AIR CONDITIONING CHARLES R. DOWNS Calorider Corporation, Weiss and Downs, Inc., New York, N. Y
C
ONTROL of environment, of weather, now known as air conditioning has always been a pressing human need. This paper is the first in a series on the subject. Later papers will contain technical information covering various phases of the development. Relief from winter cold by burning fuel was practiced by the prehistoric savage. Even relief from summer discomfort was attained by the “sixty families” of the early Persians, Greeks, Romans, Egyptians, and Moors by means that have been improved only recently. Mechanical refrigeration, developed late in the machine age, was the first step towards providing man with a tool to make his summertime atmospheric environment suit his fancy whether for personal comfort or for its effect upon the products of his labors. Who first accomplished this is controversial, but it is interesting to note that Baekeland controlled the dry-bulb temperature and the moisture content of the air in his factory in Yonkers in 1895. His primary objective was to condition the air for the sake of his product, photographic paper, rather than for personal comfort which was of secondary importance to him. Baekeland described the method ( I ) in 1903 and stated that several manufacturers of photographic papers had adopted it in the interim. Baekeland first drew the air through a spray of water to eliminate dust, cooled the air by contact with ammonia coils to remove the moisture, and then reheated it by means of steam coils. It is reported that the neighbors doubted the sanity of a man who would build a factory without windows, illuminate it with red lights, and go to the expense of cooling the air to a very low temperature and then of reheating the cooled air by steam radiators. But chemists are habitually pioneers and do strange things not understood by their contemporary lay brothers. This method has since become common summer air-conditioning practice. The chemical engineer Gayley, like Baekeland a recipient of the Perkin Medal, had previously proposed the same sort of treatment of air for the production1of dry blast for iron blast furnaces. This was strictly for the dehumidification of air for industrial processing. The value of air for this purpose containing a reduced but uniform content of water was early recognized by several metallurgists. It had been mentioned by Truran (6) in 1862. Later in England it was proposed to extract moisture from the air by passing it over lumps of calcium chloride but Lowthian Bell showed the impracticability of the process as then developed. Charles Cochrane, another English iron manufacturer, obtained a patent on the extraction of moisture by bringing the air into contact with a calcium chloride solution, but later gave up the process. With this and other proposals in mind, Gayley adopted refrigeration of the air to a selected dew point as offering the best promise of success and carried out experiments with this objective from 1890 to 1895. From then until 1900 he and his assistants collected the data necessary to design equipment for large volumes of air because the refrigerating firms lacked experience for this purpose, but installation of a full-size unit was delayed until 1904. Proof was obtained of the increased efficiency of blast furnace operation by using air with a low, uniform moisture content. The process proposed by Gayley 134
was not adopted, however, because of the large investment involved and the cost of operation using refrigeration. Carrier (2) stated in 1934 that he experimented with conditioning the air of a lithographic plant in Brooklyn as early as 1902. Later he tried to use calcium chloride for this purpose but abandoned it, and in 1906 “the &st real airconditioning installations were installed based upon practically the same principles of equipment and operation used today”-namely, by refrigera tion. I n later years various inventors proposed the use of calcium chloride and other moisture absorbents for the direct dehumidification of air for industrial purposes. A few installations were made, but the majority have long since been dismantled for various reasons. The variety of hygroscopic liquids suitable for this purpose has been increased recently by the commercial preparation of new products and the reexamination of the physical properties of the older materials. Solid adsorbents such as silica gel, activated alumina, etc,, are also recent developments.
