Optmizing Fume Hood Efficiency

JERRY KOENIGSBERG

Principal
GPR Planners/Jacobs Engineering
www.gprplanners.com

2900 Westchester Ave.
Purchase, NY 10603
Tel # 914/ 253-6744
Fax # 914/ 2536836
E-mail: #jerryk186@aol.com

Introduction

In our role as laboratory designer and planner we are constantly challenged to develop unique equipment solutions to resolve difficult problems. No problem has received more attention than the perceived notion than the laboratory fume hood is an energy hog that must be tamed. In the past we responded by developing the HOPEC fume hood design that utilizes sash limit to reduce hood exhaust demand by 50%. Today, in light of continued growth in the size and number of hoods being purchased, clients have challenged us to push the energy efficiency envelope even further. This article will describe the results of that effort.

Background

The HOPEC IV fume hood is the fourth generation of a hood design that was originally introduced in the late 1970's during the oil crisis in response to client demand for an energy efficient hood. Up to that time, the traditional method of determining a hood's exhaust demand was to measure the area of its fully opened sash multiplied by the speed that the air entered the chamber (face velocity)- typically 100 feet per minute (FPM). In applying this approach, the average 6-ft hood with a 13 square foot face opening and operating with a 100-FPM face velocity would consume +/- 1300 cubic feet of conditioned air per minute (CFM). This could equate to as much as a 5 tons of air conditioning capacity, depending on local weather conditions. The original HOPEC solution simply rationalized that by lowering the vertical sash to the half opened position and establishing it as the face opening criteria while maintaining the 100-FPM face velocity; one could cut their hood's exhaust capacity and related energy consumption in half (650 CFM in lieu of the traditional 1300). Today, almost 50% of all fume hoods sold in the United States use some form of sash limit as a means of minimizing energy consumption.

The Evolution of HOPEC

In the ensuing years, the HOPEC hood design continued to evolve in response to demands to make the hood more user friendly and improve performance. Today, the latest version HOPEC IV incorporates the following characteristics: chamber depth increased to 30" to allow for large pieces of equipment to be housed behind the fully closed sash, a trough/airfoil assembly located at the front of the counter top to contain spills, sash height increased from 30" to 36" high to allow for visual access to the upper region of the chamber regardless of the counter top height and finally, a combination vertical/horizontal sash mechanism was added to allow the sash panels to be used as full body shield. However, in spite of all the physical changes to the hood structure in the last twenty years, the recommended 100 feet per minute face velocity has remained constant.

The Great Face Velocity Debate

The issue of what constitutes a proper face velocity criteria, the recommended speed that room air enters the hood chamber through its sash, has probably been the subject of great debate ever since the first electrical fan was installed on a fume hood. Up to that time, hoods relied merely on the suction generated by the chimney effect of their exhaust stacks to evacuated heat, smoke and vapors just like their fireplace cousins.

In the last forty years various professional organizations have offered face velocity recommendations ranging from 60-150 feet per minute. In fact, there was a time that many supported raising the face velocity even higher as a means of increasing user safety. This "more is better" approach was eventually debunked when investigators determined that air passing by the hood operator's torso created counter flows similar to a rock in a stream. This counter-flow or turbulence was observed to increase dramatically in relation to the increase in face velocity and with it, the potential for increased exposure to hazardous chemical vapors.

With the issuance of its final rule in 1990, 29CFR Part 1910: "Occupational Exposures to Hazardous Chemicals in Laboratories", OSHA refrained from recommending a specific fume hood face velocity, opting for a range of 60-100 FPM. In spite of this recommendation, most have and continue to select 100 FPM because it is "SAFE". Unfortunately, in this case "safe", refers to the avoidance of potential liability rather than the science.

Evaluating Face Velocity Options

In the last two years, a number of clients have asked us to study how best to optimize their face velocity criteria in order to increase energy savings and lower operating costs. Others have expressed concern that if they bought into the HOPEC sash limit approach, their people would simply raise the sash to full open position and possibly negate any level of containment as the face velocity drops off. Therefore our challenge was two-fold. First, determine the minimum face velocity required to maintain the generally accepted .05 ppm hood performance criteria that is typically achievable at 100 FPM. Second, to determine the impact on hood performance when the combination sash is raised to the full open position and the face velocity falls off to +/- 60 FPM.

