ΔT_lm in the heat transfer context stands for what?

• A. Log Mean Temperature Difference
• B. Temperature Difference at the Hot Inlet
• C. Average Temperature Difference
• D. Temperature Rise

Answer: (A)

ΔT_lm is the logarithmic mean temperature difference. In a heat exchanger the temperature difference between the hot and cold streams changes along the length, so the total heat transfer is governed by a weighted average of that driving force rather than a single value. Defining the end differences in the appropriate way for the flow arrangement (for counterflow, ΔT1 = Th,in − Tc,out and ΔT2 = Th,out − Tc,in), the logarithmic mean is ΔT_lm = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2). The heat transfer rate follows Q = U A ΔT_lm, with U the overall heat transfer coefficient and A the area. This makes ΔT_lm the correct measure of the temperature driving force across the exchanger. The other options describe only an inlet difference or a simple arithmetic average, which do not accurately represent the changing ΔT along the length, and a temperature rise isn’t the driving force used in this context.