Thermal comfort

“That condition of mind in which satisfaction is expressed with the thermal environment ” ANSI/ASHRAE Standard 55-1992:

Thermal Environmental Conditions for Human Occupancy

The following gives a few facts about human beings:

• Heat generated from a human body:

• Human body temperature:

O

• One would wake up from sleep if this person loses 24 W or more. In this case, the skin temperature decreases 2.8 °C.

• One would feel uncomfortable or sick, if the person’s body temperature is 1 K higher.

• O

• O

Human body energy equation

Human body obeys the first law of thermodynamics. Energy balance for human body: Heat production — Mechanical work = Heat losses

2

M — Rate of metabolic heat production (W/m body surface area) (Table 4-1, textbook)

W — Rate of mechanical work

Q — Heat losses

C — Convective heat losses

E — Evaporative heat losses

Sk — Skin

Res — Respiration

Heat production: measured in metabolic or met units.

1 met = 58.2 W/m2 = 18.4 Btu/(hft2) body surface area

Table 4 Typical Metabolic Heat Generation for Various

Activities

 W/m2 Met" Resting Sleeping 40 0.7 Reclining 45 0.8 Seated, quiet 60 1.0 Standing, relaxed 70 1.2 Walking (on level surface) 3.2 km/h (0.9 m/s) 115 2.0 4.3 km/h (1.2 m/s) 150 2.6 6.4 km/h (1.8 m/s) 220 3.8 Office Activities Reading, seated 55 1.0 Writing 60 1.0 » Typing 65 1.1 •VI Filing, seated 70 1.2 Filing, standing 80 1.4 Walking about 100 1.7 Lifting/packing 120 2.1 Driving/Flying Car 60-115 1.0-2.0 Aircraft, routine 70 1.2 -1 Aircraft, instrument landing 105 1.8 Aircraft, combat 140 2.4 .• Heavy vehicle Miscellaneous Occupational Activities 18S 3.2 Cooking 95-115 1.6-2.0 Housecleaning 115-200 2.0-3.4 Seated, heavy limb movement Machine work 130 2.2 Sawing (table saw) 105 1.8 Light (electrical industry) 115-140 2.0-2.4 •a Heavy 235 4.0 i Handling SO-kg bags 235 4.0 7i Pick and shovel work 235-280 4.0-4.8 »t Miscellaneous Leisure Activities Dancing, social 140-255 2.4-4.4 Calisthenics/exercise 175-235 3.0-4.0 T Tennis, singles 210-270 3.6-4.0 H Basketball 290-440 5.0-7.6 % Wrestling, competitive 410-505 7.0-8.7 3

More and Dumin (1967), and Webb (1964). al met = 58.2 W/m2

: Buskirk (1960), PM**.

•jq*

Environmental and personal comfort variables Environmental factors

O

Air temperature ~

Relative humidity ~

Air velocity near a human body, V ~

Surface temperature of the enclosure and other objects ~

Personal factors

Activity (rate of metabolic)

Clothing (insulation)

1 clo = 0.88 F-ft2-hr/Btu

Determination of individual heat loss/gain

2

W = Active work and shivering (involuntary work) (W/m body area)

•2

M = Rate of metabolic heat production (W/m )

(M is given in Table 4-1 of the textbook. 1 met = 58.2 W/m body area)

Csk = he (Tdoth — Tair) Acioth/Abody (W/m2 body area)

 )0’25 > 12.1 V0’5 )0’25 < 12.1 V0’5

Hc = 2.38 (Tdoth — Tair)0’25 when 2.38 (Tcioth — Ta„-

Hc = 12.1 V0’5 when 2.38 (Tdoth — Tair

Tdoth = 35.7 — 0.0275 (M — W) — {(M — W)

— 3.05 [ 5.73 — 0.007 (M — W) — pv]

— 0.42 [ (M — W) — 58.15] — 0.0173 M (5.87 — pv)

— 0.0014 M (34-Tair)}

Abody = 0.202 m0’425 10’725 (m2)

M — Body weight (kg)

1 — Height (m)

= 1.0+ 0.3 Id

Rcioth = Cloth thermal resistance (m K/ W)

Rcioth = 0.155 Id

1 clo = 0.155 m2 K/W

Ici can be found in Table 8, Chapter 8 of ASHRAE Fundamentals, 1997

Rsk G Ecloth Eeiiclosure [ ( Tcl0th + 273 ) — (Tenclosure + 273 ) ] Adoth/Abodv

G — Stefan-Boltzmann constant (5.67×10"8 W/m2K4) e — Emittance (a value between 0 and l)

Rsk = 3.96xl0’8 [ ( Tdoth + 273 )4 — (Teiidosure + 273 )4] Adoth/Abodv (W/m2)

Esk nisk ifg

= 3.05 [ 5.73 — 0.007 (M — W) — pv]

+ 0.42 [ (M — W) — 58.15] (W/m2)

Pv — Vapor pressure (kPa)

Cres — nires Cp. a (Tres — Tair)

= 0.0014 M (34 — Tair) (W/m2)

Eres nires ifg

= 0.0173 M (5.87 — pv) (W/m2)

Note: Body energy balance ^ Always thermally comfort Thermal comfort = Always body energy balance

Only in a limited range of environmental parameters, thermal comfort can be achieved.

