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© RACA Journal such as investment costs for the clients , duct sizing versus space restrictions , capacity for future expansions , rates of air to specific spaces with varying requirements , impact on operational costs , and partnering with specific system components .
MANAGING AIRFLOW The primary function of a duct system is essentially the conveyance of air – be this supply or return form . The flow of air is a result of pressure differential between two points . Air flow will originate from an area of high energy ( or pressure ) and proceed to area ( s ) of lower energy . Air thus moves according to three fundamental laws of physics :
• conservation of mass
• conservation of energy , and
• conservation of momentum .
Conservation of mass : an air mass is neither created nor destroyed . From this principle it follows that the amount of air mass coming into a junction in a ductwork system is equal to the amount of air mass leaving the junction , or the sum of air masses at each junction is equal to zero . In most cases the air in a duct is assumed to be incompressible , an assumption that overlooks the change of air density that occurs as a result of pressure loss and flow in the ductwork . In ductwork , the law of conservation of mass means a duct size can be recalculated for a new air velocity using a simple equation .
The law of energy conservation states that energy cannot disappear - it is only converted from one form to another . This is the basis of one of the main expressions of aerodynamics , the Bernoulli equation . Bernoulli ' s equation in its simple form shows that , for an elemental flow stream , the difference in total pressures between any two points in a duct is equal to the pressure loss between these points .
Conservation of momentum is based on Newton ' s law that a body will maintain its state of rest or uniform motion unless compelled by another force to change that state . This law is useful when one needs to understand air flow behaviour in a duct system .
SYSTEM COMPONENTS Any air distribution structure will include components based on the required function of the overall HVAC system . In a broad and general consideration , there are five typical functions of ductwork : 1 . Supply air ductwork supplies conditioned air from an air handling unit to a conditioned space .
2 . Return air ductwork removes air from a conditioned space and returns that air to an air handling unit , which then re-conditions the air . In some applications , part of the return air is required to be exhausted to the exterior of a space too .
3 . Fresh air ductwork supplies outdoor air directly to a space or to an air handling unit . Outdoor air is used for ventilating , among others , any occupied spaces of a building .
4 . Exhaust ( also termed relief ) air ductwork carries and discharges air to a designated area outside of a building or space . Exhaust air is typically taken from toilets , kitchen , laboratories and other areas requiring air extract for the particular application .
5 . Mixed air ductwork mixes air from outside air and return air .
Diffusers , grilles and louvres These components of a typical system are generally terminal devices that force or manipulate air flow in various directions through the use of their deflecting vanes or profiled blades . They are simply designed to mix supplied conditioned air with air already in a space . Grilles are defined as air devices that are typically used to supply or return air to or from a space without any deflection . Grilles are generally not used in supply distribution due to their inability to control air flow .
www . refrigerationandaircon . co . za RACA Journal I October 2022 27