“This ventilation removes the contaminated indoor air from industrial workplaces and brings in fresh outdoor air.”
- Total Ventilation (Dilution Ventilation)
This method does not completely remove toxic materials from the pollutant sources, but it reduces the concentration of the contaminants by dilution.
This method requires ventilation of all workplace and hence it supplies and exhausts large amounts of air to and from a workplace. In winter or summer times, it requires excessive energy costs due to more makeup air to heat or cool. In addition to such inefficiency, it is difficult to remove the toxicity completely.
- - Purpose of Total Ventilation:
- 1. To use as an alternative to local exhaust ventilation (when the work requires a lot of moving or carrying / When the emission sources are spread all over the worksite);
2. To reduce the concentration of toxicity and maintain and improve health;
3. To prevent fire or explosion; and
4. To adjust the temperature and humidity within the workplace.
- - When to Use Total Ventilation:
- 1. When the toxicity of contaminant is moderate;
2. When the amounts of contaminants generated are normally consistent;
3. When multiple pollutant sources are spread all over within the same workplace; and
4. When the sources of pollution are in motion.
Local Exhaust Ventilation
Local exhaust ventilation system is placed in the vicinity of the pollutant sources (such as dusts, gases, vapors, fume, and mist). This is a method that traps the sources as soon as they are generated and lets them out of the workplace.
The facilities consist of a series of system whose components are hood, transporting duct, air cleaner, exhaust fan, and exhaust stack. The system must be connected in this order of the components.
- Advantages :
- This method captures contaminants at source and removes them from the workplace. It also reduces the toxicity near the workers’ breathing zones even as small amounts of air are exhausted.
- Disadvantages :
- It can disturb the work process and cause potential product defects. The method is also difficult to apply for works that involve a lot of moving and carrying.
- - Design Process
- - Choice of Materials and Size for the Hood
- · Calculation of air flow
- · Duct line design in consideration of minimum air speed inside the ducts and minimum pressure loss
- · Optimal dust traps according to the description of contaminant
- · Simulation program operation
- · Best solution determined
- · Design of local exhaust ventilation system
- 1) Hoods
A hood intakes toxic materials and leads them to a duct. This is a device that determines the performance of the local exhaust ventilation system. The type of hood must be decided in consideration of manufacturing facilities, the shape of worktable, the direction of contaminants’ emission, and the working method and movement of the workers. In general, the hood should surround or enclose the source of pollution or be placed as close to the source as possible to produce good ventilation results. The less the amounts of exhaust are, the more you can save in terms of installment and operation costs.
- - Basic Types
- Enclosing (Cover Type) HoodAn enclosing hood has relatively small opening so that the contaminated air will not be leaked. It can offer maximum results with minimum amounts of exhaust.
- Capturing Hood If the working conditions make it impossible to surround the emission source, a capturing hood will capture the air within the hood so that contaminants can be removed through it. One disadvantage this type is that it is rather ineffective when the airflow outside is not constant.
- Receiving Hood A receiving hood receives or catches the contaminated air from an emission source when a contaminated airflow occurs to a certain direction, such as rising air due to thermal buoyancy and inertial air current due to revolution. This may look similar to the capturing hood as it takes in and exhausts contaminated air to the same direction. However, there are certain functional differences between the two types.
- - Performance of Hood
- The performance of a hood is represented by capture velocity, which is a major factor in designing hood.
Capture Velocity of Hood: Air speed required to catch and transport the contaminants some distance away from the hood opening; air speed in consideration of the effects of various air currents nearby.
|Velocity Control by Working Conditions and Process|
|Dispersion of Contaminants||Examples||Capture Velocity (m/sec)|
|No actual velocity; emission is made into the calm air||Gases, vapors and fume created from the liquid surface of the container||0.25-less than 0.50|
|ow velocity; emission is made into the calm air||Paint-spraying, intermittent container filling, welding low-speed conveyor transfers, welding, plating, acid cleaning||0.50-less than 1.00|
|Relatively high velocity; emission is made into the fast airflow||Paint-spraying at a shallow booth, barrel filling, conveyor loading, crushers||1.00-less than 2.50|
|Very high initial velocity; emission is made into the high-speed airflow||Grinding, abrasive blasting, tumbling||2.50-less than 10.00|
|Consider the following influencing factors when you apply the capture velocity above.|
|Higher capture velocity applies when:||lower capture velocity applies when:|
- - Exhaust air volume of hood
- Capturing Hood : Q=capture velocity at the source(V)*[10* distance from the source to the hood(X)
|Shape of hood||Title||Length to width ratio of wetted perimeter||Calculation of exhaust air volume|
|Slot||0.2 or less||Q=3.7LVX|
||0.2 or less||Q=2.6LVX|
||0.2 or greater and round||Q=2.6LVX|
||0.2 or greater and round||Q=2.6LVX|
P : Wetted perimeter of aperture
D :Distance to a workstation
|Plain Multiple Slot Opening or More Slot||0.2 or greater||Q=V(10X2+A)|
|Flanged Multiple Slot Opening or More Slot||0.2 or greater||Q=0.75V(10X2+A)|
- - Effect of Flanges
- A flange is a board placed at the opening of the hood to block airflow into the hood from behind and to increase the speed of air that comes in. This can reduce the flow rate by 25 percent.