A COMPARISON OF FUME CUPBOARD EXHAUST PLUMES USING CFD

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1 A COMPARISON OF FUME CUPBOARD EXHAUST PLUMES USING CFD MICHAEL BONSHEK AECOM NICKI PARKER, M.AIRAH Norman, Disney & Young ABSTRACT This paper investigates the plume height and dispersion effectiveness of mixed flow impeller exhaust systems alongside traditional individual and manifolded high-level stacks for fume cupboards. This investigation was triggered following updates to AS2243.8:2014 which now permit the use of manifolded fume cupboard exhaust systems that are not hazardous when combined. This simplifies laboratory design and reduces construction costs by minimising the number of individual fume cupboard exhaust ducts reticulated to building roofs. Computational fluid dynamics (CFD) analysis is compared with empirical methods such as the ASHRAE graphical method and Briggs equation for varying wind speeds on a simple building. Several typical installation location scenarios are analysed along with the effects of wind velocity on plume height and dispersion. The simulations are performed using ANSYS CFX based on a k-epsilon turbulence model. Findings include presenting evidence on the importance of designers needing to account for co-locating discharges and consider the effects of high velocities on plume dispersion during the design process. INTRODUCTION The fundamental purpose for using fume cupboards within laboratory environments is to provide safe working conditions for users and other laboratory personnel (Standards Association of Australia, 2014). Fumes contained in a cupboard under negative pressure are transported to an outdoor discharge where contaminants can be safely dispersed away from occupied spaces and outside air inlets to acceptable concentrations. There are a number of options for discharging contaminants that are discussed under AS2243.8:2014, with corresponding recommendations provided for roof mounted stack heights and discharge velocities. Alternative solutions can be used which typically have a considerably higher discharge velocity with lower stack heights which mix and/or entrain air at the discharge point to provide further dilution. These systems, in theory, result in similar plume heights to traditional fume cupboard systems without the large stack heights that are required on taller buildings. The effectiveness of such systems can be demonstrated during the building design process using tools such as computational fluid dynamics (CFD) and industry accepted empirical models, with resulting contaminant concentrations compared against more traditional fume cupboard exhaust systems. 1. SYSTEM DESCRIPTION This study is a result of a currently proposed design for a new building where mixed flow impeller fans are proposed for a circa 10-storey building in an urban environment to minimise stack heights 1

2 and simplify fume cupboard ductwork. The inputs have been generalised for this study to allow high level conclusions to be drawn. Limitations of this are discussed in the relevant sections. Best practice design guides for laboratory exhaust systems discuss a number of benefits in manifolding fume cupboards, with substantial energy and capital cost savings as well as increased fume dilution, enhanced personnel safety, augmented redundancy, improved design flexibility and potential energy recovery listed (Bell, 2007). With almost a 50% reduction in energy consumption possible when compared to individual stacks running at constant volume, manifolded systems should be considered wherever possible during design phases. Figure 1. Mixed Flow Impeller Fan System Flow Distribution Mixed flow impeller fan systems operate in a similar manor to manifolded systems, however discharge velocities are higher and additional air is entrained at the stack exit, increasing the total mass flow rate exiting the stack. The typical proportion of entrained air to fume cupboard exhaust air is shown in Figure 1. The implied benefit of this is that shorter stack heights can be used to achieve the same effective plume height, which is particularly advantageous for taller buildings. 1.1 Stack design For this comparative study, three different fume cupboard stack arrangements are simulated, as shown in Figure 2 and dimensions detailed in Appendix A. 1. A traditional system with a single stack and fan per fume cupboard; 2. A manifolded system with one lead stack and one backup stack 1 ; 3. A mixed flow impeller fan system with one backup stack1. 1 Backup stacks are a requirement under AS2243.8:2014 to ensure fume cupboards still operate during maintenance or a fan failure. Backup stacks are represented as geometry only. The traditional system have no backup stacks and all are modelled as outlets. 2

