A double-wall heat exchanger is used to transfer heat between liquids flowing through semicircular copper tubes. Each tube has a wall thickness of r i = 20 mm and an inner radius of r = 2 0 mm . and good contact is maintained at the plane surfaces by lightly wound straps. The lube outer surfaces are well insulated. (a) If hot and cold water at mean temperatures of T h , m = 330 K and T c , m = 290 K flow through the adjoining tubes m h = m c = 0.2 kg/s, what is the rate of heat transfer per unit length of tube? The wall contact resistance is 10 − 5 m 2 ⋅ K/W . Approximate the properties of both the hot and cold water as μ = 800 × 10 − 6 kg/s ⋅ m , k = 0.625 W/m ⋅ K , and Pr = 5.35. Hint: Heat transfer is enhanced by conduction through the semicircular portions of the tube walls, and each portion may be subdivided into two straight fins with adiabatic tips. (b) Using the thermal model developed for part (a), determine the heat transfer rate per unit length when the fluids are ethylene glycol. Also, what effect will fabricating the exchanger from an aluminum alloy have on the heat rate? Will increasing the thickness of the tube walls have a beneficial effect’?
A double-wall heat exchanger is used to transfer heat between liquids flowing through semicircular copper tubes. Each tube has a wall thickness of r i = 20 mm and an inner radius of r = 2 0 mm . and good contact is maintained at the plane surfaces by lightly wound straps. The lube outer surfaces are well insulated. (a) If hot and cold water at mean temperatures of T h , m = 330 K and T c , m = 290 K flow through the adjoining tubes m h = m c = 0.2 kg/s, what is the rate of heat transfer per unit length of tube? The wall contact resistance is 10 − 5 m 2 ⋅ K/W . Approximate the properties of both the hot and cold water as μ = 800 × 10 − 6 kg/s ⋅ m , k = 0.625 W/m ⋅ K , and Pr = 5.35. Hint: Heat transfer is enhanced by conduction through the semicircular portions of the tube walls, and each portion may be subdivided into two straight fins with adiabatic tips. (b) Using the thermal model developed for part (a), determine the heat transfer rate per unit length when the fluids are ethylene glycol. Also, what effect will fabricating the exchanger from an aluminum alloy have on the heat rate? Will increasing the thickness of the tube walls have a beneficial effect’?
Solution Summary: The author explains the rate of heat transfer per unit length of the tube. The temperature of hot water at mean is T_h=330K.
A double-wall heat exchanger is used to transfer heat between liquids flowing through semicircular copper tubes. Each tube has a wall thickness of
r
i
=
20
mm and an inner radius of
r
=
2
0
mm
. and good contact is maintained at the plane surfaces by lightly wound straps. The lube outer surfaces are well insulated.
(a) If hot and cold water at mean temperatures of
T
h
,
m
=
330
K and
T
c
,
m
=
290
K flow through the adjoining tubes
m
h
=
m
c
=
0.2
kg/s, what is the rate of heat transfer per unit length of tube? The wall contact resistance is
10
−
5
m
2
⋅
K/W
. Approximate the properties of both the hot and cold water as
μ
=
800
×
10
−
6
kg/s
⋅
m
,
k
=
0.625
W/m
⋅
K
, and Pr = 5.35. Hint: Heat transfer is enhanced by conduction through the semicircular portions of the tube walls, and each portion may be subdivided into two straight fins with adiabatic tips.
(b) Using the thermal model developed for part (a), determine the heat transfer rate per unit length when the fluids are ethylene glycol. Also, what effect will fabricating the exchanger from an aluminum alloy have on the heat rate? Will increasing the thickness of the tube walls have a beneficial effect’?
Water must be heated from 15 to 50°C in a simple double-pipe heat exchanger at a rate of 3,500 kg/h. The water is flowing inside the inner tube with steam condensing at 110°C on the outside. The tube wall is so thin that the wall resistance may be neglected. Assume that the steam-film coefficient h_0 is 11 kW/m^2 °C. What is the length of the shortest heat exchanger that will heat the water to the desired temperature, considering an optimal diameter for the tube?
Checking several table values, the following properties of water for this scenario are:
p = 993 kg/m^3
k = 0.61 W/m °C
\mu (viscosity) = 0.78 cP
Cp = 4.19 J/g °C
In a heat exchanger, water flows through a long copper tube (inside diameter 2.2 cm) with an average velocity of 2.13 m/s. The water is heated by steam condensing at 150 degree celsius on the outside of the tube. Water enters at 15 degree Celsius and leaves at 60 degree Celsius . What is the heat transfer coefficient, h, for the water? Write the given, required and solution
Concentric tube heat exchanger (tubular or tube in tube) is used for a
large industrial gas turbine. The dimensions and values are given. One of
the steps is not required to find the required length of the HX, if the oil
leaves at 60 C? Oil and water inlet temperatures are 100 and 30 C,
respectively.
Thermal entry length to validate the steps
O None of the above
Log mean temperature difference
Reynolds number, Prandtl number and Nusselt number
O Overall heat transfer coefficient
Heat and Mass Transfer: Fundamentals and Applications
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