Ventilation Systems

By | June 8, 2021

Specification of Industrial Ventilation Systems

To keep the air temperature low enough are work place to easel sooty to dissipate by conventions to prevent excessive humanity. The functions which ventilation has to perform can be classified as

  • maintain the oxygen content of the air and to prevent C02 concentrations from rising,
  • To prevent harmful concentration of aerosols, and
  • To maintain reasonable conditions of comfort for operators.Ventilation Systems

Of these, the first two functions are insignificant in industrial environments. in the requirements of air achieving the objectives in item (ii) and (iv) will far outweigh the requirements “for the objective set out in item (i) and (ii) The chef need for ventilation is to maintain the body heat balance and to provide reasonable conditions of comfort. The heat balance of the body can be expressed by the equation:

M = E+R+C+S Where M = Rate of metabolism E = Rate of heat loss by evaporation C = Rate of heat loss by convection, and S = Rate of heat storage in body The standard rate of metabolism for sedentary adult” is about 100 kcl/hr while for adults occupied in very heavy work; it is much as 775 to 1000 kcl/hr. Factors determining the imposed thermal stress, as could be seen from the given equation are

  1. Air temperature
  2.  Degree of activity and
  3. Clothing worn
  4. Temperature of surrounding
  5. Humidity
  6. Air velocity

Ventilation should aim at:

  • Keeping the air temperature of the work room low enough to enable body to be dissipated by convection.
  • Preventing excessive humidity so as to assist body heat loss by evaporation, and
  •  Regulating the rate of air movement so that loss of body heat by convention is facilitated.

From the foregoing discussion. It is evident that ventilation has to perform three function-firstly to supply sufficient quantity of fresh air; secondly to distribute the air satisfactorily throughout the workroom and thirdly; out maintain reasonable conditions of comfort for the operators.

These functions are interrelated and are to be considered in the design of factory buildings from the point of view of ventilation. Although every case has to be considered according to its own characteristics and conditions prevailing. The amount of ventilation required is softened governed chiefly by physical consideration for controlling air temperature distribution and air velocity and depend generally on the following factors.

  • Size (including height) and type of room or building and its usage.
  • Duration and type of occupants and their activities
  • Heat gains from sun, hot manufacturing process, machinery and occupants
  • temperature conditions desired inside the building in relation to outside conditions prevailing.
  • The operation of the ventilating system.
  • Ventilation systems are divisible into main groups.
  1. Natural ventilation.
  2. Mechanical ventilation.

Many times mechanical ventilation is employed to augment the ventilation obtained by natural means.

Natural ventilation:

Forces which operate to induce natural ventilation in buildings are due to

  • pressure exerted by the outside wind. i.e. wind action and
  • the temperature difference of the air within and without the building, i.e. the thermal heat causing chimney effects.Natural ventilation

The force exerted by wind strikes a building, /I positive pressure” is created or the windward side of the building, and “negative pressure” on the lowered side, i.e. at the lee of the building, a the side and over the roof immediately behind the WinWord wall. If adequate openings, suitable placed in relation to these pressure areas, are available i should be possible to combine the effects of the varying wind. Pressures and move air rapidly through and within the building.

At or near the windows the rate of ‘air movements induced owing to other pressure differences will be high, but at the center the air movement will be considerably diminished. It is, therefore, important that such areas of low air movement should be eliminated by increasing his proportion of effective ventilating openings in the wall and roof and by suitable orientation of the building in relation to prevailing winds. Thermal head sets up the usual convection currents with the movement of warmed air upward air upward to leave openings in or near the roof, and be replaced to cool air entering at a lower level.

Cross ventilation:

In factory building where the width are not large, good cross ventilation can usually be obtained by the provision of large and suitable placed window or combination of windows and wall ventilators for the inflow and outflow of air. Since the prime consideration designing buildings from point of view of ventilation is to provide reasonable conditions of thermal comfort, air movements will have to be sufficiently high during summer months to bring down the effective temperature of the factory environments.Cross ventilation Considering the psychological principles involved’ in man’s reaction to hot weather, it is felt that air movement of 45 to 60 meters ( 150 to 200 ft.) per minutes should be aimed at to improve comfort.

