The unit consists of a vessel with class wool insulation ail around. A helical coil made of copper is fixed inside the vessel. A motorized agitator is fixed at the center of the coil. The vessel is provided with an electric heater and a controller is also provided for vessel. Heater heats water in the jacket while cold water is circulated through coll. Temperature of circulated water and water In vessel Is measured and thus heat transfer coefficient at various agitator speeds can be determined.