In the three posts preceding this one, we have been exploring the “2/3 rule” for locating the sensors controlling pump and fan speed in variable flow systems. While we have been exploring this topic in the context of a simple variable flow chilled water distribution loop, the concepts also apply to variable flow air handling systems.
The bottom line is that the further down the distribution network (towards the load) that we move the sensor controlling a variable speed pump in a variable flow application, the better the optimization of the system will be. If there was only one load in the system, then the ideal location would be at the point where the load connected to the mains. Unfortunately, most of our systems are much, much more complicated than this and contain tens or even hundreds of loads.
Figuring out which load represented the worst case pressure drop requirement could be quite time consuming and would probably vary with the operating conditions. So, being practical sorts and being a bit math phobic, we application engineers came up with a rule of thumb that serves us well; locate the sensor 2/3rd of the way from the pump to the load. Why exactly someone picked 2/3 instead of 3/4 or 7/8 or 5/8? I don’t really know; any of those ratios would probably have worked out.
Like all good rules of thumb, the 2/3 rule has enough slop in it to keep us out of trouble most of the time. For instance, no matter what you interpret 2/3 to mean, be it physical distance, hydraulic distance, radial distance from the pump location, etc. a sensor located based on any of those interpretations will most likely deliver a working system that uses less energy than one with the sensor at the pump discharge.
But, like all good rules of thumb, the 2/3 rule is not the optimized answer for each and every situation. For a one load system, having the sensor at the load is the best answer. For a modestly complex system, the perfect answer and the 2/3 rule could be with in fractions of a decimal place of each other. For a complex systems with complex hydraulics, a bit of analysis may reveal that multiple sensors at multiple locations can be justified.
How do you know which way to go? Engineering judgment, which to some extent is what they are paying us for when they hire us. How to do you obtain engineering judgment? By reading things like this blog and other resources and by taking the time to occasionally analyze something back to the fundamentals until you own it at that level. Once you own something at that level, you will find that your intuition will lead you in the right direction.
And, there is always something to be learned in this business by taking the time to do a little math and think things through, even though we don’t have the luxury of doing that on every project.
So, in parting, I would encourage you to take the time to do the math to answer your own questions on occasion, or at least see if somebody else has answered them. If you have a burning curiosity to understand why something works use that fire to understand it or explain it to someone.
That’s all I did to create this string of posts, and guess what, I learned something in the process.
Senior Engineer – Facility Dynamics Engineering