Types of Air Conditioning The fields of air conditioning may be divided arbitrarily into industrial, commercial, and residential classifications. The industrial field may be further subdivided into (a) the treatment of air or other gases to make them suitable for use i r a process or for sale, and (b) the treatment of air in manufacturing spaces where the minor constituents, such as dust, moisture, foreign gases, and the like, may adversely affect the quality of the product. Temperature control, independent of moisture control, is also of importance. The commercial field is concerned with hotels, stores, theaters, land, marine, and air transportation, and offices, and may even include s~hools,churches, and other large buildings not strictly of a commercial nature. The above classes have made the greatest progress as to the number and capacity of installations. Practically all of them until recently have been based upon the use of refrigeration. Many authorities believe, however, that the mass market resides in the residential field, whether detached houses or apartments. The creation of a new industry such as this would provide a vast amount of employment similar to the record of the automobile, but the problems involved are radically different. The purpose of this paper is to discuss this residential problem and one method of providing comfort a t low cost. Any one who has the temerity to discuss air conditioning must expect to be confronted by instances which follow no set rule because house construction presents as many vagaries as the weather in any locality, and the atmospheric conditions of various sections of the country vary widely. It seems clear that no one method will satisfy all these conditions perfectly. We find, however, that large numbers of people with available purchasing power reside in areas where the saying has long been current, “Itk not the heat, it’s the humidity” that causes discomfort. Although there have been sporadic attempts to produce human comfort by placing especial emphasis on humidity reduction, these were unsuccessful either from a misunder-
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standing of all the factors of equipment design, or for some other cause. Hence no widespread adoption of the principle has been attained. Another reason for lack of progress in this direction was that the air-conditioning field was preempted by the manufacturers of ice-making machinery who aggressively promoted the equipment they had to sell. They emphasized the reduction in dry-bulb temperature, and the fronts of theaters were decorated with papier mbch6 icicles and polar bears. The “ice house” atmospheres produced with low dry-bulb temperatures and high relative humidities were more conducive to sales of equipment than to public comfort and health. Experience has shown that this was an unwise policy, and the public has been restrained from universal acceptance of “air conditioning” as it understands the term. In fact, the public is a little uncertain as t o its meaning but wants relief from discomfort.
Requirements for Home Air C o n d i t i o n i n g The chilling shock experienced when one enters a space whose temperature is markedly below that outside the space gives the thrill of a cold shower, but it is a burden for any one but a vigorous person to withstand. One becomes slowly acclimated to the new environment, unless it be too clammy, but upon leaving the chilled space he emerges into an oven. Youth will pay to ride the “loop the loop,” but air conditioning should be devised to provide relief from discomfort to young and old alike with the least possible awareness of its presence. Ultimately anything else will be relegated to the limbo of great-grandmother’s music box and this is now being realized by the important designers of equipment. Methods, which might be permissible for spaces where people spend a considerable portion of the day and thus have time t o become adjusted, may be very undesirable for other spaces which people are frequently entering and leaving. In other words, the conditions may be satisfactory for store employees and not for customers. In private residences, especially where children are running in and out of the house a t frequent intervals, there is danger to health in great temperature differentials. Yaglou in 1934 (6) reviewed the work of W. F. Tyler. This was carried out in Shanghai in 1902 and resulted in a theater installation where the air was dehumidified by cooling it but the dry-bulb temperature of the conditioned space was not purposely depressed below that prevailing outdoors. The entire cooling effect was therefore due to the lowering of the dew point, and the conditions were stated to be satisfactory. Other investigators have since corroborated his findings. The conditions to correspond to this common sense viewpoint may not lie in the center of the present-day summer comfort zone as determined by experiments on relatively few persons, but they are a compromise between the theoretical ideal and the practically suitable. The necessary objective is to devise a method for summer conditioning that will be accepted by the majority of home owners. It must, therefore, fulfill the following requirements: 1. 2. 3. 4.
A minimum first cost of equipment plus installation. A reasonable operating cost per summer.
Apparatus of the greatest simplicity. A system that can be installed by the least number of
trades, preferably requiring no greater skill than for the installation of warm air heating equipment. 5. A method which uses a minimum of water for heat release. 6. A system which does not consume large amounts of electric power because the electric utilities cannot afford to install powergenerating equipment solely for a summer peak load of short duration. 7. Equipment where repairs and service calls will be a minimum. This point may be satisfied by a method using a material
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that has t o be replenished occasionally whereby the local agent can derive an annual income from its delivery and at the same time check the machine for satisfactory performance. The conditioning of air in summer is a matter primarily of relief from discomfort whereas winter conditioning is important from a health standpoint. At both seasons the moisture content of the air is important, and any satisfactory device should provide for extracting moisture in summer and introducing moisture in winter. Suitable temperatures, air circulation (air motion in summer, cold floor removal in winter), sterilization, and odor and dust removal, preferably by scrubbing the air by a suitable liquid, are assumed as necessary. If, therefore, a means can be devised whereby the parts of a winter heating system can be adapted to summer use, the equipment will have a double function, and the equipment cost chargeable against summer air conditioning can be reduced to comply with public acceptance. Mechanical refrigeration satisfies only the summer requirement whereas chemical methods seem inherently suitable for year round moisture control and for the other objectives.
