UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA MILK COOLING ON CALIFORNIA DAIRY FARMS B. D. MOSES and JAMES R. TAVERNETTI BULLETIN 495 JUNE, 1930 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1930 FOREWORD This bulletin is a contribution of the Division of Agricultural Engineering", and the Committee on Dairy Refrigeration of the Cali- fornia Committee on the Relation of Electricity to Agriculture. It is the fifth of a series planned to report the results of investigations con- ducted jointly by the Agricultural Experiment Station, College of Agriculture, University of California, and the California Committee on the Relation of Electricity to Agriculture. 1 The committee repre- sents the agricultural and electrical industries in California that are working together for the purpose of making available some reliable information concerning the use of electricity on the farm, and the purpose of cooperating with similar committees in other states. C. B. Hutchison, Director, California Agricultural Experiment Station. * The personnel of this committee for 1929-30 is : H. B. Walker, College of Agriculture, Chairman. Alex. Johnson, California Farm Bureau Federation, Vice-chairman. N. E. Sutherland, Pacific Gas & Electric Company, Treasurer. B. D. Moses, College of Agriculture, Director-secretary. J. R. Tavernetti, College of Agriculture, Field Engineer. C. L. Cory, Dean, College of Mechanics, University of California. H. M. Crawford, Pacific Gas & Electric Company. W. J. Delehanty, General Electric Company. J. J. Deuel, California Farm Bureau Federation. A. M. Frost, Great Western Power Corporation. Charles Grunsky, Standard Management and Operating Corporation. T. H. Lambert, Agriculturist. R. C. McFadden, Southern California Edison Company. W. C. McWhinney, Southern California Edison Company. E. G. Stahl, San Joaquin Light & Power Corporation. George Tenney, McGraw Hill Publishing Company. MILK COOLING ON CALIFORNIA DAIRY FARMS B. D. MOSES2 and J. R. TAVERNETTI3 INTRODUCTION According to the figures of the State Department of Agriculture, in 1928 there were 626,000 dairy cows in California that produced 485,112,970 gallons of milk ; of this amount 128,827,209 gallons were used for market milk purposes. In the production of market milk, state and city laws require the dairyman to use some means of cooling the milk immediately after drawing it from the cow. This requirement has given rise during the past few years to the use of mechanical refrigerating machines for milk cooling on the dairy farm. Because this application is relatively new and because there has been a lack of information regarding this particular use, the California Committee on the Relation of Electricity to Agriculture undertook a study of milk cooling on California dairy farms. The study consisted in obtaining information and conducting observations on farm milk cooling plants in operation in various parts of the state. The data and recommendations in this bulletin are the results of this study. PURPOSE OF COOLING MILK The purpose of cooling milk is to retard increase of bacteria which it may contain, and thus preserve its quality for a longer period of time. Bacteria enter the milk from the udder of the cow, with dirt, dust, insects, and such foreign material, and from unsterilized utensils. Most of the bacteria found in milk increase most rapidly at tempera- tures between 70 and 100 degrees Fahrenheit. Below 70 degrees the rate of increase of bacteria decreases with the lowering of temperature until the freezing point is reached, where it is practically stopped. Some bacteria, however, continue to develop even at freezing tem- peratures. Low temperatures do not kill most bacteria but cause them to remain dormant until warmed again, when they resume their activity. 2 Associate Professor of Agricultural Engineering and Associate Agricul- tural Engineer in the Experiment Station. 3 Field Engineer for the California Committee on the Eelation of Electricity to Agriculture. 4 University op California — Experiment Station REFRIGERATION RETARDS BACTERIAL GROWTH 4 SINGLE BACTERIUM .0. AT 50° F. AT '70° F. 5 BACTERIA (,N MILK) 7S0 BACTERIA INCREASE IN 24 HOURS Fig. 1. — The effect of temperature upon bacterial growth. TABLE 1 The Effect of Time and Temperature upon Bacterial Increase in Milk^ Temperature of milk, Bacteria at the end of degrees Fahr. 24 hours 48 hours 96 hours 168 hours 32 36 42 50 30,000 38,000 43,000 89,000 187,000 900,000 4,000,000 14,000,000,000 25,000,000,000 27,000 56,000 210,000 1,940,000 38,000,000 168,000,000 25,000,000,000 24,000 4,300,000 5,760,000 19,000 38,000,000 55 60 68 86 94 Each sample contained 30,000 bacteria per cubic centimeter at the start. Since milk coming from the cow at approximately 100 degrees Fahr. contains bacteria, immediate cooling to a low temperature is essential for preserving the quality. Freezing of milk, however, should be avoided. In order to obtain a low bacteria count, not only is it necessary to cool the milk, but precautions should be taken to prevent the entrance of bacteria during the milking and handling operations. 4 Adapted from : Bowen, John T. The application of refrigeration to the handling of milk. U. S. Dept. Agr. Bui. 98:1-88. 1914. s Data from: Bowen, John T. The application of refrigeration to the handling of milk. U. S. Dept. Agr. Bui. 98:1-88. 1914. Bul. 495] Milk Cooling on California Dairy Farms DAIRY LAWS The State of California has established requirements for the cool- ing of milk to be sold for human consumption, these in many instances are amplified by cities having their own ordinances. For the purpose of regulation, milk is classified by state law as follows : Milk Market /Ungraded \Graded Manufacturing Certified Guaranteed < _ x . , (Pasteurized Grade A ( R aw \ Pasteurized Grade B Pasteurized Unfit for Human Consumption The state requirements for the various grades of market milk are as follows: "Certified milk shall be cooled to 45° F and shall not contain more than 10,000 bacteria per cubic centimeter when delivered to the consumer. ' ' Guaranteed raw milk shall be cooled to 50° F immediately after being drawn from the cow and so maintained until delivered to the consumer, when it shall contain not more than 15,000 bacteria per cubic centimeter. "Guaranteed pasteurized milk shall be cooled to 50° F immediately after being drawn from the cow and so maintained until delivered to the consumer. It shall not contain more than 15,000 bacteria per cubic centimeter before pasteurization and 3,000 after pasteurization. "Grade 'A' raw milk, when distributed twice a day by the pro- ducer, must be cooled to a temperature of 60° F or below, immediately after being drawn from the cow, and so maintained until delivered to the consumer. Grade ' A ' raw milk bottled at a distributing plant must be cooled to a temperature of 55° F, or below, immediately after being drawn from the cow and so maintained until delivered to the consumer. "All Grade 'A' raw milk shall not contain more than 50,000 bacteria per cubic centimeter when distributed to the consumer. ' ' Grade ' A ' pasteurized milk shall be cooled to as low a tempera- ture as practical immediately after being drawn from the cow and so maintained until delivered to the plant. After pasteurization, milk must be cooled to a temperature of 50° F. or below and so maintained until delivered to the consumer. It shall not contain more than 6 University of California — Experiment Station 150,000 bacteria per cubic centimeter before pasteurization and 15,000 after pasteurization. ' ,6 The main difference between the state and city regulations is in the requirement for cooling Grade A milk for pasteurization. In the * production of this grade, the state law does not specify a definite tem- perature below which the milk must be cooled