Laboratory ventilation is an indispensable component of laboratory design. To prevent laboratory personnel from inhaling or ingesting toxic, pathogenic, or toxicologically unknown chemical substances and organisms, the laboratory must have good ventilation to safeguard the physical and mental health of laboratory personnel. To prevent the absorption of certain vapors, gases, and particles (smoke, soot, dust, and aerosols), pollutants must be removed by fume hoods, exhaust hoods, or local ventilation. The fume hood is one of the most important pieces of safety equipment in the laboratory: it is an effective means of local protection, keeping users and other laboratory personnel away from chemical reagents and other hazardous substances, and it is fire- and explosion-proof.

I. Fume Hoods

1. Structure

A fume hood has an upper-lower structure: the top has an exhaust opening where a fan can be installed. The upper cabinet contains a baffle (airfoil deflector), a circuit-controlled touch switch, power outlets, etc.; the viewing sash is made of tempered glass and can move left/right or up/down for operation. The lower cabinet is in the style of a laboratory side bench, with a worktop on top and the cabinet body below. A small cup sink and faucet can be installed on the worktop. See Fig. 7-39.

Fume hoods are made of materials such as all-steel, steel-wood, all-wood, aluminum-wood, plastic-steel, and PVC construction; the worktop, which directly contacts experimental samples, is usually made of epoxy resin board, solid compact laminate (physical-chemical board), stainless steel, PVC, ceramic, and other materials. See Fig. 7-40.

Old-style fume hoods were mostly made of wood, but modern ones are mostly made of epoxy-coated steel. Common cabinet widths are 1200 mm, 1500 mm, 1800 mm, and 2000 mm, with a depth of 700–900 mm and a height of 1900–2400 mm. They can be used by 1–3 people simultaneously. Fume hoods are generally installed against a wall.

In the middle of the front is a vertically movable transparent sash (usually glass) with an opening height of generally 100–550 mm. Behind the sash is the working surface on which experiments are performed, with connections needed for experiments such as water pipes, drains, power, vacuum pumps, and gas lines, and shielded lighting above. Air is drawn from the upper front of the cabinet interior by the exhaust fan, then either ducted elsewhere (called full exhaust ventilation) or filtered and recirculated (called ductless ventilation).

Most fume hoods sound an alarm when the exhaust volume is too low or the front glass sash is opened too far, alerting the operator. Both the internal exhaust fan speed and the light have adjustable switches.

During use, the operator stands or sits in front of the cabinet, keeps the glass sash as low as possible, and reaches into the cabinet under the sash to perform the experiment. Because the exhaust fan draws air inward through the open sash, under normal conditions hazardous gases do not escape in quantity.

[Figure: Fig. 7-39 Fume hood structure (sectional structural drawing; dimensions: width 1800 mm, depth 800 mm, upper cabinet height 1500 mm, total height 2350 mm, base cabinet height 850 mm; components: 250 mm diameter exhaust port, sink and faucet, worktop of 8 mm porcelain-white PP board, touch switch, cabinet body of 8 mm porcelain-white PP board, transparent viewing window of 5 mm tempered glass, access door, baffle, outlets, blue PP handles, PP spill containment lip, blue PP hinges, 4 leveling feet at the cabinet base; note: the fume hood contains a fluorescent lamp with a protective shield)]

[Figure: Fig. 7-40 All-steel fume hood (photograph of the actual product)]

2. Types

2.1 Top-extraction fume hood: characterized by simple structure and easy fabrication; suitable for applications that generate heat.

2.2 Slot (slotted) fume hood: exhaust slots are provided at the top and rear side; good results are obtained in a variety of situations, but the structure is more complex and fabrication more troublesome.

2.3 Bypass fume hood: ideal when the laboratory needs to exhaust large volumes of indoor air through the fume hood, because the room's air exchange is unaffected even when the sash is fully closed.

2.4 Auxiliary-air (make-up air) fume hood: ideal for laboratories with air-conditioning systems or clean laboratories, as it both saves energy and does not affect the room's airflow organization.

2.5 Natural-draft fume hood: advantages—consumes no electricity; can ventilate continuously day and night (measured indoor air exchange can reach 6 air changes/h), benefiting room ventilation; no noise or vibration; with no mechanical equipment, easy to maintain; simple construction and low cost. However, its use is limited by certain conditions: it should not be used for highly toxic experiments or experiments that do not generate heat, and in some rooms it is unsuitable in summer.

