The use of one- and two-stage liquid ring vacuum pumps of a combined type for agricultural processes is justified: drying, extraction, vacuum evaporation and transportation of plant materials. The purpose of the article is formulated, which is to develop a method of theoretical calculation for the selection of liquid ring vacuum pumps of a combined type for the presented agricultural processes. Scientists who have been and are currently continuing research on this problem are presented. The general methodology for calculating the speed of action, adjusted for the flow rate and geometric parameters of a liquid ring vacuum pump (LVN), is described. The technological scheme of the processes of two-stage convective-vacuum-pulse drying (DCVID) and extraction is described and the use of a two-stage liquid-vitrochemical drying agent for them is justified. A method for calculating the speed of action based on the technological parameters of the devices used is presented. A method for determining the speed of action for DKVIS and extraction in the case of using pulsed action on the processed plant material is also described. The rationality of creating pulses during DCVIS in an additional container or one housing (when using a multi-body apparatus) is substantiated. A technological scheme of the process of vacuum transportation of dry bulk plant material is described. It is proposed to use a single-stage liquid liquid pump for this process. A method for selecting a vacuum pump based on the required performance and air flow is described. Based on the presented materials, conclusions and conclusions are drawn.