Selected Highlights of the Labs21 2008 Annual Conference

Side-by-Side Evaluation of High Performance Fume Hoods for the University of Texas

Kevin Fox, P.E., CEM, LEED, Jacobs Engineering Group, Bernard Bhatti, P.E., Project Management and Construction Services, The University of Texas at Austin

Introduction

Laboratory buildings are among the most demanding energy users of all facilities on a University campus because of the large quantities of air that require conditioning for ventilation and exhaust makeup. Typically, the size and quantity of fume hoods installed in a laboratory determines the amount of energy used by the facility. The use of high performance fume hoods, those specifically designed for superior fume-capture performance at reduced exhaust airflow rates, offers potentially significant energy savings while ensuring a safe and healthy laboratory environment. 

The High Performance Fume Hood Evaluation program reviewed and tested commercially available high performance chemical fume hood technologies for potential use at the University of Texas (UT). The program was conducted to help determine an optimal fume hood design standard for use in the new Experimental Sciences Building, to be located in the heart of the Austin campus. The new Experimental Sciences building is being designed to Labs21 performance guidelines for energy efficiency and sustainability. More than 100 new fume hoods are currently programmed for the facility. Furthermore, high performance hoods deemed acceptable by this program will be considered for replacement and retrofit opportunities to improve the energy efficiency and exhaust system capacity of existing laboratory buildings on campus. Currently, there are over 1,000 fume hoods installed on the Austin campus. 

Background

The main purpose of a chemical fume hood is to contain toxic and/or odorous materials generated within the hood to keep exposure to laboratory hood operators below established health hazard exposure guidelines. Consequently, testing fume hood containment performance is a very important safety issue. It has long been recognized that many factors affect the hood's ability to contain fumes, including inward airflow (commonly measured by face velocity through the sash opening), hood design, room airflow patterns and user activities. In the past, visual smoke observations and face velocity airflow measurements were used as primary indicators of hood performance. Recent studies have indicated that face velocity measurement in and of itself may not adequately predict hood performance and containment. ANSI Z9.5-1992, Laboratory Ventilation, imparts to the owner responsibility to determine internal standards, which define safe and satisfactory fume hood operation and performance. Many organizations and lab user groups, including the University, have adopted a standard fume hood performance criterion of 100 feet per minute face velocity through an 18 inch sash opening as a means of defining a properly functioning fume hood.  Numerous other standards exist, but 100 feet per minute (FPM) has generally been adopted as an unofficial, arbitrary standard of hood performance by the industry. The goal of this program was to determine acceptability for use of high performance fume hoods to avoid the energy and performance penalties associated with the prescriptive use of arbitrarily high hood face velocities.

For the purposes of the UT Evaluation Program, a high performance fume hood was defined as one designed to offer superior fume-capture performance using ASHRAE 110 testing guidelines with a face velocity less than or equal to 60 feet per minute with a fully opened sash.

Hood Selection

A survey of commercially available high performance fume hoods was conducted to allow the university to evaluate available fume hood options.  A matrix was created to document commercially available high performance fume hoods and their physical and operating characteristics.  In order to narrow the focus to hoods deemed worthy of further performance testing, a screening evaluation was conducted to select only those hoods that best fit the university's functional and operational criteria. The screening criteria consisted of the following parameters:

  • Fume hood depth not exceeding 35 inches for ergonomics and to ensure user safety.
  • Advertised face velocity and total CFM less than or equal to 60 FPM with a fully opened sash.
  • Static pressure requirements minimized for energy performance.
  • Maintenance requirements (e.g., evaluation of moving parts, serviceable equipment).
  • Multiple available hood widths to meet a wide variety of end-use applications.

When the University's screening criteria was applied to the matrix of available manufacturers, three fume hoods were identified as candidates and were selected for further containment performance testing. The three selected manufacturers each furnished the University with a standard production model, five foot long, high performance fume hood. As a basis of comparison to the three high-performance fume hoods being tested, a standard 100 FPM fume hood was added to the test group to evaluate “standard” containment performance for comparison to the three high performance hoods.

