A heat exchanger cools a process liquid in a paper-making machine using a source of cooling water. A first tank includes a plurality of first walls. A first inlet is connected to at least one of the first walls and the source and receives the cooling water from the source. A first outlet is connected to at least one of the first walls and the source. The cooling water is transported from the first outlet to the source. A second tank includes a plurality of second walls. The second tank has a second inlet connected with and receiving the process liquid from the paper-making machine, and a second outlet connected with and transporting the process liquid to the paper-making machine. An intermediate chamber is defined by and disposed between at least one of the first walls of the first tank and at least one of the second walls of the second tank. The intermediate chamber has a heat transfer liquid therein for transferring heat from the process liquid to the cooling water.