An internally-cooled compressor comprising: a casing defining at least in part a stage inlet and a stage outlet; and a diaphragm disposed in the casing and comprising: a diaphragm box formed from a plurality of box components (138, 140, 142), wherein one or more of the plurality of box components defines a plurality of box channels; a bulb formed from a plurality of bulb components (132, 134, 136), wherein a first bulb component of the plurality of bulb components defines a plurality of primary bulb channels, and a second bulb component of the plurality of bulb components (132, 134, 136) defines a plurality of secondary bulb openings around and proximate to the perimeter of the second bulb component; and a plurality of return channel vanes extending from the first bulb component and connecting the diaphragm box and bulb in fluid communication, wherein each of the plurality of return channel vanes defines a plurality of return vane conduits coupled in fluid communication with the plurality of box channels and the plurality of primary bulb channels, thereby forming a first section of a cooling pathway, wherein the cooling pathway is configured such that a cooling agent introduced from an external coolant source into the diaphragm box and flowing through a first box channel flows through a first return vane conduit into and through a first primary bulb channel and back through a second return vane conduit into a second box channel before flowing back to the external coolant source. a casing defining at least in part a stage inlet and a stage outlet; and a diaphragm disposed in the casing and comprising: a diaphragm box formed from a plurality of box components (138, 140, 142), wherein one or more of the plurality of box components defines a plurality of box channels; a bulb formed from a plurality of bulb components (132, 134, 136), wherein a first bulb component of the plurality of bulb components defines a plurality of primary bulb channels, and a second bulb component of the plurality of bulb components (132, 134, 136) defines a plurality of secondary bulb openings around and proximate to the perimeter of the second bulb component; and a plurality of return channel vanes extending from the first bulb component and connecting the diaphragm box and bulb in fluid communication, wherein each of the plurality of return channel vanes defines a plurality of return vane conduits coupled in fluid communication with the plurality of box channels and the plurality of primary bulb channels, thereby forming a first section of a cooling pathway, wherein the cooling pathway is configured such that a cooling agent introduced from an external coolant source into the diaphragm box and flowing through a first box channel flows through a first return vane conduit into and through a first primary bulb channel and back through a second return vane conduit into a second box channel before flowing back to the external coolant source. a diaphragm box formed from a plurality of box components (138, 140, 142), wherein one or more of the plurality of box components defines a plurality of box channels; a bulb formed from a plurality of bulb components (132, 134, 136), wherein a first bulb component of the plurality of bulb components defines a plurality of primary bulb channels, and a second bulb component of the plurality of bulb components (132, 134, 136) defines a plurality of secondary bulb openings around and proximate to the perimeter of the second bulb component; and a plurality of return channel vanes extending from the first bulb component and connecting the diaphragm box and bulb in fluid communication, wherein each of the plurality of return channel vanes defines a plurality of return vane conduits coupled in fluid communication with the plurality of box channels and the plurality of primary bulb channels, thereby forming a first section of a cooling pathway, wherein the cooling pathway is configured such that a cooling agent introduced from an external coolant source into the diaphragm box and flowing through a first box channel flows through a first return vane conduit into and through a first primary bulb channel and back through a second return vane conduit into a second box channel before flowing back to the external coolant source.