Which of the following variables are needed to calculate the percent outdoor air when air volumes are unknown?

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Multiple Choice

Which of the following variables are needed to calculate the percent outdoor air when air volumes are unknown?

In a simple air-mixing scenario, the mixed-air temperature is a weighted average of outdoor and return air temperatures, with the outdoor-air fraction as the weight. If you know the mixed-air temperature (what you actually measure after the two streams blend), the outdoor temperature, and the return air temperature, you can solve for the outdoor-air fraction x using the equation: T_mix = x*T_out + (1 - x)*T_ret. Rearranging gives x = (T_mix - T_ret) / (T_out - T_ret). This means you need all three temperatures to determine the percent outdoor air, even when the total air volumes are unknown. Without any one of these temperatures, the fraction can’t be uniquely found.

In a simple air-mixing scenario, the mixed-air temperature is a weighted average of outdoor and return air temperatures, with the outdoor-air fraction as the weight. If you know the mixed-air temperature (what you actually measure after the two streams blend), the outdoor temperature, and the return air temperature, you can solve for the outdoor-air fraction x using the equation: T_mix = x*T_out + (1 - x)*T_ret. Rearranging gives x = (T_mix - T_ret) / (T_out - T_ret). This means you need all three temperatures to determine the percent outdoor air, even when the total air volumes are unknown. Without any one of these temperatures, the fraction can’t be uniquely found.

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