Buildings for paper mills. Part II: Some fundamental considerations

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Publisher’s version / Version de l'éditeur:

Pulp and Paper Magazine of Canada, 63, 9, pp. T449-T454, 1962-10-01

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Buildings for paper mills. Part II: Some fundamental considerations

Latta, J. K.

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Buildings

for

Paper

Mills

Part

II

:

Some Fundamental Considerations

PU1,P AND P A P E R MAGAZINE

O F

CANADA

Tf'ol. 63, A70.

9 (Se]1te7nOe?'

1962)

1). T-449-T-454

RESEARCH I'APER

NO.

172 O F TEIE

D I V I S I O N

014'

B U I L D I S G I213SICARCH

Ottawa October 1962

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A building is constructed so that some operation can be carried out under ideal, or near ideal, conditions. It i s , therefore, a basic function o f the building t o separate the controlled internal conditions from the uncontrolled ex- ternal climate. W a t e r is the most important single fac- t o r that can cause the building t o deteriorate and fail in this function. An account i s given o f some o f the means whereby water may move through and accumulate in building materials and assemblies. T h e great difficulties o f accurately assessing these movements are described, and methods are given for calculating, under some condi-

I

tions, temperature gradients and moisture migration.

!

H E BASIC REASON f o r constr~icting a building is t o rovide shelter f o r a reasonable length of time reasonable cost. The s t a r t i n g point f o r any design is, therefore, to decide what is t o be and to what extent. Once this has been es- t h e building envelope can be designed so a s t h e forces acting upon i t and t h u s ensure a In most cases, including paper mills, t h e primary objective is to protect a controlled internal climate against t h e effects of t h e external weather. The conditions inside the building can be established in two ways, either by accident according to the oc- cupancy, or by design whereby t h e most satisfactory conditions in which to carry out t h e various processes a r e determined and t h e environment controlled to give these conditions. The latter method is t h e more satisfactory one, and so i t becomes necessary t o decide upon t h e optimum temperature and humidity conditions within which to carry out t h e process and the permissible variations f r o m these optimum conditions.

While t h e internal conditions can be controlled, t h e external ones will be set by the climatic conditions a t t h e location of the mill. It is possible to avoid t h e effects of a harsh climate by constructing the plant un- derground. As f a r a s is known, this has not been done with a paper mill and the usual procedure is t o design the building envelope to act as a separator of two different climates, one of which i t is desired to control ar.d the other which cannot be controlled. I11 acting a s a separator, the building will be subjectecl to differences in conditions from inside to outside ~vhich may become important in determining t h e durability of any particular constr~~ctioii. The a r t . & the designer is in

Contribution by the Division o f Building Research, N a -

tional Research Council, Canada,.and published w i t h the

approval o f the director o f the division.

providing this separation with the most economical combination of building envelope and mechanical equipment which he can devise. To do this, he will have to make many choices and juggle the many vari- ables to reduce the total annual costs to a minimum while meeting the requirements of t h e building owner. The annual cost of the building is made up of f o u r components. The capital outlay 'epresented by the building a n d its equipment is usually amortized over a number of years and so can be expressed as a n annual cost. To this must be added t h e operating costs f o r such services as heating and ventilating. I t is usually possible to calculate these two items with a reasonable degree of accuracy during t h e design of the building. T h e remaining two a r e not so easily calculated, since they consist of t h e maintenance charges and the value of t h e lost production during t h e time when the building is being repaired or when i t i s being de- molished and a new one erected. Of these four items the f i r s t two a r e well known and require no further comment. T h e last one is bound up with the use to which the building is put, and the marketing conditions f o r t h e product, which place i t outside the scope of this paper. The cost of maintenance, on the other hand, is closely related to t h e rate of deterioration and the ease w i t h which repairs can be made.

W a t e r has been found t o be the most significant single factor in the deterioration of buildings. The various harmful effects which moisture, in conjunction with other factors, can have on building materials is the subject of a subsequent paper, and will not be pur- sued f u r t h e r here. Instead i t is intended t o discuss some of t h e factors governing the movement of mois- t u r e through building materials and assemblies.

