Air density is a critical parameter in HVAC calculations as it affects the performance of fans, blowers, and other air-handling equipment. The density of air varies with temperature, humidity, and pressure, which in turn impacts the volume flow rate and pressure drop in ducts. Accurate calculations of air density are essential to ensure proper sizing of equipment, optimal system performance, and energy efficiency. The equation for air density (d = 0.075 lb/cu ft) is a fundamental relationship in HVAC engineering, and its application is crucial in designing and operating HVAC systems.