Cooling by natural ventilation

a AL

I

V~TI

Gordon 0, MoOutohan

a thesis

in partial fulfillment of the requiremente for the MASTER OP ARCHITWTURE

degree...September 1950

Massaohusetts Institute of Technology. Oambridge

0

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285 Westgate West

Cambridge 39, Mass.

September 1, 1950 Professor Lawrence B. Anderson, Head

Department of Architecture

Massachusetts Institute of Technology

Canbridge 39, Massachusetts

Dear Professor Anderson:

This thesis entitled "Cooling by Natural Ventilation" is respectfully submitted in partial fulfillment of require-ments for the "Master of Architecture" degree.

Sine'rely yours,

TABLE OF CONTENTS

INTBONOT ION PART I

PART II

AIR MOVEMENT AND BODY COQLING

Heat Loss from the Body

Conduction Convection

Radiation

Evaporation

Relationships under Varying Conditions

The Ideal Climate Comfort Chart, ASHVE

Comfort Zone and Air Movement BEHAVIOR OF MOVING AIR

Air and Temperature

Velocity and Altitude

Changes in Velocity and Direction Summary

PART III AIRFLOW AROUND OBSTRUCTIONS

Bernoulli and Venturi Streamlines

Simple Geometric Forms

Wind Around Fences Around Buildings

Two and Three Dimensions Pressure Distribution Change in Shape

Change in Wind Angle The Sheltering Effect

Summary

PART IV - AIRFLOW THROUGH BUILDINGS

Objectives (1) Currents at Body Level

(2) Reduction of Radiant Temperatures

(3) Maintenance of Air Temperature

Temperature Difference Forces

Wind Forces

Shape of the Building The Air Wash

The Unknowns BI BLIOGRAPHY

IliTROIJCTION

With the remarkable present-day air conditioning equipment it is poss-ible to create almost any thermal environment desired by man. However, the cost of installing and operating the necessary equipment prohibits its use in a great many buildings. Those of us who have experienced the long hot summers of the Southwest are probably more acutely aware of the desirability of any successful cooling method, however crude or inefficient.

Realizing that some cooling can be had without recourse to mechanical deivces, architects have provided greater comfort at lower cost by experimenting with various shapes and materials. But in contrast to the work of air conditioning engineers, architects have had to content themselves with trial-and-error methods. This has been due to either an absence of information or to the fact that existing information has been scattered and fragmentary. In designing to take maximum advantage of the climate, architects need more precise information in more useable form.

This report is a first step in providing such information. It is only that--a first step. It will give some of the required information, but in the great majority of cases it will only point a direction. This can be understood from the fact that few other studies have attempted to bring together the many bits of information from the several broad fields that can contribute to the subject.

Henry J. Kaiser, the fabulous industrialist, was quoted in a recent periodical as saying that he had no problems. One of the "secrets

of his success," he said, was the substitution of the word "oppor-tunity" for the word "problem." The big opportunity of this study was to become acquainted with the literature of five relatvely unfamiliar fields. Physiology was delved into rather deeply, for a layman, in the investigation of the effects of air movement on the human body. In dealing with the behavior of moving air, meteorology was most help-ful. Aero-dynamics entered both the section on natural air currents and airflow around obstructions. Civil engineering offered the in-formation about pressure distribution on buildings. And of course, mechanical engineering (heating, ventilating and air conditioning) was a thread through the whole discussion.

It appeared that the general subject of cooling by natural ventilation fell naturally into four divisions. Since the object we are trying to cool is the human body, it is first necessary to determine how air movement effects the body. Since the medium for this cooling is the air that nature provides, something should be known of the properties of this air and the manner in which it is delivered. Part III, airflow around obstructions, was a necessary preliminary to Part IV, airflow through buildings.

The principle objectives of this thesis are to define the problems (i.e. opportunities) of the architect in designing for natural air movement and to give some specific criteria with which to work. Therefore, emphasis is given to the first three sections. Some

ex-amples of application are given in the last section, but no attempt was made to recommend architectural solutions. It was felt best to devote the limited time to supplying the elements that would enter such decisions.

1'..----THE IPORTANCE OF COOLING

In a certain sense, it would be a waste of time to dwell on the importance of more cooling in hot climates. Certainly the individual requires no charts or formulas to tell him when he is uncomfortably hot; he senses this without instruction and, in the extreme cases at least, instruction will not change his opinion. Neither are statistics necessary to show that people desire a cooler environment than nature offers them in most of the United States during the summer months.

Despite these obvious facts, there has not been sufficient compulsion to cool adequately a large majority of work places. And only a very small percentage of residences can boast of comfortable conditions during the warmest days. This neglect can be attributed partly to ignorance of the

price paid for inadequate cooling; a price paid in unhappiness, ill-health and inefficiency.

Happiness is difficult to measure, and the relationship of this complicated subject to the thermal environment cannot be included in this study.

However, there is little doubt that a sumnary of all the little irritations caused by hot weather would show significant correlation between heat and

conduct. On the other hand, efficiency can be measured in dollars and

cents, and it is believed that proof of increased efficiency will do more toi bring about adequate cooling than any similar facts regarding personal pleasure. There is much work to be done in this field also, but there

are some studies which show definite results.

influences health. Winslow and Herington devote a full chapter to the

subject of Climate, Season, and Health in their book on "Temperature and Human Life". After analysis of a wealth of data (cited in the book), their general conclusions are as follows: (1) Even the extremes of heat and cold in the United States may increase mortality rates as much as double their normal volume; (2) Minimum mortality rates occur with a daily mean temperature of 600 to 700 F with a mean daily relative humidity of 60 to 80 per cent; (3) Achievement is highest in climates where the above conditions prevail, with a stimulus given by moderate variations toward the cooler side of optimum; (4) The human body has regular annual rhthms of physiological activity; (5) Seasonal and climatic variations in morbidity from communicable diseases are particularly significant, and

always tend to show an increase in intestinal infections under hot con-ditions.

