Cost scaling in shell-and-tube exchangers – Purchased cost per unit area For a shell-and-tube heat exchanger, how does the purchased cost per unit heat-transfer area typically vary as the total area is increased (all else equal)?

Introduction / Context:
Capital cost estimation often uses scaling laws. In heat exchanger design, understanding how the purchased cost per square meter changes with size influences selection between one large unit versus multiple smaller units and helps optimize total installed cost.

Given Data / Assumptions:

• Similar design and materials as size scales up.
• No exotic constraints (e.g., pressure extremes) that would change scaling.
• Market conditions aside, typical vendor pricing behavior applies.

Concept / Approach:
Equipment exhibits economies of scale: total purchased cost increases sublinearly with area. Therefore, the purchased cost per unit area generally decreases as area increases. This reflects fixed costs (heads, channels) being spread over more tubes and larger shells, reducing cost per square meter of heat transfer surface.

Step-by-Step Solution:

Verification / Alternative check:
Vendor quotes and cost indices show diminishing $/m^2 as exchanger surface area increases within a product family.

Why Other Options Are Wrong:

• Increases/constant: Contradict typical economies of scale for standard designs.
• Passes through a maxima: Not characteristic of purchased cost per area; unusual.

Common Pitfalls:
Confusing purchased cost with installed or lifecycle costs; piping, plot space, and maintenance can alter the economic optimum.

Final Answer:
decreases