Containment Testing

The ASHRAE 110-1995 testing standard includes three parts: face velocity measurements across the sash opening, local and large volume smoke tests, and tracer gas containment testing. Containment testing consists of inserting an ejector emitting four liters per minute of sulfur hexafluoride (SF6) tracer gas inside a hood with a fully open sash. A mannequin equipped with a tracer gas detector in its breathing zone is placed in front of an empty fume hood three inches behind the sash opening, with the breathing zone located 26 inches above the hood work surface. The test is repeated with the tracer gas ejector located in three different sections of the hood to evaluate containment performance across the width of the sash opening. ASHRAE 110 testing also includes sash movement effect (SME) and space pressure effect (SPE) trials, measuring containment performance in response to sash cycling and lab door opening and closing, respectively. 

ASHRAE 110 is the standard performance testing methodology for chemical fume hoods, but it does have limited effectiveness in testing dynamic hood operation. Further, it does not specify a pass/fail threshold for hood containment performance. ANSI Z9.5 establishes acceptable fume hood performance at a threshold detection level (at the breathing zone detector of the mannequin) of 0.05 parts per million (ppm) concentration of tracer gas when released at a rate of four liters per minute for an “As Manufactured” (AM) condition, and 0.10 ppm concentration for an “As Installed” (AI) condition. 

The containment performance evaluation for this program included standard ASHRAE 110 testing, but in light of ANSI Z9.5 guidance for acceptable levels of containment, two additional custom testing procedures were developed, which are geared toward evaluating effective containment performance in a variety of normal and abusive user operating conditions. These custom-developed hood testing procedures included a modified ASHRAE 110 test, which introduced dynamic challenges including cross drafts, varying mannequin heights and an increased tracer gas flowrate, as well as a dynamic Human as Mannequin (HAM) test. Highlights of these custom containment testing criteria procedures are outlined below. 

All containment tests were conducted with a fully opened sash to simulate the worst-case potential “sash-abuse” operating condition. The three high performance hoods were each balanced to an exhaust airflow yielding a face velocity of 60 feet per minute with a fully opened sash. The “standard” hood was tested at 100 feet per minute face velocity.

Modified ASHRAE 110 Testing

The Modified 110 testing followed the same general format of the standard 110 test protocol, with the following deviations:

  • SF6 Tracer gas flowrate increased to eight liters per minute, twice that of the standard 110 test, intended to simulate a chemical boiling experiment.
  • Two mannequin heights used, with breathing zones at 18 inches and 26 inches above the hood work surface, to simulate different user heights.
  • Introduction of cross drafts across the face of the fume hood to simulate room air disturbances from walking traffic and operation of the air distribution system. Each of the modified tests was conducted with 30- and 75-foot-per-minute cross drafts.
  • SME test, where the sash was cycled open and closed in two minute intervals. Each SME test was conducted at both mannequin heights with the two different cross drafts.
  • SPE test, where the door to the room was cycled open and closed in two minute intervals. Each SPE test was conducted at both mannequin heights with the two different cross drafts.

Human as Mannequin (HAM) Testing

Human as Mannequin testing was conducted to evaluate the containment performance of the fume hood when subjected to dynamic movement and manipulation of standard lab apparatus within and around the hood. HAM testing consists of a series of choreographed movements that were executed using highly structured steps and sequences to demonstrate the comparative performance of the fume hood containment technologies. HAM testing sought to simulate actual laboratory operating conditions in a controlled and repeatable environment. HAM testing is not addressed by the ASHRAE 110 Method or the ANSI Z9.5 Ventilation Standard, and there are no industry standards for HAM dynamic tests, with no established pass or fail threshold values. For the purposes of this evaluation program, regimented dynamic fume hood user operations were prescribed for use as adapted from previous testing procedures developed by the Lawrence Berkeley National Laboratory. The HAM test procedures consisted of the following lab user operations:

  • Four liter per minute SF6 tracer gas flowrate.
  • No cross drafts.
  • Three ejector positions—right, center, and left—on hood work surface.
  • Breathing zone tracer gas detector on human tester.
  • Lab apparatus loaded in hood.
  • Choreographed repeated movements, including hand insertion and removal, moving apparatus front to back, moving apparatus right to left, and removing an apparatus from the hood.

