Revenue bonds issued under the provisions of this act shall mature not later than 40 years after the date of issuance. The bonds shall bear interest at a rate not to exceed the maximum rate prescribed by Kan. Stat. Ann. § 10-1009, and amendments thereto. In no case where revenue bonds are issued under and by virtue of this act, after the project has been completed, shall the total amount received therefrom be in excess of the actual cost of the project. In the case where bonds are issued prior to completion of the project and the total amount received therefrom exceeds the actual cost of the project when completed, then the excess shall be deposited in a separate fund and shall not be used except for the purpose of paying the principal of and the interest upon the revenue bonds issued under this act. The board of county commissioners of such county shall not have any right or authority to levy taxes to pay any of the principal of or interest on any revenue bonds or any judgment against the board on account thereof.