Moisture E x t r a c t i o n When moisture is extracted from air, its latent heat is converted to sensible heat, and this must be removed from the process; otherwise the air will be discharged at an increased temperature. Many moisture-extracting agents have been proposed for this purpose. The common presentday solid adsorbents are silica gel, activated alumina, and others which must be regenerated frequently i n situ by hightemperature heat for the removal of water as they approach saturation. They are not suited for winter air conditioning. Hygroscopic salt solutions have been proposed, such as solutions of calcium chloride, mixtures of salts such as calcium chloride and calcium bromide, zinc chloride, and lithium chloride. Such salt solutions must be kept a t constant strength to continue functioning. It is therefore necessary to supply a source of heat for the regeneration of the moisture-extracting properties of such solids and liquids. The salt solutions may be heated and cooled with greater facility than the soIids, and they may be regenerated with Iow-temperature heat. They are also ideally suited to the winter humidification of air and may be conveyed by simple means without deterioration. They are not damaged by entrapped dust, and this can be removed by settling or filtering, These important advantages favor the use of salt solutions. Methods for drying air by salt solutions and the automatic regeneration of their absorptive capacity in a self-contained system for summer use, as well as moistening air for winter use, will be discussed in a later paper since they appear to be more suitable for industrial and commercial installations of considerable size. Upon this premise, apparatus has been developed and tested over a period of years which employs a solid deliquescent material, preferably based on hydrated calcium chloride in lump form along with a solution of the chloride, for this purpose. It seems natural for a chemical engineer to select a cheap material such as calcium chloride, which is a waste product, for the purpose. The problem of using it in the solid form under varying conditions of load for drying large volumes of air a t low pressures in a small apparatus, where the time of contact must be a fraction of a second, presented difficulties which were not anticipated. Solid calcium chloride of various forms, compositions, and crystalline structure have been tested for performance but we prefer to use it in the form of cubes, with dimensions of 1.5 to 2 inches, containing about 72 per cent calcium chloride and of a uniform nonporous crystalline structure so that upon absorption of moisture, liquefaction takes place
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upon the external surfaces. The character of this product and a suitable method for producing it will be discussed in a later paper aa well as the comparison of its suitability with other forms.
Air-conditioning Apparatus I n order to describe the use of such lumps for conditioning air, Figure 1 shows a longitudinal vertical section of an apparatus which haa been found to operate successfully over a period of years in actual practice.
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a week a t a time. In the period of idleness the bed temperatures of a unit, even in a cellar where atmospheric temperatures are ordinarily quite constant, may be many degrees below their operating temperatures at full load. At times a small amount of “solidified drainage” is formed between portions of the residual kernels at the bottom of the bed. This causes no trouble because the bed above this point is open to horizontal air flow and immediately begins to generate heat upon restarting; the “solidified drainage” a t the bottom then melts and drains out.
I@
AUTOMATIC ,THERMOSTAT
1
AUTOMATIC CONTROL ASSEMBLY ACTUATED BY HUMIDISTAT IN LIVING ROOM SCREEN! >q H.P. MOTOR
I AIR
INLET
SPENT CALORIDE IISCHARGE TO STORAGE TANK f 3
-
CITY WATER INLET
.‘/4 H.P. MOTOR
,-
CIRCULATING
PUMP TEMPERING COILS
PUMP INLET
FIGURE1. VERTICAL SECTION OF “CALORIDER” (3)
The lumps (called “Caloride”) are dumped into the apparatus to fill the hoppers and the underlying compartments which are separated from each other by extended surface radiators marked “tempering coils.” The path of the air passes horizontally through the lump-containing compartments. As the lumps liquefy, fresh lumps feed down from the hoppers. After operating for a sufficient time for the mass to come to size equilibrium, we find an ascending series of lump sizes and void dimensions from bottom to top of the beds; the smallest residual kernels and smallest voids are a t the bottom. Since the liquid is heavy and viscous, it drains substantially crosscurrent to the air flow; and if the bed rests upon a perforated support which is not chilled to the crystallizing temperature, the concentrated liquid drains into the dilute liquid on the underlying trays. The air takes the course of least resistance, and its rate of flow is lowest through the least open part of the bed, but here the contact surface areas are the greatest. Hence, despite the fact that the air flow is reduced through the bottom of the bed, the small residual kernels continue to liquefy and finally disappear. I n this way the bed cannot be plugged by “solidified drainage” sufficiently to prevent air, even a t low pressure, from passing through it. A practical machine for residential air conditioning must be foolproof and operate perfectly without supervision and solely a t the call of the control instrument. The weather changes in summer are such that it may have to be operated a t full load for long periods followed by idleness for perhaps
