the angle of their opening (for doors open 90o, μ=0,6; for 30o – μ=0,32);
F1– the area of the lower cavities, m2
Then determine the pressure loss, Pa, in the lower cavities:
H1= 0,5V12 ρ ex/g
Define by the formula Рn = Р1+Р2 =h(ρ ex - ρ m), taking the temperature of the air to be removed tre=twa+Δ(10 - 15оС) and determine the density ρ exand ρ m, corresponding to the temperature tex і tm
Determine excess pressure in the plane of the upper exits:
Р2 = Рn- Р1
Determine the non-occupied area of the upper openings (m2):
F2 = L /(μ 2V22) = L /(μ 2 (2Р2g/ρ ср)½)
Calculation of channel natural ventilation
Calculation of natural ventilation channel type is reduced to the determination of the dimensions of the living section of the air ducts providing the passage of the necessary air resistance, corresponding to the design pressure. Pressure loss in air ducts is defined as the sum of pressure loss in the sections of the longitudinal tract of the network. At each section of the loss of pressure, Pa, consist of friction losses (RI) and to overcome the locus of resistance (Z):
р = Rl + Z,
where R - specific loss of pressure over the length of the plot, due to friction, Pa/m;
l - length of the plot, m.
The area of the living section of the air lines, m2, is determined by the formula:
F = L / (3600v),
where L - estimated air flow, m3/h;
v - air velocity in the air line, m/s (usually it is taken equal to 0,5 ... 1,0 m/s).
The area of the living section and the size of the air line are found, asking the speed of air in it. Losses of pressure on friction are determined by means of special tables or nomograms, which are compliant for round steel air ducts. If the air ducts for ventilation duct systems are supposed to be rectangular, a diameter dE for each section is calculated for a uniform (equivalent to friction) round duct:
dЕ = 2 а b / (а + b),
where а і b - length of sides of rectangular duct, m.
The specific loss of friction pressure R, determined by the nomogram for steel air ducts, in the case of non-metallic air ducts is corrected by multiplying the value found on the coefficient k, next:
-for slag gypsum channels is equal 1,1;
-for slag concrete - 1,15;
-for those who are looking - 1,3.
Losses of pressure, Pa, to overcome local resistance for each site are calculated by the formula
Z = Σξv 2ρ/2
where2ρ/2 - dynamic pressure, Pa, determined by nomogram.
In the system of air ducts, the loss of locomotive supports is 80 ... 90% of total losses, so when constructing a natural ventilation system, avoid sharp turns, excess valves, valves on the airway.
Natural ventilation is simple in design and relatively easy to maintain, which is an advantage, but the main disadvantage is a short radius of its operation, especially for rooms with non-specific excess heat.