3 3. Mixed flow impeller fan fume discharge 1. Traditional fume discharge 2. Manifolded fume discharge Figure 2. Geometry used for each stack arrangement The building has 25 fume cupboards distributed throughout the building. The traditional fume discharge system has individual fans and ducting for each of the 25 fume cupboards. The manifolded and mixed flow impeller system have the 25 fume cupboards linked to one fume exhaust fan and duct, with another fan installed as backup in the same duct system within the building. Physical dimensions for the mixed flow impeller fan units have been based on manufacturers data, with the stack discharge height being approximately 5m above roof level. Whilst AS requires that the minimum discharge height is 3m above both the roof at the point of penetration, as well as above any access walkway, it is also recommending that the point of discharge should be above the aerodynamic wake of the building, which typically extends to at least 125% of the building height above the ground (Standards Association of Australia, 2014). Based on this recommendation, the comparison traditional and manifolded stack systems have stack heights of 11.25m above roof level, which essentially provides a Deemed to Satisfy approach summarises the inputs for each of the stack arrangements simulated. 3

4 Parameter Single fan per fume cupboard (Traditional) Manifolded fan system Mixed flow impeller fan system Comments Stack height above 11.25m 11.25m 5m roof level Number of outlets Manifolded and mixed flow fans have 1 duty and 1 standby stack Diameter of stack 0.24m 1.20m 0.80m outlet Fan outlet velocity 10.4m/s 10.4m/s 23.3m/s Total fume cupboard 11.73m³/s 11.73m³/s 11.73m³/s discharge Entrained air flow (peak operation) Nil Nil 4.1m³/s Fume discharge temperature 24 C Typical indoor dry bulb design temperature Ambient air temperature 30 C Typical warm day Building geometry and stack location Table 1. Summary of inputs used for the stacks The aerodynamic wake of a building is highly dependent on the geometry of the building envelope and the surrounding buildings, as well as the wind speed and direction. For simplicity, a simple building has been created, with footprint measuring 45m x 45m, and a height of 45m to remove any dependency on local building features which will disrupt the plume dispersion. Figure 2 shows the three different locations for each stack arrangement that have been simulated; stacks located close to the leading edge of the roof, stacks place in the centre of the roof, and stacks located towards the trailing edge of the roof. This is because the height of the building wake on the roof is different for each location, and has the potential to impact the plume dispersion characteristics. Each stack arrangement has an obstruction at roof level to represent associated equipment, plenums or plantrooms required to operate the fans. This is not expected to have a significant impact on the flow fields but is incorporated to simulate a realistic arrangement. 2 Analysis of the sensitivity of the plumes to the ambient temperature was performed for a low reference temperature of 4 C and was found to not be significant. 4

5 B Building Centre A. Leading Edge C. Trailing Edge Wind Direction 1.3 Computational domain and mesh setup Figure 3. Stack locations used The building was placed in the centre of a 1,530m x 720m x 450m (length x width x height) domain as shown in Figure 5. The width and height of the domain is 16 and 10 times that of the building s respective dimensions to ensure the boundary does not adversely affect the flow field around the building. The length of the domain is 34 times the building length (17 times upwind and 17 times downwind) to allow sufficient wake regions to form depending on the simulated wind speed. These dimensions exceed the recommendations for virtual wind tunnels as quoted in multiple best practice guidelines used in the computational wind engineering community (Franke, et al., 2004). Surrounding buildings have not been included in this model as depending on the building being design, local geometrical features will differ considerably, and will therefore impact the flow field and wind microclimate in very different ways. Site specific models therefore need to be created when considering local plume dispersion in practice. This is a limitation of this exercise, however indicative conclusions drawn are still valid. A maximum cell size of 25m has been used in this simulation, with local refinement of 0.1m used where appropriate. Appendix B provides further detail on the meshing parameters used. Figure 4. Model domain and building model geometry with discharges (only one discharge type used per model) 5

