I claim the ornamental design for a supporting rack for dust collector, as shown and described.
FIG. 1 is a perspective view of a supporting rack for dust collector showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left view thereof;
FIG. 5 is a right view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an embodiment, perspective view of my design for a supporting rack for dust collector in use. The dust collector and the wheels of supporting rack are shown in broken lines for illustrative purposes only and form no part of the claimed design.