Principles in the design of cabinets for controlled environments

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Principles in the design of cabinets for controlled environments

Solvason, K. R.; Hutcheon, N. B.

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PRINCIPLES

IN THE DESIGN

OF CABINETS

FOR

CONTROLLED

ENVIRONMENTS

A N A L Y Z E D

BY

K. R. SOLVASON AND N. B. HUTCHEON

R E P R I N T E D F R O M

I N T E R N A T I O N A L S Y M P O S T U M O N H U M I D I T Y A N D M O I S T U R E P R O C E E D T N G S , W A S H T N G T O N , D . C . , ! 9 6 3 .

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REGLES A SUIVRE POUR LA CONCEPTION DE CABINETS CONDITIOI\TNES SOMMAIRE L e c h o i x d ' u n c a b i n e t c o n d i t i o n n 6 n e d e v r a i t 6 t r e a r r G t 6 q u ' a p r b s u n e 6 t u d e c o r n p l E t e d e s b e s o i n s . S i o n n e t i e n t p a s c o f i r p t e d e t o u s l e s 6 l 6 m e n t s f o n d a m e n t a u x d u p r o b -l E r n e o n r i s q u e d e n e p a s 6 t a b -l i r -l e s e x i g e n c e s q u ' i -l f a u t e n c e q u i c o n c e r n e c e r t a i n s f a c t e u r s e t d ' i g n o r e r c e r t a i n s a u t r e s f a c t e u r s q u i p e u v e n t 6 g a l e r n e n t i n f l u e n c e r l e s c o n -d i t i o n s -d 6 f i n i t i v e s . D a n s l e c o n c e p t d e l a c l i r n a t i s a t i o n o n d o i t d o n n e r u n e i r n p o r t a n c e s p 6 c i a l e i l a r 6 d u c t i o n d e s 6 c h a n g e s d e c h a l e u r e t d ' h u m i d i t 6 e n t r e I e c a b i n e t e t I ' a i r a r n b i a n t e t h l a c i r c u l a t i o n o u ) I ' a g i t a t i o n d e I ' a i r p o u r r 6 d u i r e l e s v a r i a t i o n s i n t e r n e s d e t e r n p 6 r a t u r e e t d ' h u m i d i t 6 b u n r n i n i m u r n p r a t i q u e . L a t e r n p 6 r a t u r e d u p r o d u i t s e r a c o n t r 6 l 6 e n o n s e u l e r n e n t p a r l a t e m p 6 r a t u r e d e I ' a i r r n a i s a u s s i p a r u n 6 c h a n g e d ' 6 n e r g i e r a d i a n t e a v e c I e s p a r o i s d u c a b i n e t e t l e s i n s t a l l a t i o n s , L a c o n -c e p t i o n d o i t d o n -c p e r r n e t t r e u n t r a n s f e r t r a d i a n t r n i n i r n u r n e t u n t r a n s f e r t t h e r r n i q u e c o n v e c t i f 6 l e v 6 v e r s l e p r o d u i t I o r s q u r o n d 6 s i r e c o n t r 6 l e r I e p r o d u i t b I a t e m p 6 r a t u r e d e l r a i r . U n c o n t r 6 l e p r 6 c i s d e I ' h u m i d i t 6 , p a r t i c u l i E r e m e n t l o r s q u l o n a r e c o u r s h u n e p e r c e p t i o n d e 1 ' h u r n i d i t 6 r e -l a t i v e , e x i g e 6 g a l e r n e n t u n c o n t r 6 l e p r 6 c i s d e I a t e r n p 6 -r a t u -r e p a r s u i t e d e l a d 6 p e n d a n c e d e I ' h u r n i d i t 6 r e l a t i v e s u r I a t e r n p 6 r a t u r e . T o u t e s l e s s u r f a c e s d u c a b i n e t e t d e s o n 6 q u i p e r n e n t d o i v e n t 6 t r e c o n g u e s p o u r f o n c t i o n n e r a u - d e s s u s d e I a t e r n p 6 r a t u r e d u p o i n t d e r o s 6 e d 6 s i r 6 e . L a p r 6 c i s i o n d u c o n t r 6 l e n e p e u t 6 t r e o b t e n u e q u e p a r u n e a t t e n t i o n p r 6 c i s e a u x c a r a c t 6 r i s t i q u e s d e r 6 a c t i o n d u d i s -p o s i t i f de conditionnernent e t d u c a b i n e t I u i - r n 6 r n e a i n s i q u ' b c e l l e s d e s d i s p o s i t i f s d e c o n t r 6 l e . Q u o i q u e l a p e r f o r r n a n c e s o i t i n f l u e n c 6 e p a r c h a q u e 6 l 6 r n e n t , c ' e s t l r i n t e r -a c t i o n d e I ' u n e -a v e c I r -a u t r e e t -a v e c I e p r o d u i t e n t r -a i n d ' 6 t r e c o n d i t i o n n 6 q u i d 6 t e r m i n e l a p e r f o r m a n c e u l t i . m e .