immediately after com- ing from the cow. The city regulations, in most cases, however, do state a definite temperature, ranging from 50° to 65° F., below which this grade of milk must be cooled on the farm. TYPES OF MILK COOLING AND STORING PLANTS Two general types of milk cooling and storing plants are in use, the 'wet' and the 'dry.' The 'wet' type, which is used but little in California, consists of an insulated tank of either brine or water, cooled by a refrigerating machine. In some cases an aerator (milk cooler) and a small pump for circulating the brine or water through the aerator are used. When no aerator is used, the milk, after being drawn from the cow, is placed in either five or ten-gallon cans and submerged in the brine or water, where it is cooled and stored until shipped. When an aera- tor is used, the milk is first cooled over the aerator by well water, brine, or water from the tank, or by a combination of these, and then placed in cans and stored in the tank until shipped. This type of plant has the advantages of being (1) low in first cost, (2) easy to build and install, (3) easily accessible for making small quantities of ice, and (4) able to store refrigeration. It has the disadvantages of (1) being limited to storage in cans, (2) being slow in cooling, (3) requiring occasional stirring of the milk unless an aerator is used, (4) being messy, and (5) making it necessary to lift the cans in and out of the tank. With the ' dry ' type of plant, the milk is cooled over an aerator, is placed in either cans or bottles, and then is shipped to the distributing plant or stored in a dry storage box. With this type of plant, two systems of cooling may be used, the brine and the direct expansion. With the brine system, the refrigerating machine cools a tank of brine, and this in turn cools the milk and storage box. With the direct expansion system, the refrigerating machine cools the milk and storage box directly. G Special Publication No. 78, Dairy Laws of California, State Department of Agriculture, Sacramento. Bul. 495] Milk Cooling on California Dairy Farms Legend Liquid refrigerant Vapor/zed refrigerant {high pressure) m Ifoporized mrrigenjnt (tow pressure) Water high pressure trough Fig. 2. — Parts of a milk cooling and storing plant using the brine system. Legend E233 Liquid refrigerant ^23 Vaporized refrigerant (high pressure) HvHI Voporized refrigerant (/otv pressure) (C^: 5) € <= ^ S 3 Evaporator coils storage box ~ t^ Fig. 3. — Parts of a milk cooling and storing plant using the direct expansion system. University of California — Experiment Station COMPARISON OF BRINE AND DIRECT EXPANSION SYSTEMS Advantages of the brine system are : 1. The compressor is smaller. 2. Refrigeration can be stored in the brine tank. 3. A fairly uniform storage box temperature is maintained. Disadvantages of the brine system are : 1. More equipment is necessary — a brine tank and brine pump and usually a brine pump motor. 2. Three heat transfers are required — milk to brine, brine to refrigerant, and refrigerant to compressor cooling medium. 3. Extra care is necessary for cleanliness. Advantages of the direct expansion system are : 1. Less equipment is needed. 2. Only two heat transfers are necessary — milk to refrigerant and refrigerant to compressor cooling medium. 3. The system is usually cleaner. 4. The operating cost is lower. Disadvantages of the direct expansion system are : 1. The compressor is larger. 2. Refrigeration cannot be stored. 3. It is difficult to keep a uniform storage box temperature. 4. The aerator cannot readily be removed for sterilization. 5. The aerator temperature is difficult to control. 6. The danger of freezing milk is greater. REFRIGERATING MACHINE The function of the refrigerating machine is to extract the heat from the milk and storage box and transfer it to some external cool- ing medium. The refrigerating machine is composed of five parts: namely, compressor, condenser, liquid tank, expansion or float valve, and evaporator. The medium of heat transfer is the refrigerant. The purpose of the compressor is to compress the vaporized refrig- erant to a high pressure which raises the temperature and makes it possible to extract the heat taken up in evaporation. There are many different makes and types of compressors on the market, but in all of them the principle of operation is approximately the same. But. 495] MlLK COOLING ON CALIFORNIA DAIRY FARMS 9 The purpose of the condenser is to condense the vaporized refrig- erant to a liquid by extracting* the heat taken up in the evaporator. Two different cooling mediums are used, air and water. The advantages of the water-cooled type are greater efficiency and a smaller motor. The disadvantages are that it requires a constant flow of water when running, it is more complicated, and there is danger of water pipes freezing in cold weather. Fig. 4. — Section of a 2-ton ammonia compressor, showing internal mechanism. The gaseous refrigerant enters the compressor through port A, is drawn into the cylinder through the valve B in the piston head, at the downward stroke of the piston. At the upward stroke of the piston it is compressed, is forced out through valve C in the cylinder head, and leaves the compressor through the port D. The advantages of the air-cooled type are that it has simple appar- atus, is easier installed, and the place of installation is not limited by water supply. The disadvantages are its lower efficiency and larger motor. The purpose of the liquid tank is to store the liquid refrigerant from the condenser. In some cases the condenser acts as a liquid tank also. The purpose of the expansion or float valve is to release the liquid refrigerant from a high pressure to a low pressure, lowering the boiling point and permitting it to evaporate and absorb heat. The expansion valve is used with the 'dry' type of evaporator, the float valve with the ' flooded ' type of evaporator. The expansion valve may 10 University of California — Experiment Station be either hand-controlled or automatic, while the float valve is automatic. The purpose of the evaporator, as the name implies, is to evapo- rate the refrigerant. As soon as the liquid refrigerant is released into the evaporator, its boiling point is lowered to a temperature below that of the substance surrounding the exterior of the evaporator. The refrigerant, in consequence, absorbs the heat from this substance Fig. 5. — An air-cooled sulfur dioxide refrigerating machine used for cooling milk on a 30 -cow dairy farm. It is equipped with a 1% -horsepower motor and during the summer operates about 12 hours per day to cool 75 gallons of milk. and produces the desired refrigeration. As mentioned under expan- sion valves, there are two types of evaporators, 'dry' and ' flooded.' In the 'dry' type, the refrigerant evaporates almost immediately after entering the evaporator. In the 'flooded' type, the evaporator is partly full of liquid at all times, the float valve being regulated by the quantity of liquid. The 'dry' system has the advantages of requiring less refrigerant and giving an easier starting load. The 'flooded' system has the advantage that the heat is transferred more rapidly through sur- faces in contact with liquids than through surfaces in contact with gases or mixtures of gases and liquids. Bul. 495] Milk Cooling on California Dairy Farms 11 CHARACTERISTICS OF THE MOST COMMON REFRIGERANTS IN USE Ammonia. — Ammonia (NH 3 ) a compound of nitrogen and hydro- gen, is a colorless gas with a pungent and offensive odor. It does not support combustion but may form an explosive mixture under high pressure when mixed with oil vapor. It will attack copper and copper alloys if water is present, but has no effect on iron or steel. It is very soluble in water. Under atmospheric pressure liquid