The book of ice-cream

In the making of ice-cream, refrigeration is necessary. In the following discussion only the underlying principles as applied to operation will be taken up. The details of construction are discussed by refrigerating engineers in various text-books.⁠[21]

Refrigeration is the interchange of heat units. The cooling for ice-cream-making may be obtained either from natural ice or from mechanical refrigerating machines. These will be discussed under their separate heads.

89. Terms used.

—In order easily to comprehend the principles of refrigeration, it is necessary to understand the terms used.⁠[22]

“British thermal unit.—A British thermal unit (B. T. U.) is the quantity of heat required to raise 1 pound of pure water 1 degree Fahrenheit, at or near its maximum density, 39.1° F. Some authorities consider a British thermal unit as the heat required to raise 1 pound of pure water from 61° to 62° F. For practical purposes, however, it may be considered the heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.

“Sensible heat.—Sensible heat is the heat that may be felt by the hand or measured by a thermometer.

“Latent heat.—Latent or ‘hidden’ heat is the heat which is expended in molecular work of separating the molecules of the substance and can not be measured by a thermometer. Every substance has a latent heat of fusion, required to convert it from a solid to a liquid, and another, latent heat of vaporization, required to convert it from a liquid to a gas or vapor. Thus, if heat is applied to a pound of ice at 32° F. it will begin to melt, and no matter how much heat is applied, the ice will not get any hotter. After every particle of ice has melted, we will have 1 pound of water at 32° F., the same temperature as the ice before heat was applied. Experiments have shown that it requires 144 British thermal units to melt 1 pound of ice at 32° F. into water at 32° F.; hence the latent heat of fusion of ice is said to be 144.

“If heat is applied to 1 pound of water at 212° F., the water will remain at 212° F. under atmospheric pressure until all of it has been evaporated into steam at 212° F. This has been found to require 970.4 British thermal units; hence the latent heat of vaporization of steam at atmospheric pressure is said to be 970.4 B. T. U.

“Specific heat.—The specific heat of a substance may be defined as the ability of that substance to absorb heat compared to that of water. Water being one of the hardest of all substances to heat, its specific heat is taken at unity. A better understanding of latent and specific heat may be had by studying the diagram in figure 42 which shows graphically the relation of heat to temperature.

“Ton refrigeration.—Refrigeration, or ice-melting capacity, is a term applied to represent the cold produced, and is measured by the latent heat of fusion of ice, which is 144 B. T. U. per pound. In other words, it is the heat required to melt 1 pound of ice at 32° F. into water at the same temperature. The capacity of a machine in tons of ‘ice melting’ or ‘refrigeration’ does not mean that the machine would make that amount of ice, but that the cold produced is equivalent to the melting of the weight of ice at 32° into water at the same temperature. Therefore 1-ton refrigeration is equal to 144 × 2,000, or 288,000 B. T. U. A 1-ton refrigerating machine is a machine that has a capacity sufficient to extract from an insulated bath of brine 200 B. T. U. per minute, 12,000 B. T. U. per hour, or 288,000 B. T. U. per 24 hours.

“Absolute pressure.—Absolute pressure is pressure reckoned from a vacuum. Pressure gauges in general use are arranged to indicate pressure in pounds per square inch above atmospheric. To convert gauge pressure to absolute pressure, 14.7 pounds, the weight per square inch of air pressure at sea level, must be added.”

NATURAL ICE

Only in the cold or northern latitudes can a supply of natural ice be obtained. In the warm regions a refrigerating machine or ice made by an artificial refrigerating system must be used. The harvesting of ice is a very simple process, yet it involves a large number of details. Success in obtaining a crop of ice requires careful attention to each detail.

90. The ice field.

—It is important that the water from which the ice is to be made be free from contamination. If weeds grow in the pond in the summer, they should be removed in the fall. If green spawn or algæ grow profusely, they can be eliminated by the use of copper sulfate.⁠[23] The crystals may be placed in a cloth sack, which is hung to a pole and trailed through the water until the salts are dissolved. One or two treatments of the sulfate in the season, at the rate of 1 pound to 100,000 gallons (13,000 cubic feet) of water will be sufficient to keep down such growth and make the water clear and pure. The area of the ice field or pond should be large enough to fill the ice-house at a single cutting, some allowance being made for waste. The water should be deep enough so that there will be at least from eighteen inches to two feet under the ice at the time of harvesting. Snow often interferes with the ice formation. If the ice is thin, and the fall of snow heavy, the latter may sink the ice. If the snow remains on the ice, it acts as an insulator and so prevents the freezing. The snow may be handled in either of two ways; it may be scraped off the ice by hand or with a horse scraper, or the snow may be soaked with water. In the latter practice, there is danger of a crust forming and so preventing the formation of ice and hindering future scraping.

