The causes of chimney draft

The same principle applies to cork floating underwater – it’s due to the difference in density

At high temperatures, the density is low; thus, a suction force is generated during subsequent cycles

I mean, the air is outside the chimney – how can it affect what’s inside the chimney?

I mean, the air is outside the chimney – how can it affect what’s inside the chimney?

I mean, the air is outside the chimney – how can it affect what’s inside the chimney?

Differences in gas temperature inside and outside the chimney cause variations in gas density; these density differences generate a pressure difference, namely the chimney draft, which overcomes resistance to drive the flow of flue gases. The negative pressure at the bottom of the chimney is what causes a suction force there. According to the draft force formula h_draft = H(γ_air – γ_gas), it can be seen that there are mainly three factors that affect the draft force of a chimney, namely H, γ_air, and γ_gas. (1) Effect of height H: As can be seen from the formula, the greater H is, that is, the taller the chimney, the greater the suction force; the smaller H is, that is, the lower the chimney, the smaller the suction force. (2) Effect of air temperature: As can be seen from the formula, when H and γ are constant, the greater γ_air is, that is, the lower the external air temperature, the greater the suction force. For the same chimney, with the same opening degree of the gate, the suction force is greater in winter than in summer, and it is greater at night than during the day. This is because the temperature of the outside air is lower in winter and at night compared to summer and during the day, resulting in a larger value for γ_air. (3) Effect of flue gas temperature: As can be seen from the formula, with H and γ_air remaining constant, the greater γ_gas is, that is, the lower the flue gas temperature, the smaller the suction force; the smaller γ_gas is, that is, the higher the flue gas temperature, the greater the suction force. When a new kiln is put into operation, the suction force in the chimney is very low; therefore, workers often light a fire at the bottom of the chimney to raise the temperature of the gases inside it, thereby increasing the suction force – that’s how it works. When designing chimneys, it is necessary to take into full account the impact of the aforementioned factors on the draft force; one cannot focus only on one aspect while ignoring the others. For example, the higher the chimney, the greater the suction force, but it cannot be too high. Because the taller the chimney, the more robust the foundation is required to be, and the quality of masonry must also improve, which in turn increases the cost. Furthermore, the higher the flue gas temperature, the greater the suction force, but at the same time more heat is carried away by the flue gas, increasing heat loss and reducing the thermal efficiency of the furnace. The temperature of the surrounding air is independent of human will, but when designing chimneys, the climate of the area should be taken into account, and the chimney size should be determined based on the highest summer temperatures in that region. Therefore, when designing chimneys, it is necessary to take all relevant factors into consideration, weigh the pros and cons, and carry out a rational design. When determining the chimney draft, to ensure that the minimum draft meets the requirements, calculations should be carried out using the highest summer temperatures and the local maximum air humidity.

I mean, the air is outside the chimney – how can it affect what’s inside the chimney? Is there a concrete and steel inner cylinder in between?

Well, fill a cylindrical object with some gasoline and put it in water to see if the oil will float to the surface. :)

I poured half a bottle of gasoline into the water, but nothing happened with the gasoline?