All about fume hoods

Hoods used in conjunction with a heating or air conditioning system as room exhaust, should generally be located on the opposite side of the room from the inlet registers, discusses Salil Sansare, Head of Marketinng Division, Labguard

What is a laboratory fume hood?

A laboratory fume hood is a ventilated enclosure where harmful or toxic fumes of vapours can be handled safely. The purpose of the hood is to capture, contain and remove contaminants, preventing their escape into the laboratory. This is accomplished by drawing contaminants within the hood’s work area away from the operator, so that inhalation and contact are minimised.

Airflow into the hood is achieved by an exhaust blower which ‘pulls’ air from the laboratory room into and through the hood and exhaust system. This ‘pull’ at the opening of the hood is measured as face velocity. A baffle, and other aerodynamically designed components control the pattern of air moving into and through the hood. Contaminated air within the hood is then diluted with room air and exhausted through the hood’s duct system to the outside where it can be adequately dispersed at an acceptably low concentration.

Laboratory exhaust systems and types of laboratory hoods

All laboratory fume hood’s operational airflow can be described as one of two types: conventional and by-pass. Auxiliary-air and variable air volume (VAV) hoods are variations of the by-pass hood. All the types of hoods are described below:

Conventional

The conventional hood is a basic enclosure with an interior baffle and movable front sash. The conventional hood generally operates at a constant exhaust volume with the majority of exhaust air entering the hood through the sash opening.

Closing the sash increases the speed of the air through velocities are to be expected with the sash in the near closed position (figure 1).

The conventional hood is generally the least expensive, but its performance depends largely on sash position.

With the sash in the near closed position, high velocity air passing position, high velocity air passing through the sash opening can damage fragile apparatus, disturb instrumentation, slow distillation rates, cool hot plates, disperse valuable sample materials or result in turbulence inside the hood. Conventional-type hoods comprise the majority of hoods in the market.

By-pass

The by-pass hood generally operates at a constant volume and is designed so that as the sash is closed, the air entering the hood is redistributed, thereby minimising the high velocity air streams encountered in conventional hoods. The by-pass openings above and below the sash is raised or lowered (figure 2). Therefore, the face velocity in by-pass hoods does not generally reach levels which might be detrimental to lab fume hood procedures.

Auxiliary-Air

A variation of the by-pass hood, the auxiliary-air hood offers a means of providing up to 50 per cent of the air for the hood exhaust from outside the laboratory, and limits the volume of tempered air removal from the laboratory (figure 3). This hood type has many names including induced air, add-air, balanced air and make-up air.

One advantage to auxiliary air hoods is that they feed air-starved laboratories, where room supply air volume is not adequate to support a laboratory hood. Another advantage to auxiliary-air is that, when properly applied, it can provide energy saving by limiting the volume of cooled room air exhausted by the hood. The level of savings depends on the degree to which the auxiliary air must be tempered.

VAV Fume Hoods

VAV Fume hood represents higher level of fume hood safety and energy efficiency over other type of fume hoods. It can dramatically reduce the volume of air being extracted from the laboratory while still retaining the required airflow, through the open sash. Salil Enterprises’ VAV system alters the exhaust volume using a damper that opens and closes based on airflow and sash position By reducing the air volume exhausted when the sash in lowered, VAV control offers significant energy saving opportunities.

Walk-in Fume Hoods

This hood accommodates laboratory procedures that require cumbersome equipment or maximum work area. It directly mounts on floor and uses the laboratory floor as the bottom work surface. Each hood is provided with water tap and gas service fixtures, easy moving sash and appropriate illumination arrangement.

Special Application Laboratory Fume Hoods

Unique features may be added to the hood and exhaust system to accommodate special procedures in the hoods. Below are description of a few of the special purpose hoods offered:

Perchloric-Acid Hoods: Perchloric acid hoods are dedicated for use with perchloric acid only. Organic materials should not be used in a perchloric acid hood because an explosion may occur when perchloric acid reacts with organic materials. It must be constructed of relatively inert impervious materials such as type 316 stainless steel or polyvinly chloride (PVC) or ceramic-coated materials. Hoods used for these application have integral work surfaces, covered interiors and a drain for easy and through cleaning. Washdown features are required since the hood and duct system must be thoroughly rinsed after each use to prevent the accumulation of potentially reactive perchloric salts. Horizontal ducts runs and sharp turns are avoided so that washdown residue drains thoroughly. Each perchloric acid hood requires its own dedicated exhaust system with washdown capability.