Area of openings:

The method of calculation of ventilating area based on number of occupants does not have a sound basis for the following reasons. As pointed out earlier, the main consideration in providing ventilation in an industrial building is to maintain conditions inside the work rooms, which assist the ‘maintenance of thermal equilibrium of the body and also afford reasonable comfort to worker.

The heat load from persons within the room is generally very small considering processes, machinery and lights and therefore; it is not sound to base the rate of ventilation on the number of occupants. It is difficult to evolve a common standard, which will be applicable to all cases. For factories where there is no excessive heat gain either dour to manufacturing area of 15% in arid regions and hall the ventilating area should between floor level and a height of 2.25 meters (7′ 6”) from the floor and so arranged that there of the ventilating openings may be provided under the caves or at the glazing of the roof.

Roofed ventilation:

Cross ventilation by provision or windows and wall ventilators is suitable only for narrow factory buildings or work places. In large buildings roofed ventilation would be required. The most common means adopted for effecting ventilation induced by thermal head when.

The wind blows straight against these roof openings. Such interrelating openings in the case of pitched-roof and saw-tooth-roofed buildings. In the case of monitor roofed buildings, the ventilators in the glazing should be of opening and closing type so that the ventilators on. The windward side can be kept closed to prevent direct draughts of air interfering with upward flow of air. Wind action in any directions does not cause ‘interference with chimney effect but supplements ventilation by creating suction through wind jump

Cowl type roof ventilators:

The provisions of cowl type roof ventilators can obtain natural extraction of air from buildings also. The performance of the roof cowls depends on the differences- between the temperature of the external and internal air; on the height of these Ventilators above air intake, on the velocity of the external wind and on the cross-sectional area of the ventilators.

An example of a modern roof cowl is the Robertson ventilators shown in figures given in the next page. As will be noted the circular wind band around the ventilators causes a wind jump resulting in an upward draught of air from inside the building which supplements the ventilation due to chimney effect depending on the height of the ventilator above the intake openings ill the side walls.

Capacity of this ventilator for a 60cm (24in) diameter cowl in ‘relation to temperature difference; height of ventilator above intake and wind velocity is given by the equation:

Q=A (3.3/5.91 R (t 1-10) + 0.0035v Where Q is expressed in cu. Meters I min., A is the cross- sectional area of the ventilator in sq. meters, His height of the ventilator above the inlets in meters, 1 and to are the internal and outside temperature in DC, and V is the wind velocity in kilometers/hr.

Rotating type cowl ventilator:Rotating type cowl ventilator

Windows and other openings near the fans should kept closed as ‘otherwise fans will draw air supply from these opening sand thus cause short circuiting which a very common fault with this type of ventilation Exhaust ventilation can provided in larger darkroom by having a fan of adequate capacity extracting air from the interior of the room by means of suitable placed ducting. Exhaust ventilation often used in combustion with natural means alone will not sufficient to provide adequate ventilation in all performance of the rooms.

Plenum ventilation:

Compared with extraction system, plenum ventilation affords’ more completed control of atmospheric conditions inside the factory. Plenum system useful where extraction system cannot readily applied as in very large. Departments or workrooms, In this system of ventilation, air forced into the building by means of a centrifugal or other type of pressure fan and passed into the plenum chamber and ducts. This system has various advantageous.Plenum ventilation The volume of air can conditioned, if desired, before it passed into the plenum chamber and ducts. The slight positive pressure set up with the building prevents inward leakage of warm or cold outside air. The volume of air movement that can set up by plenum ventilation is many times more than possible by exhausting. Also better dilution of contamination and lower operator exposure can achieved. With a well-designed supply system than with exhaust, since the supply can directed to the important sources of contamination and good circulation affected without accessory equipment.

