There is no simple answer here because exactly how duct is suspended is determined by various factors. For example, the construction of the building will play a large part i.e. what kind of fixing is used will be determined by the construction of the ceiling. The distance from ceiling to duct is another factor, as well as duct size. Aesthetics may also play a part (if the duct will be on show), and of course personal preference i.e. what's easier for the installer. The method of suspension may have been already determined by an authorised body responsible for overlooking the work being carried out.
None of this means hanging duct need be in anyway complicated. Below is a brief outline of various methods commonly employed.
Suspension rings and split rings are similar in that they both comprise of a rigid steel ring that's fixed around the duct. Suspension rings come with a nut that is fixed centrally to the upper half of the ring. One length of threaded rod can be screwed into this and then fixed by whatever means are necessary into the ceiling above. The lower half of the ring is screwed to the upper half. These screws are provided as part of the unit.
Suspension rings offer great benefits in that they are simple and easy to use, requiring just one stud and one fixing. The disadvantage however is that any height adjustments that may need to be made on the newly installed duct are made difficult by the design of the ring; the duct height cannot be altered at the low level, the point at which the stud screws into the ring. Height adjustments may be possible at the other end of the stud, at the ceiling, but this will depend on the type of fixing used. If this isn't possible, such as in the case of an anchor in concrete slab, the stud will need to be removed and either cut down (ducting raised) or replaced with a longer stud (ducting lowered). This problem isn't encountered with split ring supports.
Split rings differ in that both halves of the ring are the same, they have no nut on top. Instead two studs(threaded rod) are dropped through holes on each side of the ring. Nuts on the rod are tightened to clamp the ring around the duct. Height adjustments can be made easily by winding the nuts up or down the studs as required.
An alternative to suspension and split rings is support banding. Support banding is an inch wide light-gauge mild-steel banding with pre-punched holes for bolts and fixings. It's generally supplied in rolls from which you cut the lengths required with a pair of tinsnips. It is most commonly used in a loop fashion as seen in the drawing below. A rough noose shape can be formed in the banding prior to lifting the duct into place.
Alternatively a u-shape can be used if space is tight between the ceiling and duct:
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