System Effects and

Stacks

BA-5-3 2

System Effects

A System Effect Factor is added to the calculated system resistance and the fan is selected for a higher pressure point.

The fan will operate at the originally calculated system resistance.

“Industrial Ventilation” and “Fans and Systems” AMCA Publication 201 contain additional information on System Effects.

Non-uniform flow into a fan inlet by a 90º round section elbow – no turning vanes (Fig. 7-23)

Excerpted from the Industrial Ventilation Manual, 23rd Edition

Example of a forced inlet vortex – spin or swirl (Fig. 7-24).

Controlled diffusion and establishment of a uniform velocity profile in a straight length of outlet duct (Fig. 7-18).

Outlet Duct Elbows (Fig. 7-20)

Position C, No duct, 1.20VP

System Effect – Elbow at Inlet

Elbow with no straight duct R/D = 2.0

From Figure 7-23

System effect of R-S

From Figure 7-26

“R” = 1.2

“S” = 0.8

Fsys = (0.8 + 1.2)/2 = 1.0 VP

Restricted Inlet

22” duct = 2.6398 sqft

26” fan inlet = 3.687 sqft

Restricted Area = 2.6398/3.687

= 0.716

From Figs. 7-25 & 7-26

System Effect = 0.8VP

System Effects - Fan Outlet

No duct on fan outlet

Figure 7-18

Blast Area/Outlet Area = 0.9

SEF = “V-W”

From Figure 7-26

“V” = 0.26

“W” = 0.18

Fsys= (0.18 + 0.26)/2 = 0.22VP

Total System Effect

On inlet – 1.0VP

On outlet – 0.22VP

Restricted Inlet = 0.8VP

Total = 2.02VP

If V = 3500 fpm, VP = 0.76”wg

System Effect = 1.54”wg

Review

System effect is the estimated loss in fan performance from non-uniform airflow

System Effect Factor (SEF) is used to determine a correction value, in inches of water gauge, to be added to the system pressure losses and are represented in terms of VP

System effects are in addition to the system losses due to friction

How Can We Eliminate System

Effects?

Relocate the fan to eliminate elbow at inlet

Resize inlet duct to eliminate restriction

Add a no loss stack with a transition

VP will be lower due to larger inlet duct

Eliminating the System Effects

Exhaust Stacks

Do not use weather caps! – Losses associated with them

– May also have other system effects

No-loss stacks – Stack diameter = D + 1”

– Stack height = 4D + 6”

– D = duct diameter

Maintain discharge velocity

of > 3,000 fpm

Weather Cap

No-Loss Stack

D

Stack Height Above a Building

c

Discharge Stacks

• Exhaust beyond building envelope to avoid recirculation into air intakes • Provide sufficient dispersion

Proper Stack Design

Stack velocity should be 1.5 times wind velocity to prevent downwash

A good stack velocity is 3000 fpm, prevents downwash up to 2000 fpm (22mph)

High exit velocity poor substitute for stack height

Proper Stack Design

Stack velocity above 2600 fpm should prevent rain from entering stack while running (Rain Vt approx 2000 fpm)

Locate stacks on highest roof when possible

Don’t use rain caps

Separate exhaust points from intakes

Modeling

Simple one in ACGIH Vent Manual

More complex computer models

–Simplest

–Most complex

Do any give the real answer?

BA-5-3 28

BA-5-3 29

BA-5-3 30

BA-5-3 31

BA-5-3 32

BA-5-3 33

BA-5-3 34

BA-5-3 35