A brass tube of inner and outer diameters is 25 mm and 28 mm, respectively, is used in a crossflow heat exchanger. The i
Posted: Sat May 21, 2022 12:10 pm
A brass tube of inner and outer diameters is 25 mm and 28 mm,
respectively, is used in a crossflow heat exchanger. The inner
fluid is water with inlet temperature of 70 oC and velocity of 0.5
m/s. The crossflow fluid is air at 20°C with a velocity of 15 m/s.
The fouling factors, R′′f, for the inner and outer surfaces are
0.0004 and 0.0002 m2 ·K/W, respectively. (a) Make a simple sketch
to demonstrate the problem. Determine the overall heat transfer
coefficient based on the outside area of the tube (Uo) and the
water outlet temperature. Compare the thermal resistances due to
convection, tube wall conduction, and fouling. Assume constant
thermal conductivity of the solid material. Validate the referenced
temperature used for the thermal-physical properties of the fluids.
(30 marks) (b) The air velocity is varied between 5 and 25 m/s:
calculate and plot the corresponding heat transfer between the two
fluids. (20 marks) (c) Assume the above fouling factors increase
40% every year. Air velocity is fixed at 15 m/s: calculate and plot
the overall heat transfer coefficient which reduces over 5 years.
If the equipment is to be serviced when the overall heat transfer
coefficient is reduced by 25% from its initial value, how long
after installation should the first cleaning be scheduled for
continuous operation? (20 marks)
respectively, is used in a crossflow heat exchanger. The inner
fluid is water with inlet temperature of 70 oC and velocity of 0.5
m/s. The crossflow fluid is air at 20°C with a velocity of 15 m/s.
The fouling factors, R′′f, for the inner and outer surfaces are
0.0004 and 0.0002 m2 ·K/W, respectively. (a) Make a simple sketch
to demonstrate the problem. Determine the overall heat transfer
coefficient based on the outside area of the tube (Uo) and the
water outlet temperature. Compare the thermal resistances due to
convection, tube wall conduction, and fouling. Assume constant
thermal conductivity of the solid material. Validate the referenced
temperature used for the thermal-physical properties of the fluids.
(30 marks) (b) The air velocity is varied between 5 and 25 m/s:
calculate and plot the corresponding heat transfer between the two
fluids. (20 marks) (c) Assume the above fouling factors increase
40% every year. Air velocity is fixed at 15 m/s: calculate and plot
the overall heat transfer coefficient which reduces over 5 years.
If the equipment is to be serviced when the overall heat transfer
coefficient is reduced by 25% from its initial value, how long
after installation should the first cleaning be scheduled for
continuous operation? (20 marks)