ammonia has a boiling point of -28° F, which makes it well adapted for low temperature operations. Its latent heat of vaporization is 565 B.t.u. per pound, which is an advantage in large-capacity machines because of the smaller amount of refrigerant evaporation required to produce a given amount of refrigeration. It is, however, somewhat of a dis- advantage in low capacity machines because of the necessary small- ness and fine adjustments of the regulating devices. The operating pressures at standard rating, 5° F suction temperature and 86° F condensing temperature, are 19.57 and 154.5 pounds per square inch (gauge), respectively. Leaks are easily located either by the odor or by the ' smoke' test. The 'smoke' test consists of burning a sulfur stick in the vicinity of the leak, causing an apparently dense smoke to form. Sulfur Dioxide. — Sulfur dioxide (S0 2 ) is a compound of sulfur and oxygen, made by burning sulfur. It is a colorless gas with a pungent, suffocating odor. It is non-poisonous and non-inflammable, and does not support combustion. If mixed with water it forms sul- furous acid, which attacks both iron and copper. It is fairly soluble in water. Under atmospheric pressure liquid sulfur dioxide has a boiling point of 14° F. The latent heat of vaporization is 169.4 B.t.u. per pound, a fact which makes the gas adaptable to small refrigerat- ing units. The displacement necessary is about 2.6 times that of an ammonia machine, for equal refrigerating effects. The operating pres- sures at 5° F suction temperature and 86° F condensing temperature are -2.88 and 51.75 pounds per square inch (gauge), respectively. Leaks may be located by the odor or by the 'smoke' test, ammonia water being applied with a brush. Methyl Chloride. — Methyl chloride (CH 3 CL) is a compound of carbon, hydrogen, and chlorine, made from hydrochloric acid and methyl alcohol. It is a colorless gas with a sweet odor resembling 12 University of California — Experiment Station chloroform. It is inflammable in concentrations of between 10 and 15 per cent in air, but does not attack copper or iron. It is only slightly soluble in water. Under atmospheric pressure liquid methyl chloride has a boiling point of -10.66° F, which makes it well adapted for fairly low operating temperatures. Its latent heat of vaporiza- tion is 178.5 B.t.u. per pound, so that it is well adapted for smaller refrigerating units. The displacement necessary for equal refrigerat- ing effects is 1.9 times that of an ammonia machine. The operating pressures at 5°F suction temperature and 86° F condensing tempera- ture, are 6.19 and 80.83 pounds per square inch (gauge), respectively. Leaks may be located by the odor or by use of an alcohol flame, to which methyl chloride gas imparts a green color. Bating of Refrigerating Machines. — The American Society of Refrigeration Engineers and the American Society of Mechanical Engineers' standard for refrigerating machine ratings is as follows: "The capacity of any refrigerating machine shall be expressed in terms of 2,000 pounds of ice-melting effect for 24 hours (288,000 B.t.u.) with 5° F saturation temperature in the suction side and 86° F saturation temperature at the discharge side." This method of rating is used by practically all the manufacturers of ammonia-refrigerating machines ; but a large number of the manu- facturers of other types have no standard rating. The ice-making capacity of an ice machine is approximately 65 per cent of the rated capacity. Ton Refrigeration. — A ton refrigeration is equivalent to the heat removed by the melting of one ton of ice or the removal of 288,000 B.t.u. The capacity of a machine in tons or pounds of ice does not mean that the machine is capable of making that much ice, but means that it is capable of extracting the same number of B.t.u. as would be taken up in the melting of that quantity of ice. A one-ton refrig- erating machine has a capacity sufficient to extract 200 B.t.u. per minute, 12,000 B.t.u. per hour, or 288,000 B.t.u. per 24 hours. Horsepower Requirements. — For water cooled refrigerating ma- chines about two horsepower per ton capacity is required, while air cooled machines require about three horsepower per ton capacity. Bul. 495] Milk Cooling on California Dairy Farms 13 STORAGE BOX The function of the storage box is to supply low-temperature stor- age for the milk between the time it is cooled and the time it is taken away from the farm. On dairies where the milk is taken away an hour or so after milking, a storage box is usually not a necessity, but if the milk is held for several hours, it is essential. There are two general types of dry storage boxes, "the walk-in" and i ' non-walk-in. ' ' They are fundamentally the same in structure Fig. 6. — Aerator, "walk-in" type storage box and 120-gallon brine tank in use on a 25-cow dairy near Davis. Note that the brine tank is in such a posi- tion that cans can be placed underneath, and also is accessible for making ice. This plant is equipped with a ^-ton refrigerating unit. but differ in size. The "walk-in" type has a large door and is made so that a man may walk inside. It has the advantages of being more convenient and of providing more storage per unit of floor space because of the possibility of placing the cans or bottle cases one above another. It has the disadvantages of being larger and more expensive to construct and of having a greater wall area. The advantages of the "non-walk-in" type are its smaller size, its cheapness of construction, its smaller wall area, and its ease of being made portable. Its dis- advantages are its greater inconveniences and less storage per unit of floor space. 14 University of California — Experiment Station The "walk-in" type is usually built in as part of the milk house, it is used in most of the dairies where more than 100 gallons of milk per day are cooled. The " non-walk-in " type is built as a separate unit from the milk house and is used mostly on dairies where less than 100 gallons of milk per day are cooled. /Sheofhinc} 2"*-4 joists, a-o o c hx#£&' £-£"< plate ■fef- n3^C7-/oa/ Through A- A Fig. la. — Detailed construction plans of "walk-in" type storage box shown in figure 6, except that plaster is used as the finish inside instead of tongue-and- groove lumber. The insulation of a storage box should be the equivalent of 3 to 4 inches of sheet cork. Care should be taken to prevent moisture from coming in contact with the insulation. A good insulation is very essential for an efficient storage box. Bul. 495] Milk Cooling on California Dairy Farms 15 The size of the box depends upon the quantity of milk to be stored, the type of box, the location of the brine tank, the method of storing milk, and the type of plant. The usual practice when milk is stored in bulk is to allow a floor space of about 15 x 15 inches for each 10-gallon can to be stored. When the milk is stored in bottles, a floor space of about 16 x 20 inches is allowed for each stack of bottle cases (12 quarts per case) ; P. LAM Fig. Tb. — Detailed construction plans of " walk-in' ' type storage box shown in figure 6, except that plaster is used as the finish inside instead of tongue-and- groove lumber. the height of the stacks will depend upon the height of the box usually four or five cases with the ' ' walk-in ' ' type box. The quantity of milk to be stored depends upon the time of delivery of the milk, but it is usually desirable to have sufficient storage space for both milkings. In general, the "walk-in" box will have from 50 per cent to 100 per cent more storage capacity than the " non-walk-in ' ' type for the same floor space. 