Chimney Construction for Houses. Pt. 1: Structural Principles. Pt. II: Thermodynamic and Aerodynamic Principles

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Publisher’s version / Version de l'éditeur:

Technical Translation (National Research Council of Canada), 1958

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Chimney Construction for Houses. Pt. 1: Structural Principles. Pt. II:

Thermodynamic and Aerodynamic Principles

Grolle, J. H.; Adam, A.

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The study of domestic heating systems forms

an important part of the work at the Division of

Building Research.

A special phase of this study

is the evaluation of domestic chimneys on the

basis of performance.

The Division of Building Research is

privi-leged to maintain close liaison with other research

organizations on problems of common interest.

The

publication of this translation of the proceedings

of a meeting held in The Hague in February 1956, at

which the problems of chimney construction and

per-formance were discussed, is presented as part of

this liaison.

The Division of Building Research records its

thanks to Mr. H.A.G. Nathan of the Translations

Section of the N.B.C. Library for translating this

paper.

Ottawa

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Technical Translation

728 Title:

Chimney construction for houses·

(De schooreteen in de won1ngbouw)

Authors:

Pt. I:

Structural principles, by J.H. Grolle

Pt. II:

Thermodynamic and aerodynamic principles

by A. Adam

Reference:

De Ingen1eur, 68 (40): G.51-G.60, 1956

Translator:

H.A.G. Nathan, Translations Section, N.R.C. Library

Translated with permission

*Lectures and discussion on the occasion of the combined

meeting of the Division of Sanitation and the utility-Building

Section of the Division of Architecture and Hydraulics (T.N.O.)

on February 21, 1956, in The Hague.

On July

7,

1955, the above

division spent a day on the subject "heating problems in

work1ng-class houses" (the lectures and discussion have been published

in De Ingen1eur no. 40 and 43, 1955).

It was first intended to

include "chimney construction" as well.

When the programme for

the meeting was drawn up it appeared that it would be more

appropriate to discuss this subject at a separate meeting in

view of the more general character of the other sUbjects.

(4)

Part I:

Structural Principles

by J.H. Grolle

Flues must meet the following requirements:

1. Flues must be as vertical as possible.

2. Any bends in the flue should not be too

ｳｨｾｲｰＮ

3. The height of a flue should not be less than five

metres under any circumstances.

4. The profile of a flue should either be round or square,

and it must be the same throughout (including the outlet and the

bends).

5. The cross-section should be neither too small nor too

large.

6. The walls of the flue must have no leaky joints which

permit infiltration of air.

7. The exterior of flue walls should not be exposed to

moisture and should be exposed as little as possible to great

differences in temperqture.

8. The inside surface of flue walls must be capable of

withstanding temperatures up to 600°C.

It must be cmooth and must

be capable of absorbing and giving off moisture.

9. Heat losses in the flue walls should not be excessive.

10. The flue outlet on the roof should extend high enough

above nearby obstructions so that the draught will not be

ad-versely affected.

1. Flues Must be as Vertical as Possible

Since contact between the flue walls and the external air

should be kept to a minimum it is very important to determine the

locations of heating units in the plan so that wherever possible

an outlet is provided at or near tre highest point of a building.

(5)

In conventional r-oom heating, the fireplace, or stove,

and the flue are placed close to each other.

However, if for

reasons of comfort the heating unit is placed close to the front

of a house, the possibility of locating the flue at some distance

from the front should be considered.

Thus for certain roof types

it may occasionally be necessary to move the outlet away from the

ridge of the roof.

In such cases preference must be given to a

position that differs but little from the vertical position of

flues in attics.

Of course, this difficulty does not arise in the case of

central heating, where the flue is connected to one

central-heat-ing furnace.

The placing of the radiators is in no way affected

by this.

A number of other factors which may affect the location of

the heating units in the layout of a bUilding are given below.

Furthermore, the choice of the flue design is not affected by the

vertical position.

The flue design is of greater importance in

the case of bends.

2. Any Bends in the Flu.e Should Not be too Sharp

In order to prevent eddying and frictional resistance,

bends in flues must be uniformly smooth.

Central-heating flues

usually begin ttoJith an almost horizontal section.

In

ｶｩ･ｾｊ

of the

chimney draught and the draught which forms when the

central-heat-ing furnace is started, it i3 expedient that from the outset this

section make an angle as large as possible with the floor so that

on changing to the purely vertical section the angle will be as

obtuse as possible.

If the first section of such a flue is lined

With firebrick in anticipation of high temperatures, wedge-shaped

brick especially

ｭｾｮｵｦ｡｣ｴｵｲ･､

for this purpose should be used.

With the bricks properly laid the bends will then be sufficiently

smooth to prevent eddying and high frictional resistances.

If

ordinary brick, or worse still sand-lime brick,is used, the result

is less satisfactory in many cases.

Frequently an attempt is

(6)

courses and allowing each consecutive course to project somewhat

above the preceding one. At best the bricks will then appear

more or less cut and are patched up as much as possible by parging. Sooner or later, after the chimney has been in use for some time the parge will appear to have lost its cohesion, and little or nothing then remains of the initial smoothness.

Experience has shown that it is very difficult to cleave or cut sand-lime brick and therefore burnt brick is preferable for

bends in a flue. Bricks cut with an axe should be laid in the

bend in the same way as in arches and archways. Thus, the bricks

should always be laid in layers perpendicular to the axis of the

flue. The bricks must be cut to shape ｾｴｊｩ th the axe as accurately

as possible in order to obtain a thickness of the joints which is

uniform and not too great. Joints always were and still are the

weak point in chimney construction. The author intends to describe

this later in greater detail.

3.

The Height of a Flue Should not be Less Than Five ｍ･ｾｲ･ｳ

Under Any Circumstances

It should be noted that this paragraph refers to the chimney height and not to the flue length, since the difference in height of the outlet of the heating unit and that of the flue beyond the

roof is the determining factor. Non-vertical parts of a flue

mercly ｯｾｳｴｲｵ｣ｴ its proper operation. This applies particularly

to a flue of minimum height.

However, it should be emphasized that this applies only to ordinary stoves and fireplaces, whereas for central-heating

furn-accs a chimney height of five metres is nearly ｡ｬｾ｡ｹｳ insufficient.

