TAILIEUCHUNG - HVAC Systems Design Handbook part 5

In most HVAC systems, the final energy transport medium is moist air—a mixture of dry air and water vapor. This is conveyed through filters, heat exchange equipment, ducts, and various terminal devices to the space to be air-conditioned. The power to move the air is supplied by fans. This chapter discusses fans and duct systems, together with related subjects such as grilles, registers, diffusers, dampers, filters, and noise control. | Source HVAC Systems Design Handbook Chapter 5 Design Procedures Part 3 Air-Handling Systems Introduction In most HVAC systems the final energy transport medium is moist air a mixture of dry air and water vapor. This is conveyed through filters heat exchange equipment ducts and various terminal devices to the space to be air-conditioned. The power to move the air is supplied by fans. This chapter discusses fans and duct systems together with related subjects such as grilles registers diffusers dampers filters and noise control. Fans According to Air Moving and Conditioning Association AMCA Standard 210 1 A fan is a device for moving air which utilizes a power-driven rotating impeller. The three fan types of primary interest in HVAC systems are centrifugal axial and propeller. The fan motor may be directly connected to the impeller directly connected through a gearbox or indirectly connected by means of a belt-drive system. Fan law equations The fan law equations are used to predict the performance of a fan at some other condition than that at which it is tested and rated. The HVAC designer is particularly interested in the effects on horsepower pressure and volume consequent to varying the speed of the fan in a system. 95 Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures Part 3 96 Chapter Five The fan laws expressed in the following equations relate only to the effect of varying speed assuming that fan size and air density remain constant. RPM CFM - CFM1 rpmJ SP SP fRPMsY SP2 SP1 RPM1 RPM 2 TP TP 2 2 1 RPmJ RPM2 3 BHp2 - BHp1 pmJ where CFM airflow rate ft3 min SP static pressure TP total pressure BHP brake horsepower bhp Expressed in simple language the fan laws say that when fan size and air density are unchanged the airflow rate varies directly as the

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