30. Principles

in the Design

of Cabinets

for

Control led Envi

ronrnents*

K. R. Solv.nsou ervn N. B. HurcnnoN

Dioi,sion of Buil,iling Resurch, Nafinnnl Resea,rch

Council, Cana.ila

ABSTRACT

The ilesign or seledion oJ a wnd,i,tinneil enbinet slwuld, only be mad,e after thnrough, can-s'i,ileralion of the requiremcnts. Failure to

ree,ognize all the basic el,emnnts oJ the problem

may often lead. to unreasonabk requirem,ents in resper;t to some fadors, while ,i,gnoring others which ma,y .equally influence the fi.na.l

per-form,o,ne,e. In the a'i,r-cnnilit'ioning design

special emphasis shpuld, be pl,aned, on reilucing

heat and, mo'i,sture encha,nges between the cabinet

and, amhierrt and, on a'ir circu.lalion or stirring to reilu,ce spatial aariations in temperalure and, humid,ity to a prant'i.enl minitnum.

The prod,unt ternperalure will be goaerned' not

onlg by air temperature btd also by rad,'i,ant erlergA eachange wi,th t|rc mb'i,net walls anil

furnishings. The ilesign shoulil, therefore,

ensure mi,nirnurn ra,iliant transfer anil high conaectiae hcnt transter to the prod,uet when it 'is

d.esird, to c,orrtrol the proifud ttair temperature.

Close conlrol of humidity, especiallg where relqtiae lwnri,iliry sansing 'Ls trceil, also requires close temperalure oordrol bem,use oJ the de-penilence of rel'atitse humid'ily on temperaiure. AII cabi,net and, equipment surfanes mtrct be

d,esi,gned, to operole abol)e the ilesi,red' ilew-point

temperature.

Precision in wntrol ean only be obtainetl by careful altent'i,on to the resTtowe characteristics of the conditioning equipment and' the enbinet itself , as well, as to tlwse of the oontrol deuices.

* This paper is a, contribution from tho Division of Building Rosearch, Netional Rosoarch Council, Canada and ie publishod with the approval of the Director of the Division.

Tlwugh thn pertormance 'i,s i,nfl,uence,il bg each wmponent, it ;s the intera,c.tion of one u:ith the other anil with th,e proilu.ct being e.ond,itioned tlmt determines thn uki.mnte'perJormance.

