For a 7.5-ton rooftop unit with fresh air dampers set to 10%, what are the quantities of fresh air and return air?

In a rooftop unit with a specified capacity of 7.5 tons, you can determine the overall air flow required by using the relationship between the unit size and the air flow in cubic feet per minute (cfm). A general rule of thumb is that 1 ton of cooling typically requires around 400 cfm of air flow. Thus, for a 7.5-ton unit, the total air flow would be approximately:

7.5 tons × 400 cfm/ton = 3000 cfm.

With the fresh air dampers set to 10%, the total amount of fresh air being introduced into the system can be calculated by taking 10% of the total air flow:

10% of 3000 cfm = 300 cfm.

The return air can then be found by subtracting the fresh air amount from the total air flow:

Total air flow - Fresh air = Return air

3000 cfm - 300 cfm = 2700 cfm.

Thus, the correct values for outdoor air (OD) and return air (RA) are 300 cfm and 2700 cfm respectively. This makes the answer aligned with the first option.