Thoughts On Engineering A Piping System
A closed-loop water system may look easy enough, but that vision evaporates quickly if you forget some key design details.
Years after an HVAC engineer starts to design closed-loop water systems and then progressively moves up the ladder of engineering success, he can forget and/or overlook many of the issues, concerns, details, opportunities, etc. associated with engineering these systems. After all, it is just pipe sizing to allow water to flow to terminal units, transfer its energy, and return to the source of the energy to begin again. Pretty straightforward, you’d think, but as I look back at my experience learning to design a closed-loop heating system, I wrote down many things that need to be done regarding water systems beginning with the design phase.
Always start with a simple one-line flow diagram of the design to get an overview and to determine if the system will be primary, primary-secondary, or primary-secondary-tertiary based on application. The designer, using the building system heat loss or heat gain calculations, then decides on the water temperature difference: 10 or 20 degree delta-T, or maybe a larger delta-T to reduce the individual gpms and in turn reduce the overall pipe sizes to reduce first cost and possibly operating cost. Next, the designer has the option of using the lazy person’s pipe sizing spin-wheel that is based on gpm, velocity, and pressure drop, or take the time to set up a time-tested, value-added gpm range pipe chart noting pipe size and minimum-to-maximum gpm (i.e., 1.25 in 7 gpm to 16 gpm) per pipe size.
This approach raises one’s awareness of when to go to the next size pipe and/or avoid that office specification that arbitrarily states, “2.5-in and larger pipe shall be welded pipe.” A pipe value-added trick of the trade I have used in the past, because the system size required no larger than 3-in pipe, is to specify soldered copper pipe instead of welded pipe to avoid a firewatch person and a required major relocation of occupants while welding was going on, to mention just two benefits.
Design engineers frequently overlook the benefit of a direct return system versus a reverse return system. In the past, several more experienced design engineers have told me that a reverse return system is self-balancing. Wrong!
Next, a designer can allow/calculate head pressures to be higher than normal based on the number of operating hours a year (i.e., heating season only, 24-7-365, etc.) thus minimizing the pipe system to a more energy efficient, lower total pump. Think about it. Why conservatively size a larger pipe installation with smaller pump horsepower for a system that is only going to operate 2,000 hours a year? Do the math: 15 horsepower for those hours versus 20 horsepower. Will the electrical cost savings be more or less than the first cost difference of installing the larger piping system versus a smaller pipe system? There is a time to engineer based on first cost and a time to engineer based on lifecycle cost.
Design engineers frequently overlook the benefit of a direct return system versus a reverse return system. In the past, several more experienced design engineers have told me that a reverse return system is self-balancing. Wrong!
A theoretically correct reverse return system may, if the system serves only one level and every terminal unit has its own branch runout to the unit. Once the system has to serve a second, third, or additional floors with multiple supply and return risers to the floors above and/or below, then the perfect, self-balancing reverse return system is gone. Each floor and each riser will require its own attention when it comes time to TAB for the individual terminal units and risers. Heck, if self-balancing truly existed, then why do you need a TAB contractor? A more value-engineered solution to the design layout of the piping system is to eliminate the extra pipeline that is intended to return all the flow back to the boiler, heat exchangers, or chiller bundle.
Years ago, I took the National Environmental Balancing Bureau TAB test (written test followed by a field test), and the experience was not only great, it was a real eye-opener to the world of the TAB technician. If designers were to increase their knowledge on the “how to” of water balancing, they would find that they seldom can say “this system will be self-balancing.”
To all these points, why haven’t value-engineering estimators offered these suggestions to the design engineer who seldom (if ever) took the time to include their own value-design in their own projects? You could write a book for practitioners on piping.
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