Skid mounting is one term that implies that a process is built and operates as a single module. A broader term would be Modular Design, where modules of a process are pre-assembled, tested, and delivered. This approach is more costly than buying a pile of components, but is almost always less costly when all of the project costs are added. Modular Design is a method we use to satisfy several key concerns on bulk material projects:
Insuring that everything fits
While 3-D design has improved the likelihood that components will fit and operate as designed, the laws of reality state that sometimes things just don't work as planned. Better to find and fix those things in our shop, than on your plant floor. Pre-assembly and testing can't be done for everything, but when it is physically practical, it provides high value for modest cost.
Reducing mechanical installation cost
Moving one or two large skids or platforms, and coupling the other few pieces that don't quite fit the module, usually less labor than handling, placing, lagging, fitting, and connecting every component at the job site.
Reducing field wiring
Most bulk processing and packaging systems will require multiple skids (or decks, as in a multi-level tower), so fully pre-wiring the system isn't possible. But wiring and plumbing each module to junction panel or manifold will reduce the cost of electricians and pipefitters on site.
Functionality and startup
Sometimes you must see it, to "see it". Maybe the infeed is a little sluggish in the chute, or the transition into a feeder doesn't fit just right. Pre-design, pre-fab, and pre-assembly helps identify many problems. Solving those issues in our shop means that startup in your plant can focus on just the issues that could not be foreseen (there is always something), and on optimizing the system performance. Startup done in a few hours, or a few days, instead of a few weeks.