Comparing furnace draught systems: in a furnace employing a forced-draught fan (as opposed to induced draught), which statement correctly describes the in-furnace pressure condition and related implications?

Introduction / Context:
Draught arrangements determine how combustion air and flue gases move through a furnace. Forced draught (FD) pushes air into the furnace, while induced draught (ID) pulls flue gas out. The pressure regime affects leakage, fan duty, and hardware exposure.

Given Data / Assumptions:

• Forced-draught fan located on the air side.
• Induced draught fan (when used) is on the flue-gas side.
• Furnace and ductwork designed accordingly.

Concept / Approach:
FD systems create a slightly positive pressure inside the furnace because air is being pushed in; this reduces cold air in-leakage from outside but can increase hot gas leakage outward if casing is not tight. ID systems run the furnace negative, pulling in tramp air but keeping hot gases contained.

Step-by-Step Solution:
1) Define pressure sign: FD → positive; ID → negative.2) Identify which fan sees hot gases: ID fans see hotter, potentially corrosive gas; FD fans handle cooler air.3) Therefore, the correct description for FD is “positive pressure exists in the furnace.”

Verification / Alternative check:
Plant P&IDs and operating guidelines consistently state slight positive furnace pressure under pure FD systems.

Why Other Options Are Wrong:
air is sucked in …: describes ID, not FD.the fan operates hot …: applies mainly to ID fans on hot flue gas.none of these: incorrect because option (c) is correct.

Common Pitfalls:
Confusing FD and ID roles, leading to wrong leakage expectations.

Final Answer:
positive pressure exists in the furnace.