Local Exhaust Ventilation

One of the most effective engineering approaches for the control of air contaminants in industry is the use of local exhaust ventilation (LEV). LEV is a ventilation system comprised of:

 

    1. hood(s),
    2. ductwork,
    3. (an) air cleaning device(s)
    4. and air moving device.
These component parts are identified in the illustration below.
Figure 6-1
 
LEV systems are designed to collect air contaminants at the source or point of generation before these air contaminants can enter the worker's breathing zone. If LEV systems are designed correctly nearly 100% of the air contaminants can be captured and removed from the work environment so that employee exposures can be kept at or below acceptable levels.

 

LEV Advantages

There are several reasons why safety professionals and industrial hygienists should consider LEV.
For example, LEV systems:
 
1. remove the air contaminant at the source before it can enter the workers' breathing zones. This control is especially useful when employees are working with highly toxic chemicals that may cause harm from short term (acute) or long term (chronic) exposures.
 
2. usually requires less volumetric air flow (Q) , measured in cubic feet of air per minute or ft3/min to control employee exposures to air contaminants . Usually less air is required to capture and remove the contaminant at the source than would be required to dilute or exhaust the contaminant after it has been released into the ambient workroom air.
 
3. can effectively control multiple types/forms of air contaminants. Gases, vapours, and particulate may be effectively removed from the workplace using this method.
 
4. operating costs are usually less with LEV than general dilution/exhaust ventilation because less air is needed to control the air contaminant. This can result in tremendous cost savings to the employer in terms of energy costs. Remember, workroom air may need to be heated in the winter and cooled in the summer. The cost of heating and/or cooling air is directly related to the volume of air that needs to be treated each day. 

LEV Disadvantages
The use of LEV to control air contaminants has some disadvantages. However, in many situations the advantages far outweigh the disadvantages.
 
Some disadvantages are that LEV systems:
 
1. must be designed following good industrial hygiene practice to control air contaminants cost effectively and efficiently. 
 
2. can require a larger capital investment up front than general ventilation systems to pay for design, construction and installation materials,
 
3. may be more complex in design than general ventilation systems,
 
4. may require a more rigorous maintenance schedule than general ventilation systems.
 

2. Air Flow and Pressure
In order to effectively evaluate LEV systems it is important to be comfortable with the manner in which LEV systems are designed. 
 

Air Flow (Principles & Pressures)

With ventilation systems designers attempt to create pressure differences to control exposures to air contaminants. In LEV systems, designers attempt to create a low pressure zone at the hood opening so that air is drawn into the hood along with the air contaminants released from a process. Furthermore, after the fan, a high pressure zone is created so that the collected air and contaminants can be discharged.
 
By applying these basic principles and understanding some additional concepts, simple or complex systems can be designed.
 
In physics most of us learned that one (1) atmosphere of pressure at standard temperature and sea level is equivalent to the pressure exerted on one square inch by a column of mercury 29.92" high. This pressure would be equivalent to a pressure of 14.7 pounds per square inch.
 
In industrial ventilation, pressure differences are described and measured in terms of inches of water (" of H2O) or inches water gauge ("wg). One atmosphere of pressure (at standard temperature, at sea level) would be equivalent then, to the pressure exerted on one square inch of surface by a column of water approximately 407 inches high. Pressures above 407" of H2O are referred to as positive pressure while those below are referred to as negative pressure conditions.
 
Like most gases, air flows from an area of high pressure to an area of low pressure. Designers of ventilation systems create high and low pressure areas in workrooms using various kinds of air moving devices. In LEV systems this is usually accomplished with a fan. Consider the following illustration.

 
Atmospheric or ambient air pressures inside the duct and outside the duct are the same with no appreciable movement of air into or out of the duct.
 
When a fan is added inside the ductwork (and turned on), the following situation could result.

 
In this situation a low pressure zone or vacuum is created ahead of the fan. If ambient atmospheric pressure (at sea level) were 29.92" of mercury (equivalently ~407" of water), the pressure in this "zone " of the duct would be less than atmospheric ambient pressure. How much less would depend upon the size and power of the fan, the duct length, diameter, material of duct construction and other parameters.
 
After the fan, the air pressure in the duct would be greater than atmospheric. That is, a high pressure zone was created when the fan is turned on and the air pressure in this region of the ductwork would be greater than 29.92" of mercury (equivalently ~ 407" of water).

 

Static Pressure (SP)

Static pressure is defined as potential pressure exerted in all directions by a fluid at rest. For a fluid in motion it is measured in a direction normal or perpendicular to the direction of flow. Furthermore, it can be visualized to some degree as the tendency to collapse or burst the ductwork. In short, depending upon where one measures static pressure, it can be 0, positive or negative. That is it can be equal to, above or below atmospheric ambient pressure, respectively.

Velocity Pressure (VP)

Velocity pressure is defined as the kinetic pressure exerted in the direction of flow necessary to cause a fluid at rest to flow at a given velocity.
 

