element b10 / 1 thermal comfort - ohs.me.uk · relative humidity hygrometers psychrometer wet and...
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Element B10 / 1
Element B10 / 1
Thermal Comfort
Thermal Comfort
HSE
A person’s state of mind in terms of whether they feel
too hot or too cold
If 80% employees comfortable – employer reasonable
Element B10 / 1
Thermal Comfort
Not Just Temp
Other environmental and personal factors to consider
Environmental
Air temp
Radiant temp – eg. From sun, fires, furnaces, molten metal
Air velocity – cooling effect on skin
Humidity – higher = feels warmer
Sweat slow to evaporate – feels warmer
Personal
Clothing
Metabolic heat
More physical activity – more heat
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Thermal Comfort
Workplace (Health, Safety and Welfare)
Regs 1992
Reg 7
Reasonable temperature in the workplace
Reasonable number of thermometers
Comfort
ACoP – reasonable comfort
Where impracticable – as close as possible to
comfortable – i.e. clear requirement to manage
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Reasonable Temperature
WHSWR ACoP
At least 16 degrees
13 degrees for work which “involves severe effort”
No Max
But all reasonable steps should be taken through design of
building
Insulating hot pipes
Air cooling plant
Shading windows
Locate workshops away from radiant heat
Where no reasonable temp achieved
Provide local cooling and heating
Also protective clothing, rest facilities, SOW (task rotation)
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Effects of High and Low Temperature
Unit of heat
Joule (J)
Rate of heat emission
J per sec = watt (W)
1kW = 1000w
Body heat
Normal range = 36.4 to 37.2 degrees
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Effects of High and Low Temperature
Heat transfer / heat loss
Conduction
Body in contact with sources
Convection
Heat transferred by current of moving air
Radiation
Radiant energy in the environment. Furnace etc.
Evaporation
Heat transferred to sweat
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Effects of High and Low Temperature
High Temperature
Body temp regulated by rate skin radiates heat and
evaporation
Radiant process – vasodilatation
Skin temp raised – sweating
Excessive sweating – muscle fatigue, cramps – loss of salts
Overheating – heat stroke, coma, death.
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Effects of High and Low Temperature
Low Temperature
Body cold – blood vessels contract – muscles under
skin vibrate (shiver) to generate heat
Extreme cold
Frostbite
Toes and fingers. Blood and oxygen diverted from
extremities to vital organs
Hypothermia
Fall in body temp to below 35 degrees
Severe – below 30
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Effects of High and Low Temperature
Humidity
High
Limits body's ability to regulate heat through sweating
Low
Moisture taken from skin
Dry cracked skin, lips
Eyes and throat dry
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Wet / Dry Bulb
Air Temp
Dry bulb
Humidity
Wet bulb
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Air Temperature
WHSWR Reg 7
Sufficient number of thermometers
Does not show where temp coming from, only temp in
vicinity
Dry bulb thermometer – most basic type
Thermocouples – very accurate measurements
Personal Thermal Stress Monitor
Audio and visual alarms if unsafe levels
Tells worker to stop work immediately
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Radiant Temperature
Globe thermometer
Suspended at point of measurement for at least 20
mins
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Relative Humidity
Hygrometers
Psychrometer
Wet and dry bulb. Both readings plotted onto psychrometric
chart – RH readings
Whirling Hygrometer
Wet and dry bulb side by side
Rotated by hand
Static
Not accurate
Forced Draught Hygrometer
Extremely accurate
As static but has fan , is insulated. Less susceptible to env
effects
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Metabolic Rate
Rate calories are burned to maintain bodily functions
Basal Metabolic Rate – BMR
Number of calories used by inactive person
60% calories used for normal body functions
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Heat Balance Equation
M = K +/- C +/- R +/- E +/- S
Measures heat equilibrium in body
Depends on Heat absorbed, heat evaporated, metabolic rate,
calories used
M = metabolic rate
K = Heat exchange via conduction
C = Heat exchange via convection
R = radiation
E = evaporation
S = rate of heat storage (lost or gained)
+ = heat loss / - = heat gained. M always positive.
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Heat Stress Indices
Effective temperature
Wet and dry bulb temp
Combines temp, humidity and air movement
Corrected Effective Temp
As above but corrected for radiant heat
Heat Stress Index
Upper limits
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Heat Stress Indices
Predicted 4 hour sweat rate
Environmental and personal factors
Target below 2.7 litres in 4 hrs
Not exceed 4.5 litres
Wind Chill Index
Work in low temp
Air temp and velocity
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Wet Bulb Globe Temp (WBGT)
Quantify thermal stress
Takes into account 4 main thermal factors affecting
heat stress
Humidity WB (without whirling – natural reading)
Air temp DB
Air velocity
Radiant heat GT
Outdoor work
WBGT = 0.7WB + 0.2GT + 0.1DB
Indoor work
WBGT = 0.7WB + 0.3GT
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Wet Bulb Globe Temp (WBGT)
Readings
Compared against psychrometric chart
Cross referenced with metabolic rate (work load)
tables
Defines safe working temperatures
Continuous work
Defined rest periods
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Controls
Control heat / cold source
Screens, ventilation, insulation,
Control environment
Humidity, air movement, draughts
Workplace design
Insulate hot plant, air cooling, shaded windows,
workshops away from heat, enclose product, pre-chill
Job design
Work rate, rotation, rest breaks, mechanical aids – hot
environments
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Controls
Clothing
Suitable for temp, dress codes flexible
Health surveillance
Regular for those in extreme environments, also
pregnant, ill, disabled, medicating. Monitor
dehydration
Training
To recognise early symptoms of exposure to extreme
temp
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Controls
Hydration
Frequent cool drinks, small volumes
Thirst not good indicator – already too late
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