The three types of centrifugal fans covered in this specification apply to fans which are not integral parts of equipment designed as complete units by manufacturers, and are intended to be used as follows. Type I, forward-curved fans, should be used where compactness of site is a factor in packaged unit applications (example: in a heating or a high efficiency air conditioning system). Type II, radial blade fans, should be used in systems handling such materials as heavy dust (example: in a woodworking shop). Type III, backward-inclined blade fans, which may be used in air conditioning applications, should be used when high efficiency and nonoverloading characteristics are required to move air containing no condensable fume or vapors (example: in a power generating plant).

a) Fungus resistant varnish conforming to MIL-V-173 should be used to coat electrical components and circuit elements, including terminal and circuit connections, when the fan is to be installed in humid conditions. Components and elements inherently inert to fungi or in hermetically sealed enclosures or current-carrying contact surfaces should not be coated.

b) Electromagnetic interference suppression (EMI), when required, should conform to the EMI suppression requirements and test limits for class C3, group I equipment as specified in MIL-STD-461. The fan should be subjected to tests to determine conformance in accordance with MIL-STD-462.