1. Industrial Ventilation

2. What is industrial ventilation?
Ventilation is the mechanical system in a building that brings in "fresh" outdoor air and removes the "contaminated" indoor air.
In a workplace, ventilation is used to control exposure to airborne contaminants. It is commonly used to remove contaminants such as fumes, dusts, and vapours, in order to provide a healthy and safe working environment.
Ventilation can be accomplished by natural means (e.g., opening a window) or mechanical means (e.g., fans or blowers).

3. Industrial systems are designed to move a specific amount of air at a specific speed (velocity), which results in the removal (or "exhaust") of undesirable contaminants. While all ventilation systems follow the same basic principles, each system is designed specifically to match to the type of work and the rate of contaminant release at that workplace.

4. Provide a continuous supply of fresh outside air.
What is the purpose of a ventilation system?
There are four purposes of ventilation:
Provide a continuous supply of fresh outside air.
Maintain temperature and humidity at comfortable levels.
Reduce potential fire or explosion hazards.
Remove or dilute airborne contaminants.

5. Why have an industrial ventilation system?
Ventilation is considered an "engineering control" to remove or control contaminants released in indoor work environments.
It is one of the preferred ways to control employee exposure to air contaminants.
Other ways to control contaminants include:
eliminate the use of the hazardous chemical or material,
substitute with less toxic chemicals,
process change, or
work practice change.

6. What are the parts of an industrial ventilation system?
Systems are composed of many parts including:
an "air intake" area such as a hood or an enclosure,
ducts to move air from one area to another,
air cleaning device(s), and
fan(s) to bring in outside air and exhaust the indoor contaminated air.

7. What are the basic types of ventilation systems?
There are two types of mechanical ventilation systems used in industrial settings:
Dilution (or general) ventilation reduces the concentration of the contaminant by mixing the contaminated air with clean, uncontaminated air.
Local exhaust ventilation captures contaminates at or very near the source and exhausts them outside.

8. What are main features of dilution ventilation?
Dilution, or "general", ventilation supplies and exhausts large amounts of air to and from an area or building. It usually involves large exhaust fans placed in the walls or roof of a room or building.
Dilution ventilation controls pollutants generated at a worksite by ventilating the entire workplace. The use of general ventilation distributes pollutants, to some degree, throughout the entire worksite and could therefore affect persons who are far from the source of contamination.

9. Dilution ventilation can be made more effective if the exhaust fan is located close to exposed workers and the makeup air is located behind the worker so that contaminated air is drawn away from the worker's breathing zone.

10. When used to control chemical pollutants, dilution must be limited to only situations where:
the amounts of pollutants generated are not very high,
their toxicity is relatively moderate, and
workers do not carry out their tasks in the immediate vicinity of the source of contamination.
It is therefore unusual to recommend the use of general ventilation for the control of chemical substances except in the case of solvents which have admissible concentrations of more than 100 parts per million.

16. What are the limitations of dilution ventilation?
As a method for protecting workers, it is important to know that dilution ventilation:
Does not completely remove contaminants.
Cannot be used for highly toxic chemicals.
Is not effective for dusts or metal fumes or large amounts of gases or vapours.
Requires large amounts of makeup air to be heated or cooled.
Is not effective for handling surges of gases or vapours or irregular emissions.

17. Regular "floor" or "desk" fans are also sometimes used as a method of ventilation, but these fans typically blow the contaminant around the work area without effectively controlling it. Opening doors or windows can be used as dilution ventilation, but again, this method is not reliable since air movement is not controlled.
As a general note, the air or "volumetric" flow rate of dilution ventilation depends largely on the how fast the contaminant enters the air as well as the efficiency that fresh air mixes with workroom air.

18. What is local exhaust ventilation?
Local exhaust system is used to control air contaminants by trapping them at or near the source, in contrast to dilution ventilation which lets the contaminant spread throughout the workplace.
Local exhaust is generally a far more effective way of controlling highly toxic contaminants before they reach the workers' breathing zones.