I N T R O D U C T I O N

Conditioned cabinets are used to provide small spaces of less than room size rvhere one or more atmospheric factors, including tempera-ture, humidity, air motion, and pressure, can be controlled u'ithin acceptable limits. The practical and technical considerations as-sociated v'ith them almost alt'avs pose a challenging problem for the designer. Too often the potential user, rvho may not be aware of the problems involved, l'ill set un-realistic requirrements, thich may be met b1-the designer or supplier at irnnecessarill'high cost or ignored at the risk of subsequent dis-satisfaction and recrimination. Ideally, all requirements should be carefully explored in extensive discussions betn'een the user'and the supplier or designer to establish clearll' the minimum performance actually required, unless the user himself has lr,ad extensive experience. This paper u'ill be concerned mainly u'ith some of the basic principles and considerations u'ith rvhich the user or pur-chaser, as rvell as the designer, ought to be farniliar. It rr-ill be limited also to cabinets for use at atmospheric pressure at temperatures from -60 to 200'F.

The provision of the desired conditions u'ithin a cabinet has all the engineering problems of air-conditioning. The system

242

EN VIRON ]I EN TA L CHA MBDRB

be considered in three parts: the enclosure, the conditioning equipment, and the control s;*stem. Though each of these merits con-sideration individually, it is their inter-dependence and the interaction of one u'ith the other and u'ith the product being con-ditioned that is the real core of the design problem and ultimately governs the final performance.

T H E B A S I C C O N D I T I O N I N G PROBLEM The elements of the basic problem, in-volring heat, moisture, and rad.iant energy eschange betrveen the product, the cabinet, atmosphere, and the cabinet, enclosure, are illustrated in Fig. l. The product may be rnaterial being conditioned, or it may be an apparatus for \-hich a controlled environment is being prorided. In either case, both the product and the cabinet enclosure ma1' be exclranging sensible heat, H, and rrater vapor, -11. uith the cabinet air. In addition, if the product and tire cabinet rrall are not at the same temperature, there u.ill be a radiation exchange, .R, bett'een them. Such other contents of the cabinet as furnishings and equipment mav at an5' given time be ex-changing both heat and moisture rrith the cabinet air and radiant energy with the enclosure surface in a manner similar to that of the product.

The net result ofthese exchanses rvill be the addition of an amount of sensible heat, fI, and an amount of moisture, ,11, in unit time, to the cabinet air being circulated through the cabinet at a rate IIr, based on rveight ofdry air in unit time. H and -'11 may be positive or negative. If b and m are the enthalpy and moisture content, respectively, for a unit rveight of air, and assuming that conditions are not changing rapidly, simple mass and energy balances may be written as follows:

H : W(hz - h) (l) M : W(mz - m) (2) Examination of these equations leads to three im portant. eonclusions :

(1) Conditions throughout a conditioned space can neo-er be uniform so long as there are heat or moisture exchanges bets.een product or cabinet and the condit.ioned air

(2) For a given air circulation rate, the spatial variations in conditions u'ill be in pro-portion to the heat and moisture exchanges involving the air.

(3) For constant heat and rnoisture ex-changes, the differences in air condition throughout the space will vary inversely with the air circulation rate.

Thus, rvlr.en spatial variations must be kept small. it is necessary either to keep heat and moisture loads, i.e. H and M, as small as possible or to provide a high circulation or stirring rate, or both. As will be shown, there are other substantial advantages to be gained in this direction, so that minimizing the con-ditioning load and maintaining a high circulation rate become important basic objectives in cabinet design for highly uniform conditions. The primary entering airstream rvill sho'rv the greatest departure from average conditions, but this effect is modified rapidly along its path as mixing oecurs as a result of the induced secondary circulation. The con. dition of the leaving air u'ill generally reflect more closely the average cabinet conditions. Consider, next,, the conditioner sholvn in Fig. I. Its function is to reverse the change in heat and moisture conditions of the airstream from those at 2 to those at l, but an additional heat load, B, which is always positive, musl

P R 0 D U

c

30. CONDITIONED CABINETS

243

be introduced. This is the energy input to the fan or blower required to circulate the con-ditioned air. The conditioner must then reject sensible heab at, a rate H' : H * S and water vapor at a rate M' : M. It maY, in the process, condense the water vapor' rejecting it as liquid, in which case a further amount' of sensible heat', L, representing that released on condensing, must be added ta H', t'he reverse being true if water is added.

