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Environment

Thermal comfort revision

What is air conditioning?an outline

Steve King

UNSW

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Environment

Thermal comfort

... absence of irritation or discomfort due

to heat or cold;

a state involving pleasantness.

A range of conditions, the comfort zone

the limits of the comfort zone have a

physiological basis ... the thermoregulatory

mechanisms of the body are in a state of

minimal activity. (Givoni)

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Environment

Thermal comfort

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Environment

Thermal comfort

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Environment

The body's thermalProcesses

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Environment

The body's thermalProcesses

deep body temperature

constant 36.6C 20% of the energy produced

by the body, by the

oxidisation of food, is

utilised

remaining 80% must bedissipated

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Metabolic heat production

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Metabolic heat production

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Environment

Metabolic rates

Activity Watts

Sleeping min. 70

Sitting, moderate movement, e.g. typing 130-160Standing. Light work at machine or bench 160-190

Sitting, heavy arm and leg movements 190-230

Standing, moderate work. Some walking 220-290

Walking moderate lifting or pushing 290-410

Intermittent heavy lifting and Digging 440-580Hardest sustained work 580-700

Maximum heavy work for 30 minutes duration max. 1100

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Environment

Factors

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Environment

Heat exchange with the environment

convection

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Environment

Heat exchange with the environment

convection

radiation

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Environment

Heat exchange with the environment

convection

radiation

evaporation

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Environment

Heat exchange with the environment

convection

radiation

evaporation

conduction

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Environment

Climate variables for thermalcomfort

temperature humidity

air movement

intensity of radiation

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Environment

Non-climate variables forthermal comfort

Metabolismand variations in

individual response

Age

Sex

Body shape, subcutaneous fat

Foodand drink

Skin colour State of health

Acclimatisation

Clothing

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Environment

Achieving thermal comfort

http://pmvtool.exe/

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Environment

Thermal comfort

Warm or cool air

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Environment

Thermal comfort

Warm or cool air

General radiant

environment

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Environment

Thermal comfort

Warm or cool air

General radiant

environment

Radiant effectof windows in

perimeter zones

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Environment

Achieving thermal comfort

Temperature range

acclimatization and non-climate factors

within 4C range for 80% of people

Around 19-24C depending on acclimatization

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Environment

Achieving thermal comfort

Temperature range

Radiant temperature

not important if near air temperature

If different by >2C can make big difference

Each 1C change in MRT as influential as 2Cchange in DBT

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Environment

Achieving thermal comfort

Temperature range

Radiant temperature

Humidity

Air movement

affects convective loss or gain

increases evaporation

draftin cold conditions

if insufficient, stuffy, sultry, dank

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Environment

Achieving thermal comfort

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Environment

Achieving thermal comfort

Under still conditions, with the DBT comfortablefor most people, the contribution of the different

factors to the sensation of thermal comfort isapproximately:

Evaporation (including respiration) 25%

Convection (loss or gain from air) 30%

Radiation (to and from surroundings) 45%

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Environment

Comfort Indices

Effective Temperature

ET = 2/3(MRT) + 1/3(DBT)

ET = 6/5(GT) 1/5(DBT)

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Environment

Comfort Indices

Olgays Bioclimatic Chart

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Environment

Comfort assessment

The Psychrometric Chart

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Environment

Air conditioning

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Environment

Air conditioning

Air conditioning conditions:

temperature,

humidity and

air quality.

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Environment

Air conditioning

Refrigerative AC basicallylowers temperature,

but only cooling produceselevatedRelativeHumidity.

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Environment

Air conditioning

Most AC units cool belowcondensation temperature,ie lower Absolute Humidity

therefore also dehumidify.

Except for sophisticated

centralized systems,mostAC does not controlHumidity within specifiedlimits.