16 University of California — Experiment Station Fig. 8. — A "non-walk-in " type of milk cooling and storing plant in use on a 40-cow dairy. This plant has a 190-gallon brine tank and a 10-can storage space. Fig. 9. — A factory-made "non-walk-in" type of milk cooling and storing plant capable of storing eight 10-gallon cans. Bul. 495] Milk Cooling on California Dairy Farms 17 The type of plant, whether brine or direct expansion, affects the size of the box in that, with the brine system (on the smaller dairies) the brine tank is usually placed inside the box and cuts down the storage space. With the direct expansion system, coils are placed on the walls of the box, and the storage space is but slightly affected. BRINE AND BRINE TANK The purpose of the brine is to absorb the heat from the milk and storage box and transfer it to the compressor refrigerant. The tank is usually made of galvanized iron and is large enough to hold from iy 2 to 2 gallons of brine per gallon of milk cooled per day. The brine Fig. 10. — Compressor, brine pump and brine tank used for cooling milk on a dairy where no storage is necessary. The brine tank is enclosed in the insulated box, and the brine is pumped through the aerator which is located inside the milk house. used may be either sodium chloride (common salt) or calcium chlo- ride. The latter has the advantage of a lower freezing temperature with equal concentrations but is more expensive. The concentrations by weight are usually 20 to 25 per cent for common salt and 18 to 20 per cent for calcium chloride, with which it is possible to obtain tem- peratures of from 0° to 7° F without freezing. The average tempera- ture of the brine is from 20° to 25° F. The brine tank with the "walk-in" box may be on the floor, at the top, midway between the floor and ceiling, or in a separate box. When it is located on the floor, the floor space for storage is reduced and a larger box is necessary. This location has the advantages, however, 18 University of California — Experiment Station of simplifying installation and providing an easily accessible tank for making ice. When the brine tank is placed at the top, a higher box is required, which increases the area for heat leakage, the construction and installation costs, and the labor of making ice. This location has the advantages of having better air circulation, having more space available for storage, and making possible a neater and more con- venient box. When the tank is midway between the ceiling and floor, there are the disadvantages of more difficult installation and decreased accessibility for making ice. The advantages of this location are that all the floor space is available for storage, a smaller box is required, and there is less area for heat loss. When the tank is placed in a special insulated box by itself, as is usually done when there are 500 or more gallons of brine, an extra box is required which increases the cost and requires more equip- ment. The advantages of this location are that it is more convenient, is easily accessible for making ice, and makes a. neat storage box. With the ' ' non- walk-in " type of storage box, the brine tank may be placed in the upper portion with storage below, in the center with storage on both sides, or at one side with storage on the other. BRINE PUMP The function of the brine pump is to circulate the brine through the aerator and through the coils in the box when the brine tank is outside of the storage box. The common practice is to use a V2-i ncn centrifugal pump driven by a % -horsepower motor, when less than 200 gallons of milk are cooled a day, and a %-inch pump driven by a ^-horsepower motor for cooling quantities between 200 to 400 gal- lons per day. In some cases the pump is driven by the belt to the compressor. The pump should be located below the top level of the brine in order to be self -priming. AERATOR The purpose of the aerator is to expose to the air and to quickly cool the milk to a low temperature (about 40° F) immediately after it comes from the cow. The aerator usually consists of a series of hori- zontal tubes placed one above the other. The cooling is accomplished by allowing the milk to run in a thin sheet over the outside of the tubes, while the cooling medium runs through the inside. The cooling medium enters the aerator through the bottom tube and leaves through Bul. 495] Milk Cooling on California Dairy Farms 19 the top tube. When plenty of water is available, the aerator is usually made in two sections, the milk being* cooled by water over the upper portion and by brine or refrigerant on the lower portion. Fig. 11. — Aerator, brine pump and non-spill can filler used on a 30-cow dairy. The aerator is of the two-way type, the milk being cooled by water to within 5° F of the water temperature on the upper half and by brine to about 40° F on the lower half. The can tipper is operated by a float, which is lifted when the milk rises and tips the trough into the other can. Usually when direct expansion is used the lower portion consists of only one large tube about eight inches in diameter. Care should be taken to see that the milk runs in a thin sheet over the aerator and not in streaks. This can be accomplished by collecting 20 University of California — Experiment Station the first milk passing over the cooler in a sterile milk bottle or dipper, and pouring it rapidly over the top pipes of the cooler. This wets the entire surface of the cooler with milk, and permits the remainder of the milk to spread more uniformly over the surface. * Fig. 12. — Direct expansion type aerator and vats used for cooling milk on a 75-cow dairy. Note the large drum in which the refrigerant is evaporated and on which the milk is cooled to the desired temperature after being precooled by the water portion above. A rate of flow through the aerator of one gallon of water per minute to each ten gallons of milk per hour will cool the milk to within about 5 degrees of the original temperature of the water. The aerator should be approximately six inches in horizontal length for each ten gallons of milk to be cooled per hour. Bul. 495] Milk Cooling on California Dairy Farms 21 Fig. 13. — Aerator and special trough for filling six cans at a time. This eliminates the necessity for a large vat to catch the milk. Note that the milk is running in streaks and that only a portion of the aerator surface is in use. TABULATED RESULTS OF INVESTIGATIONS Investigational work on the use of ice machines for cooling milk on the dairy farm was carried on during 1928 and 1929. A survey was conducted during the first part of 1928, in the vicinities of Davis, Sacramento, Stockton, Tracy, Modesto, Turlock, Fresno, Chino, and Ontario. Seventy-three dairies were visited, and data were obtained either by personal observations or by questioning the dairymen. All of the dairies visited were selling market milk, either bottled or in bulk. A summary of the data obtained is given in tables 2, 3, and 4. 