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SECTION 14. Industrial Ventilation

General Features of Ventilation Systems Definition Ventilation refers to the continuous supply and removal of air with respect to a space. In

industry this is done to control chemical and some physical hazards, as well as to maintain conditions of temperature and relative humidity which are compatible with human habitation and industrial operations.

Air motion To move air requires creating a pressure difference between two points. Air will then

move from the region of higher to the region of lower pressure, at a rate that depends on the magnitude of the pressure difference and on the impedance to air flow offered by ducts, objects and friction. The pressure difference is created with a fan, or blower, or by inducing a density difference through differential heating. The latter mechanism is the source of wind outdoors, but is also used in workplaces where heating sources exist, as in metal refining, melting and casting.

System performance requirements The desired performance of the ventilation system will impose requirements on the

magnitude of the pressure difference to be created by the fan or heat source.

Capture In many systems the moving air must capture the airborne contaminant by entraining the

surrounding air and conducting it in the desired direction. This leads to requirements for air velocity (speed and direction) and volumetric flow rate.

Transport

Once captured the contaminant must be carried away from the inhabited space to a

suitable collector or disposal point. This is generally done by forcing the air to flow through ducts where impedance to air flow must be overcome.

Dilution

In some cases the principal goal of the system is to dilute the contaminant to safe levels

by supplying a sufficient volumetric flow rate. Again, there is often a requirement that the direction of air motion be controlled to achieve the desired dilution and worker protection.

Behavior of contaminants

Industrial Ventilation page 14.2

As pointed out earlier, nearly all airborne contaminants of hygienic importance will follow air motion, so that controlling the movement of air will cause contaminants to move in the same direction. The only important exceptions to this occur when particles are emitted at high velocity from industrial operations such as grinding, where metal particles leave the work in the direction of rotation of the grinding wheel, independent of the movement of air. This effect is important for distances of not more than about two feet, beyond which the particles are forced to decelerate and again follow air currents. Further, contaminants which are in the particle size range that is important for inhalation are not influenced to significant degree by gravitation and inertial forces, so they will remain airborne for extended periods in the workroom. Even dense gases such as chlorinated solvents will not settle appreciably due to gravity unless they are present at extremely high concentrations, generally above 1000 parts per million. This means in turn that the physical properties of air which are important in causing it to move, density and viscosity, are not affected by the presence of contaminants in the concentrations important to human health. It should be noted here, however, that when one or more contaminants are present at volume fractions in the range of 1 to 10 percent (10,000 to 100,000 ppm) which is the explosive range for flammable contaminants, the density and viscosity of air will be affected.

General Ventilation This type of system provides air flow to the entire volume of a space, either by natural or

mechanical (forced) means. It is commonly used in office and public buildings, but also is used in some industrial applications. These are often referred to as heating, ventilating and air conditioning (HVAC) systems, although they may often be designed for other purposes as well.

Dilution This form of general ventilation is intended to dilute the contaminant generated in the

workplace with uncontaminated air so that the concentration never approaches a hazardous level.

Volume requirements

The volumetric flow rate of dilution air depends primarily on the evolution rate of the

contaminant to be controlled, but also on the efficiency with which fresh air mixes with workroom air. In order to keep the concentration of contaminant in the mixed air at or below the Threshold Limit Value (TLV), the flow of dilution air supplied is given by:

Q = ER x K 14.1 TLV where Q = dilution air flow rate, m3/min ER = contaminant evolution rate, mg/min TLV = Threshold Limit Value, mg/m3 K = mixing factor


The factor K accounts for imperfect mixing in the workroom, which means that more

dilution air is required than in the case of complete mixing, where K = 1.0. Values chosen by the designer for K generally range from 3 to 10, but are based on guesswork and experience rather than hard experimental data. Because of this uncertainty, dilution ventilation is not recommended for control of contaminants of high toxicity, or for situations where the rate of evolution is not steady over time. It may be used, however, in unoccupied spaces where the main hazard is explosion due to a contaminant present in high concentration. In this case the TLV in the equation is replaced with the Lower Explosive Limit (LEL), defined as the lowest contaminant concentration in air at which an external ignition source could produce an explosion. For all flammable vapors, the LEL is 1% or more, so that dilution volumes required to control this type of hazard are much lower than those required to control toxic exposures. Conversely, if the concentration of a flammable contaminant is kept below the TLV by any form of control, there cannot be an explosion hazard.