In the latter case the proper dimensions must of course be deter-mined by a heating consultant.

4.

The Profile of a Flue Should Either be Round or Square, and

It Must be the Same ｔｨｲｯｵｾｾｨｯｵｴ (Includin::r, the Outlet and

the Bends)

Flues having a round profile are seldom encountered in build-ing practice, although this type of profile is usually considered

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to be slightly more favourable than the square one.

However, a

rectangular profile, which differs appreciably from the square

one, is even less favourable and must therefore be avoided as far

as possible.

Blocks of burnt clay (e.g. Solidus blocks) suitable for

flues of great diameter are available.

By rounding off the

in-terior angles of these blocks approximately round profiles are

obtained.

The wall thicknesses of round vitrified clay conduits

themselves are too small and result in too much loss of heat.

Therefore, insulation

wi t

h brick or some other material is required

here.

Such conduits shoulc be unglazed.

The objection to square flues with a round flue lining is

the large amount of mortar required.

As mentioned above, this is

a weak point in chimney construction.

Flues of asbestos cement and those of vitrified clay have

the same drawback.

We just have to keep on looking for a

manufac-turer who can produce, at a price suitable for low-cost housing,

the proper unit of burnt clay with all the necessary accessory

parts, such as pipe connections for the heating unit, cleanout

doors, unavoidable bends, etc.

5. The Cross-Section Should be Neither too Small Nor too Large

With respect to flue cross-sections the specifications

in-corporated in the building codes of the various Dutch

municipali-ties differ to some extent.

In Rotterdam and Haarlern the

specifi-cations require a profile such that a sphere having a diameter of

12

cm. can be freely moved up and down in the flue.

Most of the

other municipalities and also the

19S2

Model Building Code require

a size which corresponds to a

ＱＵｾ

to

16 cm. side for a square

pro-file or to a

ＱＷｾ

to 18 cm , diameter for a round profile.

Although as far as flues of smaller cross-section are

con-cerned there have been no complaints for several years, it would

be safe in any case to use dimensions no smaller than

11

x

11

cm.

for normal flues in houses.

If a round profile is chosen a diameter

(8)

no great resistance due to bends, etc.

For open fireplaces a proportionally larger flue cross-section is needed, since the amount of intake air is greater

here than that required for stoking only. A diameter of

18

em.

or a square profile of

16

em. side is the minimum here.

It is obvious that the cross-section of a flue for a

central-heating furnace should always be determined with the ad-vice of a consulting engineer.

6.

The Walls of Flues Must Have No Leaky Joints Which Permit

Infiltration of Air

Air leaks may be eliminated primarily by selecting good

material. In conventional housing sand-lime brick is the chief

material used. This applies to flue walls as well.

A detailed investigation carried out a few ｹ･ｾｲｳ｡ｾｯ by

Rqtiobouw on behalf of the Ministerie van Waterstaat en Verkeer

(Department of Transport) showed that cracks had formed in

50

per-cent of the cases in which flue walls had been built of sand-lime

brick. Another d rawba ck of thi8 material is the fact that the

brick is nearly always too dry when it is laid, so that the

ad-herence of the mortar to the ｾｲｩ｣ｫ i3 practically zero. It is

true t hat attempts are aomct Lmes made to maintain some mortar bond

in sand-lime brick walls by recessing the bricks on the two largest sides, but air leaks can scarcely be prevented in this way.

When the flue wall is built of ordinary brickwork preference should be gi ven to a burnt brick that is not too soft, "bright gray"

or best of all "farmers' Ｌｾｲ｡ｹＢ and not too fat a mortar, ･ＮｦｾＮ 1

part cement per 1 part lime and

6

parts sharp sand. For flue walls

exposed to especially ｨｩｾｨ temperqtures a mortar consisting of 1

part cement per 1 part trass and

5

parts eround chamotte is

recom-mended. The same mortar ratio also applies vrhen flue 'l1alls are

built of blocks of burnt clay, such as Solidus blocks.

A ｲｾｯｯｲｊＮ '/Jay of av ot dLng air Leaks is supervision vinen the

(9)

for each bricklayer watching carefully to make sure that each

brick is completely surrounded by mortar and worked into place,

not tapped. However, in order to ensure perfect operation of

the flue, a smoke test is required each time the ch tmney has

"risen" another storey. If the chimney then appears to be leaky

and must therefore be pulled down, the preventive effect of the above measure soon becomes apparent.

The next point is the connection of the heating unit to the

chimney. The iron smoke-pipe connection of a ce nt r'a Lc-hea.t Lng

furnace is usually well insulated with brick. However, aulte

fre-quently this does not apply to ordinary stove pipes.

The author was once asked to check why in a particular case

a chimney would not draw. When for this purpose the stove was

re-moved, i t wa s found that the stove pipe had been pushed so deep into

the chi mney opening that it butted against the back wall of the flue,

practically closing the flue. This seems to be a frequent occurrence

as the author learned later. However, the best smoke connection is

a double thimble consisting of two rings fitted into one another in such a way that the inner ring is internally connected \,li th the outer

one. If a stove pipe is chosen whose diameter is such that the pipe

can be pushed in between the two rings of the thimble the air leak is reduced to a minimum.

Another measure is described in the next paragraph. However,

it should be pointed out first that a large number of investigations have shown that the largest and most troublesome air leaks are found

at the chimney base. This is due to a structural error to which

every attention must be paid in view of the chimney draught.

Direct-ly below each horizontal smoke-pipe connection there should be a

Soundly built soot catcher. However, it should be remembered

that although soot catchers have a useful function, they nevertheless

produce resistance and t hus have a det r'Lmerrt a.L effect on the chimney

drausht, especially if they are very deep. Therefore, a depth of

(10)

In individual cases it happens that concrete floors support

the flue walls. An opening corresponding to the inner

cross-section of the flue is then made in the concrete floor and when the chimney is built up the flue is continued on top of the

concrete floor. It is self-evident that in such a case the ｣ｨｾｮ｣･

of contracted joints between the brickwork and the concrete floor

on top of i t is very great. Of course, brick-enclosed flnes must

be kept completely away from the concrete floor structure. This

is not only to avoid damage to the chimney but also to the floors,

since i t is a well-established fact that cracks form in concrete floors which have been integrated with chimneys due to expansion of the chi:nney.

