CHE425: Problem set #1
1) Run the program Microplant as an Expert Troubleshooter and turn in the last display of
the program.
You need to copy the Microplant folder from the CHE425 distribution folder into your H:
drive or your flash drive. Open the Microplant folder and double click on Microplant.exe to
run the program. Turn in the last page of the program with performance number.
2) 1 Ethylene glycol and water, at 60 and 10°C, respectively, enter a shell-and-tube heat
exchanger for which the total heat transfer area is 15 m2. With ethylene glycol and water
flow rates of 2 and 5 kg/s, respectively, the overall heat transfer coefficient is 800 W/m2K.
Ethylene glycol: cp = 2474 J/kgK, water: cp = 4186 J/kgK.
(a) Determine the rate of heat transfer and the fluid outlet temperatures.
(b) Assuming all other conditions to remain the same, plot the effectiveness and fluid outlet
temperatures as a function of the flow rate of ethylene glycol for 0.5  mh  5 kg/s. Use
Matlab and put your name on the graph using the Title command.
Ans.
q = 2.0213×105 W, Tc,o = 19.66oC, Th,o = 19.15oC
1
Incropera and DeWitt, Fundamentals of Heat and Mass Transfer, Wiley, 6th ed., 2007
3) The compartment heater of an automobile exchanges heat between warm radiator fluid
and cooler outside air. The flow rate of water is large compared to the air, and the
effectiveness,, of the heater is known to depend on the flow rate of air according to the
0.2
relation,   mair
.
a) If the fan is switch to high and mair is doubled, determine the percentage increase in the
heat added to the car, if the fluid inlet temperatures remain the same.
b) For the low-speed fan conditions, the heater warms outdoor air from 0 to 30oC. When the
fan is turned to medium, the air flow rate increases 50% and the heat transfer increases 20%.
Find the new outlet temperature.
Ans.a) Heat rate increased by 74%, b) Tco = 24oC
4. Estimate the temperature of saturated steam that is required in the shell of a new (i.e. no
scale deposit) single-pass shell-and-tube heat exchanger when water on the tube side, for
either the new design or the original design that included scale formation, is heated from 21
to 65.5oC at a flow rate of 15.75 kg/s. The exchanger contains 60 steel tubes, each with an
inside diameter of 0.0186 m and an outside diameter of 0.0254 m. The unit is designed with
sufficient tube area to permit the heating of the water as specified. In the original design, an
hd of 8250 W/m2∙K was used to allow for scaling on the water side of the tube. The film
coefficient for the steam was taken as 11,400 W/m2∙K, and the temperature of the saturated
steam required to account for the heating of the water was found to be 153.3oC. No scaling
factor for the shell side was used in the original design, and it can be neglected in the new
design.
Data: cp,w = 4179 J/kgK, w = 0.00063 Pas, w = 991 kg/m3, kw = 0.641 W/mK, ksteel = 45
W/mK. The inside heat transfer coefficient can be estimated from
kw
(0.023)Re0.8Pr1/3
Di
o
Ans. 125.5 C
hi =
5. A heat exchanger is to be constructed by forming copper tubing into a coil and placing the
latter inside an insulated steel shell. In this exchanger, water will flow inside the tubing, and
a hydrocarbon vapor at a rate of 0.226 kg/s will be condensing on the outside surface of the
tubing. The inside and outside diameters of the tube are 0.0127 and 0.0152 m, respectively.
Inlet and exit temperatures for the water are 10 and 32oC, respectively. The heat of
condensation of the hydrocarbon at a condensing temperature of 88oC is 335 kJ/kg, and the
heat transfer coefficient for the condensing vapor is 1420 W/m2K. Heat losses from the shell
may be neglected. What length of copper tubing will be required to accomplish the desired
heat transfer?
Data: cp,w = 4151 J/kgK, w = 0.000994 Pas, w = 997 kg/m3, kw = 0.61 W/mK, kcopper =
385 W/mK. The inside heat transfer coefficient can be estimated from
k
hi = w (0.023)Re0.8Pr1/3
Di
Ans. 18.5 m
6. A shell-and-tube heat exchanger with two tube passes is to heat 20,000 kg/h of water from
25 to 84oC by hot engine oil flowing through the shell. The oil makes a single shell pass,
entering at 160oC and leaving at 94oC, with a heat transfer coefficient of 500 W/m2K. The
heat exchanger contains 150 steel tubes of 22.9-mm inside diameter and 25.4-mm outside
diameter. The inside heat transfer coefficient is 3210 W/m2K. Neglecting fouling
coefficients, determine the required tube length.
Data: Water: Cp = 4182 J/kgK, density = 998.1 kg/m3
Steel: k = 40 W/mK
Ans. 4.37 m
7. A concentric tube heat exchanger uses water (Cp = 4178 J/kgK), which is available at
15oC to cool ethylene glycol (Cp = 2650 J/kgK) from 100 to 60oC. The water flow rate is
0.5 kg/s and glycol flow rate is each 1.0 kg/s. Determine the effectiveness of the exchanger.
Ans. 0.597