The logarithmic mean temperature difference (LMTD) is represented by which symbol in the overall heat transfer equation?

The logarithmic mean temperature difference (LMTD) is represented by which symbol in the overall heat transfer equation?

• A. Delta T
• B. Q
• C. U
• D. A

Answer: (A)

The main quantity driving heat transfer in the overall equation is the temperature difference between the two streams, expressed as the logarithmic mean temperature difference. In the standard form Q = U A ΔT_lm, that driving term is represented by ΔT_lm. In many texts this is written simply as ΔT (with the understanding that it’s the log-mean difference, i.e., not just a single instant difference but the LM-Diff across the exchanger). So the symbol used for the LMTD in the equation is ΔT. Heat transfer rate Q, the coefficient U, and the area A are other parts of the equation but they represent different quantities: Q is the amount of heat transferred, U is the overall heat transfer coefficient, and A is the heat transfer area.

The main quantity driving heat transfer in the overall equation is the temperature difference between the two streams, expressed as the logarithmic mean temperature difference. In the standard form Q = U A ΔT_lm, that driving term is represented by ΔT_lm. In many texts this is written simply as ΔT (with the understanding that it’s the log-mean difference, i.e., not just a single instant difference but the LM-Diff across the exchanger). So the symbol used for the LMTD in the equation is ΔT.

Heat transfer rate Q, the coefficient U, and the area A are other parts of the equation but they represent different quantities: Q is the amount of heat transferred, U is the overall heat transfer coefficient, and A is the heat transfer area.