Basic Concept of Heat Stress

Hendra

Iron and steel foundries

Non-ferrous foundries

Brick-firing and ceramics

plants

Glass products facilities

Rubber products factories

Electrical utilities

Bakeries

Confectioneries

Commercial kitchens

Laundries

Food canneries

Chemical plants

12/17/2009Temperature Extreme - hendra 2007 2

Mining sites

Smelters

Steam tunnels

Fires (firefighting)

Outdoor operations

Surface mines

Agriculture sites

Construction sites

Merchant marine ships

Hazardous waste sites

Military training sites

Athletic competitions

Heat from activity

Heat from environment

12/17/2009 3Temperature Extreme - hendra 2007



MESIN

Evaporasi

KonveksiRadiasi

Radiasi

Benda Panas

Konduksi

Konduksi Proses perpindahan panas karena terjadi kontak langsung secara

fisik

Perpindahan panas terjadi jika :

Terjadi kontak langsung

Terdapat perpedaan panas antara permukaan yang kontak

Konveksi Proses perpindahan panas melalui media cairan (uap air) dan gas.

Perpindahan panas secara konveksi dibantu oleh pergerakan udara

(angin)


Radiasi Proses perpindahan tanpa adanya media penghantar ataupun

kontak langsung.

Panas berpindah melalui gelombang energi panas

Evaporasi Proses perpindahan panas dari tubuh (kulit) ke lingkungan seiring

dengan pengeluaran keringat


Model of Thermal Balance


H = M ± R ± C – E atau H + E = M ± R ± C

H = Jumlah panas tubuh (Suhu inti tubuh harus selalu konstan)

M = Panas akibat proses metabolisme

R = Panas Radian / Infra red (Berasal dari lingkungan)

C = Panas akibat konveksi (Berasal dari lingkungan)

E = Panas yang hilang karena Evaporasi

Jumlah panas tubuh merupakan jumlah panas yang dihasilkan dari proses

metabolisme ditambah (dikurangi) panas radiasi dan panas konveksi dari

lingkungan dikurangi dengan panas yang hilang melalui evaporasi

Model of Thermal Balance


S––HEAT STORAGE RATE

If the value for S is zero, the body is in thermal

equilibrium, and heat gain is balanced by loss from the

body.

If S is positive, the body is gaining heat at the rate

indicated by the value of S.

If the value of S is negative, the body is losing heat,

and body temperature is decreasing.


M––METABOLIC RATE

Chemical reactions occur continuously inside the body.

These serve to sustain life (basal metabolism) and meet

the demands of work (muscle metabolism).

As muscle metabolism increases to meet work demands,

the rate of energy conversion from chemical energy to

kinetic energy increases.

Because the energy conversion from chemical energy to

kinetic energy is inefficient, increased metabolism

results in increased rates of heat gain to the person.

The rate of metabolism depends directly on the rate

and type of external work demanded by the job.


W––EXTERNAL WORK RATE

W is the amount of energy that is successfully

converted from internal chemical energy to mechanical

work on external objects.

The rate of external work depends directly on forces

applied against external resistance and distance moved.

W is usually about 10 percent of M.


R––RADIANT HEAT EXCHANGE RATE (RADIATION)

Solid bodies of different temperatures have a net heat

flow from the hotter surface to the cooler surface by

electromagnetic radiation (primarily infrared

radiation).

The rate of heat transfer by radiation depends on the

average temperature of the surrounding solid surfaces,

skin temperature and clothing.


C––CONVECTIVE HEAT EXCHANGE RATE

(CONVECTION)

The exchange of heat between the skin and the

surrounding air is referred to as convection.

The direction of heat flow depends on the temperature

difference between the skin and air. If air temperature

is greater than skin, C is positive and heat flows from

the air to the skin. If the air is cooler than the skin, C is

negative and heat flows from the body. The rate of

convective heat exchange depends on the magnitude of

the temperature difference, the amount of air motion,

and clothing.


