January/February 2013
Your Letters

Sparking an interest

"The need for enterprise control" (November/December 2012 InTech) was very well written. I'm a 28-year-old engineer from Brazil, in the middle of my Ph.D. course in control systems. My study is about optimal tuning of PID parameters. I am part of the team of developers of BR-tuning - a PID tuning software in use at PETROBRAS, the main oil company in Brazil - with a great interest in business and management, and I want to start an M.B.A. as soon as possible. This piece was very motivational for me. Knowing that control theory is useful in enterprise control makes me want to learn more.

Thiago Antonio Melo Euzébio

Critical business variables

Finally! As an electrical engineer and business school graduate, I truly appreciate [Peter Martin's] perspective and insight ("The need for enterprise control," November / December 2012 InTech). I have been considering those concepts and theories for years, and this is the first article that really resonated with me.

Travis DeBenedetto

Better results

As a former principal publications engineer for Compressor Controls Corporation, I find it interesting that Rick McLin ("A guide to effectively executing compressor control retrofits," November/December 2012 InTech) states, "If the compressor installation is more than ten years old, the control system involved is likely based on outdated anti-surge techniques that may not be as efficient as those in common use today," and then describes only techniques that have been around for at least 25 years. Newer research has shown the polytropic head vs. volumetric flow algorithm is not generally invariant to changes in the molecular weight and compressibility of the compressed gas. In most cases, characterizing the surge limit as a function of the compression ratio, flow measurement, and rotational speed will give a better result while requiring fewer process measurements. These algorithms work for centrifugal and axial compressors air compressors but are not applicable to positive-displacement (i.e., screw and reciprocating) machines.

Joel

A question of applicability

Regarding "A guide to effectively executing compressor control retrofits" by Rick McLin (November / December 2012 InTech): [This is] a nicely written article. Is it applicable to air as well as gas compressors?

Mahmood

Rick McLin, Turbomachinery Controls business manager replied:

The techniques described in the article apply to air machines as well as process gas machines (after all, air is just another type of gas). The section on inlet guide vanes (IGV) is particularly applicable since the majority of IGVs are installed on air compressors.