1-88 FIGURE 5-8
5.15 EVASE DISCHARGE
An evase discharge is a gradual enlargement at the outlet of the exhaust system (see Figure 5-16). The purpose of the evase is to reduce the air discharge velocity efficiently; thus, the available velocity pressure can be regained and credited to the exhaust system instead of being wasted. Practical considerations usually limit the construction of an evase to approximately a 10° angle (5° side angle) and a discharge velocity of about 2000 fpm (0.25 "wg velocity pressure) for normal exhaust systems. Further streamlining or lengthening the evase yields diminishing returns.
It should be noted, however, that for optimum vertical dispersion of contaminated air, many designers feel that the discharge velocity from the stack should not be less than 3000-3500 fpm. When these considerations prevail, the use of an evase is questionable.
The following example indicates the application of the evase fitting. It is not necessary to locate the evase directly after the outlet of the fan. It should be noted that, depending upon the evase location, the static pressure at the fan discharge may be below atmospheric pressure, i.e., negative (-), as shown in this example.
EXAMPLE
Duct No. Dia. Q V VP SP
1 Fan Inlet 20 8300 3800 0.90 -7.27
2 Fan Discharge = 8300 3715 0.86 16.5
x
19.53 Round Duct Connection 20 3800 0.90
4 Evase Outlet 28 1940 0.23 0
To calculate the effect of the evase, see Figure 5-16 for expansion at the end of the duct where the Diameter Ratio, D4+D3
=
28+20 = 1.4 and Taper length LID = 40+20 = 2.R = 0.52 x 70% (since the evase is within 5 diameters of the fan outlet)
VP3 = 0.9 as given
SP4
=
0 (since the end of the duct is at atmospheric pressure)SP3 = SP 4 - R(VP3)
= 0 - (0.52)(0.70)(0.90")
=
-0.33 "wgFSP
=
SPoutiel - SPinlel - VPinlel=
-0.33 - (-7.27) - 0.9=
6.04 "wg 5.16 EXHAUST STACK OUTLETSThe final component of the ventilation system is the ex- haust stack, an extension of the exhaust duct above the roof.
There are two reasons for the placement of an exhaust stack on a ventilation system. First, the air exhausted by a local exhaust system should escape the building envelope. Second, once it has escaped the building envelope, the stack should provide sufficient dispersion so that the plume does not cause an unacceptable situation when it reaches the ground. This brief description of stack design wilI address only the first concern.
When placing an exhaust stack on the roof of a building, the designer must consider several factors. The most impor- tant is the pattern of the air as it passes the building. Even in the case of a simple building design with a perpendicular wind, the air flow patterns over the building can be complex to analyze. Figure S-28a shows the complex interaction be- tween the building and the wind at height H. A stagnation zone is fonned on the upwind wall. Air flows away from the stagnation zone resulting in a down draft near the ground.
Vortices are formed by the wind action resulting in a recircu- lation zone along the front of the roof or roof obstructions, down flow along the downwind side, and forward flow along the upwind side of the building.
Figure S-2Sb shows a schematic of the critical zones fonned within the building cavity. A recirculation zone is formed at the leading edge of the building. A recirculation zone is an area where a relatively fixed amount of air moves in a circular fashion with little air movement through the boundary. A stack discharging into the recirculation zone can contaminate the zone. Consequently, all stacks should pene- trate the recirculation zone boundary.
The high turbulence region is one through which the air passes; however, the flow is highly erratic with significant downward flow. A stack that discharges into this region will contaminate anything downwind of the stack. Consequently, all stacks should extend high enough that the resulting plume does not enter the high turbulence region upwind of an air intake.
Because of the complex flow patterns around simple build- ings, it is almost impossible to locate a stack that is not influenced by vortices fonned by the wind. Tall stacks are often used to reduce the influence of the turbulent flow, to
release the exhaust air above the influence ofthe building and to prevent contamination of the air intakes. Selection of the proper location is made more difficult when the facility has several supply and exhaust systems and when adjacent build- ings or terrain cause turbulence around the facility itself.
When locating the stack and outdoor air inlets for the air handling systems, it is often desirable to locate the intakes upwind of the source. However, often there is no true upwind position. The wind in all locations is variable. Even when there is a natural prevailing wind, the direction and speed are constantly changing. Jfstack design and location rely on the direction of the wind, the system will clearly fail.
The effect of wind on stack height varies with speed:
• At very low wind speeds, the exhaust jet from a vertical stack will rise above the roof level resulting in significant dilution at the air intakes.
• Increasing wind speed will decrease plume rise and consequently decrease dilution.
• Increasing wind speed will increase turbulence and consequently increase dilution.
The prediction of the location and form of the recirculation cavity, high turbulence region and roof wake is difficult.
However, for wind perpendicular to a rectangular building, the height (H) and the width (W) of the upwind building face detennine the airflow patterns. The critical dimensions are shown in Figure S-2Sb. According to WilsonY6) the critical dimensions depend on a scaling coefficient (R) which is given by:
[5.9]
where Bs is the smaller and BI is the larger of the dimensions Hand W. When B1 is larger than SB" use B1 = S Bs to calculate the scaling coefficient. For a building with a flat roof, Wi 1- son(57) estimated the maximum height (HJ, center (Xc), and lengths (Lc) of the recirculation region as follows:
He
=
0.22 RXc
=
0.5 RLc = 0.9 R
[5.10]
[5.11J [5.12]
In addition, Wilson estimated the length of the building wake recirculation region by:
Lr= 1.0 R [5.13]
The exhaust air from a stack often has not only an upward momentum due to the exit velocity of the exhaust air but buoyancy due to its density as well. For the evaluation ofthe stack height, the effective height is used (see Figure S-29a).