For the general case, the four basic

func-humidifying to defunc-humidifying. Heating may be accomplished readily with electric heaters orheated liquid coils. Cooling may be obtained with cooled liquid coils or with the new thermoelectric cooling devices. Dehumidifica-tion may be accomplished by chemical me&ns or by condensation on cooled surfaces.

THE STIRRING PROBLEM

It is recognized that there is potential conflict in the desired objectives of minimizing loads and keeping circulation rate high. High circulation can only be accomplished by the expenditure of energy to drive a f9,n that' in turn, eventually appears as sensible heat in the air. A distinction may be made, however, between the heat load, 8, introduced by the fan, and that portion of it, called here the stirring load, that appars in the cabinet proper.

-

tn the arrangement thus far discussed,

this respect only when velocities are high' The dissipatlion of an entering velocity of 2000 fpm resulis in an increase in air temperature of 0.1"t'. In other arrangements separate fans

may be installed for stining in the working space, contributing their total energy input to the space.

THE RADIATION PROBLEM

It was noted earlier that the product and the interior surface of the eabinet enclosure might be at different temperatures' There would, therefore, be a radiant energy ex' change between them as shov'n in Fig. l. This mighi readily occur under transient or cyclical conditions of operation or during periods n'hen the product is adjusting to cabinet conditions. It is not so readily appreciated that such a difference can exist even under certain steady-state conditions, with the result that the product temperature may differ appreciably from the cabinet air temPerature'

When cabinet conditions are markedly difrerent from ambient' there rvill be a heat flow through the enclosing v'all of the cabinet, depending upon the differences in conditions rtt-d th" *aU th".*"I conductance. This sill result in an interior rvall surface temperature different from the interior air temperature at which the combined convective and radiative heat exchanges of the surface are in balance rvith the heJt exchange through the rvall' It can read.ily be demonstrated by caleulation that for a relatively poor cabinet rvall, con' sisting of I in. of wood and ambient tempera-tures di$ering by 20 or 30"F from eabinet air, the product temperature may differ by as much as I"F or more from air temperature'

Under these conditions the product is in thermal equilibrium, but it is exchanging heat by radiation rvith the enclosing wall surfa-ce at a rate balanced by its exehange of heat, opposite in sign, by convection rvith the air' Ttiese differences &re calculable under steady-state conditions provided that the appropriate convection coefficients and emissivities for both product and wall surfaces are knorul' Somewhat similar conditions may result rr-hen heating or cooling surfaces, v'arm motors, lisht bulbs, or rvindow surfacps at tempera' trires different from cabinet air temperaturd are located so that they can exchange thermal energy by radiation rvith the product.

ft wlttte evident that, t'hen precise control over the product condition is to be main-tained, the product must be protected from

2e

EN Y I RON M EN T A L Cfl A &BEEg

such radiative exchanges. It will be apparent also that thermostats and humidistats must be similarly protected. Radiation shielding may be employed to advantage, but it u'ill usually be best to eliminate the basic cause of the difficult5', u'henever possible, by removing heating or cooling devices from the space and by using a u'ell insulated enclosure so that its surface temperature u'ill be closer to cabinet air temperature.

Once again, the maintenance of high rates of air circulation is shorru to be desirable. The higher the velocity, the higher n ill be the cou-vection coefrcients, thus increasing the im-portance of convective heat exchange over radiative exchange and bringing all surfaces closer to the cabinet air temperature.

T H E H U M I D I T Y P R O B L E M The requirement for simultaneous control of temperature and humidity poses special problems. The main one arises out of the temperature dependence of relative humidity. For a constant, humidity ratio (vater vapor to drv air, by n-eight) and changing temperature at constant barometric presswe, the u'ater vapor partial pressure, e, is also constant. Relative humiditv (eleot), therefore, varies inversell'' as the saturation pressure, esar, corresponding to the temperature. Changes in eo, for I F deg change at various temperatures ean be obtained from tables. The chanse in relative humidity for I F deg temperature ehange decreases as temperature increases, at high humidities being roughly 5 per cent per F deg at 0"F and 3 per cent at 100'F. The proportional change in relative humidity, (not p.ercentage points) for I F deg temperature change from a particular temperature is independent of relative humidity.

