Thermal comfort Thermal comfort is a complex entity.Much work was done to determine whatconstitutes “thermal comfort”. Severalindices have been put forward from timeto time to express thermal comfort andheat stress. These are as follows: AIR TEMPERATURE – it was usedfor a long time as an index of thermalcomfort, but it was realized that airtemperature alone was not an adequateindex.
AIR TEMPERATURE ANDHUMIDITY - later, air temperature andhumidity were considered together toexpress thermal comfort, even this wasfound to be satisfactory. COOLING POWER – still later, airtemperature, humidity and air movementwere considered together and expressed as“cooling power” of the air. These indicesplus mean radiant heat are used by theBulgarian standards to evaluate the thermalcomfort.
EFFECTIVE TEMPERATUTE – anarbitrary index, which combines into a single valuethe effect of temperature, humidity and air movementon the sensation of warmth or cold felt by the humanbody. The numerical value of effective temperatureis that of the temperature of still, saturated air whichwill induce the same sensation of warmth or cold asthat experienced in the given conditions. Forexample, if the environment has an ET value of 30deg.C, it implies that the subjective sensation will bethe same as in a saturated atmosphere of 30 deg.Cwith no air movement. A criticism of the ET is that itignores the effect of a radiation from the surroundingstructures.
CORRECTED EFFECTIVETEMPERATUTE – it deals with all the fourfactors namely, air temperature, humidity, velocityand mean radiant heat. Thermal comfort is a function of manyvariables, including the season of the year,cultural practices and habits, differing fromcountry to country. Nevertheless, describingcomfort zones is necessary for the proper design ofheating and air conditioning systems.
In choosing optimal conditions forcomfort, knowledge of the energyexpended during the course ofroutine physical activities isnecessary, since body heat productionincreases in proportion to exerciseintensity.
Evaluation of the information relatingthe physiology of a person to the physicalaspects of the environment is not a simpletask. Considerably more is involved thansimply taking a number of air temperaturemeasurements and making decisions on thebasis of this information.
Whenever temperature differencesexist between two bodies, heat can betransferred. Net heat transfer is alwaysfrom the body (or object) with highertemperature to the body with lowertemperature and occurs by one or more ofthe following mechanisms.
Conduction – the transfer of heat from one point toanother within a body, or from one body to another whenboth bodies are in physical contact. Normally, this term isinsignificant and can be disregarded except in specialcases, such as swimming. Convection - the transfer of heat from one place toanother by moving gas or liquid. Natural convectionresults from differences in density caused by temperaturedifferences. Radiation – the process by which energy,electromagnetic (visible of infrared) is transmitted throughspace without the presence or movement of matter in orthrough this space
There are two sources of heat that areimportant to anyone working in a hot environment:1) internally generated metabolic heat and 2)externally imposed environmental heat. The net heat exchange between a person andthe ambient environment can be expressed by: H=M±R±C–EWhere: H = body heat storage load M = metabolic heat gain R = radiant or infrared heat load C = convection heat load E = evaporative heat loss
Metabolic heat gain is composed ofthe basal or resting metabolism thatprovides the energy necessary to keep thebody functioning, as well as the workingmetabolism that provides the energynecessary for the body to accomplishspecific tasks. Metabolism can only add energy to thebody; therefore, M is always positive.
Radiant heat load is energy in theform of wavelengths that aretransformed into heat when they strikean object. Whether the human bodyemits or receives radiant energydepends on temperature of the body,and the surrounding objects. Thus, Rcan be either positive or negative.
Convective heat load is the amountof heat energy transferred between the skinand air. Air temperature excess of skintemperature will warm the body; airtemperature less than skin temperature willcause the body to be cooled. The evaporative heat loss from thebody (perspiration) reduces body heat,therefore its value is always negative. Theuse of fans to increase E is a commonmethod of cooling workers.
The body tries to maintain a balancebetween the heat gained by work, radiantand converted heat imposed on the body,and the heat loss by sweating (evaporation).Ideally, the change in body heat contentshould be zero. If this balance cannot bemaintained by evaporation, then heat canbuild up in the body, causing a rise internaltemperature.
Measurement Thermometers are the instruments used formeasuring 1. temperature. Mercury thermometersare widely used, as mercury boils at a hightemperature, has a regular expansion and its levelcan be easily seen. The essential conditions for theuse of thermometers are that 1. the air should haveaccess to the bulbs of the thermometers and 2. thethermometer should be protected against radiantheat. Dry bulb thermometer - this is an ordinarythermometer shielded from direct radiant energysources, which measures the air temperature.
