Case Study and Calculations of Heat Transfer Applications

Part-1 (40%)

Requirement: Write a report to answer all questions:

Problem-1 (12%)

Justify what heat transfer mode should be used for designing a domestic hot water boiler capable of producing hot water of 90 -95°C. Describe the heat transfer mode with aid of the pool boiling curve.

Problem-2 (12%)

What is heat pipe?

Why is it referred as a super thermo-conductor?

Consider a copper/water heat pipe. The evaporator and condenser heat flux is 4.9 W/cm²Â and the axial heat flux of the vapour space is 124 W/cm². What is the total temperature drop?

Problem 3 (16%)

A condenser contains Four tube passes with tubes 3 m long and 25 mm internal diameter, each pass containing 100 tubes. Cooling water enters the tubes at 20°C at the rate of 80 kg/s when the shell side vapour is at 50°C. Before cleaning, the fouling factor on the water side is 0.0005 m²Â K/W; the outside of the tubes may be taken to be clean. Neglecting the thermal resistance of the fluid film on the outside of the tubes and the thermal resistance of the tube wall, calculate, using the data below:

• the effectiveness of the heat exchanger;
• the condensation rate;

• the fouling factor required on the water side if the effectiveness is to be increased to 0.7 for the same mass flow rate of water.

For heat transfer in the tube side: Nu=0.023Re^0.8Pr^1/3 can be assumed.

Data: Specific enthalpy of the vaporisation for shell side, 300 kJ/kg; mean properties of water for the temperature range considered: density, 1000 kg/m³; specific heat capacity, 4.19 kJ/kg K; thermal conductivity, 0.6 W/m K; dynamic viscosity, 0.9x10^-3 kg/m s.

A food factory uses hot water to wash and clean foods. The drained water, which has a mass flow rate of 12 kg/min, still have a temperature of 45°C.