Evaporation for Dry Screw Vacuum Pumps

Evaporation for Dry Screw Vacuum Pumps

Evaporation is the process by which specific products are concentrated through the reduction of moisture content in the intermediate to final steps of treatment. Some products are heat sensitive and require careful attention to the amount of heat being applied. To reduce the risk of a failed procedure, vacuum evaporation is employed.

Vacuum evaporation is the process of utilizing vacuum to lower the boiling point of the liquid for a smoother process. The atmospheric pressure exerted on the liquid prevents it from evaporating until the liquid molecules are excited to a certain extent by applied heat energy. As the liquid increases in temperature, it approaches the point at which its vapor pressure is equivalent to the pressure of the ambient air. The liquid molecules continue increasing in excitation, attempting to escape from its own intermolecular bonds while the ambient pressure prevents it from doing so. After the specific temperature is reached, the liquid is able to break from the intermolecular bonds, releasing kinetic energy as it travels through the air. When the pressure that the ambient air exerts is lowered, the amount of heat energy required to excite the liquid molecules enough to break the intermolecular bonds and evaporate decreases as well.

The difference between vacuum drying and vacuum evaporation is the result. The goal of vacuum drying is to have a solid product of a calculated minimal moisture content, applicable in products such as leather and pharmaceutical powders. The goal of vacuum evaporation is to reduce the moisture content of a liquid mixture or solid to a specific degree in a controlled and precise manner without undesirable reactions between the material and the heat. Vacuum evaporation is applicable in industries such as pulp and paper, nuclear energy, pharmaceuticals, and jewelry.

Applications of Dry Screw Vacuum Pumps

Dry screw vacuum pumps are often used for vacuum evaporative processes. In the pulp and paper industry, dry screw vacuum pumps are used in tandem with heating elements in order to concentrate the produced black liquor to be further processed and dried to manufacture white paper. When entering the evaporator, the black liquor is approximately 85% liquid. The vacuum pump evacuates the vessel and the condenser that is in between the two. The heating element applies a uniform temperature gradient on the bottom of the vessel, which causes the water to evaporate. Ideally, the resulting liquor should be approximately 15-30% liquid. The vacuum is applied to accelerate the evaporative process while saving energy. The dry screw vacuum pump greatly improves the rate of paper production and cuts operation costs on energy.

Dry screw vacuum pumps are used as part of vacuum evaporators for concentration of radioactive wastewater in nuclear energy. The wastewater is produced mostly by purifying the primary refrigerants of the nuclear vessel in pressurized water reactor (PWR) and boiler water reactor (BWR) plants.

Advantages of Dry Screw Vacuum Pumps

A dry screw vacuum pump is perfect for vacuum evaporation applications. It is a robust pump that brings the evaporator vessel pressure to the recommended levels of vacuum without having to use the maximum allowable RPM for its ultimate achievable vacuum level. Operating a vacuum pump constantly at the highest RPM recommended by manufacturers will wear the pump faster and increase the chance of failure or hindrance in performance. Repairs and maintenance will be needed more often, and replacement will be required soon after. The dry screw vacuum pump brings the vacuum level required for all kinds of vacuum evaporation at lower RPMs, extending its service life and requiring less repair and maintenance. In the case of maintenance, the dry screw requires much of it, but maintenance and cleaning are simple with easy gas purging and an easily accessible inner chamber. The dry screw vacuum pump boasts a high CFM to HP ratio, boosting plant efficiency, and the design can be configured with variable frequency for even more energy savings, cutting costs and minimizing environmental impact.

There are no working fluids in a  dry screw vacuum pump. This means that there is no limitation on vacuum level by working fluid vapor pressure, allowing the dry screw to reach the high levels of vacuum. There is no need for working fluid maintenance and changing like with an oil-flooded rotary vane vacuum pump, nor are there any costs with installation of proper working fluid circulation, like with the liquid ring vacuum pump. There is no working fluid contamination in any of the process gasses, so there is no requirement for extensive filtration and disposal of working fluid or process gas.

NES Company Inc. is proud to offer the NSP dry double screw vacuum pump series. This series is offered optionally with various kits, such as gas ballast kits and jacket type exhaust silencer kits, to design and construct a dry screw vacuum pump system that is tailored to the vacuum-evaporated process gasses in any industry and application. Out of interest for efficiency and the decrease of downtime, NES Company Inc. also offers the NSP Series spare parts kit, which makes simple repairs even quicker.