Selecting Laboratory Stack Designs
Necessary measures must be taken to protect the laboratory building and adjacent buildings from reingestion of toxic laboratory chemical hood exhaust back into a building air supply system. The 10 ft (3.0 m) minimum stack height called for in the body of this standard is primarily intended to pro- tect maintenance workers from direct contamina- tion from the top of the stack. However, the mini- mum height of 10 ft (3.0 m) is not enough by itself to guarantee that harmful contaminants would not be reingested. Similarly, a minimum 3000 fpm (15.2 m/s) exit velocity is specified in the body of this standard, but this exit velocity does not guarantee that reingestion will not occur.
This appendix describes general stack design guidelines and three analysis methods for deter- mining an adequate stack design. The first analysis method is termed the “Geometric” method, which ensures that the lower edge of an exhaust plume stays above the emitting building and associated zones of turbulent airflow. The geometric method is fully described here and is accompanied by an example. The second analysis method, briefly described, predicts exhaust dilution at downwind locations. The dilution equations are not presented here but can be obtained from Chapter 43 of the ASHRAE 1999 Handbook – HVAC Applications. A dilution criterion is presented in this appendix to judge the adequacy of the predicted dilutions in minimizing
reingestion. The third analysis method described is wind tunnel or water flume modeling.
General Guidelines
Laboratory chemical hood exhaust stacks should have vertical, unobstructed exhaust openings. Chapter 43 of the ASHRAE 1999 Handbook –
HVAC Applicationsdescribes appropriate rain pro- tection devices. Goosenecks, flapper dampers, and rain caps are unacceptable as they deflect the exhaust sideways or downward, making it much more likely that reingestion will occur.
For a given exhaust flow rate, reducing the exit diameter with an exhaust nozzle is recommended to increase the exit velocity and rise or throw of the exhaust over the building. However, exit veloc- ities much larger than 3000–4000 fpm (15.24 to 20.32 m/s) may result in high noise and vibration. Too small of a nozzle, or one with too rapid a decrease in area, could result in excessive pres- sure loss in the exhaust and the resulting combi- nation of reduced flow due to fan system effect and reduced dilution and safety.
Combining exhausts into a common stack, either by manifolding exhausts or with very close group- ing of stacks, will enhance the rise of the exhaust plume. Close grouping of stacks can be used for specialty exhausts that cannot be manifolded because of their chemical nature. Manifolding or combining exhausts can generally give greater benefit than installing an exhaust nozzle on a stack serving a single laboratory chemical hood.
Manifolding of exhausts can also provide some internal dilution of laboratory chemical hood exhausts when the majority of chemical emissions are from an upset condition or large release from a single laboratory chemical hood. Such upset or large release conditions are the primary cause of odor complaints and potential health effects. However, this internal dilution is partially offset by the decreased atmospheric dilution due to the larg- er plume size. Nevertheless, manifolding of exhausts is still beneficial and recommended. Variable exhaust flow rates, used to reduce energy costs, can periodically result in low exit velocities. Minimum exit velocities below 1500 fpm (7.62 m/s) are discouraged because for such low exit veloci- ties, high winds can cause the exhaust to travel down the side of the stack instead of rising verti- cally. Makeup air or variable nozzles are recom- mended to maintain high exit velocities. Adding makeup air is preferred because it provides the larger plume rise and some internal dilution. Air intake placement is as important as stack design. Intakes on the side of the building or at grade will usually provide greater protection from rooftop exhausts. Intakes on the roof may work if placed a sufficient distance from the exhausts.
When only a single tall stack is present, an intake location near the base of the stack may be a good location. The advantage of this location is dimin- ished if there are sources of toxic or odorous exhausts at other locations on the roof. Nearby intakes elevated above a laboratory exhaust stack should be avoided.
Rooftop obstacles, such as parapets or architec- tural fences, and penthouses on the same roof as the hazardous exhaust stack can also act as adja- cent buildings causing wind flow disturbances that reduce the rise of the exhaust. Note that it is the dif- ference in roof heights that is particularly important when analyzing the adjacent building effect.
First Stack Design Method— The Geometric Method
Chapter 43 of the ASHRAE 1999 Handbook –
HVAC Applications describes the geometric method. This is a conservative simplified method most appropriate for use by laboratory building ventilation designers.
