Human Comfortbk

8/12/2019 Human Comfortbk

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HUMAN COMFORT

SUBMITTED BY : RUPIKA ASHTT


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WHAT IS HUMAN COMFORT?

Condition of a man mind which expresses

satisfaction with the thermal environment.

Conditions influenced by environmental

conditions surrounding a person by

impression of some natural elements.

A perception (condition of mind) is best

assessed by askingpeople (occupants)

how they feel.


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Heat Flow

Mechanisms

three external to mechanisms; four from mechanisms


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Heat Flow to/from Human Body

Sensible Heat

Flows via conduction, radiation, and convection

Flow rate is generally related to space temperatures

Latent Heat

Flows via evaporation

Flow rate is generally related to space humidity

Total Heat Flow= sensible + latent flows


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HVAC

Air moving through these systems can be

divided into three categories:

Outside or fresh air;

Return air (brought into the system from

the occupied space);

Supply air (the mix of outside and returnair that is delivered to the space).


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HVAC System

HVAC controls vary, as do the systems

they control. Most new systems will rely on

direct digital controls (DDC).

This control method operates from a

centrally located computer that allows the

operator to modify system parameters

from a terminal.


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BASIC FACTORS OF HUMAN COMFORT

Environmental factors:

Air temperature

Radiant temperature

Air velocity

Humidity

Personal factors:

Clothing Insulation

Metabolic heat
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Environmental factors

Air temperature

This is the temperature of the air surrounding the body. It is usuallygiven in degrees Celsius (C) or degrees Fahrenheit (F).

Radiant temperature

Thermal radiation is the heat that radiates from a warm object.

Radiant heat may be present if there are heat sources in anenvironment.

Radiant temperature has a greater influence than air temperature onhow we lose or gain heat to the environment. Our skin absorbsalmost as much radiant energy as a matt black object, although thismay be reduced by wearing reflective clothing.

Examples of radiant heat sources include: the sun; fire; electric fires;furnaces; steam rollers; ovens; walls in kilns; cookers; dryers; hotsurfaces and machinery, molten metals etc.


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Humidity

If water is heated and it evaporates to the surrounding environment, the resulting amountof water in the air will provide humidity.

Relative humidity is the ratio between the actual amount of water vapour in the air and themaximum amount of water vapour that the air can hold at that air temperature.

Relative humidity between 40% and 70% does not have a major impact on thermalcomfort. In some offices, humidity is usually kept between 40-70% because of computers.

However, in workplaces which are not air conditioned, or where the climatic conditionsoutdoors may influence the indoor thermal environment, relative humidity may be higherthan 70% on warm or hot humid days. Humidity in indoor environments can vary greatly,and may be dependent on whether there are drying processes (paper mills, laundry etc)where steam is given off.

High humidity environments have a lot of vapour in the air, which prevents the evaporationof sweat from the skin. In hot environments, humidity is important because less sweatevaporates when humidity is high (80%+). The evaporation of sweat is the main method ofheat loss in humans.

When vapour-impermeable PPE is worn, the humidity inside the garment increases as thewearer sweats because the sweat cannot evaporate. If an employee is wearing this type ofPPE (eg asbestos or chemical protection suits etc) the humidity within the microclimate ofthe garment may be high.


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Air velocity

This describes the speed of air moving across the worker and mayhelp cool the worker if it is cooler than the environment.

Air velocity is an important factor in thermal comfort because peopleare sensitive to it.

Still or stagnant air in indoor environments that are artificially heatedmay cause people to feel stuffy. It may also lead to a build-up inodour.

Moving air in warm or humid conditions can increase heat lossthrough convection without any change in air temperature.

Small air movement in cool or cold environments may be perceived

as draught. If the air temperature is less than skin temperature it willsignificantly increase convective heat loss.

Physical activity also increases air movement, so air velocity may becorrected to account for a person's level of physical activity.


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Personal factors

Clothing insulation

Clothing, by its very nature, interferes with our ability to lose heat to theenvironment. Thermal comfort is very much dependent on the insulatingeffect of clothing on the wearer.

Wearing too much clothing or personal protective equipment (PPE) may bea primary cause of heat stress even if the environment is not considered

warm or hot. If clothing does not provide enough insulation, the wearer maybe at risk from cold injuries such as frost bite or hypothermia in coldconditions.

Clothing is both a potential cause of thermal discomfort as well as a controlfor it as we adapt to the climate in which we live and play. You may addlayers of clothing if you feel cold, or remove layers of clothing if you feelwarm. However, many companies remove this ability for their employees to

make reasonable adaptations to their clothing. It is important to identify how the clothing may contribute to thermal comfortor discomfort. It may also be necessary to evaluate the level of protectionthat any PPE is providingcan less or other PPE be used?


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Work rate/metabolic heat

The work or metabolic rate, is essential for a thermal riskassessment. It describes the heat that we produce inside ourbodies as we carry out physical activity.

The more physical work we do, the more heat we produce.

The more heat we produce, the more heat needs to be lost sowe dont overheat. The impact of metabolic rate on thermalcomfort is critical.

When considering these factors, it is also essential toconsider a person's own physical characteristics.

A person's physical characteristics should always be borne inmind when considering their thermal comfort, as factors suchas their size and weight, age, fitness level and sex can allhave an impact on how they feel, even if other factors such asair temperature, humidity and air velocity are all constant.


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Thermal comfort

may be influenced by:

air temperature

mean radiant temperature air velocity

humidity

metabolic rate clothing


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Local thermal discomfort

may be caused by:

draught

high vertical air temperature differencebetween head and ankles

too high radiant temperature asymmetry

too warm or cold surface temperature ofthe floor


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| Slide: 15

Specific goals

the body is neutralslightly cool

vigilance better

mean and local skintemperatures do not

reach discomfort limits

absence of local thermaldiscomfort

no or minimal draught

Tsk > 31C

>20C

> 17C


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Air Temperature


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The daily variation in air temperature is controlledby incoming energy (primarily from the sun) and

outgoing energy from the earths surface.

Where incoming energy exceeds outgoing energy

the air temperature rises. Where outgoing energy

exceeds incoming energy the air temperature falls

Diurnal Temperature Variation


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Incoming energy exceeds outgoing energy (orange shade), the air

temperature rises. Where outgoing energy exceeds incoming energy (blue

shade), the air temperature falls


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20 MET 112 Global Climate Change

On a sunny calm day the air near the surface can be substantially


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On a sunny, calm day, the air near the surface can be substantially

warmer than the air a meter or so above the surface.


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Air Temperature and Human Comfort

The human body loses heat by emitting infrared energy

Conduction and convection also cool and heat the body.

On a cold day, a thin layer of warm air molecules forms close to the skin,

protecting it from the surrounding cold air.

Once the wind starts to blow, this insulating air is swept away from the skin and

heat is rapidly removed. The faster the wind blows, the greater the heat loss, andthe colder we feel.

How cold the wind makes us feel is expressed as a

wind-chill index.


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Air Temperature and Human Comfort

High winds, in below-freezing air, can remove heat from exposed skin so quickly

that the skin may actually freeze and discolor. The freezing skin, called

frostbite, usually occurs on the body extremities first because they are thegreatest distance from the source of body heat.

The rapid loss of body heat may lower the body temperature

below its normal level and bring on a condition of

hypothermia!


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Documents

Human Comfortbk