Figures for accident frequency in coal mines show a minimum at a point near 600 . For semi-skilled labor in a munitions factory, accident frequency is at a minimum near 670 with an increase in frequency of accidents on both sides of this figure. Recent information from the Royal Navy shows that mental tasks are affected as well. Their data show that in radio code reception errors are more frequent under conditions of thermal stress.

But it is quite possible that heat, like sound and light, can do damage when the individual is not conscious of being uncomfortable. On same of the very hottest days in Washington, D.C. some officials have seen fit to

stop the work of hundreds of office workers, that they might seek more

efficiency of these workers was not at a peak during the hour or two it took someone in charge to make such a big decision; and in all probability there were many days before and after the unexpected vacation when climatic conditions were very similar. Dr. L.P. Herrington has said, "what we call

climatic fatigue is not comparable to the fatigue a man gets from digging

a ditch. It is worse, because it frequently involves the sense of tired-ness without the reward of work accomplished".

Having been convinced that a hot environment is neither pleasant nor "good

for you", there are some rather striking contrasts between the amount of

effort put into winter heating and the amount of effort given to summer cooling. _{Look at the average man in winter. He sleeps comfortably,} because his house is heated or because he uses such accessories as a hot water bottle or electric blanket. He rides to work comfortably because his automobile, bus or subway is heated. He works comfortably because his

office or shop has an expensive heating plant. His wife at home can also

go about her work with a high degree of thermal comfort. In the summer

this same man sleeps in a pool of his own sweat, arising unrested to

start the day. _{His conveyance to and from work offers him little relief} from the heat. _{At his place of business he is uncomfortable both because} the air and walls are too hot and also because he is wearing clothes

ill-suited to the climate. His wife has the choice of either becoming extremely uncomfortable at her work or of simply not doing it.

Another example comes to mind; perhaps extreme, but true. Take the case

of the backward child who must go to sumer school to keep up with his

ideal conditions. Secondly, he would much rather be out playing with

the other children. Finally, he must attempt to learn in an uncomfortable environment. The poor kid has three strikes against him before he enters the door.

But the illustration of the school child need not be so extreme to

illus-trate a need for adequate cooling. In many parts of the South and

South-west it is very hot during the regular school session. And even the

average or brilliant child deserves an environment conducive to his best

efforts. It is little consolation to say, "well, he has only one strike

against him".

Thinking beyond the individual, the widespread practice of summer cooling

might influence the development of whole world regions. Climatologists

and historians have shown that the highest civilizations have developed

in either a naturally favorable climate, or have developed with the advent of efficient heating. In the past century the powerful and efficient

nat ons have been those of Northwestern Europe and Northern North America.

In these climates the temperature rarely goes above 750 and winter

con-ditions are controlled by adequate heating. With as efficient and

wide-spread utilization of summer cooling, there is reason to believe that the tropic and sub-tropic areas might develop to the same high degree.

PART I

Air Movement and Body Cooling

Since our primary concern is with cooling the human body, it seems wise to

consider first the process of heat production by the human mechanism. As

Winslow and Herrington put it, "the whole life process is a form of slow

combustion". The "fuel" df food and oxygen is utilized for the regeneration

of tissues, for work, and for heat.

Physiologists use the term metabolism to mean the quantitative relationship between the intake of this fuel and the resultant work and heat produced by the body. Metabolism is influenced by such factors as individual body build, age, sex, and the amount of muscular work performed. Therefore any reference to metabolism should allude to the special conditions which pro-duced this metabolism.

Most of us are familiar with the term, if not the meaning, of basal

metabolism. This concept was introduced to eliminate the many variables

connected with metabolism alone. As defined by Winslow and Herrington, it is "the level of metabolic activity displayed by a subject at rest at an air temperature of about 700 F and at a period long enough after a meal to avoid the specific dynamic action of food". After looking at a number of references on body temperature with a laynan's point of view, other non-physiologists are cautioned to be aware of the important difference between metabolism and basal metabolism. For persons of average weight and build,

basal metabolism is roughly 60 calories per hour.

Women have slightly less metabolism than men, and in both sexes the maximum rate is achieved at about 10 years of age, dropping off thereafter. For

reasons which appear later, metabolism is related to surface area of the

body. The surface area of men varies from approximately *91 square meters

to.2.1 square meters. Most people have a metabolic rate of within 101/1. of 39.7 calories per square meter.

The variance with age and sex is indicated in the following chart:

AGE METABOLISM PER SQ. METER (CALORIES PER HOUR)

Male - Female

14-16 49I~ 43.0

70-80 35.5 33.0

These figures refer to basal metabolism. While they may have some value in the design of buildings, (for example, the contrast between bedrooms designed for a boy's school as against those designed for an old ladies' hame) by far the most significant variable is muscular work. The range

is from about 60 calories per man hour when sleeping, up to a maximum of

around 1200 calories per man per hour when performing extremely strenuous

tasks such as rowing.

The following chart gives some indication of the metabolic rates for a man of average weight and stature when involved in various activities:

Occupation Calories per

man per hour

Sleeping 65 Sitting at Rest 100 Typewriting rapidly 140 Walking 2.6 m.p.h. 200 t 3.75m.p.h. 300 Stone working 400 Swimming 500 Walking upstairs 1100

It is interesting to note the very high figure for walking upstairs. This

is attributed to the effort required to overcome gravity. It should be

I -

--Only a figure of between 500 to 1000 calories can be maintained for as

long as an hour. The 2.6 mph figure for walking is a rate about normal

to most people. Military marches are continued at approximately this

rate for many hours a day, with only very short rest periods.

