Engineers are under pressure like never before to generate mechanical designs that are energy-efficient and environmentally friendly. Of course, first costs and liability issues never go away. Emerging energy-efficient mechanical systems often are based on hydronics, most likely because water uses less than one-tenth of the energy that air expends to move a British thermal unit 1 ft. Engineers are being pushed to design piping systems that use fewer resources during construction and operation and have minimal cradle-to-cradle impact on the earth. Piping-system engineers will play a pivotal role in solving the energy challenges facing the United States and the environmental issues threatening the planet.

DISTRIBUTION ENERGY

Because pumping systems often run continuously and unnoticed, they can consume many times their initial cost in energy. A small improvement in their performance can reduce their electrical consumption and associated costs and lessen their environmental impact significantly. Engineers can reduce pump-power demand more than 50 percent by utilizing the following:

• Dual pumps

  Main and backup heating pumps sized for worst-case scenarios sometimes are run full bore during summer. Instead, two pumps should be selected at 50-percent flow and full head. If one pump goes down, the other typically can ride the pump curve up to 80 percent or more flow, handling the emergency and reducing first and operating costs.

• Controls

  MATERIAL-SELECTION CONSIDERATIONS

  Automatic controls can be engineered to sense when a pump is not needed and shut it off. Instead of an off/on/auto switch that can get left in the “on” position, an override timer that resets to “auto” after 24 hr can be specified.

• Plant placement

  Historically, architects have banished mechanical equipment to remote nether regions of facilities. This is like locating a kitchen far from the dining room it serves. Cutting the distance of the longest pipe run in half reduces distribution energy by 50 percent.

• Delta-T selection

  CONCLUSION

  Say goodbye to rules of thumb, such as a 10°F delta-T for chilled water. When slightly more effective cooling coils are specified, a 12.5°F delta-T, which requires 20-percent less flow, is possible. If critical-run pipe size is left unchanged, distribution head loss can be reduced by 36 percent and distribution energy by nearly 50 percent.

• Variable flow rates

  REFERENCE

  Variable-frequency drives have become inexpensive, quickly paying for themselves in energy savings. They also increase system life.

• Piping layout

  Determining the most direct route to the most hydronically remote location with the least amount of pipe and fittings is important. Considering new piping systems that use one pipe instead of two can reduce not only head loss in piping, but parasitic head loss of control and balance valves. Additionally, these types of systems are self-balancing, further reducing head loss.

• Critical-pipe sizing

  Critical runs should be optimized. Head loss can be saved by oversizing pipe or specifying long-radius elbows. The increased cost of oversizing a critical run often can be offset by undersizing non-critical legs.

• Pipe friction

  Pipe roughness is measured via the Hazen-Williams equation by using Hazen-Williams coefficients to calculate friction loss in ducts and pipes. Common materials can have a coefficient of 100, while very smooth materials can have a coefficient of 150. In calculating actual pipe friction, the coefficient is raised to the 1.85 power, meaning smooth materials can generate less than half the pipe friction of common materials at the same water velocity. Pipe materials or linings should be specified with a Hazen-Williams friction coefficient of 140 or higher.

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