Gases and liquids, together with 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. A solid body subjected to a small shear force 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.

Gases, liquids, solids

Afluid, when subjected to an arbitrarily small shear force 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. A fluid may be either a liquid or a gas. A gas differs from a liquid in that it will expand to completely fill the container. A gas 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.

The absolute viscosity is defined as the shearing stress for a unit rate of change of velocity. It has the units of Newton-sec per metre squared in the SI system. The shearing stresses are proportional to the ratio of absolute viscosity to density, called kinematic viscosity. Viscosity (the ability to flow)is a property of fluids (both liquids and gases) treated under the heading of rheology.

The work rheology derives from the Greek “rheos” meaning flow. Between two layers of fluid flowing at different speeds, a tangential resistance, a shear stress, is developed because of molecular effects. We say that the shear stress is caused by the internal friction of the fluid or conversely that the fluid transmits shear forces by reason of its internal friction. A liquid in motion is continuously deformed by the effects of these shear forces.

The magnitude of the stress depends on the rate of shear deformation and the sluggishness of the liquid, i.e. the viscosity. Another health hazard, sometimes not recognised as such, is noise. Some countries have regulations stipulating the acceptable noise levels and exposure times. It must be remembered that the fan duty will largely determine the fan noise. High pressure fans will be noisier than low pressure fans. System resistance should therefore be kept as low as possible. Some fan types are inherently noisy.

Gases and liquids, together with  solids, are our names for the  various forms in which substances naturally occur
Large equipment, in general, is noisy. Noise levels can be attenuated by fitting acoustic enclosures. However, these tend to drastically diminish the maintainability of the equipment by hindering access. In some instances, costly acoustic enclosures have been removed at site and scrapped in order to achieve acceptable access. One easy solution to this hazard is to declare certain areas “Ear Protection Zones”.

Environmental hazards

We are becoming more aware of the limitations of our environment. The Earth’s resources and waste disposal capabilities are finite. Stricter limitations will be imposed gradually on the amount of pollutant which can be released, while the list of pollutants will become longer. The fan user must be aware of the full consequences of leakage of gas from the fan and installation.

The nature of the hazards will also dictate the type of duct connections to be used. Spigot, flat-face, flanged, raised-face flanged, ring-type joints. Process upset conditions must be considered as part of the assessment. Upset conditions which last for more than one or two hours may have a significant impact on pump and ancillary equipment selection.

How to Understand Indoor Air Quality | Ask This Old House

The physical location of the fan, indoor or outdoor, will decide the behaviour of the leakage once outside the fan. Will any vapour cloud quickly disperse on a breeze which always blows over the un-manned site or will a manned enclosed fan house gradually build up a dangerous concentration of gas? Only the user can assess these questions and specify the necessary precautions. It is the responsibility of the user to define exactly what the fan is intended to do. It is the responsibility of the fan manufacturer to supply equipment to meet the required performance.