In furnace and chimney calculations, draught provides the driving head that moves air and flue gases through the grate, furnace, and passes. Draught is defined as the __________ of pressures above and below the fire grate.

Introduction / Context:
Draught (draft) is the small pressure difference that causes flow of air into the furnace and movement of flue gases through the boiler and up the chimney. Understanding the definition is essential before applying chimney or fan draught equations.

Given Data / Assumptions:

• Pressures considered: just below and just above the grate/furnace region.
• Steady operation; small pressure differences relative to atmospheric.
• Either natural draught (chimney) or mechanical draught (fans) may be present.

Concept / Approach:
Draught is defined as the difference between the static pressure of gases below the grate (or inside the furnace path) and the surrounding/adjacent reference pressure (often atmospheric or upstream/downstream locations). This difference, expressed in units like mm of water column or Pascals, provides the net driving force for flow through the system resistance.

Step-by-Step Solution:
Identify relevant points: below the grate and above (or at chimney base).Compute pressure difference: Δp = p_below − p_above (sign per convention).Use Δp to size air openings, grates, and chimney/fans by matching system resistance curves.Conclude: definition clearly uses “difference,” not sum/product/ratio.

Verification / Alternative check:
Chimney equations show Δp ∝ (ρ_air − ρ_gas) * g * H, explicitly a pressure difference.

Why Other Options Are Wrong:
Sum/product/ratio/vector addition: none matches the formal definition or units used in draught calculations.

Common Pitfalls:
Confusing velocity head with pressure difference; misreading gauge versus absolute pressures.

Final Answer:
difference