Application Considerations
This section assists the system designer in application and control of Trane Q and Super Q II fans. Satisfactory distribution of conditioned air requires a properly chosen fan and a well designed duct system.
Abbreviations
sp ...... static pressure (in. of water)
vp ...... velocity pressure (in. of water)
tp ....... total pressure (in. of water)
ov ...... outlet velocity (ft per minute)
rpm ... fan speed (revolutions per min.) bhp ... brake horsepower
p ........ air density (lbs/ft3)
db ...... decibel (sound power or sound
pressure level) cps .... cycles per second
cfm .... cubic feet of air per min. at any
density
scfm .. cubic feet per min. of standard air clean, dry air with a density of
0.075 lbs/ft3 at 70 F and a barometer reading of 29.92- inches Hg)
The System
System Curve
At a fixed volume flow rate (cfm) through a given air system, a corresponding pressure loss, or resistance to this flow, will exist. If the flow rate is changed, the resulting pressure loss, or resistance to flow, will also change. The relationship governing this change for most systems is:
PRESSUREc/PRESSURE = (CFMc/CFM)2
The characteristic curve of a typical “fixed system” plots as a parabola in accordance with the above relationship.Typical plots of the resistance to flow versus volume flow rate are shown with normalized duct system curves, Figure
For a fixed system, an increase or decrease in system resistance results from an increase or decrease in the volume flow rate along the given system curve only.
Refer to Duct System A, Figure
100 percent volume and 100 percent resistance. If the volume flow rate is increased to 120 percent of design volume, the system resistance will increase to 144 percent of the design resistance in accordance with the system equation. A further increase in volume results in a corresponding increase in system pressure. A decrease in volume flow to 50 percent results in a 75 percent reduction in design resistance.
An air system may consist of a fan, ductwork, air control dampers, cooling coils, heating coils, filters, diffusers, noise attenuation, turning vanes, etc. The fan is the component in the system which provides energy to the airstream to overcome the resistance to flow of the other components.
System Component Losses Every system has a combined resistance to flow which is usually different from every other system and is dependent upon the individual components in the system. The determination of the “pressure loss” or “resistance to flow,” for the individual components can be obtained from the component manufacturers. The determination of pressure losses for ductwork and branch piping design is well documented in standard handbooks such as the ASHRAE Handbook of Fundamentals.
Figure
13
