Industrial Ventilation for Fume Hood Systems

March 1, 2001
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A laboratory fume hood has just minimal openings for the operator and maintenance


The exhaust system may be VAV or CAV.
Energy managers in manufacturing and laboratory applications often overlook the energy-saving potential of industrial ventilation. While human safety and comfort are the primary design concerns, managers must also ensure that the work process is supported and that exhaust systems be non-polluting and energy efficient.

System Design: Fresh Air and Supply Ventilation

Industrial fresh air intakes must be located to avoid drawing in hazardous chemicals or products coming from either the laboratory or manufacturing building itself or from other structures and devices.

The supply air quantity is determined by room size, physical arrangement of the conditioned space, occupancy, and type of exhaust system. Air may be supplied at high-, medium-, or low-pressures (generally the lower the better) through ceiling diffusers, wall grilles, or registers, or perforated ceiling panels. Perforated plate or ceiling diffusers are designed for large volumes, and, when properly applied, they allow more hoods in the space because the diffusion is better.

Air may be supplied through single- or dual-duct constant air volume (CAV) or variable air volume (VAV) systems. The systems may consist of individual or central air handling units. When considering a VAV system, the design and air balance must not change room pressure in a way that affects the performance of the exhaust hoods.

Individual Ventilation Air-Handling Units

Each individual ventilation air-handling unit consists of a fan and air treatment apparatus, such as filters and heating and cooling coils. Each unit should have adequate capacity to maintain conditioned space temperatures and balance the exhaust air requirements.

Individual air handling units are primarily used for isolated or separate industrial or laboratory spaces where hours of operation may be irregular. Individual units can be shut down when their operation is not required without affecting other units and other spaces. Systems comprising individual units can be designed to match exhaust fan capacity exactly because it is easier to balance to exhaust requirements.

Individual air-handling units can be used to supplement central systems in areas where the central system would be overloaded, when heat gains are high and subject to variation, or when the exhaust quantities are greater than the supply air requirements for cooling. Individual units provide more control and more flexibility than central units, but they take up more space, cost more, and have greater maintenance costs than a central system of the same capacity.

Central Ventilation Air-Handling Units

The central ventilation air-handling unit consists of one or more fans and air treatment apparatus, such as filters and heating and cooling coil. The simplest central air-handling unit is a constant air volume system in which ventilation air is conditioned to a dry bulb temperature to maintain conditioned space temperature and humidity within an acceptable range.

Central units are economical, especially if close tolerances on humidity are not required, internal conditioned space heat loads are moderate and fairly constant, exhaust air quantities are constant and in balance with the supply, and the hours of operation or occupancy are approximately the same for each conditioned space. However, central units may allow less control and provide less flexibility than individual units.

VAV and Constant Air Volume Ventilation

Variable air volume (VAV) systems use less energy compared to constant air volume (CAV) systems. VAV systems are very flexible: supply air can be added or subtracted without affecting the rest of the system. VAV systems can be very complex, making it difficult to balance and control air. CAV systems, on the other hand, are relatively easy to control and balance, but since air volume is constant so is the energy required to operate the fans.

Industrial Hoods: Basic Design Principles

The basic goal of industrial hood design is to enclose the industrial process as completely as possible, allowing only enough access for the user and for maintenance. When complete enclosure is not practical, the hood should be designed to accommodate the work process while remaining as closed as possible. The hood should be located close to the work process to minimize air volume.

The access openings of well-designed and built hoods minimum are located away from the natural path of contaminant travel to eliminate or minimize air motion around the work process. The hood should also be positioned so contaminants are removed away from the user.

Velocity

Proper hood design includes components to provide necessary air velocity. Velocity is the speed of the air through the various exhaust components, given in feet per minute (fpm). Capture velocity is the air velocity at any point in front of the hood or at the hood opening necessary to overcome opposing air currents and to capture contaminated air at that point by causing it to flow into the hood.

Velocity Range

Lower velocities are acceptable when minimal room air currents are present. Lower velocities are also acceptable when there are other conditions that are favorable to capture contaminants or when the contaminants are of low toxicity. Additionally, lower velocities are used when the work process is intermittent or there is a low production of contaminants and the hood is large with a large air volume.

