ANS:B - 25

In the context of fuel oil combustion in conventional burners, the viscosity of the fuel plays a crucial role in its atomisation process. Atomisation refers to the breaking up of liquid fuel into fine droplets, which is essential for efficient combustion.

Explanation:

• Viscosity: It is a measure of a fluid's resistance to flow. Higher viscosity means the fluid is thicker and flows less easily, while lower viscosity means it is thinner and flows more easily.
• Centistokes (cSt): This is a unit of kinematic viscosity. It describes how a fluid flows under gravity. One centistoke is equivalent to 1 mm²/s.
• Redwood I seconds: This is another unit of measuring kinematic viscosity, used particularly in the oil industry. The conversion between centistokes and Redwood I seconds is approximately direct for many fuel oils.

Atomisation in Burners:

For efficient atomisation in conventional burners:

• The fuel must be atomized into fine droplets to ensure complete and efficient combustion.
• If the viscosity is too high, the fuel won't atomise properly. It will result in larger droplets which burn less efficiently, leading to incomplete combustion, higher emissions, and potential damage to the burner.

Maximum Viscosity:

• 100 centistokes (or 100 Redwood I seconds) is identified as the maximum viscosity for easy and efficient atomisation.
• At this viscosity, the fuel remains fluid enough to be easily pumped and atomised into fine droplets by the burner.
• Fuels with viscosities higher than 100 centistokes may require pre-heating to reduce their viscosity to a suitable level for efficient atomisation.

Thus, maintaining the viscosity of tar/PCM/fuel oil at or below 100 centistokes ensures that the fuel can be effectively atomised, promoting better combustion and efficiency in conventional burners.