19. This type of system is usually the preferred control method if:
Air contaminants pose serious health risk.
Large amounts of dusts or fumes are generated.
Increased heating costs from ventilation in cold weather are a concern.
Emission sources are few in number.
Emission sources are near the workers' breathing zones.
In a general way, a local exhaust system operates similar to a household vacuum cleaner with the hose as close as possible to the place where dirt would be created.

20. What are the components of local exhaust ventilation?
A local exhaust system has six basic elements:
A "hood" or opening that captures the contaminant at the source.
Ducts that transport the airborne chemicals through the system.
An air cleaning device that removes the contaminant from the moving air in the system (not always required).
Fans that move the air through the system and discharges the exhaust air outdoors.
An exhaust stack through which the contaminated air is discharged.
Make up air that replaces the exhausted air.

22. How do I know which type of ventilation system is best for my workplace?
All industrial ventilation systems, when designed properly, should be able to provide long-term worker protection. The two types of ventilation, dilution and local exhaust, are compared in the following table.

23. Comparison of Ventilation Systems Local Exhaust Ventilation
Dilution Ventilation
Disadvantages
Advantages
Higher cost for design, installation and equipment.
Captures contaminant at source and removes it from the workplace.
Does not completely remove contaminants.
Usually lower equipment and installation costs.
Requires regular cleaning, inspection and maintenance.
Only choice for highly toxic airborne chemicals.
Cannot be used for highly toxic chemicals.
Requires less maintenance.
Can handle many types of contaminants including dusts and metal fumes.
Ineffective for dusts or metal fumes or large amounts of gases or vapours.
Effective control for small amounts of low toxicity chemicals.
Requires smaller amount of makeup air since smaller amounts of air are being exhausted.
Requires large amounts of heated or cooled makeup air.
Effective control for flammable or combustible gases or vapours.
Less energy costs since there is less makeup air to heat or cool.
Ineffective for handling surges of gases or vapours or irregular emissions.
Best ventilation for mobile or dispersed contaminant sources.

24. In general, what are limitations of any ventilation system?
Some limitations include:
The systems deteriorate over the years because of to contaminant build-up within the system, especially filters.
Require ongoing maintenance.
Regular and routine testing is needed to identify problems early and implement corrective measures.
Only qualified persons should make modifications to a ventilation system to make sure the system continues to work effectively.
The following is an example of changes that can affect how a system works.

26. What should I know about make-up air?
An important and sometimes overlooked aspect of local ventilation is the need to provide enough air to replace the air that is exhausted from the workplace. If enough make-up air is not provided when large volumes of air are exhausted, the workplace becomes "starved" for air and negative pressure is created.
Negative pressure in the workplace increases resistance on the ventilation system causing it to move less air.

27. Air will also enter a building through cracks around doors or windows or other small openings to try to "equal" the rate of air being removed.
The result is that workers may be exposed to cold air in the winter, and additional heating costs may occur. One simple way to judge if a building is under an excessive negative pressure is if you have difficulty opening a door that pushed into the room or building (the air wants to force the door closed).

28. A separate intake fan, located away from the exhaust fans, should be used to bring in fresh, uncontaminated air from outside. This air must be clean and heated in winter or cooled in summer, as needed.
For example if the air flow rate required in a workspace which is 40 feet long, 40 feet wide and 12 feet high, volume of the work space is 40 x 40 x 12 = 19,200 cubic feet.
Air flow rate required per ACH = 19,200 / 60 = 320 cfm
Or, air flow rate required per ACM = 19,200 cfm

29. Or, if the ceiling height is 20 feet high then the room volume is 40 feet X 40 feet X 20 feet high= 32,000 cubic feet and the required air flow rate will be as follows:
Air flow rate required per ACH = 32,000 / 60 = 533 cfm
Or, air flow rate required per ACM = 132,000 cfm
The required air change rate is sometimes given in ventilation regulations and ventilation design standards. For example, a flammable storage room requires six air changes per hour according to US OSHA requirements.

30. What is a duct system?
The ventilation system in a building consists of air moving devices such as fans and blowers and a network of ducts to exhaust the contaminated indoor air and to bring in air from the outside of the building.