Mention has been made of the hoppers above the compartments for “Caloride.” These are shown as decreasing in size for the following reasons: The greatest amount of moisture is removed in the first compartment, and the air passing through the successive compartments contains progressively less moisture until it is discharged from the apparatus. Hence, the lumps in the first compartment are used up most rapidly. I n order to obviate the necessity for frequent charging of the apparatus, the hopper above this compartment must have the greatest capacity; the succeeding hoppers decrease in size in a ratio related to the rate of consumption within the convenient dimensional limitations set by practicable construction arrangements. With a suitable ratio of hopper capacities and air volume handled, all of the hoppers may be replenished with lump a t the same time; the frequency of deliveries is thereby reduced. When a bed of lumps is used to dry large volumes of air per unit time, the latent heat of condensation plus a small heat of solution is converted into sensible heat amounting to about 1150 B. t. u. per pound of water removed. The removal of this heat at the point of formation is impractical or at least difficult by direct means. The bed temperature therefore rises and heat is transferred to the flowing air. Since air is a poor carrier of heat, it restrains the rise in temperature of the bed only moderately. If the bed is too long in the direction of air flow, the air and bed temperature increase in the downstream portion to a point where further moisture is not absorbed by the lumps. The maximum
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length of a bed of lumps should therefore be limited by the relation of the amount of heat generated to the heat that can be carried away by the air. Any excessive length of the bed also increases the air flow resistance. The mass is therefore divided into compartments by cooling coils so that the air passes in series through compartments and coils. Thus the air is dried and heated and then recooled for further drying, etc. A compromise must be made as to the minimum lengths of the beds, in that they must be long enough to prevent the lumps from bridging over as they shrink down as a result of liquefaction. The operating beds are warmer than the air. The liquefied calcium chloride is formed on the surfaces of the lumps and is close to saturation a t approximately the bed temperature. Any slight cooling tends to cause recrystallization which is termed “solidified drainage.” Hence, the solution must be removed as soon as possible after it is formed, without being chilled by contact with cooled surfaces or otherwise until it has been diluted to a safe point. The concentrated solution flows into the diluted solution on the trays, and thus automatically restores the strength of the latter and prevents lwystallization of the concentrated increment by diluting it. The temperature and moisture content of air to be dehumidified by an air-conditioning device fluctuate greatly from time to time. If raw air is blown directly into a bed of solid lumps, the heat load varies markedly. If the moisture content and temperature of raw air can be stabilized by pretreatment, this serious difficulty is nullified. This is provided for by first passing the air into intimate contact with a cooled calcium chloride solution to give it a relatively constant temperature and moisture content before it is permitted to pass through the bed of lumps. Intimate contact of the air and liquid is accomplished by circulating the liquid over the trays, preferably at a high enough rate to form cascades and by continuously removing the heat resulting from moisture absorption by means of a cooling coil in the liquid through which city water, well water, water from a cooling tower, etc., may be passed in series with the coils separating the compartments. The excess solution may be discharged to waste or collected in a storage tank a t the right of the air-conditioning unit shown in Figure 2. The solution may be pumped once or twice during the summer into a tank truck and carried to the point of disposal. The moisture content of the discharged air is necessarily a compromise between various factors, one of which is the time of contact between air and dehydrating agent. The equipment should be small enough to be moved into a space through any door or passageway that may be encountered. This sets a limit to the width and length unless the equipment is sectional and easily assembled. The height is generally limited by the headroom available in a cellar. The amount of air to be treated may vary’from around 200 to 1,500 cubic feet per minute for residential work, with 500 to 1,000 cubic feet per minute the most common requirement. Therefore, we must accept a higher moisture content in the discharged air than the theoretical equilibrium between the vapor pressure of the moisture in the air and that of a saturated solution on the surfaces of the lumps in the last bed. A total contact time of the air while passing through the liquid and solid phase sections of about 0.5 second has been found to be a suitable compromise, but this may be decreased considerably if pressure blowers are used. When the principles of air treatment described above are used, the moisture content of the discharge air is surprisingly constant within the dry-bulb temperature ranges met in summer. This varies from about 2.75 grains per cubic foot at 74” F. to 3.0 grains a t 87” F. I n other words, the relative humidity decreases from about
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31 per cent at 74’ F. to 22 per cent at 87” F. This will be discussed in more detail in a later paper, but the fact is very useful in relation to the size of the equipment and the consumption of cooling liquid required to absorb the latent heat of condensation of the extracted moisture. It is only occasionally that the air in a residence in summer should be maintained below 40 per cent. In that case the relative humidity spread between the air in the space and the discharge air may be 18 per cent, which provides sufficient differential to take care of infiltration and internal generation of moisture. On cooler days the relative humidity in the space would be maintained a t a higher level, and then the reduced moisture load is amply cared for by the higher relative humidity of the discharge air. A further attribute is that this air contains sufficient moisture so that the equipment will operate a greater proportion of