At the immediate entrance of a shell-and-tube exchanger tube, the tube-side heat transfer coefficient is_________________?
Show answer Hide answer
Answer: A. Infinity
Explanation
Thermal boundary layer thickness is theoretically zero right at the exact entrance of a tube. Local heat transfer coefficient is inversely proportional to boundary layer thickness. As boundary layer thickness approaches zero, the local heat transfer coefficient approaches infinity. Boundary layer development dictates convective heat transfer rates inside circular conduits. Entrance regions exhibit much higher heat transfer coefficients than fully developed flow zones. Thermal and velocity boundary layers grow progressively as fluid moves down the tube length.
Short heat exchangers benefit from high average coefficients found in entrance regions. Thermal design calculations must account for entrance length effects when sizing short tubes. Precision cooling systems utilize entrance region heat transfer enhancement strategies. Zero heat transfer coefficient would imply no thermal energy is crossing the boundary wall. Equaling the average coefficient ignores the distinct profile of thermal boundary layer growth. Option D is incorrect because the entrance coefficient is theoretically infinite due to zero thickness.
- Subject
- Engineering
- Category
- Chemical Engineering
- Topic
- —
- Difficulty
- medium
Related questions
Q1. During _______________ boiling, the bulk liquid remains below saturation temperature while boiling occurs near the heated boundary?
Show answer Hide answer
Answer: B. Local
Local boiling occurs when heat is added to a boundary while bulk liquid remains subcooled. Vapor bubbles form at the superheated solid wall but collapse in the colder surrounding liquid. This specific subcooled boiling r...
Q2. In extended-surface heat exchangers, the fluid exhibiting the lower coefficient should be routed________________?
Show answer Hide answer
Answer: B. Flows outside the tubes
Fluids with lower convective coefficients offer higher thermal resistance to heat transfer. Routing such fluids outside the tubes provides larger surface area and permits extended fins. This design strategy compensates f...
Q3. During _____________ boiling, vapor bubbles generated on a submerged hot surface dissolve back into the bulk liquid?
Show answer Hide answer
Answer: B. Pool
Pool boiling takes place when a heated surface is submerged in a relatively large, quiescent liquid body. Vapor bubbles nucleate on the hot surface and rise through the pool of liquid. If the bulk liquid is subcooled bel...
Q4. Applying a vacuum inside a single-effect evaporator system causes the steam economy to ________________?
Show answer Hide answer
Answer: A. Increases
Applying a vacuum reduces the boiling point of the liquid solution inside the evaporator vessel. Lowering the boiling point increases the effective temperature difference between steam and boiling liquid. Larger temperat...
Q5. For a supercooled solution on the brink of crystallization, the solution's free energy compared to the solid phase is ________________?
Show answer Hide answer
Answer: A. More
Supercooled or supersaturated solutions possess higher Gibbs free energy than stable solid crystals. This thermodynamic excess free energy acts as the primary driving force for spontaneous crystallization. Molecules in s...
Q6. The characteristic length applied in the Biot number is defined as the ratio of the solid's _______________?
Show answer Hide answer
Answer: A. Volume to surface area
Biot number compares internal conduction resistance to external convection resistance of a body. The characteristic length in Biot number is defined as the body volume divided by its surface area. This specific ratio nor...