Choice of a Low-
Temperature Insulating Material HE first item of consideration K. M. RITCHIE AND C. C. VOGT The thermal conductivity will vary with the mean temperature, when a low-temperature insuArmstrong Cork Co., Lancaster, Penna. usuallv decreasing as the mean temlating material is to be selected peratire decreasis, but the rate of is the heatflow to or from the strucchange may vary with the material. Within the range ture or article to be insulated. The reduction of this heat flow from 90" down to 32" F., it is permissible to extrapolate from is affected by the type of insulating material and by the thickknown values, assuming that a straight-line relation holds ness or amount employed. The total reduction required is true. However, there are few published data on actual deusually determined by economic factors, although there may terminations below a mean of 32" F., and more would be debe processes which would be unworkable without insulation. sirable, since installations where the arithmetical mean is It follows, therefore, that thermal conductivity, which is a below 32" are becoming increasingly common in the frozen specific property of a material, is not the deciding factor in food industry and in certain chemical processing. many cases; for by using greater thickness a material of higher In addition to heat flow, the other economic factors which conductivity may serve as well as another material with a must be considered are cost of insulation per increment of lower conductivity or greater insulating value. reduction in heat flow and the corresponding costs of heat or refrigeration, whichever is being supplied to the insulated structure. This makes each job a specific problem in itself, This paper will be useful to writers of owing to variation in unit costs of heating or refrigeration. In practice it is reduced to a decision to use a certain number specifications for insulating materials to be of inches of insulation of a type best adapted to the general utilized on refrigerated structures or equipconstruction. For certain types of work, the recommendament. The proper insulating material has tions are well established and are published in handbooks a low thermal conductivity, is able to withfrom which one may determine the economic thickness for stand the service conditions to which it will any particular job, provided it is a fairly common type. Unusual or new applications, however, involve special treatbe subjected, is relatively impermeable to ments which include consideration of the secondary effects, air and water vapor, can be installed with a on the process or product, of a lack of refrigeration or a failure minimum of difficulty and expense, and of the insulation due to breakdown.
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will have a long life. The effects of many service conditions on insulating efficiency and permanence have not been completely evaluated. The factors affecting thermal conductivity are a direct result of the movement of air and vapor and the deposition of water in the insulation. Quantitative values of these effects are not available.
Type of Service The type of service or service conditions to be considered: (1) the desired temperature differential, (2) whether the temperature differential is likely to include a dew point or freezing point, (3) the relative humidity conditions likely to be encountered, (4) air pressure or wind pressure, (5) whether the service is intermittent or continuous, and if intermittent the frequency of change, (6) whether the insulation is to be applied to the inside or outside of the structure or unit, (7) whether the insulation is required to be easily removable and replaceable, and (8) whether the insulated surface must be capable of withstanding sterilizing, washing, steaming, or similar treatment while in service. If the temperature differential includes the dew point or freezing point of water or both, which will probably be true of the majority of low-temperature jobs, a host of problems are encountered; the solutions are far from perfect a t the present time. Since the movement of air through the insulation represents heat loss by convection, and the presence of water vapor a t the dew point or the freezing point means deposition of
This factor cannot be carried to extremes since the items of bulk and heat capacity become greater as the insulating efficiency decreases. The range of thermal conductivity of commercially employed materials for low-temperature work is from 0.22 to 0.35 B. t. u. per square foot per hour per inch thickness per " F. temperature difference a t a mean temperature of 75" F. Materials of higher conductivity, such as wood, molded plastics, etc., are not usually classed as lowtemperature insulators, and are used primarily because of some other property. 821
INDUSTRIAL AND ENGINEERING CHEMISTRY
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liquid water or frost in the insulation, with a resultant loss in insulating value and eventual deterioration of the insulating material, movements of either should be eliminated or restricted to the greatest possible degree. Some insulating materials have greater resistance than others to the movement of air and water vapor, and again others are more resistant to the aftereffects of the deposition of water. After a material is selected with the best resistance, it must be supplemented by the best known installation methods to prevent ingress of air and moisture. In the case of insulated pipe lines, tanks, ’ and similar items, the problem is concerned with only one side of the insulation, but in building structures both sides must be given attention. Whether the service is intermittent or continuous is a feature which affects the ultimate life, since the alternate contraction and expansion, due to temperature changes or alternate wetting and drying, usually deteriorate the insulation or loosen it from its fastenings. I n the case of some materials, wetting may remove some of its ingredients by solution, may cause damage by freezing, or may promote chemical