2.6 Mobile fume hood: in modern laboratory buildings, a general-purpose laboratory (laboratory hall) is sometimes configured in which the laboratory workbenches, sink-integrated fume hoods, and other equipment can be moved at any time and pushed into an adjacent storage room when not in use. Such fume hoods should be made of wood, plastic, or light metal for ease of movement. See Fig. 7-41.

[Figure: Fig. 7-41 Mobile fume hoods (photographs of two mobile fume hoods)]

3. Classification

3.1 By Exhaust Method

Divided into three categories: top exhaust, bottom exhaust, and combined top-and-bottom exhaust. To ensure uniform air velocity in the work zone, bottom exhaust should be used for cold-process fume hoods and top exhaust for hot-process fume hoods; for processes with unstable heat generation, exhaust openings can be provided at both top and bottom, with the ratio of upper and lower exhaust volumes adjusted as the heat output inside the cabinet varies, thereby achieving uniform air velocity.

3.2 By Air Intake Method

Full-exhaust fume hood: indoor air enters, circulates in the cabinet, and is then exhausted outdoors—called full-exhaust type; this is a very widely used type.

Auxiliary-air (make-up air) fume hood: when a fume hood is installed in a heated or temperature/humidity-controlled room, to save heating and air-conditioning energy, make-up air is taken from outdoors, circulated through the cabinet, and then exhausted outdoors—called the auxiliary-air fume hood.

Variable air volume (VAV) fume hood: an ordinary constant air volume system requires manually adjusting the damper with fixed blades to regulate the fume hood's exhaust volume; when the damper is adjusted to a certain angle, the desired face velocity is achieved. VAV control changes the air volume via a sensor on the damper to achieve the set face velocity. Standard systems are of course low-cost, while VAV costs more and is suitable for applications requiring high precision.

3.3 By Usage Configuration

Divided into integral type with open lower section, floor-standing type, two-sided type, three-sided glass type, bench-top type, and combined (modular) type, as well as special-purpose fume hoods designed for radioactive experiments, synthesis experiments, and perchloric acid experiments according to different experimental needs.

3.4 By Whether External Ductwork Is Connected

Divided into ducted (external-exhaust) fume hoods and ductless fume hoods. A ducted fume hood extracts the toxic and hazardous gases generated during experiments directly outdoors through external ducts. A ductless fume hood adsorbs the toxic and hazardous gases generated during experiments directly with an activated carbon filter. See Fig. 7-42.

[Figure: Fig. 7-42 Ductless fume hoods (photographs of two fume hoods)]

3.5 Main Functions of a Fume Hood

The most important function of a fume hood is its exhaust function. In a chemistry laboratory, experimental operations generate various harmful gases, odors, moisture, and flammable, explosive, and corrosive substances. To protect users' safety and prevent experimental pollutants from spreading into the laboratory, a fume hood must be used near the pollution source. In the past, fume hoods were used in small numbers, only for experiments with especially hazardous and dangerous gases and those generating large amounts of heat; the fume hood only served an auxiliary function to the laboratory bench. With the aim of improving the experimental environment, experiments previously performed on open benches have gradually moved into fume hoods, which requires the fume hood to have functions best suited to the equipment used. In particular, most newly built laboratories require air conditioning, so the number of fume hoods must be included in the air-conditioning system plan at the preliminary building design stage. Because the fume hood occupies a very important position in the biochemical laboratory, its usage count has increased dramatically in view of improving the laboratory environment, improving occupational hygiene conditions, and raising work efficiency. Along with this, ventilation ducts, piping, wiring, and exhaust have all become important topics in laboratory construction.

The primary purpose of using a fume hood is to exhaust the hazardous gases generated in experiments and protect the health of laboratory personnel—that is, to provide a high degree of safety and superior operability. This requires the fume hood to have the following functions:

Release function: there should be a mechanism that dilutes the hazardous gases generated inside the fume hood by drawing in air from outside the cabinet and exhausts them outdoors.