22 University of California — Experiment Station TABLE 2 Descriptive Data of Dairy Cooling Plants Included in Survey, Giving Number of Plants in each District Having the Items Listed Item Machines visited Different makes Sulfur dioxide machines Ammonia machines Methyl chloride machines Machines cooled by air Machines cooled by water Direct expansion systems Brine systems Home-made storage boxes Ready-made storage boxes Without storage boxes Sheet cork insulation Granulated cork insulation Sawdust insulation Rice hull insulation Having brine tank inside storage box Having brine tank in special box Having special brine tank for making ice Having calcium chloride brine Having common salt brine Bottling their own milk Delivering twice a day Delivering once a day Precooling with water Making ice for own use With compartment for storing food That have had repairs That are not satisfactory Davis and Sacra- mento 28 Stockton and Tracy Modesto and Turlock Fresno Chino and Ontario Total number covered by survey TABLE 3 Maximum, Minimum, and Average Specifications of Dairy Cooling Plants Covered in Survey, Arranged According to Refrigerant Ammonia* Sulfur dioxide and methyl chloridet Max. Min. Av. Max. Min. Av. 370 6 yrs. 4.0 7.5 60"x36" 3000 1470 50 50 $3500 15 2 mo. .25 .75 24" x 18" 150 144 30 35 $325 85 2.4 yrs. 1.6 3.15 35" x 20" 625 450 40 39 $1630 100 2 yrs. .5 1.5 30" x 25" 400 500 58 50 $1450 16 1 wk. .16 .33 18" x 18" 40 20 30 35 $400 41 .7yr. Rated capacity of machine in tons of ice.. Horse-power of the compressor motor .35 .80 25"x21" 208 160 Temperature of storage box— deg. Fahr. Temperature to which milk is cooled — 43 42 $930 Thirty-nine machines visited. t Thirty-four machines visited. Bul. 495 J Milk Cooling on California Dairy Farms 23 TABLE 4 Summary of Survey Data on Dairy Refrigeration Plants Showing Unit Performance Cubic feet of storage per gallon of milk Hours per day to cool milk Gallons of milk cooled per day Gallons of milk cooled per hour Gallons of milk cooled per day per ton capacity Gallons of brine per gallon of milk per milking Initial cost of plant per 10 gallons of milk cooled per day Initial cost of plant per cow Ammonia Max. 13 10.0 1100 110 900 14.0 $350 $80 Min. .58 10 45 15 33 1.7 $28 $7 Av. 3.2 5 4 235 43 177 5.6 $104 $26 Sulfur dioxide and methyl chloride Max. 11.9 9 360 51 1440 19.0 $190 $59 Min. .29 2 42 11 84 1.1 $20 $7 Av. 2.3 4.9 124 28 383 5.5 $85 $26.50 During" the survey, from one to five representative plants were selected in each of the vicinities, and a watt-hour meter was installed on the compressor motor and in some cases also on the brine pump motor. These meters were read at definite intervals of time, and the power consumption was obtained for a period of one year. The results of these tests appear in table 5 and figure 14. .24 .20 % v t /6 12 §.03 .04 1 1 1 Cooling and storing / u "^ '"V ^ in bottles I f \ 1 V s». in bulk t— -o' / \ \ ^ 'v / ' / _^ / / y \ \ \ \ \ i / J \ r Cooling on >y~ \ v V ,'J ^no storage ? J3 \ ^ / \„ -°-^ / May June July Aug. Sept Oct. /Yov Dec. M. /a 4 6 AM. 12 4 e P.M. /2 4 6 AM. /2 Pig. 15. — Characteristics of a milk plant when cooling 71 gallons of milk per day with a 750-pound ice machine. Taken from summer observation con- ducted on dairy No. 3. Bul. 495] Milk Cooling on California Dairy Farms 25 characteristics on dairies where the milk was cooled and not stored or where it was cooled and stored in bottles, a one-week summer test was conducted on three other dairies. The results of these tests appear in tables 6 to 12 inclusive, and in figure 15. TABLE 6 Operating Characteristics on Dairy No. 1 Milk cooling plant installed May, 1928. Ice-melting effect per 24 hours of ice machine: 500 pounds. Condenser cooling medium: air. Refrigerant: methyl chloride. Horsepower of motor: % horsepower. Type of storage box: li walk-in, r * made on farm. Inside dimensions of storage box: 4' x 4' x 7'. Insulation of storage box: 3-inch sheet cork and 4 inches rice hulls. Gallons of brine: 120. Kind of brine: calcium chloride. Location of brine tank: suspended 3' from floor inside box. Size and type of brine pump: % inch centrifugal. Horsepower of brine pump motor: 14 horsepower. Size and type of aerator: 18" x 18" two-way. Cost of complete plant: $800. Remarks: Ice machine is automatically controlled. One man did all the milking with a two-unit milking machine. Evening's milk was stored over- night and both milkings were hauled by truck a distance of four miles to the distributing plant in the morning. Number of cows milked Gallons of milk cooled per day Gallons of milk stored per day Hours compressor was run per day Kw.-hr. used by compressor motor Kw.-hr. used by brine pump motor Total kw.-hr. used Average brine temperature, degrees Fahr Degrees rise in brine temperature during cooling degrees Fahr Average storage box temperature, degrees Fahr Average milk room temperature, degrees Fahr Milk temperature before cooling, degrees Fahr Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr Rate of flow of cooling water, g.p.m Rate of flow of milk over aerator, g.p.h Temperature of cooling water, degrees Fahr Aug., 1928 16 54 28 10.3 9.4 .8 10.2 18 8 32 73 95 74 37 1.6 25 Oct., 1928 15 64 34 10.2 9.1 .9 10.0 12 8 28 69 93 68 37 2.2 29 63 Jan., 45 7.8 1.0 April, 1929 23 85 44 7.8 7.4 1.1 8 5 14 7 30 61 95 69 40 1.3 25 62 Average of all tests* 7.5 29 64 38 1.9 27 63 * Nearest whole number. 26 University of California — Experiment Station TABLE 7 Operating Characteristics on Dairy No. 2 Milk cooling plant installed May, 1927. Ice-melting effect per 24 hours of ice machine: 500 pounds. Condenser cooling medium: water. Refrigerant: sulfur dioxide. Horsepower of motor: % horsepower. Type storage box: "non-walk-in," factory made. Inside dimensions of storage box: 3' x 5' x 5'. Insulation of storage box: 4-inch sheet cork. Gallons of brine: 190. Kind of brine: calcium chloride. Location of brine tank: upper portion of storage box. Size and type of brine pump: % inch centrifugal. Horsepower of brine pump motor: % horsepower. Size and type of aerator: two-way 14" wide 24" long. Cost of complete plant: $1150. BemarTcs: The ice machine is automatically operated. The evening's milk is stored and both milkings are shipped by truck in the morning a distance of 27 miles to the distributing plant. During the summer test two men were milking by hand and during the other three tests one man did all the milking with a two-unit milking machine. Aug., 1928 Oct., 1928 Jan., 1929 April, 1929 Average of all tests* Number of cows milked 40 84 42 13.4 7.6 1.4 9.0 30 10 45 79 94 75 44 2.7 22 68 40 79 39 12.2 6 5 1.9 8.4 33 6 43 69 91 72 43 3.0 14 66 23 43 22 5.8 3.4 .9 4 3 30 5 40 56 86 71 50 6.0 18 64 29 75 43 15.3 7.8 1.6 9 4 25 8 40 65 89 73 51 5.8 32 60 33 70 36 11 7 6 3 1.5 7.8 30 Rise in brine temperature during cooling, degrees Fahr. 7 42 67 90 Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr 73 47 4.4 22 65 Nearest whole number. Bul. 495] Milk Cooling on California Dairy Farms 27 TABLE 8 Operating Characteristics on Dairy No. 3 Milk cooling plant installed April, 1928. Ice-melting effect per 24 hours of ice machine: 750 pounds. Condenser cooling medium: air. Eefrigerant: sulfur dioxide. Horsepower of motor: 1% horsepower. Type of storage box: "walk-in," made on the farm. Inside dimensions of storage box: 6' x 6' x 7'. Insulation of storage box: 3-inch sheet cork. Gallons of brine: 150. Kind of brine: calcium chloride. Location of brine tank: suspended 3 feet from floor inside box. Size and type of brine pump: % inch centrifugal. Horsepower of brine pump motor: % horsepower. Size and type of aerator: 30" wide, 18" deep, two-way. Cost of complete plant: $1100. Bernards : The ice machine is automatically controlled. A space 2' x 6' x 7' in the storage box was partitioned off for storing food stuffs. One man did all the milking with a two-unit milking machine. The morning's milk was stored and both milkings were shipped a distance of 60 miles by truck in the evening. Number of cows milked Gallons of milk cooled per day Gallons of milk stored per day Hours compressor wzfs run per day Kw.-hr. used by compressor motor per day... Kw.-hr. used by brine pump motor per day Total kw.-hr. used per day Average brine temperature, degrees Fahr Degrees rise in brine during cooling, degrees Fahr. Average storage box temperature, degrees Fahr Average milk room temperature, degrees Fahr Milk temperature before cooling, degrees Fahr Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr. Rate of flow of cooling water, g.p.m Rate of flow of milk over aerator, g.p.h Temperature of cooling water, degrees Fahr » ... Julv, 1928 Oct., 1928 Jan., 1929 April, 1929 30 30 30 30 71 73 64 74 37 38 35 41 11.9 9 1 3.7 7.6 13.6 10.2 4.1 8.5 .7 .6 .6 .5 14 3 10 8 4.7 9 20 23 25 21 9 6 1 4 37 36 30 35 75 67 45 59 95 94 90 94 79 72 55 68 39 38 38 38 1.7 2.0 2.5 2.5 25 26 26 28 75 66 50 62 Average of all tests* 22 5 35 62 93 69 38 2 2 26 63 * Nearest whole number. 