Chapter 4_Comfort and Health-Indoor Environmental Quality

8/19/2019 Chapter 4_Comfort and Health-Indoor Environmental Quality

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Comfort and Health Indoor

Environmental Quality

Comfort involves: control of temperature, humidity, air mo

radiant sources interacting with the occupants

Not everyone can be made comfortable by one set of cond

Oder, dust, noise, and vibration are additional human comf

Nonenvironmental factors such as dress and the activity le

occupants must be considered

A well-designed HVAC system manages to keep these vari

within specified limits set by customer, building codes, and

engineering judgment

Engineers are challenged to utilize all of the available infor

tools to design systems that provide a comfortable environ


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Comfort PHYSIOLOGICAL consideration

The amount of heat generated and dissipated by the huma

considerably with activity, age, size, and gender

The body has a complex regulating system acting to maint

body temperature of about 98.6 f (36.9 C)

Thermal balance with environment = comfortable

ANSI/ASHRAE standard 55-1992 specifies the thermal Env

Conditions for Human Occupancy as conditions in which 8

of the occupants will find the environment thermally accep


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The environmental factors that affects thermal comfort are

The dry bulb temperature of the surrounding air

The humidity of the surrounding air

The relative velocity of the surrounding air

The temperature of any surfaces that can directly view

the body and thus exchange radiation

The personal variables that influence thermal comfort areclothing

Animal and human body temperatures are controlled by a

that involves metabolism, blood circulation near the surfac

respiration, and heat and mass transfer from the skin


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The energy generated by a person’s metabolism varies wit

Met is a unit to express the the metabolic rate per unit of barea 1 met = 18.4 Btu/(hr-ft 2 ) (58.2 w/m 2 )

The average adult is assumed to have an effective surface

transfer of 19.6 ft 2 (1.82 m 2 ) and will dissipate approximate

(106 W)

Clothing insulation is described as a single equivalent un

over the whole body

Clo is a unit for clothing insulation 1 clo = 0.88 (F-ft2-hr)/B

C)/W]

A heavy two-piece business suit with accessories has an i

value of about 1 clo, whereas a pair of shorts has about 0.


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Environmental comfort indices

Temperature: is easily measured as air or dry bulb tempera

Humidity: can be described, for a given pressure and dry btemperature

Air motion: can be determined from measurement or predi

theories of fluid mechanics

Radiant exchange: between person and surroundings

Surroundings do not surround a person but occur on osides

Computation involves the angle factor or configuration

The basic index used to describe the radiative conditio

the mean radiant temperature


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Vernon ’ s globe thermometer is the most common instrume

determine the mean radiant temperature

The mean radiant temperature can calculated as:


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Other indices have been developed to simplify description o

thermal environment and to take in account the combined e

or more factors

The Effective Temperature: i s the temperature of an environ

relative humidity that results in the same total heat loss from

in the actual environment. It combines the humidity and the

into single index

The Operative Temperature: is the average of the mean radiaambient air temperature

The humid Operative temperature: is the temperature of a u

environment at 100% relative humidity in which a person los

total amount of heat from the skin as in the actual environm


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The Heat Stress Index: is the ratio of the total evaporative he

required for thermal equilibrium to the maximum evaporativ

possible for the environment, multiplied by 100, for steady-s

conditions, and with the skin temperature held constant at 9

The Wet Bulb Globe Temperature:


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comfort conditions

ASHRAE sensation Scale


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comfort conditions


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comfort conditions


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comfort conditions


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comfort conditions


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comfort conditions


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The basic concern of iaq


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Common contaminants

Carbon Dioxide CO 2

Carbon Monoxide (Co)

Sulfur oxides

Nitrous oxides

Radon

Volatile Organic Compounds (VOCs)

Mycotoxins (Mold Poisons)

Particulate Matter


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METHODS TO CONTROL HUMIDITY

Humidity is a significant parameter in comfort

Low relative humidity and the dry effect on nose and bronchial linings

increased incidence of disease

High relative humidity encourages condensation and growth of harmfu

such as mold

Indoor air relative humidity must be kept around 50%

Moisture must be removed (dehumidification) when cooling and added(humidification) when heating

The condensed liquid should not be blown into the supply duct


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METHODS TO CONTROL HUMIDITY

Methods to control humidity:

Lower the supply temperature low enough to remove the required a

moisture and then reheat that air back to a temperature required to

space cooling load

Reduce fan speed (reduce air flow) or bypass some of air around the

Use surface or liquid desiccants to remove water chemically from hu

makeup or recirculated air. Desiccants are particularly useful in HVA

where:

When heating steam should be injected in air stream rather than sprayi


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METHODS TO CONTROL contaminants

Four methods to control gaseous or particulate contaminants and main

Source elimination or modification

Use of outdoor air

Space air distribution

Air cleaning


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sources Elimination or modificatio

The most effective method for reducing contaminant- operates directl

this method specifies exactly what building materials and furnishings a

allowed in the buildings

Design and construction should be such that water can not condense o

building

Elimination of smoking within the buildingNo paints, solvents, cleaners, and volatile compounds within the buildi

outdoor intake

Cleanup of mold and fungus


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use of outdoor air

Outdoor air is use dilute

contaminants within a space

Supply air is the air delivered to theconditioned

Ventilation air is the portion of

supply air that is outdoor air plus

any recirculated air that has been

treated for the purpose of maintain

acceptable IAQ

Makeup air is outdoor air supplied

to replace exhaust air and

exfiltration

Infiltration and exfiltration air


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use of outdoor air

The contaminant concentration in space can be calculated as


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use of outdoor air


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use of outdoor air

Occupied Zone is the region within

an occupied space between the

floor and 72 inch (1800mm) above

the floor and more than 2 ft (600

mm) from the wall or fixed air –

conditioning equipment

Occupied Zone must be maintained

at a nearly uniform condition.

Some fraction S of the supply air

bypasses and does not enter the

occupied zone

Effectiveness Eoa is the fraction of

outdoor air entering the system


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use of outdoor air


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use of outdoor air


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Indoor Air quality procedure


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filtering: particulate removal


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Efficiency measures the ability of the air cleaner

to remove particulate matter from an airstream

(Fig 4-8)

The smaller particles are the most difficult tofilter

The air-flow resistance is the loss in total

pressure at a given flow rate

Air flow resistance is an important factor in

operating costs for the system since it is directlyrelated to fan energy requirements

The pressure loss at any required rate of flow Q

can be determined by

Where the subscript r stands for rated conditions


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Chapter 4_Comfort and Health-Indoor Environmental Quality

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