In this essay we will discuss about the introduction and types of industrial ventilation.
Essay # 1. Introduction to Industrial Ventilation:
In various industrial enterprises hazardous or potentially hazardous conditions exist which can effect the health and safety of people working there. Fumes and vapours are given off from storage tank, processing tanks, and other types of processing equipment.
Dust are given off from grinders, pulverisers, hammer mills, conveying equipment, mixers and many other types of equipment. Paint sprayer produce mist which can get into the work space if uncontrolled. But most heavy industries have large number of situations in which maintenance of a clean working environment is essential.
Four general methods to prevent exposure of workers to harmful levels of pollutants are:
ADVERTISEMENTS:
(a) Replace hazardous process by non-hazardous one.
(b) Enclose the source of pollutant, use container in place of open vats for storage of chemicals.
(c) Dilute the concentration of the pollutants by providing adequate ventilation air to the region near the sources of pollutants.
(d) Provide exhaust hood which will ensure that the creeping substances will be sucked into the hood and not allowed to escape in the work space.
ADVERTISEMENTS:
In addition to above measures protective devices such as mask, etc. should be provided whenever necessary to the workers.
The problem of industrial ventilation is distinct from that of pollution control because in that the usual method of ventilation is to remove the pollutants from the work space by exhausting them out door. Pollution control however requires that the pollutants may not be exhausted outdoors.
These two opposing requirements can both be met by installing pollution control devices prior to exhausting the ventilation stream. Thus, industrial pollution control and ventilation often go hand to hand even though the engineering principles involved in both subject are rather similar.
It can be safely said that the most desirable method of pollution control is to avoid producing the pollutants in work place, which also makes industrial ventilation a much simpler task.
ADVERTISEMENTS:
Control of contaminants in work room air for health protection or tire/explosion prevention is not only done for ventilation. Other uses include comfort material, reuse and environmental protection.
Essay # 2. Types of Industrial Ventilation:
There are two types of industrial ventilation dilution and local exhaust. In dilution ventilation contaminants released into the work space are mixed with air flowing through the room. Either natural or mechanically induced air movement can be used to dilute air contaminants.
In local exhaust system the contaminants are captured at their source before they escape into the work space. In local exhaust system contaminants are removed rather than just dilute them. The local exhaust system is more difficult to design than dilution ventilation system.
I. Dilution Ventilation Systems:
ADVERTISEMENTS:
In dilution ventilation system contaminants in the working space are diluted to acceptable level by providing sufficient air flow.
Dilution ventilation can be classified as:
(i) Natural Ventilation, and
(ii) Mechanical Ventilation.
ADVERTISEMENTS:
(i) Natural Ventilation:
It is the air movement within a work area due to natural wind temperature difference between exterior and interior of a buildings or other factors, where mechanical air movement is not used.
A 25 km/hour wind blowing directly at a window with an open area of 3.3 m2 can move 700 m3/min air through the window.
(ii) Mechanical Ventilation:
In mechanical ventilation mechanical means such as fan blowers, exhaust etc. are used for dilution ventilation or removal of air.
The amount of dilution air flow required depends upon:
(a) Rate of contaminant released
(b) Toxicity and flammability of contaminant
(c) Acceptable air borne concentrations
(d) Relative efficiency of total air volume flowing through area.
The room size is not used to calculate dilution requirements since the air flow for these system is not based on the room air changes per hour as is often used for general comfort exhaust ventilation.
Dilution Ventilation for Health:
The dilution airflow rate for toxic or irritating contaminants is,
where, Q = dilution air flow, m3/min.
W = amount of liquid used per time interval
F = concentration factor for units for W
p = Sp. gravity of liquid
M = molecular weight of the contaminant
L = Acceptable air born concentration of contaminant, TLV, range ppm
K = dimensionless safety factor to increase the calculated air flow rate over minimum in order to take non-ideal conditions, normally k ranges from 3 to 10 depending upon the number of workers exposed.
a. Dilution for Fire Protection:
Dilution is also adopted for reduction concentrate in flammable or explosive gases, vapour or dust to safe level below their lower explosive limit. (LEL) The dilution must occur before the contaminated air reaches any source of ignition.
The Air flow rate for dilution for fire/explosion prevention is,
where, W = amount of flammable liquid used per time interval
F = conversion factor for units
C = dimension less safety factor that depends on the percentage of LEL acceptable to safe condition.
M = molecular weight of contaminant
LEL = lower explosive limit of contaminant percent
B = constant reflecting that the LEL decreases at elevated temperature
β = I for temperature up to 250°F,
β = 0.7 for temperature above 280°F.
b. Dilution Ventilation System Layout:
Dilution system work in best way when the air inlet and exhaust fan are located so that as much air as possible flows through the zone of contaminant release only the air that passes through the area where contaminants are released is available for immediate dilution of contaminant to safe levels. Air supply systems with blowers and air out lets near the work area used to provide the correct amount of air at right place.