or bacterial action. Occasionally drying may assist in retaining a good thermal conductivity figure. I n general, intermittent service is more damaging to insulation, the lower the operating temperature. This is due to the fact that the alternate contraction and expansion have a tendency to open up cracks and joints and thus permit entry of heat as well as air and water vapor. Different materials will show a different reaction, however, since fill and flexible types of insulation may not be affected, while special construction methods must be used for rigid types. The mechanical feature of the insulation decided on will have a direct bearing on the cost of application. If fill or nonstructural types are used, supporting members must be supplied on both sides to keep them in place and protect them from physical damage. Rigid types can usually be applied direct to the structure or unit by adhesive or simple mechanical fastenings, For extremely irregular shapes and surfaces, such as may be present in steel ships, fill materials may be applied by the erection of a supporting wall without much regard for irregularities; rigid materials for the same service must be molded or specially fabricated. Blanket and semiflexible materials offer a compromise both ways, since they are less adapted than fill materials to surfaces with pro: jecting members, such as bosses, but require more substantial fastenings than rigid materials when used on plane surfaces. Blankets are best used where there is likely to be a necessity for periodic removal and reapplication due to inspection or repair of the insulated chamber. The labor and other items making up the cost of application will vary with the complexity of the specific job and often in ratio with the severity of the conditions or the temperature differential which it is desired to maintain. For building structures, the methods and specifications are well standardized as a result of experience, and the costs can be predicted accurately in advance. Estimates for this type of job can readily be secured from insulation contractors, and this is the usual procedure. The actual erection methods for rigid-type materials usually employ either adhesives, nailing, or mechanical fastening, depending on the base material. For partitions and structures of small size, the rigid insulation material can often carry its own weight, so that no structural framing is necessary. For fill and nonrigid materials, a confining wall or face must be constructed, in addition to the actual load-bearing walls of the structure.
Finish An important part of the erection specifications for any job is the means of protection, commonly called “finish”, both against mechanical abuse and against the entry of air and
VOL. 31, NO. 7
water vapor. Service conditions will dictate the necessity for protection against mechanical abuse, and this ranges from no finish a t all to elaborate masonry tile, steel sheets, and similar products. When rigid insulating materials are used, a relatively thin portland cement plaster or asphalt mastic coating is usually all that is required. For flexible and fill materials the supporting wall or face is usually mechanical protection as well. Prevention of entry of air and water vapor calls for different requirements, the chief one being impermeability and permanent freedom from cracks. Films of paint, asphalt plastics, and flexible sheet materials of various types are employed. The amount of air and vapor leakage should be reduced to a minimum, first to minimize heat flow by convection, and secondly to reduce deposition of water in the insulation. The mechanics and physics of this requirement are probably the least understood of any because little concrete information is available regarding corresponding conditions in actual installations. Published figures are available on air leakage and lately on water vapor transfer, but their reliability in calculating heat losses, and more particularly rates of moisture deposition, remains to be demonstrated. Correlation with service results and past experience is exceedingly difficult, and the value of much test work has been vitiated by the inability to evaluate the variables in workmanship and standard building design. The permanency of the insulation is the result of the foregoing factors, plus the inherent resistance to the action of all the deteriorating influences to which it may be subjected. These might be listed as temperature changes, wetting, alternate freezing and thawing, bacterial action, vibration, mechanical abuse, and oxidation or chemical attack. Failure of supplementary erection materials is often a contributing factor; this may be in the finish, the means of fastening, or occasionally in the structural members themselves. There are appreciable differences between the various commercial materials in their inherent resistance to these influences; and although they may be partially compensated for by variation in construction methods, such a course is, in the long run, more expensive and less satisfactory than the use of the best available material. It is the duty of the specification writer to obtain the necessary information to enable him to choose the most suitable material for the specific condition with which he has to deal. From the foregoing it might be concluded that little information regarding correct insulation practice has been accumulated. What we have intended to show mas that there are gaps in our knowledge that need filling. Actually, over the past fifty to seventy-five years an accumulation of experience has been built up by the industries in which refrigeration plays a major part, that can be applied with satisfactory results to practically all cases. This information is available in the literature of the industry, in engineering handbooks, and particularly in the Data Book of the American Society of Refrigerating Engineers. I n addition, new experience with new methods and materials is becoming available with the rapid expansion of refrigeration into industries where it was formerly but little used.