The daily totals for various activities vary from about 2000 to 5000 calories. A normal day for a carpenter would run something like this:

8 hours sleep @ 65 calories per hour 520 calories

6 " sitting at rest 6 100 cal. per hr. 600

2 " light exercise @ 170 " "t 340

8 " carpentry work 0 240 " " 1920

24 hour total 3380 calories

The influence of atmospheric conditions on metabolism is very strange indeed. Within the comfort range, metabolism seems to be unaffected by the environment, but outside this range the metabolic rates increase. Below the comfort level (cold weather) the increase is clearly an adaptive reaction, useful in maintaining body temperature. Above the comfort level

(hot weather4 the increase in metabolism is not an adaptive process, but

is a vicious circle which may be detrimental to health. To quote Winslow

3'&

and Herrington, "under extreme conditions of heat, the warming up of body

tissues was accompanied by an increase in metabolism, which, in turn, accentuated heating up. The body temperature rose to 1050 -1100 F and

death promptly ensued".

It appears that the whole purpose of temperature regulation by the body is to maintain a stable deep-body temperature. The control mechanism (both thermostat and thermometer) is in the brain, and seems to get its

all normal conditions the body produces more heat than it can use for the

7 regeneration of tissues and for work. Excessheat is stored in the tissues for gradual discharge over a period of hours. This heat must be eliminated at approximately the same rate at which it is produced if the temperature of the tissues is to remain at a level which affords comfort.

We see that the problem of the themal environment is to control the heat

loss from the body, so that the rate of loss is approximately equal to

the rate of production. In winter the natural environment will drain

heat fram the body at a faster rate than the body can produce it. In

summer the difficulty with the natural environment is two-fold. Not

only does the atmosphere keep body-heat-loss below the optimmn rate, in

sane cases it is adding heat to the body, thus causing the vicious circle

mentioned above.

HEAT LOSS FROM THE BODY

There are four methods of heat transfer utilized by the body. These are:

convection, conduction, radiation, and evaporation. Of course the four

avenues of heat emission are connected in a rather complex way. At times all four are put into play at once. At other times the heqt loss from the body is channeled through only one of these methods. Let's consider each one separately, then in combination, and finally analyse in detail only

those concerned with air movement. Conduction

Conduction is the transfer of heat between two surfaces of unequal

temperature by actual contact of the two. Probably the most conmon

example of this is taking a bath. _{If the water is cooler than the skin,}

is heated. The reason conduction is considered here first is that this method of heat transfer is the one least used; that is, the periods of

time when conduction is used make up such a small percentage of a person's Activities that this method is not usually shown as a separate percentage of total heat loss. In most discussions, conduction losses are included

in the figures shown for convection, or the total for conduction and

con-vection is indicated as one sum.

For el1 general purposes this seens a very reasonable way of handling con-duction. However, it should be borne in mind that this method is the mzst rapid of all, and would probably play a significant role in very extreme

conditions. It also appears possible to make more use of the principles of

conduction in the design of furniture and gadgets for human confort. It

may not be too fantastic to consider, for example, a mattress cooled by some

mechanical means which would do the same thing for the sleeper in summer that the electric blanket does for him in the winter - or even circulating

cool water thru the tubular frame of a metal lawn chair. Convection

Here we are concerned with the transfer of heat between the body and the air. Convection, like conduction, can work both ways: either the body gives up heat to the air or receives heat from it, depending upon the

relative tenperatures of the two.

There are two components to the measurement of heat interchange due to

convection. The minor component is the amount of heat required to warm

the air we breathe. The major component is the heat transfer between

the surface of the body and the air which circulates by it.

temperature of the body surface, (2) the area of surface exposed to con-vective heat loss, (3) the mean dry bulb temperature of the air, and (4)

the rate of air movement.

Radiation

Heat transfer by radiation does not depend upon contact with the air or any other material. Radiant heat waves are transmitted thru the air from one surface to another without directly warming the air in between. The amount of this transfer depends upon (1) the relative surface temperatures of the two bodies, (2) the distance between them, (3) the amount of surface area exposed to transmit and receive radiant energy, and (4) the radiant properties of each surface.

The so-called steam radiator actually warms people less by radiation than

it does by convection. It exposes a relatively small amount of surface to

the individual, generally has a poor surface for transmitting radiant heat

(especially when painted a dark color) and does most of its heating by

raising the temperature of the air. The fireplace and the electric

radiant heater are our most familiar exanples of a primary radiant source.

Of course nearly every true radiant source also heats the air indirectly by first heating the objects which intercept its rays.

The human skin is remarkably efficient when it comes to this method of

heat transfer, being almost 99 per cent amnissive and receptive to the

infra-red (heat) rays.

Evaporation

con-tinually gives off moisture to the air. In this process it is also transferring heat to the atmosphere. Variables in this process are:

(1) air temperature, (2) air movement, (3) relative hunidity of the air, (3) area of evaporative surface, and (4) the available moisture for evaporation.

Everyday examples of the evaporative process are hard to find. An interesting use of this method was found, however, by troops in the

tropics during the last war. If you put a can of beer in a helmet with

some gasoline and force a jet of compressed air thru the gasoline, you

have to be careful to stop soon enough or you'll have frozen beer.

Unlike the preceding three methods, heat transfer by evaporation from the hunan body is always cooling. Under basal conditions, that is with the subject at rest in an average environment, nearly one half of the heat

loss due to evaporation may be contributed by evaporation from the moist

membranes of the nose and throat. This percentage is materially altered

under varying atmospheric conditions, but all evaporative loss has two

components - one from the mucous menbranes, the other from the sweat given off at the skin surface.

RELATIONSHIPS UNDER VARYING CONDITIONS

We have seen that there are five factors influencing the heat exchange

between the body and its environment. These are the heat production, or

metabolism, and the four avenues of heat loss or gain from the surround-ings. _{As is customarily done by physiologists, we shall combine conduction}

with convection, and since conduction is the lesser of the two, the term convection shall be understood to include both. That gives us: (1)

heat production by metabolism, (2) heat gain or loss by convection, (3)

gain or loss by radiation, and (4) loss by evaporation.