I n fulfilling its function as a separator, the building must prevent the entry of wind and r a i n and contain, without damage to itself, the moisture which is seeking i t s way through t h e construction. These factors a r e over and above t h e structural requirements which a r e well known and can be resolved without difficulty. T h e movement of moisture through the enclo- sure is largely determined by the temperatures, hrl- miclities and pressures existing inside and outside the building; t h e control of this movement is a prime consideration in the design of paper mill buildings. Thus the characteristics of t h e internal and external atmospheres and t h e influence of temperature, humidity and pressure differences on t h e movement of moisture will be considered.

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ith mean values of about 65 deg. F. in July and 10 in J a n u a r y f o r Central and E a s t e r n Canada, .esponding mean relative humidities being 50 and 80 per cent. F o r design purposes, how- is necessary to take temperatures lower than ean J a n u a r y value; f o r many purposes t h e 10 per t e r design temperature will be satisfactory. The atmospheric pressure will vary slightly from clay to day, but f o r practical purposes can be consid- ed to be constant. The agitation of t h e air, however, f a r from constant, ranging from dead calm to a full le, with a mean value over most of the inhabited Quebec and Ontario of about 10 m.p.h. Pres- sures exerted on a building due to wind will vary w i t h wind speed, height above ground, and the shape and orientation of the building. These may be of signifi- cance i n producing structural loads on the building and its parts, in influencing the pressure differences across walls, windows, doors and roofs which will determine t h e d ~ r e c t i o n and extent of a i r leakage through them, and i n influericing t h e extent and na- t u r e of the wetting action by rain. The distribution of pressures over a building may be found approxi- tely with t h e aid of coefficients such as those now de available in Supplement No. 3 [2] to t h e 1960 tion of t h e National Euilding Code of Canada. ese coefficients, although intended primarily f o r culating structural loads, may be used also to esti- mate other wind effects. Values f o r many climatolog- cal phenomena can be obtained from the Climatolog- ?1 Atlas of Canada [3

1

and a discussion of their use relation to building is given in Canadiail Building igest 1 4 14) and also In Supplement No. 1 151 to the ational Ijuildi~lg Code 1960.

Eecause t h e differences i n temperature, humidit51 and pressure t h a t exist across the external fabric of

e building contribute to the movement of moisture, ner in which this occurs will be discussed. This

some consideration of psychrometry.

1 1

PSYCHROMETRY

I

!

Psychrometry is t h e study of the physical propertie?

$ of a i r and water vapour mixtures; a n introduction to

this subject is given in Canadian Building Digest No.

1

1 r6]. Many engineers and architects will already be familiar w i t h t h e subject, but f o r the benefit of others who a r e not, several of t h e principal points will be re-

-I viewed here.

4

kt 1. The air and the water vapour associated with it behave

$ as two 'ndenendent gaces with a few important excep-

4

!ions: ( a ) Moving air will carry the water vapour with

4 ~ t . and ( b ) Heat can be transferred from the air to the 4 water v a p ~ u r and vice versa. This means that in n~osc

cases met in building problems they both have the - -

sanle temperature.

2. Since the two conlponents act independently, the va- ppur will exert a pressure which is independent of thr air pressure.

3. Vapour pre-sure is independent of the tempzrature for a given air-vapour mixture, but is proportional to thz weight of water associated with unit weight of air, i.e., to the humidity ratio.

-1. At any given temperature, there is a limit to the quantity of water which can be held by the air. Thus, it follows that as a given air-vapour mixture is cooled, a temperature will be reached a t which the air is saturated and if cooling is continued below this temperature, called the dewpoint temperature, water will condense out of the mixture.

5. At any given temperature, the weight of water vapour present in the air expressed as a percentage of the weight of water vapour required to saturate the air at that te~nperature is called the relative humidity. Since the vapour pressures are proportional to the quantity of water vapour present, the relative humidity is usually expressed as the rat;o of the existing vapour pressure to the vapour pressure of a saturated mixture a t the same temperature.

