In a forced circulation evaporator, liquor is pumped from the bottom cone of the vapor body through the tubes of the heat exchanger, where heat is added, and back into the vapor body where evaporation occurs.
Sufficient liquid height (submergence) is maintained above the liquor inlet on the vapor body and above the heat exchanger tubes to suppress mass boiling in the inlet and prevent boiling at the tube surface. This is necessary to prevent precipitation in the tubes which would lead to fouling of the heat transfer surface.
A high circulation rate is provided for adequate tube velocity to achieve good heat transfer. Therefore, lower temperature rises are assured which minimize supersaturation of the solution. The circulating pump is usually of the axial-flow, single-elbow design, well suited for the high flow rates and low pressure drops in Swenson-designed circulating systems. These heavy duty pumps operate at low speeds, which minimize erosion and reduce maintenance.
Circulating piping interconnects the vapor body, the heat exchanger, and the pump. Conical liquor chambers provide gradual, low pressure drop transitions from the circulating piping to the tube bundle which is particularly important for the establishment of uniform feed to the tubes. To provide for thermal expansion without expansion joints, the circulating pump base is spring-mounted. As an alternative, the entire pump may be hung from the circulating piping.
The vapor body is conservatively designed both in diameter and height. It is important to have an adequate free space above the liquor level to allow the liquor droplets entrained in the vapor leaving the boiling surface to reach equilibrium and return by gravity to the circulating slurry. The large diameters result in low vapor velocities which minimize entrainment. A mesh-type entrainment separator is typically installed in the upper portion of the vapor body to reduce entrainment.
Heat exchangers can be located on the discharge or suction side of the circulating pump. In some cases, both discharge and suction side heat exchangers have been provided in the same circulating system to maximize heat transfer surface. Multiple circulating systems can be supplied for the same vapor body on large evaporators.
A wide variety of forced circulation evaporator designs are available, tailored for the particular application.