the “conditioned time” than if it were highly dried; thus more continuous air motion and cleaning are provided. The tempering coils may be used for heating air in the winter time and applying this air to heating the house. For this purpose the heating fluid in winter may be steam, but hot water from a boiler is preferably recirculated through the coils since it gives more uniform heating conditions. A fortuitous circumstance enters here. In winter there is a great temperature differential between the hot water and the incoming air. If the capacity of the radiators is made large enough for heating a house in winter, they are suitable in size for summer air treatment, even when ordinary city water is used for cooling, when there is a differential of only a few degrees between the incoming air and the water. This is one example of designing for a double function of the parts of the system. The quantity of cooling water to be used in summer depends upon its temperature and the objectives of the air treatment. For example, if it is desired to reduce the drybulb temperature of the air being treated in order to lower the temperature of the conditioned space appreciably, well water or other coolant of suitable temperature must be used for heat release. Then a sufficient volume of air must be treated to produce the specified temperature of the conditioned space. If the dry-bulb temperature of the latter is of no importance or it is adjusted by other means, the treatment of the air can be such that moisture removal alone is involved. This is the simplest example, in so far as heat load on the equipment is concerned-namely, that the inlet and outlet air temperatures are the same. Then the only heat generated within the apparatus is that resulting from dehumidification. If the air entering the apparatus is initially drawn from an unconditioned house, its moisture content will be high at the start of operation and will be reduced to a relatively constant amount after the desired conditions are established. The quantity of coolingwater needed for heat release will vary from a maximum a t the beginning to a minimum later. Water consumption may be conserved by placing a thermostat in the wind box, which will energize a valve to regulate the supply of water and thus prevent the outlet air from exceeding a selected temperature. A considerable saving of water is provided by this arrangement and may be justified where water costs are high. Toward the end of the summer, the “Caloride” lumps may be permitted to become exhausted without replenishment of the bins. Between the end of the humid season and the beginning of the heating season when no residential air conditioning is required, the small residual lumps and the solution may be removed if desired, and the latter replaced by water for humidifying the air for winter conditioning. The residual lumps, however, may be left in the beds with no great loss because during the winter season the air passing through the beds, if heated for indirect warm air heating purposes, is
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hot. Regardless of the presence of residual lumps, the calcium chloride solution may be used for humidifying the winter air, and it possesses certain advantages over water for this purpose. The only precaution to be taken is that water must be added to the solution in step with its evaporation.
Odor Removal During the development of this system of air conditioning, it was found that commercial calcium chloride imparted a disagreeable odor to the treated air which was first detected after installation in the test residence and not suspected in the laboratory. Some described the odor as that of wet plaster and others as musty. Many materials were tested with the hope of successfully masking this odor, but all were deemed objectionable from a psychological standpoint. The public, especially the gentler sex, is extraordinarily fussy on the subject of “fresh air” and no amount of argument will suffice to combat this attitude, A fortunate solution of this problem was found by the use of a finely divided solid adsorbent such as activated carbon. This could be suspended in the calcium chloride solution because of the high gravity of the latter plus the agitation resulting from its rapid circulation. The air, however, comes into final contact with the layer of concentrated solution on the surfaces of the lumps, and deoaorization of the air solely in the pretreating liquidphase zone is not sufficient. It was found, however, that the carbon could be dispersed throughout the solid lumps themselves during their preparation. While in use these lumps wear away, and fresh carbon is continuously exposed upon their surfaces, mixed with the solution as a sludgelike coating which ultimately drains into the diluted solution to be used in pretreating the air. If desired, sufficient carbon can be used to remove odors from the incoming air itself. I n addition, a granular activated carbon filter can be inserted in the treated-air discharge duct, and its effective life is increased by the pretreatment of the air as described. The finely divided carbon can be removed from the spent solution by settling or atering if it is desirable to recover “Caloride.” One precaution, however, should be taken. Slight traces of ammonia compounds are found in certain natural brines, and these should be removed or otherwise fixed to prevent the liberation of ammonia during use. This objectionable impurity has not been found in by-product calcium chloride derived from the Solvay ammonia-soda process. Impurities which are responsible for bad corrosion troubles should be eliminated from the moisture-absorbing material, or proper construction materials should be employed. This will not be discussed here since it is more fitting under the subject of hygroscopic salt solutions. Suffice it to say that corrosion due to oxygen is generally less with concentrated than with dilute calcium chloride solutions. Other gases or vapors of an acidic nature, such as sulfur dioxide, may be removed by adjusting the alkalinity of the calcium chloride.