If a chimney is built against an eXisting wall or if, as frequently happened in the past, the chimney is not built

simul-taneously ""ith the walls, then it is e s s crrtLa.L that this flue

en-closure be provided with a back wall. The existing or previously

erected wall should not be' used under any circumstances. Nor can

leaks be avoided if, during the erection of the chimney, an

attempt is made to link it up immediately at gaps left previously or at toothings already constructed.

7. The ＩｾｙＺＮＮＮｾｾｲｩｯｲ of Flue Walls Should Not be Exposed to Moisture and Ｕｨｯｵｾ､ be EXEoscd as Little as Possible to Great

Differeqqes in Tem2erature.

If a chimney is located outside a house, the chimney walls are usually half a brick thick and get thoroughly soaked through

during the rainy season. The sulphur dioxide (502) present in the

flue ga s has a detrimental effect on the mortar of the joints, the brickwork loses its bond, particularly on the side exposed to the

rain. Therefore, the brickwork must be repaired at regular

inter-vals. For the same reason a high chimney located outside a house

may become warped toward the slde exposed to the rain and may even

Collapse if no anchorage or support is provided. A chimney wall

Which is thus completely soaked through with rain cannot absorb any mOisture from the flue gases, while the heat-insulating effect

(11)

decreases sba rpLy where the external t empe r-a tu r-e exerts its

greqtest influence. The moist, ｧｲ･ｾｴｬｹ cooled flue gases can no

longer rise, and there is no chimney draught.

Another difficulty is presented by heqt stresses, which

occur if one or more walls are ｣ｯｮｳｩ､･ｲｾ｢ｬｹ warmer than the other.

This may occur even in cases where flues in use are adjacent to flues not in use.

It is thus expedient to place as much of a chimney as possible inside a bUilding and allow the top of the chimney to extend above

the roof just far enough to prevent downdraughts. In fact, in

order to overcome moisture trouble above the roof, holloW

construc-tion is necessary. If this type of construction consists of two

half-bricks with an air ｳｬｾ｣･Ｌ a considerably larger amount of

brickwork is required, which in many cases cannot be supported in

a simple manner. Especially for a chimney extending freely above

a loft to the ridge, such an additional load may have ｳ･ｲｬｯｵｾ

con-sequences. Therefore, the author suggests a much simpler hollow

construction, which was reported in the weekly paper Bouw. This

construction consists of an ordinary chimnoy,the walls of which

are half a brick thick and into which, through a small opening,

un-,glazed earthenware pipes are placed from approximately

25

cm.

inside the roof area up to the chimney pot. A plate of lead, ｰｬｾ｣･､

in the outer brickwork prevents the rain water absorbed by this brickwork from penetrating downwards and inNards.

Furthermore, proper attention must be paid to an absolutely watertight finish of the part of the chimney extending above the

roof. Stucco, which is regul'Olrly dam9.ged by frost, must therefore

be replaced at intervals of a few years, and hence should not be

used. What is needed here is a reinforced concrete slab of

suffi-cient thickness fabricated at a yard capable of producing a good

waterproof concrete (1 part cement per

Ii

parts sharp sand and

2t

parts gravel, or 1 part cement per

4

parts coarse sand).

As mentioned above, the exterior of flue walls should not be

e xpo ce d to temperatures that are too Low , Thus, not only is too

(12)

on the chimney draught, but, ovn.ng to the great differences in temperature, considerable stresses will occur in the cold external walls and above all in concrete walls and the floors of basements. These stresses in turn will c'J.use damage to the structure of a

building. For example many leaks in basements are due to this

di fference in tempe rature. Therefore, it is inadmissable to place

flues simply in external \'73.11s. Above the g r-oun d an unventilated

air space is required. In basements, heating flues must be kept

entirely away from concrete structures, all the more so if con-crete walls and floors come in contact with the ground water at

the other side. Thus, ventilated floors should be placed not only

under heating stoves but also under flues. Carelessness in this

respect may result in damage which is very difficult to repair. Especially for high chimneys for central heating, a linear

cont.r-acti on and e xpan s Lon of the flue wa Ll.s by a f'evr centimeters

as a result of differences in temperature cannot be ｩ｢ｾｯｲ･､Ｎ Such

flues must be 8urrounr l e d by an unventilated s pa ce , The chimney cap

must be of waterproof concrete and must be designed in such a way

that it can properly adapt iself to the ､･ｦｯｾｭ｡ｴｩｯｮ occurring in the

interior ｾＧｬ｡ｬｬ and not in the external lila 11. On the strength of

It/hat has been said above, the rear:ler '1'1111 corne to the c onc Lusi on

that these cases are still another ｲ･ｾｳｯｮ for making flues vertical.

8. The Ins ide Surfe.ce of Flu.e ltlalls Hus t be CaDable of

ｾｩｴｨｳｴ｡ｮ､ｩｮｧ Temperatures Up to 600°C. It Must be Smooth and Must be Capable of Absorbing and Giving off Moisture These points are directly related to the desie=,-ning of flue

8nclo3ures anj their material. Bricks and blocks of burnt clay,

such as Solidus blocks, provided they ar3 neither too hard nor too

so

r

t (IIhard gray" and. best of all "f·'3rrner sI gray" q1l..'3..11 ty), sa tl sfy

all these requirements. The mortar is a weak point here, since it

is in the Long run mor-e or less affected by the hlgh te,-nperat ur-e s

an-} harrnf'uL compon ent s of th<:: flue Gases, rJepend1ns on the ｦｕｾＳＱ u se d.

T:-18r-efor-8, th8 amount; of mortar US<::Q rnust be r-edu ced to a m1nimum.

(13)

while the joints are protected by the material of the blocks to

the greatest extent possible.

By bUilding the brickwork with the proper care and tcsting

it for leaks after completion as well as by using good material,

the penetration of soot with all its unpleasant consequences is

prevented.