6 1.4 Wind conditions The wind inlet was modelled with a logarithmic profile, which creates a wind boundary layer profile based on the assumption that wind speed increases with height from the ground. Appropriate roughness heights were used following the recommendations made by Blocken et al. (Blocken, Stathopoulos, & Carmeliet, 2007). The wind outlet was modelled as a zero-pressure outlet to prevent airflow back into the domain. Three wind velocities were selected for analysis; 1m/s, 3m/s and 7m/s, all measured at 10m above ground level as per typical weather station data. A range of speeds were selected to assess how each stack arrangement performed with varying wind speeds. A still condition was not analysed as in this scenario, plumes (in general, depending on stability class) will project vertically upwards from the stack before dispersing in the atmospheric boundary layer. Further, higher concentrations around roof level are likely to be experienced when there is the presence of some local wind flows. 1.5 Other model inputs and assumptions The approximation of the discharge from the fan outlets will impact the dispersion of the contaminant plumes. A uniform discharge velocity across the outlet of the stack has been assumed. To capture the inherent turbulence in the air flows around the building, a steady state k- epsilon turbulence model has been used with ANSYS CFX physics definitions. This allows a reasonable level of accuracy, whilst being able to generate results within an acceptable time frame. This analysis is restricted to contaminants that do not pose a risk when combined in a manifolded fan system, nor for fumes that could result in hazards such as fire or explosion. Convergence criteria for these models was an RMS residual below 1e-4 as well as stable velocities measured at various user points within the domain, namely at low level, adjacent to the fan discharge and in the general domain. Contaminants from the fume discharge are represented as a traceable scalar contaminant at 100% concentration at each outlet that represents the mixture of the air with evaporated or gaseous chemical contaminants in the fume cupboard system. No allowance for specific dispersion characteristics has been made due to the simplified assumptions used in this paper. The impact of a variable density mixture should however be considered during individual building design. 2. EMPIRICAL PERFORMANCE Empirical approaches are traditionally used to calculate the potential plume height from a building mounted stack which allows the mechanical designer to understand the risk of contaminant entrainment and pollution concentration in occupied areas. Two of the most popular desk based approaches in fume cupboard and stack design use the Briggs Equation and the ASHRAE Graphical Method. Both of these methods have been used as a comparison for the CFD analysis and the methodologies are summarised in Appendix C. Neither method accounts for the effect of colocating discharges and is further discussed in this paper. Gaussian dispersion models can then be used to calculate contaminant concentrations based on atmospheric stability class and a large number of wind conditions, however these models can fail to capture microclimatic effects when the built form is complex (Dharmavaram & Hanna, 2007). 6

7 3. RESULTS In order to provide a robust comparison, 27 different CFD models were simulated based on the following variables and combinations thereof: Stack location on building; A) leading edge, B) building centre, C) trailing edge. Stack arrangement; 1) traditional fan system, 2) manifolded system, 3) mixed flow system. Wind speed; i) 1m/s, ii) 3m/s and iii) 7m/s. The prefix of each variable is used to easily identify and reference each case, i.e. A1 refers to a traditional system located on the leading edge of a building, B3 refers to a mixed flow system at the centre of a building, etc. The CFD models (suffix X) are compared to both the Briggs equation estimates (suffix Y) and the ASHRAE method estimate (suffix Z). The labels used in the graphs in this paper therefore correspond to this suffix approach. The plume height has been extracted from the CFD models based on the average plume height at a fixed concentration as the plume stabilises and additional height increases are minimal (see Figure 5 for an example) which aligns with the approach taken in Gaussian plume modelling (Figure 6) which have been reviewed by Stathopoulos et al (Stathopoulos, Hajra, & Bahloul, 2008). This approach replicates the original wind tunnel measurements via tracer smoke used to develop the empirical methods (Briggs, 1969). Figure 5. Example plume height measurement from CFD results. Plumes represent a scalar trace element at a concentration of 1%, 2% and 5%. Case shown is B2 iii (i.e. Central, Manifold, 7m/s) Figure 6. Gaussian plume model plume height estimates (Beychok, 2005) 7

8 Plume height above roof level (m) 3.1 Empirical plume heights Both the simple Briggs equation and ASHRAE method predict the effective height based on stack diameter, exit velocity, and wind speed, with the ASHRAE method taking in to account the position of the stack on the roof. A comparison of these is shown in Figure 7 for the central roof location (prefix B). Corresponding pairs of curves, e.g. B1Y and B1Z, B2Y and B2Z, and B3Y and B3Z correlate well with each other at wind speeds of 7m/s, however there is a weaker correlation at the lower wind speeds with the Briggs equation consistently estimating a lower plume height. This is expected as the Briggs equation does not account for contributing factors such as entrainment, plume rise and building geometry that are considered in the ASHRAE method which are more prevalent at low wind speed. Figure 8 shows the effects of discharge locations and types on plume heights based the ASHRAE method. Several observations can be made: The plume heights from manifolded and mixed flow discharges located at the leading edge of a building are lower than at the central or trailing edge location. The traditional system discharge results indicate that location has little to no effect on the plume height when utilising the ASHRAE calculation method. There is a significant difference in height between the traditional discharge case and the mixed flow and manifolded cases at lower wind speeds, even though exit velocity between the single fan case and the manifolded system is consistent. The manifolded system and high-speed fans show that plume heights are likely to be twice that of those calculated for a single fan per fume cupboard, which raises the question as to whether this method of calculation is suitable for fume cupboard stacks that are grouped together Wind speed (m/s) B1Y Traditional Briggs B2Y Manifold Briggs B3Y Mixed flow Briggs B1Z Traditional ASHRAE Middle B2Z Manifold ASHRAE Middle B3Z Mixed flow ASHRAE Middle Figure 7. Comparison of plume height for empirical models with central discharge location 8