91. The ice-house.

—The main essentials of a good ice-house are insulation, ventilation, and drainage. The house should be so located that there will be good drainage. If proper drainage is not provided, the water acts as a conductor of heat and so causes the ice to melt faster. It is desirable, but not necessary, that the ice-house have a north exposure and shaded with trees to keep off the heat of the sun. It should be located as near the place where the ice will be used as possible. There is a wide range of variation in type of construction and cost of materials in the erection of a satisfactory ice-house. The walls may be insulated so that the ice is simply piled in the house. This is the most expensive type of construction. In contrast to the insulated house, a bin may be built and the cakes of ice piled close together in it so that there will be a space about one foot to eighteen inches around the sides between the ice and the bin. This space should be filled with sawdust or hay and an equal amount placed over the top, which acts as an insulator. This is the cheapest form of construction and is somewhat wasteful since the top of the pile is exposed to the direct rays of the sun and the rains. The usual type of ice-house is a form of construction between the two extremes mentioned above. It consists of a cheap board frame to hold the insulation on the sides and a roof. The gables should be partly open to give a circulation of air.

A cubic foot of ice weighs about 58 pounds and requires about 35 cubic feet for a ton. Allowance for the spaces between the cakes of ice should be made when figuring the capacity of the house. The usual practice is to figure from 43-46 cubic feet for each ton of ice.

92. Harvesting and storing.

—Ice is not usually harvested until at least 8-12 inches thick. This will depend on the location and the season. The size of the cakes vary, but the usual sizes are 22 × 32 inches; 20-22 × 28; 22 × 42-44. The cakes of ice may be sawed with a hand-saw. (Fig. 37.)

On a large field, the ice may be cut with an ice-plow drawn by horses. (Fig. 38.) On this plow is a marker to show where the next cut should come. Thus if the first cut is straight, a straight mark will be made to be followed for each succeeding cut. The ice-plow does not cut entirely through the ice, but it should be adjusted to cut nearly through. This will make the breaking off of the cakes easy. If the field is large enough, it is usually plowed each way. This cuts the ice into cakes. Therefore, after plowing all that is necessary is to separate the cakes. This is accomplished by breaking or splitting with a splitting-fork. (Fig. 39.) On a small field, the ice is sometimes plowed only one way. In this case the cakes are sawed off with the hand-saw.

Either a perpendicular or inclined elevation with their conveyors should be used to put the ice into the house. Only regular shaped cakes should be stored, all broken ones being rejected because they do not pack closely, hence allow too much waste and air space. The cakes should be packed closely together and yet allow air circulation. In order to secure this, it is best to run the rows of cakes one way in one tier, the other way in the next tier and so on. Each tier should be planed smooth before the next is placed on it. This may be done by a large planer on the ice incline or by hand in the house. The insulation should be put around the sides as the house is filled. When all the ice is in the house, the insulation, either hay or sawdust, should be immediately placed over the top. The ice should be watched and if the hay or sawdust has settled so that the ice is exposed to the air, more should be added. It is best to fill the house when it is freezing temperature. If it is thawing the cakes of ice will have water on them, later this will freeze and it will be almost impossible to remove the cakes without breaking them. When taking the ice out, each tier should first be removed before the one below is disturbed. After each removal of ice the covering over the top should be replaced.

93. Amount of ice needed.

—It is difficult to know exactly how much ice to store to meet the needs of the summer. Some ice goes farther than others; some years there is more waste than others. The following figures compiled by the “Ice Cream Trade Journal” will give a fair idea of the amount required to a gallon of ice-cream: Amount of ice used for 100 gallons, 1,928 pounds. This was divided as follows: freezing, 614 pounds; hardening and storing, 914 pounds; shipping, delivering, and icing cabinets, 400 pounds.

