Unlike chimneys, the heads of the vertical shafts of the exhaust ventilation are always covered with umbrellas. Rainfall inside the duct is undesirable - the water has nowhere to go. The essence of the problem: the protective cap creates additional aerodynamic resistance to air flow. The work of the natural hood deteriorates, and with a lack of traction it stops altogether.
The question is solved as follows: at the end of the pipe, instead of the traditional "fungus", a ventilation deflector is mounted. Installation is done with your own hands, but first you need to choose the design of the exhaust device.
Why do you need a deflector
For a better understanding of the issue, we cite data from the reference literature. The value of local resistance to air flow in ventilation systems is characterized by a dimensionless coefficient ξ. The greater its value, the stronger the shaped element - an umbrella, elbow, gate - slows down the movement of gases through the pipeline.
In relation to our cases, the coefficient is:
- at the outlet of the air flow from an open pipe of any diameter ξ = 1;
- if the channel is covered with a classic hood, ξ = 1.3–1.5;
- Grigorovich’s umbrella with a diffuser (section expansion) is installed on the pipe, ξ = 0.8;
- Volper nozzle cylindrical or star-shaped "Shenard", ξ = 1;
- TsAGI deflector, ξ = 0.6.
Note. There is no mistake - even with a free ejection from the mine, an air stream overcomes local resistance from sudden expansion. Source: Handbook of Heat Supply and Ventilation, 1976 edition
So, the deflector is a nozzle that, under the influence of the wind, creates a vacuum at the outlet of the vertical ventilation duct and thus reduces aerodynamic resistance to flow. That is, it acts as a traction amplifier.
In addition, the exhaust device solves the following problems:
- protects the duct from precipitation;
- does not allow the wind to blow into the pipe;
- prevents the occurrence of reverse thrust (tipping).
The principle of operation of any deflector is based on two effects: rarefaction from a wind load and ejection (entrainment) of a slower gas flow by a faster one. Although some foreign manufacturers implement a mechanical impulse, they simply equip the umbrella with an electric fan. Consider the device of each design separately.
Comment. On the Internet, the operation of such caps is often explained by the action of the Bernoulli law or the Venturi effect. Both physical phenomena involve a narrowing of the duct, an acceleration of the flow, and a drop in pressure. In fact, the deflectors do not reduce the channel cross section (see above in the photo) - rarefaction is created solely by wind.
Varieties of nozzles
Now you can buy ready-made or make yourself the following types of caps - traction amplifiers:
- TsAGI deflector with ventilation duct extension - diffuser;
- cylindrical "fungus" Volper;
- H-shaped pipe manifold;
- cap - weather vane (popularly - "sneak");
- spherical rotary nozzle - the so-called turbo deflector;
- Statodynamic open device of the "Astato" type.
To list and consider ordinary umbrellas is pointless - such products do not improve traction, they only cover the pipe section from the rain.
The device caps TsAGI
This design was developed during the USSR by a specialized research institute (scientific institute). The deflector consists of such parts (shown in the drawing):
- bottom glass with a diffuser (extension) at the end;
- outer case - a shell made of cylindrical roofing steel;
- umbrella cover;
- rack fastening covers made of metal strips.
The scheme of the product’s operation is simple: wind blowing the case on either side creates a vacuum zone above the diffuser open from above. Exhaust gases coming from the mine are carried away by this vacuum, go outside and are picked up by the wind - the principle of ejection is triggered.
The table below shows the characteristics of typical TsAGI deflectors - sizes, performance depending on the speed of the wind flow.
Comment. Performance is shown without taking into account the resistance of the duct system crossing the roof. The actual exhaust volume depends on the height of the pipe and the temperature difference of the indoor / outdoor air.
Of all the static traction amplifiers, the TsAGI cap is recognized as the most effective, despite the venerable age of development. Design advantages:
- simplicity in manufacturing, installation;
- maximum protection against rain and snow, overturning traction;
- reliability, lack of rotating parts;
- the direction of the wind flows does not matter;
- lowest coefficient of resistance (ξ = 0.6).
The disadvantage of the deflector is its dependence on wind speed. If flows move slower than 2 m / s, the efficiency of the device tends to zero. However, calm has a negative impact on the operation of any nozzle, designed to enhance natural traction in the ventilation duct.
Please note: in modern versions of TsAGI factory-made insulation of the lower glass is provided if the cap is attached to the roof sandwich pipe. Under the "fungus" we see a skirt, although the passage section of the channel does not decrease.
Volper Static Umbrella
This deflector is more likely a windproof device than a natural traction amplifier. Although the pressure loss at the outlet of the flow nozzle successfully compensates. The design includes the following elements:
- lower branch pipe (glass);
- top cylindrical glass with concave walls;
- cone umbrella;
- connecting stripes.
The cap is installed on a circular duct or a rectangular shaft through an adapter. How does the Volper ventilation deflector work:
- Direct wind flows are reflected up and down by the concave surface of the upper shell.
- The jet passing between the umbrella and the cut of the glass creates a region of reduced pressure inside the body.
- Exhaust air changes the direction of movement - flows through the gap under the "skirt".
H-shaped nozzle
This original design is a tube assembly in the form of the Russian letter “H”, the hood is connected to the middle of an imaginary crossbar. Whichever way the wind blows into open pipes - from above or from below - a faster flow will eject (entrain) the air stream from the ventilation riser.
The advantage of the H-shaped deflector is almost one hundred percent protection against wind blowing, reverse traction, moisture and freezing. These pluses are crossed out by no less significant disadvantages:
- Problems with aerodynamics - to go out, the air overcomes 2 turns of 90 °. Losses are compensated by wind flow, but traction increases only minimally. Hence the low performance of the exhaust nozzle.
