In a shell-and-tube heat exchanger, baffles are placed inside the shell to shape the flow and support the tubes. Their main job is to hold the tubes in place and direct the shell-side fluid to pass across the tube bundle rather than moving straight along the shell. This cross-flow path increases the contact between the fluid and the tube surfaces, which boosts heat transfer and helps distribute temperature more evenly. The baffles also provide structural support, helping prevent tube sagging and vibration. Because guiding the flow across the tubes adds friction and creates a longer flow path, baffles typically increase, not decrease, the pressure drop. They do not measure temperature—that’s done by sensors—nor are they intended to “roughen” the flow; they create turbulence to enhance heat transfer, not to roughen the surface.

Roof drains are provided to remove rainwater and condensate from the roof so water does not pond there, adding weight, promoting corrosion, or creating unsafe conditions. The best practice is to keep the drain path unobstructed so water can flow out freely. If a drain were sealed shut or blocked, water would back up on the roof, increasing the risk of structural damage and other hazards. By leaving the drain effectively open (not blocking the opening with a cap or plug), drainage occurs as intended and pooling is prevented, which is why this approach is the correct management for roof drains.

Percent concentration is parts per hundred, while ppm is parts per million. That means 1% equals 10,000 ppm, because 0.01 × 1,000,000 equals 10,000. So you multiply the percent value by 10,000 to get ppm. For example, 0.75% becomes 7,500 ppm. Dividing by 10,000 would give a much smaller number, multiplying by 1,000 would yield a result far too small, and subtracting from 100,000 isn’t a valid conversion.

Polymers are large molecules built from many repeating units called monomers linked into long chains or networks. This creates a macromolecular structure with high molecular weight and unique properties like strength and flexibility, which is why polymers are used to make plastics, fibers, and rubbers. The other options describe states or sizes that don’t capture what a polymer is—the description that fits is a large molecule.