The geometric method is designed for isolated rec- tangular buildings that do not have taller buildings, dense taller trees, or taller hills close to the labora- tory building. Also air intakes on the emitting build- ing should be no higher than the top of the physical exhaust stack opening. Provided these conditions are met, the geometric method can be applied as follows:
1) Calculate the length of the recirculation zone (R) downwind of the building for each of the four basic approach wind directions. For a given direction, R = (Bsmall0.67) (Blarge0.33), where Bsmall is
the smaller of the building height and width, and Blargeis the larger of the two. As used here, the
recirculation zone height is the height of the emitting building.
Table A1 presents recirculation zone length for various building dimensions.
2) Calculate the added plume rise (throw) due to exhaust momentum and add it to the stack height, to obtain the effective stack height. The added plume rise due to momentum (h-added)
equals 3 × (stack diameter) × (stack exit veloci- ty/1%-wind speed). The 1%-wind speed is a high wind speed exceeded only 1% of the time. These wind speeds are available for numerous locations in the ASHRAE 1997 Handbook –
Fundamentals, Chapter 26.
3) The effective height of the stack is the physical stack height plus the added plume rise due to momentum.
4) The geometric method, as stated here, specifies that the bottom of an exhaust plume should clear the emitting building, including penthous- es, and the recirculation zone downwind of the building. The bottom of the plume extends down- ward at a 5:1 slope (5 units horizontal and 1 unit downward) from the effective stack height (physical height plus added plume rise). This should be done for all four of the basic approach wind directions. Table A2 shows flowrates required to meet the geometric method, given a 10 ft (3.0 m) stack height and a 3000 fpm (15.2 m/s) exit velocity (as per this standard), a 1%-wind speed of 15 mph (24 km /h), and various horizontal distances to clear. The horizontal distance is the distance between the stack and the downwind building edge plus the recirculation zone length.
The same method can be used to determine a taller stack that also complies.
Example Calculation for the First Stack Design Method—The Geometric Method
A laboratory building is 100 ft (30.5 m) wide, 200 ft (61.0 m) long, and 60 ft (18.3 m) high. A manifold- ed laboratory exhaust with a flowrate of 10,000 cfm (4719 L/s) is located in the center of the roof. For wind approaching the 100 ft (30.5 m) wide side, Bsmallis 60 ft (18.3 m) and Blargeis 100 ft (30.5 m).
The length of the recirculation zone is R = (600.67)(1000.33) = 71 ft (21.7 m). The horizon-
tal distance that must be cleared by the plume equals 100 ft (30.5 m) from the center to the edge of the building plus 71 ft (21.6 m) for the recirculation zone, or 171 ft (52.1 m). The required effective stack height to clear the building and recirculation zone is 171/5 (using the 5:1 slope) = 34.2 ft (10.4 m).
The added stack height due to momentum is cal- culated next. The stack diameter is 2.06 ft (0.63 m) based on a 3000 fpm (15.2 m/s) exit velocity and a 10,000 cfm (4719 L/s) flow rate. Using a 15 mph (24.1 km/h), 1320 fpm (6.7 m/s) 1%-wind speed, the added stack height = 3 × 2.06 × 3000/1320 = 14 ft (4.3 m). Given a physical stack height of 10 ft (3.0 m) based on the minimum required to meet this standard, the effective stack height is 14 + 10 ft = 24 ft (7.32 m).
The required effective height computed above is 34.2 ft (10.4 m), which is not met with a 10 ft (3.0 m) physical stack height. The designer can increase the physical height to 20 ft (6.1 m). As an alternative, the designer can increase the momentum of the air by introducing outside air to the system. If the physical stack height remains at 10 ft (3.0 m), the diameter would need to increase to 3.5 ft (1.1 m), increasing flow rate to about 30,000 cfm (14158 L/s). Also,
increasing to 30,000 cfm (14158 L/s) will increase in-stack dilution by a factor of 3:1. This in-stack dilu- tion, whether achieved by manifolding exhausts in the building or by adding roof air, can be very valu- able to achieving safe results. The other wind direc- tion (aimed toward the long side of the building) should be checked, but for this example this wind direction is the worst case.
