This potential solution stops the sound emission at the source, but enclosures can make machine tending or maintenance operations far more difficult. Also, machine enclo- sures can cause the machines to overheat unless additional cooling is developed. Before enclosing machines, check with the manufacturers of the machines to see if there is any downside to enclosing. Sometimes enclosure walls are too thin and light and loud sounds come directly through them. Consider going to a heavier wall construction or a double wall construction so that structural members on the inside of the wall near the machines are separated from those in the wall away from the machines.
Principle 4.13
Increase the distance between the source and nearby people.
Moving the machine operation away from nearby personnel or moving the personnel away from the operation will help attenuate the sound before it reaches the people around it. Remember, noise levels decrease approximately with the square of the distance between the source and the receiver.
Principle 4.14
Place sound-absorbing and reflecting barriers in the noise path.
Absorbing materials will help reduce the sound level energy and reflecting barriers will direct some of that energy away from surrounding personnel so that the sound wave will need to go a large distance before encountering people and, hence, attenuate.
Hearing protection
Earplugs and earmuffs are commonly used in loud environments to protect people from excessive exposure to noise. Examples of both forms of personal protective equipment are shown in Figure 4.4. Earplugs are made out of soft materials, such as cotton, wool, plastic, or wax. When inserted into the ear, earplugs significantly reduce the amplitude of particu- lar sounds. The amount of reduction in decibels (dB) is given by the noise reduction rating (NRR).* For example, if the NRR is 10 dB, then wearing the plugs in a 90 dBA noise environ- ment would reduce the exposure to 80 dBA, assuming that the earplugs fit properly. The earplug should fit snugly in the outer ear entrance without leaving any openings around the plug through which sound might intrude. Earplugs also should have a retaining ring, protrusion, or other feature to both keep them from being inserted too far into the ear and make it easier to remove them. Along these lines, some designs attach a cord or bracket to the plug, which allows them to be easily removed.
Like earplugs, earmuffs often come in a wide variety of designs, as also shown in Figure 4.4. In some cases, earmuffs are combined with other forms of personal protec- tions, such as helmets or face shields. Earmuffs also sometimes include speakers and microphones. Traditionally, earmuffs have provided a passive form of protection by act- ing as a barrier between the source of the noise and the exposed ear. More recently, the passive aspect of earmuffs has been supplemented with electronic features, such as noise cancellation and sound amplification (see Best’s Safety & Security Directory—2003).
*The U.S. Environmental Protection Agency (EPA) requires that hearing protectors be labeled with the NRR, measured in decibels.
Figure 4.4 Some examples of hearing protection devices. (Reprinted with permission from Conney Safety Supply, Madison, WI.)
Both approaches process sounds from the environment before transmitting them over speakers within the earmuffs in a way that allows the wearer to hear normal conversa- tion and other sounds they want to be able to hear, while simultaneously providing hearing protection. Noise cancellation becomes feasible when the sound is periodic or repetitive. This is done using sophisticated signal-processing circuits that cancel out periodic, predictable noise sources. Sound amplification is useful in environments where high-intensity, intermittent sound noise sources are present, such as stamping presses or gunshots. The latter systems use amplification circuitry within the earmuff to keep the sound levels within safe limits.
As implied by the previous discussion, there are a number of ergonomic issues associ- ated with the use of hearing protection. One issue is that human heads and ears vary in size and shape. This variability can make it difficult to fit certain people, and may impact both the effectiveness and comfort of hearing protection. Another point is that wearing earplugs or earmuffs can interfere with verbal communication. As pointed out by Morata et al., interference with communication and the ability to do the job are two of the most common reasons given by workers for not using hearing protection (2001).*
Temperature and humidity
The atmospheric environments people work and live within vary greatly. In tropical set- tings, people face high temperatures and humidity. In polar settings, people deal with temperatures well below the freezing point of water. The human body performs remark- ably well under various atmospheric conditions. The key issue is that the human body intelligently adapts to different thermal conditions to maintain the core body temperature within a narrow range of 36°C–37°C. This process is referred to as thermal regulation.