Winslow and Herrington have expressed this relationship as a formula:

Metabolism minus Evaporation plus or minus Convection plus or minus

Radiation equals Zero; or

bI-E-C-tR:o

when a state of equilibrium exists.

The following chart gives all the relationships between the three methods of heat transfer and the nine physical and physiological factor's involved

in the heat interchange between the human body and its environment.

Evapo Conv4 Rad. Physical

Air Temperature

X

Air Movement X

Relative Humidity X

Mean Radiant Temperature X

Physiological

DuBois Surface Area X

Effective Radiation Area _X

Area of Evaporative Surface _X

Mean Skin Temperature _{X X}

Available Moisture for Evap. X

Frca the foregoing chart it can be seen that air movement across the body

turn, are in some way altered by seven of the nine physical and

physio-logical factors. It will be impossible to eliminate radiation from our

considerations, but emphasis will be placed on those elements which are enclosed in boxes on the chart.

However, in the total picture of body heat loss, air movement can influence

radiation. That is, temperature of the radiant surfaces which add or

take-away heat from the body can be altered by the movement of air. This will

be taken up later, and will not be considered at this time.

Apparently several attempts have been made to develop a single figure for the combined influence of air temperature, air movement, relative humidity,

and mean radiant temperature. _{These attempts have been abandoned because,}

according to inslow and Herrington, "only an independent determination of the four distinct factors mentioned above can give a real measure of the thermal demands of the environment".

There have been, however, several more-or-less successful efforts to

combine two or more of these factors. The American Society of Heating and

Ventilating Engineers have developed, and use, a tea odled "effective

temperature". Effective temperature shows the relationships of air

temperature, relative humidity, and air velocity which give an equal sensation of comfort. This is of course a subjective measurement, and

was determined from votes of a numberof subjects tested by the ASHVE under

varying conditions. This quantity makes no reference to radiation, since

the temperature of air and walls was always the seme in the experiments.

Operative temperature is a figure representing the combined influence of air temperature and mean radiant temperature. A formula involving con-stants for both radiation and convection has been devised, but for most

suual situations a mean between air and wall temperature gives approximately

the same figure as operative temperature.

Effective temperature and operative temperature should not be thought of

in terms of one being better than the other. They simply give figures

for different things, and are useful in different ways. They are cited

here to reduce any confusion that might result from reference to them in this discussion or elsewhere.

The "ideal" climate

Since this thesis is limited to the cooling effects of air movement, it is necessary to attempt to locate the comfort zone as a point of departure.

Some physiologist and climatologist seem to be especially wary in

dis-cussing the ideal thermal environment. Considering the number of

variables involved, this is a natural attitude. However, we can get

some indication of the ingredients of the ideal climate from a reference

made to it by Dr. L.P. Herrington in a speech to the Building Research Advisory Board conference held in Washington, D.C. In this talk Dr. Herrington said, "The ideal indoor climate occurs in this general latitude

in the months of October, early November, or in May, when it is not necessary

to use any artificial heating or cooling at all. Under these circumstances

the air outside is about 600. _{We have sunny days and, due to radiation}

effect, the structure itself has a radiation temperature 40 to 50 above the air temperature. _{Due to the moderate temperature outside, the windows are} open, air movement is considerable, and _{the air changes through a structure}

are perhaps 15 to 20 an hour, but variable from moment to moment.

"In this internal environment with wall temperatures having infra-red

radiation values, characteristic of radiation frcm surfaces around 700

to 720, and with air tenperatures about 40 lower, people feel very good. This is the ideal indoor climate for this latitude, and for people with

the kind of seasonal weather experience typical of our geographical zone".

Wolfgang Langewiesche, in one of his articles for the Climate Control

21

Project of House Beautiful magazine, gave a less technical but more

generalized criterion of the ideal climate. He said that the ideal

climate is simply where the individual is not conscious of climate at all. _{You don't want to take off your shirt or put on an overcoat;} you don't want to move in front of the fan or over the floor register;

you don't want to take a cool shower or a hot bath; you just don't think

about the climate.

Assuming that both the above observations are correct, we need to know whether other combinations of the factors involved in heat interchange

will give equivalent sensations of comfort, and how far we can deviate from the optimum before the climate becomes intolerable or unhealthy.

Cofort Chart, ASHVE

Any amount of physiological data would be valueless unless the conditions

it prescribed also produced a feeling of comfort for the individual. Therefore, the comfort zone should be determined primarily by subjective

measurement. _{The Research Laboratory of the American Society of Heating}

and Ventilating Engineers has used such a system of measurement in the

development of their comfort chart, shown on the following page. The

special conditions to which this chart apply, given in the note

accompany-ing the chart, should be carefully considered. In addition to those

qualifications, recent information has indicated other possible variations. There is some evidence to indicate that the comfort zone could be extended beyond the 30% and 70% relative humidity lines.

There seems to be a rather wide spread of effective temperature called the summer comfort zone, ranging from a preference of 30% on the cool side to

50%

will know how many people are likely to be satisfied in any given condition.

This chart makes no provision for the influence of radiant heat. In the

experiments condt.cted in connection with this chart, air and wall

tempera-tures were approximately the same. The A.S.H.V.E, _{Guide states, "Radiation}

from occupants to room surfaces and between the occupants has an important

bearing on the feeling of warmth and may alter to some measurable degree

the _{optimum conditions for comfort previously indicated. Since the mean}

radiant temperature of a space is affected by cold walls and windows, as well as by the warm surfaces of heating units placed within the room or

'?K. D01

0

DRY BULB TEMPERATURE*F

A.S.H.V.E.