All three containment test procedures were conducted without a rigid pass/fail criterion.  Rather, each hood was evaluated in terms of its performance relative to the AI containment performance threshold for comparison with the performance of the other fume hoods in the program.  The goal of this testing methodology was to provide comparative observation of hood containment performance.

Fume hood containment performance testing was conducted at the Center for Energy and Environmental Resources on the UT Austin J.J. Pickle Research Campus, and facilitated by ENV Services of San Antonio under the direction of the University with third party review and observation provided by Jacobs.  Tracer gas samples were recorded in 10 second intervals throughout the duration of the test and collected by data acquisition hardware.  Each test was recorded by videotape.

Containment Performance

Comparison of hood containment performance using the standard ASHRAE 110 testing protocol revealed virtually no differences between any of the fume hoods.  All containment performance was below the 0.10 ppm AI threshold. Figure 1 displays a side by side comparison of containment performance using the standard 110 test. For testing and confidentiality purposes, the three high performance hoods have been identified as A, B, and C. 

Figure 1. Standard ASHRAE 110 Test Results

Figure 1. Standard ASHRAE 110 Test Results

When the modified tests were conducted for different cross drafts and mannequin heights, significant differences in containment performance between hoods was observed. Figure 2 shows a comparison of hood performance with a 30-foot-per-minute cross draft and a mannequin breathing zone height set at 18 inches above the hood work surface.

Figure 2. Modified ASHRAE 110 Test Results

Figure 2. Modified ASHRAE 110 Test Results

As observed in Figure 2, hoods A and B exhibited brief excursions beyond the 0.10 ppm AI containment threshold, but hood C and the standard hood displayed considerable containment stability problems. Similar results were observed with the 30-foot-per-minute cross draft with the taller mannequin height, although the performance was generally less erratic. At the taller mannequin height, the breathing zone is in close proximity to the bottom of the sash, whereas the lower mannequin height breathing zone is near the middle of the open sash field. It was concluded that the containment performance in the hoods is generally more stable closer to the sash and higher above the work surface. This demonstrates the importance of proper sash management for all modes of fume hood usage, particularly for hood users shorter than 67 inches.

When the containment tests were conducted with the 75-foot-per-minute cross draft, containment performance was significantly diminished. The purpose of the testing at this high face velocity (two-and-a-half times in excess of that recommended by ASHRAE guidelines for an acceptable laboratory design) was to evaluate fume hood performance at an abusive operation condition. As might be expected, when the cross draft past the hood opening exceeds the exhaust flowrate into the hood, unpredictable results can occur. Operation with such excessive face velocities represents a severely compromised fume hood installation environment, and should be prevented at all times. (Nonetheless, one overall observation from the 75-foot-per-minute cross draft tests was that one hood, A, performed consistently better than the others.)

HAM testing results, conducted with the lower tracer gas flowrate and with no cross drafts, exhibited much more stable containment performance among all the hoods. Figure 3 shows the containment performance comparisons between hoods for the case where lab apparatus and objects were moved from the front to the back within the hood. Generally, all hoods performed well below the 0.10 ppm AI containment threshold for each of the different HAM tests.

Figure 3. HAM Test Results—Object Move Front to Back

Figure 3. HAM Test Results—Object Move Front to Back

Table 1 shows the containment performance results of all hoods for all the tests conducted. For the SME and SPE tests, the recorded AI containment readings were peak recorded values across the range of the test; all other readings were averaged between the left, center and right ejector positions, consistent with ASHRAE 110 recording protocol. Values in red indicate containment excursions in excess of the 0.10 ppm AI threshold.


