Question

The Number of Transfer Units (NTU) method is primarily used to:

• A. Design heat exchangers based on effectiveness
• B. Calculate pressure drop in pipes
• C. Determine fluid velocity in convection
• D. Analyze radiation heat transfer

Hint:

Use LMTD when all four temperatures are known.
Use the NTU method when only the inlet temperatures are known, as it directly solves for performance using heat exchanger effectiveness without trial-and-error.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the main application of the Number of Transfer Units (NTU) method in thermal engineering.

Step 2: Key Formula or Approach: The NTU method is a standard thermodynamic approach used for the analysis and design of heat exchangers, especially when the outlet temperatures of the fluids are not known beforehand.

Step 3: Detailed Explanation:

• In heat exchanger design, two primary methods are utilized: the Log Mean Temperature Difference (LMTD) method and the Effectiveness-NTU method.

• The LMTD method is simple to apply when all inlet and outlet temperatures of both the hot and cold fluids are known.

• However, if only the inlet temperatures are specified, utilizing the LMTD method requires an iterative, trial-and-error approach.

• To avoid iterations, the NTU method was developed. It introduces a dimensionless parameter called the Number of Transfer Units (NTU), which is defined as:
\[ \text{NTU} = \frac{U A}{C_{\text{min}}} \] where \(U\) is the overall heat transfer coefficient, \(A\) is the heat transfer area, and \(C_{\text{min}}\) is the minimum heat capacity rate.

• This method expresses the effectiveness (\(\epsilon\)) of a heat exchanger as a function of the NTU and the capacity ratio \(C_{\text{r}} = C_{\text{min}} / C_{\text{max}}\). This enables direct calculation of heat transfer performance and area requirements.

Step 4: Final Answer:
The NTU method is primarily used to design heat exchangers based on the effectiveness-NTU relationship.