moving air for comfort

8/2/2019 Moving Air for Comfort

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1 8 A S H R A E J o u r n a l a s h r a e . o r g M a y 2 0 0 9

By Edward Arens, Ph.D., Member ASHRAE; Stephen Turner, P.E., Member ASHRAE; Hui Zhang, Ph.D.;And Gwelen Paliaga,Associate Member ASHRAE

Air movement can be an energy-eicient alternative to air cooling. However,

in recent years it has been regarded more as a possible source o un-

desirable drat. Thermal comort standards set room air speed limits low, even

or temperatures as warm as 26C (79F). An exception was granted i the air

speed source was under individual control, such as a window in a private ofce,

or a desk an, but only above 26C (79F).

Recent studies o occupied buildings

provide consistent evidence that large

percentages o occupants preer more air

movement than what they currently have,

and small percentages preer less. This

is true in all conditions that occupants

perceived as warm, thermally neutral,

and even slightly cool. In terms o tem-

perature, above 22.5C (72.5F) there is

a small risk o drat and a strong preer-

ence or more air movement. This article

describes these feld study fndings.

Responding to such indings ,

ANSI/ASHRAE Standard 55-2004,

Thermal Environmental Conditions

for Human Occupancy, is being up-

dated with new provisions1 that allow

elevated air speed to broadly oset theneed to cool the air in warm conditions,

Moving Air

For Comfort

About the Authors

Edward Arens, Ph.D., is professor and director

of Center for the Built Environment, University of

California at Berkeley. He is a member of Standing

Standards Project Committee (SSPC) 55. Stephen

Turner, P.E., is managing director, NE, CTG Ener-

getics, Providence, R.I. He is chair of SSPC 55. Hui

Zhang, Ph.D., is research specialist, Center for

the Built Environment. Gwelen Paliaga is senior

mechanical designer, Taylor Engineering, Alameda,

Calif. He is a member of SSPC 55.

The ollowing article was published in ASHRAE Journal, May 2009. Copyright 2009 American Society o Heating, Rerigerating and Air-

Conditioning Engineers, Inc. It is presented or educational purposes only. This article may not be copied and/or distributed electronically or inpaper orm without permission o ASHRAE.


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respondents preerence votes, one can

see that the unacceptable air movement

is due to too little air movement, not too

much. 84% wanted more, 7% wanted

less (Figure 3).

When people elt the air speed wasnot acceptable, and when they were

experiencing the higher range ($0.2

m/s [39 pm]), did they eel the speed

was too much and, thereore, not ac-

ceptable? Figure 4 shows that or the

higher speed range, when people said

that they elt the speed was not accept-

able, the unacceptability was still due

to insufcient air movement. The per-

centage wanting more (73%) is still ar

more than the percentage wanting less

(17%). The dierence is now smaller,

Figure 1 (let): Air movement preerence (sensation 0.7 to 1.5), all air speeds (n = 3,230).

Figure 2 (right): Air movement preerence (sensation 0.7 to 1.5), air speed$0.2 m/s (n = 924).

No Change: 45%

Want More: 52%

Want Less: 3%

No Change: 49%

Want More: 47%

Want Less: 4%

No Change: 9%

Want More: 84%

Want Less: 7%

Acceptable Air Movement 71%

Unacceptable Air Movement 29%

Figure 3: Air movement preerence or people (sensation 0.7 to 1.5)

who said the air speed was unacceptable. (n = 2,091).

No Change: 10%

Want More: 73%

Want Less: 17%

Acceptable Air Movement 78%

Unacceptable Air Movement 22%

Figure 4: Air movement preerence or people (sensation 0.7 to

1.5) who said the air speed was unacceptable, V$0.2 m/s (n = 324).

but it is still clear that the majority ound the speed unac-

ceptable because there wasnt enough, even with the speed

in the higher range.

One may also calculate the drat risk percentage (DR)

or all the databases occupants based on the equations pro-

vided in the previous ASHRAE standard, using air speed,

temperature, and turbulence intensity as inputs. Looking

at the air movement preerence or those people when their

DR exceeded 20% (thermal sensation again within 0.7

and 1.5, n=172), the percentage wanting less air speed

was 8%; 59% wanted no change, and 33% wanted more.

The result is that 92% o a population predicted to be at

an unacceptable risk o drat accepted their air speed oractually wanted it higher.

