King Saud University
College of Engineering
Chemical Engineering Dept.
ChE 313
Tutorial # 7
23/2/1426
Heat Transfer Operations
Dr. M Al Ahmed, Eng. M Gaily
Air at a temperature of 290 K is being heated in a heat exchanger and flowing in a tube at
a mass velocity of 24 kg/m2s. Waste gases flowing outside the tube enters at a rate of 2.7
gm/s and a temperature of 480 K is used to heat air. The average bulk temperature of the
air is considered to be 320 K and its mass flow rate and physical properties are the same
as the waste gases flowing outside the tube. The average inside wall temperature is given
by:
Tw = Tai + 0.8 (Tbw –Tba)
Where:
Tai inlet air temperature (K)
Tbw average bulk temperature of waste gases (K)
Tba average bulk temperature of air (K).
Calculate:
(1) The outlet temperature of air and waste gases.
(2) The average wall temperature (T).
(3) The inside heat transfer coefficient.
(4) The length of tube of the heat exchanger.
(5) The log mean temperature difference and comment on that value.
(6) The outside heat transfer coefficient, assuming that the wall of the metal tube is
very thin.
Heat transfer data:
For NRe < 2100, NNu =1.86 ( NRe NPr D/L)1/3 (μb/μw)0.14
For NRe > 10000, NNu = 0.023 NRe0.8 NPr1/3 (μb/μw)0.14
1. Water entering at 10 ºC is to be heated to 40 ºC in a tube of 0.02 m ID at a
mass flow rate of 0.01 kg/s. The outside of the tube is wrapped with an
insulated electric heating element that produces a uniform flux of 15,000 W/m2
over the surface (NNu = 4.36). Neglecting any entrance effects, determine:
a. The Reynolds number.
b. The heat transfer coefficient.
c. The length of the pipe needed for a 30 ºC increase in average
temperature.
2. Air at a pressure of 6 kN/m2 and a temperature of 300 ºC flows with a velocity
of 10 m/s over a flat plate 0.5 m long. Estimate the cooling rate per unit width
of the plate needed to maintain it at a surface temperature of 27 ºC. The
physical properties of air at operating conditions are: k = 0.0364 W/m.K, Pr =
0.687, μ/ρ = 5.21 x 10 –4.
3.
Air at 2 bar and 200 ºC is heated as it flows through a tube with a diameter of 25
mm at a velocity of 10 m/s. Calculate the heat transfer per unit length of tube if a
constant heat flux condition is maintained at the wall and the wall temperature is 20 ºC
above the air temperature, all along the length of the tube. How much would the bulk
temperature increase over a 3 m length of the tube?
4. Water at 60 ºC enters a tube of 25 mm diameter at a mean velocity of 20 mm/s.
Calculate the exit water temperature if the tube is 3 m long and the wall temperature
is constant at 80 ºC.