Deflector area calculation

Fans

For
mechanical ventilation systems
usually use radial
(centrifugal) fans. Selection
radial fan perform
according to performance targets
L_v,
m/h,
and pressure drop Р_v,
Pa, according to the summary schedule presented
v . If the intersection point
coordinates do not match the working
characteristic, then it is demolished vertically
to the underlying performance curve,
determine the corresponding full
pressure P_at
and recalculate the system for this pressure
(usually increase the cross section of one or two
main sections). Maybe
transfer of the operating point to the located
higher performance with increase
system resistance. Individual
characteristics of fans, knowing
L_v
and P, find the rotation frequency n
, rpm, efficiency
in the work area. The fan must
work with maximum efficiency, deviation
of which should not exceed 10%,

So
how the characteristics of the fans
compiled for standard conditions,
when selecting fans
pre-calculate:

R_v
= 1.1 Rset [( 273 + t)
/ 293] 
(1010/R);

L_v
= k
L_set

where
R_set
- design resistance of the ventilation
networks with equipment (filters,
air heaters, louvered grilles, etc.) Pa;

t
is the temperature of the air passing through
fan, ° С;
R_bar
- barometric pressure in place
fan settings, kPa; .

TO
- correction factor for suction
air in the exhaust and leakage in the supply
systems, adopted depending on
fan installation locations: k \u003d 1.1
for systems with metal air ducts,
plastics and asbestos-cement pipes
up to 50 m; K=1.15 for systems with air ducts
from other materials, as well as for systems
with a channel length of more than 50 m;

L_set—
estimated air flow in the system
ventilation, m3/h.
Duct length when determining
the value of K is determined by the length
channels laid out of service
premises. Power consumption per
motor shaft is determined by
formula, kW

N
= 0,728_vR_set
10/
(_P),

where
_P
— efficiency transmission received by .
Minimum installed power
electric motor, kW

N_At=k
N,

where
kz
— power reserve factor,
accepted by .

On the
based on the value of N_at
and the number of revolutions by pick up
fan motor.

Fan
is a mechanical
device designed for
air movement through ducts
air conditioning and ventilation systems,
as well as for direct delivery
air into the room or suction from
premises, and creating the necessary
this pressure difference (at the inlet and outlet
fan).

By
design and principle of operation
fans are divided into axial (axial),
radial (centrifugal) and diametrical
(tangential).

V
depending on the value of the total pressure,
that they create when they move
air, fans are low
pressure (up to 1 kPa), medium pressure
(up to 3 kPa) and high pressure (up to 12 kPa).

By
direction of impeller rotation
(when viewed from the suction side)
fans can be clockwise rotation
(wheel rotates clockwise)
and left rotation (the wheel rotates
counterclock-wise).

V
depending on the composition of the transported
environment and operating conditions fans
subdivided into:

ordinary
– for air (gases) with temperature
up to 80С;
corrosion resistant
– for corrosive environments;
heat-resistant
- for air with a temperature above 80C;
explosion-proof
– for explosive environments;
dusty
– for dusty air (solid
impurities in the amount of more than 100 mg/m3).

By
fan impeller connection method
and motor fans can
be:

With
direct connection with
electric motor;
With
connection on an elastic coupling;
With
V-belt transmission;
With
regulating stepless transmission;

Main
fan characteristics are
the following options:

consumption
air, m3/h;
complete
pressure, Pa;
frequency
rotation, rpm;
consumed
drive power
fan, kW;
efficiency
- efficiency factor
fan, taking into account mechanical
power losses for various types
friction in the working parts of the fan,
volume loss due to leaks
seals and aerodynamic losses
in the flow part of the fan;
level
sound pressure, dB;

Deflector calculation

Features of ventilation of buildings for various purposes: Ventilation of residential buildings, 4

Aerodynamic calculation of natural ventilation consists in determining the dimensions of the cross section of the channels and their resistance to air movement.

Given the speed of air movement and, m / s, calculate the preliminary section of the channels f, m 2, according to the formula:

f = V / (3600*v), (III.52)

where V is the specified volume of exhaust air in the design area, m 3 / h.

