ANS:D - passes through a maxima

For a shell and tube heat exchanger, the purchased cost per unit heat transfer area typically decreases with increasing heat transfer area. Explanation:

• Economy of scale: As the heat transfer area increases, the manufacturing costs per unit area decrease due to economies of scale. Manufacturers can achieve efficiencies in production, materials, and labor costs when producing larger heat exchangers.
• Reduced material costs: Larger heat exchangers require less material per unit of heat transfer area compared to smaller ones. This reduction in material usage can lead to lower costs per unit area.
• Reduced fabrication costs: Fabrication costs per unit area tend to decrease for larger heat exchangers because larger components are often easier and more efficient to manufacture.
• Volume discounts: Purchasing larger heat exchangers often qualifies for volume discounts on materials and components, further reducing the cost per unit area.

As a result, the purchased cost per unit heat transfer area typically decreases with increasing heat transfer area for shell and tube heat exchangers. However, there might be a point where the cost reduction starts to diminish due to other factors such as increased complexity, transportation costs, or design considerations, leading to a "passing through a maxima" scenario.