K––CONDUCTIVE HEAT EXCHANGE RATE

(CONDUCTION)

When two solid bodies are in contact, heat will flow

from the warmer body to the cooler body.

The rate of heat transfer depends on the difference in

temperatures between the skin and the solid surface,

the thermal conductivity of the solid body that the

person contacts, and clothing that may separate the

person from the solid surface.


CRESP––RATE OF CONVECTIVE HEAT EXCHANGE BY

RESPIRATION

The fact that air is moved in and out of the lungs,

which have a large surface area, means there is an

opportunity to gain or lose heat.

The rate of heat exchange depends on the air

temperature and volume of air inhaled.


ERESP––RATE OF EVAPORATIVE HEAT LOSS BY

RESPIRATION

The large surface area of the lungs provides an

opportunity to lose heat by evaporation.

The rate of heat exchange depends on the air humidity

and volume of air inhaled.


E––RATE OF EVAPORATIVE HEAT LOSS

Sweat on the skin surface will absorb heat from the skin

when evaporating into the air.

The process of evaporation cools the skin and in turn

the body.

The rate of evaporative heat loss depends on the

amount of sweating, air movement, ambient humidity,

and clothing.


Because W, K, Cresp and Eresp are small relative to the

other routes of heat exchange in industrial

applications, they are usually ignored. When

calculating heat storage, Equation 1 becomes Equation

2 as a general statement of heat balance.


Excessive heating or cooling of a small portion of the skin can

occur when it comes in contact with a hot or cold surface. The

contact can be either intentional or incidental. Injury occurs when

there is sufficient heat gain to cause a burn or heat loss to cause

the tissue to freeze (or at least become very cold for a period of

time). In these cases, the local storage rate (Slocal) becomes

important.


where K is conductive heat transfer between the skin and an object,

and D is the rate of heat transfer to or from the local area by

conduction through the local tissue and by the heat supplied or

removed via local blood flow.

Climatic conditions of the environment Climatic conditions are widely used to describe the degree of

stress, as seen in casual descriptions by air temperature, relative humidity, and wind chill.

Work demands In heat stress, metabolic rate can add 10 to 100 times more

heat to the body than radiation and convection combined. In cold stress, metabolic rate affects heat balance on the same order as radiation and convection losses.

Clothing.. Clothing has three characteristics: insulation, permeability, and

ventilation.


The deep organs, especially heart,

lungs, and other vital organs

The arms, legs, and the tissues close to

the skin are referred to as the

periphery

The core and the periphery compete

with each other for blood supply


The body does, however, maintain a constant

temperature at its centre, that is, in the interior of the

brain, the heart and abdominal organs. This constant

temperature is known as the core temperature, and

fluctuates very slightly at around 37OC. Maintaining this

core temperature is necessary for the normal function

of important vital organs.


Schematic Diagram of the

Human Temperature

Regulation System

(Grandjean 1986)


So much sweat is lost that

dehydration results

the body cannot cool itself by

sweating, and the core temperature

rises

Salt loss causes heat cramps

So much of the blood flow goes to

the skin that other organs cannot

function properly


When body core temperature rises

Blood flow to skin increases

Sweating increases

Heart rate increases to move blood – and heat - to the

skin

When this works well

Core temperature drops or stabilizes at a safe level


Thermal stress is a combination of environmental,

work, and clothing factors, heat stress is a combination

that tends to increase body temperature, heart rate,

and sweating. These physiological adaptations are

collectively known as heat strain. (FIH-Chapter 12)

Heat stress is the burden or load of heat which must be

dissipated if the body is to remain in thermal

equilibrium. It is represented by the sum of the

metabolic rate (minus external work) and gain or loss

by convection and radiation.


Heat strain is the change resulting from heat stress. It

may be an adaptive response e.g. increased sweat

rate, increased heart rate or body temperature, or a

harmful effect such as heat exhaustion or heat stroke.

