02 - thermal comfort

8/9/2019 02 - Thermal Comfort

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Confort

Energy Balance to Human Body

Operative Temperature

Confort Conditions

Other aspects of confort:Ilumination, Sound level, Air quality

Thermal Comfort

• “that condition of mind that expresses satisfaction ” .

• The main objective of the HVAC systems is to provide thermal comfort and is one of the main causes for energy consumption in buildings.

• The thermal comfort felt by individuals depend not only on the ambient conditions (general) as well as conditions from the persons (particular).


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Human Metabolism and

Body Temperature

• Metabolism is the energy produced by the aliments

oxidation that is used for work and the remainder is

dissipated as heat to the environment.

• The usefull work can be quantified by the variation of

kinetic or potential energy but in many activities is ~0!

• The metabolism and hence the heat to dissipate

depends a lot on the human activity and person mass

while heat transfer depends on body surface, clothes.• The body temperature should be close to 37ºC while

the skin temperature is about 32‐33ºC.

Heat Balance to Person

M‐W=qsk+qres +S

Heat is transfered to the environment

through the skin or respiration and the

rest is used to accumulate energy (S)

The heat transfer has sensible and

S = rate of energy acumulation in the body W/m2

• All values are expressed by body area for convenience and the body area is

correlated with height and mass of the person.

Extracted from ASHRAE (2009) F09‐C.9 latent contributions.

DuBois area

• The ‘standard’ person is 1.73 m tall,

weights 70 kg and has an area of 1.8 m2.


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MetabolismExtracted from ASHRAE (2009) F09‐C.9

Values of metabolism are available for

different activities and for convenience

the unit met is defined as correspondent

to a person sitted and to a value of 58.1

W/m2 or 104 W for the standard person.

(Sensible) Heat loss through skin• The heat loss through skin in part of the body as to cross

the clothes and therefore is directly affected by these.

• The heat conduction throuh the clothes is characterised by

the clothes heat transfer resistance (Rcl in m2K/W) that also

increases the external area available for heat transfer ( f cl ).

• The heat is then transfered from the external surface of

the clothes in parallel by convection and radiation to the

environment.

K m

W T F h ccer 2

3 7.44

hc is calculated from correlations for natural or forced convection


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Convection heat transfer

There are (very) different correlations proposed.

In genera epen on velocity and distinguish the effects of natural or forced convection.

The ISO 7730 (2nd Ed. 1994) considers the maximum from the following correlations:

h=2.38 (tcl‐t

air)0.25

h=12.1 v0.5

Radiation heat transfer

• The building materials in general have large emissivities and

due to the larger dimensions of the room surfaces compared

to the person are aproximated as black bodies.

• Due to the small differences in the absolute temperatures the

radiation heat transfer rate can be linearized and a typical

value for clothes emissivity is 0.95 leading to hr = 4.5 W/m2K

• The radiation temperature can be calculated from shape

factors and the temperature of surfaces (floor, walls, ceiling)

Nas su erfícies em contacto com o

exterior podem existir diferenças até

3oC em relação à temperatura do ar.


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Measurement of temperature

• The average radiation temperature can be obtained from

measurements made with a globe thermometerC e n

air globeglober vt t t t 273273 dependente do aparelho

Fotos de José Luis

Alexandre, FEUP

The figures show the measurement of the three temperatures:

Air (dry); humid temperature and global temperature.

A anemometer is also required and there is a model for omnidirection velocity.

Clothes I• The heat transfer resistance of

clothes are expressed in clo units 1 clo = 0.155 m2K/W

• 1 clo ~ winter indoor clothes

• The cloth factor f cl represents the area increase for heat transfer and in ASHRAE is aproximated by

• In ISO7730 it is given by:

• f cl = 1 + 1.29 Rcl for Rcl 0.078

• ASHRAE defines also mass transfer characteristics not considered here


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Clothes II• Heat transfer resistances may be calculated from the sum of

contributions of individual garnments or by

When the person is sitted for example an aditional insulation should be considered of 0.1 clo

Operative and clothes temperature

• The definition of the radiation convection coefficient allows

the definition of an operative temperature (t 0):

• From the equality of the heat through the clothes and the

convection/radiation the clothes temperature can be

eliminated and the sensible heat transfer rate is thus:

• The ISO 7730 keeps the general formulation of the heat

balance and the clothes temperature is obtained from:

44 273273 r clceair clclclsk cl t t F t t h f Rt t

896.3 E F ce W M t sk 0275.07.35


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(Latent) Heat loss through skin

• The latent heat loss through the skin is due to transpiration and depends on the partial pressures of moisture close to the skin ( p ) and in the air ( p ), the fraction of wetted surface (w) and ,the mass transfer resistance of the cloth (Re,cl ) and convection (he). The calculations are complex and we will use ISO7730.

