> Does anybody know if a height adjustable ceiling fixture exists?
Google "chandelier lift" or "chandelier hoist". These get used in
residential applications to hang a chandelier from a high ceiling and
allow it to be lowered to change the lamps, but some are available with
"industrial" safety approvals.
Some of them use a curled cord to supply the fixture; some have a
disconnect at the ceiling that automatically separates when you winch
the fixture down and reconnects when you winch the fixture up.
You can get manually-operated ones that are designed to have a
mechanical cable or rope come down to near floor level, or ones with an
electric motor. Typically you buy a lift with the weight rating you
need, and then attach whatever fixture you want to the lift.
> Because of the layout of the plant a ladder cannot be used to change
> out the lamps when they burn out.
I don't know if your plant is like this, but often there will be aisles
between the machinery. You could put a horizontal rail near the
ceiling, with a sliding block attached to the rail. One end of the rail
is where the fixture needs to be normally, and the other end of the rail
is over one of the aisles. You run a rope horizontally from the block,
around a pulley at one end of the rail, around a pulley at the other end
of the rail, and back to the block. The power comes to a receptacle
near the aisle end of the rail. You bolt the fixture to the block and
wire it to a flexible cord that runs over to the receptacle (possibly
with some springs/hooks to hold up the middle of the cord).
When you want to change the lamp, you put a ladder in the aisle, climb
up, unplug the fixture, and then pull on the rope to haul the fixture
over to you. After you change the lamp, you pull on the other side of
the rope to haul the fixture back to its normal location, and plug it
back in again.
Or, if the aisle is wide enough, you may be able to use a boom lift in
the aisle to reach up and over the machinery to change the lamps.