In the case of chimney fires the temperatures in the flues

may be from

1100

to

1300o

C. According to fire-fiehting experts

these scarcely occurring temperatures need not be taken into account

in chimney construction for houses.

9. Heat Losses in the Flue Walls Should Not be Excessive.

In order to satisfy this requirement the material of which

the flue walls are built should not contain too much moisture,

other-wise the conduction would become too great and the heat losses would

increase too much.

ｂｲｩ｣ｾｎｯｲｫ

half a brick thick and cavity-wall

material of burnt clay with air spaces of equal thickness closed at

regular distances are the most suitable constructions in this respect

as well, since the temperature of the outside of the flue wall

re-mains reasonable and the flue gases are not cooled too rapidly.

Still more favourable in this respect is a flue wall, for cxample,

hard burnt brick and insulating concrete with the same wall

thick-ness.

However, the inner surface of the flue wall is not so smooth

when this material is used and the result will mean greater

resist-ance to flow than where burnt clay with air spaces is used.

The

use of thin-walled pipes, such as, for example, pipes of asbc3tos

cement or earthenware, requires insulation with brick or some other

material in view of the rapid cooling.

At the

ｳ｡ｾ･

time care must

be taken to ensure that any air spaces are interrupted at uniform

intervals.

10.

The Flue Outlet on the Roof Should Extend High

ｅｮｯｾｨ

Above Nearby

ｏ｢ｳｴｲｵ｣ｾｩｯｮｳ

So That the Draught Will Not

3e

Adversely Affected

(14)

pressure at the different sides of a building and obstructions in the vicinity of the chimney top are discussed in the second part of the present paper.

In this connection, a few general wor-d s may suffice here. It

is essential that the chimney top be at least one metre above a f'La.t roof and in tho case of sloping roofs i t must ha ve a propor-tionally higher outlet above the ridge.

Part II: Thermodynamic and. Aerodynamic Pr:-1.IJ..Qlp].cG.

by

A.

Adam

1. The Oneration of a Chimney

The operation of a chimney is illustrated by Fig. 1. 'I'he air

for combustion flows into the heating room at A and into the heating

unit at B. At C the combustion gases pass from the he8ting unit

through tho chimney to the outside by way of the chimney top. It

seems to be like a cycle, part of which is formed by the atmosphere outside the building.

In this cycle there are the following resistances: W

A

=

resistance encountered by the air on ｰ｡ｳｳｩｮｾ into the

heating room. W

B

=

resistance encountered by the air on ｰ｡ｳｳｩｮｾ into the

heating unit. This can be controlled by valves or flue

dampers. W

K = resistance encountered by the air and combuGtion gases

in the heating unit (furnace, stove or fireplace).

W

_c

=

resistance encountered by the combustion gases in the

smoke flue. This can be controlled by a valve or damper.

W

=

total of all the resistances encountered by the combustion

gases in the chimney (including those in a possible chim-ney pot or cap).

The atmosphere outside a bui t di.ns offers no resistance of

(15)

open atmosphere the wind may blow and result in an additional

resistance in the cycle or in the opposite of this.

For the time being, irrespective of the effect of the wind,

the following applies for the cycle outlined above:

available pressure

=

total resistance

(1 )

where the available pressure equals the difference in weight per

unit of basal plane of the column of outside air (of height h) and

column of combustion gases in the chimney (also of height h).

Since Ye denotes the specific gravity of the atmosphere and

Yr the mean over the height of the specific gravity of the combustion

gases in the chimney, then

available pressure

=

hYe - hyr

=

h( Y

_e

- Yr) .

On account of the above,

(2)

(3)

The draught measured behind the heating unit is the difference

between the pressure in the heating room and that in the smoke flue

behind the valve or damper, thus

(4 )

or, on account of

(3),

(5)

The available pressure h (Ye - Yr) is denoted here as the

"static draught", i.e., the pressure which is measured with a valve

or damper closed so that the flow is completely arrested so that the

resistances vanish.

At the same time it is assumed that the

(16)

remain unchanged, which, of course, will only be the case for

very short Dcriods of time. Indeed, on the ｳｴｲ･ｮｾｴｨ of this the

static ､ｲｲｬｕ［ｾｨｴ and the resistances may be estimated, and by quickly

closing the air-supply valve or slide the heating unit will be

reasonably free from air leaks. This method may be useful when

lookinG for the cause of draught trouble. The resistance W

A encountered by the air on passins into the

heating room should entail a negligible reduction in ､ｲ｡ｵｾｨｴＮ In

tbe heating of living rooms, the joinings of windows and doors

generally suffice to admit the small amount of air required for the

combustion. However, in large boiler rooms the air supply is often

limited. If such a heating room also contains an ascending

ventila-ting flue the situation becomes worse, since this flue absorbs an appreciable amount of air, Which, of course, must be supplied

some-where. It is recommended that the cross-sections of air-supply ducts

be 2.t le8.st equal to the sum of the cross-sectionsJf the chimney and ventilating flue.

Assuming that the air-supply ducts are large enough and that

ｾＮ thus is negligible, then

(5)

becomes

ti.

whe r-c

draught :::;: static draught - W

W :::;: total resistance in the chimney.

(6)

In order to obtain a better knOWledge of the behaviour of a chimney, the static drau3ht and Ware investigated first.

For a given chimney and given atmospheric conditions the

static ､ｲ｡ｵｾｨｴ depends exclusively on ｾｲＧ the specific 0ravity of

the combustion gases in the chimney, which in turn depends chiefly

on their ｴ･ｭｰ･ｲｾｴｵｲ･Ｎ As 0.. result of the heat losses through the

chimney vraL'l s , the temperature of the combustion gases gradually

de-creases in the direction of the flow. The lower the quantity of

(17)

draught. This has been depicted in Fig. 2, based. on D. ,:1 ven

temper3.ture in the smoke flue. In Fig. 2, v

n denotes the

velocity of the combustion gases reduced to

aOc.

and 760 mm. Hg.

As the quantity of combustion gases, and hence v , Lno r-ea s e , the

n

static pressure increases at a gradually decreasing rate.

The resist3.nce in the chimney is approxim3.tely proportional

to the square of v

_n

, whence the curve for v

_n

is obtained (of. FiP.'. 2).