Note.-Both summer and winter comfort zones apply to inhabitants of the United States only. Applica-tion of winter comfort line is further limited to rooms heated by central staApplica-tion systems of the convecApplica-tion type. The line does not apply to rooms heated by radiant methods. Application of summer comfort line is limited to homes, offices and the like, where the occupants become fully adapted to the artificial air con-ditions. The line does not apply to theaters, department stores, and the like where the exposure is less than 3 hour.. The optimum summer comfort line shown pertains to Pittsburgh and to other cities in the northern portion of the United States and Southern Canada, and at elevations not in excess of 1000 ft above sea level. An increase of one deg ET should be made approximately per 5 deg reduction in north latitude.

densely occupied spaces, such as classrooms, theaters and auditoriums,

temperatures somewhat lower than those indicated by the comfort line may

be desirable because of counter-radiation between the bodies of occupants

in close proximity to each other. Such radiation will also elevate the

mean radiant temperature of the room".

The Guide does not indicate any method of determining the quantitative compensation which should be made for significant variation in radiation.

A short discussion of how this might be done will be given in later paragraphs.

Comfort Zone and Air Movement

This comfort chart is for minimal air movement of 15 to 25 feet per minute. We are, however, given data vhich can be used to convert the comfort

chart for higher air movement. The fish-shaped chart shows how effective temperature varies with air velocities up to 700 ft. per min, Consider the line A-B on the chart. We see that for a dry bulb temperature of 760 and a wet bulb temperature of 620 , the effective temperature at an

air velocity of 20 to 30 ft. per min. is slightly over 700 When air

velocity is increased to 600 ft. per min., effective temperature drops

to 640.

It should be remembered that effective temperatures are equivalent

com-fort conditions. _{In the above example, we have an air temperature,}

humidity, and air movement producing 640 ET. This would give the same

feeling of comfort as still, saturated air at 640. Obviously, effective

2 -120 Do-6o- -6o 70-1 0709 50 o* ~ kii

EFFECTIVE TEMPERATURE CHART SHOWING NORMAL SCALE OF EFFECTIVE

TEMPERATURE, APPLICABLE TO INHABITANTS OF THE UNITED STATES

UNDER FOLLOWING CONDITIONS:

A. Clothing: Customary indoor clothing. B. Activity: Sedentary or light muscular work. C. Heating

Methods: Convection type, i.e., warm air, direct steam or hot water radiators. plenum systems.

I

comfort chart. Few people have any occasion to know how it feels to be in still, saturated air at various temperatures.

Our objective is to determine the influence of air movement on the comfort

zone. Assuming that we l-ave the conditions to which the ASRVE comfort

chart applies, let's take one example for illustration. Suppose we want

80% of our tenants to be comfortable (we're mad at the other 20%). The

relative humidity remains constant at 70%. The chart indicates that an

effective temperature of 730 must be maintained.

For still air, our wall thermometer must not go above 760 ; the

corres-ponding wet bulb temperature will be 690 .. Now if we can in some way,

natural or otherwise, increase air velocity to 700 feet per minute

(approximately 8 mph), the room thermometer may rise to 830 and our tenants

will be equally comfortable. _{It's something of a nuisance to check it,}

but the figures on the effective temperature chart which give this con-dition are: 83.50 dry bulb, 75*50 wet bulb, giving 730 ET at 700 ft. per min.

Thus, an increase of 8 mph in air velocity allows us an increase of 70 dry

bulb temperature for an equivalent comfort condition. The 30% relative

humidity at the other end of the 730 _{ET line does not give quite as good}

results. By increasing to the same velocity, dry bulb temperature goes

up only 60 , from 820 to 880.

These figures give us, very roughly, the upper limits of the effective-ness of air movement for an almost ideal (80o satisfied) comfort condition.

That is, 880 at low humidity, and 830 at high. However, it is believed that the importance of natural air movement will not be in satisfying an

ideal condition, but rather in bringing an intolerable heat down to an acceptable one. This would lead us to believe that even higher dry bulb temperatures will enter the comfort picture, and they might under certain conditions. But there is another joker in the deal. Allen and Walker

say, "the maximum air velocity which can be tolerated with comfort by

human beings at rest is approximately two feet per second". Converted,

two ft. per sec. equals 120 ft. per min. or about 1.4 mph.

This low air velocity may be a desirable goal, but it. is believed that if the choice is between a higher air velocity and disagreeable heat, most

people would "tolerate" at least twice that velocity. W7hile no data have

come to the authorts attention regarding the effects of air velocity on

various normal activities, such data undoubtedly exist; and it would seem

advisable for the designer to determine such things as: at what velocity

does a newspaper become unmanageable; when do letters and other papers

leave the desk; what air movement makes lighting a cigarette difficult;

etc.

All the preceding discussion has been on the basic assumption that the

temperature of air and walls is the same. It would apply directly to

those rare situations when a building ig well insulated, well oriented,

well shaded, etc. In other words, air and wall temperatures will not be

the same in most situations, and especially at the elevated temperatures with which we are primarily concerned. Therefore, it seems reasonable to

look for data showing the influence of radiant heat on the comfort zone.

A thorough search has revealed no comprehensive information on the subjective measurement of radiation and comfort. The physiological reactions of the human body to radiant heat are well documented by Winslow and Herrington, but references to which conditions produce a feeling of comfort are rather sketchy. We can get a hint by comparing their charts relating skin temperature to operative temperature and skin temperature to sensations of pleasantness.

Remembering that, for engineering purposes, operative temperature may be taken as the mean between air and radiant temperature, the above com-parisons would give some indication of how radiation affects comfort.

However, generalizat*ins from this data will not be hazarded in this study.

In reference to the ASHVE comfort chart, Winslow and Herrington have this to say, "'%ork now in progress under the Society's direction will probably

result in twro scales. One of these, the familiar effective temperature

scale in present use, will quite possibly be restricted to use insituations -where traffic in and out of conditioned spaces places a premium on short-period contrast sensations. It is expected that to this will be added a

similar scale for equilibrium conditions applicable to the comfort-conditioning of spaces with relatively long periods of occupancy. This added equilibrium scale will probably base its equivalent combinations of dry-bulb temperature and relative humidity on lines of equal skin temperature, experimentally determined by methods similar to those which we have developed in connection

3'C

with partitional calorimetryt

.

