Air is a homogeneous mixture of several gases. These different gases have a good, bad or neutral impact on the earth. Air is of undefined shape and volume; doesn’t possess any kind of odour. The two types of air composition explain the air composition entirely.
Types of Air Composition
- Percentage of gas by mass
- Percentage of gas by volume
|S. No.||Gas Name||% by Volume||% by Weight|
The composition of air varies with places on Earth. Air composition is highly dependent on the source of pollution, environment and source of fresh air. The vapour in the air affects this percentage of gas by volume whereas the mass percentage composition remains unaffected. The volume and the density of the air changes due to the presence of vapour in the air which leads to change in percentage by volume.
Gases with different densities if mixed, stay mixed forever. This is known as diffusion. This process of mixing of gases is also achieved if the two gases are separated by a thin porous membrane.
Due to the emission of gases from the exhaust machinery, the seam and the human working under the ground, mine composition of air not similar to the surface composition of air. The mine composition of air consists of non-toxic gases(Hydrogen, Methane and hydrocarbons). It also has some mildly toxic gases (Carbon Dioxide) and toxic gases (Carbon monoxide, Nitrogen oxide and Sulphur Dioxide). And, along with dust particles, explosive particle, acid vapour, humidity and lubricated oil.
Impurity in mine air mixture defines damp. Impurities can be classified as following –
- Toxic gases like Carbon Dioxide and Radon
- Nontoxic and explosive gases like Acetylene and Methane
- Solid impurities like smoke, dust and other small living organisms
- Acutely poisonous gases like Nitrous fumes, Carbon monoxide, Arsene and Sulphur Dioxide
- Suspended liquid droplets
- Other miscellaneous impurities like water vapor and lubricants.
Due to the additional different types of impurities in mine, the mine air composition changes. Some of the mines which require cooling plants have Freon 11 and Freon 12 in its air composition.
In mine air, different types of damps exist. These damps are the results of the above gases.
Kinds of Damps
- Firedamp – Firedamp is a combination Methane and other gases. Presence of methane leads to reduction of oxygen.
- Afterdamp – This damp originates because of firedamp or other coal dust explosion.
- Blackdamp – Blackdamp is also called as Chokedamp. It is a combination of carbon dioxide and nitrogen.
- Whitedamp – Whitedamp consists of Carbon Monoxide originating from engine exhaust and strata emission.
- Stinkdamp – The main gas of this damp is Hydrogen Sulphide
- Sulphur Dioxide – As the name suggests, the sulphur dioxide is the main gas in such damps.
- Nitrous Fumes – Nitrous gases originated from the blasting are present in such damps.
Threshold Limit Value/Maximum Allowable Concetration
All the mentioned damps are harmful to the human life if their concentration is above the ceiling. This limit is termed as Threshold Limit Value (TLV) and it represents the concentration in several ways. TLV defines the exposure levels of the above-mentioned gases to the humans.
The permissible limits of the various gases for a time period of eight hours shift of a worker is TLV. These Threshold Limit Values are based on the information of experiences of different industries and experimental studies on animals and human beings.
TLV is measured in parts per million (ppm) or percentage. 1 ppm = 0.0001%
To make use of TLV, the following three values play a vital role –
- Short-term Exposure Limit
- Time Weighted Average
- Ceiling Limit
Threshold Limit Values for gas mixture is a dimensionless value. It is determined using this formula –
(C1/T1) + (C2/T2) + ____ + (Cn/Tn)
n – Number of pollutants
C – Measured concentration
T – Corresponding TLV