ｾ

According to (6) the draught is the difference be twe en s t.a tic

draught ana. W, furnishing a curve '.t1ith a fairly flat maximum, whi ch

for brick-built vertical ch Lmney s normally lies bet.ween v

=

a.

5

n

and 1.0 m./sec. or slightly more.

This shows that very low velocities, i.e., chimneys which

are too \olide, are unfavourable because the static draught ts

in-adequate, and very high velocities, i.e., excessively narrow

chimneys, are also unfavourable because the static draught is too

great. This must be taken into account in attempts to Impr-ove a

chimney which does not draw properly, so that proper measures can be taken.

h _{detailed report on favourable passage in chimneys of}

dwellings has recently been published by Van de Beek in Bouw(l).

In Fig. 2 the draught curves for low velocity extend to the

positive-pressure region. This condition results if the combustion

gases are definetely heavier than air (at the same temperature and

pressure). At very low loads and in the absence of air currents

the wind may actually be transformed into pressure with the result that the combustion gases produced penetrate into the heating room

and the building beyond. Although this is mor-e Ll keLy to occur

vrhen chimneys are too wide, it may also take place in chf.mneys

having proper passage. The beqaviour of the heating unit would

then have to be Lnve s t.Lga t.e d more closely(2 ,3). If the combustion

gases contain carbon monoxide, the phenomenon described here woul.d

constitute a life hazard. Therefore, at very low loads it is

sug-gested that the fire be kept low. The chance of car-bon monoxide

(18)

2. The Effect of the Wind

The effect of the wind makes itself felt at the chimney pot

and at the point where the air is nupplied to the heating room. The

3hape of nearby structures or other obstructions, such as high trees, may cause updraughts or downdraughts at chimney tops.

Downdraughts produce an additional resistance to the escape

from an open chimney top. The steeper the angle of incidence of the

wind the greater this resistance will be. Under unfavourable

condi-tions there may be a back-flow into the chimney. Horizontal winds

and updraughts have a draught-promoting effect, which increases as

the velocity in the chimney decreases. This greatly facilitates the

lighting of the fire, since a flow in the right direction is already

present.

The unfavourable effect of downdraughts may be reversed by

suitably designed chimney caps, which do not interfere with the

chim-ney draught in the case of downdraughts. The behaviour in the case

of updraughts must therefore also be known. A report on a detailed

investigation into the operation of chimney caps was published in

1936

by the Rijksinstituut voor Brandstoffen-Economie en de Gasstichting(4).

A serious obstruction of the draught is provided by the

forma-tion of a positive-pressure region, of which a repeatedly occurring

case has been drawn in Fig.

3

(pressure increases in the direction

convex to the flow path). A chimney cap seldom brings results here,

since the velocity at the chimney top is in most cases too low. A

solution to the problem is to increase the height of the _chimney to a point above the region of positive pressure or to use an electric

fan for the chimney top.

Negative pressure at the point where the air is supplied to the

ｨ･ｾｴｩｮｧ room has the effect of an additional resistance in the cycle

mentioned above and may also bring about a back-flow. Under

unfavour-able conditions this negative pressure may approach the dynamic

pressure of the wind. If the chimney top, whether provided with a

cap or not, i8 exposed, the effect of the wind may entirely or

(19)

room on the draught.

It should be noted here that the

draught-increasing action of the best chimney caps, wh8re the transport

through the chimney is zero, barely exceeds the dynamic pressure

of the wind and continues to decrease during passage through the

｣ｨｩｾｮ･ｹＮ

Thus it is not expected that the favourable effect of

the Wind on the chimney top will cancel the effect on the heating

room in all cases.

The vertical velocity gradient of the incident

wind also has an effect on this.

The more the wind velocity at the

chimney top exceeds that at a lower level (i.e., where the heating

room is located), the more effective the chimney top may become.

Where the problem will not be solved by a chimney cap an

electric or turbine-driven

｣ｨｩｭｮ･ｹｾｴｯｰ

fan may be the answer.

In

some cases there is a possibility of having the air supply to the

heating room enter at a more favourable location.

An example taken

from practice is shoTNn in Fig.

4. It presents a row of detached

houses on a Wide avenue, behind which a large open field is located.

The wind from the indicated direction causes a negative pressure due.

to the constriction of the flow between the houses.

A low wall right

in front of

ｾｨ･

intake supplying the air to the heating room in the

basement increases this effect.

The result was a back-flow of

com-bustiongases which spread through the house.

No fault could be found

with the location of the chimney top.

Transfer of the air intake

to A was the solution here.

Although it sometimes is difficult to judge local situations

With respect to such effects, there are nevertheless many cases like

the present one where this is possible Without any difficulty.

3. Positive Pressure in a Chimney

In the first report attention was drawn to the importance of a

properly built chimney haVing no air leaks.

This is necessary not

only because of the draught but also in order to prevent the

com-bustion gases, which may contain dangerous carbon monoxide, from

penetratinG the house.

In fact, it is possible that a positive pressure will form

locally with respect to an adjacent room, for example, if negative

(20)

pressure ls caused ln thls room due to the effect of the wlnd.

Furthermore, the vertlcal pressure gradlent lnslde and outslde

the chlmney (cf. Flg.

5)

must be taken lnto account.

_{In F1G.}

5,

a

ls the curve for the statlc draught and b that for the

ｲ･ｳｬｳｴｾｮ｣･Ｎ

In Flg.

5A,

whlch shows an open chlmney top, there ls ne6ative

pressure throughout the chlmney.

In Flg.

5B

a posltive-pressure

region can be seen above the helght h

1•

In a vertical flue of

uni-form cross-sectlon such a

ｳｩｾｵ｡ｴｬｯｮ

can arise only ln exceptionally

slender chlmneys.

In the case shown ln Flg.

5C

the chimney has an

additlonal reslstance (poorly deslgned bend, horizontal

ｰｾｲｴＬ

con-strlctlon, wlnd pressure) at a hlgh level.

In thls case the top

part of the chlmney ls under posltlve pressure With respect to lts

surroundlngs.