9 Plume height above roof height (m) Wind Speed (m/s) B1Z Traditional ASHRAE Middle B2Z Manifold ASHRAE Middle B3Z Mixed flow ASHRAE Middle A1Z Traditional ASHRAE Leading A2Z Manifold ASHRAE Leading A3Z Mixed flow ASHRAE Leading C1Z Traditional ASHRAE Trailing C2Z Manifold ASHRAE Trailing C3Z Mixed flow ASHRAE Trailing Figure 8. Comparison of effects of discharge location and discharge type on plume heights based on ASHRAE method. 3.2 Traditional (single fan per fume cupboard) systems Figure 9 shows the estimated plume heights from the CFD models for the traditional fume discharge case, with the Briggs and ASHRAE results included for comparison. At lower wind speeds, a large disparity in plume heights is evident when compared to the ASHRAE empirical method (of up to +8m) and the Briggs Method (+11.5m). This variation is less at 3m/s, with the simulated plume heights varying by up to +3.8m when compared to the empirical models. At higher wind speeds, significant variation between the methods is seen with only the plume height from the stack located at the leading edge being higher at +2.1m. This variation in plume heights, in comparison to the Briggs equation and ASHRAE graphical method is attributed to the co-location of the individual plume sources in the CFD model which is not accounted for in the empirical models. This was investigated by simulating fume cupboard discharge from only 1 of the 25 outlets in the traditional discharge case located at the centre of the building, which perfectly correlated with the B1Y Traditional Briggs results shown in Figure 9. Further, upon comparison of the simulated plume heights for the traditional fume cupboard system (all 25 stacks running) and the manifolded system (Figure 10), there is very little difference between each case at all wind speeds, and for all stack locations. This demonstrates that designers must be aware of the increased plume heights that will occur when a number of stacks are located together, and not necessarily estimate the plume height based on flow from a single stack as this may be overly conservative, depending on the operation of the building. 9

10 Figure 9. Comparison of plume height for a traditional fume cupboard system Figure 10. Comparison of Traditional and Manifolded discharge plume heights 3.3 Manifolded systems Figure 11 shows the estimated plume heights from the CFD models for the manifolded fume cupboard case, with the Briggs and ASHRAE predictions included for comparison. The results from the CFD simulations indicate that, at low wind speeds, plume heights vary much less with stack location than the estimates given by empirical methods predict, however both the CFD and empirical estimates correlate much more strongly as wind speeds increase. Interestingly at low wind speeds, the ASHRAE estimate for the stack located on the leading edge is consistent with both the Briggs equation and all of the CFD simulations, especially that for the leading edge. The ASHRAE prediction of plume height as the stack moves further away from the leading edge tends to significantly over predict plume height at low wind speed, by around 20% when you reach the trailing edge of this building. 10

11 Plume Height above roof (m) Wind Speed (m/s) A2X Manifold Leading B2X Manifold Middle C2X Manifold Trailing B2Y Manifold Briggs A2Z Manifold ASHRAE Leading B2Z Manifold ASHRAE Middle C2Z Manifold ASHRAE Trailing Figure 11. Comparison of plume height for a manifolded fume cupboard system 3.4 Mixed flow impeller fan systems Figure 12 shows the estimated plume heights from the CFD models for the mixed flow impeller fan case, with the Briggs and ASHRAE predictions included for comparison. The results from the CFD models show that plume heights are generally not impacted by the stack location on the roof for this type of fan arrangement but are more driven by the wind speed. Again, the ASHRAE method estimates a much larger difference in plume height than is observed in the CFD models at low wind speeds for various stack locations. This indicates that the ASHRAE method may overemphasise the buildings effects on plume heights in some cases and may not be conservative, particularly at lower wind speeds. Furthermore, this also indicates that the Briggs empirical model is generally conservative when considering the height of plume discharges in this simplified case with no obstructions. As wind speeds increase, the plume height is likely to be consistent across the methods of calculation. 11