Thermal C o n d u c t i v i t y Of all the factors affecting the value of a low-temperature insulating material, its thermal conductivity is the most important from a basic standpoint. We wish to consider somewhat more in detail what this property is and how it may be affected by practical conditions under which an insulating material may be installed. Heat transfer is accomplished by conduction, radiation, and convection; all three play a part in transfer through an insulating material which is made up, as most low-tempera-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ture products are, of air spaces surrounded or enclosed by solid barriers. The normally used methods of measurement of thermal conductivity usually eliminate the factors of surface radiation coefficients, except in reflective types, and convection factors, both of which are of great importance in heat conductance through an insulated wall section. Values for these coefficients are available which can be used for calculating conductance, although they have not been as fully substantiated by experiment as would be desirable. In most calculations in which the commercial conductivity, K , is employed, an attempt is made to calculate for dry conditions only, and usually no correction is made for air movement due to differential air or vapor pressure across the thickness of the sample. The problem existing when the insulation has become wet due to deposition of moisture has been recognized; but little progress has been made as a result of inherent difficulty in determining the conditions that actually exist in service, and in applying the customary methods of heat flow measurement which are based on establishing equilibrium conditions through the sample. Since the conditions existing in service are usually unbalanced or not in equilibrium, it appears likely that a solution of the problem must await the development of methods for practically instantaneous measurement of heat flow, such as was attempted in the heat flowmeter. Insulating materials containing liquid or solid water are known to have lost their insulating value to some extent, although very little of a quantitative nature is known. The degree of loss is not only proportional to the amount but will vary greatly with the distribution; i. e., if the material is absorbed into the cell walls or solid portion of the insulation, it will have a different effect from what it will if present in drops between the cells or interstices. Whether or not the moisture is a t one face or in one section of a given area will also affect the results. Much more positive information on the effect on thermal conductivity of these variable conditions is badly needed if the accuracy of heat flow calculations is to be improved. As stated above, most measurements of thermal conductivity eliminate the effects of wind pressure as related to movement of air and water vapor through the material itself. Under service conditions the surface of the insulation is usually covered with a finish and backed by a wall of a relatively impermeable nature; yet there is usually some leakage wherever differential pressures exist. There is some effect due to height on account of differential temperatures, but this can be shown t o be less than possible wind pressure effects
ERECTION OF CORKBOARD INSULATION
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in most cases. The negative pressure created by wind effects on the lee side of a building may reverse the direction of air movement through a wall. It would be desirable to have more information showing the effects on conductance due to air leakage, such as occurs with varying wind velocities and differential pressures. The same considerations apply to water vapor pressure relations which are apt to be much more complicated by temperature relation, dew points, and ultimate effects on thermal conductivity due to deposition or absorption of water in the material itself. The simple effect of vapor passing through the insulation due to differential pressures would be much the same as that of air, except for the difference in specific heats, as long as (1)the dew point was not reached, (2) the material was not hygroscopic, and (3) the material was already in equilibrium with moisture a t the temperature existing in it. However, these conditions do not usually exist together, and the situation becomes extremely complicated when we try to predict what condition will exist after a service period under practical conditions. I n the case of organic materials containing cellulose, according to some recent work the amount of water present is proportional to relative humidity rather than absolute humidity, but only within certain temperature limits. This indicates the necessity for experimental determinations under controlled conditions. At the temperatures occurring in insulated low-temperature structures, there is no information available as to the variation of moisture content for specific materials, with relative humidity or the corresponding effect on thermal conductivity. A related factor is the rate of condensation of water vapor within the insulation. The result of such condensation is to decrease the insulating value, and if conditions are such as to permit continued deposition, the amount will increase steadily. KOinformation is available, however, as to the rate a t which moisture is*apt to be deposited under typical service conditions, and a study of this kind, to determine what sort of a curve would be obtained, by plotting refrigeration input against time, would give information of considerable value. Many questions in connection with the service life and efficiency of low-temperature insulation remain unanswered, The majority of these are concerned with deterioration of insulation by absorption or deposition of water vapor; conversely it appears that improvement of insulation efficiency may well be made in the construction methods whereby such absorption may be prevented. It is hoped that such advances may be forthcoming.
APPLYISGCORK INSULATIOX TO LOW-TEMPERATURE PIPELINES