10C

50 60 70 80 90 100

RADIANT TEMPERATURE (*F)

1. RADIANT AND AIR temperatures are inter-related in their effect on winter comfort zone. Rise in either temperature

requires corresponding drop in temperature of opposite factor to maintain equivalent comfort condition. The resulting comfort temperature is simply the arithmetical average of the two factors.

40 60 80 100 0

RELATIVE HUMIDITY (%)

2. HUMIDITY as it affects comfort band. Using preceding graph as basis for temperatures, lower humidities are shown to raise upper comfort levels but to have almost

no effect on lower. Upper level is further raised by increased air movement. Greatest cooling effect is obtained when air is relatively dry and moving rapidly.

100 200 300 400 500 VELOCITY OF AIR IN FEET PER MINUTE

3. AIR MOVEMENT as it affects body cooling. As velocity increases, upper comfort limit is raised. Air movement

becomes still more effective as relative humidity goes

down. These graphs are not intended to show precise

lines of upper and lower comfort range, but nature and trend of band under influence of various factors.

10 901 s0 Ws7 C (r6 0 .5 5

we might make some adjustment for radiant heat by a rough combination of

the ASHVE comfort chart and the concept of operative temperature. That

is, take operative temperature (mean of radiant and air) and use it as

dry bulb temperature in the comfort chart.

One might assume that because we can simply average radiant and air

tem-peratures and somehow relate this figure to comfort, that it wculd be equally desirable to have high radiant and low air temperature as it would

be to have low radiant and high air temperature. This is not so. The

human body, marvelous machine that it is, can adjust for either condition

and bring about thermal equilibrium. In doing so, the amount of heat

dissipated by each method of heat-loss varies over a wide range. The

cue is that the stress on the body is not the same when losing heat by

evaporation as it is when losing it by radiation or convection.

As a starting point, let's see what the porcentage of heat loss is for

the various avenues _{of heat interchange under basal conditions; that is,}

for a resting subject, moderate temperatures, little air movement, low

relative humidity, and air and walls at about the same temperature. In

this situation, radiation accounts for 2/5 of the total heat loss from the

body, convection 2/5, and evaporation 1/5. But that's a whole mouthful

of conditions. It seems almost impossible to make any generalization about

these percentages. Some results of experiments made by the Pierce

Air Tem. Wall Temp. Percent Heat Loss Due to

Series U 3 _Evag. Rad. Cony.

A 17.1 19.0 10 40 50

B 16.0 49.1 21 79

C 22.8 22.8 17 13 70

D 29.4 52.4 78 22

E 35.4 36.6 100

In Series A - Body temperature was falling.

" It - _{There was more gain by radiation than was produced by metabolism.}

" "t - _{High air movement of 264 cm per sec.}

" " D - Gain by radiation was 66% above metabolic rate.

"

" E - Heat gain from both air and walls.

These figures show how widely the percentages can vary under different

conditions. _{But although generalizations are hard to make, we can tell} what will happen if we know the conditions beforehand. To do this we must know how each avenue of heat loss varies as temperature, humidity,

air movement, etc. are altered.

The chart on the following page shows how changes in operative temperature

influence the heat loss or gain by evaporation, radiation and convection. This chart is for a single subject, so the values given should not be applied to people in general, but the relationships would be similar.

Figures above the zero horizontal line indicate either heat production by metabolism or heat gain from the environment. _{Those below indicate heat}

loss. _{Radiation and convection are shovm as one sum because operative} temperature governs both processes.

Considering the curve for radiation and convection, we notice that as

120 100 80 60 40 20

(n

0

o

o

* EVAPORATION

Factors in heat balance between the unclothed human body

and its environment at various Operative Temperatures.

a! 0.

In this case, at an operative temperature of about 840 F no heat is lost, and the body is beginning to gain heat by radiation and convection.

Evaporation, on the other hand, remains fairly constant at lower

tempera-tures, until an operative temperature of about 800 F is reached. At this

point the rate of increase by this method jumps very rapidly. Obviously,

evaporation is beginning to take over all the heat losses from the body

-not only that demanded by metabolism, but that gained from the environment

thru radiation and convection. Thus we see why it is said that heat loss

by evaporation is accomplished only by greater stress on the body.

Note where the curve for evaporation crosses the curve for radiation and

convection. This area has been termed the "zone of thermal equibibrium".

Operative temperatures below equilibrium has beeniermed the "zone of body cooling" and above equilibrium the "zone of evaporative regulation". Mean values for thermal equilibrium, using a group of subjects, have been determined to be as follows:

Oper. Temp. for

Equilibrium, OC

Nude subjects in reclining position - 29 to 33

Clothed subjects in reclining position - 25 to 29

Nude subject performing active work 19 to 21

The question might well be asked: what significance do the preceding facts

have in a discussion of air movement? They tell us that at operative

tem-peratures above 210 C to 330 C (or 700 -90o F) we are dealing only with

evaporation, and that evaporative regulation produces the greatest stress

on the body. Therefore, we should give attention to means of reducing

PART II

BEHAVIOR OF MOVING AIR

Before attempting to control the wind, it seemed wise to try to get a fairly

accurate picture of what the wind is like. And before studying the wind,

the properties of air must be known. So the first step was to review the

physics of air. Following this, a search was made for information regarding

the characteristics of unconfined, natural breezes flowing without obstruction.

Finally, the effects of obstructions, such as buildings, will develop.

In this section, we will be concerned only with the air and natural air move-ment. The following section will deal with the flow of air in and around

buildings.

Air and Tenperature

It might be well to make a brief summary of the dlementary physics relating to

air and temperature, in order to have this well-known informationfresh in our

minds. Air is composed of about 80% nitrogen, 18% oxygen, small amounts of

carbon dioxide and other gases, and water vapor. When it comes to cooling,

the most important variable is that water vapor.

