A digester apparatus is provided for delignifying wood chips to form wood pulp comprising: a vessel, a first downcomer, an outer fluid supply downcomer and structure defining an insulating region between the first and outer downcomers. The first downcomer extends into the vessel for delivering a first fluid into the vessel. The first fluid is generally at a first temperature when passing into the first downcomer. The outer fluid supply downcomer extends into the vessel and may be located about the first downcomer for supplying a second fluid into the vessel. The second fluid may be generally at a second temperature different from the first temperature when passing into the outer downcomer. The structure defining an insulating region between the first and outer downcomers functions to reduce heat transfer between the first and outer downcomers.