The easiest manner i n which to follo which take place in these interrelated

by means of the psychrometric chart, a s is outlined i Canadian Euilding Digest No. 1 r6]. A c c o ~ ~ n t s of application of these principles to the problems of c densation on inside window surfaces, condensation tween panes of clouble windows, and vapour barrie in honse construction, a r e given in Digests No. 4, and 9 17, 8, 91. These digests do not deal specificall with the conditions and t h e type of construction no mally found in paper mills, but the pri

have to be applied a r e the same.

MOVEMENT OF MOISTURE

The factors t h a t govern t h e movement of moisture in building materials will now be considered. T h i s i complicated subject and does not lend itself to a n e solution, but i t is desirable, nevertheless, t h a t t h e signer have some understanding of the general prin ples which govern this movement in orde

make a suitable assessment of w h a t mig in t h e proposed construction.

T o do this, he n i l s t t r y to visualize the v gradients through the materials which will cause t u r e to m i g r a t e i n one form or another. T h e foll points m u s t be kept in m i n d :

1. Vapour flows a s a result of a vapour pressure pressure side to the lower.

2. Liquid w a t e r moves in a homogeneous material as a result of a moisture content difference, t h e flox being from t h e area of higher m o i s t ~ ~ r e conte t h e area of lower moisture content, g r a v i t a t and other forces being neglected.

3. The moisture content of a material i n a i r is con trolled principally by t h e relative humidity of the a i r in contact with it, moisture being given off or reached. T h e moisture content rises a s t

humidity increases.

enced by t h e temperature, usually decreasing with increasing temperature. T h i s effect is small a t low relative humidities but is of more importance at high humidities.

Consider now t h e situation when a material is sep- a r a t i n g two atmospheres, which have the same tem perature b u t different relative humidities. Because of the difference in vapour pressures in t h i s case vapour will flow f r o m the side of the h i g h relative humidity to t h a t of the low relative humidity. A t the e r on the side of high relative humidity t h a n i t h e side of low, and consequently t h e r e will be t

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ity on t h a t side will rise and i n c

right, it is possible t h a t t h e moisture content gradient will be reversed and the liquid moisture flow will then be in opposition to the vapour flow. I t is this interac- between the flow of vapour and the flow of liquid

1- which makes t h e calculation of t h e quantity of

r passing through building materials very diffi- these examples i t has been assumed t h a t the va- ow passes right through t h e material, but in stances in practice the vapour is cooled t o be- ewpoint before i t reaches the cold side and within t h e material. I n such cases, t h e mois- n t a t t h e point of condensation is raised and moisture flow could be in both directions

.

If t h e cold surface of t h e material is n impermeable membrane, such as a It-up roof, t h e moistlure collteilt will build up until

r n flow to the warm face is possible.

a r it has been assumed t h a t no freezing takes ce if it does, t h e whole situation is upset, a s ation of t h e liquid water will be stopped. A n of this problem and the many difficulties in-

ubject has been published rll]. at, then, a r e the situations in which a t least ap-

calculations can be made? These situations d to those in which the liquid migration is ted o r reduced to a small factor relative t o t h e migration. Materials which are not wetted by provide such conditions and t h e introduction of r gap into t h e construction will also be effective in preventing moisture from moving a s liquid. The temperature gradient must be determined a s the f i r s t step i n these calculations and the method of doing SO

given in Appendix I . Following this t h e method used determine t h e moisture gain due t o water vapour igration is given in Appendix 11.

I n addition to vapour migration, moisture may also be transported by a mass movement of a i r caused by a difference in pressure between t h e inside and the outside. If t h e outside pressure is higher t h a n that inside t h e building, cold d r y outside a i r will enter through any cracks but normally this will not have any harm- f u l effects on t h e building. I f , on t h e other hand, t h e reverse is true, and warm moist a i r is forced out through t h e cracks, serious trouble can be expected. As t h e temperature decreases through t h e wall or roof the dewpoint temperature may be reached and a t very low outside a i r temperatures t h e freezing point will be reached. Thus the moisture from t h e room may condense within t h e structure and cause damage. If freezing occurs, f u r t h e r damage may result. If there a r e interconnected passages within t h e structure, such as cavity malls or unintentional passages around insulat- ing material, then the a i r flow may carry t h e moisture considerable distances away from i t s point of entry into t h e wall before i t meets a cold surface on which

will condense.

