The challenge of choosing green materials and using fewer resources
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.
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.
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
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
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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- Other losses
Control and balancing valves, coil losses, etc. on a critical run should be assessed to see if oversizing is beneficial. Internal pipe fittings can have more than 10 times the pressure drop of external fittings and should be examined closely. Likewise, central-plant components and arrangements should be analyzed for possible savings.
Perhaps the most wasteful practice in HVAC engineering is the oversizing of central-plant equipment. Design teams need to work with owners to reach a better understanding of the benefits of more accurate system sizing.
Engineers should not fall into the trap of thinking that if a little pipe insulation is good, a lot would be better. There is an extreme law of diminishing returns in insulating pipes.
- Commissioning and verifying
Engineers should verify that actual flow and pressure correspond to design. If they do not, the engineer should work with the contractor to determine why.
- Measuring and comparing results
The engineering community needs to develop metrics that define distribution-system efficiency.
Energy-efficient design is only half the story. The other half is the materials (e.g., pipe, fittings, glues, solvents, solder, gaskets, sealants, tapes, and lubricants) used to create systems. The following issues should be considered when choosing piping material:
Potable water for human consumption should not be exposed to potentially toxic materials. Lead, heavy metals, polyvinyl chloride (PVC), dioxins, and other toxins should be avoided.
If something is built using half the resources, but its useful life is half as long, the net benefit is zero. When this occurs with piping systems, the net benefit is less than zero because piping-system failures almost always result in collateral damage, such as mold. Making matters worse, pipes often are concealed, allowing damage to progress unnoticed for years, ultimately requiring substantial additional costs for repair or replacement. It is more important to specify top-quality systems that last for the life of a building.
Recyclable piping systems do not contribute to landfills and can be made into other useful items. Recycled materials, however, never should be used to make new pipes. By nature, recycling introduces impurities. Impurities affect the toxicity and quality of pipe.
- Thermal integrity
When hot or cold fluids are being transported, the piping materials chosen should not be able to conduct heat out of the fluid, allowing for the reduction or elimination of pipe insulation and associated costs and the lessening of environmental impacts.
Chemical interactions between fluids and piping materials can result in premature failures of piping systems and water contamination. Historically, engineers have tried to protect piping systems from the fluids they carry by adding inhibitors. However, this entails adding chemicals to the mix. More chemicals mean greater chances for unintended consequences, such as chemical incompatibility or environmental contamination. To protect people and the environment, engineers must strive to select the toughest, most inert, and least harmful materials and use as few materials as possible.
A detailed analysis of steel, copper, and plastic drinking-water piping systems was conducted by Technical University in Berlin.1 The ecological impact of each type of piping material from raw-material sourcing through manufacture, installation, and disposal was quantified. The study also evaluated the relative energy required to produce the piping systems. The results showed more intensive ecological pollution loads associated with metal pipe systems. Pipe systems made of polypropylene (PP), cross-linked polyethylene, polybutylene (PB), and chlorinated PVC plastics were found to represent a more ecologically beneficial solution. In terms of energy balance, recycling, and waste disposal, PP and PB piping systems were found to present the most environmentally beneficial alternative to metallic piping systems.
Piping engineers are in the position of being stewards over vital natural resources, and they make decisions every day that have long-term effects on the health of people and the planet. They are being called on more than ever to seek better answers to the challenges facing us. If we are supposed to consider the environment before printing an e-mail, how much more should we consider it before designing a piping system?
- Kauter, H., Weinlein, R., & Jokel, C. (1994). Final report: Comparative analysis of drinking water systems. Berlin: Technical University Berlin.
President for North America for plastic-pipe manufacturer Aquatherm Inc., Steven J. Clark, PE, P.Eng., has 30 years of experience in building-energy optimization, including performance of design and energy studies for commercial, institutional, and industrial facilities. His building-system designs have won energy-efficiency awards in the United States and Canada. He holds several patents on HVAC and piping systems.