The psychrometric chart (page ) shows how an increase in the temperature of the air increases its ability to hold moisture. This chart plots air tempera-ture against grains of moisture in the air. Among other things, it shows the amount of moisture required to saturate the air at various temperatures. This is shown by the top curve of 100% relative humidity, which is saturation.

Notice that only about 80 grains of moisture are required to saturate air at

a: < 160 0

I

~

z

DRY BULB TEMPERATURE F

As air is heated, it expands - that is, it becomes less dense, lighter, and has a tendency to rise. The low pressure area thus created must be filled by the cooler, heavier air adjacent. This is the basic cause of all air movement. The worldwide air currents have been shown by meteorologists to

be the result of the changing temperatures of the air. Strong upward currents

occur over the hot land masses, particularly deserts, and cooler air from over water or from polar regions rushes in to fill the low pressure area. Of course this is a super-simplification, but it illustrates the basic principle.

These two facts about the air-expansion and ability to hold moisture - account for most weather phenomena, on large or small scale. The proper utilization

of these facts can be extremely helpful in an effort to provide greater thermal comfort.

This study will not attempt to discuss whether a certain locality will have natural air movement. There are existing works which give comprehensive pictures of regional climates throughout the United States. However, these studies are-for sizeable regions, and may not give a true picture of the

microclimate, that is, the climate of the immediate neighborhood, block, or

lot on which a building is to be erected. Therefo;e, a few facts about why

the air acts as it does will be given. This is not an attempt to survey microclimatology, but rather to give same hints as to the value of this

field to design.

The most significant relationship is between air, land, and water. The land changes temperature at a much faster rate than water, gaining heat fran the sun more rapidly during the day, and radiating it to the sky faster at night. This accounts for the sea breeze. As the land heats during the day,

it raises the temperature of the air next to the ground. This air rises,

and the low pressure thus created must be filled by the cooler air from over the water. This breeze may extend inland from the ocean as far as twenty miles, whereas in the case of a small lake, its effects may not be felt over a few hundred yards. The whole procedure is reversed at night when the

water is warmer than the land.

This sea breeze may not be the"prevailing" breeze in some areas, and its benefits may be cancelled by attempting to oppose a larger force. Chicago

has been cited as a good example of this opposition of forces. In summer

the prevailing breeze is toward Lake Michigan. Due to the conditions mentioned above, the purely local "sea" breeze is of course fraam Lake

Michigan. Therefore, the two have a tendency to cancel each other, leaving the city with little natural air movement. And when the rush of cool air fram the lake is felt at all, it carries for only a few blocks. Obviously, on the opposite side of Lake Michigan, where the two forces are cooperating,

the weather is much more ideal.

Another phonomenon worth mentioning is the downward flow of cool air.

While this effect is either negligible or non-existmnt during the summer, in areas where the land is relatively flat, it contributes to significant velocities in mountainous areas. The so-called "mountain breeze" is a result of air being cooled' by the lower temperature surfaces at high

altitude, and descending into the valley. It has been said that residents of such areas seek to place their houses at the mouth of a valley in order to benefit from this breeze.

It should be noted that the air gains relatively little heat from direct

solar radiation. Finch et. al. state "... the atmosphere absorbs directly only about 10 to 15 per cent of the solar energy that comes to it. Such absorption takes place mainly in the upper layers of the air. This process, therefore, is not very effective in heating the layers of air close to the

earth." However, the same reference goes on to say that the re-radiation of solar energy from the earth is readily absorbed by the atmosphere - "some 90 per cent" is estimated. It would seem that this method of heat transfer from the earth to air by radiation would heat the air more or less uniformly.

That is, the radiant energy would have to travel same distance before it

would become completely absorbed.

On the other hand, air is a very poor conductor. Therefore, the air which

comes in contact with the heated surface of the earth trqnsmits this heat

very slowly to adjacent air, end a film of warm air forms. One might

assume that this film would always rise immediately, allowing more air to come

in contact with the heated surface. This is not true in all cases. In one of his articles for the House Beautiful Magazine Climate Control Project,

Wolfgang Langewiesche refers to the "stagnation" of hot, low,air.

Apparently, this hot air forms in a sort of bubble, and in open areas it balloons away periodically. However, when this air is confined in a snall

space such as a patio or enclosed garden, this bubble sometimes doesn't

rise. The comparison is made to a soap bubble, which will remain stationary until activated by some small breeze. The renedy for such stagnation as

mentioned above is to allow access to even a small air movement - by a gate,

Many more interesting examples of the influence of the surroundings on

air movement and temperature could be made - the cooling effect of trees

and lawns, radiation from paved areas and buildings, etc. - but they do

not relate directly to this subject; so let it be sufficient to say that

the importance of the surroundings to the otal design picture cannot be overestimated.

Velocity and Altitude

In the search for information about the behaviour of natural breezes, an

interesting study came to light. The practical application to the cooling

field of the results of this study are obscure, but since so little informa-tion of any kind is readily available, it is cited here for the possible

bearing it might have on future work.

In their paper, "Air Conditions Close to the Ground, and the Effects on

35

Airpla.Ine Landings," the authors report finding that average air velocities

will be greater at higher altitudes than they are at ground level at the same time. They have even expressed this relationship in terms of a formula

.

These experiments were conducted on an airfield where the breeze could travel for about mile over unobstructed, flat terrain before hitting the test instruments. Measurements were made at intervals fram six to fifty

one feet above the ground. Fluctuations in vertical and horizontal direction were recorded, as well as fluctuations in velocity. The test runs were of some 36 seconds duration, and results were computed from

- ) 0 Q-2 _.36 o, -3 20/0 0 ., 4 '?4 0 / 0 0

...

-Wind speed and inclination fluctuations for an average ground wind of 8 niles per hour. Run 1, July 8, 1932.

112

1z ExperimnentaI

')1V

50 60

Average wind speeds expressed in terms of the average wind specd

at altitude of 51 feet. b

0~

0 0 /0 20 30 40 Height above gr-ound, feet

photographs. _{Results of one test run of average velocity of 8 mph at ground} level are reproduced here as an example.