Higher velocity ranges are needed for high velocity room air currents or for other conditions that are unfavorable to the capture of contaminants. Higher velocities are required when contaminants have a high toxicity or when the work process has a high production of contaminants or the hood is small.

The exhaust system design must also account for duct velocity, face velocity, slot velocity, and plenum velocity. Duct velocity is the air velocity through the duct. Minimum duct velocity is the minimum air velocity required to move particles or contaminants through the duct. Face velocity is the air velocity at the hood opening. Slot velocity is the air velocity through the openings in a slotted hood, and plenum velocity is the air velocity in the plenum.

Volume

Air volume through the fume hood and exhaust system is measured in cubic feet per minute (cfm). The volume of exhaust air necessary to safely remove contaminants from the work process must be adequate but should not be excessive. A high volume of air may increase velocities to an extent that the effectiveness and safety of the hood is compromised. Increased air volume increases horsepower requirements by as much as the cube of the volume. Therefore, the hood should be as close to the work as possible, as the volume of the exhaust air varies with the square of the distance from the process. For example, a work process requiring 1,000 cfm of exhaust air would actually need 4,000 cfm of exhaust air if the process were twice the distance from the hood.

Ductwork

The types of effluents or waste materials that will be in the hood and exhaust ducts and discharged into the atmosphere must be known when designing ductwork.

The highest possible dry bulb and dew point temperatures must be known to prevent condensation. The ambient temperature of the space in which the ductwork is installed is also important because temperature affects the condensation of the vapors in the exhaust system, and condensation can cause corrosion of ductwork metals. Consideration must also be given to the length and arrangement of duct runs. The longer the duct the longer the exposure to effluents and, therefore, more condensation. When condensation is likely, sloped ductwork and condensate drains should be provided. Condensate drains that may accumulate hazardous materials must be given special consideration. Exhaust ductwork should be of adequate strength and construction to accommodate the type of waste material flowing through the duct and the air pressures generated by the fan system. Proper design does not use flexible duct connectors in hidden spaces or with corrosive materials.

Stacks

Common sense and good design directs all exhaust discharge away from any present or future air intake. Stacks should be placed on the highest roof of the building so that exhausts are discharged above the building envelope and not on the side of the building. As with ductwork, exhaust stacks must also be constructed with adequate strength for the effluent in the system. Before discharging exhaust air, filters, collectors, scrubbers, or other means required by local code are used to reduce contaminants. When architectural enclosures are used, the stack extends over the enclosure and above any other obstruction. Follow local building codes for stack height requirements. Rain caps should not be used on exhaust stacks, as they tend to deflect air downward, increasing the chances that contaminated air would lay on the roof and circulate into the building. Additionally, rain caps have high friction losses and may actually provide less rain protection than a properly designed stack head.

Fans

Fans are selected to meet requirements for exhaust volume, fan pressure, fire hazard, explosion hazard, and resistance to corrosion. Fans carrying corrosive materials are lined with or constructed of corrosive-resistant materials. Fans carrying combustible materials, must be explosion proof and have wheels and housings constructed of nonferrous or non-sparking materials. Motors mounted in the air stream must also be explosion proof.

Constant volume exhaust fans are usually belt-driven, which allows easy changes of fan speed. Labeling fans to identify the exhaust system served and to indicate correct fan rotation supports effective maintenance. When flammable, combustible, or other harmful materials or substances are conveyed, fans are placed outside of buildings.

Fans operated intermittently require special consideration. Airflow may become unbalanced and cause unsafe conditions and longer periods of wetness due to condensation. A time delay can be specified to allow wet surfaces to dry before the fan shuts off. Fans should be accessible and easily inspected, decontaminated, repaired, or replaced. Components such as belts, bearings, and shaft seals should also be easily accessible.

Exhaust Systems

Exhaust systems include individual, central, combination, constant air volume, and variable air volume types.