Thermal regulation
Thermal regulation involves several physiological responses taken to balance the heat generated within the body by metabolic processes against the flow of heat from the envi- ronment to the body. When the flow of heat out of the body exceeds that generated by metabolism, people start feeling cold, and the body makes several responses to reduce the net flow of heat out of the body. These responses include shivering, which increases the heat generated by metabolism, and reduced flow of blood to the extremities and skin, which reduces the rate at which heat is transferred out of the body. Conversely, when the flow of heat out of the body is less than that generated by metabolism, people start feeling hot, and the body makes several responses to increase the net flow of heat out of the body.
These responses include lethargy, which causes people to be less active and, consequently, reduces the heat generated by metabolism, and increased flow of blood to the extremities and skin, which increases the rate at which heat is transferred out of the body.
If the physiological responses of the body are not adequate to maintain a balance between the heat coming in and the heat going out, the system will no longer be in equilib- rium and there will be a net change in the amount of heat stored in the body. In the initial stages of this process, the physiological responses of the body still maintain core body tem- perature within the critical range by distributing the heat unevenly between the extremi- ties and deep body tissues—depending on whether the heat storage is positive or negative, the outer extremities and skin will be, respectively, warmer or colder than the core body
*Workers in the Morata et al. study also mentioned itching, headaches, and other discomfort.
temperature.* At some point, however, the core body temperature will eventually begin to change. A change of less than 1°C in core body temperature is generally viewed as the acceptable limit from a safety and health perspective. Larger changes can result in serious or fatal consequences.† Any situation where environmental conditions are severe enough to cause core body temperature to change significantly should be carefully monitored, and obviously will be uncomfortable or stressful to most people.
Heat transfer
As discussed herein, people produce heat and also receive heat from the air and other sources. The exchange of heat between the human and environment can be described with the following equation:
S M C= ± c±Cv± ±R E (4.16) where
S represents body heat storage
M represents metabolic heat produced Cc represents conductive heat exchange Cv represents convective heat exchange R represents radiative heat exchange E represents evaporative heat exchange
When the body is in equilibrium with the environment, body heat storage is zero, and the air temperature is typically between 21°C and 32°C. Outside of this range, additional cloth- ing, heating, or cooling may be required, depending on the individual and other factors.
Metabolism
The heat generated by metabolic processes within the human depends on how active people are.‡ As discussed in Chapter 2, people produce anywhere from 60 kcal/h when at rest to over 4000 kcal/h during heavy exertion. Metabolic rates for several different tasks or activities are given in Table 4.7. Note that the values in the table are given in met units.
To calculate the metabolic rate of a particular person, doing a particular task, the values in the table must be multiplied by a conversion factor and the skin area of the evaluated person. (For an example, see Box 4.7.)
When performing these calculations, the DuBois approximation formula is frequently used to compute skin areas. This formula is as follows:
Askin =0 202. W0 425. H0 725. (4.17) where
Askin is in m2
W is the person’s weight in kg H is the person’s height in m
*For example, in extremely cold environments, people’s hands, feet, ears, and noses can freeze well before the core body temperature drops below the critical range. Similarly, when people are vigorously exercising or in hot environments their hands, feet, and skin can be much warmer than the core body temperature.
† Core body temperatures below 33°C or above 42°C are extremely dangerous and are likely to result in fatal consequences.
‡ The heat generated by the human body can actually be large enough to affect the thermal environment itself.
The combined functioning of many metabolisms can generate sufficient heat to warm a cool auditorium!
BOX 4.7 CALCULATION OF METABOLIC RATE USING METS
Note that the metabolic values in Table 4.7 are given in met units. Met units reflect both the size of the person being evaluated and the level of activity. Size is an impor- tant consideration because larger people produce more heat. Size is closely related to skin area—that is, the larger the person, the larger the surface area. Consequently, skin area provides a way of measuring or estimating differences in size. A metabolic rate of 1 met corresponds to the basal or resting rate of energy production of a per- son, in units normalized by skin area. The maximum possible expenditure rates vary from about 12 mets for young men to 7 mets for 70-year-old men. The corresponding rates for women are about 30% smaller.
