Role of the draft in heat transfer: Chimney effect

What is the draft? The draft is defined as the difference between absolute air/gas pressure at any point in a gas flow passage and the ambient (same elevation) atmospheric pressure. The draft is achieved by a small pressure difference which causes the flow of air or gas to take place. We will discuss three broad categories of the draft in this post {1] Natural draft [2] forced draft and [3] induced draft

Natural draft: Natural Draft is a thermal draft. It occurs when hot gases expand. Since the combustion/hot gases have a density lower than room air or outdoor air, they tend to rise. The rising is contained and increased by enclosing the gases in a tall chimney. This creates a vacuum in the chimney which sucks more hot gases. This is generally called the stack effect or chimney effect.

There are three factors that control chimney draft:

[1] Chimney height - the higher the chimney, the greater the draft. [2] The density of hot gases - the hotter the gases, the greater the draft and [3] The density of ambient air outside the chimney - the colder the outside air, the greater the draft.

Since the outside temperature and chimney gas temperature vary, the draft will not be constant. In SI unit draft is expressed as N/m2 (Newtons per square meter)

?P = g x H x [ρ ambient – ρ exit] H is the required height for a chimney to discharge hot gases when ρ ambient and ρ exit are the density of ambient air and flue gases or gases in the chimney at the exit.

Chimney effect in a tall building to save energy: How does it work?

Every building has a neutral pressure level (NPL), where the pressure difference between the building and its environment are the same. Air movement into or out of the building is reduced along this plane and increases further from it. Equipment that actively moves air in or out of the building, such as exhaust systems, will move the NPL location in the building. Knowing the NPL of a building allows designers to focus on control measures where they are most needed. Natural ventilation can save owners money, up to 30 percent in some cases.

Forced draft/Induced draft: In the forced draft, air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan. In the induced draft, the fan sucks the air,

Boiler draft: The main objects of producing draught in a boiler are:

[1] To provide an adequate supply of air for the fuel combustion, [2] To exhaust the gases of combustion from the combustion chamber, [3] To discharge these gases to the atmosphere through the chimney. For, the proper and optimized heat transfer from the flue gases to the boiler tube draft holds a relatively high amount of significance. The combustion rate of the flue gases and the amount of heat transfer to the boiler are both dependent on the movement and motion of the flue gases. A boiler equipped with a combustion chamber that has a strong current of air (draft) through the fuel bed will increase the rate of combustion (which is the efficient utilization of fuel with minimum waste of unused fuel). The stronger movement will also increase the heat transfer rate from the flue gases to the boiler (which improves efficiency and circulation). A balanced draft system combines the features of the forced draft and the induced draft. In case of a forced draft, when the furnace doors are opened high-pressure air rushes outside and even blows out the fire entirely.

Role of the draft in air-cooled heat exchanger:

 Forced Draft air-cooled heat exchangersFans are located below the tube bundles. Air is pushed across the finned tube surface. This provides better accessibility to the fan for maintenance and fan-blade adjustment. The two key advantages are [1] fan and V-belt assembly are not exposed to the hot-air stream that exits from the unit and [2] needs less structural support and capital cost is lower. Finally, these advantages give longer mechanical life. An important point to note, in theory, the primary advantage of the forced-draft unit is that less power is required but this is true only when the air temperature rise exceeds 30 deg C, because the fan is pumping denser air.

Induced draft air-cooled heat exchangers

Fans are located above the bundle. Air is pulled across the finned tube surface. Since air velocity approaching the bundle is relatively low, it provides more even distribution of air across the bundle. This is better suited for exchangers designed for a close approach of product outlet temperature to an ambient air temperature. It is less likely to recirculate the hot exhaust air since the exit air velocity is much higher than the forced-draft unit. In a service in which sudden temperature change would cause upset and loss of product, the induced-draft unit gives more protection in that only a fraction of the surface (as compared with the forced draft unit) is exposed to rainfall or snow. Will transfer more heat by natural convection with fans off because of the stack effect.

Credit: Google