High volume flow in itself is not a guarantee of ade- quate dilution. For a given source spill rate in kilo- grams/second, a higher exhaust volume flow Qe increases the in-stack dilution, but somewhat reduces the atmospheric dilution because the atmosphere is now presented with a larger volume of gas to disperse.
The following Tables A1 and A2 allow for rapid esti- mates of required dilution to be made where numerical calculation is not possible at the time.
Table A1 Length of Downstream Recirculation Zone (feet and meters)
Each story is 15 ft (4.6 m) high
Bldg.
Dimensions 1 Story 2 Stories 3 Stories 4 Stories 5 Stories 6 Stories 7 Stories Height in Feet (meters) (4.6 m)15 ft 30 ft (9.1 m) 45 ft (13.7 m) 60 ft (18.3 m) 75 ft (22.9 m) 90 ft (27.4 m) 105 ft (32.0 m) Length or Width 50 ft (15.2 m) 22.3ft (6.8 m) 35.5 ft (10.8 m) 46.6 ft (14.2 m) 53.1 ft (16.2 m) 57.2 ft (17.4 m) 60.7 ft (18.5 m) 63.9 ft (19.5 m) 75 ft (22.9 m) 25.5 ft (7.8 m) 40.6 ft (12.4 m) 53.3 ft (16.2 m) 64.6 ft (19.7 m) 75.0 ft (22.9 m) 79.7 ft (24.3 m) 83.3 ft (25.4 m) 100 ft (30.5 m) 28.1 ft (8.6 m) 44.6 ft (13.6 m) 58.6 ft (17.9 m) 71.0 ft (21.6 m) 82.5 ft (25.1 m) 93.2 ft (28.4 m) 101.6 ft (31.0 m) 150 ft (45.7 m) 29.8 ft (9.1 m) 51.0 ft (15.5 m) 67.0 ft (20.4 m) 81.2 ft (24.7 m) 94.3 ft (28.7 m) 106.5 ft (32.5 m) 118.1 ft (36.0 m) 200 ft (61.0 m) 29.8 ft (9.1 m) 56.1 ft (17.1 m) 73.6 ft (22.4 m) 89.3 ft (27.2 m) 103.7 ft (31.6 m) 117.1 ft (35.7 m) 129.9 ft (39.6 m) 250 ft (76.2 m) 29.8 ft (9.1 m) 59.6 ft (18.2 m) 79.2 ft (24.1 m) 96.1 ft (29.3 m) 111.6 ft (34.0 m) 126.1 ft (38.4 m) 139.8 ft (42.6 m) 300 ft (91.4 m) 29.8 ft (9.1 m) 59.6 ft (18.2 m) 84.2 ft (25.7 m) 102.0 ft (31.1 m) 118.5 ft (36.1 m) 133.9 ft (40.8 m) 148.5 ft (45.3 m) 500 ft (152.4 m) 29.8 ft (9.1 m) 59.6 ft (18.2 m) 89.4 ft (27.2 m) 119.2 ft (36.3 m) 140.3 ft (42.8 m) 158.5 ft (48.3 m) 175.7 ft (53.6 m) 1000 ft (304.8 m) 29.8 ft (9.1 m) 59.6 ft (18.2 m) 89.4 ft (27.2 m) 119.2 ft (36.3 m) 149.0 ft (45.4 m) 178.8 ft (54.5 m) 208.5 ft (63.6 m)
Formula for figure is:
Length of downstream recirculation zone is Bsmall(0.67) × Blarge(0.33)where Bsmallis the
smaller of height and width or length and Blargeis the larger of the two (from ASHRAE, 2001).
Where Blargeis > 8 Bsmall, use Blarge= 8 Bsmall
84
Table A2 Volume Necessary to Achieve Throw Off Edge of Building and Recirculation Zone, cfm and L/s
Assume stack is 10 ft (3.0 m) high and fan exit velocity is 3000 fpm (15.2 m/s) with 15 mph
Distance to Edge of Building and Recirc. Zone Feet to throw horizontally Meters to throw horizontally Flow needed, cfm Flow needed, L/s 75 22.9 1,267 598.0 100 30.5 5,068 2392.0 150 45.7 20,272 9567.3 200 61.0 45,612 21526.5 250 76.2 81,088 38269.3 300 91.4 126,699 59795.3
Second Stack Design Method— The Numerical Method
A more detailed analysis that accounts for dilution within the plume can be used if the required stack heights or flowrates are too large from the geomet- ric method. Minimum dilution can be predicted using equations from Chapter 43 of the ASHRAE 1999 Handbook – HVAC Applications. The equa- tions are not discussed in detail here. The equation numbers of most interest are equations 25 to 30 in Chapter 43. These equations apply only to intakes below stack top. The stack height used in these equations is the physical stack height only. “Effective stack height,” including the effect of plume rise, should not be used. The EPA screening dispersion model, SCREEN3, can also be used in certain situations to supplement the ASHRAE
Handbookequations.
