thermal comfort and ac
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Thermal comfort revision
What is air conditioning?an outline
Steve King
UNSW
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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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Thermal comfort
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Thermal comfort
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The body's thermalProcesses
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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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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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Factors
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Heat exchange with the environment
convection
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Heat exchange with the environment
convection
radiation
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Heat exchange with the environment
convection
radiation
evaporation
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Heat exchange with the environment
convection
radiation
evaporation
conduction
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Climate variables for thermalcomfort
temperature humidity
air movement
intensity of radiation
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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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Achieving thermal comfort
http://pmvtool.exe/ -
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Thermal comfort
Warm or cool air
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Thermal comfort
Warm or cool air
General radiant
environment
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Thermal comfort
Warm or cool air
General radiant
environment
Radiant effectof windows in
perimeter zones
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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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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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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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Achieving thermal comfort
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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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Comfort Indices
Effective Temperature
ET = 2/3(MRT) + 1/3(DBT)
ET = 6/5(GT) 1/5(DBT)
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Comfort Indices
Olgays Bioclimatic Chart
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Comfort assessment
The Psychrometric Chart
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Air conditioning
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Air conditioning
Air conditioning conditions:
temperature,
humidity and
air quality.
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Air conditioning
Refrigerative AC basicallylowers temperature,
but only cooling produceselevatedRelativeHumidity.
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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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The Psychrometric Chart
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> Dry Bulb Temperature
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> Heating and cooling
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>Absolute Humidity
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> Relative Humidity
Saturation
line 100%
50%
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> Cooling and condensation
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The refrigeration cycle
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The refrigeration cyclelarge scale (chillers)
C
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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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Window unit air conditioner
All components in a single casing
R C l
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Reverse Cycle
Can both heat and cool
S lit t
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Split system
Compressor
stays with
inside unit
S lit t
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Split system
The relatively noisy
compressor is located
outside, and the
interior unit isrelatively compact and
silent
S lit t
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Split system
The relatively noisy
compressor is located
outside, and the
interior unit isrelatively compact and
silent
S lit t
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Split system
Multiple fan-coil units
from one condenser unit
S li
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Split systemwater cooled
Unlikely in
museum
applications
S lit t
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Split systemwater cooled
Typical application is 'ultra-
flat' package unit in falseceiling of office.
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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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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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Unitary rooftop package units
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Unitary rooftop package units
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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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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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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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Components of central plant
Components of central plant:
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ventilation
Components of central plant:
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air quality
Components of central plant:
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ventilation
Components of central plant:
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cooling and heating
Components of central plant:
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humidity
Components of central plant:
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humidity
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Components of central plant
Humidification:
Spray
Steam
Evaporation
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Components of central plant
Humidification: jet spray
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Desiccant dehumidifiers
reduce humidity in the air by
using solid or liquid
desiccant materials
Need another source of heat
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Desiccant dehumidifiers
reduce humidity in the air by
using solid or liquid
desiccant materials
Need another source of heat
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Summary
Air conditioning
conditions temperature,
humidity and air quality.
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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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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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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!