12 Figure 12. Comparison of plume height for a mixed flow impeller fan system 3.5 Plume contaminant concentrations and dilution Figure 13 shows contaminant plumes from a mixed flow discharge type located at the centre of the roof to visualise the effects of wind velocity on plume concentrations. Typically, the higher wind velocity cases showed the plumes diluting much more quickly but potentially results in higher concentrations of contaminant at roof level, whereas lower wind speeds produce a significantly longer plume that is projected higher in to the atmosphere. This highlights that a number of wind speeds need to be assessed to understand contaminant concentration, depending on where sensitive locations might be. Figure 13. Scale comparison of contamination plumes of 1% (dark blue/outer plume), 2% (light blue/middle plume), and 5% (green/inner plume) as wind speed increases. Case shown is B3X i, ii & iii (i.e. Central, Mixed Flow, 1, 3 & 7m/s) The percentage concentrations shown in Figure 13 are the inverse of the dilution ratio that exits the stack, i.e. a concentration of 1% means that the mixture at the exit is diluted by 99%. For the mixed flow systems, the fume cupboard exhaust has already been diluted by the entrained air, and so dilution rates are in fact even higher. 12

13 To provide a meaningful output, consider a 250mL container of hydrochloric acid 3 that spills in one of the fume cupboards and proceeds to evaporate within 1 hour, rather than being cleaned, therefore removing the risk of evaporated contaminants. Assuming that no other hydrochloric acid is spilt in the remaining 24 fume cupboards, the mixture of air and acetone that emits each of the fume cupboard systems tested is shown in Table 2. The corresponding concentration exiting the stack is also shown, as is the parts per million concentrations at 95% dilution, i.e. what would be contained within the outer plume in Figure 13. Lastly, the Time Weighted Average (TWA) from the Workplace Exposure Standards for Airborne Contaminants ( (Safe Work Australia, 18 April 2013)) is provided to demonstrate the limits that are deemed acceptable in practice. Although this estimate shows that all stack arrangemnets result in acceptable air quality, there is a significant reduction in concentration achieved using the manifolded and mixed flow system. Parameter Single stack per fume cupboard Manifolded fume cupboard Mixed flow impeller stack Concentration at stack exit (PPM) Concentration at 95% dilution (PPM) TWA (PPM) 5 (Standards Association of Australia, 2014) Table 2. Potential concentrations following a hydrogen chloride spill What we can infer is that because of the dilution of contaminants as they are mixed within a manifolded system and with fresh air when discharged, results in pollutant concentrations that are typically lower than the TWA soon after exiting the discharge. This further supports the use of the empirical methods of plume height analysis as a simple conservative design approach, but for a more accurate understanding of the entrainment of contaminants in specific cases, CFD should to be utilised where this detail of data can be approximated. 4. DISCUSSION AND IMPLICATIONS Computational fluid dynamics (CFD) analysis has been undertaken for a number of stack arrangements for fume cupboard exhausts exiting through the roof of a simplified building. This has been compared to widely accepted empirical methods for estimating plume heights above stack outlets with varying success. At wind speeds above 3m/s, the CFD results typically correlate strongly with the corresponding Briggs equation and ASHRAE graphical method estimates of plume height, especially for manifolded and mixed flow impeller fan type systems. This suggests that, for high wind speeds, empirical methods can be used confidently to estimate potential plume height from fume cupboard stacks. At lower wind speeds however, empirical methods tend to overestimate plume height in some scenarios, and underestimate plume heights in others. Most notably is the situation when a number of stacks are co-located with all stacks operating, and, in practice, plume heights more closely resemble those from a manifolded system than that of a single stack. The simulation results also indicate that, for all exhaust strategies, the location of the stacks on the roof of the building for the stack heights tested do not significantly impact the resulting height of 3 Hydrochloric acid is used as an example as it has a relatively low threshold of acceptable concentration and is commonly used in laboratories. 13

14 the plume. This is disparate from the results from the ASHRAE method which overestimated plume heights at the middle and trailing locations. For all wind speed conditions, the Briggs model results for the manifolded and mixed flow impeller fans tend to underestimate the plume height from the stack, suggesting that estimates of local contaminant concentration are conservative; however, the sensitivity of this should be explored on a site by site basis as this is a very simplified approach. CONCLUSION Analysis indicates that, should the stack discharge momentum be sufficient and constant such as in a manifolded or mixed flow impeller system, the recommendation under AS to maintain stack heights of 25% above the building height can be reviewed and optimised. Whilst empirical methods of estimating plume height have their place in some situations, such as when wind speeds are expected to be higher, they have the potential to be inaccurate at lower wind speeds, which is often when concentrations of contaminants at roof level or near adjacent buildings are higher. When this is the case, CFD or traditional wind tunnel methods are likely to give a much more complete understanding of the risk around contaminant dispersion, and assist design teams in providing a mechanical solution that reduces the likelihood of contaminant ingestion, be it by occupants or by the mechanical system itself. 14