The plant should be arranged so that the air movement is from cleaner to director area. Locate the process or locate the fan so that the air unit that release contaminants are as close as possible to the fans.
c. Proper Hood Selection:
The hood is the important part of a ventilation system. No local exhaust system will work properly unless enough of the contaminant are retained or captured by the hoods so that the concentration of concentration in work room air is less than the acceptable limits.
Both the design and location of hoods are crucial in determining whether a system will work or not. A poor design may prevent the ventilation system form ever working adequately. It may also result in excessive power cost as fan size and speed are increased to compensate for the initial poor hood selection.
Various Types of Hood:
There are three types of hoods which work on different principles:
(i) Enclosures Hoods:
That surrounds the contaminant sources as much as possible. Contaminants are kept inside the enclosure by air flowing in through opening in the enclosure.
(ii) Receiving Hoods:
Some process through a stream of contaminants in a specific direction. For example, a furnace emits a hot stream of air and gases that rises above the unit.
The ideal hood for this type of process is one that is positioned so that it catches the contaminant thrown at it.
(iii) Capturing Hoods:
Hoods that reach out to capture contaminant in the work room air. Air flow into the hood is calculated to generate sufficient capture velocity in the airspace in from of the hood. The needed capture velocity depends on the amount and motion of contaminants and contaminated air.
Different types of above hoods are shown in Fig. 11.1.
The design of different hoods with their hood velocities and volumes are shown in Fig. 11.2 In figure 11.2 CO is air flow rate in m3/min H is heat flow from heat source (BTu/min), A is the area of hot surface and Z is the distance between hot surface and hood. The expansion of the rising gases is allowed for in a canopy hood designed by providing an overhang of 0.4 Z at each of the hood opening.
Guidelines for Hood Selection:
For the selection of hood to suit a particular system following guidelines are followed:
(i) Minimize airflow requirement
(ii) Protect workers breathing zone
(iii) Follow design recommendation
(iv) Make the hood useable by workers, and
(v) Avoid common hood selection fallacies.
Depending upon the process, ventilation system may have one or more hoods in the work area. Design of multiple systems is more complex. In multiple system air provided with dampers to control flow through hoods.
II. Local Exhaust:
a. Air Cleaner:
Exhaust air from ventilation system is to be cleaned before discharging into atmosphere. Various air cleaning devices such as, fabric filters, electrostatic precipitation, wet scrubbers, cyclones are widely used to separate contaminants from gas stream. The choice for air cleaning device depends upon the nature of contaminant, their toxicity, and quantity.
b. Exhaust Fans:
Fans are required for the movement of air in ventilation system, flic fans generates the suction in the system that draws contaminated air in through the hoods. Fans have some built in flexibility since their capacity increases with higher fan speeds. Although speeding up the fan is the standard remedy for systems with inadequate air flow, this also increases fans power consumption.
For the selection of a right fan for a ventilation system following information’s are required:
(i) Air volume to be removed
(ii) Fan static pressure
(iii) Type and concentration of contaminants in the air
(iv) Importance of noise levels as a limiting factor.
The fan size in a ventilation system must be specified by both volumetric air flow (m3/min) and fan static pressure (in cm of water). Together these both parameters specify how much air the fan has to move against the system static pressure or resistance. Fan static pressure is the amount of static pressure which the fan must develop to move the required amount of air through the system.
Fan static pressure is given as:
FSP = SP/at fan inlet + SP/star – VP inlet
FSP = fan static pressure, cm of water
SP = static pressure
VPintet, — velocity pressure at fan inlet, cm of water
Since the fan may be located anywhere, in a local exhaust system, then
FSP = SP/inlet + SP/outlet – VP inlet
The velocity pressure is the amount of kinetic energy in the moving air and is related to velocity as,
VP = (V/4005)2
where, VP = velocity pressure cm of created
V = velocity, m/min
c. Ducts:
Ducts size and length are important factor in the design of the ventilation system. Duct diameter and number, type of bends affect the resistance in the duct network. In multiple hood systems a design which results in too much resistance in one branch will interfere with air flow through the entire system.
d. Exhaust Stacks:
An exhaust stack on a ventilation system serves two purposes, it helps to disperse the contaminants, in the exhaust stream by discharging the exhaust air above roof level and it improves fan performance since the uneven velocity distribution at the fan outlet causes a high velocity pressure at the outlet. This higher pressure can result in higher discharge losses if the system has no stack on the fan.
All system should have at least a short straight stack on the fan. A high stack discharge velocity (1000 m/min or higher) helps to disperse contaminants since the air jet action can increase the effective stack height under severe wind conditions.