CHAPTER I REFRIGERATION UNITS AND THEORY
Heat Unit— A heat unit is an arbitrary standard or unit of measurement which expresses the capacity of a given body to absorb and retain heat energy under a given increase of its sensible heat. Water has a greater heat capacity than ahnost any other common substance and it has been used in framing the definition of a heat unit.
British Thermal Unit.— A British thermal unit (B.t.u.) is the quantity of heat required to raise the temperature of one pound of pure v^ater one degree Fahrenheit at or near its temperature of maximum density, 39.1° F. For practical work it may be considered as the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.
Sensible Heat.—Sensible heat is the heat which goes to increase the temperature of a body without afifecting its state, whether it be that of a solid, liquid or gas. Thus the addition of sensible heat to a body may be felt by the hand or be indi- cated by a thermometer.
Latent Heat.— Latent heat is the amount of heat that must be supplied to a body to change its state from a solid to a liquid, or from a liquid to change it to a gas. This heat sepa- rates the molecules of the substance and cannot be indicated by a thermometer since it produces no change in temperature. Every substance has a latent heat of fusion, required to con- vert it from a solid to a liquid, and another, a latent heat of vaporization required to convert it from a liquid to a gas or vapor. Experiments have shown that it requires 144 B.t.u.
11
12 HOUSEHOLD REFRIGERATION
to melt one pound of ice at 32° F. into one pound of water at 32° F.; thus we have 144 B.t.u. as the latent heat of fusion of ice.
If heat is applied to one pound of water at 212° F. the water will remain at this temperature under atmospheric pressure until all of it has been evaporated into steam at 212° F. This has been found to require 970.4 B.t.u.; therefore, 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 is the ratio of its heat capacity to that of water. One pound of water requires one B.t.u. to raise its temperature one degree F. One pound of cast iron requires only 0.13 B.t.u. Therefore, the specific heat of cast iron is 0.13. The specific heat of ice is 0.504; of air, 0.240, of anhydrous ammonia, 1.10. The specific heat of materials usually stored in a refrigerator averages about 0.80.
Refrigeration. — Refrigeration is a term used to represent the cold produced or rather the amount of heat removed. It is measured by the latent heat of fusion of ice. 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° F. into water at the same temperature.
One ton of refrigeration is equal to 144x2,000 or 288,000 B.t.u. per 24 hours, or 12,000 Bt.u. per hour or 200 B.t.u. ])cr minute.
Absolute Pressure. — Absolute pressure is the pressure reckoned from a complete ^•acuunl. Gauges in common use indicate the pressure, in pounds per square inch, above atmos- pheric which is 14.7 at sea level ; this reading is called gauge pressure. To convert gauge pressure to absolute pressure, 14.7 pounds, per square inch, must be added to the gauge reading.
Absolute Zero. — Absolute zero is the point at which mole- cules lose all motion ; in other words, the temperature at which
REFRIGERATION UNITS AND THEORY 13
there is an absence of all heat. This temperature has not been reached but is assumed to be 460 degrees below 0° F.
Mechanical Equivalent of Heat.— The mechanical equiva- lent of heat has been determined by accurate experiment. If the heat energy represented by one B.t.u. be changed into mechanical energy without loss, it would accomplish 778 foot- pounds of work. One hp. represents 42.416 B.t.u. per minute.
Refrigerating Machine Capacity Rating. — In December 1920, the A. S. R. E. and A. S. M. E. adopted a standard method for rating the capacit}^ of any refrigerating machine which is concisely as follows :
"The capacity of any refrigerating machine shall be ex- pressed in terms of 2,000 lbs. ice melting effect for 24 hours (288,000 B.t.u.) with 5° F. saturation temperature in the suc- tion side and 86° F. saturation temperature at the discharge side."
Heat and Temperature. — Heat is a form of molecular energy. All bodies are composed of large numbers of ex- tremely minute particles, known as molecules. These mole- cules have an attraction for each other, which is greater in solids than in liquids and greater in licjuids than in gases. These molecules are in a state of continuous and irregular motion, the rate of which depends upon the temperature, be- ing more rapid at higher temperatures. Absolute zero is sup- posed to represent the condition of matter where there is no kinetic energy of the molecules, and therefore no temperature. Absolute zero is — 460° F., or —273° C.
Heat, being a form of energy, may be converted into elec- trical, chemical or mechanical energy. The two terms, heat and temperature, are frequently confused. Heat is a measure of quantity. Two pieces of iron may have the same tempera- ture, however if one piece is larger than the other it will con- tain a larger quantity of heat. A cake of ice may contain more heat than a smaller quantity of boihng water. Heat is con- stantly passing from warmer objects to colder ones, just as water always tends to flow down hill. There is no natural process in which heat passes from a colder to a warmer object without the expenditure of outside work.
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Temperature is a term used to denote the degree of hotness or coldness of a body and as explained above, it depends upon the amount of sensible heat contained in the body. Since our sensation of warmth and cold is not sufficiently accurate and trustworthy for technical purposes the physical change of ex- pansion of a mercury, for example, accompanying its change in temperature has been agreed upon as a method of measur- ing temperature.