Wet bulb thermometer - precisely thesame as the dry bulb thermometer exceptingthat the bulb is kept wet by a muslin cloth.The evaporation of water from the muslincloth lowers the temperature of the mercury.The wet bulb thermometer therefore shows alower temperature reading than the dry bulbthermometer.
The drier the air, the lower the wetbulb reading. It the wet and dry bulbthermometers record the same temperature,it means that the air is completely saturatedwith moisture, which is rare. In conclusion, the wet bulbthermometer measures the effect ofhumidity on evaporation and effect of airmovement on ambient temperature.
2) Humidity, the amount of watervapour in a given space is commonlymeasured as relative humidity (RH). That isthe percentage of moisture present in theair, complete saturation being taken as 100.It could be determined using an Assmannpsychrometer giving accuratemeasurements of the wet and dry bulbtemperature of the air.
In this instrument, air is drawn at aspeed higher than 5 m/s by a click-workfan. The bulbs of the thermometers areprotected from the effects of solarradiation. By use of suitable psychrometriccharts or tables the RH of the air may beobtained from the readings of thepsychrometer.
Kata thermometer. The word “kata” isa Greek word meaning “down”. The katathermometer is an alcohol thermometer with aglass bulb. The readings on the stem aremarked from 35 to 38. Before taking thereadings, the bulbs are immersed in hot waterto warm them when the alcohol rises into asmall reservoir at the top of the instrument.Then the instrument is suspended in air at thepoint of observation.
The time in seconds required for the spiritto fall from 38 to 35 is noted with a stop watch.The length of time depends upon the “coolingpower” of the air. Each Kata has a “factor”called Kata Factor marked on the stem. Thisfactor is determined for each instrument by themanufactures. Kata Factor divided by thecooling time gives the rate of cooling. Theinstrument is used for recording low 3) airvelocity. High air velocity is measured by aninstrument, called anemometer and is expressedin meters per second (m/s).
The globe thermometer is used for directmeasurement of the mean radiant temperature ofthe surroundings. The instrument consists of ahollow copper bulb 15 cm in diameter and iscoated outside with mattblack paint which absorbsthe radiant heat from the surrounding objects. Aspecially calibrated mercury thermometer isinserted, with its bulb at the center of the globe.
This thermometer registers a highertemperature than the ordinary air onebecause it is affected both by the airtemperature and radiant heat. The globethermometer is also influenced by thevelocity of air movement.
Location of thermal sensors Thermal-sensing instruments should belocated at the workstation so that the actualconditions of heat exposure are measured. In thosezones where the worker spends substantial amountof time, measurements should be takenperiodically, three or four times for a work shiftcould be adequate.
Where the employee moves through alarge area, several zones may be involved.In such cases the thermal sensors could belocated in several points to collect datafrom these different zones.
Heat stress indices Heat stress is the load of heat that must bedissipated if the body is to remain in thermalcomfort. The guidelines currently used for workerexposure to heat are based on indices developedthrough subjective and objective testing ofworkers or from combinations of external heatmeasurements.
Three of the more commonly used indicesare - EFFECTIVE TEMPERATUTE, the - WET BULB GLOBETHERMOMETER INDEX (WBGT) and the - HEAT STRESS INDEX.
For indoor exposure, or outdoorexposure without a solar load, the formula is: WBGT = 0.7twb + 0.3tg The necessary measurements requirerelatively simple instrumentation. Heat stressmonitors that measure all three temperaturesand calculate WBGT index are also available.Sensors should be at least the mean height ofthe worker, or at the levels of the ankles, theabdomen and the head.
A practical application of the WBGTindex is to recommend the percentage oftime that the individual is permitted toperform the task according to theseverity of the environment andmetabolic demands of the task For example, a task requiring light tomoderate work of 200 kcal/hr could beperformed continuously in environment upto a WBGT of 30 C, but only 25 % of thetime at a WBGT of 32.2 C.
The WBGT index – (ISO 7243) iscommonly used because is easy to determine. It isrecommended by NIOSH in the United States.The WBGT index is calculated from themeasurements of the wet bulb (WB), the blackglobe (G) and the dry bulb air (A) temperatures. For outdoor exposure with a solar heatsource, the WBGT formula is: WBGT = 0.7twb + 0.2tg + 0.1tawhere: twb = wet bulb temperature tg = globe temperature tg = dry bulb air temperature
Heat stress index (HSI) The determination of HSI results inmore knowledge about the environmentand possibility to perform efficient controlmeasures than the use of the WBGT. Tocalculate the HSI, measurements of the wetbulb (WB), the black globe (G), the drybulb air temperatures and the air velocityare necessary for each jobsite, andestimation of the metabolic rate (M) of theworkers.
