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LECTURE NOTES ON ENVIRONMENTAL ENGINEERING

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This amount should be calculated with due regard to estimated future needs. The particular method to be adopted for a particular case or for a particular site depends largely on the factors discussed in the methods, and the choice is left to the judgment and intelligence of the designer. The catchment must be in a cleaner area of ​​the source, in order to draw the best quality water from the source and thereby reduce the load on the treatment plant.

The intake must never be located downstream or near the sewage disposal site. In the first stage, water is transported by gravity or by pumping or by the combined action of both, depending on the relative heights of the treatment plant and the source of supply. In such a conduit, the hydraulic gradient line coincides with the water surface and is parallel to the bearing of the conduit.

Since, d is the same for all pipes of the considered loop, it can be omitted from the summation. The numerator in the above equation is the algebraic sum of the head losses in the various closed loop pipelines calculated with the assumed flow.

Settling

Plume rise under stable and calmconditions

Non- buoyantplumes

The primary requirement of such a device would be a chamber in which the carrier gas velocity is reduced so that the particles can settle out of the moving gas stream under the action of gravity. The gas velocities in the settling chamber must be sufficiently low for the particles to settle due to gravitational force. Literature indicates that gas velocity less than about 3 m/s is necessary to prevent the settled particles from being entrained again.

The velocity of the particles in the settling chamber can be obtained by Stokes' law as follows: The advantages of settling chambers are: .. i) low initial cost, ii) simple construction, iii) low maintenance cost, iv) low pressure drop, . v) dry and continuous removal of solid particles, vi) use of any material for construction, and .. vii) Temperature and pressure limitations will depend solely on the nature of the material of construction. Because of the above advantages and disadvantages, settling chambers are often used as a pre-cleaner.

The velocity of this incoming gas stream is then converted into a confined vortex from which centrifugal forces attempt to push the suspended particles onto the walls of the cyclone. The vortex turns upward after reaching the bottom of the cylinder in a narrower inner spiral. Centrifugal forces used in modern designs vary from 5 to 2,500 times gravity, depending on the diameter of the cyclone.

Cyclone efficiency is greater than 90% for the particles with the diameter of the order of 10 µ. For particles with a diameter higher than 20 µ, efficiency is about 95%. The efficiency of a cyclone can be increased by using cyclones either in parallel or in series. The second cyclone removes the particles not collected in the first cyclone, due to the statistical distribution over the inlet, or accidental re-entrainment due to eddy currents and re-entrainment in the vortex core, thus increasing the efficiency.

The disadvantages of cyclones include: .. i) low collection efficiency for particles with a diameter less than 5 – 10 µm, . ii) serious scouring problems may occur during particle impact on the cyclone walls, and iii) a decrease in efficiency at low particle concentration. The large potential difference between the two electrodes creates a powerful ionizing field. The precipitator funnel is an integral part of the precipitator housing and is made of the same material as the housing.

Single stage or two stage

Transport of particles to the collection surface. iv) Neutralization or removal of charge from dust particles. v) Removing dust from the collection surface. The main components of electrostatic precipitators are: .. ii) Discharge and collection electrodes. iii) Gas inlet and outlet. iv). All cages provide an inlet and outlet connection for gases, pits to collect precipitated particles, and the necessary discharge electrodes and collection surfaces.

There is a weatherproof, gas-tight enclosure over the separator housing the high voltage insulators. Electrostatic precipitators also usually have a number of auxiliary components which include access doors, dampers, safety devices and gas distribution systems. The doors can be closed and bolted under normal conditions and can be opened when necessary for inspection and maintenance.

It usually consists of a conductor, one end of which is grounded to the enclosure, and the other end is attached to the high voltage system by an insulated operating lever. Since ESPs require a very high voltage direct current source of energy for operation, transformers are required to step up normal service voltages to high voltages. In the two-phase precipitator, particles are ionized in the first chamber and collected in the second chamber.

For example, the "Penny" - two-stage filter system uses DC voltages of 11,000 to 14,000 volts for ionization and is called a low-voltage filter.

Pipe type or Plate type

Dry and Wet Precipitators

However, for materials such as carbon black, which have very low efficiency due to very low collection capacity, very high efficiency can be achieved by properly combining an ESP with a cyclone. The gas entering the ESP can be pre-treated (i.e. remove some of the particles) by using certain mechanical collectors or by adding certain chemicals to the gas to change the chemical properties of the gas to increase their capacity to collect the discharge electrode and thus increase the efficiency. The factors that affect the efficiency of electrostatic precipitators are particle resistivity and particle recovery.

Particle Resistivity

Particulate-entrainment

The particle must be transported to the surroundings of the water droplets which are usually 10 to 1000 times larger. This involves removing the droplets, which contain the dust particles from the gas phase. The physical principles involved in the operation of the scrubbers are: i) impingement, ii) interception, iii) diffusion and iv) condensation.

Condensation of the liquid medium on the particulate matter increases the size and weight of the particles. Fabric filtration is one of the most common techniques for collecting particles from industrial waste gases. The number of bags can vary from a few hundred to a thousand or more depending on the size of the bag house.

This is done to prevent moisture or acid mist from condensing inside the unit, causing corrosion and rapid deterioration of the filter housing. When selecting the filter medium for filter bags, the characteristics and properties of the carrier gas and dust particles should be taken into account. The abrasion resistance, chemical resistance, tensile strength and permeability and cost of the fabric must be taken into account.

This is the pressing of the fabric through rollers under high pressure to flatten, smooth or decorate the material. This involves scraping the filter surface over metal points or burrs on a rotating cylinder. Once formed, the pre-layer becomes part of the filter medium, which aids in the further removal of the particles.

The accumulation of dust increases the air resistance of the filter and therefore filter bags must be cleaned periodically. This is the penetration of the dust by fine particles, which is common in dust filtration. The nature of the gas and the dust determines the frequency of such maintenance work.

References

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