Combined plenum and extraction systems:

Better control of ventilations obtained by this system of ventilation in wider buildings, by supplying appropriate quantities of air and. As suitable velocities at the required areas by plenum ducts and extracting the air into return ducts and sometimes recirculation this air after properly mixing it with cooled fresh air, completely satisfactory ventilation can be obtained. In an combined supply and exhaust system it is preferable to provide slightly excess of exhaust, if there are adjoining occupied spaces and slight excess of supply if there are no such Spaces.

Mechanical roof ventilation:

Power mechanical roof ventilators of unit type increasingly used for augmenting natural ventilation in buildings with large width or where the heat load very heavy. When these provided it important that the openings within their area of influence should closed to avoid short circulating exhaust fans exercise very little influence beyond a velocity contour of about 15m/min.(50 ft/on). Which is just short instance from the fan.

The amount of air required:

Base on heat gain:

The amount of ventilation required can calculated on the basis of total heat gains from sun, not manufacturing processes machinery and occupant within the buildings, determining in advance the temperature rise which would acceptable. The volume of air required in removal of sensible heat gain (in keel. Per hour) can calculated from the formula.

 Q=Kca/hr.x.0577 °C () / Temperature rise in = B. Th U/hr. °f / ()

or

Q=1.08xTemporize in Where Q is the volume of air in cubic meters/min. (or where Q is the volume of air in cu. ft./ min. In majority of the cases; the sensible heat load will for exceed the latent hat load(resulting from moisture given off by occupants and processes); so that the capacity often ventilating equipment; can calculated in most cases on the basis of the above equation.

ALLOWABLE TEMPERATURE RISE VALUE

Roof elevation in meters Rise in °C
12 (40 feet)  4.4 to 6.7 (8 to 120F)
9 (30 feet)  4.4 to 6.7 (8 to 120F)
9 (20feet)    2.8 to 4.4 (5 to 80F)

Conditions limited to [a supply air outlets not more that 3 to 4.5 meters above floor b or medium heavy manufacturing operations and c] Freedom radiant heat. Temperature rises = roof exists temperature – outdoor temperature. The above values refer mainly to exit. Temperature rise roof and is not indicative of t temperatures. The recommended maximum allowable temperature rise for air stream as it leave the grills and reaches the working level is 1.70C{3.00F)

Based on contaminants:

Though general ventilation not appropriate method in dealing with toxic chemicals contaminants arising out of processes, this type of ventilation (dilution ventilation) used generally to control contaminants such as the vapor from less toxic solvents. The amount of ventilation required for dilution can determined if the amount of the contaminant escaping into the environment per hour and its toxic know

Cooling of supplied air:

The quantity of air required for ventilation as indicated above could reduced, if the outside air cooled before the air discharged into the buildings, Refrigeration very expensive; however, evaporating cooling may be adopted with advantage particularly in. acid regions where summers are dry with lowest bulb temperatures.

Control of heat explosives

Besides providing good general ventilation, it would be necessary in may industrial situations to adopt additional means for giving relief from heat to the workers?

Control at source:

The first obvious step mitigating the effects of heat is to eliminate the heat wherever possible or to segregate them. Some of the hot process equipment could placed out of doors with o a protective roof. The location of fume in separate wings rather than in large single building will simplify problem of supplying air for general ventilation. These wings may have high roof to provide better chimney effect for the removal of hot air over the furnaces.

Local exhaust ventilation

Another method of removing the heat from the source by providing ventilated enclosures such as canopy or exhaust hoods by which natural convention column of heated air raises from a hot process environment. “This will minimize the temperature rise in the space around the not pressure.

Insulation

The insulation or furnace and other heat producing equipments’ will not only reduce the amount of heat exposure but also result in the consequent saving in fuel consumption. If all the heat produced by a process released into a work room and the heat production constant, no amount of insulation around the process warbling about reduction in heat exposure.