Once measurements have been made, rates ofheat exchange between the worker and theenvironment by convection (C) and radiation (R)are calculated, and with M, are used to estimate theamount of sweating required to stay at equilibrium(E req). Calculations of C, R, E req, and E max,and estimation of M lead to a knowledge of therelative contribution of each, and hence, may wellsuggest possible means of solving the problem.This can not be done with WBGT.
An interpretation of the HISvalues is given below: 0 No thermal stress 10-30 Moderate to mild heatstrain 40-60 Severe heat strain Very severe heat strain Upper limit of heat tolerance
Subjective and physiological methodfor thermal comfort evaluation Inquiry method of Begford – methodto collect votes from a group of people,exposed to certain thermal environmentabout their thermal sensation, estimatedaccording to the 7-point thermal sensationscale.
Index of Fanger (PMV and PPD indices)(ISO 7730). It could be calculated usingmathematical equations, as well as using anintegrating sensor - Bruel & Kjäer mod. 1212. The PMV-index can be determined when theactivity (metabolic rate-1 metabolic unit = 1 met =58 W/m2) and the clothing (thermal resistance – 1unit of thermal resistance of clothing = 1 clo=0,155 m2 . ºC/W) are estimated – from tables, thefollowing environmental parameters are measured:air temperature, mean radiant temperature, relativeair velocity and partial water vapor pressure.
In moderate environment man’sthermoregulatory system will automatically tryto modify the skin temperature and the sweatsecretion to maintain heat balance. In the PMV-index the physiological response of thethermoregulatory system has been relatedstatistically to thermal sensation votes collectedfrom more than 1 300 subjects. It is recommended to use the PMV-indexonly for values of PMV between –2 and + 2 andthe main parameters of the equation are insidefixed intervals.
The PMV-index predicts the meanvalue of the thermal votes of a largegroup of people exposed to the sameenvironment. But individual votes arescattered around this mean value and it isuseful to predict the number of peoplelikely to feel uncomfortably warm or cool.
The PPD-index establishes aquantitative prediction of the number ofthermally dissatisfied persons among alarge group of people. It is recommendedthat the PPD be lower than 10 %. Thiscorresponds to the following criteria for thePMV: - 0,5 < PMV < +0,5
Preventive measures and control of heatstress Modifying one or more of the followingfactors can reduce heat stress: metabolic heatproduction, heat exchange by convection, heatexchange by radiation, or heat exchange byevaporation. Environmental heat load (C, R and E) canbe modified by engineering controls (e. g.ventilation, air conditioning, screening, andmodification of process or operation) andprotective clothing and equipment.
Engineering approaches to enhancingcovective heat exchange are limited tomodifying air temperature and airmovement. When the air temperature is lessthan the mean skin temperature, increasingair movement across the skin by increasingeither general or local ventilation willincrease the body heat loss.
Metabolic heat production can bemodified by work practices andapplication of labor - reducing devices– mechanization of the physicalcomponents of the job, reduction ofwork time (reduce work day, increaserest time) to reduce the duration ofexposure to a hot environment,increased work force.
Work and hygienic practices andadministrative controls. Situations in industries exist where thecomplete control of heat stress by applicationof engineering controls may betechnologically impossible or impractical,where the level of heat stress can beunpredictable. In such cases other solutionscould be applied. Preventive practicesinclude:
1. Limiting of modifying theduration of exposure time - whenpossible, schedule hot jobs for thecooler part of the day (early morning,late afternoon), provide cool areas forrest and recovery etc. 2. Enhancing the heat toleranceby heat acclimatization.
3. Adequate water supply to maintain theelectrolyte balance. It is widespread that extrasalt intake is needed to prevent the ill-effects ofheat. The normal intake of salt in some nationaldiets is far more than is actually needed.Therefore there is no need to add salt to water;only unacclimatized persons need extra saltduring the first some days of their exposure toheat. 4. Protective clothing and equipment –they should be light, loose and of light colors.
5. Protective devices – goggles,shields, helmets. 6. Medical screening of workers todiscern individuals with low heat toleranceand/or physical fitness. The screeningshould include a history of any previousincident of heat illness or other pathologicalconditions that could influence the health ofworkers in unfavorable thermalenvironment.