For reasons of safety it ls thus also necessary to pay

atten-tlon to the prevenatten-tlon of leaks, to low reslstance (primarily at a

hlgh level) and to properly lnsulated chlmney walls (favourable

shape of the statlc-draught curve a).

(21)

References

1. Van de Beek, E.H. Schoorstenen voor l<amerverwarming

(Chimneys for room heating). Bouw, 10 (51):

1042-1045, 1955.

2. Van de Beek, E.H. Trekstoringen bij lage belastinc; (Draught

trouble at low loads). Ve rwa rtm.ng en Vonti1atie, 10

(2): 25-29, 1953.

3. Adam,

A.

Het gassen van ketels bij lage belasting (Gases

escaping from boilers at low load). Verwarming en

Ventilatie, 10 (11): 241-243, 1953.

4. Report no. 9 by the Gasstichting, Beschouwingen naar

aanleiding van een onderzoek naar het eedrag van schoorsteenkappen bij wind en regen, ingesteld door het Rijksinstituut voor 3randstoffen-Economie en de Gasstichting. (Detailed study of the results of an

investigation into the behaviour of chimney caps in wind and ra1n set up by the Rijksinstituut voor Brandstoffen-Economie en de Gasstichting), February 1936.

(22)

Part III:

Discussion

A.J. Dorrenboom, Chief Engineer of the Administration of

ｗｯｲｫｩｮｾ

Class Housing, Rotterdam, (guest speaker), makes the

following additional statement to what has been said in the reports.

The building of properly working chimneys in dwellings is a

difficult problem, with very many aspects.

However, it is much more

difficult to find and remedy the cause of complaints in cases where

chimneys do not

ｔｾＡｏ

rk properly.

The correct operation of a chimney in a dwelling actually

depends on two factors, i.e., the chimney unit (including smoke

in-take and outlet), its construction and arrangement, and the wind

(particularly, the differences in pressure at the intake and outlet

brought about by the wind).

The building of a proper flue may be controlled, so to speak;

this problem can be solved, but not that constituted by the effects

of the wind.

Heterogeneous factors and uncertain conditions are involved

here.

The effect on the pressure differences, for example, is neither

completely and accurately known nor can it be predetermined or

com-pletely controlled.

I am thinking here of wind direction and wind pressure, the

shape of the building, the situation of nearby buildinBs, the location

of the heating unit,

th

e arrangement of the

ｾＧｊｩｲ［､ｭＧｬ

units arid the Nay

in

ｾｨｩ｣ｨ

the windows and doors are used in the dwelling, how they are

han c l e d ,

As can be seen in Fig.

6,

both the heatinG unit 'and the outlet

of the flue (ilK) may be in a positive-pressure ree:;ion (if the movable

pa

r-t

s of the windo'tl at the weather side provide good commun.ica

t i

on

betwsen the atmosphere and the air inside the house), but it is

still open to question whether the pressure above the outlet is

actu-ally Lowe

r'

than that in front of the heat Lng unit.

If all the windows on the windward side (including the joints)

arc l!lil"c1proof,

vih i

Lc the hee.t Lng unit commun

i

ca

t

c s "'ith the

(23)

the pressure in front of the heating unit will be low and there will

be a downdraught in the chimney.

If the wind shifts, the condition

will be as shown in Fig.

7 and there is chimney draught.

These cases frequently occur in practice.

It is also possible for an adjacent ventilating flue (VK) to

affect the window draught unfavourably.

With respect to the ten requirements for a good flue laid down

by Mr. Grolle the following may be added.

Good tightness is indeed necessary.

This applies particularly

to the connection of the thimble which is not always ideal in practice.

I should like to add three additional requirements:

(1)

The material must be incombustible.

(2)

The material must be fireproof.

(3)

The material must be resistant to the effects of flue gases.

The last two requirements are important now that new ch Lmney-we.Ll,

materials are beine or have been introduced into the building trade

in place of brick, i.e., sand with lime or cement as the binding

material, and various other materials for admixing such as

Klinkeri-soliet, Hollith, pumice.

This has created a number of new problems.

In this connection it may be noted that:

quartz undergoes a change at

600°C.

vn

t

h an

increase in volume,

carbon dioxide is liberated from limestone at

+ 900°C.,

lime and cement (containing lime) are affected by the

acids of flue gases.

There also is the question of whether c':''''1ent is entirely

in-sensitive to high temperatures.

High

t

empe r-atu r-e may affect the compressive strength

unfavour-ably.

British Standard

476 (1942) requires thqt the compressive strength

must not have decreased by more than

.50

percent after heating to

600°c.

(24)

and shape of a flue must be such that a spherical object of at least

12 ｣ｾＮ diameter can be moved with ease up and down the entire flue,

but also that the cross-section of a flue must be at leant 0.025 sq.m. This is supplementary to what Mr. Grolle mentioned on this

subject.

Finally in order to build an efficient chimney, a good and

care-fully studied plan and good workmanship are required. Both may be

effectively supported by regulations, but then only good and modern ones.

A.M. Asselbergs, N.V. Technisch Bureau voor Economisch

Kolen-verbruik v/h Asselbergs

&

Nachenius, Breda, (guest speaker), makes the

following statmment.

Architects and contractors are not convinced of the importance of a good chimney and are not ready to sacrifice money or

architec-tural considerations for this purpose. It is to be hoped that the

detailed publication of the lecture given by Mr. Grolle contributes to progress in this direction

I consider the requirements laid down by Mr. Grolle for the building of a chimney as excellent.

Mr. Grolle gave a flue-wall thickness of 11 em. as normal for

a chimney. I find this dangerously thin in view of the possibility

of leaky joints. There are 120 to 150 courses and each course contains

a number of vertical joints. In onp.-brick walls the chance of leaks

is at least reduced.

Mr. Grolle mentioned a municipal r-egu l a.tLon requiring a 17 em.

diameter for a flue. For stoves I consider this much too wide, 14 em.

is ample. I consider this important because a narrow flue makes it

impossible to obtain the maximum output from the stoves, whereas too wide a flue constitutes a hazard-since one may easily get involved with the unsteady region where the gases cool too much with the possibility

of the draught ｣ｨｾｮｧｩｮｧ to pressure, and since too Iowan exh3ust

velocity of the gases from the chimney top produces an unsteady flow,

(25)

where the warm gases flow out.