31. What are some basic principles of duct design?
Duct systems should be designed to have air flow through the ducts with as little friction or resistance as possible.
The amount of air that flows through a duct depends on the cross section area (duct opening area) of the duct and the air speed. Air moving too slowly will allow contaminants such as dusts to settle and accumulate and these particles will eventually clog the duct.
Air moving too fast wastes power, can create noise problems, and may cause excessive abrasion by dust particles hitting the ducts.

32. Duct Design Principles
Avoid design that causes more resistance to air flow
Design for less resistance for air flow
Principle
Streamline the system as much as possible to minimize air turbulence and resistance.
Round ducts provide less resistance than square ducts (less surface area)
Smooth, rigid ducts provide less resistance than flexible, rough ducts.
Short runs of ducts provide less resistance than long runs.

33. Duct Design Principles
Avoid design that causes more resistance to air flow
Design for less resistance for air flow
Principle
Straight runs offer less resistance than runs with elbows and bends.
Duct branches should enter at gradual angles rather than right angles. Duct branches should not enter the main duct at the same point.
Elbows with gradual bends provide less resistance than sharp bends.
Large diameter ducts provide less resistance than small diameter ducts.
                                                                                                                                                                              
                                                                                                                                                      
                                                                                                                             
                                                                                                                      

34. Duct systems typically require large amounts of air to move relatively small amounts of contaminants. The required volume of airflow depends of the acceptable concentration of air contaminants in the inside workspace.
A carefully designed system can achieve the required air concentration while using the least amount of power.
Other design considerations include initial capital costs, reliability, maintenance, and durability of air handling equipment.

36. Airflow changes direction abruptly:
Deposits are more common in short radius elbows and "T" type branch connections. The figures below shows the examples of abrupt air direction changes.

37. However, below are some tips for conducting a simple inspection
However, below are some tips for conducting a simple inspection. Before you start, be sure you have a drawing of the ventilation system (or make one as you go). While you walk through the entire system take note of the following:
Reduced ability to capture the contaminants ("fugitive" contaminants can be measured or sometimes been seen).
Constant plugging of a duct. Tap the duct with a stick to see if it has layers of build-up,
Damaged ducts (dents, holes).
Damaged or missing gaskets.

38. Visible dust on equipment connected to ventilation system.
Obvious add-ons to system (especially those that were added on after the initial installation of the system).
Opened blast gates or other openings.
Ducts cut off and covered with blank flanges.
Document any of the above problem(s) and possible causes from your walk around the system. Bring these problems to the attention of the building maintenance staff, your supervisor, or a ventilation expert if possible.

39. What is a hood?
A hood - correctly called a local exhaust hood - is the point where contaminated air is drawn into the ventilation system. The sizes and shapes of hoods are designed for specific tasks or situations. The air speed (velocity) at the hood opening and inside the hood must be enough to catch or capture and carry the air contaminants. To be most effective, the hood should surround or enclose the source of contaminant or be placed as close to the source as possible.

40. What are the common types of hood?
The three common classes of hoods are:
enclosing,
receiving, and
capturing.
Enclosing Hood
Enclosing hoods, or "fume" hoods, are hoods surrounding the process or point where the contaminants are generated. Examples of completely enclosed hoods (all sides enclosed) are glove boxes and grinder hoods. Examples of partially enclosed (two or three sides enclosed) hoods are laboratory hoods or paint spray booths. The enclosing hood is preferred whenever possible.

42. Receiving Hood
These hoods are designed to "receive" or catch the emissions from a source that has some initial velocity or movement. For example, a type of receiving hood called a canopy hood receives hot rising air and gases as shown in Figure 2. An example is a canopy hood located over a melting furnace.

44. Capturing Hood
These hoods are located next to an emission source without surrounding (enclosing) it. Examples are a rectangular hood along the edge of a tank or a hood on a welding or grinding bench table or a downdraft hood for hand grinding bench.

48. What is meant by "capture velocity"?
The ventilation system removes contaminants by "pulling" the air (and the contaminant) into the exhaust hood and away from the worker or the source.
Airflow toward the hood opening must be fast or high enough to "catch and transport" the contaminant until it reaches the hood and ducts. The required air speed is called the "capture velocity".