Note that equation 14.1 indicates that the flow rate of dilution air does not depend on the volume of the space to be ventilated. This is at variance with the common practice of specifying ventilation requirements in terms of "number of room air changes per minute or per hour" which directly involves room volume. The fact is that the rule of thumb based on room air changes per minute, though in widespread use over many years, has been used improperly in the control of workplace hazards.

Air flow pattern

Apart from the volumetric flow rate, the other requirement of a dilution system is that the

direction of air flow in the ventilated space be controlled. This is necessary to achieve the most uniform mixing, but also to carry contaminants away from the breathing zones of the workers. Figure 14.1 shows several examples of dilution ventilation systems in which the air flow patterns differ depending on the location and configuration of the air inlet and outlet. The best arrangements conduct the contaminant in the proper direction, and also include a fan and distributor for inlet air giving good mixing, as well as a fan for the outlet. Such dual fan systems are also referred to as "push-pull" designs. On occasion dilution systems incorporate provision for recirculation of the outlet air, and still others permit inadvertent recirculation when the outlet is placed too close to the dilution air inlet. When toxic contaminants are evolved in the workroom, recirculation must be avoided.

Applications

As an example of the design of a dilution ventilation, assume that methyl ethyl ketone is

used in a solvent cleaning operation, and on the basis of consumption of MEK over time it is estimated that 1 gallon of solvent is evaporated per hour of operation. This is the same as 3.8 liters/hr, and using the density of liquid MEK the mass evaporated is 55.1 g/min. The TLV for MEK is 590 mg/m3. If good mixing is assumed (K = 3), then the flow rate of dilution air required to control at the TLV is

Q = ER x K TLV


= 280 m3/min The Lower Explosive Limit for MEK is 2% by volume, or 20,000 ppm, one hundred

times higher than the TLV. Therefore under similar assumptions, the dilution flow rate required to control only the explosion hazard is 2.8 m3/min.

Energy requirements

All forced (fan-driven) ventilation systems require energy to create air flow and

overcome friction. This is estimated using techniques described below for local exhaust systems. There is a large additional energy requirement for dilution systems associated with the volume of outdoor air usually used as the source of clean dilution air. When the outdoor air temperature and relative humidity are not suitable for indoor human habitation, air conditioning is necessary. For the example worked above with MEK, if outside air is used for dilution and its average annual temperature is 55 F while the desired indoor temperature is 70 F, the heating costs for tempering the air can be estimated. Using oil heat at $1.25 per gallon of fuel, the annual cost of air tempering for the high flow, TLV target case is $162,000, which would be considerably higher than the cost of the energy required to move the air into and out of the workroom. For the explosive hazard case, the air tempering cost would be 1/100 this figure.

Comfort /Heat Control This form of general ventilation is used to maintain the temperature and relative humidity

of the air in a workplace within the usual limits for the comfort of the occupants. At the rate of airflow usually necessary to do this, odors, carbon dioxide and other man-associated contaminants are readily diluted to acceptable limits. In addition, this flow of air will deliver far more oxygen than is required for respiration even at the highest physiological work rates.

Although carbon dioxide produced by human metabolism does not reach hazardous levels in office buildings, it is often a useful indicator of the adequacy of building ventilation. Concentrations of CO2 above about 1,000 ppm indicate low exchange with outside air, and are associated with excess complaints from building occupants.

Comfort ventilation systems do not, however, employ air flow rates adequate to control health and safety hazards presented by other chemicals, and are not recommended for that purpose. In the design and construction of modern office and public buildings, there has been and understandable but unfortunate trend to reduction of the flow rate of fresh, outdoor air used in ventilation systems. Energy costs are the main factor driving this trend, but the consequence of this is that such systems often fail to dilute some of the chemicals commonly evolved in office and light industrial operations. The result is complaints from occupants of non-specific health problems which are probably related to the accumulation of airborne residues of cleaning solvents, copy machine fumes, carpet and drapery emissions and a variety of other unidentified chemicals emitted by many materials of construction.

Local Exhaust Ventilation


A local exhaust system is used to control air contaminant by trapping it at or near the source, in contrast to dilution ventilation which lets the contaminant spread throughout the workroom, later to be diluted. Local exhaust is generally a far more effective way of controlling highly toxic contaminants before they reach the workers' breathing zones. This type of system is usually the proper control method if:

air sampling shows that the contaminant is a serious health hazard. emission sources are large in magnitude, few in number, and/or widely dispersed. emission sources are near the workers' breathing zones. emission rates vary widely with time. there are several contaminants emitted by the process having different levels of

toxicity. components The four major components of a local exhaust system are shown in figure 14.2.

hoods

The hood is the point where air containing the captured contaminants enters the system.

