Department of Industrial Engieering. Climate: Chap: 22

Transcription

1 Department of Industrial Engieering Climate: Chap: 22 Learning Outcomes: After careful studying this chapter, You should be able to: Define key terms like sick building syndrome, heat stress, cold stress, Explain different ways of heat transfer, Discuss the effects of heat as well as cold on workers, Distinguish between area ventilation and local ventilation, Explain the comfort factors related to work environment, Discuss ways how to reduce heat stress as well as how to reduce cold stress, Compare between heat stress and cold stress, 1

2 work environment A work environment in which the temperature is not properly controlled can be uncomfortable. Extremes of either heat or cold can be more than uncomfortable they can be dangerous. Heat stress, cold stress, and burns are major concerns of modern ergonomics & safety & health professionals. Temperature, humidity, air distribution, personal preference, and acclimatization are all determinants of comfort in the workplace. Determining optimum conditions is not a simple process. Overview Sources of heat High air temperatures Radiant heat sources High humidity Direct physical contact with hot objects Energetic physical activities Sources of cold Low air temperatures Wet and windy environments Direct physical contact with cold objects 2

3 Industries Iron and steel foundries Brick-firing plant Ceramic plants Glass products facilities Rubber products factories Electrical utilities Bakeries Commercial kitchens Laundries Food canneries Chemical plants Mining sites* Smelters Steam tunnels Construction sites* Asbestos removal Hazardous waste site activities* * cold or heat exposure THERMAL COMFORT Conduction is the transfer of heat between two bodies that are touching, or from one location to another within a body. Convection is transfer of heat from one location to another by way of a moving medium (gas or liquid). Metabolic heat is produced within a body as a result of activity that burns energy. Environmental heat is produced by external sources. Radiant heat is the result of electromagnetic nonionizing energy transmitted through space, without the movement of matter within that space. 3

4 Key Definitions Heat a measure of energy Metabolism - the system of chemical activities by which a living thing gets power (energy) from other things, such as food Conduction- the transfer of heat between materials that contact each other Convection- the transfer of heat in a moving fluid Key Definitions (cont.) Radiation - the transfer of heat energy through space Evaporative cooling- takes place when sweat evaporates from the skin Hypothermia systematic effect of cold stress; reduced body temperature Hyperthermia systematic effect of heat stress; increase in core body temperature 4

5 Fundamental Factors Primary: Environment/climatic conditions Clothing Work Demands Secondary Age Gender Weight Degree of physical fitness Degree of acclimatization Use of alcohol or drugs Medical conditions Prior heat/cold injuries Responses of Humans to Heat Increased flow of blood Sweating Illnesses/disorders Acclimatization to heat Susceptibility to diseases 5

6 Metabolic Expenditures sleeping desk work light machine work moderate assembly work lifting, manual handling pick & shovel work Air Volume and Quality Volume of air required is proportional to local contaminants. Carbon dioxide may be limiting. Body odors and cigarette smoke can be contaminants. For forced-air heating and cooling, ventilation volume may be determined by room temperature. 6

7 Sick Building Syndrome A problem of tight buildings. Interior pollutants, mites and fungi are not isolated. Carbon monoxide may enter through supply air ducts (Channels). Shop Environments Local contaminants may pollute the air. Solution strategies: 1. Decrease the concentration of the contaminant. 2. Reduce worker exposure duration. 3. Consider ventilation. 7

8 Area Ventilation Use when contaminant sources are diffuse (spread out). Keep the contaminant source downwind from the person. Locate the source as close to the exhaust as possible. Local Ventilation Capture the contaminant directly. Dump exhaust air outdoors or filter it. Make filter systems fail-safe. For explosive substances, TLV is limiting. Consider reusing the air after filtration. The threshold limit value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse health effects. Strictly speaking, TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). 8

9 Comfort Factors Dry bulb temperature/ Wet-bulb temperature Water vapor pressure Air velocity Radiant temperature Metabolic rate Clothing Time of exposure Dry bulb temperature/ Wet-bulb temperature Dry bulb temperature/ The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. Temperature is usually measured in degrees Celsius ( C), Kelvin (K), or Fahrenheit ( F). Wet-bulb temperature Wet-bulb temperature is largely determined by both actual air temperature (dry-bulb temperature) and humidity, the amount of moisture in the air. 9

10 Comfort for Standard Conditions Use psychrometric chart to find comfort zone. Use percentage of people dissatisfied (PPD) calculation. Remember that sweating is not comfortable. Psychrometrics or psychrometry or hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilating, and air-conditioning and meteorology. In human terms, our comfort is in large part a consequence of not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor. Psychrometric-Chart 10

11 Psychrometric-Chart Adjustments for Nonstandard Conditions Clothing Activity Air velocity Mean radiant temperature Time of exposure Time of day and season of year Gender and age of occupant 11

12 Individual Adjustment Conventional approach is to set conditions to match comfort zone. Desktop controls allow individuals to set their own conditions. Occupancy sensors turn off equipment (moving sensor). Energy savings may be substantial. Women are comfortable at a temperature 2.5C warmer than men - between 24-25C. (WHY)!!!! Heat Stress Problem is effects on performance and health. Performance declines well before physiological limits are reached. Design using comfort as the criterion. For health, use heart rate, blood pressure, or body temperature criteria. 12