Radioisotope Hoods: Hoods used for radioactive applications have integral work surfaces and covered interiors to facilitate decontamination. Liner materials such type 316 stainless steel, are impermeable to radioactive materials. Cupsinks are provided in the integral work surface. The safe disposal of radioactive effluents should be observed. These hoods are sturdy enough to support lead shielding bricks in instances where they are required. They can also be installed to facilitate the use of High Efficiency Particulate Arrester (HEPA) filter in the ductwork. The laboratory’s competent authority should determine which (if any) filters are required to trap the radioactive materials emitted during a particular applications.

Laboratory Hood Specification

Hood Size: The working space inside a laboratory hood is defined as that part of the hood interior where apparatus is set up and vapours ar generated. The working space required deter-mins the width of the hood needed. One sources recommends that 5 linear feet of hood space be provided for every two workes if they spend most of their time working with chemicals.

Laboratory hood sizes are commonly expressed by the outside width and not by working space. The most common hood widths are 4,5 and 6 feet. 4 feet models are ideal as individual work stations whereas 5 feet models are spacious enough to handle huge apparatus. 6 feet models prove to be excellent choice when greater ventilated workspace is required. Dept of all models remain constant. Custom designed fume hood may have large widths. The actual working space is approximately 3” to 10” less than the expressed width of the hood, depending upon the size.

Hood Linear Materials

The liner materials selected should be durable and resist chemicals, heat and open flame. A description of common liner materials and their characteristics follows (table 1).

The best liner materials for a hood should be determined by the applications, types and concentration of chemicals, gases, vapours, fumes or smokes and qualitatively as fume acids, alkalis, solvents or oils. Hood liners are subjected to attack from such effluents by corrosion, dissolution and melting.

Compatibility chart (table 2) is intended as a guide for selection of right liner material. Please select the liner suitable to your tasks.

Work Tops

It is the actual surface on which the reactions are carried out. It is fabricated to contained spillage and splashes. Apart from regular PP, PVC, SS 316 work surfaces; following options are also available:

Ceramic Tiles

• Most preferred option
• Highly resistant to aggressive chemicals
• Excellent stain and heat resistance

Granite

• Hard and uniform surface
• Moderately resistant to aggressive chemicals
• Excellent stain and heat resistance.

Sashes

Sashes provide physical protection from splashes and reactions and are transparent to allow viewing. Sashes rise vertically, slide horizontally or combine both: horizontal and vertical characteristics. (figure 7,8 and 9) Sash configuration selection is a matter of preference. Vertical rising sashes are the most popular and allow large apparatus to be loaded in the hood. Horizontal sliding sashes allow the operator to reach around both sides of the sash while using the sash as shield. Because the sash opening is smaller, they conserve energy by limiting the volume of air exhausted.

Service Fixtures

Utility services include connections to gases, air, water and vacuum. If service fixtures are required, they are installed to allow the connection of service supply lines on the hood itself. All service valves are accessible for maintenance. Inner those connectors are corrosion resistant. For safety and convenience, all service fixtures and remotely controlled from outside the hood and clearly identified with different colours.

Electrical Receptacles

Electrical receptacles are located on the hood exterior, away from the corrosive effects of the fumes inside the hood structure. Provisions are made so that all electrical wiring is isolated and physically separated from vapours handled within the hood.

Lighting

Light fixtures are Vapour-proof. These light fixtures are fluorescent, installed outside the hood liner and protected from the hood interior by a transparent, impact - resistant shield. Access for replacing or cleaning is from the exterior whenever required.

