For producing pulps by cooking wood chips in a manner similar to a commercial digester. Ensure good repeatability of results. Heating effected through electrical heaters. Centrifugal type circulation pump provided. Voltage, current, heating rate is indicated through digital readouts. Digester volume 20 liters. Max pressure 20 kg/cm2, max temperature 200oc, pump capacity 20 liters/min. A circulation digester consists of a cooking vessel with chip basket, a heat exchanger with circulation pump, separate vessel with independent heating system for preheating the cooking and all necessary piping. The exchanger and vessels are equipped with heating elements and pid logic controllers. A sampling line with cooling attachments allows liquor samples to be withdrawn safely during the cooking process and a control valve determines the circulation rate of cooking chemicals.