Heat, cold and the design of the physical environment BMFP 3553

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 oC.•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 oC.


• Goal is to maintain 37 Celcius.• 39.5 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.


• 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..


• 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.


• Convection = transfer of heat through the movement of air/fluids

• Radiation = heat transfer by electromagnetic waves or photons.


• Sweat production and evaporation (E) is a mechanism by which heat is lost to the environment.

• Surrounding temp > Body temp, no heat loss occurs.


• 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.


• 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.


• Mean Radiant Temperature (MRT)= is simply the area weighted mean temperature of all the objects surrounding the body.

• Almost equivalent to Globe Temperature.


• Air movement moderates the effects of high temperatures and exacerbates the problems of low temperatures (causing ‘wind chill’).


• 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

• Fanger’s comfort criteria• developed by Prof. P. O. Fanger (Denmark)• Fanger’s comfort equation:

f (M, Icl, V, tr, tdb, Ps) = 0where M = metabolic rate (met)

Icl = cloth index (clo)

V = air velocity (m/s)

tr = mean radiant temp. (oC)

tdb = dry-bulb temp. (oC)

Ps = water vapour pressure (kPa)


• Fanger’s 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)


– 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

- - 1 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*[5733-406,7*(M-W)-pa]-24,21*[(M-W)-1]-10-3*M*(5867-pa)-0,0814*M*(34-ta)-3,96*10-8*fcl*[(tcl+273)4 - (teq+273) 4] - fcl*hc,eq*(tcl-teq)]

hc,eq = 2,38*(tcl - teq )0,25 fcl

M [MET)] Icl [CLO]

1,00+0,2*Icl for Icl <0,5 clo

1,05+0,1*Icl for Icl >0,5 clo

PMV = (0,303e-2,100*M + 0,028)*[(M-W)- H - Ec - Cres - Eres]

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.

Heat illnesses

• Heat stroke

• Heat exhaustion

• Heat syncope = inadequate venous return

• Heat hyperventilation = can occur while wearing protective clothing

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 = 20-24 C– Summer = 23-26 C

• RH values:– 60-80 % at 20 C– 50-70 % at 22 C– 45-65 % at 24 C

Documents

Heat, cold and the design of the physical environment BMFP 3553