Centrifugal Blowers

Of all the additional components needed, the blower is the most crucial to the performance of the hoods. Fume hood installations utilizing remote blowers are the most common type. Since the entire duct length is under negative air pressure, any leakage in the duct is drawn in and contained rather than pushed out into the building environment. The exhaust blower is positioned on the building’s exterior, usually on the roof, where noise is less noticeable. By creating suction within the ductwork, blowers draw air from the laboratory room, through the hood and out the duct system. Blowers components are constructed from a variety of materials to resist corrosion from chemical fumes. Impellers are made of stainless steel or PVC or PP as per model specifications. For weather proofing, roof-mounted blowers have protective housings.

Fire Extinguishers

Hoods can be scenes of fires due to the nature of some applications. We offer fire extinguishers that mount adjacent to the hood and can be used at the emergency providing round the clock protection. ABC Dry powder extinguishers are most preferred.

Ductwork

Ductwork includes rigid and flexible pipes, couplings, elbows, reducers and weather cap. Round diameter duct offers the least static resistance. Like the liner materials of a laboratory hood, duct materials must be resistant to the fumes exhausted through it. Hence usually ductwork of similar liner material is used.

Optional Accessories

Base Cabinets

Fume hoods are designed to rest on a bench-high base stand or cabinet. Existing platform may be used as long as it provides adequate depth and height for the structural support of the hood. Special base cabinets of powder coated steel are available to store chemicals.

Base Stand

The base stand supports fume hoods. It is constructed out of corrosion resistant epoxy polyester coated steel and equipped with adjustable foot rests. Available in various sizes.

Airflow Monitor

Face velocity measurements are often used to gauge the fume hood’s ability to contain and exhaust harmful vapours. Standards written by NFPA, ANSI and eve. U.S. OSHA require specific acceptable fume hood face velocities, with local alarms to warn if face velocities drop to an unsafe level.

To ensure the safety of fume hoods, Salil Enterprises’ airflow monitors directly measure the average face velocity. Its sensor, mounted on the fume hood, detects face velocity disturbances caused by moving sash, splash shield use, or even user standing in front of the hood. If the face velocity falls outside the acceptable range, audible and visual alarms warm users of unsafe condition.

Fume scrubber

Fumes exhausted from fume hoods are usually disposed of by dilution into the atmosphere. Scrubber uses water as the scrubbing liquid and incorporates a water purification procedure. The chamber contains one or more spray nozzles. It produces water jets across the chamber, providing liquid screen and causes impact with dispersoids dropping them from the gaseous waste. The scrubber removes particles, aerosols and gases, performing most effectively with latter if they have a high solubility in water.

Planning Lab Space

The planned location of the fume food within the laboratory affects both the safety and convenience of the user and the airflow patterns of the room.

Keep these basics in mind when planning your space:

• Hoods should not be located near windows, doors or air conditioning registers because potential cross drafts will interfere with the airflow in to the hood.
• Never locate the hood adjacent to an exit or where the user would forced to work in a high traffic area.
• If a common exhaust or supply system is used for several hoods, the fume hoods should be arranged to require a minimum amount of ductwork.
• Hoods used in conjunction with a heating or air conditioning system as room exhaust, should generally be located on the opposite side of the room from the inlet registers. The air from the registers should sweep through the laboratory first, then be exhausted through the hood for both safety and HVAC efficiency. (see diagram for an example).

Basic Features

Aerodynamic design Automatic by-pass type air flow Interior of selected liner with preset baffle Exterior of powder coated steel with S. S corner covers Sink and drain arrangement with a tap Two remotely operated service fixtures for gasses Flourescent lights with vapour proof fitting Counter-balanced easy moving sash Two electrical points with blower switch and light switch on front fascia Silent blower of suitable capacity* Flow control valve to regulate airflow Fire extinguisher

Aerodynamic design Aerodynamic Design Auxillary type air flow Interior of selected liner with preset baffle Exterior of powder coated steel with S. S corner covers Sink and drain arrangement with a tap Two remotely operated service fixtures for gasses Flourescent lights with vapour proof fitting Counter-balance easy moving sash Two electrical points with blower switch and light switch on front fascia Silent blowers (2 Nos.) of suitable capacity* Flow control valve to regulate airflow Fire-extinguisher

* Blower suction capacity (CFM) and motor power (HP) vary with hood width. Our blowers are designed for maximum efficiency at minimal power consumption

E-mail:labguard@vsnl.net