13 Signs and Symptoms: Heat Stress Markedly raised temperature of about 40 C (104 F) or more. Headache, sickness, vomiting; loss of appetite Fast heart rate (tachycardia); fast breathing Red skin; skin may feel hot to touch Strong sweating which eventually leads to an absence of sweating Chills and shivering is possible Confusion; coma/unconsciousness Temporary Blindness is possible Heat-Related Illnesses Heat stroke Heat exhaustion Heat cramps Heat collapse Heat rash Heat fatigue 13

14 HEAT STRESS Widely used heat stress-related terms: Heat exhaustion - A physical state in which skin becomes clammy & moist and body temperature is still normal, or slightly higher than normal. Results from fluid & salt loss through sweating, that are not properly replaced during exertion. Heatstroke - Skin becomes hot & dry, there is mental confusion, and may be seizures or convulsions Heat cramps - muscle cramps that can occur when workers exert themselves sufficiently to lose fluids & salt through sweating, but replace only fluids by drinking large amounts of water containing no salt. HEAT STRESS Widely used heat stress-related terms: Heat rash - Workers in a hot environment in which sweat does not evaporate can develop a prickly rash. Periodic rest breaks in a cool environment that allows sweat to evaporate will prevent heat rash. Heat syncope or fainting - Workers who exert themselves in a hot environment will sometimes faint. Heat strain is the overall physiological response resulting from heat stress. Acclimatization is a gradual physiological adaptation that improves an individual s ability to tolerate heat stress. 14

15 Setting Environmental Limits Measure physiological responses during work Predict the stress beforehand based on predicted environments and tasks WGBT combines effects of the four environmental comfort factors. Ways to reduce heat stress Engineering controls Ventilation Shielding Cooling fans, etc. Work practices Drinking plenty of water Training Move to cooler place Reduce work pace or work load Removing or loosening clothing Acclimate 15

16 Reduction of Heat Stress Consider human body heat storage. Reduce metabolic rate. Consider work being done. Reduce radiant load. Maximize convective heat loss. Increase evaporative heat transfer. Consider conductive cooling. Radiant Heat Transfer Driving force is difference between the two temperatures, each to the 4th power. Reduce radiant load by working in the shade. Use clothing (hats and long-sleeved shirts). Use a heat shield with ovens, welding, and molten glass. 16

17 Convective Heat Transfer Driving force is the difference between the temperatures. Keep temperature of the environment below 35º. Increase air velocity on the skin. Permit air circulation around clothing. Humid moving air can cool a worker by convection. Evaporative Heat Transfer Sweating capacity increases with acclimatization. Prevent dehydration by replacing water. Evaporation can be limited by humidity. Reduce effect of humidity by increasing air velocity or decreasing water vapor pressure. Non-permeable clothing prevents sweat from evaporating. 17

18 Cold Stress Cold causes discomfort, reduced mental performance and dexterity, pain, loss of extremities, and death. Hand skin temperature critically affects dexterity and tactile sensitivity. Vigilance decreases when the core temperature drops. Signs and Symptoms: Cold Stress Dry, cold skin Shivering, but only during the early stages Slow pulse; slow breathing Drowsiness, which can lapse into coma. 18

19 Effects of Reducing the Core Body Temperature Environmental Limits Wind chill index combines all factors for cold. Wind chill is based on cooling water, not a clothed living human. Use wind chill for livestock and for a crude index for precautions. Wind chill (popularly wind chill factor) is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of cold air. 19

20 Preventing Cold Stress Whether employees are exposed to cold air or are immersed (deep) in cold water, wind can magnify the level of cold stress. The phenomenon often referred to as wind chill. Protection Against Cold Stress Layer clothing. Shoes should accommodate two pairs of socks. Keep clothing dry. Protect the head. Warm the hands by wearing a jacket. For exercise, use breathable fabrics. Reduce air velocity. 20

21 Other Factors in Cold Stress Cover openings or use diffusers or deflectors. Keep floor temperature at 73.5º in offices. Avoid contact with cold metal. Drink fluids to prevent dehydration. Avoid caffeine (a vasodilator). Ways to reduce cold stress Engineering controls Heating Shielding (from wind) Work practices Training Move to warmer place; avoid winds and wet environments Clothing Adequate insulating dry clothing Protect extremities 21

22 Preventing Cold Stress Employees should be medically certified as suitable for work in such conditions if routinely exposed to: higher than 24 deg C, with wind speeds less than 5 mph. Less than 18 deg C, with wind speeds greater than 5 mph. Summary of Identification Is the environment recognized as being hot/cold, is protective clothing required, or are work demands high (in a hot environment)? Are worker behaviors indicative of attempts to reduce heat or cold stress, is morale low or absenteeism high, or are people making mistakes or getting hurt? Do medical records show a pattern of the symptoms of exposure? Are body temperatures (or in the case of heat stressheart rate or sweat losses high) on a sample of workers? 22

23 End of Chapter 23