Prediction of thermal comfort

 PMV =

Predicted Mean Vote (PMV)

 + 3 Hot + 2 Warm + 1 Slightly warm 0 Neutral -1 Slightly cool -2 Cool -3 Cold

 O O

L — Thermal load on the body

L = Internal heat production — heat loss to the actual environment

• Predicted Percentage Dissatisfied (PPD)

PPD = 100 — 95 exp [ — (0.03353 PMV4 + 0.2179 PMV2)]

 PMV PPD 0 5% ± 0.5 10% ± 1.0 25%

Example:

The environmental parameters in an office are:

Tair = 25 °C Twaii = 25 °C

Pv = 2500 Pa he = 3 W/m2K

Niperson — 70 kg lperson — 1.86 m

Id, person =1.0 clo (Business suit)

Determine the comfort level in the office.

Solution:

From Table 4-1 of the textbook (or handouts), M = 60 W/m2 Since Tair is 25 °C and person does not do physical work, W = 0.

Rdoth = 0.155 1 = 0.155 x 1.0 = 0.155 m2K/W

Abodv= 0.202 m0-425 10’725

= 0.202 x 70°’425 x 1,860-725 = 1.927 m2

Adoth/Abody = 1.0 + 0.3 I = 1.0 + 0.3 x 1.0 = 1.3

Tdoth = 35.7 — 0.0275 (M — W) — Rdoth {(M — W)

— 3.05 [ 5.73 — 0.007 (M — W) — pv]

— 0.42 [ (M — W) — 58.15] — 0.0173 M (5.87 — pv)

— 0.0014 M (34-Tair)}

= 35.7 — 0.0275 x (60 -0) — 0.155{(60 — 0)

— 3.05 [ 5.73 — 0.007 (60 — 0) — 2.5]

— 0.42 [ (60 — 0) — 58.15] — 0.0173 x 60 (5.87 — 2.5) -0.0014×60 (34-25)}

= 26.86 °C

Csk — hc (Tdoth — Tair) Adoth/Abodv = 3 x (26.85 — 25) x 1.3 = 7.254 W/m2

Rsk — 3.96xl0’8 [ ( Tdoth + 273 )4 — (Tenciosure + 273 )4] Adoth/Abody = 3.96xl0"8 [(26.86 + 273)4 — (25+273)4] x 1.3 = 10.23 W/m2

Esk =3.05 [5.73 -0.007 (M-W)-pv]

+ 0.42 [(M-W) -58.15]

= 3.05 [ 5.73 — 0.007 (60 — 0) — 2.5]

+ 0.42 [(60-0)-58.15]

= 9.348 W/m2

Cres = 0.0014 M (34 — Tair)

= 0.0014 x 60 (34 — 25)

= 0.756 W/m2

Eres =0.0173 M (5.87-pv)

= 0.173 x60( 5.87-2.5 )

= 3.50 W/m2

L = M — W — [( Csk + Rsk + Esk ) + ( Cres + Eres )]

= 60 — 0 — [( 7.254 + 10.23 + 9.348) + (0.756 + 3.5)]

= 29 W/m2

PMV = [0.303 exp ( -0.036 M ) + 0.028 ] L

= [0.303 exp ( -0.036 x 60 ) + 0.028 ] x 29

= 1.825 => Warm

PPD = 100 — 95 exp [ — (0.03353 PMV4 + 0.2179 PMV2)]

= 100 — 95 exp [ — (0.03353 x 1.8254 + 0.2179 x 1.8252)]

= 68%

Ure 2.4: At lower temperatures heat loss by convection and radiation dominate, but >ve 80°F air and surface temperature evaporation begins to dominate. (After. Flynn ISegil, 1970.).