Theory of Refrigeration. — Refrigeration implies the reduc- tion of the temperature of a body below the surrounding en- vironment temperature. It further implies the maintaining of this temperature difference. This requires the constant extraction of heat from the space in which the temperature is already lower than the surrounding environment temperature.
Example. — The food compartment of a refrigerator is being maintained at a temperature of 45° F., and the room tempera- ture is 70°. The refrigerator will continually absorb heat from the room. It is therefore necessary to "pump" this heat out of the refrigerator, as well as the heat supplied by placing rela- tively warm food or containers inside the refrigerator. To extract this heat from the 45° F. food compartment, it is neces- sary to have a still colder object such as a cake of ice, a brine tank, or cooling coil to continually absorb heat. The ice melts and the heat in the refrigerator is used to supply the latent heat necessary to change ice into water. With a brine tank in which are immersed the evaporator coils, the heat in the re- frigerator is used to vaporize the liquid refrigerant in the coils, and a small amount to superheat the gaseous refrigerant, after being vaporized. The refrigerant is then compressed, and this heat passes into the condensing medium which is usually water or air.
Refrigeration Constants. — A number of the commonly used refrigeration constants are shown in Tables I to IX inclusive. Table I contains the interrelation of tons of refrigeration, pounds of refrigeration, and heat units (B.t.u.).
Table II gives the units of refrigeration, tons of refrigera- tion, and pounds of refrigeration expressed in B.t.u. per day, hour, minute and second. Due to the fact that the British
REFRIGERATION UNITS AND THEORY 15
ton is 2,240 pounds, the corresponding British ton of refrigera- tion is therefore equal to 2.240X144=318,080 B.t.u. The cor- responding American ton of refrigeration, 2,000X144=288,- 000 B.t.u.
Table III gives the tons of refrigeration required per ton of ice made when approximately 20 per cent is allowed for the losses occurring in the ice freezing process. Some of the common properties of ice are given in Table IV, while the weights of water per cubic foot and per gallon are given in Table V. Table VI contains some useful hp. equivalents. Some of the useful atmospheric pressure equivalents are given in Table VII. Some average weights of cork insulation are given in Table VIII. The heat transmission through one square foot of surface is found by dividing the total heat in B.t.u. transmitted per hour by the production of the mean temperature difference, and the heat transfer rate expressed in B.t.u. per square foot per degree of temperature difference per hour. Some of the fixed points in thermometry and other tem- peratures are given in Table IX.
TABLE I. CONVERSION FACTORS
^ Tons Pounds K.t.u.
Ton of Refrigeration 1 0.0005 O.GG0003507
Pound of Refrigeration 2,000 1 0 007014
B.tu 288,000 144 1
TABLE II. TONS AND POUNDS OF REFRIGERATION
1 Ton Refrigeration = 288000 B.t.u. per day 1 Ton Refrigeration = 12000 B.t.u. per hour 1 Ton Refrigeration = 200 B.t.u. per minute 1 Ton Refrigeration = 3j^ B.t.u. per second 1 Pound Refrigeration = 144 B.t.u. per day 1 Pound Refrigeration = 6 B.t.u. per hour
1 Pound Refrigeration = 0.1 B.t.u. per minute 1 Pound Refrigeration = .001^ B.t.u. per second
TABLE III. — RELATION OF REFRIGERATION TONNAGE TO ICE MAKING
Temperature of Condensing Tons Refrigeration
Water degrees F. Per ton ice making
50 1.46
60 1.53
70 1.60
80 1.67
90 1.74
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TABLE IV. PROPERTIES OF ICE
Weight per cubic foot 57.5 pounds
Specific Heat 0.504 B.t.u.
Latent Heat 144 B.t.u.
TABLE V. — WEIGHT OF WATER
Weight per cubic foot 62.5 pounds
Weight per gallon 8.35 pounds
TABLE VI. — liORSEPOWER EQUIVALENTS
One mechanical horsepower = 33,000 foot pounds per minute One mechanical horsepower = 2545. B.t.u. per hour One mechanical horsepower = 746. watts
TABLE VII. — ATMOSPHERIC PRESSURE EQUIVALENTS
One Atmosphere = 14.67 pounds per sq. in. One Atmosphere = 33,9 feet of water One Atmosphere = 29.92 inches of m^ercury
TABLE VIII. — CORK INSULATION DATA
Weight per cubic foot, granulated = 6.5 pounds Weight per cubic foot, regranulated = 8.0 pounds Weight per cubic foot, corkboard = 12.0 pounds B.t.u. heat leakage of one square foot corkboard 6.5 X temp, difference
per 24 hours =
Thickness in inches
TABLE IX. FIXED POINTS IN THERMOMETRY
Fehrenheil Degrees
Absolute zero (theoretical) —460°
Mercury freezes —38°
Water freezes -{-32°
Household refrigerator (ideal temperature) 40° to 50°
Room temperature 68° to 70°
Pasteurizing milk 145°
Water boils 212°