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article_pratt.jpg

©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 7, July 2017

By Russell Pratt P.E., Member ASHRAE

About the Author
Russell Pratt P.E. is a senior mechanical engineer at CSHQA Architects and Engineers in Boise, Idaho.

An irony for engineers and architects is that the very computer programs used to design energy-efficient buildings also generate significant energy loads. To help mitigate these loads, an Idaho architecture and engineering studio designed a CPU exhaust system to capture and manage waste heat as part of its renovation of a long-vacant 1950s railway warehouse.

Prior to renovation, the old building in downtown Boise was best described as so nondescript most locals had to be reminded it was there. But CSHQA decided that with some improvements the building would make a good home for the firm. They targeted LEED Platinum Certification and a budget ($2.2 million) comparable to a typical new construction, Class A office space. The project achieved LEED Platinum Certification in 2015.

 

Innovations

CPU Exhaust

Historically, personal computers in an office environment account for somewhere between 20% and 25% of the internal cooling loads. The main method of treating these loads is to size cooling equipment with enough capacity to treat the additional waste heat generated. In recent years, the power requirements for monitors have been dropping due to LCD and LED technology, while the power requirements for CPUs have been rising due to faster chipsets and graphics card technology. This shift in generated cooling loads has made it easier to capture most waste heat at one source (CPU) rather than two (CPU and monitor). The intent of the CPU exhaust system is to capture the heat generated at the source so it can be managed rather than treat the load at the space level.

CSHQA's CPU exhaust system is a network of exhaust ductwork routed throughout the building and along the spine of each group of cubicles to each workstation. The exhaust duct is connected to a centrally located variable speed fan. The fan selected uses an electronically commutated motor so the speed can be controlled using direct feedback from a pressure transducer without using a VFD. The discharge of the fan is fitted with two dampers and branch ducts. When the building is in heating mode, the airflow is routed through the exhaust side of the energy recovery wheel on the DOAS. When the building is in cooling mode, the airflow is routed to the outside.

Each personal computer at CSHQA is fitted with a warm air collector box and small control damper. The damper is open when the computer is on and closed when off. Each collector box is connected to the exhaust ductwork under the desk. With a damper at each personal computer, the system can operate using a variable air volume strategy to reduce fan energy. The exhaust fan varies speed to maintain constant negative static pressure in the ductwork during occupied hours.

The primary benefit to the CPU exhaust system is the savings in reduced cooling costs. A personal computer in an architectural and engineering office has the potential to produce 340 Btu/h (100 W) peak per hour with the use of graphic-intensive programs. This value was measured at the CSHQA office by using a watt meter connected in line with a computer workstation operating BIM software. The estimate for this project including diversity is a savings of 2 tons (7 kW) per hour of cooling with every workstation connected to the system and the occupancy of the office at maximum capacity.

 

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