For the example case discussed above [10 ft (3.0 m) stack, diameter = 2.06 ft (0.63 m), exit velocity =
3000 fpm (15.2 m/s), flowrate = 10,000 cfm (4719 L/s), receptor at end of wake recirculation zone 171 ft (52.1 m) away], the predicted minimum dilution from Chapter 43 is 455:1. If the diameter is increased to 3.5 ft (1.07 m) associated with a larger flow rate of 30,000 cfm (14152.4 L/s), the minimum dilution decreases to 264:1.
At first glance, the smaller flowrate stack that yields the larger dilution would seem to be preferred. However, the larger 30,000 cfm (14152.4 L/s), flowrate provides an internal dilution of 3:1 com- pared to the original 10,000 cfm (4719 L/s). When comparing the two cases, the larger flowrate case has a total dilution of 3 × 264 = 792:1, which is bet- ter than the lower flowrate case and would provide lower chemical concentrations at an air intake for a given chemical release rate. Allowable spill rate to meet the 0.05 ppm at the receptor location would be 11.2 L/m of toxic vapor. The original design with d = 2.06 ft (0.63 m) has a higher dilution Dcritof 455
volume rate of 6.4 L/m. In effect, the factor of 3 vol- ume flow increase in the stack with the fan allows about a factor of 1.75 increase in allowable spill rate.
In conceptual terms, exit velocity and volume flow rate are “equal partners” in plume rise and the resulting increase in safety through greater dilution. However, in practical terms, exit velocities can only be increased by doubling or tripling while manifold- ing or adding roof air to the stack can easily result in a 10-fold increase in dilution.
Dilution in the context of dispersion of laboratory exhaust is a deceptively difficult concept because one must account for both the dilution within the exhaust system, De, which is present at the stack
and the dilution from the stack to a downwind loca- tion, D. The concept can be simplified by normaliz- ing D by the volume flow rate through the exhaust stack, Q. By normalizing D, only the dispersion, which occurs between the exhaust stack and the downwind location, needs to be considered. The normalized value can be presented in one of two ways, either as a normalized dilution or a nor- malized concentration value. A normalized dilution value can be obtained by multiplying D by the ratio of the actual volume flow rate and a standardized volume flowrate [i.e., 1000 cfm (472 L/s) × (Qact /
Qstd)]. The result is a dilution value that is indepen-
dent of the actual volume flowrate through the exhaust stack, making it possible to compare the effectiveness of various exhaust stacks with differ- ent volume flowrates, because all of the values are referenced to the same 1000 cfm (0.47 m3/s) vol-
ume flowrate.
A normalized concentration value is obtained by applying the definitions of concentration and dilu- tion provided in the ASHRAE 1999 Handbook –
HVAC Applications,Chapter 43 [C/m = 1/ (D × Q)]. The result is a normalized concentration value that is the ratio of the concentration present at the downwind location and the mass emission rate of the emitted chemical, expressed in units of µg/m3
per g/s. This value is completely independent of the volume flowrate through the exhaust stack, and thus can be used to readily compare the effective- ness of exhaust stacks with various volume
flowrates. Another advantage of this method is that if the emission rate of a chemical is known, you can simply multiply the emission rate by the C/m value to obtain a pollutant concentration. This concentra- tion can then be compared directly with established health and odor limits.