Its purpose is to direct the air flow so that its direction and distribution are appropriate for the conditions at the site of contaminant generation. Capturing hoods create a directed air current with sufficient velocity to draw contaminants from outside the hood itself. An example is the downdraft hood often incorporated into modern kitchen range units which captures cooking fumes by drawing them sideways and downward before they can escape into the general room atmosphere. Enclosing hoods surround the contaminant source as completely as possible, and include laboratory fume hoods and glove boxes. Receiving hoods are placed and shaped to receive the contaminant as it is thrown out by a source, as in the canopy hood over hot work (not recommended) and in the hood around a grinding wheel.


ductwork The ductwork is a network of piping which carries the captured contaminant out of the

workroom to its final disposition. The primary goals in designing ductwork are to maintain sufficient air velocity in the piping, and to minimize the resistance to flow created by bends, junctions and changes in cross-sectional area. Maintenance of air velocity is important for transport of particulate contaminants, since if the air velocity falls below a critical (empirically determined) value, the dust will deposit on the inner surfaces of the ducts and impede flow. Resistance can be the major part of the energy required to operate the system, and a poorly designed duct system may cause so much resistance that the fan is unable to move the required volume of air.

air cleaner

The air cleaning device removes the captured contaminant before the exhausted air is

discharged, either outdoors or into the recirculation pathway if provided. Collectors may range from simple centrifugal collectors much like the cyclone used to sample respirable dust, to elaborate filters and gas/vapor adsorbers specially designed for the contaminant present. All air cleaners add to the total resistance of the system; generally, the more efficient the collector, the greater the resistance added.

fan

The final component provides the energy to accelerate the air as it enters the hood and to

overcome friction and dynamic losses (eg, from elbows, junctions) between the moving air and the surfaces of the ductwork and cleaning device. If possible the fan is almost always placed downstream of the air cleaner, to prevent deposition of contaminant on the fan blades, or damage due to contact with corrosive contaminants.

capture velocity For a contaminant to be captured effectively at a capturing hood, before it is released into

the workroom air, the local exhaust system must create a pattern of air flow of adequate speed and direction. The minimum air velocity necessary to capture the contaminant, or capture velocity, depends on the conditions at and near the point of release, and on the nature of the contaminant. The range of recommended capture velocities, in feet per minute (fpm), for common local exhaust applications is given in Table 14.1. Laboratory fume hoods, as an example, are generally designed to operate with a minimum capture velocity of 100 fpm at the face of the hood opening. The influence of air currents in the room, and especially in the area near the hood entrance, should be noted. Cross drafts due to other operations in the room can impair the performance of a local exhaust system to a severe degree.

flow pattern The direction of air flow near the hood entrance is also important in effecting capture. In

all suction systems the influence of air moving into the hood reaches out only a short distance from the entry point. This is in marked contrast to flow of air out of a hood or duct, where


the velocity of the air in the jet is still measurable several duct diameters away, as indicated in figure 14.3. The short reach for suction systems is shown in further detail in figure 14.4a and 14.4b. For a plain duct opening the velocity of air near the duct is less than 7.5% of that at the duct opening at a distance of one duct diameter from the entrance. Adding a flange to the duct opening, making it a crude hood as in figure 14.4b, provides slight improvement, but the point to be made is that capture is only possible at very short distances from the hood opening, even in the best designed systems. Generally speaking, local exhaust systems will only be effective in capturing material within one foot of the hood opening.

energy losses As suggested earlier, energy is required to move air into and through the local exhaust

system, and this is provided by the fan. The two components of energy use are the acceleration of the (nearly) still air in the workroom to the velocity of travel through the hood and ductwork, and the friction created by air rubbing against the sides of the ductwork, and the dynamic losses caused by sudden changes in direction or separation from surfaces such as elbows and junctions. Both components of energy are "lost" in that they ultimately appear in the environment as heat and cannot be recovered, so the energy requirement is referred to as energy loss. Knowledge of the energy loss components of a system is critical in determining the power requirements for the fan, which dictates in turn the size of the fan motor and the amount of electric power needed. The components of energy loss can be determined by measuring two types of pressure within the system, as described below.