Dust Removal The removal of dust from air is important because it is a nuisance. It also acts as a suspending agent for air-borne bacteria and viruses, especially when its particles form nuclei for droplets of moisture resulting from coughing and sneezing. Bacteria are surprisingly long-lived when existing in the moist condition on air-borne suspended droplets. Most of the viruses that affect man enter and leave through the upper respiratory tract. Air should be cleansed of pollens and
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other solid respiratory irritants, especially for people susceptible to their effects. I n passing through a properly designed liquid-phase contact zone, such materials are removed by being entrapped in a viscous liquid such as a 40 per cent solution of calcium chloride which dries as well as cleans the air. This air then passes through a bed of liquid-coated lumps, and any remaining suspended particles are subjected to countless direct impacts upon highly viscous surfaces. As the lumps liquefy, the particles drain away along with the concentrated solution. From this standpoint this method becomes a self-renewable viscous filter, and its efficiency is very high. The quantitative results as to dust removal will be reported later. Whenever this method of air treatment is discussed, the entrainment of calcium chloride into the air is questioned. Although the air passes through the open voids of the bed at high velocity, the solution coating on the lumps is difficult to entrain because of its high viscosity. I n fact, any spray entrainment from the liquid-phase pretreatment zone is effectively eliminated by the upstream portions of the first bed of solid lumps. As a precaution, an efficient spray eliminator may be used before the blower or in the wind box. Extensive tests have been made, using silver chromate paper exposed to the discharged air, which show a negative chloride content in the air. The removal of dust and the absence of entrained chlorides are matters of vital importance in telephone exchanges where apparatus of this type has been used successfully to control the moisture content of the air and thereby eliminate telephone operating troubles.
Economics Mention was made above that the solution produced by the use of calcium chloride may be discarded or recovered in the form of regenerated lumps. The adequacy of supply is ample with no recovery of spent solution. Enormous quantities of calcium chloride are discharged to waste from brine operations and from Solvay-process soda ash factories. If the cost of summer air-conditioning equipment can be reduced by mass production methods and by designing it for a winter as well as a summer function so that the investment for summer comfort will be little greater than the ordinary winter heating equipment, its adoption should be widespread. As soon as a locality has a considerable number of units in operation, the cost of delivery of lump will be reduced; but more important, the collection of the spent solution by tank truck becomes advantageous. The spent solution containing about 38 to 40 per cent calcium chloride may be recovered at low cost in the form of lump and recharged to the apparatus. Offhand, solution collection and lump delivery appear to be a roundabout and expensive method, but in no event should the former cost more than the delivery of fuel oil nor should the latter substantially exceed the delivery cost of similar quantities of coal. One marked advantage is that the fuel used for this recovery a t a central station may be the cheapest available, and regeneration of lump can be conducted under factory efficiencies. I n contradistinction to this method, the regeneration of solid adsorbents or hygroscopic salt solutions must be performed a t the point of use. At present these require either high-temperature heat or excessively complicated apparatus and control devices for residential use that are likely to demand frequent free service calls, which are the bate noire of the installer. Domestic fuel is relatively expensive and highly variable as t o type. When gas is used as a source of heat, the operating income goes to the gas company rather than t o the local agent, whereas by the lump calcium chloride method he should receive a yearly income from each installation and be able to inspect the
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equipment a t each delivery. The method proposed here appeam to offer ultimately a possibility that the residential operating cost may be as low as one-half that when lump made from virgin calcium chloride is used and the solution wasted. An alternative is to collect the solution, credit the householder on some fair basis, and dispose of it in the locality for various uses both known and potential.
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complished by hinging them a t the top and swinging them out a t the bottom. Unfortunately the glass is not colorless, since it contains ferrous iron. In winter such storm sash perform their normal function.