The insulation will of value only when heat produced has several avenues of escape, the dissipation into the workroom being only one of these. In such cases, a combination of insulation and exhaust or gravity ventilation will often be quite effective.

Control of radiant heat:

In Some industries commonly referred to as “hot” industries, the frequency of very hot objects and surface such as furnaces, ovens, furnace stacks, molten materials, hot ingots of metal, castings, and forging is such that the major environmental heat load is in form of radiant heat which may be several times greater than the convective head load.

No amount of ventilation with or without air-cooling will reduce the hat exposure since air temperature has no significant influence on the flow of radiant hat. The only effective control the direct one of decreasing the amount or radiant heat impinging on the” exposed workers.

High reflected shield

Roofs of corrugated iron shorts; or of asbestos cement sheets get heated by sun in hot weather and act as content source of heat. The major portion of which is radiant heat, control of this type of near exposure may achieved by lowering the surface temperature’ of the roof by one or more the following methods.

  • Insulating roof by providing of a false roof or ceiling with conductivity.
  • Shielding by the provision of a false roof or ceiling with sufficient air space Ventilated to outside atmosphere.
  • White-washing or white painting the exterior surface of the roof,
  • Spraying of water intermittently over the roof to cool it y evaporation.

Openings and glazing in the walls and roof through which sun’s rays could directly penetrate summer as far as practicable be screened.

LOCAL RELIEF:

In certain situations when a general reduction of heat exposure throughout the plant is not feasible and at the’ above methods such as exhaust ventilation and radiation shielding have not brought down the thermal environment to a tolerable degree, relief may be provided locally. i.e. at the work-places by surrounding the exposed workers with an acceptable thermal environment by providing positive ventilation. The following three methods may employed depending on the work situations:

  • Providing a completed enclosure around the worker with separate ventilation in order to maintain cooler working conditions. This may in the form of air- conditioned control room small shelter booth, or ventilated crane cab:
  • Surrounding the worker with a relatively cool atmosphere; by a direct supply of air introduced at the working level or over a small area of the plant.
  • Directing a high velocity air steam at the worker when the air is capable or absorbing heat i.e.; either its dry bulb is reasonably lower than the body temperature; or its humidity is low enough to allow evaporation of swat.

Personal protection and shielding

In certain operations, such as glass enameling particularly by dry process; the workers may required to work within reach of hot objects; which may be at temperature of 8000 C (14720 F) or higher and the time; required for operation may reach 20 minutes. In such situations, protective clothing and protective shields of the heat reflecting type can be used for controlling heat exposures. The protective clothing generally made of aluminized asbestos cloth usually backed up with felt or other insulating material. Aluminized fiber glass, aluminized cotton duck or aluminum sprayed asbestos has also proved effective. The operator may equipped with mitts. Leggings, aprons jackets, helmets and special arm sleeves according to specific operations.

Local exhaust ventilationLocal exhaust ventilation

Local exhaust ventilation is one of the most important methods of control of atmospheric contaminants. The principle is to create a sufficient movement of air to withdraw contaminants at point of origin; and convey them to a’ safe point for; disposal. An exhaust consists of four major parts.

  • Hoods or enclosures near source of contaminant
  • Ducting to connect hoods into system
  • Collection equipment
  • Fan

Dust is usually more difficult to control than. Gases, Campos, mists, and fumes. Dusty operation, tens to project particles as that the hood; must provide velocities sufficient to draw them info the exhaust system, Exhathoods should enclose the process as completely as possible; or the head should be located to take advantage of directional effects of the dust flow. Dust removal systems generally requires higher air velocities; and dusts or heavier gauge metal than those designed for gases.

Hoods down drafts through grill openings below the process or slot-typed hoods. The object is to remove the contaminants without drawings it thought the breathing; zone of the operators and with minimum interference with. Ducts connect the hoods to the, central fan, distributed the airflow in direct proportion to the requirements; of each inlet and maintain adequate pipe velocity; to convey the contaminant to the point of discharge.

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