The metabolic rate (M) of a particular individual, performing a particular activ- ity, is simply
M=(number of mets for particular activity) *CV A* where
the number of mets for a particular activity is obtained from a source such as Table 4.7
CV is a conversion factor
A is the surface area of the particular person
Table 4.7 Metabolic Rates of Typical Human Activities Measured in mets
Activity mets
Seated quietly 1.0
Walking on level surface @ 2 miles/h 2.0 Walking on level surface @ 3 miles/h 2.6 Using a table saw to cut wood 1.8–2.2
Handsawing wood 4.0–4.8
Using a pneumatic hammer 3.0–3.4
Driving a car 1.5
Driving a motorcycle 2.0
Driving a heavy vehicle 3.2
Typing in an office 1.2–1.4
Miscellaneous office work 1.1–1.3 Filing papers while sitting (standing) 1.2–1.4
Light assembly work 2.0–2.4
Heavy machine work 3.5–4.5
Light lifting and packing 2.1
Very light work 1.6
Light work 1.6–3.3
Moderate work 3.3–5.0
Heavy work 5.0–6.7
Very heavy work 6.7–8.3
Conduction and convection
The human body also gains or loses thermal energy from the environment through con- duction and convection. Heat enters the body when hot air molecules are cooled by contact with the skin, and leaves the body when cool air molecules are warmed by the skin. This cooling and heating process causes convective air currents that greatly increase the rate of heat transfer. In this process, air molecules warmed by skin contact become lighter than the surrounding molecules and start to rise. As they rise, they bump into cool molecules.
Some heat is conducted during each contact, and the formerly warm molecules begin to fall. This cycle of rising and falling molecules results in convective air currents that carry heat from or to the skin.
The rate of heat loss or gain through conduction–convection depends upon the exposed area of the person’s skin, skin and air temperature difference, insulative values of the clothing covering the nonexposed skin, and the air velocity about the body. For a con- stant amount of skin exposure, the conduction–convection heat exchange rate is approxi- mately given by the following equation:
TC=kV0 6. (tair−tskin) (4.18) where
TC is the thermal exchange rate due to conduction–convection k is a constant*
V is the air velocity
tair and tskin are the temperatures of the air and skin, respectively
*Note that the constant k depends on the units used to measure air velocity and temperature, the skin area of the body, and the presence of clothing. When air velocity is measured in meters per minute and temperature in °C, the constant k is approximately equal to 1, if we ignore the effect of wearing clothes and assume a skin area of 1.8 m2.
The conversion factor (CV) depends on the energy units used. The three most-commonly used cases are
CV1 2 CV
2 2
50 18 4
= kcal h m/ , = . BTU/h m
CV3=58W/m2
The surface area of a particular person can be estimated using Equation 4.17, or by other means. Suppose that we wanted to estimate the metabolism of a typical young male who weighs 70 kg (154 lb) and has a height of 1.73 m (5 ft 8 in.). The first step is to estimate this person’s skin area, using Equation 4.17, as shown here:
Askin =0 202. (70 kg)0 425. (1 73. m)0 725. =1 8. m2
If this person is sitting quietly, his metabolic rate, M, is 1 met (see Table 4.7). Using conversion factor CV1, we get
M=1 8. m2(50kcal/h m2)=90kcal/h
Equation 4.18 gives TC in kcal/h when air velocity is measured in m/min and temperature in °C. Note that the exchange rate is a linear function of the temperature difference. Air veloc- ity has a nonlinear effect—that is, increasing the air velocity by a factor of four roughly dou- bles the heat exchange rate. Another important consideration is that the thermal exchange rate increases when more skin is exposed to the air and with lower insulative clothing.*
Radiative effects
The human body emits and receives thermal energy in the form of infrared radiation. If the objects in the environment are colder than the surface of a person’s skin, more heat is radiated out from the body than is received. The opposite situation occurs when hot objects in the environment radiate significant amounts of heat to the body. Almost every- one has felt the direct radiation of a bonfire on a cold night; the side of the person facing the bonfire is warmed and the side away from the bonfire is still quite cold.