In order to accomplish this task quickly and with minimum exposure to our clients, we enlisted the aid of the fume hood community. We requested that each build a HOPEC IV combination sash hood prototype and make arrangements to have it tested at their facilities in accordance with the ASHRAE-1995 hood test protocol. Of the six firms contacted, four agreed to participate.

The test protocol was broke down into two distinct components. The first effort dealt with determining optimum face velocity. To accomplish this task, the hood was tested at three sash positions, horizontal, half-open vertical, full open vertical. Each sash position was ASHRAE tested with five different face velocities: 100,90,80,70,60 FPM and the minimum hood performance standard was established at 0.05 ppm. While we were confident that we could have gotten acceptable results at even lower velocities, we purposely limited our effort to 60 FPM since it is currently the lowest value recommended by OSHA and other government and professional organizations.

The second phase of the test evaluated the impact on hood performance when raising the combination sash to the full open position. The test began by establishing the hood's performance with its horizontal panels fully opened and the face velocity set at 100 FPM. Once the initial test was completed and the results tabulated, the test was replicated at the half-open and full open sash position while keeping the hood's total exhaust constant. This meant that the face velocity declined in relation to the increased open area of the sash. At each sash position face velocity readings were taken and tabulated.

Test Results

The test results were quite revealing yet not unexpected. In the first phase, all hoods exceeded the commonly used .05 ppm criteria regardless of sash position or face velocity. In the second phase, the face velocity at the full open sash position fell off to +/- 60 FPM and yet the hood still produced test results in excess of the .05 ppm. These results appear to support the notion that face velocity alone is not a good predictor of hood performance.

Economic Impact Analysis

The following chart illustrates the potential economic benefits of incorporating sash limit into your hood design be it vertical or combination approach when compared to a fully opened fume hood. In addition, it should be obvious that additional significant savings can be realized by combining sash limit with diminished face velocity.

Fume Hood
Sash Position

Face Velocity
(feet/Minute)

Exhaust Demand
(Cubic Feet/Minute)

Air Conditioning Load
Based on:
(200 cfm = 1 Ton)

HOPEC Hood: Combination Sash Horizontal Open

100

650

3.25

HOPEC Hood: Combination Sash Horizontal Open

90

585

2.92

HOPEC Hood: Combination Sash Horizontal Open

80

520

2.60

HOPEC Hood: Combination Sash Horizontal Open

70

455

2.28

HOPEC Hood: Combination Sash Horizontal Open

60

390

1.95

HOPEC Hood:
Vertical Sash:

Half Open

100

650

3.25

HOPEC Hood:
Vertical Sash:

Half Open

90

585

2.92

HOPEC Hood:
Vertical Sash:

Half Open

80

520

2.60

HOPEC Hood: Vertical Sash:
Half Open

70

455

2.28

HOPEC Hood: Vertical Sash:
Half Open

60

390

1.95

Vertical Sash:
Full Open

100

1300

6.50

Vertical Sash:
Full Open

90

1170

5.85

Vertical Sash:
Full Open

80

1040

5.20

Vertical Sash:
Full Open

70

910

4.55

Vertical Sash:
Full Open

60

780

3.90

Conclusion

As a result of these tests, it is fair to assume that a properly designed fume hood, regardless of manufacturer, is capable of producing extremely high levels of vapor containment, as defined by the ASHRAE test protocol; even at lower face velocities when the sash is fully opened.

All results are consistent with earlier research that pointed to the positive impact on hood containment by increasing the hood chamber depth from the traditional 25" to 30". Again this is consistent with classical fireplace design which stipulates a direct relationship between the size of the hood face and the chamber depth.

One word of caution is appropriate. By merely attaining outstanding ASHRAE test results of +/- 0.5 PPM or any other arbitrary value, it should not give license to the well intentioned or the huckster to declare their hood as "Safe". It is this author's opinion that in light of the many potential hazards associated with hood operations that go way beyond mere vapor exposure (fire, flying debris, etc) no hood is truly safe to operate with the sash fully opened regardless of face velocity. The key to optimizing personal safety is to use the hood's sash to shield the body as well as the breathing zone and not rely merely on airflow.


"Reprinted by special permission from R&D Magazine's Laboratory Design
Newsletter. Copyright 2000 Cahners Business Information. All rights
reserved."

Lowering a fume hood's face velocity criteria without compromising performance