15 REFERENCES 1. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. (2011). Chapter 45 Building Air Intake and Exhaust Design. In ASHRAE Handbook - HVAC Applications. 2. Bell, G. (2007). Manifolding Laboratory Exhaust Systems. Lawrence Berkeley National Laboratory. 3. Beychok, M. R. (2005). Fundamentals Of Stack Gas Dispersion (4th ed.). author published. 4. Blocken, B., Stathopoulos, T., & Carmeliet, J. (2007). CFD Simulation of the atmospheric boundary layer: wall function problems. Atmospheric Environment 41, Briggs, G. A. (1969). Plume Rise. In USAEC Critical Review Series. 6. Briggs, G. A. (1972). Discussion: Chimney plumes in neutral and stable surroundings. In Atmospheric Envir. 6 (pp ). 7. Dharmavaram, S., & Hanna, S. (2007). Computational Fluid Dynamics (CFD) Modelling of Toxic Gas Dispersion in the Vicinity of Complex Buildings, Structures and Topography. IChemE Symposium Series No Franke, J., Hirsch, C., Jensen, A., Krüs, H., Schatzmann, M., Westbury, P., et al. (2004). Recommendations on the use of CFD in predicting pedestrian wind environment. Proceedings of the International Conference on Urban Wind Engineering and Building Aerodynamics COST Action C Safe Work Australia. (18 April 2013). Workplace Exposure Standards for Airbourne Contaminants. Canberra: Safe Work Australia. 10. Standards Association of Australia. (2014). Australian/New Zealand Standard: AS/NZS : Stathopoulos, T., Hajra, B., & Bahloul, A. (2008). Analysistical Evaluation of Dispersion of Exhaust from Rooftop Stacks on Buildings. Montreal: Institut de recherche Robert-Sauve en sante et en securite du travail (IRSST). 15

16 APPENDIX A Figure 14 records the dimensions of each fume cupboard discharge type. They are shown as mixed flow impeller fan system, manifolded system and traditional system from left to right. Figure 14. Dimensions of each discharge type. 3. Mixed flow impeller, 2. Manifold and 1. Traditional (left to right) 16

17 APPENDIX B Mesh resolution Based on the experience of the authors, a number of mesh settings have been applied to the domain: Maximum cell size of 25m throughout the domain 5m maximum cell size on the ground boundary, with inflated prism layers to capture the resulting boundary layer 1m maximum cell size on the building, with inflated prism layers to capture the resulting boundary layer 0.1m maximum cell size applied to stacks, with proximity and curvature control to capture geometrical features. Figure 15 shows the above resolution for one of the stack arrangements. Figure 15. Example of building (top) and stack (bottom) mesh resolution of traditional stacks. 17

18 The Briggs Equation APPENDIX C Briggs developed a number of equations covering four general categories (Briggs, 1969) (Briggs, 1972): Cold jet plumes in calm ambient air conditions Cold jet plumes in windy ambient air conditions Hot, buoyant plumes in calm ambient air conditions Hot, buoyant plumes in windy ambient conditions. It follows that colder jet plumes are dominated by discharge momentum, with hot plumes dominated by buoyancy, and discharge velocity having minimal effect on the plume height. For fume cupboard stack applications, where the exhaust temperature is of a similar magnitude to ambient air conditions, Briggs cold plume formula are most relevant for use. Simplistically, the following equation can be used for isothermal plume rise, and is often quoted by manufacturers for estimating the effective plume height for their respective fan and stack arrangements: h = 3V e U h d e Where And h is the effective plume height above the stack exit (m) V e is the stack discharge velocity (m/s) U h is the cross wind velocity (m/s) d e is the effective stack diameter (m) The ASHRAE Graphical Method The method outlined in the 2011 ASHRAE Handbook (American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc, 2011) uses a formulation by Wilson based on flow visualisation studies, and estimates the size of recirculation and high turbulence zones around a building mounted stack. A more in-depth calculation methodology is used and the reader is recommended to refer to the 2011 ASHRAE Handbook for further details. Figure 16. Extract from the ASHRAE Graphical Method after Wilson. 18