49. Any air motion outside of the hood and surrounding area may affect how the air flows into the hood.
The ventilation system will require a higher airflow speed to overcome air disturbances.
As much as possible, the other sources of air motion should be minimized or eliminated effectively

50. Common sources of external air movement include:
thermal air currents, especially from hot processes or heat-generating operations,
motion of machinery such as by a grinding wheel, belt conveyor, etc,
material motion such as dumping or filling,
movements of the operator,
room air currents (which are usually taken at 50 fpm (feet per minute) minimum and may be much higher), and
rapid air movement caused by spot cooling and heating equipment.

51. Most of the capture velocities are around 100 feet per minute (fpm)
Most of the capture velocities are around 100 feet per minute (fpm). How fast is 100 fpm? Blowing lightly on your hand so that you can just barely feel air movement is about 100 fpm. It is easy to see how it will take very little air movement from other sources to affect how well a hood can capture contaminants.

52. What are general rules for hood design?
The shape of the hood and its size, location, and rate of airflow each play an important role in design considerations. Each type of hood also has specific design requirements, but several general principles apply to all hoods:
The hood should be placed as close as possible to the source of contamination, preferably enclosing it. The more completely enclosed the source is, the less air will be required for control. The required volume varies with the square of the distance from the source.
The air should travel from source of the contaminant and into the hood with enough velocity (speed) to adequately capture the contaminant.

53. The hood should be located in a way that the operator is never between the contaminant source and the hood.
The natural movement of contaminants should be taken into consideration. For example, a hood should be placed above hot processes to trap rising gases and heat. A grinding wheel or woodworking machine should be equipped with a partial enclosure to trap the flying particles where they spin off.
The flanges or baffles should be used around the hood opening to increase the capture effectiveness and reduce ventilation air requirements.

55. Air-cleaning Devices

56. What are air-cleaning devices?
In a ventilation system, an air-cleaning device removes or captures the contaminants that are present in the air. The type of air cleaner used will depend on:
type of air contaminant to be removed,
concentration of the contaminant in the air,
how much contaminant must be removed to meet any regulations or standards,
type and concentration of toxic chemical contaminants,
type and size of dust particles,
temperature, humidity, etc.,
fire safety and explosion control, and air pollution control regulations.

57. What are air-cleaning devices for gases and vapours?
Gases and vapours can be removed by using the following processes:
Adsorption
The removal of a contaminant by contact with other materials such as activated alumina, activated charcoal and silica gel (referred to as adsorbers).
Absorption
Absorbers remove soluble or chemically reactive gases from the gas stream by close contact with an appropriate liquid so that one or more of the air contaminants will dissolve in the liquid.

58. Thermal Oxidation (Combustion)
Catalytic conversion
In this process, a catalyst converts a contaminant to a chemical form not considered to be hazardous. Catalysts are substances that alter the rate of a chemical reaction without being affected by the chemical reaction.
Thermal Oxidation (Combustion)
The combustion process (also called incineration) converts volatile organic compounds (VOCs) to carbon dioxide and water vapour by burning them. It is a very effective means of eliminating VOCs. Typical applications for incineration devices include odour control, reduction in reactive hydrocarbon emissions, and reduction of explosion hazards.

59. What should be considered when selecting an air-cleaning device?
Following are some tips for selecting an air-cleaning device in your workplace. Remember that a qualified professional should make final decisions regarding the suitability of an air-cleaning device.
Before the air cleaning device is selected, it is very important to know maintenance and access requirements, the physical size of the equipment and how it will be installed in the plant as well as the methods of removing the collected contaminants.

60. The air cleaner must be reliable
The air cleaner must be reliable. Many installations require monitoring or proof of continual operations by measuring conditions in the system.
Maintenance and operating costs must be considered. The air cleaner must operate in stable conditions as well as variations such as plant start-up and shut down. Considerations also include if it must be accessible for maintenance or if the air cleaner must continue to operate while maintenance or repairs are being done.
The device must meet local and national regulations (at start-up and over time)

61. Thank You