• ISO7730 defines the heat losses due to transpiration and moisture diffusion as a function of metabolism and moisture pressure in air ( pa):

• The total heat loss through skin is then

ask pW M E 394.6)(4.0

sk sk E RC q

Respiration• The respiration has also two contributions due to temperature

and moisture variations between the ambient air and the air expelled from the lungs. This is a minor contribution for heat loss.

• The heat losses are proportional to the metabolism and are defined in the same way by ASHRAE and ISO7730

• The moisture partial pressure in the air can be calculated from the product of the relative humidity and the saturation pressure

with M in W/m2, ta in ºC and pa in kPa

• The total heat transfer from the body to the environment is:

)(t p RH p sat a and 235403065.16exp)( air sat t t p

resressk ressk E C E RC qq


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Energy accumulation and PMV

• The difference between the metabolism and the work done

and heat loss is the energy accumulation that in ASHRAE is

separa e n wo par s w e n on y n one:

• The rate of energy accumulation is directly related with the

comfort sensation and in ISO7730 is used to define the

Predicted Mean Value (PMV)

resressk ressk

E C E RC W M qqW M S

028.0303.0 036.0 M eS PMV that corresponds to:

‐3

Cold

Insuportável‐

mente frio

3

Hot

Insuportável‐

mente quente

1

Slightly Warm

Ligeiramente

quente

0

Neutral

Neutro

2

Warm

Quente

‐1

Slightly cool

Ligeiramente

frio

‐2

Cool

Frio

Predicted Percentage of Dissatisfied • Based on a statistical analysis of the comfort sensation of

persons when subject to a given set of conditions the PPD

index was defined and correlated with the PMV by:

24 2179.003353.0exp95100 PMV PMV PPD

PPD

e x a n d r e ,

F E U P

Frio Calor

There is always a PPD even if PMV =0; for|PMV|


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Condições de conforto PMV ‐ PPDFixing the relative humidity comfort temperature maps can be

produced as a function of clothes and metabolism (occupants). Note that operative temperature depends on t, tr and v

From ISO7730, Optimal values correspond to |PMV|


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EN 15 251

• Based on ISO7730 defines 4 levels of comfort:

|PMV|; PPD


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ASHRAE 55

• Comfort conditions are defined in regions of the psycrometric

diagram for winter and summer based on assumed Met, Clo

and air velocity

When velocities are

changed there is a

correction for the

temperature values.

Adaptative comfort• The comfort conditions perceived by the persons depend on

the ambient air conditions outside the buildings.

Comfort temperature as a function of

the average outside temperature

(retirado de Vitor Leal, MIT)


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Standard Effective Temperature

• ASHRAE also defines the SET Index that expresses the

result of temperature and humidity on discomfort

This type of graphs is used

to evaluate the discomfort

of workers in harsh

environments.

There is other index: Heat

stress index defined by the

capacity to dissipate heat by

transpiration that defines

the maximum time periods.

Conclusions• The definition of thermal comfort conditions

depends on human activity and clothes.

• There are quantitative methods to evaluate comfort

and statistical information to evaluate the level of

dissatisfied persons that will always exist.

• The heat balance allows to

– Analyse the impact of individual variables and to see how

they can be compensated by others.

– Evaluate the heat loads due to occupancy (sensible+latent)

In terms of legislation the reference conditions inside buildings are

25oC and 50% relative humidity in summer and 20oC in winter.


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Efficiency of light sources

Tipo de Lâmpada

Potência

Eléctrica (W)

Fluxo

Luminoso

(lm)

Eficácia

luminosa

(lm/W)

Eficiência

Eflum/668,45

Farol de bicicleta 3 30 10 1,5 %

Incandescente 100 1360 13,6 2,0 %

Incandescente (?) 60 620 10,3 1,5 %

Compacta … 11 570 51,8 7,7 %

Fluorescente 58 5400 93 13,9 %

Sódio de alta pressão 100 10500 105 15,7 %

Sódio de baixa pressão 131 26000 198 29,6 %

Va or de mercúrio 1000 58000 58 8 7 %

Iodetos metálicos 2000 190000 95 14,2 %

Lâmpadas compactas novas

Lâmpadas compactas de projector 60 5700 95 14,2 %

ASHRAE ‐ F09, Ch. 10


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Manuel Duarte Pinheiro, ISTJosé Luis Alexandre, FEUP

Desenhos de José Luis

02 - thermal comfort

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