\Yhen precision in relative humidity is required. the temperature control must also be preciSe. This applies not only to the temperature of the conditioned air but also to the temperature of the product and the humidity sensing devices, spatial temperature variations, as well as radiation exchanges, being particularly important in this respect.

The physics and thermo.d-l'lamics of the air-vapor s;rstem are the basis for air-condi-tioning theory and must be fully appreciated as a prerequisjte in design for humidity

con-ditioning. Only a few ofthe practical consider-ations u'il] be discussed.

Advantage is often taken of the constancy of vapor pressure at constant barometer with changing temperature in the dew-point method of control. A saturated mixture is produced at the dew-point temperature of the desired final condition so that, upon heating to the required final dry-bulb temperature, the required relative humidity is produced. Account must always be taken of the heat released rvhen water vapor is adsorbed or con-densed and of the opposite effect when it is desorbed or evaporated.

Cooling below the dew-point is a means of dehumidifying. No portion of the cabinet or conditioner system must operate unintention-ally at temperatures below the desired finat dew-point, or this v'ill produce dehumidifiea-tion and thus prevent the attainment of the disired relative humidity. This is particularly important in cabinets operated at tempera-tures well above ambient and in systems in which heat and moisture adjustments are made separately in different devices. In such cases a large coil operating at high air veloeity to produce the required heat exchange at an acceptably small differential with air tempera-ture may be employed, or cooling may be follorved by rehnmidifying. It may be noted that in a cabinet having a cooling requiremeut at high humidity provided by a conditioned airstream, the relative humidity that can be carried can never be greater thaa that corresponding to saturation at the entering air temperature. Conversely, when extremely high humidities must be produced, spatial variations in temperature, includ-ing those involved in the conditioning airstream, must be reduced to appropriate low levels. Heat gains to the cabinet must be eliminated, as far as possible, and high rates of air circulation maintained. The maintenance of precisely 100 per cent RH without foggrng is impossible in practice.

THE CONTROL PROBLEM All possible variations and solutions of the control problem cover & broad range, which has become a full-time specialty in itself. The main features of a system, however, that make the control problem easy or difficult can be

30. CONDITIONED CABINETS

245

readily appreciated. Consider a simple cabinet requiring heating only, with an air thermostat operating an electric heater on a,nd off as re-quired.-The first determinant, is the reliability and sensitivity of the control itself. Next to be considered is its time of response to a given change in air temperatute, in relation to the rate at which the heater is capable of raising the air temperature as well as the speed of response of the heater to the on-off switching. These same considerations apply equally well to humidity control with the sensitivity, reproducibility, and speed of response of the humidity sensing element being of paramount importance. Again the correcting device, humidifier, or dehumidifier m-gst respond rapidly to the on-offswitching, and its output must be controlled in relation to the moisture loss or gain. The moisture capacity of the air in small cabinets is very small, and the cabinet and its furnishings may not provide ap' preciable moisture storage unless a hygro-scopic material is purposely provided.

Over-shooting of the control point will result with on-off systems controlling tempera-ture or humidity if the heat or moistempera-ture capacity of the cabinet is small, the correcting device is oflarge capacity and slow to respond, and the sensing element is sluggish. Occasion' ally the extreme cycling eontrol provided in this manner may be acceptable if it maintains a constant mean value. The time response of the cabinet may change, horvever, if ambient conditions change, thus shifting the mean control condition.

On-off control is widely used in conditioned cabinets and leads to simple control and eon-ditioning equipment. Precision is increased if:

(f ) the response time of the sensing element can be decreased,

(2) the response time of the correcting devices can be decreased,

(3) the response time of the slntem to the restoring action can be increased.