The buildup in the body of heat

generated by the muscles during work

coming from warm and hot environments


Heat stress is the net heat load on the body with

contributions from both metabolic heat

production, and external environmental factors

including temperature, relative humidity, radiant

heat transfer and air movement, as they are

affected by clothing.

Heat strain refers to the acute (short-term) or

chronic (long-term) consequences of exposure to

environmental heat stress on a person’s physical

and mental states.


Heat flow through the body, beginning with heating the body core by metabolism, the transfer

of heat by blood flow to the skin, heat gain or loss to the skin from the environment by

radiation and convection, heat loss by sweat evaporation, and cooler blood returning to the

core.Skin blood flow (sbf) to promote heat transfer is proportional to metabolic rate (M)

divided by the difference between core and skin temperatures (∆Tc-s).


Heat Stress

Worker

- Acclimatization

- Hydration

- Clothing

- Medical Condition

Environment

•Air temperature

•Airflow

•Humidity

•Radian heat (e.g. sun, kiln) Work

• Workload

• Work rate


Work safe British of Columbia)

(Occupational Safety and Health Service Department of Labour Wellington New Zealand)

There are six main factors:

Air temperature is how hot or cold the air around us is. It is what

we measure with a thermometer. It will have a direct warming or

cooling effect on a person. In situations with a high radiant heat

level, air temperature alone is not a good indicator of the thermal

environment.

Humidity is the moisture content of the air. Relative humidity is

the moisture content expressed as a percentage, with 100% being

total saturation for that temperature. The warmer the air, the

more moisture is able to be carried in the air.




Radiant heat is emitted from anything that is hot. Radiant heat

will in time heat the air, but people will absorb heat far more

quickly. Radiant heat will affect people anywhere there is direct

sunlight, or where a person is close to a process that emits heat.

Air movement in most situations will cool a person. This will

provide some relief to people in a hot situation, but extra chill to

people in a cold situation. In hot environments increasing the air

speed can be used as a control measure. In cold environments, a

wind chill factor can make a person considerably colder than if

there was no wind.




Physical activity will increase the generation of heat in the body.

In a cold environment, physical activity can help to warm a person.

In a warm or hot environment, physical activity can increase the

load of heat on a person. A high level of physical activity on a hot

day can place a worker at risk of heat strain, where the heat of

the day alone would not cause a problem.

Clothing aids or prevents heat transfer from our bodies to the

surrounding environment. In a cold environment, a person should

wear clothing that will prevent as much heat transfer as possible.

Ideal clothing in a hot environment will allow a worker to freely

dissipate heat. Clothing can also be used to shield a person from

factors such as radiant heat or a high wind speed.



Other Factor (personal factors)

Weight

Overweight people are more at risk of harm in both hot and cold

environments. This is due to an imbalance in heat transfer.

Health

There are a number of medical conditions that increase the risk of harm

to people working in an extreme hot or cold environment.

Level of fitness

A physically fit person will acclimatise better and generally cope with

heat or cold stress better than an unfitperson.



Other personal factors

Age

As a person reaches middle age (45+), lifestyle health issues can start to

emerge. These can make people more susceptible to harm caused by

extreme hot or cold environments.

Use of prescribed substances

Use of some prescribed medications will adversely affect people

working in extreme hot or cold environments.

Use of non-prescribed substances

Use of many non-prescribed substances such as alcohol or cannabis will

adversely affect people working in extreme hot or cold environments.


Thermal comfort

What an individual feels in an ideal thermal environment. They are

not conscious of being either too hot or too cold.

Thermal discomfort

A person experiencing thermal discomfort feels either too hot or

too cold. They can often be very uncomfortable, but the body’s

temperature control mechanisms are working adequately, and

there is low risk of harm.

Heat or cold stress and strain

This is where harm or serious harm can occur to a person as a

result of working in a temperature extreme situation.


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Basic Concept of Heat Stress

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