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Log Mean Temperature Difference Application To Heat Exchangers | Up
Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3 | Next
Overall Heat Transfer Coefficient |
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Uo Ao Tlm
Uo
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Heat Transfer
HEAT EXCHANGERS
Rev. 0
Page 37
HT-02
transfer rate of the steam generator can also be determined by comparing the temperatures on
the primary and secondary sides with the heat transfer characteristics of the steam generator
using the equation
.
Condensers are also examples of components found in nuclear facilities where the concept of
LMTD is needed to address certain problems. When the steam enters the condenser, it gives up
its latent heat of vaporization to the circulating water and changes phase to a liquid. Because
condensation is taking place, it is appropriate to term this the latent heat of condensation. After
the steam condenses, the saturated liquid will continue to transfer some heat to the circulating
water system as it continues to fall to the bottom (hotwell) of the condenser. This continued
cooling is called subcooling and is necessary to prevent cavitation in the condensate pumps.
The solution to condenser problems is approached in the same manner as those for steam
generators, as shown in the following example.
Overall Heat Transfer Coefficient
When dealing with heat transfer across heat exchanger tubes, an overall heat transfer coefficient,
U , must be calculated. Earlier in this module we looked at a method for calculating U for both
o
o
rectangular and cylindrical coordinates. Since the thickness of a condenser tube wall is so small
and the cross-sectional area for heat transfer is relatively constant, we can use Equation 2-11 to
calculate U .o
Example:
Referring to the convection section of this manual, calculate the heat rate per foot of
tube from a condenser under the following conditions. T = 232- F. The outer
lm
diameter of the copper condenser tube is 0.75 in. with a wall thickness of 0.1 in. Assume
the inner convective heat transfer coefficient is 2000 Btu/hr-ft -- F, and the thermal
2
conductivity of copper is 200 Btu/hr-ft-- F. The outer convective heat transfer
coefficient is 1500 Btu/hr-ft -- F.
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