Program €or Humidity Adj ustment
For r e a s o n s given above the reader will understand that air dehumidification is proposed for those sections of the country where excessive atmospheric humidity is the primary cause of discomfort and where mildew and swollen woodwork cause nuisance and loss. In those localities the dry-bulb temperature outdoors rarely exceeds 90" to 95" F. and then only for short periods. This is generally followed in the evening by progressively reduced outdoor temperatures which reach a minimum during the early morning hours. During the night the outdoor relative humidity rises as the temperature drops, but generally by 9:00 P. M. the outdoor air, especially in suburban districts, has become sufficiently reduced in temperature to provide a marked relief from bodily discomfort. The atmosphere within the house is far less bearable and, even with all windows open, remains so until the early morning hours, especially on the upper floors. A fan or preferably a positive pressure blower of large capacity, located in the attic, is the cheapest investment one can make for nighttime comfort, and its operating cost is negligible. It should be arranged not only to draw air in through the windows of the first or second floor rooms during the night but also to sweep the hot air out of the attic during the daytime, which often reaches a temperature of 130" F. The latter function combats the effect of sun load on the ceilings of the bedrooms. Insulation above these ceilings also helps in this regard and reduces heat losses in winter as we1l.l Awnings on the east and west windows in particular are a great aid in keeping heat out of the house in summer, but they are a nuisance to care for and have a short life. A new glass has been developed recently which possesses the property of absorbing radiant heat. When this is substituted for ordinary glass in storm sash, they may be left in place during the summer to absorb the sun load. Then the space between the ordinary window sash and storm sash must be ventilated to the outdoor air to carry away the heat: this is easily ac1 T o prevent moisture condensation in winter, i t is best not t o fasten the insulation t o t h e roof itself. The subject of winter condensation will be disoussed in a later paper. It should be noted in passing that this is a matter of great importance a8 there is the serious danger of harming the struoture of the house..
FIGURE2 . "CALORIDER" Ar~~ STORAGE TANK FOR DISCHARGED
SOLUTION
Although an attic blower is a valuable asset, it is not alone sufficient for summer conditioning. Dehumidification of the air in the house in the daytime is required. If these two means are provided, the following operating program around the clock has been found to give adequate relief from discomfort for weather conditions typical of large sections of the country: At about 10:00 A. M. on days which promise to be very uncomfortable, the windows are closed. When further relief is required-for example, around noon-the dehumidifying apparatus is started with the control humidistat set a t about 50 to 55 per cent relative humidity. When the air in the house is reduced to this condition and if the occupants are satisfied, the humidistat may be left a t that setting; otherwise it may be reduced to 45 or 40 per cent, or below, but rarely does it require a lower setting. It is assumed that adequate air motion is provided, either by a central blower and ducts or by noiseless room fans which are less expensive. Moving air permits the dried air to come in contact with the body to promote the evaporation of perspiration. A dry-bulb temperature of 80°F. and relative humidity of 50 per cent are satisfactory for comfort conditions when the air is practically stagnant (has a velocity of about 15 to 25 feet per minute in the occupied space). By circulating air a t a more rapid rate, higher dry-bulb temperatures are made comfortable. A velocity of air motion in excess of 100 feet per minute is not usually employed. If this is the maximum velocity permitted, the relative humidity can be reduced to about 35 to 40 per cent, and relief from discomfort provided even a t dry-bulb temperatures of 85" F. or somewhat greater, as compared to more trying outdoor conditions. By keeping the windows closed, the hot outdoor air is prevented from entering the house, and with good modern construction these provisions cause an inside-outside temperature differential of as much as 10" F. at midafternoon without cooling equipment for purposely reducing the inside dry-bulb temperature. At the approach of nightfall or during the evening when outside conditions become more bearable, the
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dehumidifier is shut off, the windows are opened, and the attic blower is changed to draw night air through the house. The attic blower may be operated until rising time and the program repeated. In this way the air in the house quickly approaches outside conditions in the evening and follows them throughout the night. This program fits in with precedent. Few people will object to spending their waking hours with closed windows, but there is a universal objection to sleeping with closed windows. It is fruitless to argue the right or wrong of this attitude. Custom dictates it, and air-conditioning methods should be arranged to satisfy public habit when possible. Artificial dry-bulb reduction of the air throughout a house by refrigeration is expensive to provide and operate. I n some localities it may be essential but in many places its cost is not justified. One reason for the high cost is that cold air stratifies and hence ducts should be led to and from each room. Moisture will not stratiyy but diffuses through still air a t a very rapid rate. If dry or moist air is introduced into one part of a house, the absolute humidity of the air throughout the house, even in rather remote rooms, will quickly become uniform if the room doors are left open. Hence conditioning air by humidity-adjusting devices permits a minimum of ducts. This is of great importance in old houses heated by standing radiation where there are no ducts running through the partitions.
