Can logarithmic mean temperature difference be negative?
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.
How do you find the mean temperature difference of a log?
Formula for LMTD calculation –
For a Parallel Flow Heat Exchanger, Hot fluid entering at temperature 100 degree Celsius and exiting at 90 degree Celsius . Cold fluid Entering at 30 degree Celsius and exiting at 50 degree Celsius. Find the LMTD . LMTD = ((100 – 30)-(90-50)) / ln (100-30/90-50) = 53.6 degree Celsius .
Can the LMTD of a heat exchanger be a negative quantity?
According to the definition modeled above, LMTD cannot be negative.
Why do we use logarithmic mean temperature difference?
The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.
What is the limitations of LMTD method?
However, there are some limitations found in this method. In the case of “specific heat changes”, this method is not found to be accurate. This method is used for liquids in a steady-state and hence is not applicable in the case of dynamic evaluation. There is no change in the passes during the transfer of heat.
What happens when LMTD is zero?
Zero LMTD means no further heat transfer is possible and if heat transfer is to be done than infinite area is required, which is not possible.
Why do we use LMTD correction factor?
Why do we use LMTD Correction factor? Explanation: In a shell and tube heat exchanger we observe both counter current and concurrent flow at different regions of the shell. This phenomenon makes the use of complete countercurrent LMTD unsuitable for calculations.
Why NTU method is preferred over LMTD?
The LMTD method is convenient for determining the overall heat transfer coefficient based on the measured inlet and outlet fluid temperatures. The ε-NTU method is more convenient for prediction of the outlet fluid temperatures if the heat transfer coefficient and the inlet temperatures are known.
What are the assumptions made during LMTD analysis?
One of the fundamental assumptions adopted in the derivation of the LMTD method is that the fluid specific heats are constant and the fluid temperature variations only result from heat exchange.
Why LMTD is introduced in a heat exchanger?
LMTD is introduced due to the fact, the temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. Here the value of overall heat transfer coefficient can be assumed as a constant.
When effectiveness and NTU method is used instead of LMTD method?
In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.
Why do we need correction factor for LMTD?
The flow conditions are neither parallel flow nor counter flow type. In these exchangers flow is a combination of both parallel and counter flow types. Hence a correction factor ‘F’ must be introduced in the general heat equation and the equation is modified as Q = UA (F) LMTD.
Why is NTU more accurate than LMTD?
How do you calculate LMTD correction factor?
Q = U x A x LMTD, The value of Q will increase if LMTD increases, LMTD = Q / [ U x A ], So from the above form it is clear that if Heat Transfer Area increases then the LMTD will decrease, and same is the case of Overall Heat Transfer Co-efficient also.