Individual Exhaust Systems

Individual fume hood systems are used in selective applications requiring special exhaust filtration, special duct or fan construction for corrosive elements, or to exhaust fumes that have very hazardous elements. Each fume hood has its own exhaust connection, duct, and fan. Individual units are increasingly used only on single-story buildings when only a few hoods are needed. Normally, the exhaust fan is always on and is interlocked electrically with the supply fan so that when the exhaust goes off the supply fan does too.

When installed with constant air volume fans, individual fume hood exhaust designs provide a stable and easily controlled system that is simple to air balance. Simply starting or stopping the fan motor operates each hood, and the operation of any one hood does not affect any other hood. Since each hood has its own ductwork, exhausted air from fume hoods does not mix and shutdowns for repairs or maintenance are localized.

Individual fume hood exhaust systems are inexpensive for small systems having only a few fans. However, if the system is large, the initial investment and the operating costs are high because of the greater number of fans, roof penetrations, and controls, and the more extensive ductwork and wiring that must be installed and maintained.

Central Exhaust Systems

Facilities that use many fume hoods usually have a central exhaust system. A central system might have a primary fan, a standby fan, a common suction plenum, and branch connections to multiple exhaust terminals. Grouping exhaust devices by type, proximity, fire pressurization, or contamination zones minimizes cost.

Compared to an individual system of equivalent size, a central exhaust system requires a smaller initial investment and has a lower operating cost. The air is more diluted before being exhausted into the atmosphere. The central system has a standby fan for safety and provides greater flexibility for future expansion.

Central exhaust systems are more difficult to air balance, and they require more periodic re-balancing to ensure proper airflow. Air balancing of central systems is more difficult when there are various types of exhaust devices installed on common duct runs.

CAV vs. VAV

VAV exhaust systems are more energy-efficient than constant air volume (CAV) systems. VAV systems are very flexible and can easily exhaust from different types of devices such as fume hoods and safety cabinets. Since the air flow and pressure in a VAV system changes constantly, the different pressure losses associated with each type of device are not a problem as is the case with CAV systems. VAV exhaust systems can become very complex and very difficult to air balance and control.

Laboratory Fume Hood Systems

A laboratory fume hood is a ventilated, box-like structure enclosing a workspace. Laboratory fume hoods capture, contain, and exhaust contaminated fumes, vapors, and particulate matter generated inside the enclosure. The basic laboratory fume hood is usually mounted on a bench or table and has two side panels, a front area which is open or partly open, a back panel, a top panel, an exhaust plenum with a baffle, and an exhaust collar. Laboratory fume hoods are made of various materials, such as epoxy-coated steel, stainless steel, fiberglass, epoxy resin, polypropylene, and PVC. Some older hoods were made of transite-a cement and asbestos material that is no longer used.

The front of the hood is called the face and is usually equipped with a movable, transparent sash. Sashes may be vertically moving only or a combination sash that has horizontally sliding panels set in a vertically moving sash. For either type of sash, the vertical sash is put in the full up position for easier setup or removal of laboratory apparatus in the hood. For normal hood use, other than setup or removal of apparatus, the vertical sash is closed when someone is not using the hood and is only opened high enough to allow for proper operation. With the combination sash, the vertical sash is closed completely and the horizontal sash is opened only wide enough for proper operation.

The laboratory fume hood has a baffle across its back, which helps control the pattern of air moving into and through the fume hood. The baffle is usually adjustable so airflow through the hood can be directed up for lighter-than-air fumes, or down for heavier-than-air fumes. The baffle is normally built so it is impossible, by adjustment, to restrict airflow through the fume hood by more than 20%.

The top panel of the fume hood has an exhaust collar to connect the exhaust duct to the fume hood. The exhaust duct may have a manual or automatic damper for control of the total volume of air through the hood. Total volume of air may also be controlled by changing the speed of the exhaust fan or by moving volume dampers at the exhaust fan.

Most fume hoods have an airfoil, called a deflector vane, at the entrance to the work surface. The design of the vane smoothes the flow of air across the work surface and deflects it to the lower baffle opening. The vane also functions as a standoff to help keep the user away from the hood face. When a deflector vane is installed, there is a fixed opening between the work surface and the vane so that when the vertical sash is fully closed and the exhaust fan is on, there is still airflow into the hood.