VP is also described and measured in inches of H2O or inches water gauge. Velocity pressure is exerted in the direction of flow, is always positive (i.e. greater than atmospheric pressure) when a fluid is moving, and is measured in the direction of airflow. It is also related to velocity by the following formula:

V = 4005 ( Square Root of VP)
 
Using this formula , the velocity of air at any point in the ventilation system can be determined by taking a velocity pressure reading. 
 
It is important to understand that VP is not uniform across the duct. In short, air tends to move faster near the centre and slower near the walls of the duct. Therefore, VP measurements will vary when a series of measurements are made when conducting a duct traverse using a pitot tube. (This topic will be discussed in greater detail later under the Ventilation Testing section of this module).
 

Total Pressure (TP)

The last head component that evaluators of LEV systems must be aware of is total pressure. Total pressure (TP) is simply the algebraic sum of SP and VP. That is:

TP = SP + VP
 
TP, like SP and VP, is described or measured in terms of inches of H20 water or inches water gauge ("wg). Therefore, TP can be zero, positive or negative when compared to ambient atmospheric pressure.

The relationship between TP, VP and SP can be found in the illustrations below.

 

3. Design Considerations

When an LEV system is fitted the designer must take into consideration several important factors. These factors include:
 
    • Process & Nature of the Air Contaminant
    • Hood Design, Selection & Placement
    • Volumetric Airflow Rate (Q) Requirements
    • Capture Velocity
    • Transport Velocity
    • Other Exhaust and Supply Considerations

Process & Nature of Contaminant
Safety professionals must consider the process when designing any ventilation system. The location, size, number and design of hoods, duct work, air cleaning device(s), fan(s), and discharge stack(s) will be influenced by process parameters such as the number of emission points, available space, employee access needs, and the specific chemical/physical properties of the air contaminant(s).
 
For example, systems can be designed to remove gases/vapors, particulate, or both. The volumetric air flow rate required to collect these air contaminants and transport them through the LEV system, with adequate duct velocity, can vary considerably. Generally LEV systems require less air to control gases/vapors than heavy particulate (e.g. grinding dust).
 

Hood Placement and Design

There are many different hood designs to choose from. 
Hood placement is also an important consideration. Generally, hoods should be located as close to the point of air contaminant release as practicable to maximize air contaminant capture and removal. 
 
In general LEV hoods fall into four categories:
 

(1)Booth-type hoods -have three sides and an open face (front). Examples include spray booths and laboratory hoods.

 
 

(2) Enclosures -typically used for high toxicity and/or radioactive substances. These hoods are completely enclosed and are typically held under negative pressure so that if leakage is inward. Examples include the glove or dry box and some abrasive blasting cabinets that utilize sand containing crystalline free silica as the abrasive material.
Glove Box
 
 

(3) Receiving hoods -designed and positioned to take advantage of the contaminant release characteristics of the process or operation they serve. Examples include canopy-type hoods that extend above open surface tanks. Generally, they are less preferred than other hoods especially where the operator must reach over or into the tank. The canopy hoods can draw the contaminant through the worker's breathing zone and their capture of contaminants is readily influenced by cross-drafts. Another example is the hood provided on pedestal type grinders.
Canopy Hoods
 
 

(4) Exterior hoods -reach beyond their own boundaries to draw in and capture the contaminant before it reaches the worker's breathing zone. Slotted hoods provided at open surface tanks (e.g. electroplating tanks ) are examples.

Volumetric Airflow Rate (Q)

Within an LEV system it is important to determine the volumetric air flow rate (Q) that is needed to create a sufficient capture velocity in front of the hood. The capture velocity that is created in front of the hood entry must be sufficient to capture the contaminant and draw it into the hood and away from the worker's breathing zone.
 

Volumetric airflow rate (Q) is described in cubic feet of air per minute or ft3/min. In LEV systems it is not usually measured directly but is calculated. The formula which follows is used to make this determination:
 

Q= AV
 
Where: Q = the volumetric airflow rate in ft3/min
 

A = the cross- sectional area of the opening through which air is moving and is measured or defined in square feet or ft2
 

V = the velocity or speed or air moving past a point in the system expressed, measured, or described in feet per minute (ft/min)

 

5. Testing LEV Systems

After a LEV system is designed and installed, the system should be tested to determine how well the installed system compares with the designed system. Volumetric air flow rates (Q) , capture, face, and transport velocities should be tested as should static pressures at various locations in the system. This initial testing will serve as a baseline against which all future measurements can be compared.
 
