1 What are the heat transfer mechanisms involved during heat transfer from the hot fluid to the cold fluid?
1.Find the Overall heat transfer coefficient for a shell and tube counter flow heat exchanger where the heat exchanged is 11.27 GJ/hour with the heat transfer area of 96 m2. Assume the LMTD as 76 oC.
2.. In a double pipe heat exchanger hot fluid is entering at 220°C and leaving at 115°C. Cold fluid enters at 10°C and leaves at 75°C. Mass flow rate of hot fluid 100 kg/hr, Cp of hot fluid 1.1 kcal/kg°C. Cp of cold fluid 0.95 kcal/kg°C. Calculate LMTD?
- If the flow is parallel
- If the flow is counter current
- Find the mass flow rate of cold fluid if the heat loss during the exchange is 5%.
3. When a heat exchanger was designed its overall heat transfer co-efficient was 600 kcal/hr mt²°C. The heat transfer area provided = 10mt². Over a period of time, its overall heat transfer co-efficient has fallen to 450 kcal/hr mt²°C due to fouling. Assume Counter flow.Data:
- Specific heat of hot fluid = 1 kcal/kg °C
- Hot fluid entering temperature = 80°C
- Hot fluid leaving temperature = 60°C
- Cold fluid entering temperature = 25 °C
- Cold fluid leaving temperature = 40°C
Calculate: How much additional area is to be added to maintain the same rate of heat transfer?
4. In a parallel flow double-pipe heat exchanger water flows through the inner pipe and is heated from 20°C to 70°C. Oil flowing through the annulus is cooled from 200°C to 100°C. It is desired to cool the oil to a lower exit temperature by increasing the length of the heat exchanger. Determine the minimum temperature to which the oil may be cooled?
5. The flow rates of hot and cold water streams running through a parallel flow heat exchanger are 0.2 kg/. and 0.5 kg/s respectively. The inlet temperatures on the hot and cold sides are 75°C and 20°C respectively. The exit temperature of hot water is 45°C. If the individual heat transfer coefficients on both sides are 650 w/m2°C, calculate the area of the heat exchanger.
6. In a counter flow double pipe heat exchanger, water is heated from 25°C to 65°C by an oil with a specific heat of 1.45 kJ/kg K and mass flow rate of 0.9 kg/s. The oil is cooled from 230°C to 160°C. If the overall heat transfer coefficient is 420 W/m2°C, calculate the following:
- The rate of heat transfer
- The mass flow rate of water, and
- The surface area of the heat exchanger
7. An oil cooler for a lubrication system has to cool 1000 kg/h of oil (cp = 2.09kJ/kg°C) from 8°C to 4O°C by using a cooling water flow of 1000 kg/h at 30°C. Give your choice for a parallel flow or counter-flow heat exchanger, with reasons. Calculate the surface area of the heat exchanger, if the overall heat transfer coefficient is 24 W/m2°C. Take Cp of water = 4.18 kJ/kg°C.
8. A counter-flow double pipe heat exchanger using superheated steam is used to heat water at the rate of 10500 kg/h. The steam enters the heat exchanger at 180°C and leaves at 13O°C. The inlet and exit temperatures of water are 30°C and 80°C respectively. I f overall heat transfer coefficient from steam to water is 814 W/m2°C, calculate the heat transfer area. What would be the increase in area if the fluid flows were parallel?
9. The amount of F¬12 used in compression refrigeration system is 4 tonnes/hour. The brine flowing at 850 kg/min, with inlet temperature of 12°C is cooled in the evaporator. Assuming F¬12entering and leaving the evaporator as saturated liquid and saturated vapour respectively, determine the area of evaporator required. Take the following properties: For F¬12 : Saturation temperature : - 23°C; cp = 1.1 7 kJ/kg°C; hfg = 167.4 kJ/kg Cp(brine) = 6.3 kJ/kg°C; U = 8368 kJ/m2h°C.
10. An after cooler of shell and tube type with single pass is used for cooling compressed air from 85 oC to 35 oC. The compressed air generated is 1350 m3/h at mean air temperature. Calculate:
- The amount of cooling water to be circulated at a temperature of 30 oC. Assume the cooling water outlet temperature as 35 oC.
- The LMTD and no of tubes if the dia of the tube is 25.4 mm and 2500 mm length. Assume overall heat transfer coefficient for air to water is 150 W/m2.K.
- The hp of the pump required if the pressure required is 3.5 kg/cm2.g.