Govt. Exams
Entrance Exams
Condensation is phase change from vapor to liquid, releasing latent heat. This occurs when steam condenses on cooler tube surfaces. Pool boiling and convective boiling involve liquid-to-vapor phase change.
Q = hA·ΔT = 50 × 2 × (100-25) = 50 × 2 × 75 = 7,500 W = 7.5 kW
For fully developed laminar flow in a circular tube with constant wall temperature (thermal entry), Nu ≈ 4.36. For constant heat flux, Nu ≈ 3.66. The question refers to the thermally developed region.
Biot number classifies transient conduction regimes: Bi << 1 indicates uniform internal temperature (lumped model valid), while Bi >> 1 indicates significant spatial temperature gradients requiring solution of differential equations.
In parallel flow, both fluids flow in the same direction. The smallest temperature difference (approach) occurs at the outlet end where the hot fluid has cooled and cold fluid has heated.
Thermal conductivity is expressed as W/(m·K) in SI units, which is equivalent to J/(s·m·K) since 1 W = 1 J/s.
Q_loss = U × A × ΔT = 8 × 4 × (80-20) = 8 × 4 × 60 = 1920 W. The overall U value already accounts for both convective and radiative losses.
According to Stefan-Boltzmann law, Q = σAT⁴. When T increases from 500 K to 1000 K, the factor becomes (1000/500)⁴ = 2⁴ = 16 times. Radiation is highly temperature-dependent due to the fourth-power relationship.
The value of 3.66 is the fully developed Nusselt number for laminar flow in circular pipes under constant wall temperature (T_s constant) boundary condition, representing the thermal development region.
Baffles in shell-and-tube heat exchangers create turbulence by forcing the fluid to flow in a cross-flow pattern, significantly increasing the convective heat transfer coefficient on the shell side, which enhances overall heat transfer effectiveness.
