## NTU Effectiveness Method

The **log mean temperature difference** (LMTD) method discussed in previous section is easy to use in heat exchanger analysis when the inlet and the outlet temperatures of the hot and cold fluids are known or can be determined from an energy balance. Therefore, the LMTD method is very suitable for determining the size and performance of a heat exchanger.

When direct knowledge of the LMTD is not available and the NTU method (**Number of Transfer Units method**) can be used. This method is based on a dimensionless parameter called the heat transfer effectiveness, defined as:

As can be seen, the *effectiveness* is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate. To define the effectiveness of a heat exchanger, we must first determine the maximum possible heat transfer rate, q_{max}, for the heat exchanger.

**NTU Effectiveness Method**

Further reading:

- Fundamentals of Heat and Mass Transfer, 7th Edition. Theodore L. Bergman, Adrienne S. Lavine, Frank P. Incropera. John Wiley & Sons, Incorporated, 2011. ISBN: 9781118137253.
- Heat and Mass Transfer. Yunus A. Cengel. McGraw-Hill Education, 2011. ISBN: 9780071077866.