What is the 'approach' temperature in heat transfer, and why is it important when selecting heat exchangers?

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Multiple Choice

What is the 'approach' temperature in heat transfer, and why is it important when selecting heat exchangers?

The approach temperature is the smallest temperature difference between the two fluids inside a heat exchanger, which sets the driving force for heat transfer at the point where the streams are closest in temperature. In practice, it’s often taken as the difference between the hot fluid leaving the exchanger and the cold fluid entering it, or the minimum gap somewhere along the flow path. This value matters for selecting heat exchangers because a smaller approach temperature means a stronger driving force for transferring heat, so you can achieve the required duty with more efficient heat transfer. However, achieving a smaller approach typically requires more surface area, resulting in a larger, more expensive exchanger. Conversely, a larger approach reduces the required area and cost but sacrifices some temperature matching and efficiency. The other options describe different concepts ( outdoor–indoor difference, total system drop, or inlet–outlet differences of a single fluid) that are not the driving force between the two streams inside a heat exchanger.

The approach temperature is the smallest temperature difference between the two fluids inside a heat exchanger, which sets the driving force for heat transfer at the point where the streams are closest in temperature. In practice, it’s often taken as the difference between the hot fluid leaving the exchanger and the cold fluid entering it, or the minimum gap somewhere along the flow path.

This value matters for selecting heat exchangers because a smaller approach temperature means a stronger driving force for transferring heat, so you can achieve the required duty with more efficient heat transfer. However, achieving a smaller approach typically requires more surface area, resulting in a larger, more expensive exchanger. Conversely, a larger approach reduces the required area and cost but sacrifices some temperature matching and efficiency. The other options describe different concepts ( outdoor–indoor difference, total system drop, or inlet–outlet differences of a single fluid) that are not the driving force between the two streams inside a heat exchanger.

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