Question

After expansion from a gas turbine, the hot exhaust gases are used to heat the compressed air from a compressor with the help of a cross-flow compact heat exchanger of 0.8 effectiveness. What is the number of transfer units of the heat exchanger?

• A. 2
• B. 4
• C. 6
• D. 8

Hint:

For cross-flow heat exchangers, use the formula \( \varepsilon = \frac{NTU}{1 + NTU} \) to find the number of transfer units (NTU).

Answer 1

The Correct Option is B

Step 1: Use the formula for heat exchanger number of transfer units (NTU).
The effectiveness \( \varepsilon \) of a heat exchanger is related to the number of transfer units (NTU) by the following formula for a cross-flow heat exchanger: \[ \varepsilon = \frac{NTU}{1 + NTU} \] Step 2: Solve for NTU.
Given that the effectiveness is 0.8: \[ 0.8 = \frac{NTU}{1 + NTU} \] Solving for NTU, we get: \[ NTU = 4 \] Final Answer: \[ \boxed{4} \]