28 University of California — Experiment Station table 9 Operating Characteristics on Dairy No. 4 Milk cooling plant installed March, 1928. Ice-melting effect per 24 hours of ice machine: 750 pounds. Condenser cooling medium: air. Eefrigerant: sulfur dioxide. Horsepower of motor: iy 2 horsepower. Type of storage box : ' ' walk-in, ' ' made on f arm. Inside dimensions of storage box: 6' x 6' x 7'. Insulation of storage box: 6 inches of rice hulls. Gallons of brine: 200. Kind of brine: common salt. Location of brine tank: on floor at one end of the storage box. Size and type of brine pump: % inch centrifugal. Horsepower of brine pump motor: % horsepower. Size and type of aerator: 24" wide, 15" deep, two-way. Cost of complete plant: $1200. Remarks: Ice machine is automatically operated. During the summer and fall tests, three men did the milking by hand. During the winter and spring tests, two men did the milking by hand. The evening's milk was stored over night and both milkings were hauled to the distributing plant. During the year new brushes were put on the compressor motor, new valves put in the compressor, and a new expansion valve installed. Number of cows milked Gallons of milk cooled per day Gallons of milk stored per day Number of hours compressor was run per day Kw.-hr. used by compressor motor per day Kw -hr. used by brine pump motor per day Total kw.-hr. used per day Average brine temperature, degrees Fahr Degrees rise in brine temperature during cooling degrees Fahr Average storage box temperature, degrees Fahr Average milk room temperature, degrees Fahr Milk temperature before cooling, degrees Fahr Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr Rate of flow of cooling water, g.p.m Rate of flow of milk over aerator, g.p.h Temperature of cooling water, degrees Fahr Aug., 1928 Nov., 1928 Feb., 1929 May, 1929 Average of all tests* 80 65 52 69 67 213 120 100 185 155 103 60 50 • 91 76 19 3 12 2 6 6 20.0 14.5 21.5 15 4 5.8 21.1 16.0 1.0 10 .8 1.6 1.1 22 5 16.4 6.6 22.7 17.1 22 20 28 25 24 4 3 4 6 4 43 40 34 43 40 71 58 47 65 60 96 92 91 94 93 65 63 57 64 62 43 35 34 38 38 6.0 4.4 4.0 5.0 4.9 35 22 21 26 26 63 60 56 62 60 Nearest whole number. Bul. 495] Milk Cooling on California Dairy Farms 29 TABLE 10 Operating Characteristics on Dairy No. 5 Milk cooling plant installed August, 1927. Ice-melting effect per 24 hours of ice machine: one ton. Condenser cooling medium: water. Eefrigerant: ammonia. Horsepower of motor: 2 horsepower. Type of storage box: "walk-in, " made on the farm. Inside dimensions of storage box: 5' x 6' x 6%'. Insulation of storage box: 6 inches granulated cork. Gallons of brine: none. Size and type of aerator: 30" wide, 20" deep, two-way direct expansion. Cost of complete plant: $1450. BerrwrJcs: Between the winter and spring tests, the storage box was increased in size to 5' x 12,' x 6' with 2y 2 'x5'x6' partitioned off for food-stuffs, and a 140-gallon brine tank was placed on the floor in one corner of the storage box. It was used only to keep a uniform box temperature and to make ice and not to cool the milk. The ice machine was moved and recharged with 50 pounds of ammonia. During the spring test about 50 pounds of ice was made per day. Three men did all the milking by hand. The evening's milk was stored over- night and both milkings shipped by truck a distance of 10 miles each morning. During the winter test the ice machine was used for cooling the milk only in the evening. Aug., 1928 72 225 112 7.4 7.1 7 1 50 77 95 65 52 8.0 36 64 Oct., 1928 Feb., 1929 May, 1929 Average of all tests* 72 189 97 6.3 6 5 6 5 45 60 94 62 44 8.0 36 61 72 77t 77 3 1 2.8 2.8 42 49 90 59 41 8 31 57 80 235 123 9.7 10.6 10.6 39 67 94 65 44 8.0 41 61 74 Gallons of milk cooled per day 182 Gallons of milk stored per day 102 Hours compressor was run per day 6 6 Kw.-hr. used by compressor motor per day 6 8 Kw.-hr. used by brine pump motor per day o Total kw.-hr. used per day 6 8 Average storage box temperature, degrees Fahr 44 Average milk room temperature, degrees Fahr 63 Milk temperature before cooling, degrees Fahr 93 Milk temperature after water cooled, degrees Fahr Milk temperature after totally cooled, degrees Fahr Rate of flow of cooling water, g.p.m 63 45 8.0 36 61 Rate of flow of milk on aerator, g.p.h , Temperature of cooling water, degrees Fahr * Nearest whole number. t Evening's milking only cooled by ice machine. 30 University of California — Experiment Station TABLE 11 Operating Characteristics on Dairy No. 6 Milk cooling plant installed July, 1927. Ice-melting effect per 24 hours of ice machine: 1.5 tons. Condenser cooling medium: water. Refrigerant : ammonia. Horsepower of motor: 3 horsepower. Type of storage box : ' ' walk-in, ' ' made on farm. Inside dimensions of storage box: 6' x 11' x V. Insulation of storage box: 4 inches of rice hulls. Gallons of brine: 350 gallons. Kind of brine: common salt. Location of brine tank: on floor at one end of box. Size and type of brine pump: % inch centrifugal. Horsepower of brine pump motor: % horsepower. Size and type of aerator: 36" wide, 15" deep, two-way. Cost of complete plant: $1500. BemarTcs : The plant is manually controlled. A space 4' x 6' x 7' was parti- tioned off at one end of the storage box to store food-stuffs. The morning's milk was stored 24 hours, and the evening's milk 12 hours. Both milkings were hauled a distance of 10 miles by truck in the morning to the distributing plant. Three men did the milking by hand. Except during the winter test, about 50 pounds of ice was made a day. The compressor was charged with ammonia twice during the year. The compressor motor burned out once in consequence of the power being turned off and on. Aug., 1928 Oct., 1928 Feb., 1929 May, 1929 Average of all tests* 80 214 214 99 22.9 2 24 9 25 5 40 77 93 73 47 2.0 53 63 75 233 233 7.9 15.8 2.0 17.8 20 2 39 60 92 72 46 2.5 53 61 55 102 102 4.2 8 1 10 9.1 18 32 47 84 65 43 1.0 53 52 95 238 238 8 8 17t 2.5 19.5 20 3 35 67f 92 73 47 1.0 44 62 76 Gallons of milk cooled per day 197 197 7.7 16.0 1.9 17.9 21 Degrees rise in brine temperature during cooling, 3 36 63 90 Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr Rate of flow of cooling water, g.p.m , 71 46 1.4 Rate of flow of milk over aerator, g.p.h 50 Temperature of cooling water, degrees Fahr 60 * Nearest whole number. t Estimated. Bul. 495] Milk Cooling on California Dairy Farms 31 Operating Characteristics on Dairies 7, Observation Was Made TABLE 12 , and 9 on Which Only a Summer Tests, July, 1929 Milk cooling plant installed Ice-melting effect per 24 hours of ice machine Condenser cooling medium Refrigerant Horsepower of motor Type of storage box Inside dimensions of storage box Gallons of brine Insulation of brine tank or storage box Kind of brine Location of brine tank Size and type of brine pump motor.. Horsepower of brine pump motor Size of aerator Cost of complete plant Dairy No. 7 March, 1929. 1000 pounds... Water S0 2 1 horsepower. None 200 3" sheet cork Calcium chloride.. Special insulated box Yi" centrifugal x /i horsepower 30"x24" $900 Dairy No. 8 500 pounds Water S0 2 Yi horsepower. None 160 4" shavings Calcium chloride.. Special insulated box ¥2" centrifugal 14. horsepower 18"x24" Dairy No. 9 May, 1929. 500 pounds. Water. SO2. Yi horsepower. "Walk-in." 5' x 5' x 8' 120. 2" celotex. Calcium chloride. Upper half of stor- age box. l A" centrifugal. 34 horsepower. 24" x 18". Number of cows milked Gallons of milk cooled per day Gallons of milk stored per day Number of hours compressor was run per day Kw.-hr..used by compressor motor Kw.