Design Criteria
When designing stacks with the numerical method, it is necessary to have a design criterion for select- ing a stack design. Development of a dilution crite- rion can be difficult since the types and quantities of laboratory chemicals can vary significantly from laboratory to laboratory. As a starting place, it is suggested here to have the stack provide protec- tion similar to what a laboratory chemical hood would provide a worker standing at the hood. As described in this standard, a laboratory chemical hood should have an ANSI/ASHRAE 110 test per- formed by a manufacturer, and the ANSI/ASHRAE 110 rating should be AM 0.05 or lower. This rating translates to the worker being exposed to 0.05 ppm or lower of tracer gas while 4 L/min of tracer gas are being emitted from within the laboratory chem- ical hood. The same 4 L/min of tracer gas are being emitted from the laboratory chemical hood exhaust stack. The recommended design criterion is that the 0.05 ppm concentration also be the maximum concentration at the air intake. (The time constant for exposure concentrations mentioned in this stan- dard is measuring over a 10-minute span of time.) The detailed calculations are not presented here, but it can be confirmed that the 4 L/min. emission rate and an allowable air intake concentration of 0.05 ppm corresponds to a normalized concentra- tion design criterion of 750 µg/m3per g/s or a 2800:1
dilution for a 1000 cfm (472 L/s) flowrate exhaust, 280:1 for a 10,000 cfm (4719 L/s) flow rate, and a 93:1 dilution for a 30,000 cfm (14158 L/s) exhaust. These suggested design criteria is somewhat more lenient than the smaller criteria suggested in the ASHRAE 1999 Handbook – HVAC Applications, Chapter 13, which recommend that air intake con- centrations should be less than 3 ppm due to an evaporating liquid spill in a laboratory chemical hood and exhausted at a rate of 7.5 L/s. The ASHRAE criteria translate to a normalized concen- tration design criterion of 400 µg/m3 per g/s or a
5000:1 dilution for a 1000 cfm (472 L/s) flowrate exhaust. For facilities with intense chemical utiliza- tion, design criteria specific for that facility can be developed using the chemical inventory.
In the stack examples above, the 10,000 cfm (4719 L/s) case had a predicted dilution of 455:1, which meets the 280:1 criterion for a 10,000 cfm (4719 L/s) flowrate. The 30,000 cfm (14158 L/s) case had a predicted dilution of 264:1, which also meets the 93:1 criterion for this flowrate, by a larg- er margin than the 10,000 cfm (4719 L/s) stack.
Graphical Solution Referenced for the Second Stack Design Method Using the Halitsky Criteria
Two graphical solutions can be consulted that show a solution to the dilution calculations. The first is Ratcliff and Sandru (ASHRAE Transactions, 105, part 1, paper Ch-99-7-2, 1999) and the second is Petersen, Cochran, and LeCompte (to be pub- lished in 2002 ASHRAE Transactions). The solu- tions in both papers are for a Halitsky Criteria spill, 0.028 ppm, rather than the criterion derived from the ANSI/ASHRAE 110 test specification. Quite a bit of expertise is required to interpret the graphs. As an example, in the second paper, one point cal- culated and shown on the graph is that a zero height stack with a flow of 50,000 cfm (23597 L/s) and an exit velocity of 3000 fpm (15.2 m/s) would require an offset distance of 120 ft (36.6 m) to the nearest receptor site using the 0.028 ppm expo- sure limit at the receptor. These graphs were derived from Chapter 43 of ASHRAE 1999 Handbook – Applicationsequations for critical wind speeds and dilutions. Zero-height stacks are quite
common because stacks that end below parapet walls, below the height of adjacent penthouses, or that end below adjacent screen walls or screens will act as a zero-height stack. Receptor sites would include operable doors and windows, and any location where pedestrian access was allowed as well as to outside air intakes.
Third Stack Design Method— Physical Modeling Using the Wind Tunnel or Water Flume
If the stack heights determined from the first two methods described above are undesirable or if the geometry or topography of the building site makes simple analysis methods unreliable, a scale model of the building and surroundings should be physi- cally modeled in an atmospheric wind tunnel or water flume. Physical modeling provides more accurate, and typically less conservative, predic- tions than the numerical or geometric methods. Physical modeling is the safest method to choose stack heights in new buildings or in buildings being retrofitted. It more accurately accounts for complex building geometries, taller nearby buildings, hills, architectural screens, and several stacks placed closely together. Physical modeling should follow the guidelines given in the ASHRAE 2001 Handbook – Fundamentals, Chapter 16. Dilution criteria are still necessary to evaluate the results of physical modeling. The design criteria discussed