As we left off on the last page: If you don't need the immense pressures and force that hydraulics can give you, it doesn't make sense to pay for that capacity.
And, when you get a hydraulic leak, that's quite a visible phenomenon. A hydraulic oil leak generates issues with regards to safety as well, as high pressure oil spraying from a small orifice can hurt someone. Whether a high pressure leak or just a drip, hydraulic oil in the wrong place does create more than just a mess, but a significant slip hazard as well.
There's also an increasing concern about the potential for environmental damage from hydraulic oil leaking onto and into where it's not supposed to be.
A compressed air leak, albeit costly in wasted energy to compress that air in the first place, usually doesn't create these same hazards.
An electrician told me once that you might get killed by being well grounded and touching a 120 Volt AC power source. That same electrician said if you touch a 240 volt source, there's no more might about it, you will die. Electrical work should be performed by a licensed electrician, and if you haven't one on staff, that can get costly, which may be an issue for some businesses.
On the up side, in the urban world, electricity is ubiquitous - found almost everywhere.
Many industrial operations use a great deal of electricity in their operations, often in a control capacity or smaller manufacturing processes. However, electricity cannot economically generate the force that's available from compressed air or hydraulic power. It compares poorly in both size of components and the strength of the force generated.
Ironically, it's electricity that most often drives the industrial air compressor and the hydraulic power pack. But it is the compressed air from that compressor that provide the heavy lifting work in the typical manufacturing plant. And it's the hydraulic system that provides huge amounts of force when that force is what's called for.
Regardless, for my money, if compressed air can generate the force I need versus other energy sources, for the reasons outlined here, compressed air is the energy source that I would pick first.
Here's some more general information that may interest you.
When force is applied to a fluid in a closed vessel, it is transmitted equally in all directions.
Force is applied to the inside of the compressor tank with the compressor driving air into it, and that air pressure inside is pushing equally against the entire inside of the tank.
When a downstream air valve opens, since air is pushing equally in all areas, it will instantly blast out of the tank through the now open line, to pressurize the downstream air mains, racing towards atmosphere and what nature calls balance; where air pressure is at the same level.
The compressed air wants to maintain pressure equally so it flows from high pressure to low until pressure is equalized.
When the height and temperature are constant, if the pressure in a fluid is reduced the flow will accelerate, and if you increase the pressure, the flow will slow.
Did you know that...
"every 10 psig increase of pressure in a plant system requires about 5% more power to produce."
Source: Kaeser Compressors