-hr. used by brine pump motor Total kw -hr. used per day Average brine temperature, degrees Fahr Degrees rise in brine temperature during cooling, degrees Fahr. Average storage box temperatures, degrees Fahr Average milk room temperature, degrees Fahr Milk temperature before cooling, degrees Fahr Milk temperature after water cooled, degrees Fahr Milk temperature after brine cooled, degrees Fahr Rate of flow of cooling water, g.p.m Rate of flow of milk over aerator, g.p.h Temperature of cooling water, degrees Fahr Dairy No. 7 95 318 14 5 16.0 13 17.3 29 16 79 96 73 47 4.0 83 Dairy No. 8 55 124 17.5 10.7 1.0 11.7 20 15 77 96 73 41 3.2 25 Dairy No. 9 18 45 22 16.1 9.3 .5 9.8 22 8 46 77 93 75 43 2.5 22 67 32 University of California — Experiment Station SUMMARY AND RECOMMENDATIONS? Specific heat of milk 93 Heat removed in cooling one gallon of milk 1 deg. Fahr 8.00 B.t.u. Temperature of milk as it is started over aerator in warm weather .... 93°-98° Fahr. Temperature of milk as it is started over aerator in cold weather 85°-90° Fahr. Diameter of a 10-gallon milk can 13H inches Height of a 10-gallon milk can 24 inches Weight of a 10-gallon milk can 18-23 pounds Size of a 12-quart-bottle crate 14 x 18 x 11 inches Weight of a 12-quart-bottle crate 12 to 15 pounds Weight of a quart bottle (empty) \ l A to 2 pounds 1. Milk is cooled to retard bacterial increase and preserve quality. 2. Cooling: milk on the dairy farm is required by state and city regulations for the production of market milk. 3. Mechanical refrigerating machines are an efficient and eco- nomical means of accomplishing this cooling, if equipment of the proper size and type is used. 4. Two types of milk cooling plants are in use, the 'wet' and the 'dry.' The 'dry' type, used almost entirely in California, may be either a brine or direct expansion system. 5. Milk, after being drawn from the cow, is usually handled on the farm in one of three ways: cooled and stored in 10-gallon cans and shipped to the distributing plant once a day; cooled and stored in bottles and delivered once a day; or cooled and shipped to the dis- tributing plant after each milking. 6. The electric power consumption for the operation of a farm milk cooling plant depends upon the method of handling the milk, the time of the year, the type of plant or system, and the construction and operation of the plant. a. When the milk is cooled and stored in 10-gallon cans, the brine system being used, about .12 kw-hr. is consumed per gallon of milk. b. When the milk is cooled and stored in bottles the brine sys- tem being used, .17 kw-hr. is consumed per gallon of milk. c. When the milk is cooled and not stored, the brine system being used, about .09 kw-hr. is consumed per gallon of milk. d. When cooled and stored or not stored, the direct expansion system being used, about .05 kw-hr. is consumed per gallon of milk. 7 For convenience in calculating, a table of weights, temperatures and measurements is given herewith: Bul. 495] Milk Cooling on California Dairy Farms 33 7. Estimating electric power at 2 cents per kilowatt-hour, depre- ciation at 10 per cent, interest at 7 per cent, and upkeep at 3 per cent per annum, a milk-cooling plant, will cost from % cent to 1 cent per gallon cooled, depending on the size and type of plant and the method of handling the milk. 8. Recommendations for the selection and operation of a 'dry' type of farm milk cooling plant : a. The profit that is to be gained by the use of such a plant should be determined and the dairy inspector in charge consulted on the requirements to be met for producing the grade of milk desired. b. In purchasing the equipment one should obtain as much information as possible on the different makes and types and make a decision on the relative merits, the size of units, the price, the reputation of the manufacturer and dealer, and the service obtainable. c. Efficiency and length of life are as important as initial cost. Poor equipment usually means poor efficiency, and although the initial cost may be lower, the saving will be offset by increased operating cost, depreciation, and upkeep. d. Future increase in quantity of milk to be cooled should be considered, as it is sometimes more economical to take care of future needs by installing the original plant larger than necessary at the time. Selection of proper size of refrigerating machine is very essential. Recommended sizes are given in table 13. TABLE 13 Size of Compressor for Handling Different Quantities of Milk Size of compressor* Horsepower of compressor motor Gallons of milk cooled and stored per day using brine system Gallons of milk per day when cooled only, using brine system Gallons of milk per day when cooled and In pounds ice-melting effect per 24 hours In tons of ice-melting effect per 24 hours With water- cooled con- denser With air-cooled condenser stored, or cooled only, using direct expansion system 500 750 1000 1500 2000 3000 % M l i m 2 3 l m 2 3 50-100 100-150 150-200 200-275 275-350 350-400 75-125 125-200 200-250 250-325 325-400 400-450 50- 75 75-100 100-150 150-200 200-250 250-300 * If it is desired to make ice, the capacity of the compressor should be increased by approximately twice the quantity of ice it is desired to make per day. 34 University of California — Experiment Station /. If a storage box is needed, a floor space of 15 x 15 inches should be allowed for each 10-gallon container that is to be stored. Where the milk is stored in quart bottle cases, a space 16 x 20 x 12 inches should be allowed for each case. g. The equivalent of 3 to 4 inches of sheet cork should be used for insulation of the brine tank or storage box. h. If the brine system is used the tank should contain from 1.5 to 2 gallons of brine for each gallon of milk cooled per day. i. A ^-inch centrifugal brine pump driven by a % -horsepower motor is sufficient for circulating the brine where less than 200 gallons per day are cooled. A %-inch centrifugal pump driven by a %-horsepower motor is sufficient where 200 to 400 gallons of milk are cooled per day. j. The aerator should be of the two-way type, with water pass- ing through the upper half and brine through the lower half. Approximately 6 inches of horizontal length should be allowed for each 10 gallons of milk cooled per hour. k. The different units of the plant should be arranged so as to require a minimum of labor for cleaning, operating, and repairing. I. The dairyman should learn the various adjustments neces- sary for correct and efficient operation of the plant. It is good practice to keep a thermometer available and check the tempera- tures occasionally. m. The milk should be cooled to about 10° Fahr. below the tem- perature at which it is to be delivered to the distributing plant. About 40° Fahr. is the usual desired temperature of cooling over the aerator. n. The storage box temperature should be kept a few degrees lower than the temperature to which the milk is cooled. ACKNOWLEDGMENTS The authors wish to express their appreciation to the various dairy farmers, power companies, refrigerating machine agencies, and dairy inspectors throughout the state who aided in obtaining the data ; also to the members of the Agricultural Engineering Division who aided in preparing the manuscript. STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION BULLETINS No. 253. 263. 277. 279. 283. 304. 310. 313. 331. 335. 343. 344. 346. 347. 348. 349. 353. 354. 357. 361. 362. 363. 364. 366. 367. 368. 369. 370. 371. 373. 374. 380. 385. 386. 388. 389. 390. 391. 392. 393. 394. 395. 396. 397. 400. 405. 406. 