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Environment

The Psychrometric Chart

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Environment

> Dry Bulb Temperature

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Environment

> Heating and cooling

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Environment

>Absolute Humidity

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Environment

> Relative Humidity

Saturation

line 100%

50%

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Environment

> Cooling and condensation

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Environment

The refrigeration cycle

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Environment

The refrigeration cyclelarge scale (chillers)

C

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Environment

The Refrigeration Cycle

The simplest form of the

refrigeration cycle, found in

typical 'window rattler' packageunits.

All components are built into a

simple housing, with condenser on

the outside, evaporator facing

inwards.

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Wi d i i di i

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Environment

Window unit air conditioner

All components in a single casing

R C l

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Environment

Reverse Cycle

Can both heat and cool

S lit t

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Environment

Split system

Compressor

stays with

inside unit

S lit t

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Environment

Split system

The relatively noisy

compressor is located

outside, and the

interior unit isrelatively compact and

silent

S lit t

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Environment

Split system

The relatively noisy

compressor is located

outside, and the

interior unit isrelatively compact and

silent

S lit t

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Environment

Split system

Multiple fan-coil units

from one condenser unit

S li

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Environment

Split systemwater cooled

Unlikely in

museum

applications

S lit t

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Environment

Split systemwater cooled

Typical application is 'ultra-

flat' package unit in falseceiling of office.

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Environment

Direct expansion

The three major components

evaporator

compressor

condensor

are separated, and connected to each other

by refrigerant lines.

Such systems are limited in overall size, to

limit the refrigerant gas that may be

released to an occupied space in the event

of a failure.

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Environment

Unitary rooftop package units

A complete package unit,

rejecting heat directly from

the condenser to air. Intended

for rooftop installation,

where ductwork may be taken to

the conditioned spaces below.

Requires access arrangements

over roof for maintenance.

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Environment

Unitary rooftop package units

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Environment

Unitary rooftop package units

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Environment

Central plantwith air cooled chillers

Typically, a roof mounted

package unit, incorporating

all three parts of the

refrigeration cycle, but

producing chilled water

instead of air.

This chilled water is then

piped in a loop to fan-coilunits in the occupied spaces.

C

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Environment

Central plantwith air cooled chillers

Typically, a roof mounted

package unit, incorporating

all three parts of the

refrigeration cycle, but

producing chilled water

instead of air.

This chilled water is then

piped in a loop to fan-coilunits in the occupied spaces.

C l l

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Environment

Central plantwith water cooled chillers

The system most likely to be

found in most large central

air conditioning plantrooms,usually without the extra heat

recovery loop incorporated

here.

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Environment

Components of central plant

Components of central plant:

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Environment

ventilation

Components of central plant:

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Environment

air quality

Components of central plant:

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Environment

ventilation

Components of central plant:

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Environment

cooling and heating

Components of central plant:

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Environment

humidity

Components of central plant:

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Environment

humidity

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Environment

Components of central plant

Humidification:

Spray

Steam

Evaporation

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Environment

Components of central plant

Humidification: jet spray

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Environment

Desiccant dehumidifiers

reduce humidity in the air by

using solid or liquid

desiccant materials

Need another source of heat

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Environment

Desiccant dehumidifiers

reduce humidity in the air by

using solid or liquid

desiccant materials

Need another source of heat

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Environment

Summary

Air conditioning

conditions temperature,

humidity and air quality.

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Environment

Summary

Refrigerative AC basicallylowers temperature, therefore

also dehumidifies, ie lowers

Absolute Humidity.

But if you are only cooling, it

produces elevatedRelative

Humidity.

To lower RH, you must reheat.

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Environment

Summary

This overcooling for humidity

control is themajor source of

energy inefficiency in air

conditioning.

Except for sophisticated

centralized systems,most AC

does not control Humidity

within specified limits.

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Environment

Summary

Air quality is largely afunction of the proportion

of fresh air supply.

Choose appropriate

systems. Size ductwork to allow

full outside air economy

cycles.

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Summary

If designing for Air

Conditioning, design for

when the A/C fails!