The effective height is the sum of the actual stack height (Hs), the rise due to the vertical momentum ofthe air, and any wake downwash effect that may exist. A wake downwash occurs when air passing a stack fonns a downwind vortex. The vortex
will draw the plume down, reducing the effective stack height (see Figure 5-29b). This vortex effect is eliminated when the exit velocity is greater than 1.5 times the wind velocity. Ifthe exit velocity exceeds 3000 fpm, the momentum ofthe exhaust air reduces the potential downwash effect.
The ideal design extends the stack high enough that the expanding plume does not meet the wake region boundary.
More realistically, the stack is extended so that the expanding plume does not intersect the high turbulence region or any recirculation cavity. According to Wilson, (5.6) the high turbu- lence region boundary (Z2) follows a I: I 0 downward slope from the top of the recirculation cavity.
To avoid entrainment of exhaust gas into the wake, stacks must terminate above the recirculation cavity. The effective stack height to avoid excessive reentry can be calculated by assuming that the exhaust plume spreads from the effective stack height with a slope of 1:5 (see Figure 5-28b). The first step is to raise the effective stack height until the lower edge of the 1:5 sloping plume avoids contact with all recirculation zone boundaries. The zones can be generated by rooftop obstacles such as air handling units, penthouses or architec- tural screens. The heights of the cavities are determined by Equations 5.10,5.11 and 5.12 using the scaling coefficient for the obstacle. Equation 5.13 can be used to determine the length of the wake recirculation zone downwind of the obsta- cle.
If the air intakes, including windows and other openings, are located on the downwind wall, the lower edge ofthe plume with a downward slope of 1:5 should not intersect with the recirculation cavity downwind ofthe building. The length of the recirculation cavity (L,) is given by Equation 5.13. If the air intakes are on the roof, the downward plume should not intersect the high turbulence region above the air intakes.
When the intake is above the high turbulence boundary, extend a line from the top of the intake to the stack with a slope of 1 :5. When the intake is below the high turbulence region boundary, extend a vertical line to the boundary, then extend back to the stack with a slope of 1:5. This allows the calculation of the necessary stack height. The minimum stack height can be determined for each air intake. The maximum of these heights would be the required stack height.
In large buildings with many air intakes, the above proce- dure will result in very tall stacks. An alternate approach is to estimate the amount of dilution that is afforded by stack height, distance between the stack and the air intake and intemal dilution that occurs within the system itself. This approach is presented in the "Airflow Around Buildings"
chapter in the Fundamentals volume of the 1993 ASHRAE Handbook. (5.S)
5.16.1 Stack Considerations:
I. Discharge velocity and gas temperature influence the effective stack height.
2. Wind can cause a downwash into the wake ofthe stack reducing the effective stack height. Stack velocity should be at least 1.5 times the wind velocity to prevent downwash.
3. A good stack velocity is 3000 fpm because it:
.. Prevents downwash for winds up to 2000 fpm (22 mph). Higher wind speeds have significant dilution effects.
.. Increases effective stack height.
.. Allows selection of a smaller centrifugal exhaust fan to provide a more stable operation point on the fan curve (see Chapter 6).
.. Provides conveying velocity if there is dust in the exhaust or there is a failure of the air cleaning device.
4. High exit velocity is a poor substitute for stack height.
For example, a flush stack requires a velocity over 8000 fpm to penetrate the recirculation cavity bound- ary.
5. The terminal velocity of rain is about 2000 fpm. A stack velocity above 2600 fpm will prevent rain from entering the stack when the fan is operating.
6. Locate stacks on the highest roof of the building when possible. If not possible, a much higher stack is re- quired to extend beyond the wake of the high bay, penthouse, or other obstacle.
7. The use of an architectural screen should be avoided.
The screen becomes an obstacle and the stack must be raised to avoid the wake effect of the screen.
8. The best stack shape is a straight cylinder. If a drain is required, a vertical stack head is preferred (see Figure 5-30). In addition, the fan should be provided with a drain hole and the duct should be slightly sloped toward the fan.
9. Rain caps should not be used. The rain cap directs the air toward the roof, increases the possibility of reentry, and causes exposures to maintenance personnel on the roof. Moreover, rain caps are not effective. A field study(59) with a properly installed standard rain cap showed poor performance. A 12-inch diameter stack passed 16% of all rain and as high as 45% during individual storms.
10. Separating the exhaust points from the air intakes can reduce the effect of reentry by increasing dilution.
II. In some circumstances, several small exhaust systems can be manifolded to a single exhaust duct to provide internal dilution thereby reducing reentry.
12. A combined approach of vertical discharge, stack height, remote air intakes, proper air cleaning device, and internal dilution can be effective in reducing the
consequences of reentry.
13. A tall stack is not an adequate substitute for good emission control. The reduction achieved by properly designed air cleaning devices can have a significant impact on the potential for reentry.