94. Use of ice and salt mixture.

—Under normal conditions ice melts slowly. In order to obtain a quick change of temperature and one below that of the ice, a salt and ice mixture is used. Bowen⁠[24] gives the following discussion of cooling by salt and ice mixtures: “When two solid bodies, as salt and ice, mix to form a liquid a certain amount of heat becomes latent, called the latent heat of solution. Since this latent heat is taken from the mixture itself the temperature falls correspondingly. The temperature obtained by a salt and ice mixture depends principally on the relative proportions of the mixture, and to a less extent on the rate at which the heat is supplied from the outside, the size of the ice lumps and salt particles, and the amount and density of the resulting brine. Hence it is impracticable to give other than approximate temperatures with fixed ratios of salt and ice. The following curve (Fig. 40) shows the approximate temperature obtained with different proportions of salt and ice.

“One pound of ice, in melting, absorbs 144 B. T. U. This is known as the latent heat of fusion of ice. Salt in dissolving also absorbs heat, called the latent heat of solution, which varies in amount, depending on the density and temperature of the resulting brine.

“The heat of solution of salt in water at 32° F. varies from 58 to 16 B. T. U., depending on the final strength of the brine obtained.

“The following curve (Fig. 41) shows the amount of refrigeration available per pound of ice and salt mixture. The figures were calculated from the melting of ice at 32° F. into a liquid at the same temperature. If, however, the salt is added to the ice at a temperature varying from 32° F. or, if the resulting brine is allowed to escape at a temperature other than 32° F., the amount of available refrigeration must be corrected accordingly. These corrections are determined by multiplying weights, in pounds of salt and brine, by their respective specific heats and by their difference in temperature from 32° F. The specific heat of dry salt may be taken as 0.214, as the specific heat of salt brine varies with its density.

“Usually salt when added to ice is of a higher temperature than that of the ice; consequently the correction for its heat above 32° F. must be subtracted from the available refrigeration shown by the curve, Fig. 41; and if the brine is allowed to escape at a temperature below 32° F. the refrigeration lost in the discharge brine must be subtracted, while, on the other hand, if the discharge brine is at a temperature higher than 32° F. the correction must be added.

“If given amounts of ice and salt, at a temperature of 32° F. are mixed together and the mixture supplied with sufficient heat to melt the ice and dissolve the salt and raise the temperature of the resulting brine to the original temperature of 32° F., then the total amount of heat absorbed by the reaction will be the sum of the latent heat of the ice and the heat of solution of the salt to form the resulting brine of the density which will result from the particular proportion of salt and ice chosen. As an example, under the foregoing conditions, if 100 pounds of dry salt are added to 900 pounds of ice the total available refrigeration is 1,000 × 133 = 133,000 B. T. U. The available refrigeration per pound of mixture, 133 B. T. U., is taken from the curve in Fig. 41. If the salt added is at a higher temperature than 32° F., say 60° F., then the available refrigeration will be 133,000 - [100 × 0.214 (60 - 32)] = 132,401 B. T. U., or 132.4 B. T. U. per pound of mixture. If the resulting brine is allowed to escape at 25° F., the available refrigeration is 133,000 - [1,000 × 0.892 (32 - 25)] = 126,756 B. T. U., or 126.7 B. T. U. per pound of mixture. Or, in other words, there is lost in the first case 100 × 0.214 (60 - 32) = 599 B. T. U., and in the second case, 1,000 × 0.892 (32 - 25) = 6,244 B. T. U., or a total loss, if the salt is added at 60° F. and the brine allowed to escape at 25° F., of 599 + 6,244 = 6,843 B. T. U. Under these conditions the available refrigeration is 133,000 - 6,843 = 126,157 B. T. U., or 126 B. T. U. per pound of mixture.”

MECHANICAL REFRIGERATION

A large number of small ice-cream plants do not use mechanical refrigeration, but natural ice. It is not considered economical either in labor or cost to attempt to employ ice if seventy-five or more gallons of ice-cream are made a day. The size of the mechanical refrigerating machine varies, but the underlying principles are the same.

95. Principles of mechanical refrigeration.

—Bowen gives the following concise but plain description of these principles:⁠[25]

“When a solid or a liquid changes its state or condition, as when a solid is converted into a liquid or a liquid into a gas or vapor, the change of state or condition is in each case accompanied by the absorption of heat. This absorption of heat, as previously explained, is called ‘Latent Heat’; that is, heat that cannot be measured by a thermometer; and in order to transfer a substance from one state to another it is only necessary to supply or extract heat. For instance, if we take 1 pound of ice at zero temperature, Fahrenheit scale, and apply heat, the temperature will rise until it reaches 32°. If we continue the application of heat, the ice will begin to melt, and after we have supplied sufficient heat the 1 pound of ice will have changed to water at 32° F., the same temperature at which the ice commenced to melt. If the application of heat is continued the water will grow warmer, but at a slower rate. It now takes about double the amount of heat to raise the 1 pound 1 degree as water that it did to raise the 1 pound 1 degree as ice. In other words, the specific heat of water is approximately double that of ice.