This would immediately end the

efficient operation of the chimney.

Insofar as the new materials are concerned, we are

occasion-ally using, though with some hesitation, reinforced eternite (Ferrocal).

However, this is done primarily to avoid leaky joints.

What does

Mr. Grolle think of pipes freely suspended with a cavity around them?

New materials are also used for stone floors.

These floors are

surprisingly airtight.

If, in addition, the door and windows close

properly, the air supply to the stove is insufficient and the flue

gas penetrates the room to some extent.

Mr. Grolle suggested the

II

smoke test

II

\'lhile a chimney is being built.

Thi s is the best method

and also is most suitable in existing buildings.

If architects saw the results in almost all the chimneys tested,

they would not believe their eyes.

Wet straw fired at the bottom of

the chimney and the top suddenly closed completely!

According to the speaker, poor draught is more readily

en-countered in the case of heating boilers than for stoves, since for

the former the flue-gas temperature, and hence the static draught, is

lower and for the boiler more draught is required than for a stove.

ｾＧｬＱｴｨ

boilers where the gas outlet is low, more trouble is

en-countered than when the gas outlet is at a higher point.

In the

latter case the flue-gas temperature is slightly higher.

Boilers heated with nut coal are more inconvenient than those

heated With coke, since more draught is required in the first case.

Therefore, for small heating systems with one boiler I prefer

boilers with the smoke outlet above.

The boiler should not be too

large in order to prevent the gases from cooling too rapidly.

The

heating surfaces of radiators should not be too large either, since

otherwise the water temperatures would be too low and the gases would

cool excessively.

Tho speaker confirms Mr. Adam's experience

ｲ･ｧｾｲ､ｩｮｧ

the

un-steady character of the draught and the change of draught to pressure

when the temperature of the flue gases is too

10\'1,

and this in

con-junction with the high specific gravity of CO

_{2 •}

I find this an

(26)

occurring during the night are a result of this phenomenon. The furnace is damped, but the chimney is still warm and thus

contri-butes to maintaining a posi ti ve draught. Later when stove and

chimney have cooled sufficiently the draught changes to pressure

and within a short time the room is filled with flue gas. Since

no more oxygen is supplied to the fire, the flue gas then contains

a great amount of CO2 •

l.-lith respect to the positive-pressure region about the chi.mney top and the negative-pressure region at the air inlet of the heating room the speaker raises the question of whether the conversion of kinetic energy into pressure is so complete that, for example, at

80 km./hr. counter pressures of the order of

5

or

6

mm. are obtained.

In most cases a good chimney may well supply such a draught. If the

effect of the wind is lower than these values there is no danger. When it works, a ventilating flue in a basement heating room

moves a lot of air in the absence of considerable resistances.

There-fore, the opening of the air intake must be large. Then there is too

much draught to sui t the person stoking the furnace who \'lill eLose

the intake opening, regardless of how often this has been prohibited. The resulting condition is much more dangerous than in the complete

absence of a ventilating flue. If the latter must be effective in

order to suck up the flue gases entering the heating room, there is every reason to assume that this flue is even less successful in

this t han the chimney, Which is much warmer.

I am in favour of using a ventilating flue but I am also

opposed to it in certain respects, and i t is our duty to make it en-tirely superfluous.

The chimney tables provide0. by the manufacturers of heat ing

boilers usually give dimensions which are too large. As I said before,

too vnde a chimney is defini tely dangerous. The chimney t ob l e s

assembled by Mr. Adam should be used instead.

Reply by Mr. Gralle

The following may be said concernLig flues of ｲ･ｩｮｦｯｲｾ｣､ eternite.

(27)

ｴＸｭｰ･ｲｾｴｵｲ･ and harmful flue gases, it is necessary to ｾｾｫ･ these pipes double-walled and to provide the air spaces with the required

hori-7.ontal partitions. In this way the insulating properties of as much

dead air as possible are utilized.

With respect to the wall-thickness of brick flues I should like to point out that in the case of poorly built masonary the possibility of air leakn in one-brick walls is only slightly less than in walls

half a brick thick. After all, Mr. Asselbergs is thinking ｰｲｩｾｾｲｩｬｹ

in terms of flues for central heating and that is why I, too, find a

wall thickness of 22 cm. prefer'3.ble in view of the heat inSUlation.

ｒ･ｯｬｾ _by Mr. J\.dam

With respect to the expected positive or negative pressures due

to the Nind i t should be noted that a ｾｊｩｮ､ velocl ty of, for example,

72 km./hr., or 20 m./sec., corresponds to a dynamic pressure of 25 mm.

water column. Even in the case of very incomplete conversion of

kine-tic energy into pressure, the effect of the wind may thus exceed the

thermal draught. Even if this is not the case, difficulties ｭｾｹ arise,

since it appears from Fig.

7

that the thermal draught to the left of

the maximum dect"33.ses as the transport decreases, whereby the effect of the wind may make itself felt in an unfavourable way.

The chimney tables referred to are obtainable at Alg. Vereni3ing

voor de Centrale-Verwarmings-Industrie, Surinamestraat

24,

The Hague.

J.

Leyh. Consultant Engineer. The Hague

What are the fundamental conditions (dimensions, etc.) for flues for open fireplaces, providing a satisfactory outlet of the flue gases?

ｒｰｾｬｹ by Mr. Adam

In addition to directions for the finishing, the dimensions are also ｾｩｶ･ｮ in Building Research Station Digest No.

18

(Octcb8r

1955).

With resoect to the effect of the Wind the considerations already

(28)

A. Lcouwenbur-gh , Archltect, B.N.A. Division for ｾ･｣ｨｮｬ｣ＸＮｬ

Rf')se::1.l'ch of the Depa r trnen t of Reconstruct1oX1:md ,,.;ork1nG

ｃ｜Ｚｾ＿ＲｾｩＺｌｑＱＲｾＱｘＡｧＮｊＮ｟ｔＮｨｾ

Ji§.r:ue

.