Rice. III.38 Nomogram for selection of TsAGI-type deflectors Fig. III .37 TsAGI deflector

1 - umbrella cap, 2 - paws, 3 - cone shield, 4 - diffuser, 5 - branch pipe, 6 - body

For preliminary calculation, the following air speeds are recommended: in the vertical channels of the upper floor - 0.5 & pide.0.6, from each lower floor - 0.1 more than from the previous one, but not higher than 1, in prefabricated collectors - at least 1 , in the exhaust shaft - 1 - 1.5 m / s.

If the loss is higher than the available pressure, determined by formula (III.50), then one of the following measures must be taken: increase the cross-sections of the channels, arrange two channels instead of one, install an axial fan to increase thrust or a deflector.

The deflector (Fig. III.37) is a nozzle that is placed at the mouth of pipes or shafts, as well as directly above the exhaust openings in the roofs of buildings. The principle of operation of the deflector is based on the use of the energy of the air flow - wind. When air flows around in the frontal part of the deflector, a zone of positive pressure is created, and in the rest (approximately 5/7 of the perimeter) - a rarefaction zone, which contributes to increased air extraction from the room. The most widespread deflectors of the TsAGI type are round (shown in Fig. III.37) and square.

It is convenient to select deflectors using nomograms. On fig. III.38 shows a nomogram for selecting the diameter of the deflector nozzle with a capacity L, m 3 / h, according to wind speed without taking into account gravitational pressure.

In modern hotel buildings, as mentioned, mechanical ventilation is designed, which can be equipped with an air conditioning system. The supply unit and the central air conditioner can be located in the technical underground, on the 1st floor or the last technical floor, and the exhaust unit can be located on the technical floor. Air is distributed through floors and rooms, as well as its exhaust, through air ducts combined into vertical or horizontal collectors (Fig. III.39, a and b).

Rice. III .39 Schemes of air ducts for mechanical supply and exhaust ventilation of multi-storey residential and public buildings

a - system with vertical collectors, b - system with horizontal collectors, 1 - exhaust openings, 2 - vertical collector, 3 - branch duct, 4 - self-closing check valves, 5 - valve that automatically opens when the fan is turned on and the temperature rises at point K up to 50 degrees, 6 - fan, 7 - valve, 8 - supply openings, 9 - supply unit or air conditioner

In buildings with a height of ten floors or more, on the exhaust air ducts of the upper two floors, it is necessary to provide for the installation of self-closing check valves 4, and on the supply and exhaust shafts located on the technical floor, valve 5, which automatically opens when the fan stops and the temperature rises above 50 degrees Celsius . These measures are necessary to block the air ducts of the two upper floors and remove air from the system in the event of a fire in the building.

Big Encyclopedia of Oil and Gas

Performance - Deflector

The performance of the deflectors depends on the thermal pressure, wind force, installation height, as well as on their dimensions, design features and the length of the exhaust ducts. With an increase in wind speed, the performance of the deflector increases.

The performance of the deflectors is taken at least three times the air exchange.

Deflector performance, of course, depends on its size. To ensure the constancy of all quality indicators of the deflector, the ratio of the dimensions of its individual elements (for each type) must in all cases remain constant and a multiple of, for example, the diameter of the nozzle (dflej), which is usually the only value that determines the size of the deflector.

For accurate selection of the deflector, it is necessary to pre-calculate the aeration of the shop, taking into account the performance of the deflector and find the true value of PPOY.

According to the wind speed VB m / s, which is accepted according to SNiP for a given area, the difference in air densities and the diameter of the deflector D determine the performance of the deflector.

By analogy, for other initial temperatures, we obtain the values entered in the form. The performance of the deflectors is taken equal to three times the air exchange. Additional exhaust holes, as you can see from the entries in the form, are required only during the transitional and summer seasons.

The total area of the inlet openings located in the lower part of the building must not be less than the total area of the openings of the inlet pipes of the deflectors. The performance of a deflector usually differs little from that of any vent in a similarly located skylight. However, a simple formula for calculating the deflector performance cannot be given, since this performance is determined by the complex interaction of the four factors listed above.

When selecting deflectors, one should be guided by the data of the technical characteristics of manufacturers. The reliability of this data for determining the performance of deflectors requires special verification each time.