The response time of a thermal sensing element and of a heating or cooling element will be a function of its heat capacity and overall heat transmission characteristics- The lishter the element and the larger its ratio of .,itfu." to volume, the faster its response u'ill usually be. Elements immersed in air rvill have a much longer response time than those in liquid by a factor ofas much as 100 because of

the lower heat transmission characteristics of air or vapor compared to liquid. fncreasing the velocity of flow over an element will decrease the response time. Fortunately there are now available temperature sensors having low capacity and providing relatively fast r€sponse in air. Electric heabers usually pro-vide relatively fast response, but liquid-filled coils including their associated liquid heating or cooling devices u'ill be relatively sluggish. It will often be ofadvantage to operate cooling devices continuously at capacities just over the maximum cooling capacity required, and to follow them u'ith fast-response electric heating to provide the final precise eontrol.

The time nesponse of a cabinet rvill be a function of the heat and moisture losses or gains, its thermal and moisture storage capacity, and the capacity of the restoring action being called into play. Improvement in control capability can, therefore, be achieved if the losses or gains can be kept small, the restoring action limited onl-v to that required, and the cabinet response time in-creased.

Relative humidity sensor€ generally react like a hygroscopic material; that is, their moisture content, varies in relation to the relative humidity of the air. If the element is maintained at constant temperature, the speed of response to a given vapor pressure change is inversely proportional to the moisture capacity of the element and the resistance to rvater vapor flolv from the air to the element. Elements of the electrical resistance t3pe are norv available' having an accurately reproducible rapid response to small changes in relative humidity at normal temperatures. At lolv temperatures, horvever, as the rvater vapor pressure is very lon' and, hence, the vapor pressure difference for a given change in relative humidity is extremel.l-small, much slo'wer response u'ill result. In many cases it may be impractical to attempt humidity control using relative humidity-sensors at lorv temperatures, and some form of flelrrpoint control may be necessary.

\Ithere on-off control is not suitable, pro' portional controls capable of continuously iarying the rest,oring action in relation to the need can be used. All svstem components must again be carefullv selected rvith 'regard to .""pott.u times to achieve stable control'

246

_{EN VIRON IT EN T A L CHA MBENB}

of controls u,ith increasingly complicated characteristics can be used in the more ex-treme and more difficult cases. The designer may make use of the thermal storage capacity of liquids in the system n'hen this is an advantage and avoid this rvhen it is not.

C Y C L I N G C O N D I T I O N CABINETS The design of cycling condition cabinets depends largelv upon the objective to be aclr.ieved. but it u'ill almost ahva}rs be difficult. Some thought u'ill shou' that, u-hen rapid c5-cling is invoh'ed the conditioning loads ma;' be verv high. the spatial variations large, and the product condition t'ill not necessarily follorr the cabinet conditions closely.

It mav only be desired to expose the product to some prescribed c1 cle of cabinet conditions, s-ith the product condition cycling in response to this. A change in the product u.ith respect to kind of material, size, or shape u.ill then change the pattern of conditions produced in it b.r- a gir-en pattern of cabinet conditions.

\Yhen the product, consists of a number of samples to be conditioned uniformly, it becomes necessarv to ensure that the con-ditions are the same for each sample. Each must be exposed to the same radiation ex-changes. If the cabinet air is to be circulated over a number of samples in series, conditions l-ill change along the air florv path. For slorvly changing eonditions this change may not be signifieant. In most cases, horvever, it will be necessarv to distribute the heat and moisture sources and sinks among the samples to achieve equal exposure, unless the samples are arranged for one-pass parallel air circulation. The relative humidity at the sample will not be that of the main airstream but will be determined by the temperature and vapor pressure at the sample surface.