This outward flow of a i r can be caused by many factors one of the most important of which is mind ac-

fect by warm inside and cold outside t e m - peratures. This effect will cause air to flow in

openings in t h e lower p a r t of the building through those i n the upper. Unlike the action of _w which will vary in speed and direction, this effect be relatively constant f o r lengthy periods. A discus of the effects of wind pressure and chimney action is given in Canadian Building Digest No. 23 [12].

If the building is ventilated under pressure with t h e supply capacity being greater than t h e exhaust then t h e internal pressure produced must be added algebra- ically t o t h e effects of t h e wind and chimney action, t h u s causing a n increase in t h e exfiltration of warm moist a i r into t h e fabric of t h e building. Fortunate!y this is unusual in the case of paper mills where normally a considerable suction i s produced on t h e building.

CONCLUSION

I n this paper emphasis has been placed on the presenc and movement of water in various forms a n d unde various influences. These matters are considered to be of great importance in t h e design of any building and very often a r e not fully appreciated by designers. I t must be kept in mind t h a t water is t h e enemy of al- most all building materials, a n d it is usually a principal agent in deterioration. T h e low outdoor tempera tures characteristic of Canadian winters do not o themselves seriously affect t h e durability of material and coi~structions. Apart f r o m the increased therm expansions and contractions which may result on seasonal basis, freezing of d r y materials usually does not lead t o serious deterioration. Freezing of materials containing moisture can be very destructive, however, t h e effect increasing in proportion t o t h e degree of saturation a t t h e time of freezing.

Changes in the moisture content of many materials may produce dimensional changes which can b e as serious a s those produced by change i n temperature. These movements must be prevented where possible by limiting t h e movement of moisture or they m u s t be al- lowed f o r if this movement cannot be stopped. Liquid water moving through t h e building envelope can meak- en it by leaching out cementing materials, leading t~ t h e slow disintegration of concrete and of masonry mortars. Similarly salts dissolved at one point may be deposited on t h e face of t h e wall as a disfiguring stain when t h e w a t e r evaporates. Corrosion of metals and rotting of wood will not take place i n a d r y atmos- phere, b u t may proceed rapid!y if they a r e kept moist. Paper mill buildings and in particular paper machine rooms a r e most susceptible to these dangers because of t h e high humidities which a r e to be found in them. A t t h e same time, the designer must not lose sight of t h e fact t h a t moisture can be present in building materials from other sources such a s r a i tration a n d may be introduced i11 thz n l a n u f a o r construction process.

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all or roof construction in which all the parallel low paths from inside or outsicle are of t h e oi~structioi~, the temperature drop through each component, inclucling the internal and external a i r films, is proportional t o its thermal resistance, t h e thermal resistance being equal to the reciprocal of the thermal conductance. If only one steady-state con- dition is being examined, the temperature gradient can conveniently be determined by tabulating the components of the wall in sequence, listing their thermal resistance opposite them and then proportioning t h e temperature drop through each component in the ratio of the resistances. This is shown in Table I for a wall panel made up of two 'ji-in. thick boards of asbestos cement with 2 in. of glass fibre i n ~ u l a t i o n sand- wiched between them and with a n internal temperature of 94 deg. F. and a n outsicle temperilture of $10 deg. F. Similarly, if i t i s desired to maintain t h e surface temperature above the cle~vpoint, the maximum permissible temperature drop across the internal a i r film is known and so for a given external temperature the thermal resistance required of the wall construction can be calculatecl.

If i t is desired to determine t h e temperature gradients produced by several combinations of inside and outside temperatures, a graphical method will be less laborious. To do this, draw a cross-section of the wall

I in which t h e thickness shown f o r each component is

proportional to its thermal resistance. Then, by plot-

1

ting temperature scales on each side of the cross-sec-

1

tion, a straight line joining the inside and outside temperatures ~vill give t h e temperature gradient. This

I method is illustrated in Fig. I, using the same wall

1

section as f o r Table I.

