Question 21. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference for parallel flow is

Question 22. Pick the odd one out.

Question 23. A cross flow type air heater has an area of 50 cm². The overall heat transfer coefficient is 100 W/m² K and heat capacity of both hot and cold stream is 1000 W/m K. The value of NTU is

Question 24. Water (specific heat = 4 k J/kg K) enters a cross flow exchanger (both fluids unmixed) at 15 degree Celsius and flows at the rate of 7.5 kg/s. It cools air (C P = 1 k J/kg K) flowing at the rate of 10 kg/s from an inlet temperature of 120 degree Celsius. For an overall heat transfer coefficient of 780 k J/m² hr degree and the surface area is 240 m², determine the NTU

Question 25. After expansion from a gas turbine, the hot exhaust gases are used to heat the compressed air from a compressor with the help of a cross flow heat exchanger of 0.8 effectiveness. What is the number of transfer units of the heat exchanger?

Question 26. E – NTU method is particularly useful in thermal design of heat exchangers when

Question 27. The engine oil at 150 degree Celsius is cooled to 80 degree Celsius in a parallel flow heat exchanger by water entering at 25 degree Celsius and leaving at 60 degree Celsius. Estimate the number of transfer units

Question 28. In a surface condenser, the water flowing through a series of tubes at the rate of 200 kg/hr is heated from 15 degree Celsius to 75 degree Celsius. The steam condenses on the outside surface of tubes at atmospheric pressure and the overall heat transfer coefficient is estimated at 860 k J/m² hr degree. Find the number of transfer units of the heat exchanger. At the condensing pressure, stream has saturation temperature 0f 100 degree Celsius and the latent heat of vaporization is 2160 k J/kg. Further, the steam is initially just saturated and the condensate leaves the exchanger without sub-cooling i.e. only latent heat of condensing steam is transferred to water. Take specific heat of water as 4 k J/kg K

Question 29. When two fluids of the heat exchanger are separated by a plane wall, the thermal resistance comprises
(i) Convection resistance due to the fluid film at the inside surface
(ii) Conduction resistance
(iii) Convection resistance due to the fluid film at the outside surface
Identify the correct option

Question 30. NTU is a number of transfer units, dimensionless parameter defined as