Summarizing these ASHRAE ield study results, it is

clear that or sensations 0.7 to 1.5, air movement should be

encouraged. The air movement should not be made so great

that it leaves people eeling cold, but a certain amount o it

does answer a basic need ound in the surveys, and can oset

an increase in temperature in the space. Similar results have

been ound or a building in which occupants have personal

or group control over window ventilation.4

Provisions for Elevated Air Speed in Standard 55

Standard 55-2004 will soon provide a two-step proce-dure or setting a comort zone or temperature, radiation,

humidity, and air movement. Both steps o this process

can be carried out using the ASHRAE Thermal Comort

Tool,5 or can be interpolated rom igures in the standard.

The irst step, unchanged rom the previous standard,

combines air temperature and radiant temperature in the

index operative temperature as deined earlier, which

is then combined with humidity to produce comort zones

or still-air conditions displayed on a psychrometric chart.

This step uses the predicted mean vote (PMV) human heat

balance model, combining these environmental variables

with the occupants clothing level (expressed in the insula-

tion unit, clo) and activity level (expressed in the metabolic

rate unit, met).6,7

The second step evaluates elevated air speeds. It uses a

dierent model that better accounts or convective cool-

ing o the body. The standard eective temperature (SET)

thermo-physiological model is based on long-standing

ASHRAE research and practice.6,8 Equal heat balance and

skin-wettedness or dierent air speeds can be plotted in


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temperature, standard effective (SET):the temperature o an imaginary environment at 50% RH,


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Figure 6: Orinda City Hall plan view. Orange circles are the ceiling ans, and the light orange

circles are the areas in which 0.75 m/s (150 t/min) air velocity is supplied in the occupied zone.

Below 22.5C (72.5F), the limit is

0.15 m/s (30 pm) to avoid cold discom-

ort due to drat. 0.15 m/s (30 pm) is the

upper limit o the still-air zone, and is

coincidentally equal to the air speed sel-

generated by an ofce worker at 1.2 met.Between 22.5C and 25.5C (72.5F

and 77.9F), the allowable speed ollows

an equal SET curve dividing the light and

dark gray areas. This local-control bound-

ary between 22.5C and 25.5C (72.5F

and 77.9F) is not linked to any PMV

comort zone, but is based on temperatures

that have been observed in ofce feld

studies to cause virtually no risk o drat.9

Using a temperature-based local-

control boundary provides a substantial

practical advantage in environmental control: decentralized air

movement sources (such as ceiling ans) can be manuactured

or retroftted with temperature-based speed controllers or

automatic control o common spaces.

The 1 clo zone in Figure 5 represents a higher level o

clothing than is typical or situations in which air-movement

cooling is used. For more typical summer clothing levels such

as 0.5 and 0.6 clo, the limits to air speed and temperature will

be determined by the cool-side SET boundary o the comort

zone, not by this occupant-control boundary.

Air speed measurement. Above 22.5C (72.5F), the over-

all heat balance o the body determines comort. For this, the

standard uses the average air speed o the three measurement

heights that represent the seated occupied zone0.1, 0.6, and1.1 m (4, 24, and 43 in.). Measurements should be taken in

the occupants estimated vicinity. To prevent anyone rom at-

tempting to use the standard to justiy cold-air dumping rom

poorly designed HVAC supply air outlets, the coldest tempera-

ture in the occupied zone must be used to represent the entire

zone. This also renders it improbable that cold air-distribution

systems can be used as the source o elevated air movement

because they are likely to produce cold air jets, as opposed to

using ans, stack eect, or window ventilation as a means to

elevate air speed.

Below 22.5C (72.5F), the problem is avoiding thermal

discomort on a local, usually unclothed, part o the body. TheSET and PMV models do not distinguish between clothed and

unclothed portions o the body, so the ollowing conservative

approach is adopted: the maximum mean air speed o the three

measurement heights, and the lowestair temperature is used or

the SET calculations, thereby over-predicting the whole-body

cooling to a level that more closely approximates the cooling

o the most aected local part.

There is no longer a requirement to measure turbulence

intensity or determining drat risk.

Validation. In lab studies where air speed has been imposed

on people, conditions on the cool side o the without-local-

control (light gray zone) boundary have been ound to be

comortable, even recommended as optimal.10,11,12 Imposed

air movement does not appear to be a problem at these tem-

peratures. This is also supported in the ASHRAE feld database

described previously. For air speeds above 0.4 m/s (79 pm) and

air temperatures between 22.5 and 25.5C (73F and 78F),

32% wanted more air movement, 59% wanted no change, and

only 9% wanted less.

On the warm side o 25.5C (77.9F), ceiling an studies

(with no occupant control) have ound 1 m/s (197 pm, 2.2

mph) acceptable to 77% o subjects13 and above (95%,14 80%

to 100%15). A rontal desktop breeze o 0.8 m/s (157 pm) air

speed produced an acceptability level o 80%.12 Subjects in

preerred air speed studies16,17,18 oten chose air speeds well

above 1 m/s (197 pm).Fountain19 summarized many o thesestudies, rom which one can see that the SET-based curves in

Standard 55 represent the laboratory results well.