Ｎｾｾ｟Ｎ

__ｾ

__.._.

Ｎ｟Ｎ｟Ｎｾ｟

How 1s it possible that there can still be a difference

in oper-ati on of the chimney outlet when the height ab ov e the

r')of and the cross-section and cesign of the f'Lue a r'e the same

in LderrtLca L dwellings? For example , in some f'our- ｲＩＲＮｬｾｓＺＮｬｬｇＱＮ rC)1IJs

of hous e e it was found. that at s pe o l f io d.irect:\.ons of the 1·rj ＧｾｪＨＱ

t he ch:l.l1neys in the two outer r-ows \tJor':-;:ed sat Ls t'ac t.ort ly,

vJrlere-as back flow wvJrlere-as observed in the intermediate rows.

ｬｩｾｰｬｬ｟ｑ［ｌｊＱｸＮＺ ....

A:

ｾ

In the ca se of four parall.el rows of h0115e8 :it":.:.nl'Jot be

expected that the flow pattern all around the r-ows \,11 1.l be

ldentic8.l. The entire block of houses, Lnc Luoi ng eVl3rythine;

'n

ｴｨｾｾ t.mmed La te vicinit.y , will determine t he f'Low pattern here.

This pattern can only be ascertained e xper-LrnerrtaTly by m,o>:mn of

model tests. No doubt, it should then be possible to explain

the dlfferencesbetween the individual rows •

.J !__ＧｌＲｲｌ､ｾＺｑ

E111£.1.

ｄＱＮｶｾｾｩｯｮ of ｓｩｬｬｬＱｴ｡ＡｴＹｾ｟

...

ｾ .O ...ＱＮ｟ＮＡ｢ｾ｟ｴｌｾｾＱＲＧ＿

Is it advisable to equalt ze pressures and reslstm1cc:3?

If the heating untt is lightly charged, the effect on the

dr9.u,zht of a given chimney is unf'av our-ab Le because of ＺＺＺＺ［ｬＢｾＸＮｴ･ｲ

he'3.t 108se::>. But since the velocity is 10'lJer fop a Iljid.:) chi umey ,

the he3.t losses a.re smaller as Nell. Shoulr1 not ｴＱＭｪＨｾ .J.r·ctl).::::;ht; be

HON can leaks ever have a favourable effect even l:n tir: C:3'.;e

of 11Sht charges? Does this have a bearing on the C3.se where the

CO₂ cont.errt is hiGh so that the y values rJ.ecreas8 ｢･｣ﾷＺＺｴｬｬＺＭ［［Ｂｾ of the air leal<lnc; through'?

ｂＮｰｾｬｙＮＮ｟ｑｙＮｲｉＡＺｾ Adam.

By

resistance is meant here the conversion of mcchsnical

alLergy into heat per unit of volume of tho ｦｬｯｾＧｊｩｮｇ rnedLurn ,

::,::i:3u:n·-inc; that the charige s in vo Lume as a r-e su I t o r t.h e ｣ｬｹｵｾＺＮＺＺ［｣ｾＺＺ［ trll.:;O

pr-e s sur-o may be neglected. The r-es l Gtanc e therl ［ＢＢｾＨＧＮｏ［ｬ［［ ,; Ｈ［ＨｾｕＧｬｊＮ t C'

t.he total pCCGSIU'e c1:tf'f'e r-cnc.e on both sides 0

r

the fl Ol/J ｳＺＳＨＧＮｾＮＺＮｩｯｮ

(29)

If the charge is light, the quantity of flue gases Y3.ssing

through the chimney is small. Therefore, when a specific amount

of heat is given off the drop in temperature 1s gr8at. It is true,

when the flue-gas velocity is low the coefficient of heat

trans-miss Lon is 'lmoJer due to convection on the chimney wall. However,

the effect of this on the heat transmission through the wall is

slight. Therefore, the effect of the flue gases passing through

the chimney, is predominant.

Indeed, for light charges the favourable effect ef 8 leak is

chiefly due to the decrease in the specific gravity of the flue gases in the cases where this is initially high as the result of a

high CO2 content and a low H20 content.

L.E.

Wisse, Chlef Engineer in charge of the Public Works Department,

Rotterdam .

Pipes of unreinforced asbestos cement are less suited for

chimneys, since they burst OWing to the heat. Parging has the

dis-advantage that the parge crumbles and disappears in the course of time.

The close proximity of high and low dwellings as frequently found in modern town planning creates insoluble problems for the low building as far as the operation of the chimney in conjunction

with the effects of the wind is concerned. It should actually be

compUlsory for the low bUildings to be heated centrally from a boiler room in the high block of houses.

ReDly by Mr. Grolle

Mr. Wisse should joint the flue enclosure on the inside rather

than parge or plaster it. With this I would be in agreement.

How-ever, in practice the cross-section of the flues is too small for this in most cases.

ReDly by Mr. Adam

It has been recommended that the flue gas outlet problem of hiGh and low houses in close proximity be considered.

(30)

It may

be

expected that in various cases the solution suggested by Mr. Wisse 1s the only possible one.

Mr. Roelofsen,C.V. Fort3., ltJowJenlJerg

It is desirable to have the outlet of the actual flue at least

five centimetres above the chimney cap of the chimney ｳｴｾｊ｣ｴｵｲ･Ｎ

The finishing of the chimney top is indeed important,

particu-larly if there are several flues in one chimney. HmJ8Ver, teo

little is known about this.

Dr. F. HartQ.:;ensis, Division of Sanitation.J.--1'...N.O., The Haf,,:ue

One of the functior.s of a chimney is to conduct the flue gases

to higher strata of the a I r , Mr. Grolle st at e d that t.he outlet must

be at least one, metre above a flat roof. Should not t.he chimneys

actually be considerably higher in many ｃｓｴｓｃｾ［ ifI view of the

varia-tion in heip;ht of different b Loc k s of houses?

Reply by Mr. Adam

This is the same oroblem here as that mentioned by Mr. Wisse.

ｒ｡ｩｳｩｲｬｅＮｾ the chimney to a hei ght where no d lf'fLcuLt Len viou ld be

(31)

4

ｾ

1- . _ ._ ._ -

O.k

'nl

I ' I ' S _ J " ' " II " , to ｶＮｮｴｕｾＢＮ｟ I-f"",. k_at I I I I II I;