Source Capture and Energy Savings
When using source capture technologies in factories, to what extent is it possible to both save energy and effectively control air pollutants, while also meeting CSST requirements?
In practice, the source capture method requires the lowest airflow rates and is consequently the most energy-friendly. As far as CCST requirements go, source capture is still the preferred solution and is often required. Below we will use the example of a welding workshop to calculate our energy savings.
In a welding workshop, there are four different methods for controlling fumes, each of which extracts different air volumes.
The source capture method contains a pollutant very close to its source while expelling the least amount of air. For a welder, a fume extraction welding gun is the best way to control welding fumes at their source, as they are captured at approximately 6 cm (2.5 in.) from the welding arc. It is also the most energy-friendly method available.
The extraction arm method also complies with source capture requirements. However, it involves moderately high airflow rates and consumes more energy due to the quantity of air expelled.
A canopy hood placed above a workspace offers a certain amount of control at the source but requires a significant airflow rate. However, an employee working under the canopy hood will not be protected because the pollutant will pass through their breathing zone before being expelled by the hood. This method also expels the most air.
Forced ventilation, general ventilation, and air change all refer to the same method. It is more accurately described as dilution. Fresh air is mixed with one or more factory emissions. This type of factory pollution control can protect, to a certain extent, a portion of factory workers. However, it cannot effectively protect workers close to the source(s) of emissions. An enormous airflow rate would be required to avoid exceeding the exposure standards in the ambient air. Moreover, this method has the disadvantage of not protecting workers close to the source, including welders themselves, and therefore does not comply with regulations.
For our calculations, let’s use the following building example:
- Building height: 30 m*18 m*7.6 m (100 ft*60 ft*25 ft).
- Building volume: 4,250 m3 (150,000 ft3).
- 10 welders are in the factory.
- Estimated cost of heating fresh air is 0.028 m3/min or $1 per ft3/min per winter, based on eight-hour workdays, five days per week.
In the following section, we will quantify what each method described above requires for optimal performance.
Airflow rates required for each capture method
If fume extraction guns are used, 2.8 m3/min (100 ft3/min) are required for each welder. Based on the measurements taken with this source capture method, the welders’ breathing zone is below permissible exposure values (PEVs). The factory’s ambient air is of course well below PEVs.
For 10 welders, a maximum of 28.3 m3/min (1,000 ft3/min) is therefore required. If the extraction arm is used to capture the pollutant at the source, 17 m3/min (600 ft3/min) per arm is required to obtain an acceptable capture volume when the arm is in the proper position. Once again, if the welders use all the arms to capture the pollutant at the source, the pollutant concentration in the welders’ breathing zone and factory’s ambient air should be well below PEVs.
For 10 welders, a maximum of 170 m3/min (6,000 ft3/min) is required.
If a canopy hood is placed above each welder and the welder works on a 1.2 m x 2.4 m (4 ft x 8 ft) work table, a speed of at least 15 m/min (50 ft/min) should be achieved when facing the hood. This means that 45 m3/min (1,600 ft3/min) of polluted air should be expelled per table. An acceptable concentration level, e.g. well below PEVs for ambient air, should be obtained in the factory. However, it is not clear whether the welders’ breathing zone would be below PEVs as they would be in the fume trajectory between the extraction gun and the canopy hood.
For 10 welders, a maximum of 453 m3/min (16,000 ft3/min) is therefore required.
With a fresh air supply, for four air changes per hour (minimum required by the CSST), 136 m3/min (4,800 ft3/min) of expelled contaminated air is required. This airflow rate is calculated for the factory’s entire surface area, but only for a height of 3.6 m (12 ft), unless the workers are positioned higher up. In this case, the working height must be considered in addition to the 3.6 m (12 ft) to calculate the volume of air to be treated.
Based on our experience, without source capture, and with only four air changes per hour, it is nearly impossible to respect the PEVs in ambient air.
We would need to go up to six or eight air changes per hour, depending on the welding intensity. Remember, hourly air changes involve dilution. As pollutants increase, more fresh air must be supplied to maintain an ambient concentration below PEVs.
For six changes per hour, we would need 204 m3/min (7,200 ft3/min).
For eight changes per hour, we would need 272 m3/min (9,600 ft3/min).
In other words, if your pollutant emission rates double, the quantity of fresh air must also be doubled to maintain the same concentration. Table 1 explains the operational costs for each of these methods.
If we were to conduct the same exercise for other factory emissions, we would see that source capture remains the most affordable among all pollutant control methods. However, investments need to be considered.
There are three types of investment costs:
- Equipment purchase and installation
- The operating costs described above
- Maintenance costs, which could be significant
If dust extractors need to be installed at the time of purchase, the smaller they are, the less expensive they are. If the type of source capture involves lower airflow rates, the dust extractors will also be smaller.
During operation, the smaller the dust extractors are, the smaller the filtration surface to purchase or to clean will be.
The source capture method is advocated by the CSST to control pollutant emissions and protect breathing zones in factories and elsewhere. In conclusion, source capture technology is guaranteed to comply with legal requirements for worker protection. As a bonus, it will also allow you to save a considerable amount of money.
Table 1. Calculation of the operational costs for each method.
- GUNS : 1 000 FT 3/MIN * X 1 $ = 1 000 $/YR
- ARMS : 6 000 FT 3/MIN * X 1 $ = 6 000 $/YR
- HOODS : 16 000 FT 3/MIN * X 1 $ = 16 000 $/YR
- FOUR AIR CHANGES ** : 4 800 FT 3/MIN X 1$ = 4 800 $/YR
- SIX AIR CHANGES ** : 7 200 FT 3/MIN X 1$ = 7 200 $/YR
- HEIGHT AIR CHANGES ** : 9 600 FT 3/MIN X 1$ = 9 600 $/YR
*Airflow rates required for 10 workers using guns, arms, or hoods.
**With fresh air supply.
Note. The cost of $ 1 is only used as an example for calculations. It is possible that the real costs are higher.