Presentation on theme: "Heat, cold and the design of the physical environment BMFP 3553."— Presentation transcript:
Heat, cold and the design of the physical environment BMFP 3553
Objectives Understand how heat and cold affects our perception of temperature Learn how to manage extreme climatic conditions
Human Thermoregulation Our normal body core temperature: 37 o C. We detect heat and cold differently. – The hypothalamus sends a signal if temp > 37 C. – Cold sensors are located in the skin. Send signals when skin temperature is < 34 o C.
Human Thermoregulation Goal is to maintain 37 Celcius C can be disabling and 42 C can be fatal. Skin temperature can widely. Sources of heat are the liver, brain and the heart, and the muscles. Muscular work efficiency is 20%. The rest is all heat.
Human Thermoregulation Humans have a remarkably well-adapted ability to tolerate heat compared with other primates Thermoregulation is achieved by balancing the two main factors that determine body temperature – the metabolic heat produced and the rate of heat loss..
Human Thermoregulation How does the environment influence our body temperature? There must be a state of heat balance. (equation in the textbook). Heat may be gained or lost. Our body produces heat and loses it to the environment.
Human Thermoregulation Convection = transfer of heat through the movement of air/fluids Radiation = heat transfer by electromagnetic waves or photons.
Human Thermoregulation Sweat production and evaporation (E) is a mechanism by which heat is lost to the environment. Surrounding temp > Body temp, no heat loss occurs.
Human Thermoregulation In a cold environment, metabolic heat production takes place by shivering, or some physical activity. Heat loss can be reduced by wearing heavy clothes (convection and radiation is reduced). How about work in foundry? How to solve the problem?
Measuring the thermal environment Dry-bulb temperature (DBT) – It is the temperature measured by a regular thermometer exposed to the airstream – Does not indicate moisture in the air Wet-bulb temperature (WBT) – It is the temperature you feel when your skin is wet and is exposed to moving air. – Gives you an indication of moisture.
Measuring the thermal environment Globe temperature (GT) – Measured by a thermometer placed in a black sphere – Also sometimes referred to Mean Radiant Temperature. – Radiant heat (from the sun or from hot objects) is absorbed by the sphere and heats up the thermometer.
Measuring the thermal environment Mean Radiant Temperature (MRT)= is simply the area weighted mean temperature of all the objects surrounding the body. Almost equivalent to Globe Temperature.
Measuring the thermal environment Air movement moderates the effects of high temperatures and exacerbates the problems of low temperatures (causing wind chill).
Measuring the thermal environment WBGT incorporates the following : – Dry bulb temp – Wet Bulb temp – Globe temp
What is Thermal Comfort? - That condition of mind which expresses satisfaction with the thermal environment. ISO 7730
Prediction of Thermal Comfort Fangers comfort criteria developed by Prof. P. O. Fanger (Denmark) Fangers comfort equation: f (M, I cl, V, t r, t db, P s ) = 0 whereM = metabolic rate (met) I cl = cloth index (clo) V = air velocity (m/s) t r = mean radiant temp. ( o C) t db = dry-bulb temp. ( o C) P s = water vapour pressure (kPa)
Prediction of Thermal Comfort Fangers equation is complex – but it may be transformed to comfort diagrams – it can also be used to yield three indices: predicted mean vote (PMV) predicted percentage of dissatisfied (PPD) lowest possible percentage dissatisfied (LPPD)
Prediction of Thermal Comfort – PMV a complex function of six major comfort parameters; predict mean value of the subjective ratings of a group of people in a given environment – PPD determined from PMV as a quantitative measure of thermal comfort dissatisfied means not voting -1, +1 or 0 in PMV normally, PPD < 7.5% at any location and LPPD < 6%
Predicted Mean Vote scale - +3 Hot - +2 Warm - +1 Slightly warm - +0 Neutral Slightly cool - -2 Cool - -3 Cold The PMV index is used to quantify the degree of discomfort
Calculation of PMV index PMV = (0,303e -2,100*M + 0,028)*[58,15*(M-W) -3,05*10 -3 *[ ,7*(M-W)-p a ]-24,21*[(M-W)-1] *M*(5867-p a )-0,0814*M*(34-t a ) -3,96*10 -8 *f cl* [(t cl +273) 4 - (t eq +273) 4 ] - f cl *h c,eq *(t cl -t eq )] h c,eq = 2,38*(t cl - t eq ) 0,25 f cl M [MET)]Icl [CLO] 1,00+0,2*I cl for I cl <0,5 clo 1,05+0,1*I cl for I cl >0,5 clo PMV = (0,303e -2,100*M + 0,028)*[(M-W)- H - E c - C res - E res ] PMV ?