relation of velocity to flow rate Thus far we have focused on the velocity of air entering and passing through the system

as the major factor in performance of the capture function. A second, related factor is the volumetric flow rate of air necessary for capture, which depends on the physical dimensions of the source of contaminant, and on the evolution rate. Unfortunately, the determination of adequate flow rate for capture is almost entirely empirical, based on accumulated experience with a variety of applications in industry. The best single source of guidance on necessary air flow rates for capture is "Industrial Ventilation - A Manual of Recommended Practice" published by the ACGIH. For our purposes it will be important only to recognize that air volumetric flow rate and air velocity are related by a simple expression:

Q = A x V 14.2 where Q is the volumetric air flow rate, in cubic meters/min or cubic feet/min A is the cross sectional area of the duct or hood, in square meters or

square feet V is the air velocity in meters/min or feet/min Thus knowledge of the velocity of air flowing in a portion of the ductwork, or in the

hood, and of the corresponding cross sectional area will permit calculation of the air flow rate. This will prove useful in providing for make-up air, as described in the earlier portion of this section on general ventilation systems.


measurement of performance Performance of a local exhaust system is evaluated somewhat indirectly, by measuring

pressures at various points near the hood and in the hood and ductwork. From these measurements the velocities and volumetric flow rates are calculated.

pressures

Two types of pressure can be measured in exhaust ventilation systems, using the simple

U-tube water-filled manometer, and a specially made piece of tubing called a Pitot tube. Figure 14.5 shows the types of pressures and their relationship to energy loss. The "static" pressure is measured with the manometer connected as shown in part one of the figure. In each of the two configurations shown the manometer is exposed to the air via an orifice whose plane is oriented parallel to the air flow direction. In most cases the static pressure is the driving force causing air to move from one point to another. Thus in most parts of the ductwork, the static pressure must be lower than the pressure in the workroom. The second type of pressure is called the "velocity" pressure; its measurement is shown in part two of the figure. The second arm of the manometer is now oriented with the plane of its orifice perpendicular to the flow direction, and the velocity pressure is given as the difference in height of water in the two arms of the manometer. The velocity pressure is proportional to the square of the air velocity:

VP = k x (Velocity)2 14.3 where k depends only on physical properties of the air at existing temperature

and barometric pressure The two pressures are commonly measured by drilling small holes in the ductwork and

inserting the Pitot-static tube as shown. Part three of the figure shows the general relationships among the pressures at two points in a section of ductwork. The velocity pressure, which depends only on the air velocity, will be the same at the two points if the duct cross section is fixed. The sum of static pressure and velocity pressure at each point, sometimes called the "total" pressure, is also equal to the energy of the flowing air at each point. Because of friction, there is a loss of energy between the two points, and this loss is equal to the difference in total pressures. Total pressure must decrease in value in the direction of flow.

The relationships are also shown in figure 14.6, a diagram of a simple local exhaust system including the fan. The principle of energy conservation requires that the energy delivered to the fan be accounted for as an increase in the energy content of the air moved through the system, plus energy dissipated as heat. Starting with still air in the room outside the hood, the total energy is proportional to the sum of the static pressure (which is zero relative to barometric pressure) and the velocity pressure, also zero since the air here is not in motion. Once inside the duct, the energy content of the air is again proportional to the sum of the two pressures: static pressure is now subatmospheric or negative when measured with the manometer, but the velocity pressure will be positive because the air is in motion. However, the total energy content of the air must decrease as it passes along the duct


approaching the fan because of energy lost to friction. Thus the algebraic sum of static pressure and velocity pressure at point two in the figure must be less than the corresponding sum at any point upstream, including point one outside the hood. As energy losses in the system increase due to rough edges, sharp turns or deposited dust, the static pressure must be made more negative to achieve a given velocity, and the fan must deliver more energy.

Bibliography McDermott HJ. Handbook of Ventilation for Contaminant Control. 2nd Ed. Boston:

Butterworth Publishers. 1985 ACGIH. Industrial Ventilation. A Manual of Recommended Practice. 21st Ed.

Cincinnati, OH: American Conference of Governmental Industrial Hygienists. 1992. McQuiston FC, Parker JD. Heating, Ventilating, and Air Conditioning. Analysis and

Design. 4th Ed. New York: John Wiley & Sons. 1994.

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