Air speed requirements or industrial occupancies.Stan-

dard 55 has also added an inormative appendix (Addendum

f, orthcoming) to address more strenuous indoor activities

than ofce work, such as industrial work. For such work in hot

environments, sweating may be acceptable to the occupants.

Elevated air speeds provide high rates o cooling by sweat

evaporation. Using SET as described above, hot conditions with

high air speeds may be equated to their temperature equivalents

at lower air speeds. There is no specifed limit to air speed in

this range.

Impact in Practice

The new provisions allow designers to use ans, stack eect,

or window ventilation to oset mechanical cooling, or in some

climates to supplant it entirely. The ollowing two examples are

o ofces using ceiling ans.

City Hall, Orinda, Cali., (Figure 6). Ceiling ans are used

to enable a mixed-mode system that combines passive cooling

through operable windows and a stack vent with indirect-direct

evaporative cooling, instead o compressor-based cooling. The

building is 1300 m2 (14,000 t2) with 40 ull-time occupants

along with a community meeting room and conerence rooms.


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2 6 A S H R A E J o u r n a l M a y 2 0 0 9

Figure 7: Orinda City Hall with ceiling ans. The let photo shows open space, and the right photo shows cubicles and hallway.

The building management system controls clerestory window

opening based on indoor-outdoor temperature dierential and

illuminates signs that instruct occupants to open/close their

windows. A total o 36 ans with 1321 mm (52 in.) blade

diameter are provided in ofce, conerence, and circulation

spaces. Figure 7shows two spaces, one with our ans arrayed

in a 100 m2 (1,076 t2) open space (let image), another with

10 ans arrayed or 273 m2 (2,938 t2) o cubicles and hallways

(right image). Fans are controlled individually or in groups

o two or three rom wall-mounted switches and have three

speed levels. The ans use approximately 30 to 70 W per an.

In the design, they were assumed to allow the air temperature

Advertisement formerly in this space.


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M a y 2 0 0 9 A S H R A E J o u r n a l 2 7

to increase 2.6 K (4.7F) at 0.75 m/s (148 pm). Simulations

o indoor temperature and comort predict that the ans are

critical to maintain comort or 100 to 200 hours per year

when the evaporative cooler has limited capacity and space

temperatures rise.

The building has been occupied or a year and a hal, and theusers are happy with the ans.

Acknowledgments

The authors wish to acknowledge the members o Standing

Standards Project Committee 55 or their work undertaken to

incorporate new air movement provisions into Standard 55, as

well as urther improvements to the standard identifed and un-

der way.21

Furthermore, the authors wish to acknowledge TylerHoyt and Elliot Nahman or their assistance with the statistical

analysis and graphics.

Loisos+Ubbelohde Ofce, Alameda,

Cali., (Figure 8). This 223 m2 (2,400

t2) design ofce is cooled only by a

system o operable windows, automated

shading, and ceiling and desk ans. The

occupancy is 18, or an occupant density

o 12 m2 (133 t2) per person. Ceiling

ans are individually controllable rom

one control panel in the entry area. They

are controlled in groups o one or two.

Fans have high-efciency twisted airoil

blades designed by Florida Solar Energy

Center/AeroVironment, generating ap-

proximately 40% higher cm/W than

paddle-blade ans, drawing between 9 W

to 50 W rom low to ull speed (0.4 to 1.6

m/s [79 pm to 315 pm]).20

The system has been in place through

one summer in its present confguration,

with owner and occupants expressing

satisaction. The owner eels that the an

density shown could be increased or

more even coverage. He suggests thatair motion sectional diagrams, similar

to photometric curves, be published to

acilitate the design layout o ans in ac-

cordance with work stations.

Conclusions

Forthcoming addenda to Standard 55-

2004 include new provisions or using air

movement to oset warm air tempera-

tures. They are based on feld studies that

consistently demonstrate that in neutral

and warm environments, people wantmore air speed. The SET model is used to

generate comort zones in which elevated

air speed osets warm air temperature.

New criteria or group local control are

specifed, making it possible to use air

movement in open-plan ofces as shown

in the two examples. The new air move-

ment provisions should assist the design

o both passive and mechanical systems

such as natural ventilation, thermal mass

cooling, evaporative cooling, and mixed-

mode HVAC.



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2 8 A S H R A E J o u r n a l M a y 2 0 0 9

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