I _{Values f o r the conductivities of many building ma-}

i _{terials a r e given in the ASHRAE Guide [I01 the ap-}

I _{propriate chapter of the}₁₉₆₀_{edition being}

by the Division of Building Research a s Technical P a - I per No. 7 r131.

Contponent Con- Resis- T e m p . Inlerfnce

dliclancc frince D r o p Teitq,. O F

Internal air film 94

(still air) . . . . . 1.46 0.68 6.4

%" thick asbestos 87.6

cement board . . 16.0 0.06 0.6

87.0 2" glass fibre insulation 0.125 8.00 74.8

%(" thick asbestos 12.2 cement board.

.

. . . 16.0 0.06 0.6

External air film 11.6

(15 m p.h. wind) 6.0 0.17 1.6

-t10

-- -- -

8.97 84.0 84

/c--

2' G L A S S F I B R E

,-j

Fig. 1. Graphical determination of temperature gradient.

I

Appendix

I I

Moisture Migration

The calculation of moisture flow in a n unsaturated material can be made by using a simple equation which assumes t h a t the flow of moisture results solely from the difference in vapour pressure across the material. F o r conditions of uniform temperature and moderate humidities, the flow values given by this equation are quite realistic. Since, however, the movement of moisture even in a partially saturated material is actually the result of a combined vapour and liquid flow, the permeability factor in the equation must be modified for conditions where a n appreciable temperature gradient exists o r where t h e material is subjected to a high humidity. The difficulty arises in trying t o determine values f o r the permeability factor which will be realistic for the new conditions of combined heat and moisture flow.

The standard methods of determining permeability measure t h e total flow of moisture and a r e carried out

under isothermal conclitions. A sample of the materia! is sealed over the face of a cup and then exposed in a cabinet in which the relative humidity is maintained a t 50 per cent. The cup may contain either a desiccant which provides a 0 per cent relative humidity or i t may contain water which provides a 100 per cent relative humidity. The vapour flow either into o r out of t h e cup through the material is measured by the change in total weight. Two values f o r the permeability a r e obtained depending upon which method of measure- ment is used, i.e. either the dry cup or the wet cup method, and these are really average values with the material separating atmospheres a t 0 and 50 per cent R.H. or 50 per cent and 100 per cent R.H.

It

i s found t h a t the permeability of many materials increases a t the higher humidities a s shown in Fig. 2 and the designer must bear this in mind when selecting t h e permeability of t h e material f o r t h e conditions involved.

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Solt~tion: T h e f i r s t step is t o calculate t h e t e

pressures corresponding t o t h e calculated temperat1 through t h e wall are shown b y t h e dotted line. On assumptioil t h a t there i s a continuity o f vapour f

i t follows t h a t coiidensation m u s t take place.

Condensation does not t a k e place, however, at the point o f intersection shown since the theoretical vap- our pressure gradient h a s been calculated on t h e as- sumption o f continuity o f vapour flow, i.e, t h a t t h e vapour flowing into t h e wall f r o m the inside passes through i t and o u t at t h e outside. W h e n coiidensation t a k e s place, t h i s i s not t r u e and an adjustment must be made. T h i s is done b y assuming t h a t vapour can flow f r o m inside t h e building t o a point o f condensa- tion independently o f a n y conditions existing beyond t h a t point. T h u s a point o f condensation m u s t be as- sumed and t h e vapour pressure gradient recalculated using t h e vapour pressure differential between the room conditions and the saturation vapour pressure at t h e point o f condensation. Since condensation usually takes place a t a place where t h e permeability changes,

%

R E L A T I V E

H U M I D I T Y i t will be convenient i n t h i s case t o select f o r trial the _{inside face o f t h e outer board o f asbestos cement as}

ariation of pern~eability with relative humidity. t h e plane o f condensation. Since the temperature at

t h i s point i s below freezing condensation will be i n the f o r m o f ice.

I t should be noted that merely projecting t h e room dewpoint temperature across into the temperature gradient w i t h o u t considering t h e vapour transmission resistxnces o f t h e materials \vill not locate t h e point o f condensation nor even prove t h a t condensation will

TEMPERATURE '

place, it remains t o calculate t h e quantity o f water being deposited inside t h e panel. T h i s i s done b y con- sidering separately the f l o w f r o m the inside t o the point o f condensation and f r o m t h a t point t o t h e out- T h e vapour pressure drop f r o m t h e room t o t h e back o f t h e outer sheet is 0.855- 0.070 = 0.785 in. Hg.