Future Developments When a new house is planned, it is assumed that it will be an example of all that is modern. There are countless engineering items to be considered, distinct from architectural features, which cannot be discussed here. The body-comfort-dispensing plant is of prime importance. The perennial argument concerning winter heating methods still continues. Shall the house be heated by direct hot air; by indirect warm air; by standing radiation using hot water (thermally circulated or pumped) or steam (pressure or vacuum); or a combination of standing radiation and indirect warm air (the split and auxiliary systems) in which the bath rooms, servants’ quarters, kitchen, and garage are heated by standing radiation and the rest of the house by warm air, alone or supplemented by standing radiation. There are variations of each of the above methods and each has its proponents. Regardless of cost from the standpoint of heat alone, direct radiation seems to have the edge in the argument. The body gains or loses heat by radiation to a very important degree. The air temperature may be well within the so-called comfort aone, but this does not take into account the loss of body heat to cold walls or the gain from heated surfaces. Ordinary standing radiation, however, is highly heated and widely separated in the form of room radiators which are also convectors. The most ideal solution appears to be low-temperature panel heating which is rare in this country but increasingly common abroad. There, factories, schools, and homes have been equipped in this way, and apartment buildings containing several hundred apartments are not uncommon. Excavations of Roman cities show that rather crude panel heating was practiced before the Christian era. The installation cost of panel heating in this country has been almost prohibitive, but its operating cost is stated to be about half that of ordinary convection heating. Many European installations employ the ceilings as heating panels, thereby utilizing large unobstructed areas per room. Iron pipe coils heated by warm circulating water a t about 120’ F. are embedded in the material forming the ceiling. The air temperature need not exceed 60’ F. in winter. The greater severity of our winter climate must be considered, but provision for this appears to be well understood.
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A still further view of the future indicates the perfection of panel cooling. We may speculate that water at around 55” F., passed through the coils and distributed throughout the ceiling of the room, will be cool enough to provide summer comfort, especially if combined with air drying. It seems to present an ideal solution to the problem as compared to the present attempts to cool occupied spaces by conveying enormous volumes of cold air, a poor heat carrier at best, for the purpose. Whether or not low-temperature direct radiation is used for heating and cooling the body, it is of interest to note that white human skin is a 95 per cent perfect black body surface. The amount of heat transferred is proportional to the difference in temperature between the surface of the skin and the object from which it is absorbing heat or to which it is losing heat. The amount of heat gained or lost is also affected by the area of the surface exposed. Hence, in winter, with proper correction for customary clothing, heat is lost to a lesser extent per unit area to internal partitions, a greater amount to external walls, and the largest amount to unprotected window glass, and heat is gained by radiation from a radiator. In summer, heat is gained from surfaces warmer than the body, and even if there is no heat gain from these sources, the closer they approach clothed body temperatures the lesser amount of heat is removed. Heat loss or gain by this means takes place irrespective of the surrounding air temperatures, but this factor is not accounted for in present-day comfort charts. Recognition of this prospect opens a vista of comfort conditioning for both seasons which has not been sufficiently appreciated, especially in this country. For those who wish to go further into this subject, reference is made to papers by DuBois (6) and numerous others on this subject. Should the low-temperature radiation method prove sufficiently inexpensive to install, even in the higher cost living quarters in this country, provision must be made in summer to prevent moisture condensation in the panel structure. The air must also be cleaned of dust and odors and be sterilized. This presupposes a simple duct system and equipment in the cellar or equivalent space to treat the air. Chemical engineers-Baekeland, Gayley, and otherswere early experimenters in the modification of air quality to serve man’s needs. Later, this field was dominated by mechanical engineers, Recently chemical engineers-for example, Midgley-have made outstanding contributions in this art, and its ultimate perfection will be attained far more quickly if the pioneering characteristic of the chemical engineer is applied to the many problems still unsolved. Acknowledgment The author desires to express his appreciation for the assistance given toward the development of this work by Research Corporation and its president, Howard A. Poillon.
Literature Cited (1) Baekeland, paper presented before 5th Intern. Congr. Applied Chem., Berlin, 1903. (2) Carrier, Heating & Ventilating, May, 1934. (3) . , Downs, C. R.,U. S. Patents 2,026,935-6 and 2,027,093-4 (Jan. 7, 1936), 2,091,353 (Aug. 31, 1937), and others pending. (4) DuBois, E. F., “Mechanism of Heat Loss and Temperature Regulation,” Stanford Univ. Press, 1937. (5) Truran, “Manufacture of Iron,” 1862. (6) Yaglou, C. P., Heating, Piping, Air Conditioning, 6, 205-6 (1934). RECEIVED September 14, 1838.
