PART A
What is burnout point in boiling heat transfer? Why is it called so?
What is meant by sub-cooled and saturated boiling?
Differentiate between pool and flow boiling.
Name the types of condensation and give one example for each type.
What are the assumptions made in Nusselt theory of condensation?
How does boiling differs from evaporation?
Define effectiveness and NTU of a heat exchanger.
How heat exchangers are classified?
What are the limitations of LMTD method? How is ε-NTU method superior to LMTD
method?
What are the factors on which overall heat transfer coefficient depends?
What is fouling and how does it affect the rate of heat transfer?
Why counter current heat exchanger is more effective than co current heat exchanger.
What is meant by dropwise condensation?
Consider film condensation on a vertical plate. Will the heat flux be higher at the top or at the
bottom of the plate? Why?
Distinguish between drop wise and film-wise condensation.
PART B
Explain the different regimes involved in pool boiling?
Water is to be boiled at atmospheric pressure in a polished copper pan by means of an electric
heater. The diameter of the pan is 0.38 m and is kept at 115 oC. calculate the following: (i)
Surface heat flux, (ii) Power required to boil the water, (iii) Rate of evaporation, (iv) Critical
heat flux.
The bottom of a copper pan, 0.3 m in diameter, is maintained at 118 oC by an electric heater.
Estimate the power required to boil water in this pan. What is the evaporation rate? Estimate
the critical heat flux.
A Teflon coated stainless steel surface maintained at a uniform temperature of 106 oC is used to
boil water at atmospheric pressure. Determine the heat flux and critical heat flux for nucleate
boiling. Also find the heat flux for a water-brass system.
Saturated steam at atmospheric pressure condenses on a 2 m high 3 m wide vertical plate that is
maintained at 80 oC by circulating cooling water through the other side, Determine the rate of
heat transfer by condensation to the plate
A vertical plate of 3.2 m high maintained at 54 oC is exposed to saturated steam at atmospheric
pressure. Calculate the heat transfer rate per unit width.
In a parallel flow heat exchanger, both fluids unmixed, hot fluid with specific heat of 2300
kJ/kg K enters at 380 oC and leaves at 300 oC. Cold fluid enters at 25oC and leaves at 210 oC.
Calculate the required surface area of heat exchanger. Take overall heat transfer co-efficient as
750 W/m2K. Mass flow rate of hot fluid is 1 kg/s.
A cross flow heat exchanger with both fluids unmixed is used to heat water flowing at a rate of
20 kg/s from 25 °C to 75 °C using gases available at 300 °C to be cooled to 180 °C. The overall
heat transfer coefficient has a value of 95 W/(m2 K). Determine the area. Also find the gas flow
rate. Assume for gas, c = 1005 J/kg-K).
Hot chemical products (Cph = 2.5 kJ/kg K) at 600 oC and at a flow rate of 30 kg/s are used to
heat cold chemical products (Cp = 4.2 kJ/kg K) at 200 oC and at a flow rate 20 kg/s in a parallel
flow heat exchanger. The total heat transfer area is 50 m2 and the overall heat transfer
coefficient may be taken as 1500 W/m2 K. calculate the outlet temperatures of the hot and cold
chemical products.
A counter flow double pipe heat exchanger is to heat water from 20 0C to 800C at a rate of 1.2
kg/s. The heating is accomplished by geothermal water at 160 oC at a mass flow rate of 2 kg/s.
The inner tube is thin walled and has a diameter of 1.5 cm. The U = 640 W/m2K. Using
effectiveness NTU method. Determine the length of heat exchanger required to achieve the
desired heating.