No question is made of the validity of these results when used under the conditions outlined above. The procedure of these experiments is mentioned

to explain caution in attempting to apply the results to any other conditions

than those under which the experiments were made. Certainly an unobstructed approach is demanded, and there is some doubt in the mind of the writer

whether similar figures would come of longer test periods and measurements

at higher altitudes.

As was said, the relation of this pheonomenon to ventilation is not quite

clear. It probably has no bearing on the problem of the one-story building. But it seems that it could be important in taller buildings, particularly when the building is not divided by floors, as in some industrial structures. 'What would be the indoor effect of air entering the five foot level at 10 mph and at the seme time entering the fifty foot level at nearly 15 mph?

Changes in Velocity and Direction

Fluctuations in wind speed and inclination are of interest also. The preceding charts of these variations show changes in inclination sometimes

as often as every second, and rarely farther apart than three or four seconds. This would indicate that the wind has a sort of wave motion of rather high frequency. Changes in wind speed are not as frequent, but do

indicate a definite pulsation. It is doubtful whether these changes would be conspicuous to the observer, and records for such short periods would not indicate large gusts, but this is evidence that the behavior of wind is

far from the smooth flow found in the wind tunnel.

It was expected that the fields of meteorology and aerodynamics would offer many references on the behavior of the wind, and they do. But unfortunately,

the vast majority of this information cannot be used in connection with natural ventilation. Meteorology concentrates on the large-scale causes of winds, and the wind patterns over vast areas. Present-day aerodynamics has little use for any air movement of less than about a hundred miles per hour. Not a single comprehensive study of wind behavior at low velocities (below 20 mph) was discovered.

In the search of aerodynamics literature, after being confronted with sub-sonic, super-sub-sonic, much numbers, and many, many airfoils, it gradually dawned that the very early literature might be of a different nature. In 1893 The Smithsonian Institution published, as one of its "Contributions to

Knowledge", a pamphlet by S.P. Langley called "The Internal Work of the Wind." Aside from its bearing on this study, the paper is thoroughly fascinating. It is a story of observations made by a man who was later to make some of the

most important contributions to the development of heavier-than-air craft.

Of course the date precedes the first flight of man by about ten years.

Langley was trying to find an explanation of the soaring flight of birds. Since he had assumed that this phenomenon was due to the vagaries of the

wind, he set about to determine the precise nature of air currents. His

paper offers data on the changes in wind velocity at very short intervals of time. The technique was to build an extremely light cup-anemometer so

half-revolution, which would sometime mean as often as every second. Graphs of these recordings, one example of which is shown on the following page, are

very similar to those of Thompson, et al, mentioned above. Langley's results

were substantiated by other findings, though not extended materially, and

apparently little more is known about the wind today than was known in 1893.

Irminger and NAkentved state, "The wind having such an irregular structure,

it is obviously necessary to study it most thoroughly, and to correlate the

results with those obtained by experiment, in order that data applicable to

practical conditions may be evolved." They go on to report essentially

the same characteristics mentioned above; and in reference to pressures on structures, say "The available knowledge, however, is insufficient to enable any definite statement to be made." This same inadequacy can be applied to ventilation.

Summry

In the investigation of natural air movement, we first looked for the

basic properties of air and their relatonship to temperature. It was

found that when air is heated, it expands and rises, causing a low pressure

area which must be filled by other air. This is the basic cause of all air

movement, from worldwide currents to the draft in your fireplace. The

natural means of heating air were discussed, and reasons given for the hot

film of airthat forms on surfaces and sometimes does not rise.

This same change in temperature governs the ability of the air to retain

moisture. Hot air can hold more moisture than cold, therefore having a

greater power to evaporate moisture from a surface.

PLATE II.

5 m 5 53- 541" 55m" 56rn 57"' bw"

Wind velocities recorded January 14, 1893, at the Smithsonian Institution with a light Robinson anemometer (paper cups) registering every revolution.

Abscisso = Time.

The present state of knowledge about the behavior of unconfined, natural air movement is generally inadequate. We know that a natural breeze is

far from a auooth steady current. _{Its absolute velocity changes rapidly,}

wide fluctuations having been recorded as often as every second. Mean

values over longer periods of time also give a changing picture. At the

same time the velocity is pulsating, the wind is changing direction, both horizontally and vertically.

It was shown also that the mean velocity of the wind will increase with altitude, within certain limits at least.

With regard to the rapid fluctuation of velocity and direction, it seems

that no generalizations can be made from existing data. Only this

observation will be made here: the evidence of fluctuation is sufficient

to demand that it be accounted for in any theoretical solution to natural

ventilation. This does not mean that the phenomena will necessarily

influence engineering applications, but proof that it either does or does

not must be shown sooner or later. The already complicated procedure

of making comparisons between wind tunnel tests and full-scale results

PART III

AIRFLOW AROUND OBSTRUCTIONS

The preceding section discusses some of the properties of air, and its

behavior when unrestricted and unconfined. The following discussion

will deal with the happenings when an obstruction is placed in the path of

air currents. The possible number _{of shapes and combination of shapes is}

infinite, so _{an attempt was made to select examples which would have the most} bearing on building considerations.

Certain simple geometric shapes are discussed first, with the thought that

these will serve for generalizations about airflow around more complex

shapes. _{Another reason for using these geometric forms is that most}

experimental investigations on building shapes use only the standard

rectangle and gable roof.

It should be noted that, with rare exceptions, all of the results used in the following discussion were derived from steady-state wind conditions,

either produced in a wind tunnel or assumed for calculation. Such a

procedure ignores the actual fluctuation of the natural wind which was

shown in the preceding section. HoweVer, we must be content to use

existing information, keeping in mind the limitati ons as well as the possibilities.

We are, in general, looking for two things: (1) the path of air flow, and

(2) variations in pressure. _{Without changing the temperature of the air,}