407. Irrigation and Soil Conditions in the Sierra Nevada Foothills, California. Size Grades for Ripe Olives. Sudan Grass. Irrigation of Rice in California. The Olive Insects of California. A Study of the Effects of Freezes on Citrus in California. Plum Pollination. Pruning Young Deciduous Fruit Trees. Phylloxera-resistant stocks. Cocoanut Meal as a Feed for Dairy Cows and Other Livestock. Cheese Pests and Their Control. Cold Storage as an Aid to the Market- ing: of Plums, a Progress Report. Almond Pollination. The Control of Red Spiders in Decid- uous Orchards Pruning Young Olive Trees. A Studv of Sidedraft and Tractor Hitches. Bovine Infectious Abortion, and Asso- ciated Diseases of Cattle and New- born Calves. Results of Rice Experiments in 1922. A Self-Mixing Dusting Machine for Applying Dry Insecticides and Fun- gicides. Preliminary Yield Tables for Second Growth Redwood. Dust and the Tractor Engine. The Pruning of Citrus Trees in Cali- fornia. Fungicidal Dusts for the Control of Bunt. Turkish Tobacco Culture, Curing, and Marketing. Methods of Harvesting and Irrigation in Relation to Moldy Walnuts. Bacterial Decomposition of Olives During Pickling. Comparison of Woods for Butter Boxes. Factors Influencing the Development of Internal Browning of the Yellow Newtown Apnle. The Relative Cost of Yarding Small and Large Timber. Pear Pollination. A Survey of Orchard Practices in the Citrus Industry of Southern Cali- fornia. Grqwth of Eucalyptus in California Plantations. Pollination of the Sweet Cherry. Pruning Bearing Deciduous Fruit The Principles and Practice of Sun- Drying Fruit. Berseem or Egyptian Clover. Harvesting and Packing Grapes in California. Machines for Coating Seed Wheat with Copper Carbonate Dust. Fruit Juice Concentrates. Crop Sequences at Davis. I. Cereal Hay Production in California. II. Feeding Trials with Cereal Hays. Bark Diseases of Citrus Trees in Cali- fornia. The Mat Bean, Phaseolus Aconitifolius. Manufacture of Roquefort Type Cheese from Goat's Milk. The Utilization of Surplus Plums. Citrus Culture in Central California. Stationary Spray Plants in California. Yield, Stand, and Volume Tables for White Fir in the California Pine Region. No. 408. 409. 410. 412. 414. 415. 416. 418. 419. 420. 421. 423. 425. 426 427. 428. 430. 431. 432.. 433. 434. 435. 436. 438. 439. 440. 444. 445. 446. 447. 448. 449. 450. 451. 452. 453. 454. Alternaria Rot of Lemons. The Digestibility of Certain Fruit By- products as Determined for Rumi- nants. Part I. Dried Orange Pulp and Raisin Pulp. Factors Influencing the Quality of Fresh Asparagus After it is Harvested. A Study of the Relative Value of Cer- tain Root Crops and Salmon Oil as Sources of Vitamin A for Poultry. Planting and Thinning Distances for Deciduous Fruit Trees. The Tractor on California Farms. Culture of the Oriental Persimmon in California. A Study of Various Rations for Fin- ishing Range Calves as Baby Beeves. Economic Aspects of the Cantaloupe Industry. Rice and Rice By-Products as Feeds for Fattening Swine. Beef Cattle Feeding Trials, 1921-24. Apricots (Series on California Crops and Prices). Apnle Growing in California. Apple Pollination Studies in California. The Value of Orange Pulp for Milk Production. The Relation of Maturity of California Plums to Shipping and Dessert Quality. Range Grasses in California. Raisin By-Products and Bean Screen- ings as Feeds for Fattening Lambs. Some Economic Problems Involved in the Pooling of Fruit. Power Requirements of Electrically Driven Dairy Manufacturing Equip- ment. Investigations on the Use of Fruits in Ice Cream and Ices. The Problem of Securing Closer Rela- tionship between Agricultural Devel- opment and Irrigation Construction. I. The Kadota Fig. II. The Kadota Fig Products. Grafting Affinities with Special Refer- ence to Plums. The Digestibility of Certain Fruit By- Products as Determined for Rumi- nants. II. Dried Pineapple Pulp, Dried Lemon Pulp, and Dried Olive Pulp. The Feeding Value of Raisins and Dairy By-Products for Growing and Fattening Swine. Series on California Crops and Prices : Beans. Economic Aspects of the Apple In- dustry. The Asparagus Industry in California. A Method of Determining the Clean Weights of Individual Fleeces of Wool. Farmers' Purchase Agreement for Deep Well Pumps. Economic Aspects of the Watermelon Industry. Irrigation Investigations with Field Crops at Davis, and at Delhi, Cali- fornia, 1909-1925. Studies Preliminary to the Establish- ment of a Series of Fertilizer Trials in a Bearing Citrus Grove. Economic Aspects of the Pear Industry. Series on California Crops and Prices: Almonds. Rice Experiments in Sacramento Val- ley, 1922-1927. BULLETINS — ( Continued) No. No. 455. Reclamation of the Fresno Type of 465. Black-Alkali Soil. 466. 456. Yield, Stand and Volume Tables for Red Fir in California. 467. 458. Factors Influencing Percentage Calf 468. Crop in Range Herds. 459. Economic Aspects of the Fresh Plum 469. Industry. 470. 460. Series on California Crops and Prices: Lemons. 471. 461. Series on California Crops and Prices: Economic Aspects of the Beef Cattle 474. Industry. 462. Prune Supply and Price Situation. 464. Drainage in the Sacramento Valley 475. Rice Fields. Curly Top Symptoms of the Sugar Beet. The Continuous Can Washer for Dairy Plants. Oat Varieties in California. Sterilization of Dairy Utensils with Humidified Hot Air. The Solar Heater. Maturity Standards for Harvesting Bartlett Pears for Eastern Shipment. The Use of Sulfur Dioxide in Shipping Grapes. Factors Affecting the Cost of Tractor Logging in the California Pine Region. Walnut Supply and Price Situation. CIRCULARS No. 115. Grafting Vinifera Vineyards. 117. The Selection and Cost of a Small Pumping Plant. 127. House Fumigation. 129. The Control of Citrus Insects. 164. Small Fruit Culture in California. 166. The County Farm Bureau. 178. The Packing of Apples in California. 203. Peat as a Manure Substitute. 212. Salvagine Rain-Damaged Prunes. 230. Testing Milk. Cream, and Skim Milk for Butterfat. 232. Harvesting and Handling California Cherries for Eastern Shipment. 239. Harvesting and Handling Apricots and Plums for Eastern Shipment. 240. Harvesting and Handling California Pears for Eastern Shipment. 241. Harvesting and Handling California Peaches for Eastern Shipment. 243. Marmalade Juice and Jelly Juice from Citrus Fruits. 244. Central Wire Bracing for Fruit Trees. 245. Vine Pruning Systems. 248. Some Common Errors in Vine Pruning and Their Remedies. 249. Replacing Missing Vines. 250. Measurement of Irrigation Water on the Farm. 253. Vineyard Plans. 255. Leguminous Plants as Organic Ferti- lizers in California Agriculture. 257. The Small-Seeded Horse Bean (Vicia faba var. minor). 258. Thinning Deciduous Fruits. 259. Pear By-Products. 261. Sewing Grain Sacks. 262. Cabbage Production in California. 263. Tomato Production in California. 265. Plant Disease and Pest Control. 266. Analyzing the Citrus Orchard by Means of Simple Tree Records. No. 269. An Orchard Brush Burner. 270. A Farm Septic Tank. 276. Home Canning. 277. Head. Cane, and Cordon Pruning of Vines. 278. Olive Pickling in Mediterranean Countries. 279 The Preparation and Refining of Olive Oil in Southern Europe. 282. Prevention of Insect Attack on Stored Grain. 284. The Almond in California. 287. Potato Production in California. 288. Phylloxera Resistant Vineyards. 289. Oak Fungus in Orchard Trees. 290. The Tangier Pea. 292. Alkali Soils. 294. Propagation of Deciduous Fruits. 295. Growing Head Lettuce in California. 296. Control of the California Ground Squirrel. 298. Possibilities and Limitations of Coop- erative Marketing. 300. Coccidiosis of Chickens. 301. Buckeye Poisoning of the Honey Bee. 302. The Sugar Beet in California. 304. Drainage on the Farm. 305. Liming the Soil. 307. American Foulbrood and Its Control. 308. Cantaloupe Production in California. 309. Fruit Tree and Orchard Judging. 310. The Operation of the Bacteriological Laboratory for Dairy Plants. 311. The Improvement of Quality in Figs. 312. Principles Governing the Choice. Oper- ation and Care of Small Irrigation Pumping Plants. 313. Fruit Juices and Fruit Juice Beverages. 314. Termites and Termite Damage. 315. The Mediterranean and Other Fruit Flies. 15m-6,'30