Air Purifiers for construction sites and silica dust control
The following is an excerpt taken from the new version of the Dustcontrol Handbook on Dustcontrol Air Cleaners from Sweden.
Theory for Air Cleaners
Air cleaners are a complimentary ventilation measure to source extraction and work according to diﬀerent principles. The purpose of an air cleaner is the same as general (dilution) ventilation, that is; it is used to reduce the concentration of particulate in the work space environment.
Source extraction will eliminate a high percentage of generated particulate before it reaches the operator’s breathing zone. The air cleaner will reduce the particulate that is present in the entire work space, including the breathing zone. Air cleaners beneﬁt the plant environment generally and are particularly eﬀective in smaller spaces or in situations where the general make
up ﬂow is very low.
By circulating ambient air though a highly efficient HEPA filter, the air in the space is cleaned of airborne dust.
The air cleaner will circulate air through the HEPA ﬁlter and return cleaned air to the room. An almost total separation of particulate is acheived as ambient air is passed through the unit. To protect the HEPA ﬁlter, a pre-ﬁlter is installed for coarse separation before the HEPA. While the degree of separation for the pre-ﬁlter is much lo- wer than the HEPA, the pre-ﬁlter captures a much higher percentage of the total dust. The pre-ﬁlter should be changed much more frequently than the HEPA. Additionally, the exhaust air from the air cleaner serves to dilute and mix the air in the work space. Again, this eﬀect is stronger as the volume of the space decreases.
In premises that have insuﬃcient general ventilation for redu- cing airborne particulate concentrations or where ventilation system upgrades are cost prohibitive, the air cleaner can be used as an additional measure for controlling particulate. Please note that the air cleaner does not ﬁlter gases or reduce humdity.
A calculation of how the concentration of particulate will be reduced by using an air cleaner is basically the same as the calculation used for dilution venti- lation. Before considering the calculation, it should be noted that only airborne particulate should be considered. Larger particulate that falls to the ﬂoor or settles on other horizontal surfaces within two or three minutes is not considered airborne particulate. While it is a house keeping problem, it does not neccessarily constitute a health risk in the working environment. Airborne particulate is the hazard for the respiratory tract and human health and it is for that that the air cleaner (and general ventilation) is most eﬀective.
For air cleaners as well as for general ventilation, the particulate concentration can be calculated with the following formula;
C = C0 • e(-n*t) + (M/Q) • (1 – e(-n*t)) where
C = concentration at time t
C0 = concentration at time t=0
M = generation rate for the particulate
n = the number of air changes per hour in the room = Q/V
Q = air ﬂow through the air cleaner (/ventila- tion ﬂow)
V = volume of the room
This formula assumes complete mixing of the air – something that is rarely the case. In a realistic
case, it can also be diﬃcult to estimate the generation rate for airborne partculate. By simply
plugging in some numbers though, we can illustrate how eﬀec- tive the air cleaner (or increase
fresh air make-up) will be.
This graph illustrates the concentration of air borne particulate over time in three distinct cases. The example space is 2 x 3 x 2,5 m (15 m³) where 1000 mg/h of air borne particulate is emitted during period A (1 hour). During period B, no further particulate is introduced.
1. No ventilation of the space.
2. Dilution ventilation of two times the total volume of the space (30 m³/h).
3. Use of an air cleaner with an eﬀective ﬂow rate of 400 m³/h.
This graph illustrates how the concentration of airborne particulate (mg/m³ )changes over time during three diﬀering scenarios. The sample space is the same as the illustration above with a total volume of 15 m³ and an airborne particulate gene- ration of 1000 mg/h during period A (1 hour). By using source extraction, in this example a suction casing equipped tool with a capture eﬃciency of 95 % of total dust, only 50 mg/h of air- borne dust per hour is introduced into the space. No additional particulate is introduced into the space during period B.
1. Air cleaner only, ﬂow rate is 400 m³/h.
2. Source extraction, suction casing equipped tool with a capture eﬃciency of 95 % and a
dilution ventilation rate of 2 air changes per hour.
3. Combination of measures, source extraction as in point 2 and air cleaner as in point 1.
Be careful to note the scale diﬀerence of the vertical scale in both graphs.
The lowest possible particulate concentrations will be achieved by using both source extraction and an air cleaner.
The air ﬂow rating of the air cleaner should be selected with consideration to the total volume of the space where it will be used. The above equation shows that long term activity will result in the same particulate concentration regardless of the volume of the room. As well, it demonstates that using a small air cleaner will result in a longer duration of air cleaner operation after work has ceased, to reduce the concentration.
This graph shows how the capacity of the air cleaner will aﬀect the concentration of particulate in a space after work is comple- ted. The starting concentration is 50 mg/m³ and an air cleaner with a ﬂow of 400 m³/h is used.
1. 2 x 3 x 2,5 m = 15 m3
2. 5 x 4 x 3 m = 60 m3
3. 10 x 8 x 3,8 m = 300 m3
One or several DC AirCubes can be ceiling mounted in a bakery or woodworking shop.
Air cleaners can also be used as negative air units; devices used to create a negative pressure in one area with relation to an adjacent area. This can be done by connection a hose or duct to the inlet or outlet and leading the hose out through a wallor hoarding. The air cleaner exhausts ﬁltered air from the enclosed zone. The goal in this type of application is generally to maintain a negative pressure in the subject space and prevent any contaminant from escaping. Applying this type of measure for a hazardous dust makes it extremely important that the air cleaner is reliable, that it does not leak and that the HEPA ﬁlters meet the high H13 rating. Otherwise, hazardous dust may be spread to the surrounding spaces.
The DC AirCube can be used to maintain a negative pressure in a space by ducting the exhaust to an adjacent space.