Since most field instruments do not measure Q directly, it is necessary to determine Q by taking a series of velocity (V) measurements at some point in the system , determining an average velocity and then multiplying this average velocity by the cross sectional area (A) of the opening through which the air is moving at the location where the velocity determinations were made. Q is then determined by the formula:
 

Q= AV

where:

Q= volumetric airflow rate, cfm

V= average linear velocity, fpm

A= cross-sectional area of the duct or hood at the measurement location, ft2
 
Equipment options:
 
  • Air velocity meter to determine face or duct velocity (V)
  • Velometer to determine face or duct velocity (V)
  • Tape measure to measure (length, width) of a rectangular hood face
  • Pitot Tube and Magnehelic Gauge, U-tube manometer or inclined manometer to measure duct velocity pressure (VP)

Note: Cross-sectional area (A) of a round duct can be determined by using the formula:

A= (pi) (r2)
 

Velocity (V) Determinations

All instruments used to make velocity determinations should be handled, used, calibrated and repaired following the recommendations and directions of the manufacturer. Before using any instrument the safety professional must carefully read the manufacturer's instructions . This is to help ensure that field measurements are conducted safely and properly and all results are recorded and interpreted correctly.
 
  • Swinging Vane Anemometer
Today this instrument is less commonly used to assess air velocities (e.g. face velocity, capture velocity, and transport velocity) at spray booths, laboratory hoods, and other ventilation systems. This instrument, at one time, was popular because it was portable, rugged and provided instantaneous readings on an analog scale. The versatility of the instrument was improved by adding different probes or attachments. Diffuser and pitot probe attachments can be added to measure velocities at air supply vents/diffusers and duct velocities.
 
In most cases the velometer with its fittings was calibrated as a unit. Therefore, the fittings and attachments can not be interchanged with another instrument. With appropriate attachments swinging vane anemometers can also be used to measure static pressure at various points in the LEV system. 
 
 
  • Digital and Analog Air Velocity Meters
These light weight instruments feature a telescopic probe and a digital or analog read out display. They can be used to measure any of the velocities mentioned earlier at any point in an LEV system. The tip of the telescopic probe contains a heated ceramic sensor that cools as air passes around it. The drop in the temperature of the ceramic sensor is related to air speed or velocity which appears on the digital read out in feet per minute (ft/min).
 
Although these instruments are extremely portable, the telescopic probe can be damaged easily as can the heated ceramic sensor. Additionally, hot, caustic or dust laden air streams may damage the sensor. Furthermore, since the instrument is designed with electrical circuitry and is battery operated, safety professionals should read the manufacturers instruction manual to determine if the instrument can be used in environments where flammable or explosive gases/vapours are present.
 
  • U-tube Manometer and Inclined Manometer
A u-tube manometer contains water, oil or some other appropriate liquid that is displaced when pressure differentials in a LEV system are found. That is, when a tube is connected to one or both ends of the u-tube manometer, static pressure, velocity pressure and/or total pressure can be measured in inches water gauge.
 
Increased sensitivity and scale magnification can be achieved by tilting and extending one leg of the u-tube manometer. When this is done an inclined manometer is formed. To be used effectively, the inclined manometer must be placed on a level surface. Although accuracy is increased with the inclined manometer it becomes less practical to use in the field where industrial traffic is heavy and levelling is problematic.
U-Tube Manometer
Inclined Manometer
  • Pitot Tube
Pitot tubes and inclined manometers or similar gauge pressure measuring devices can be used to measure velocity pressure and determine the velocity of air moving through ductwork.
This is because air velocity meters may not be rugged enough to measure air velocity directly. The pitot is a device that consists of two concentric tubes. One tube measures total or impact pressure while the other measures static pressure only. Therefore, the instrument can be used effectively to measure static velocity (VP) and total pressures (TP).

Static Pressure Measurements

Static pressure (SP) can be measured at any point in the system. This can be accomplished by using the u-tube or inclined manometer with a tube attached. A hole is drilled in the side wall of the LEV ductwork and the SP is measured perpendicular to airflow.
SP can also be measured using the pitot tube attachment and inclined or u-tube manometer.
 

Velocity Pressure Measurements

VP can be used to compute velocity of the air stream if the air density is known. For standard air (where density is 0.075 Ib/ft3) the relationship between velocity and velocity pressure is given by the formula:

V = 4005 (Square root of VP)
Because airflow in a section of ductwork is not uniform, it is necessary to obtain an average by measuring VP at points of equal area across the cross section of duct work. Usually, two traverses are made one perpendicular to the other. Traverse openings should be drilled and be located at least 7.5 duct diameters downstream of any air disturbance (e.g. branch, entry, elbow etc.) At duct or transport velocities above 2000 fpm VP readings will be accurate to + 1.0%. At air velocities below 2000 ft/min, VP readings will be less accurate. 
 
Readings (recorded in inches water gauge) are taken at the centre of the annular rings of equal area. For round ducts 6" and smaller, at least 6 traverse points should be used. For round ducts larger that 6" in diameter at least ten (10) traverse points should be used. For very large ducts as many as 20 traverse points may be used.
 
To determine the average velocity in the section of ductwork where the pitot traverse was made:
  1. Record the VP readings
  2. Convert VP readings to Velocity using the formula V= 4005 (Square root of VP)
  3. Add the converted velocity readings together and divide by the number of measurements

Total Pressure Measurements

To determine TP the following pitot and inclined manometer configuration is used.