Anti-backflow function: it should prevent the airflow generated by the exhaust fan from carrying hazardous gases from inside the fume hood back into the room. To ensure this function, the best method is to connect one fume hood to one exhaust fan with a single dedicated duct; where a single-duct connection is not possible, only hoods on the same floor and in the same room

may be connected in parallel, and the exhaust fan should be installed at the end of the duct (or on the roof) wherever possible.

Isolation function: in front of the fume hood there should be a non-slipping glass viewing window separating the inside and outside of the hood.

Make-up (replenishment) function: there should be a channel or alternative device that draws air from outside the fume hood while hazardous gases are being exhausted.

Face velocity control function: to prevent hazardous gases from escaping the fume hood, a certain intake velocity is required. The factors determining the fume hood's intake velocity are the heat generated by the experimental content and its relationship to the air change rate; the main factors are the experimental content and the nature of the hazardous substances. National standards stipulate a fume hood face velocity of 0.5 m/s ± 10%. To ensure this velocity, the exhaust fan must have the necessary static pressure—i.e., the frictional resistance of air passing through the ventilation ducts. When determining the velocity, noise must also be considered: air velocity in the duct should be limited to 7–10 m/s; above 10 m/s noise is generated. The usual noise limit for laboratories (indoor background noise level) is 70 dBA; increasing the duct cross-sectional area lowers the velocity and hence the noise. Considering duct cost and construction issues, the power of the ducts and exhaust fan must be chosen carefully.

Heat, acid, and alkali corrosion resistance: some fume hoods need to house electric furnaces, and some experiments generate large quantities of highly corrosive toxic and hazardous gases such as acids and alkalis. The fume hood's worktop, liner, side panels, and the selected water and gas fittings must all be corrosion-resistant. Where strong acids such as sulfuric acid, nitric acid, and hydrofluoric acid are used in the semiconductor industry or in corrosive experiments, the entire fume hood material must be acid- and alkali-proof, usually made of stainless steel, PVC, or PP.

4. Safety

Safety is the foremost mission pursued by a fume hood; using a fume hood in the laboratory is all about ensuring the user's safety and preventing contamination of the surrounding environment.

4.1 A slotted exhaust structure effectively exhausts hazardous gases.

4.2 A streamlined handle with a gap between the handle and the glass; because the airflow over the worktop is rotational, effective air intake is ensured.

4.3 A sash anti-fall pin: if the steel cable breaks and the glass sash falls unexpectedly, the anti-fall pin catches it, preventing injury to personnel.

4.4 The glass sash is made of tempered glass; even if the glass is accidentally broken or shatters, no injury accident will occur.

4.5 Ventilation openings are provided at the top of the fume hood, so that air can enter even when the glass sash is closed, avoiding excessive negative pressure.

4.6 The fume hood shell is made of metal—fireproof and non-combustible. The interior liner uses flame-retardant or non-combustible materials such as compact laminate or stainless steel, and the worktop uses solid compact laminate or stainless steel for acid/alkali resistance, heat resistance, and flame retardancy.

4.7 The effective height of the glass sash is 800 mm, the interior cavity is 1200 mm, and the worktop height is 800 mm—ergonomic, convenient to operate, with ample usable space and improved safety.

4.8 The water and gas fittings have remote operating handles outside the cabinet, which is more convenient and safer than direct hand operation.

5. Precautions

5.1 Before starting an experiment, confirm that the fume hood is in operation before performing experimental operations.

5.2 Before ending an experiment, the hood must continue running for at least 5 more minutes before the fan is switched off, to exhaust residual gases in the duct. Installing an exhaust time-delay relay may also be considered to ensure delayed fan operation.

5.3 During experiments, do not place any equipment within 150 mm of the glass sash; large experimental equipment needs ample space and must not obstruct airflow; the front sash should be kept as closed as possible during use.

II. Articulated Extraction Arms (Universal Exhaust Hoods)

The articulated extraction arm is mainly a device for rapidly exhausting small-scale dust, hot air, volatile gases, hazardous gases, and other small particulate suspended matter or gases generated during experiments or production processes; it can completely extract all hazardous gases and is one of the indispensable pieces of safety equipment in the laboratory.

[Figure: Fig. 7-43 Single-joint articulated extraction arm]

[Figure: Fig. 7-44 Double-joint articulated extraction arm]

[Figure: Fig. 7-45 Triple-joint articulated extraction arm]

[Figure: Fig. 7-46 Wall-mounted extraction arm]