There are a lot of things we do in the air conditioning business, because that is how we have always done it. This is true in sizing ductwork.
EQUAL FRICTION DESIGN METHODOLOGY
The Equal Friction Design Methodology is what we typically use for sizing duct and have for years. By sizing duct system for a constant pressure loss per unit length, such as 0.1″ wg friction loss per 100 feet of duct, there is no equalization of pressure drops in duct branches unless the system has a symmetrical layout. Dampers are used for balancing.
STATIC REGAIN METHODOLOGY
Static Regain Methodology sizes the supply duct system to obtain uniform static pressure at all branches and outlets. Much more complex than equal friction, static regain can be used to design systems of any pressure or velocity. Duct velocities are systematically reduced over the length of the distribution layout, which allows the velocity pressure to convert to static pressure, offsetting friction losses in the succeeding section of duct. Systems designed using static regain require little or no balancing.
Static regain has been used since the 1940s, following the development of the commercial fan and air moving market. Static regain is very friendly to long-term operations but it took a lot of calculating, adjusting sizes and recalculating to get a system balanced on paper. Thus until the advent of computer programs, it had limited use until the 1980’s.
GREATER EFFICIENCY WITH STATIC REGAIN
Most VAV ductwork systems are sized for a 1% ASHRAE cooling day for 86 hrs/year of peak demand. The average VAV fan system operates at 65-75% capacity for 4000 hours/year and operates at minimum 35% capacity for 2000 hrs/year. With static regain using 30% smaller VAV spiral duct sizes, there is less BHP hrs used for part load spiral systems estimated at 5-10% less than rectangular ductwork systems.
Static Regain Methodology is more efficient with less lbs of supply ductwork, lower acoustic effect, and less noise, and part load energy savings which are very important. We have been part of a national survey of comparing systems cost and our calculations netted a 36.6% ductwork savings and a national average of 39.6%. In eleven national regions surveyed, the lowest savings was 28.7%.
It is worth a look on your next budget challenged project. Call a member of the BCH team to learn more.
BCH Mechanical has been a trusted name in Florida construction since 1976, delivering innovative design, high quality craftsmanship and excellent customer service. BCH capabilities include HVAC, plumbing, medical gas, piping, sheet metal, LEED, Design/Build, 3D-CAD/BIM coordination, and HVAC service. Visit BCH on the web: www.bchmechanical.com.