The term "revolving fund" means a "plant engineering revolving fund" maintained in the state treasury as a separate fund which can be expended by the Mississippi Agricultural and Industrial Board for costs incurred in connection with the design engineering and projected operating estimates of the proposed industrial plants so long as there is a balance in the fund provided by the Mississippi Legislature. All moneys repaid by individuals or groups of individuals in return payment for such plant engineering will be credited to the "plant engineering revolving fund" so that additional studies can be made on the same basis and under the same conditions as provided in Sections 57-11-31 through 57-11-39.