The properties of gases

For all those whose knowledge of physics is sketchy, and who wondered if the gas laws would ever be useful in their lives — here is the answer. Boyle, Charles and Dalton have centre stage. Without these you can never completely understand the underlying rules of the fan engineer.

Essential information about those properties of air and other gases which must be known for fan selection are given in this Chapter. In the case of hazards, guidance is detailed regarding legislation and safety standards.

Gases, together with liquids and solids, are our names for the various forms in which substances naturally occur. Thus we speak of the gaseous state, the liquid state and the solid state. Sometimes we call these the three phases of a substance. Gases and liquids are often grouped together as fluids. Fluids differ from solids in that they readily take up the shape of the container in which they are placed.

Asolid body subjected to a small shearforce undergoes a small elastic deformation and returns to its original shape when the force is removed. When subjected to larger shear force the shape may be permanently changed due to plastic deforma­tion.

A fluid, when subjected to an arbitrarily small shearforce under­goes a continuous deformation. This happens regardless of the inertia of the fluid. For a fluid the magnitude of the shear force and the speed of deformation are directly related. In a solid body it is the deformation itself, which is related to the shear force.

Afluid may be either a liquid or a gas. A gas differs from a liquid in that it will expand to completely fill the container. Agas at con­ditions very close to boiling point or in contact with the liquid state is usually called a vapour. Fluids are compressible; gases being much more compressible than liquids.

A substance can exist in all three states. A typical example of this is ice, water and steam. When ice is heated at constant pressure, the ice converts to water at the melting point and to steam at the boiling point. If the steam pressure is increased at constant temperature, the steam converts to water at the satu­ration (vapour) pressure.

Solid particles can be suspended in a gas. Such a combination, gas plus particles, is very common in dust control, pneumatic conveying etc. When the particles distribute themselves evenly through the gas, we speak of a homogeneous mixture. When concentration gradients occur, we speak of a heterogeneous mixture.

Gases display greatly varying properties. For the purposes of fans and fan systems, the following characteristics of gases should generally be known:

• density

• relative humidity and liquid content

• viscosity

• compressibility

• temperature and changes of state

• chemical composition and solid content

Posted in Fans Ventilation A Practical Guide


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