Answer
constant 3.09 is dimensional
and the equation is not valid in other units
Work Step by Step
where $ Q = 3.09 B H ^{3/2}$ we put dimension in the equation as follows : $ \frac{ft^{3}}{sec} = 3.09 ft ft^{3/2}$ therefor $3.09 = \frac{ft^{1/2}}{sec} $ so it is dimensional
if we change unites to meter and hour , 3.09 will change divided by 3600