S O M E F E A T U R E S O F C A B I N E T S A N D E Q U I P M E N T

The size and arrangement of a cabinet .w'ill be influenced by the intended use and the con-ditions and the control precision required. Ifit is to be used only for conditions close to room temperature, a relatively simple enclosure may suffice. On the other hand, if operation is

required at temperatures below room dew point, air locks rnay be needed to facilitate insertion and removal of samples without disturbing cabinet conditions or da,maging samples. Where samples must be manipulated inside the cabinet, gloved openings may be used, or where temperatures are not too different from ambient, say 35 to ll0'F, a rvalk-in cabinet may be considered. Even at 35oF, however, difficulty may be experienced in u'alk-in cabinets from condensation of moisture from the operator's hands and breath on samples, particularly where pre-cision weighing is required. At lower tempera-tures most manipulation may have to be done remotely; weighing, for example, ean be carried out by attaching samples to an external balance by a connection through the cabinet rvall. Remotely controlled machinery inside the cabinet may be required in some cases for tests or product manipulation.

The cabinet-conditioner system thus far discussed is fully recireulating, with the con-ditioner separated from the cabinet, but other arrangements are possible. Only a portion of the air drawn from the cabinet may be passed through the conditioner, to be mixed with the main stream and returned. Where the product is a source of contamination, which woulo increase in concentration under full recircu-lating conditions, ventilation may be intro-dueed by rejecting a part ofthe air drawn from the cabinet and replacing it with ambient air. The necessary adjustments in heat and moisture content of this entering ventilating air will then contribute to the heat and. moisture loads.

The conditioner may aiso be located within the cabinet but separated from the controlled space, as in the design shown in Fig. 2. In thh case the air duct connections between coB. ditioner and cabinet are eliminated, though the system is fundamentally unchanged. There is, however, a marked practical advan. tage. When the equipment is external to the cabinet there can be substantial loads froft air leakage into the system resulting from tha pressnre differences along the air flow circuit. This is a common fault in many cabinets. It is also possible to locate the essential com-ponents of the conditioner within the con-trolled space, particularly when the functions required and, therefore, the number of

com-Fig. 2. Cabinet, with corrditioning equipment inside.

ponents are reduced. The advantage of re-duced spatial variations resulting from proper juxtaposition of components and good mixing of the conditioned airstream may then be lost Special designs of systems can often be devised for particular requirements. When spatial variations and radiation exchanges in the conditioned space are likely to be a problem, the cabinet may be completely lined u'ith coils or hollow metal plates, or made of double-walled tank construction through which a controlled temperature liquid is circulated. Thermal loads arising from am' bien't conditions are, thus, intercepted; the conditioned space is provided with a uniform radiation environment; the small stirring load can be taken out with very small temperature differences from the large enclosing surface. In such a case it u'ill usually be adequate to control jacket liquid temperature rather than air temperature, and the large tllermal capacity of the liquid then facilitates control.

In the design ofa freeze-tharv apparatus the authors found it possible to utilize rather than minimize heat, exchange by radiation in the

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cabinet. This rvas for an automatic cycling apparatus to employ air freezing and liquid thawing of concrete specimens on a l?-hour cycle. It is a problem in such cases to achieve uniform conditions throughout a cabinet, even with high circulation, because of the heavy thermal load, the unavoidable dif-ferences in air florv over specimens, and the marked differences betrveen entering and leaving air temperature. In the design em-ployed the specimens rvere placed in ros's bet'w'een liquid-cooied plates, u-hich could be held at a uniform temperature from end to end with liquid circulated from a tank. Each specimen rvas, thus, subjected to nearly identical conditions of cooling by radiation on trvo faces. In addition, high air circulation v'as maintained parallel to the plates and rorvs of specimens to promote even further by-forced convection the rate and uniformitl' of heat exchange. As the circulating airstream 1r'as not cooled outside the cabinet. it ahraS-s entered at exactly the same temperature as it left, heat transfer being effected largelv at right angles to the direction of air florv. \rerl-uniform temperatures from specimen to specimen \l'ere achieved in this design.