“When sufficient heat has been added to raise the 1 pound of water to a temperature of 212° F., another critical point is reached at which further application of heat to the water, under atmospheric pressure, will not increase its temperature, but changes it into steam at a temperature of 212°. The relation of heat to temperature is shown in Fig. 42.

“It will be noted from Fig. 42 that to raise the temperature of the 1 pound of ice from zero to the melting point (32° F.) 16 B. T. U. were expended; in melting the ice, 144 B. T. U.; in raising the water to the boiling point, 180 B. T. U.; and to evaporate the water, 970.4 B. T. U. If the operation is reversed, the heat being extracted instead of being added, the curve will follow backward on itself to the starting point.

“The latent heat of fusion and the latent heat of vaporization are represented on the diagram by the two lines parallel to the horizontal base line, the length of the lines representing to scale the amount of heat expended in molecular work in separating the molecules of the substances. Starting from the left, the rising lines represent the heat required to raise the temperature of the ice, water, steam at constant volume, and steam at constant pressure, respectively.”

96. Materials used in mechanical refrigerating systems.

—“The same law applies to liquified anhydrous ammonia, carbon dioxid and sulphur dioxid, which are the substances most commonly used in commercial refrigerating machines. These liquids are extremely volatile, their change of state takes place very rapidly, and their latent heat is absorbed at a corresponding rate. Their boiling point is sufficiently low, under atmospheric or other conveniently produced pressure, to give the temperature desired. Although the same principles underlie the use of all such fluids, their physical properties vary, and consequently demand different treatment in order to produce the best results.

“The theoretical requirements of a good refrigerant are: A low boiling point at ordinary pressure, a large latent heat of vaporization, and a small specific volume. A low boiling point is desirable, because it makes operation possible with comparatively low pressure in all parts of the system; therefore, the machines and accessories may be of lighter construction, with smaller loss of gas by leakage. As the latent heat of vaporization is, to a certain extent, a direct measure of the cooling effect, it is obvious that the greater the heat of vaporization the better the refrigerant. The specific volume of the refrigerating agent determines the volume of the cylinders of the compressor, consequently the size and weight of the machine.

“In comparing the three refrigerating agents which are considered applicable to the dairying industry, viz., ammonia, carbon dioxid, and sulphur dioxid, it will be noted by referring to tables giving the main characteristics of the agents that, assuming the limits of operation are between 5° F. and 85° F., the absolute pressures are: Ammonia from 27 to 175 pounds, carbon dioxid from 290 to 1,000 pounds, and sulphur dioxid from 9 to 65 pounds. Taking the boiling points of the liquids at the temperature at which the liquid boils under atmospheric pressure, it will be noted that there is a wide difference in their boiling points as well as their latent heats of vaporization. Ammonia boils at 28.5° F. below zero and has a latent heat of vaporization of 572.8 B. T. U. Carbon dioxid boils at 110° F. below zero and has a latent heat of vaporization of 140 B. T. U. at a pressure of 182 pounds per square inch absolute. The latent heat at atmospheric pressure is not definitely known. Sulphur dioxid boils at a temperature of 14° F. and has a latent heat of vaporization of 162.2 B. T. U.

“For practical purposes the value of a refrigerant depends upon its boiling point, its latent heat of vaporization, and upon the pressure at which it can be used.

“To maintain a zero temperature with ammonia as the refrigerant an absolute pressure of 30 pounds per square inch is required in the evaporating coils; with carbon dioxid, 310 pounds absolute; and for sulphur dioxid, 10 pounds.

“Ammonia has a much greater latent heat of vaporization and the working pressures are not excessive, but it has the disadvantage that it corrodes brass or any other copper alloy; consequently only iron or steel can be used in the construction of those parts of the machine with which the agent comes in contact. The pressures of carbon dioxid are so high as to cause trouble in keeping the stuffing box and joints tight. A relief valve is often placed in the high-pressure side of the system in order to protect it from excessive high pressures. It is noncorrosive, nonexplosive, and is not dangerous to life when diluted with air. The high pressures necessary, combined with the small specific volume of the gas, make it suitable for use with a very compact machine. As the lower pressure of sulphur dioxid is below the atmospheric, any leakage of air will be into the system and will cause corrosion of the metal by forming sulphurous acid. The low pressures required in using sulphur dioxid as a refrigerant in connection with its large specific volume makes a large and cumbersome machine necessary. The ratios of the volumes of the cylinders necessary for a given capacity of machine, taking that of carbon dioxid as one, are approximately as follows: Carbon dioxid 1, ammonia 4.4, sulphur dioxid 13.”