 Table 2.1: Clo Values for Various Men’s Clothing Ensembles Clothing Ensemble Dovakit Nude 0 Shorts 0.1 Tropical dothing: shorts, open-neck short-sleeved shirt, Dght socks and sandals 03-0.4 Light summer dothing: long light-weight trousers, open-nsck short-sleeved shirt OS Tight working dothing: athletic shorts, wool soda, opervnsck short sleeved cotton shirt, long cotton trousers OlB Light outdoor sportswear: T-shirt, cotton shirt, cotton undaisiiorts, cotton trousers, cotton socks, shoes; and single-pty, light-weight popiin Jacket 0.7 Typical business suit 1J0 Typical business suit with cotton topcoat (or heavy wool suit without topcoat) 1J5 U. S. Army standard cold-wet uniform: cotton-wool undershirt and undershorts, wool/nylon flannel shirt, wind-resistant, water-repellent trousers and field coat, doth mohair and wool coat, and wool socks 1Л-2Л Heavy wool pile ensemble: (Polar weather suit) ЗЛ-4.0
 (Reproduced from Fanger, 1882, by permission.)

 Fig. 11 Predicted Percentage of Dissatisfied (PPO) as Function of Predicted Mean Vote (PMV)

ASHRAE comfort zone

• Operative temperature (T0)

• Effective temperature (ET*)

• ASHRAE comfort zone

Operative temperature (TO):

O

Tmrt — Mean radiant temperature Tmrt = I AjTj /1 A,

T; — Surface temperature of enclosure i A; — Area of surface i

Effective temperature (ET*):

The temperature of an environment at 50% relative humidity that results in the same total (sensible + latent) heat loss from the skin as in the actual environment. It combines operative temperature and humidity into a single index.

ASHRAE comfort zone (Fig. 4-2):

The “comfort zone” represents combinations of air temperature and relative humidity that most often produce comfort for a seated North American adult in shirtsleeves, in the shade.

Winter: T0 = 20-23.5°C (68-74°F) at 60% relative humidity and Tc = 20.5-24.5°C (69- 76°F) at 2°C (36°F) dew point. Slanting side boundaries correspond to ET* of 20°C (68°F) and 23.5°C (74°F).

Summer: To = 22.5-26°C (73-79°F) at 60% relative humidity and Tc = 23.5-27°C (74- 80°F) at 2°C (36°F) dew point. Slanting side boundaries correspond to ET* of 23°C (73°F) and 26°C (79°F).

Conditions: Typical summer and winter clothing

Light and primarily sedentary activity (<=1.2 met)

10% dissatisfaction

 OPERATIVE TEMPERATURE. "C Fig. 5 Standard Effective Temperature and ASHRAE Comfort Zones

(1) . Clothing (e. g., minimum clothing such as briefs Tc = 26-29°C (79-84°F)

(Table 4-3)

(2) . Air speed VО -> Tot (Fig. 4-3)

(3) . Activity (met)T -> Toi (Fig. 4-4)

Draft

Draft is related to air temperature, air velocity, and turbulence intensity. Percentage dissatisfied people due to draft can be expressed as:

PD = 3.143 (34 — Tair) (V — 0.05)0’622

+ 0.3696 (34 — Tair) (V — 0.05)0’622 V Tu

When V < 0.05 m/s, use V = 0.05.

When PD > 100%, insert PD = 100%.

 18% DISSATISFIED PD — Percentage dissatisfied people due to draft (%) V — Air velocity (m/s) Tair — Air temperature (°C) °-8 Tu — Turbulence intensity (%) Tu = 100 Vstandard deviation /V (0/. Fig. 13 Percentage of People Dissatisfied as function F|g u Dnrfl Conditions Dissatisfying 15% of Population Of Mean Air Velocity 22 24 AIR TEMPERATURE. *C 0 I O. t 0.9 0 4 MEAN AIR VELOCITY, n*

 10 16 20 25 30 35 40 RADIANT TEMPERATURE ASYMMETRY, *C

 W ALL

 WARM CEILING

 COOL WALL

 Fig. 12 Percentage of People Expressing Discomfort Due to Asymmetric Radiation

Asymmetry Warm ceiling (—) Cool wall (—) Cool ceiling (—) Warm wall (-)

Thermal stratification

Temperature difference < 3-4 K between head and feet

For large temperature gradients, local warm discomfort can occur at the head, and/or cold discomfort can occur at the feet, although the body as a whole is thermally neutral.

 AIR TEMPERATURE DIFFERENCE BETWEEN HEAD AND FEET "C

Rg. 15 Percentage of People Dissatisfied as Ruction of Vertical Air Temperature Difference between Head and Ankles

Warm or cold floors

 Fig. 16 Percentage of People Dissatisfied as Function of Floor Temperature

 Increased MK Textends comfort zone Dry Mb temperature (TJ Figure 2.7: Effect of mean radiant temperature on comfort zones. (After Arends etal., 1980, modified to current ASHRAE comfort zone.)

 Increased air motion extends comfort zone D/y buib temperature (TJ Figure 2.8: Effect of air motion on the summer comfort zone. (After Arends et al., 1960, modified to current ASHRAE comfort zone.)