- The fixture is rather bulky, so fastening on the pipe is difficult.
- The H-deflector does not look too pretty. Imagine a situation when 2-3 ventilation ducts with similar caps are brought to the roof.
Addition. We missed one advantage of the nozzle - it is easy to assemble it with our own hands from ready-made tees. You can use the product for ventilation of auxiliary buildings, for example, a bathhouse or a warm barn.
Turbo Deflectors and Weathercocks
We combined these 2 types of nozzles in one section due to the similarity of the principle of action:
- A spherical rotary deflector with multiple semicircular blades rotates by wind. A vacuum forms above the pipe head (inside the ball), and the drawing efficiency increases.
- A weather vane with a wing always rotates “back” to the wind, preventing blowing into the trunk.A reduced pressure zone (aerodynamic shadow) forms behind the nozzle body, the air stream more readily leaves the vertical channel.
In terms of efficiency, dynamic caps outperform static caps, but have a number of operational features:
- in calm weather, turbo-deflectors and "sneaks" do not spin, respectively, they do not improve traction;
- the rotation unit - a bearing or a sleeve - requires maintenance (lubrication); in winter, it runs the risk of freezing;
- jammed weather vane can break with a sharp gust of wind;
- nozzles weakly protect from oblique rain or snow.
Reference. Prices of weathercocks and rotary baffles are higher than static nozzles. Example: TsAGI factory umbrella, made according to the 5.904.51 series, costs from 23 y. e., a turbodeflector - 38 at. e. Conclusion: you will have to pay extra for efficiency, plus climb up to the roof annually and maintain the ventilation device.
Astato Compression Cap
This is the only type of deflector, functioning in any weather, including full calm. The nozzle is made of two truncated cones, turned vertices to each other. The upper part is equipped with an umbrella and an axial electric fan. On the side, the opening is closed by an aluminum mesh from birds.
How the deflector of the French brand Astato works:
- In windy weather, the hood acts as a static amplifier - the flow passing between the cones picks up air that rises along the exhaust shaft. The fan is off.
- When the wind subsides, a pressure sensor is triggered - a pressure switch. It sends a signal to the EOL control unit.
- The controller starts the fan at the desired speed (there are only two of them). Forced extraction from the channel begins.
Note. The sensor threshold is user-configurable. The statodynamic device can work without expensive automation - from a temperature switch or can be switched on manually.
The only drawback of the Astato active deflector is the space price by our standards. To buy a nozzle with a minimum diameter of 160 mm, you will have to pay 1395 euros. If you want to automate the operation of forced extraction - add here the cost of the EOL unit - another 1,520 euros.
Which deflector to choose
If you want to install a cap - traction amplifier at minimal cost and not to service the product during operation, we recommend staying on static models - a Volper or TsAGI deflector. The latter option is preferable for hand-made manufacturing.
Tip. Select the nozzle size according to the diameter of the exhaust barrel. If a rectangular shaft is removed from the house, the selection is made according to the equivalent circular section. That is, it is necessary to calculate the diameter of the channel, then take a circle of a similar area. An adapter is used during installation.
Recommendations for choosing different deflectors:
- With a lack or lack of traction, it is better to put dynamic versions of the hoods - rotary or weather vane.
- When buying a rotating nozzle, do not chase cheapness. In low-cost products, an open hinge is used - an ordinary sleeve that will freeze in winter. Select a weather vane or turbo deflector with a sealed bearing.
- The H-shaped cap is useful in areas with constant strong winds. In other cases, it is better to take TsAGI.
Buy Astato deflectors as desired - the amplifier will work in any conditions. But remember: moving parts of the nozzle need to be serviced periodically.
Do it yourself
We suggest explaining the assembly technology of the cap using the example of a TsAGI nozzle. Details are cut out of galvanized steel 0.5 mm thick, fastened together with rivets or bolts with nuts. The design of the exhaust element is shown in the drawing.
For manufacturing, you need a regular bench tool:
- hammer, mallet;
- scissors for metal;
- electric drill;
- vise;
- marking devices - scriber, tape measure, pencil.
The table below shows the dimensions of the deflector parts and the final weight of the product.
Reference. The most “running” diameters of the ventilation ducts are 100 or 110 mm, when the hood is made with a plastic sewer pipe.
The assembly algorithm is as follows. According to the scans, we cut out the blanks of the umbrella, diffuser and shell with scissors, fasten them together with rivets. Cutting the shell is not difficult, the sweep of the diffuser and umbrella are shown in the drawings.
The finished deflector is mounted on the head, the lower pipe is pulled together by a clamp. You will have to make or buy an adapter for a square shaft, whose flange is attached to the end of the pipe.
Can I install on a chimney
By installing a baffle, unlucky homeowners are trying to solve the problem of lack of traction. This happens when the chimney is made incorrectly - the tip fell into the area of the wind support of the roof, was raised to a low height, or the neighbor built a tall building nearby.
The best solution with insufficient traction is to raise the chimney to the desired height. Why it is undesirable to jam various nozzles onto the head:
- It is forbidden to place umbrellas and other exhaust devices on pipes that discharge the combustion products of gas boilers. These are safety requirements.
- During combustion, stoves and solid fuel boilers emit soot deposited on the inner surfaces of chimneys and hoods. The deflector will have to be cleaned, especially when spinning.
- At the bottom of a properly constructed smoke channel, a pocket is provided for collecting condensate and excess moisture. It makes no sense to close the pipe from precipitation; it is enough to attach a nozzle protecting the sandwich insulation on the end.
The heads of the furnace ducts can be equipped with umbrellas, but the turbo deflector is definitely not needed there. The topic of mounting caps on chimneys is described in detail in a separate material.