all you do is cut a hole in your roof and lay down some 4x4s around the hole screw them down and coat them with roofing tar and place the unit on top of the 4x4s
I've got an even better idea. Just call the local fire dept and have them spray about 5000 gallons of water into your shop. The net result will be the same, and you won't have to wait for the next rain to see it happen.
Carl, based on your answer to Gary, it sounds like you have a space that's 25x100. The no-duct device you're asking about is called a concentric diffuser. They'll work fine in s space that's fairly square, but not worth a hoot in a long rectangle like you have.
Ductwork is a major portion of the quotes you've got, and the fact that the ductwork has to be installed in an occupied and working space makes it an even greater part of the overall cost.
Another big cost item is the installed cost of the roof curb for the unit. Last time I had one of similar size installed was about 10 yrs ago. At that time, it cost about $1400 to have it cut in, flashed, and roofed in after we had placed the assembled curb on the roof and marked out the location. Installed cost to the owner was in the neighborhood of $2000.
I'd look at the cost of (2) 5 ton split systems with condensing units on the roof and air handlers hung within the space. These can be spaced sufficiently to eliminate ductwork with the exception of a piece 3-4' long on the discharge of each air handler, with a splitter or some turning vanes in the end. No return ductwork required. Equipment cost to the contractor will be the same or less than the cost of the 7.5T RTU. Each system will require a small roof penetration for passage of refrigerant piping and conduit, such as a pitch pocket or something similar, depending on the type of roof. But, there's no curb involved. Future maintenance like filter changes can be accessed from within the space instead of by getting on the roof. Operating efficiency is typically higher on equipment of 5 tons and under since the primary market is residential, and the equipment falls under federal residential efficiency requirements. Because this equipment is produced in such large quantities, the reliability tends to be better than that of small commercial equipment. The condensing units are available in both single and three phase. I'd go with 3 phase assuming you have the service in your shop, since it eliminates the start and run capacitors on the compressor, and they tend to be the most common trouble spots over time. Not much difference in cost from single to 3 phase for the unit itself, and not much difference in installed cost for the power wiring.
My guess would be that you can get 2 systems like this for a total installed cost in the $10K range. Normal comfort cooling is typically based on 400 sq ft per ton. 10 tons would give you 250 sq ft per ton, and when the additional loads of motors, controls, and evaporating coolant are considered, I wouldn't go with any less than this.
Don't waste your time looking for used equipment. Its no bargain even if its free in equipment this size. 15 yr old units with an EER in the 8 range will cost you about 60% more to operate than current equipment with a minimum EER of 13.
Assuming your shop is already heated, don't let anyone talk you into heat pumps over straight cooling units. The reversing valves in heat pumps tend to be one of the most troublesome parts of a heat pump, and they're a part that doesn't exist in a straight cooling unit. If your shop is like most, there's not much heat required when the machinery is running anyway.
I think all hvac salesman are crooks.
Maybe so. I sold a lot of jobs when we were in the contracting business, and a lot of them were in the million bucks a pop range. So I guess that'd make me a big crook. However, I'd rather be a crook than an idiot who runs his mouth about stuff where he obviously has zero knowledge, any day of the week