In some cases, to increase the efficiency of work, it is advisable to give the deflector an asymmetrical shape in terms of plan. However, so that this does not affect the performance of the deflector when the wind direction changes, it must be made rotating around its vertical axis, and the deflector must always automatically become in the same position with respect to the direction of the wind. Rotating deflectors of this kind are called wind vanes.

The deflector works most efficiently when it is located in a place on the roof where the kinetic energy of the wind is used to the maximum to create a vacuum in the ventilation system. If the deflectors are placed in the rarefaction zone that occurs when the wind flows around the building, in a closed space or on the roof of a low building located between two tall buildings, the performance of the deflectors is significantly reduced or completely reduced to nothing.

Deflectors provide removal of the fulfilled air outside, not allowing penetration into the room of an atmospheric precipitation. Their work is based on the use of wind and thermal pressures. The performance of the deflector depends on four factors: 1) its location on the roof, 2) resistance to air movement in the deflector itself and in the exhaust duct, 3) the height of the air column that creates thermal pressure, 4) the efficiency of the installation using the kinetic energy of the wind.

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Useful:

— Convenient program in Excel format for deflector calculation

— SP60 13330.2012 (Heating, ventilation and air conditioning)

Design features

The TsAGI deflector and other models are made according to the cross-sectional shape of the ventilation passage. They consist of the following parts:

A pipe attached to the pipe head.
Cone-shaped diffuser fixed on the branch pipe.
Ring mounted on the outside of the diffuser.
Umbrella (cap), designed to protect the channel from the penetration of foreign objects.
Legs for attaching the cap.
Fixing brackets.

The umbrella can be made in different forms:

Flat. The simplest option, which is easy to make from sheet steel or copper.
Removable. Suitable for chimneys requiring frequent cleaning.
Gable. Provides the best rain protection.
rounded. It has an aesthetic appearance.

Do-it-yourself ventilation deflector

Having good draft is a prerequisite for the normal operation of any boiler, fireplace or good old rustic stove. Otherwise, for example, in strong winds, poor draft can lead to smoke escaping back into the building or even severe carbon monoxide poisoning of the occupants.

In order to prevent such a situation from occurring, a special device called a ventilation deflector is installed on top of the pipe.

His work is based on the Bernoulli effect. Its essence is that when the air jet collides with the surface of the diffuser of the deflector and goes around it from all sides, a rarefaction is created in this place and traction improves.

The use of such a device allows you to increase the efficiency of the ventilation or chimney by 15-20%, as well as protect them from moisture or debris getting inside the pipe.

There are several designs of deflectors, but the Grigorovich model is considered the most successful of them. The design of its deflector is convenient for mounting on any type of pipe, it is not particularly difficult and can easily be made by hand.

Deflector calculation

Before proceeding with the manufacture of the components of the deflector, it is necessary to calculate their main dimensions. The basis for the calculation will be the internal diameter of our chimney. Based on this, the height of the deflector and the width of the diffuser are selected. To do this, you can use the data from the following table:

Deflector Assembly

For this we need things like:

Tin sheet or galvanized iron,
metal shears,
Aluminum rivets or screws with nuts,
metal scriber,
Medium density cardboard and stationery scissors.

To make the parts as accurately as possible, we first draw their contours on cardboard and cut out templates for the inlet pipe, deflector housing, diffuser, upper protective cap and mounting brackets from it.

After that, we estimate on the templates how the details match each other and if everything is fine, then we proceed to cutting the tin.

On finished parts, all sharp edges should be blunted and burrs removed.

The diffuser elements can be connected to each other using rivets, bolts and nuts, or you can use the services of a specialist with a semi-automatic welding machine. Arc welding is not suitable in this case, as it easily burns through thin tin.

Installing the deflector

First you need to fix the lower deflector cylinder on the pipe. The method of fastening (for example, clamps, bolts with dowels) is selected locally, depending on the material of the pipe and its condition.

Then, we fix the diffuser on the cylinder with the help of clamps. On top of it we install a reverse cone and a protective cap. If bolts with nuts are used for fasteners, it is recommended that their threads be well lubricated to protect against corrosion.