Chemical Engineering - Heat Transfer - Discussion
Discussion Forum : Heat Transfer - Section 4 (Q.No. 15)
15.
Log mean temperature difference (LMTD) cannot be used, if
Discussion:
5 comments Page 1 of 1.
Munesh said:
9 years ago
It can be used in above condition, but for flow rates change then it can not be used.
Pankaj said:
5 years ago
Explain clearly.
Rajesh said:
4 years ago
Please explain it properly.
Yash said:
4 years ago
Someone explain it, please. What is the assumption for calculation pf lmtd?
Atharv said:
4 years ago
Actually, the answer is all of them.
It has been assumed that the rate of change for the temperature of both fluids is proportional to the temperature difference; this assumption is valid for fluids with a constant specific heat, which is a good description of fluids changing temperature over a relatively small range. However, if the specific heat changes, the LMTD approach will no longer be accurate.
A particular case for the LMTD is condensers and reboilers, where the latent heat associated to phase change is a special case of the hypothesis. For a condenser, the hot fluid inlet temperature is then equivalent to the hot fluid exit temperature.
It has also been assumed that the heat transfer coefficient (U) is constant, and not a function of temperature. If this is not the case, the LMTD approach will again be less valid.
The LMTD is a steady-state concept, and cannot be used in dynamic analyses. In particular, if the LMTD were to be applied on a transient in which, for a brief time, the temperature difference had different signs on the two sides of the exchanger, the argument to the logarithm function would be negative, which is not allowable.
Steady-state flow,
No phase change during heat transfer.
Changes in kinetic energy and potential energy are neglected.
It has been assumed that the rate of change for the temperature of both fluids is proportional to the temperature difference; this assumption is valid for fluids with a constant specific heat, which is a good description of fluids changing temperature over a relatively small range. However, if the specific heat changes, the LMTD approach will no longer be accurate.
A particular case for the LMTD is condensers and reboilers, where the latent heat associated to phase change is a special case of the hypothesis. For a condenser, the hot fluid inlet temperature is then equivalent to the hot fluid exit temperature.
It has also been assumed that the heat transfer coefficient (U) is constant, and not a function of temperature. If this is not the case, the LMTD approach will again be less valid.
The LMTD is a steady-state concept, and cannot be used in dynamic analyses. In particular, if the LMTD were to be applied on a transient in which, for a brief time, the temperature difference had different signs on the two sides of the exchanger, the argument to the logarithm function would be negative, which is not allowable.
Steady-state flow,
No phase change during heat transfer.
Changes in kinetic energy and potential energy are neglected.
(3)
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