DIAGNOSTIC TESTING USING ASHRAE 110

Gerhard Knutson, Ph.D., CIHKnutson Ventilation, Inc.

952-928-0195

ASHARE 110A Method of TestingThree components

Face velocity measurementsSmoke visualizationTracer gas testing

“Owner” sets acceptance levelsThe problem

What do you do when the results are unacceptable?

General Approach

Operational ProblemsTracer gas leakageReentry

Hood problemsAerodynamicsOperations

Room Conditions Other Considerations

Gas Delivery System Leakage

SymptomsSteady increaseNo reduction when gas is turned off

AnalysisUse meter as a leak meter

SolutionStop leakPlace cylinder inside hood

Cylinder Inside the Hood

Reentry

Tracer gas released inside the hoodCharacteristics

Delayed onsetTracer gas in the roomOften slow decay

Potential Reentry

Room Concentration

Repeat with Different Wind

Smoke Visualization

ASHRAE 110 Failure – Smoke spills outDiagnostic testing

Lazy airReverse air flowInterior roll

Reverse FlowReverse flow

Lazy FlowLazy Flow

Good Sweeping Action

Good sweeping flow

Smoke Showing the Hood Roll

Roll in top of hood

Summary

Poor flow characteristic can “pass” but can contribute to poor performanceRepeat somke tests with mannequin

Vortex formed by mannequin interacts with the weak airflow in the hoodCan show spillage where not seen without the mannequin

Equipment in the Hood

Other Issues

Equipment in hoodSash Position

Mandates other than left, right, centerDifference between use and design

Heat sources or disturbances in hoodWork positionOperator position

Room Air Supply

The supply ventilation design is as important (or more important than) the exhaust ventilation designRule of thumb:

Room air currents in the laboratory near the hood should be less than half, preferably less than one third, of the face velocity

Room Air Currents

Side ViewFront View

Top View

Left Center Right

High

Midpoint

Low

VerticalDraft

PerpendicularDraft

HorizontalDraft

Cross Draft Grid & Probe Orientation

Vertical

Horizontal

Perpendicular

Stand

Minor Room Air Currents

Hood 628, Position A, 52.5 inches

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Time, Seconds

Velo

city

, fpm Vertical

Perpendicular

Horizontal

Significant Room Air Currents

Hood 628, Position AB, 52.5 inches

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Time, seconds

Velo

city

, fpm Vertical

Perpendicular

Horizontal

Case History

New 12-foot bench top hoodsAcceptance level AI 0.05Preliminary tests not acceptableDiagnostic testing

Reverse flow at work surfaceHigh Turbulence in laboratoryHigh Cross drafts near hoodTemperature variable in laboratory

Original Design

Approach

Change diffusers to reduce room air turbulenceModify airfoil on laboratory hoodsModify room temperature controlRecalibrate CV supply and exhaust boxesRewire part of Building Management System – Readout matched boxes

Diffuser Replacement

Original Diffuser and Hood

Hood Rating 0.45

Change Diffuser

Hood Rating 0.03

Changed Diffuser – Modified Airfoil

Hood Rating <0.01

Conclusions

In many instances ASHRAE 110 can be used diagnostically to determine the probable cause of poor hood performance

Other Test

Dynamic testsHuman as a mannequinArtificial clutterWalk by testsLarge apparatus (walk in hoods)Floor mounted fans

Philosophy

Provides additional information relative to robustnessOften no correlation to safetyInterpretation is difficult

Caution

Hoods are designed for specific strengthSome hood manufacturers modifies the hood to provide strength in the top of the hood

Reduce rollAirfoil that tends to “lift” the air up

Some manufacturers provide a strong baseExceptional turning action by airfoilPiston flow in lower portion of the hood

Many manufactures design for general flow

Manufacture Recommended Tests

Higher release rate is better controlled by a hood strong at the topA lower diffuser is better controlled by a hood strong along the work surfaceBoxes and induced flow may work bit the geometry may be criticalSome tests can be self serving

Conclusion

Special tests are useful and provide additional information concerning the robustness of the hoodOften suggested by user but have no comparative basisHood manufactures recommend tests that favor their hoods