The invention relates to a modular flow system having a plurality of frame elements (101, 102) configured to be combined together to form a stack for forming afunctional member such as in particular a membrane distillation stage, a vapor generator, a condenser, a heat exchanger, a filter and/or a pervaporation stage, wherein the frame elements (101, 102) each include: an outer frame (39) and an inner frame (43), the inner frame (43) encasing a central inner region (40) and being surrounded by the outer frame (39), passage openings (13 to 16) and vapor and/or liquid channels (17, 18) arranged between the outer frame (39) and the inner frame (43), wherein at least one of the vapor and/or liquid channels (17, 18) comprises at least one internal strut member (48) extending between the inner frame (43) and the outer frame (39).