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15 Apr 15: Applications 15: The Distribution of Electrical and Control Systems
This note will annoy the heck out of some of my electrical friends, but it's something that needs to be said about MCC rooms and DCS rack rooms.

The gist of this article is, are these rooms really necessary? I'll start with the DCS first.

We just recently developed a detailed capital cost estimate for a tissue mill in the US. I had all of the I/O's of the "Distributed Control System" distributed around the mill in hardened cabinets. Each cabinet was set up to provide local instruments with a means to communicate with the main controller. The only interconnecting cable between the cabinets, and to the controller, was a single bus communications cable. This setup is a lot like today's cars where I/O modules are placed throughout the car to shorten cable lengths and speed up assembly.
 
Apparently, for this mill anyway, this was unacceptable and all I/O cabinets had to be in one air conditioned room for "environmental reasons" (I'll get to that later). What this decision did was to add complexity and cost to the project in that now, we needed a dedicated footprint for the rack room, an air handling unit with temperature control, room lighting, a false floor along with steps and doors. The biggest cost of course was all of the cabling, trays and labor, from the now centralized DCS, out to each instrument. I was sure the additional cost we estimated would dissuade the client from going this way, but it didn't. Even the argument that future additions and modifications would be far less costly with local I/O's didn't work. All this to say that our light weight DCS system was now a CCS or Centralized Control System.

Now on to paper mill MCC rooms. In the beginning of each project, I usually ask the electrical engineers, "how many ping pong tables will I be able to set up in the MCC room once the mill is running". It's a running joke, but it has some truth to it. The answer from them of course is "Space for future requirements".

All joking aside, why can't we treat the motor control centers as a distributed system where MCC's are localized to avoid these huge rooms and miles of cables? Future requirements are also met with simple distribution of the MCCs. Tissue converting plants, sawmills and other areas of the business do it so why can't we in the paper mill side of the business do it?
 
We used to do it this way a long time ago. That changed when Kraft mills were integrated with paper machines (without physical separation) and the Kraft mill ambient chemistry attacked the guts of the MCCs and circuit boards. (Think Rayonier Port Cartier).

This has now changed in a big way simply because of environmental pressures. The ambient environment that caused the corrosion issues is A) Very well controlled in new Kraft Mills and B) Nonexistent in recycled and kraft bale fed systems. So why do we still stick to this approach?

If it a case of environmental hardening that prevents us from doing this, it's time to let them know what we need.
 
Bryan Creagan
Montreal, PQ 


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