Table 1. Fume Hood Containment Test Summary

AS INSTALLED READINGS (AI)†

TEST

CONDITION

CROSS DRAFT, FPM

MANNEQUIN HEIGHT, IN

HOOD A

HOOD B

HOOD C

STD HOOD

D

STANDARD

STANDARD

0

26

0.00

0.00

0.00

0.00

STANDARD

SME

0

26

0.04

0.03

0.00

1.25

MODIFIED

CROSS DRAFT

30

18

0.04

0.09

0.69

0.36

MODIFIED

CROSS DRAFT

30

26

0.03

0.00

0.03

0.11

MODIFIED

CROSS DRAFT

75

18

0.10

1.65

0.29

1.62

MODIFIED

CROSS DRAFT

75

26

0.06

0.28

0.34

0.07

MODIFIED

SME

30

18

0.12

0.10

3.03

0.00

MODIFIED

SME

30

26

0.05

0.35

0.01

0.00

MODIFIED

SME

75

18

0.05

3.93

0.29

0.99

MODIFIED

SME

75

26

0.13

0.04

0.00

1.67

MODIFIED

SPE

30

18

0.99

1.97

3.92

0.46

MODIFIED

SPE

30

26

0.08

0.36

0.00

0.01

MODIFIED

SPE

75

18

0.18

3.93

0.15

0.12

MODIFIED

SPE

75

26

0.09

0.73

2.85

0.08

HAM

HAND INSERT

0

28

0.01

0.00

0.00

0.00

HAM

MOVE F TO B

0

28

0.00

0.02

0.01

0.01

HAM

MOVE L TO R

0

28

0.00

0.00

0.00

0.00

HAM

WATER POUR

0

28

0.01

0.01

0.00

0.01

HAM

OBJ REMOVAL

0

28

0.03

0.00

0.06

0.07

OVERALL AVERAGE

0.11

0.71

0.61

0.36

AS INSTALLED AVERAGE

0.03

0.21

0.14

0.23

MAXIMUM RECORDED PPM

0.99

3.93

3.92

1.67

† For SME and SPE, AI condition represents peak tracer gas reading

Conclusions

The overall conclusions drawn from the High Performance Fume Hood evaluation program include:

  • One hood, A, exhibited a clear performance advantage across the range of all containment tests conducted.
  • The containment performance of the standard 100 foot per minute hood was generally inferior to that of the newer hood technologies.
  • Cross drafts pose a significant challenge to hood containment performance, and must be minimized and eliminated in a lab environment when possible.
  • Not all high performance hoods are equal.
  • Standard ASHRAE testing may not predict performance in actual lab operating conditions.
  • The lower 18 inch breathing zone height proved more challenging for hood containment performance.
  • High performance fume hoods can operate safely at face velocities less than 100 FPM.
  • Face velocity in and of itself is not an adequate indicator of safety.

It is important to note that while the high performance hoods used in this evaluation were designed for operation at 60 feet per minute with a fully opened sash, normal operation is recommended with the sash opened no higher than 18 inches. Since constant volume fume hoods are normally used, they must be balanced to ensure safe operation at the worst case “sash abuse” condition, which is fully opened. An 18 inch sash opening yields a net operating face velocity of around 80 feet per minute. While the containment performance was generally noted to be superior to that of the standard hood, it must be realized that the total exhaust volumes are not significantly lower than those of a standard hood. Care must be exercised not to assume a 40 percent exhaust flowrate reduction for a high performance hood that is advertised for a 60 FPM face velocity. In general, for large labs with one or few fume hoods, the total volume of air used in the lab will be driven mainly by the prescriptive air change rate designed for the lab rather than the fume hood exhaust volume itself. Reductions in hood exhaust volumes may not serve to greatly decrease lab energy use if the room ventilation rates are excessively high. In general, it is recommended that high performance fume hoods be evaluated in conjunction with lab room air change rates to maximize the energy efficiency of the laboratory.


Biographies

Kevin Fox is the Director of the Energy and Power Solutions Group with Jacobs Engineering in Fort Worth, Texas. He has been responsible for the design, construction administration and project management of mechanical systems for a wide range of projects, including central steam and chilled water utility plants, power plant cooling tower installations, campus utility infrastructure master plans, biotechnology and research laboratories, educational facilities, student dormitories and commercial HVAC systems.  Mr. Fox is a Professional Engineer in Texas, Oklahoma and Missouri, a Certified Energy Manager (CEM) and a LEED® Accredited Professional.

Bernard Bhatti leads the Project Support Engineering and Design Services for the University of Texas at Austin Project Management and Construction Services department. This multi-disciplined group provides mechanical, electrical, structural, and civil engineering services and offers design assistance for special architectural, interior design and selection projects, and CADD support for the entire department. Mr. Bhatti holds a Master's degree in Mechanical Engineering from the University of Oklahoma, and is a registered professional engineer in Texas, Arizona, and Oklahoma. He is also a member of the Advisory Board of the Industrial Energy Technology Conference.