T h e combined permeability o f t h e inner sheet and the insulation i s given b y t h e reciprocal o f t h e s u m of

-

I .o

their vapour resistances a s

1 1

= ;-- = 5.85 perms.

116.5

+

l/58.O 0.1 r 1

T h u s , the vapour flow to t h e point o f condensation =

V.P. W I T H VAPOUR B A R R I E R _5.85

_x

_0.785_{= 4.59 grains per hour per sq.}

_ft.

_Simi-

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p r e s s u r e c u r v e will n o t be a l t e r e d m a t e - w i t h a roof, n o drying o u t i s possi ese c h a n g e s a n d h a v e n o t been a d j u s t e d i n s e n t i a l t h a t t h e v a p o u r b a r r i e r s h e. T h e v a p o u r p r e s s u r e g r a d i e n t c a n b e re- as possible.

R E F E R E N C E S

TA, J. K., Buildings f o r paper mills. P a r t I: A i n g Recearch, Canadian Building Digest No. 4, A rvey of existing machine rooms. P ~ l p Paper iMag. 1960, 4p.

Can., 63, No. C: T75-T81 (Convention Issue 1962). 8. WILSON, A. G., Condensation between p a n e s of doubl

inted a s NRC 6686). ~vindoms. National Research Council, Division

ook of pressure coefficients f o r wind loads, Building Research, Canadian Building Digest No. National Research Council, Associate Comn~it- May 1960, 4p.

n the National Building Code. Supplement No. 3 9. HANDEGORD, G. O., Vapour barriers in home cons e National Building Code of Canada. 18p. NRC tion. National Research Council, Division of B i n g Research, Canadian Building Digest No. 9, AS, M. I<., Clin~atological Atlas of Canada. ( J o i n t teinber 1960, 4p.

publication of the Meteorological Division, Dept. of 10. T h e Guide of the American Society of Heating. Transport a n d the Division of Building Research, Na- f r i g e r a t i n g and A i r Conditioning Engineers. P u tional Research Council.) 253p. NRC 3151. lished annually by ASHHAE.

.

BOYD, D. W., Weather a n d building. National Re- 11. HUTCHEON, N. B., Combined heat a n d moisture f l search Council, Division of Building Research, Cana- philosophy and review of a Canadian 1-

dian Building Digest No. 14. F e b r u a r y 1961, 4p. g r a m . Proceedings, 10th International Congre Climatic information f o r building design in Canada, Refrigeration. Vol. 1. p. 283-287. Copenhagen

1961. National Research Council, Associate Commit- (Reprinted a s N R C 5329.)

tee on the National Building Code, Suppleiuent No. 12. WILSON, A. G., A i r leakage in buildings. Nation 1 t o t h e National Building Code of Canada. 36p. NRC search Council, Division of Building Research

dian Building Digest No. 23, November 1961, TCHEON, W. B., Humidity in Canadian buildings. 13. H e a t transn~ission coefficients of building ational Research Council, Division of Building Re- Chapter 9, Heating Ventilating, Air Con

h, Canadian Building Digest No. 1. J a n u a r y Guide 1960, of t h e American Society of H e

4 ~ . f r i g e r a t i n g and Air-Conditioning E n g

N, A. G., Condensation on inside window s u r - printed a s NRC 5596).

.

National Research Council, Division of Build-

B I B L I O G R A P H Y

HUTCHEON, N. B., Fundamental considerations in t h e de- LEGGETT, R. F., and HUTCHEON, N. B., The durability of

I sign of exterior walls f o r buildings. Engineering J o u r - buildings. Amer. Soc. Testing Materials Syinposium o n

I nal, Vol. 36, J u n e 1953. p. 689. (Reprinted a s NRC Some Approaches t o Durability i n Structures. A S T M

3057). Special Tech. Publication No. 236, 1958, p. 35-44.

1 1