Laboratory Fume Hood Operating Principles

The supply air handling equipment supplies the laboratory space with filtered, conditioned air for temperature and humidity control. There is enough outside air in the conditioned air to meet ventilation code requirements and maintain proper space pressurization. The supply air system may be either CAV or VAV. The exhaust system may be CAV, VAV, or a combination.

When the fume hood exhaust fan is operating, conditioned air from the laboratory space is brought into the hood to contain and exhaust the contaminants generated inside the hood. The contaminants are ducted to the outside where they are released to the atmosphere. The exhausted air must be replaced entirely by conditioned air from the supply system to maintain laboratory temperature and pressurization.

There are two basic classifications of laboratory fume hoods: conventional and bypass. The airflow through the conventional fume hood in a CAV exhaust system is really variable both in total air volume and face velocity. The conventional hood has a movable, vertical or combination horizontal and vertical sash. In the full open vertical position of the sash, the free area of the hood face is generally about 10 to 13 square feet. The volume of air exhausted is calculated using: Q = A x V, where Q is air volume in cfm; A is face area in square feet; and V is face velocity in fpm. Therefore, a Class B laboratory fume hood with a minimum average face velocity of 100 fpm would exhaust 1,000 to 1,300 cfm of air through the hood.

As the sash is lowered on a conventional fume hood in a CAV system, the face area is reduced and the velocity of the air through the face opening increases to maintain a constant air volume. However, at some point in the closing of the sash the total volume of air also reduces with the increase in face velocity. Closing the sash on a conventional hood in a constant air volume system disrupts the airflow pattern and results in high velocities and unwanted turbulence at the hood face. The air turbulence can induce contaminants out of the fume hood into the laboratory space.

Airflow through a bypass fume hood in a CAV exhaust system is constant in total air volume but has a variable face velocity. The standard bypass hood has a movable vertical or combination horizontal and vertical sash. The construction of the bypass hood is similar to the standard conventional hood described before with the addition of the bypass. The bypass provides a constant volume of airflow through the fume hood as the sash is closed.

As the sash is pulled down, air volume through the hood face is reduced. However, simultaneously, as the sash is closing, the bypass is opening, and more air is drawn through the bypass. This keeps the total airflow through the hood constant. With this design the hood face velocities stay constant. This is an improvement over the standard CAV in conventional fume hoods.

The airflow through a conventional fume hood in a VAV exhaust system varies in total air volume but has a constant face velocity. The conventional hood has a movable vertical or combination horizontal and vertical sash. This type of hood is also equipped with special controls to allow the volume of exhaust air to vary while still maintaining a constant velocity across the hood face.

As the sash is lowered on a conventional fume hood in a VAV system, the face area is reduced. The face velocity begins to increase to maintain constant volume. However, in the VAV the hood functions differently than the standard CAV hood. As the velocity of the air through the face increases, a controller in the VAV hood senses the rise in velocity and sends a signal to an air valve or to the fan to decrease air volume through the hood to maintain a pre-selected face velocity. When the sash is raised, the controller senses a face velocity below the set point and sends a signal to increase air volume to maintain the correct face velocity.

VAV hood and exhaust system maintain constant face velocities and reduce the volume of conditioned air exhausted compared to CAV as the sash is closed, which can result in considerable energy savings.

Laboratory Exhaust System Design

Laboratories where hazardous materials are handled must be maintained at an air pressure that is negative to the corridors and adjacent non-laboratory areas. Care must be exercised in the selection and placement of supply air diffusers, grilles, or registers to avoid air patterns that would adversely affect the performance of the fume hoods. Laboratory fume hood face velocities are designed to have sufficient velocity to prevent the escape of contaminants. Good design practice equips laboratory fume hoods with visual and audible alarms to warn users of unsafe airflows.

Higher face velocities do not necessarily result in improved user protection. In fact, with higher face velocities the eddy currents in the hood become greater. These eddy currents can drag the contaminants in the hood back into the face of the user. Higher face velocities also mean greater volume of supply air and increased energy usage.

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