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MEP 451
Refrigeration &
Air Conditioning
Thermal Comfort
King Abdulaziz University Faculty of Engineering, Mechanical Engineering Thermal Engineering and Desalination Technology Department
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Outline
•What is thermal comfort?
•Factors affecting thermal comfort
•Environmental factors
•Personal factors
•Indices and parameters
•ASHARE comfort chart
•Effect of air velocity, met rate and clothing
•IAQ [Indoor Air Quality]
•ASHARE recommended air qualities for different applications
•Outdoor climate conditioning
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What is Thermal Comfort ?
The condition of mind which express satisfaction with thermal environment
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Factor affecting the comfort condition in occupied spaces
•Air temperature
•
Air humidity
•
Air velocity
•
Cold or hot surfaces that view the body [Radiative heat exchange]
Additional personal factors
•Human activity
•Clothing
•
Metabolic rate changes with person’s activity
•
For sedentary person, the rate is around 58 W/m
2=18.4 Btu/(hr.ft
2)= 1 met
•
As the activity increases, the metabolic rate increases
Tables for met values corresponding to
Human activity
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Clothing
The clothing thermal resistance is measured in CLO [Cloth thermal resistance]
1 clo=0.155 °C.m2/ W =0.88 °F-ft2-hr/Btu
Tables listing CLO values
8 Resistance due
to clothing only
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Controlling the temperature of human body
•Metabolism : rate at which chemical energy is converted to thermal energy
•Blood circulation: control thermal energy to skin surface
•Respiration: air in & air out. [Air leaves human body almost saturated]
•Sweating : rate at which energy is removed from skin by heat and mass
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Thermal comfort parameters
•Dry bulb temperature, Ta
•Relative humidity, RH
•Air velocity, V
•Globe temperature, Tg
•Mean radiant temperature, Tmrt
•Effective temperature, ET*
•Operative temperature, to
Mean Radiant Temperature T
mrtThe average temperature of surrounding at which it exchanges radiation with the body
)
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(
/ 4 1 4
a g g
mrt
T CV T T
T = + −
Tmrt, Tg, and Ta are in R or K
Relation between T
g, T
aand T
mrt13
Globe temperature: The equilibrium temperature reached by a globe when exchange heat with the surrounding by radiation and
convection
Globe temperature
Tg Tmrt
Vernon’s Globe thermometer Ta
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ET
*= Effective temperature
It is the temperature of the environment at 50% RH that results in heat loss from skin as the actual environment
This temperature combines the effect of temperature and relative humidity
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t
o= Operative temperature
The average of mean radiative temperature and ambient temperature weighted with their heat transfer coefficient
Good approximation
2
a mrt o
t
t = t +
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ASHRAE thermal sensation scale
+3 Hot +2 warm +1 Slightly warm 0 Neutral -1 Slightly cool -2 Cool -3 Cold
PMV= Predicted Mean Vote
PPD=Predicted Percent Dissatisfied
A group who did not vote +1, 0 or -1
Relation between PMV and PPD
ASHRAE Rec.
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Mathematical expressions for PMV and PPD
Energy balance for a body can be used to predict PMV
[
res res]
M M W H E C E
e
PMV=(.303 −0.036* +0.028)*( − )− − − −
) 2170 . 0 03353 . 0
( 4 2
* 95
100 e
PMV PMVPPD = −
− +20
Thermal Comfort Chart ASHRAE
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Effect of velocity increase
Software
Indoor Air Quality (IAQ)
• ASHARE requirement for min. fresh air in
occupied spaces
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Outdoor Climate Conditioning
Td= Design Outdoor temperature The maximum temperature for certain time period
DR= Daily range
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