In another case, this time for a higli-humidity cabinet, the necessarv saturator u-as, in effect, rvrapped around the inside of the cabinet'. The small room was coinpletell' copper lined, the bottom forming a sump'n-ith the u'orking floor provided by slatted paneis' Water, recirculated from the sump and con-ditioned to constant ternperature. rvas sprared from the top dorrns'ard over the copper u-all lining. This provided, in effect, a jacketed room as u-ell as a yer]'pol-erful saturator. The air-pumping action of the rrater jets rras used to induce air circulation behind a light inner 'wall, gapped top and bottom' and into the room. A rvet blanteb lining lvithout free air circulation to the room rrould, in rnost cases, be preferable, because the spral-s produce an aerosol effect in the space. This rvas not un-desirable in this application for concrete curing rvhere some rvetting of the product rras acceptable as long as leaching rvas not ap' preciable.

One feature of the liquid conditioning system used in the humid room design is l'orthy of attention. Esperience over rnan]' years indicated that small refrigerator units

F A N c 0 0 L l N G c 0 l L HEATER D E H U M I D I F I E H U M I D I F I E R

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_{ENVINON M EN TA L C EA M BERS}

control ga,ve much trouble. Coneequently, used for cooling and operated on-off for

, i t has been the practice. to operate theh con. tinuously on a full-load liquid-cooling function whenever appropriatc. The cooling coil is followed by an electric heater operated from an immersion thermostat and proportioning controller. A damper motor drivee a variac and, thus, varies the electrical input to offset, the exeess capacity ofthe refrigerator.

In another case a si.miler anangement was used for air-cooling u'ith very satisfactory results. The refrigeration unit was run con-tiluously to supply a direct expansion air coil. Arr open-wire electric heater s'as constructed over the face of the coil to provide fast heater responee and fed from a proportioning electrical input anangement, as described above, actuated by a fine-u'ire resistance t54rc thermostat.

Small humidifiers, often required, have been found particularly troublesome pieces of equipment; heated $pes tend to have un-satisfactory time response or to be unreliable. Very satisfactory results have been obtained rrith a simple denice consisting of a ;water surface in a small tank over which an airstream is blova by a small electric blower of suitable capacity, operated on-off. A small float valve maintains the u'ater level. In order to prevent evaporation while the motor is shut off, gas mask valves, actuated by the blower pressures, are used.

Iluch might be said on the subject of cabinet construction. Cabinets must, in general, be heavily insulated and constructed to prolide adequate resistance to air and vapor leakage. Doors should be well gasketed and all external parts ofthe system should be reasonably air-tight to eliminate unnecessary

heat and moisture loads as far as possible. Vapor barriers may have to be incorporated to prevent condensation and, finally, special interior finishes may sometimes be necessary for special pu4)oses.

C O N C L U S I O N

The design of controlled-condition cabinets almost always presents a challenge to the designer. It is highly desirable, when purchas-ing or constructpurchas-ing a cabinet, to identify clearly the essential requirements and to appreciate the ease or the difficulty with which these can be obtained. A:r attempt has been made to present the principles and some of the more important considerations to assist this. The stated requirements rnust be realistic and capable of achievement. There is little point in specifying extreme precision in the control system if time, spatial variations, and radia-tion exchanges lr'ithin the cabinet, lead to large departures in product cond,itions from those desired. If cost is-an important, con-sideration or when extreme precision is re-quired, it rvill often be necessdry to restrict the range of conditions the cabinet must provide to permit design or selection for optimum performance. The capability of controlling conditions over a wide range wili usually necessitate compromise, so that it should only be called for rvhen actually required.

The ultimate performance of a cabinet depends not only upon the eharacteristics of the individual parts of the system but also upon the ways in which they interact in determining the final product condition. All aspects of the design must be considered for their possible effect upon the final perform-&nce.