97. Operation of refrigerating machines.

—The refrigerating material commonly used in ice-cream plants is ammonia. There are two types of ammonia machines, the compression and the absorption systems.

98. The compression system.

—The following, Fig. 43, shows the simplest compression system of refrigeration. The liquid ammonia in the small container is allowed to evaporate but it really boils. In order to boil or to change from a liquid to a gas, it must absorb heat. This heat is taken from the surrounding material, in this case brine. This cools the brine in the container in which the vessel of ammonia is placed. In this case, there is no control of the rate of evaporation of the ammonia.

An arrangement by which the evaporation or escape of gas can be controlled is shown in Fig. 44. The flow of liquid is regulated by an expansion valve and the liquid is carried into a brine tank or refrigerating room and from the coil of pipe in there gas is allowed to escape in the atmosphere. The change from a liquid to a gas in this coil of pipe cools the surrounding substance, either brine or air. This is the usual arrangement of the compression system; the remainder of the system is to return the evaporated liquid or gas back to a liquid in the ammonia tank.

99. Parts of a compression system.

—The functions and principal parts of a compression system of refrigeration are as follows:

Compressor.—This is a specially designed valve pump. It takes the gas from the evaporating coils, compresses it and forces it into the condensing coils. This reduces its volume and produces heat.

Oil-traps.—In the compressor, there is danger of some oil becoming mixed with the ammonia. The purpose of the trap is to separate the oil from the ammonia. It is usually placed next the compressor.

Condensing coils.—This consists of a double coil of pipe, one within the other. Cold water is circulated in the inner pipe and the ammonia in the space between the inner and outer pipe. In the condensor the heat is taken up by the water and the ammonia again becomes a liquid.

Ammonia receiver or storage tank.—From the condensing coils, the liquid ammonia passes into a receiving or storage tank until wanted for use again.

Expansion valve.—It is by means of this valve that the evaporation of the ammonia is regulated or, in other words, the rate of flow of the ammonia from the receiving tank is regulated by this valve.

Evaporating coils.—These coils are usually located in the material to be cooled, ordinarily the air of the refrigerator or a brine tank. In these coils, because of the reduced pressure, the ammonia liquid evaporates or boils and in doing so takes up heat. This, as has been explained before, causes the cooling. From the evaporating coils the ammonia gas goes back to the condensor. This makes a complete circuit for the ammonia.

100. Operation of direct expansion compression system.

—The following diagram, Fig. 45, shows the complete system of direct expansion refrigerating. When the evaporation coils are placed in the refrigerator and the heat is taken directly from the air, it is known as the direct expansion system.

The liquid ammonia passes from the ammonia receiver (R) through the expansion valve (X) into the evaporating coils (E). Here the ammonia changes from a liquid to a gas and in so doing takes up heat from the refrigerator. The ammonia gas passes to the compressor (C). From the expansion valve to the compressor is what is usually known as the low pressure side because here the pressure is reduced in order that the ammonia can boil or evaporate. For this reason the expansion valve is sometimes called the reducing valve. The gas is compressed in the compressor (C), then passes through the oil-trap (S) where the oil is taken out and then through the condensing coils (W) where the heat is absorbed and the gas changed to a liquid and back to the ammonia receiver (R). From the compressor to the expansion valve is what is known as the high side because of the pressure caused by the compression.

101. Location of evaporating coils.

—As explained above, the location of the evaporating or expansion coils in the refrigerator so that the heat is taken directly from the air is known as the direct expansion method of refrigeration. In order to keep a refrigerator cold with this method, it is necessary to run the compressor almost continuously. In some cases the evaporating or expansion coils are placed in brine tanks. The heat is then taken from the brine which in turn cools the air. By the use of the brine tanks, the compressor may be stopped and the cold brine will tend to maintain a more uniform temperature in the refrigerator while the compressor is not running.

A combination of the direct expansion and brine storage tanks is shown in Fig. 46. This is a common arrangement in refrigerators where a low temperature is desired and it is not economical to run the compressor continuously. The brine storage tanks are sometimes called congealing tanks.

