Proper calculation of the area of the smokestack is a fundamental step in the design of any heating system, whether it is a private house, bath or industrial boiler. Errors at this stage lead to a decrease in thrust, accumulation of condensate and, most dangerously, to the possibility of carbon monoxide poisoning. Many property owners underestimate the importance of accurate calculations, relying on approximate values or advice from acquaintances, which often leads to the need for expensive rework of the entire smoke removal system.
All calculations are based on the physics of the combustion process and the aerodynamics of gas flows. Area of section It directly affects the rate of release of combustion products and the ability of the system to create natural thrust. If the channel is too narrow, the smoke will not have time to remove, creating excessive pressure and smoke. If the cross section is excessively large, the flow rate will fall, which will lead to cooling of gases, the formation of condensation and destruction of the inner surface of the channel.
To obtain accurate data, it is necessary to take into account the type of fuel used, boiler power, pipe height and temperature of the outgoing gases. Modern building codes and regulations (SNIP) regulate minimum safety requirements, ignoring which can cause a fire. In this article, we will discuss the step-by-step algorithm of calculations, the necessary formulas and coefficients that will allow you to design an effective system.
Influence of section on traction efficiency
The efficiency of the entire heating system depends on how correctly selected the internal section of the chimney. Plunge It is the driving force that ensures the flow of fresh air into the furnace and the removal of combustion products outwards. It is created due to the difference in temperature and density of gases inside and outside the pipe. Insufficient area of the channel creates resistance to the flow, which causes the boiler to βsuffocateβ, reducing its efficiency and increasing fuel consumption.
On the other hand, an overly large pipe diameter is also harmful. In a wide channel, gases quickly lose heat by contacting cold walls. This leads to a sharp drop in flow temperature, which in turn reduces traction. There is a reverse thrust effect or its complete absence, especially in windy weather. Bricks. and metallic pipes behave differently due to different thermal conductivity of materials, which should also be taken into account when calculating.
Particular attention should be paid to the form of the section. The round shape is considered ideal from the point of view of aerodynamics, since it creates minimal resistance to the swirls of air flows. Corners in square or rectangular channels (such as brickwork) create turbulence that inhibits gases and promotes faster deposition. soot.
β οΈ Note: The use of pipes with a diameter smaller than the boiler output pipe is strictly prohibited and may lead to an emergency stop of equipment or an explosion of the furnace chamber.
Calculation of the minimum area of the pipe cross-section
To determine the required channel size, a formula is used that connects the volume of passing gases and the speed of their movement. The main parameter that you need to start from is the power of your heat generator. There is a simplified method of selection, but for accuracy it is better to use the calculation data. Area of section (F) is calculated by the formula: F = (Q Γ r) / (v Γ 3600), where Q is the boiler performance, r is the gas expansion coefficient, v is the flow rate.
The speed of the gas flow is a critical parameter. For solid fuel boilers, the speed from 2 to 4 m / s is considered optimal, for gas boilers - from 6 to 10 m / s. If the speed is below the minimum, the thrust will be weak, if higher - the heat will fly into the pipe, not having time to give energy to the coolant.
When choosing ready-made pipes (sandwich panels, ceramics) always look at the diameterNot the outside. The thickness of the insulation can significantly reduce usable space. For brick chimneys, the calculation is carried out according to the internal dimensions of the channel, cleaned of plaster and solution protrusions.
- π Measure the diameter of the boiler output pipe - it can not be less than the diameter of the pipe.
- π₯ Determine the type of fuel, as the volume of gases emitted from coal and gas varies significantly.
- π‘οΈ Take into account the temperature of flue gases at the exit of the boiler (indicated in the passport).
- ποΈ Select the pipe material to adjust the heat transfer coefficient.
If you plan to connect several devices to one chimney (for example, a boiler and a fireplace), the cross-sectional area should be summed up, but taking into account the simultaneity factor of work. Usually add 10-15% to the estimated value for the stock.
Formulas and coefficients for calculations
For a professional approach, it is necessary to operate with specific numerical values. The basic equation for calculating the area looks like this: F = V / (W Γ 3600). Here, V is the volume of flue gases per hour, and W is the flow rate. However, the volume of gases depends on the fuel burned. For solid fuels, a coefficient of 0.03 is used, for gas - 0.02. These figures are obtained experimentally and take into account the chemical composition of combustion products.
There is also a formula for calculating the diameter of a round pipe based on the area: D = β(4 Γ F / Ο). The resulting value should be rounded up to the nearest standard size. Standard pipe diameters are usually multiples of 50 mm (100, 150, 200 mm, etc.). Using non-standard sizes can create problems when docking elements and buying headers.
An important factor is the roughness of the walls. For smooth stainless steel, it is minimal, which allows the use of pipes of a slightly smaller diameter. For brickwork The resistance coefficient is higher, so the channel area should be increased by 10-15% compared to smooth metal. This compensates for the loss of speed on friction against uneven walls.
What is the excess air ratio?
The excess air ratio shows how much excess oxygen is entering the furnace. For solid fuels, it is higher (1.5-1.7), as more air is needed to burn, for gas - lower (1.1-1.2). This affects the volume of flue gases: the more air, the greater the volume that needs to be removed through the pipe.
When calculating, do not forget about the temperature regime. Hot gases have a lower density and a higher speed. If the pipe passes through an unheated attic or outside the building, the gases cool down, their volume decreases, and the thrust falls. Therefore, external pipes often require an increase in cross-sectional area or high-quality thermal insulation.
Channel geometry: circle, square or rectangle
The choice of the geometric shape of the channel affects not only the aesthetics, but also the technical characteristics of the chimney. As mentioned earlier, round-section It's the most effective. In a round pipe, flue gases move in a spiral, which creates additional swirl in the center of the flow, improving traction. In addition, in a round pipe, condensate flows evenly along the walls, without accumulating in the corners.
Square and rectangular channels are most commonly found in brick chimneys. Their main disadvantage is the presence of angles where stagnation zones are formed. In these areas, soot settles faster, narrowing the usable space. To maintain traction, the area of the rectangular channel should be about 10% larger than that of a circular similar diameter. This rule also applies to metal rectangular air ducts.
If you are rebuilding an old brick chimney under a modern boiler, a shell case is often required. In this case, a round stainless steel pipe is inserted into the rectangular channel. The area of the round sleeve should be at least 80-85% of the area of the original brick channel.To keep the passport thrust of the boiler. Too large a gap between the sleeve and brick requires filling with a non-combustible insulation.
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