PMV and PPD PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people. PPD-index predicts the number of dissatisfied people.
Thermoregulatory mechanisms Peripheral vasomotor tone – Arteries and blood vessels dilate and heat is conducted to the skin (HOT) – In the cold, vasoconstriction occurs, thus reducing blood flow. – Insulation capacity of a person is measured by CLO values. A person wearing a business suit has a CLO value of 1. Shivering = groups of motor units acting out of phase and resulting in a heat production
Work in hot climates Peripheral vasodilation increases the blood flow to the skin. Working muscles also demands blood supply As a result, the cardiovascular system is under strain.
Relative humidity If the DBT > 38 C, but the R.H < 20%, then sweating is effective. But if R.H is 90% and DBT = 32 C, with no air movement, only low level of work activity can be performed.
Heat tolerance Work in hot environments can be made more tolerable by introducing job aids or rest pauses. (metabolic heat is reduced) Workers differ in their ability to tolerate stress. – Heat intolerant, heat tolerant
Heat acclimatisation Heat acclimatisation is a physiological process of adaptation rather than a psychological adjustment to life in a hot environment. It involves an increase in the capacity to produce sweat and a decrease in the core temperature threshold value for the initiation of sweating.
Heat acclimatisation A state of acclimatisation is best achieved by exercising in the heat and drinking plenty of fluid. Heat acclimatisation occurs naturally but it may also be induced artificially. – Surface acclimatisation chambers f 31.5°C WBT and 33.5°C DBT.
Factors influencing worker ability Age = children have less sweating capacity, older person unable to tolerate high heat stress. Physical fitness = Physically fit workers are less stressed by hot conditions even if they are accustomed to a temperate climate. Body fat = Excess body fat degrades heat tolerance. Same heat load will cause a greater increase in temperature.
Heat Stress Management
Work in cold climates Core temperature can be maintained in the cold if the person is working and suitable protective clothing is provided. If the core temp < 33 C, CNS is disrupted. At 29 C, hypothalamic core temperature control breaks down completely.
Work in cold climates Peripheral temperatures and repetitive work Cooling of the peripheral tissues, particularly in the hands and feet, causes – reductions in strength – neuromuscular control, – resulting in a loss of dexterity.
Acclimatisation to cold? Local acclimatisation to cold may occur in the extremities as a reduction in the peripheral vasoconstrictor response. Increased blood flow through the hands can occur after repeated exposure to cold conditions.
Acclimatization to cold Up to 25% of heat loss takes place at the head. During cold temperatures, peripheral vasoconstriction takes place. Behavioural adaptation to the cold, through experience, is of great importance; wearing correct clothing and keeping on the move are examples.
Acclimatization to cold Perception of cold – The perception of cold seems to depend on experience. – Accustomed people = feel comfortable with layers of clothing, despite local cooling at the extremeties – Unaccustomed = may confuse being cold (low core temperature) and feeling cold ( low temp on the extremeties)
Protection against extreme climates Specify work rest cycles Design cool spots Issue protective clothing – Cooling jackets – If temp > 37 C, more clothing needed to protect from heat gain.
Thermal comfort in buildings The thermal comfort of a factory or office worker depends on there being an average skin temperature of approximately 33°C Draughts, sunlight falling on an arm or the face and sitting next to a cold wall are all causes of thermal discomfort due to uneven skin temperature distribution.
Thermal comfort in buildings ISO 9241 recommends for indoor climates: – Winter = C – Summer = C RH values: – % at 20 C – % at 22 C – % at 24 C