The Williams & Davis Series 4000D is a traditional 4-pass, Dry-back horizontal firetube boiler that offers high efficiency, flexibility, reliability, safety and ease of operation. The Dryback design provides full access to boiler tubes, tube sheet, and furnace for ease of maintenance and includes single rear tube sheet construction, which reduces tube sheet stresses. Series 4000D provides high flue gas velocities and low stack temperatures to maximize its performance. All units are factory fire tested and shipped as an integrated package, ready for quick connection to utilities.