In some cases it is desirable to have refrigeration in some place where it is not possible to use either the direct expansion or the brine storage system; for example, to freeze ice-cream. In this case the expansion coils are located in a brine tank and the cold brine pumped to the place where refrigeration is desired. Such an arrangement is shown in Fig. 47. The brine flows from the tank (T) in the refrigerator to the pump (P). It is then pumped through the ice-cream freezer (I) and back to the brine tank. The latter may be separate from the refrigerator and contain cans of water for the making of artificial ice. Most plants make artificial ice for packing the ice-cream for delivery.

102. Notes on operating compression system.

—In order to operate a refrigerating machine economically, certain factors must be given constant attention. When ammonia is passing through the expansion valve, it should be covered partially with frost or the part where the pressure is reduced will be frosted as will the pipe leading from it into the refrigerator. This cannot be prevented. The proper adjustment of the expansion valve is very important. If too wide open, the flow of liquid will be too rapid, it will not all vaporize in the evaporating coils and so will take heat from the air after leaving the refrigerator, causing the pipe from the refrigerator to the compressor to become covered with frost. This is a waste and may cause a high pressure on the low side.

Usually the low pressure side carries 10-20 pounds pressure and the high side 125-150 pounds.

If the ammonia passes the expansion valve too fast, as mentioned above, it may cause the compressor to labor too hard and so cause pounding. If not enough ammonia is passing the expansion valve, the rate of refrigeration is reduced.

The cost of operating a refrigerating machine varies. The principal items are: 1, Power; 2, water; 3, incidentals (refrigerant oil, and the like); 4, repairs. No figures can be given for the cost of a ton of refrigeration because of the variation in the price of each of the items mentioned.

Some very compact refrigerating systems are on the market especially adapted for making ice-cream in places where space is limited. The principle of operation of these machines is the same as all other expansion systems.

ABSORPTION SYSTEM

The absorption system is not as common as the compression. When used, it seems to be very satisfactory.

103. Operation of absorption refrigerating system.

—The following principles of operation, and Fig. 48 of an absorption refrigerating machine, are contributed by Henry Vogt Machine Company.

“The first step is pumping a strong charge of what is technically known as aqua ammonia, or, in plain terms, a solution of water and anhydrous ammonia, from the absorber into the bottom pipe of the rectifier. It is then forced upward through the inner pipes or tubes and out from the top through a pipe connected to the top of the exchanger where the strong liquid passes down through the inner pipes or tubes and out at the bottom through a pipe connecting with the ammonia generator.

“Within the generator the ammonia gas is driven off from the strong solution by the heat in the steam coils, leaving a weak solution of aqua ammonia in the lower part of the generator.

“The generated gas, under pressure passing out at the top of the generator, enters the rectifier through the top connection and is forced downward through the outer pipes. In transit through the rectifier, the strong aqua absorbing some of the heat in the gas, condenses whatever moisture is in it. The gas passes out of the bottom of the rectifier into a separator where baffle plates separate the moisture from the gas.

“The moisture is trapped back to the generator while the dry gas continues to the condenser, where it enters at the top of the shell or coils. Being brought into contact with the water cooled surface of the condenser, the sensible as well as the latent heat of the ammonia is extracted, and the gas quickly liquifies. This liquid ammonia is conducted to the brine cooler or refrigerating coils where it evaporates by absorbing the heat contained in the brine or air surrounding the coils, thus performing the work of refrigeration. The vapor or gas thus formed is piped to the bottom of the absorber.

“The weak aqua ammonia, in the meantime, passes from the bottom of the generator to the bottom of the exchanger and flows upward through the outer pipes for the purpose of exchanging the heat with the strong aqua ammonia flowing downward through the inner pipes.

“From the top of the exchanger the weak aqua is conducted to the bottom of the weak aqua cooler, flowing up through the outer pipes to be further reduced in temperature by cooling water passing down through the inner pipes. Finally it flows in the top of the absorber where the ammonia vapor from the refrigerating coils, owing to its great affinity for water, is rapidly absorbed by the weak aqua, forming again the strong solution of aqua ammonia. The double cycle of circulation is thus completed. The same operation is repeated indefinitely.”

104. Arrangement of double pipe and atmospheric absorption machines.

—There are several types of the absorption machines. The general arrangement and the direction of the flow of gases and liquids are shown in Figs. 49 and 50.