Example sources of infrared radiation include blast furnaces used in the basic met- als industries, infrared lamps, and the sun. The amount of radiation energy exchanged depends upon the cross-sectional area of contact. Consequently, the thermal exchange rate is considerably less when the source is overhead than if it is frontal. Radiative energy exchange is also greatly reduced when reflective clothing is worn. The rate of radiative energy exchange is approximately given by the following equation:
TR =k t(source4 −tskin4 ) (4.19)
where
TR is the thermal exchange rate due to radiation in kcal/h k is a constant
tsource and tskin are the temperatures of the source and skin, respectively, in degrees Kelvin (k)†
A typical value of k is 4.92 × 10−8 when temperatures are in degrees Kelvin. It is obvious from Equation 4.19 that even small increases in the source temperature can cause large increases in the radiative thermal rate of exchange.
Evaporation
A third avenue of heat exchange between the human body and its environment is evapo- ration. Unlike the other two avenues of heat exchange, evaporation only results in a heat loss by the body. Evaporative heat losses depend primarily on the wetted skin area, air velocities, and the saturation deficit of the air. The saturation deficit is merely the differ- ence between the existing humidity and saturated air. As each of these variables increases, greater heat losses occur through evaporation. The maximum rate of cooling that the human body can achieve by sweating is approximately given by the following equation:
Emax=2 0. [V0 6. 42−VPa] (4.20) where
Emax is the maximum evaporative heat loss in kcal/h V is the air velocity in m/min
VPa is the ambient vapor pressure of water in mm of mercury‡
*The insulative role of clothing is discussed later in this chapter.
† The temperature in degrees Kelvin = 273 + °C.
‡ A vapor pressure of 1 lb/in.2 is equivalent to 12.475 mm of mercury vapor pressure.
Also note that the constant of 42 in this equation is the vapor pressure in millimeters of mercury of water on the skin assuming a skin temperature of 35°C.
Vapor pressure and other values of interest can be determined for particular combina- tions of temperature and relative humidity by reading a psychrometric chart, many variants of which are easily available. One such chart is shown in Figure 4.5. In Figure 4.5, vapor pressure values are plotted on the second-to-last vertical line on the right-hand side of the figure. The temperature values are plotted along the horizontal axis at the bottom of the chart, and the curved lines extending from left up and to the right are the relative humidity lines.* The upward slope of each relative humidity line describes how water vapor pressure increases with temperature when relative humidity is held constant. For example, the curved line toward the bottom of the chart shows that vapor pressure is about 3 mm of mercury at a temperature of 27°C and relative humidity of 10%.
Control strategies for hot and cold environments
Several different types of control strategies for hot or cold environments directly follow from the preceding discussion and can be roughly grouped into three categories:
engineering controls, administrative controls, and protective clothing.
Engineering controls
The primary engineering controls used to modify hot or cold environments are adequate heating, ventilation, and air-conditioning. Heating and air-conditioning systems modify both air temperature and humidity and are obviously the first choice for improving many inside environments. However, it should be emphasized that heating and cooling costs can be a very important economic consideration when changes are contemplated for large facilities, such as warehouses. Determining the energy required to change environmen- tal conditions from one state to another is an important step in deciding whether such changes are economically feasible.
The energy needed can be estimated for a perfectly efficient system by reading off enthalpy values for the compared conditions from a psychrometric chart, and then performing a few additional calculations. Enthalpy is the inherent amount of heat energy held by moist air, for different combinations of temperature and humidity, and is measured in units such as kilojoules per kilogram of dry air (as shown in Figure 4.5), the number of British thermal units (BTUs) per pound, or watt-h per gram of dry air.
The difference in enthalpy between two environmental states multiplied by the mass of the dry air contained in the facility provides a measure of how much sensible heat must be added or removed to implement a proposed improvement. For pure heating and cooling, or, in other words, the addition or subtraction of heat without changing the humidity,† the change of state corresponds to a horizontal shift across the psychro- metric chart from one temperature to another. Humidifying or dehumidifying without changing the dry bulb temperature analogously corresponds to a vertical shift across the psychrometric chart.
A second strategy is to increase the air flow around the people to increase evapora- tive cooling. The latter strategy is effective only at lower humidity levels. Effectiveness
*Relative humidity is defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density and is usually expressed on a measurement scale that goes from 0% to 100%. When the air contains no evaporated water, the relative humidity is 0. When air is saturated, the relative humidity is 100%.
† This type of atmospheric change is referred to in the heating and air-conditioning literature as sensible heat- ing and cooling.