to the steam at 212°F, the steam is superheated to300°F.TOTAL HEATTOTAL HEAT is the sum of sensible heat andlatent heat. Since measurements of the total heat in acertain weight of a substance cannot be started atabsolute zero, a temperature is adopted at which it isassumed that there is no heat; and tables of data areconstructed on that basis for practical use. Data tablesgiving the heat content of the most commonly usedrefrigerants start at 40°F below zero as the assumedpoint of no heat; tables for water and steam start at 32°Fabove zero. Tables of data usually contain a notationshowing the starting point for heat contentmeasurement.DAY-TON OF REFRIGERATIONA day-ton of refrigeration (sometimes incorrectlycalled a ton of refrigeration) is the amount ofrefrigeration produced by melting 1 ton of ice at atemperature of 32°F in 24 hours. A day-ton is oftenused to express the amount of cooling produced by arefrigerator or air-conditioner. For example, a 1-tonair-conditioner can remove as much heat in 24 hours as1 ton of 32°F ice that melts and becomes water at 32°F.Figure 6-2.—Relationship between temperature and theamount of heat required per pound (for water at atmosphericpressure).It is a rate of removing heat, rather than a quantity ofheat. A rate can be converted to Btu per day, hour, orminute. To find the rate, proceed as follows:Per Day: Multiply 2,000 (number of pounds ofice in 1 ton) by 144 (latent heat of fusion perpound) = 288,000 Btu per dayPer Hour: 288,000 (Btu per day) ÷ 24 (hours in aday) = 12,000So, a "1-ton" air-conditioner would have a ratingof 12,000 Btu per hour.PRESSUREPRESSURE is defined as a force per unit area. It isusually measured in pounds per square inch (psi).Pressure may be in one direction, several directions, orin all directions, as shown in figure 6-3. The ice (solid)exerts pressure downward. The water (fluid) exertspressure on all wetted surfaces of the container. Gasesexert pressure on al I inside surfaces of their containers.Pressure is usually measured on gauges that haveone of two different scales. One scale is read as somany pounds per square inch gauge (psig) andindicates the pressure above atmospheric pressuresurrounding the gauge. The other type of scale is readas so many pounds per square inch absolute (psia) andindicates the pressure above absolute zero pressure (aperfect vacuum).Atmospheric PressureAtmospheric pressure is the pressure of the weightof air above a point on, above, or under the earth. Atsea level, ATMOSPHERIC PRESSURE is 14.7 psia,as shown in figure 6-4. As one ascends, theatmospheric pressure decreases about 1.0 psi for every2,343 feet. Below sea level in excavations anddepressions, atmospheric pressure increases.Pressures underwater differ from those under air onlybecause the weight of the water must be added to thepressure of the air.Figure 6-3.—Exertion of pressures.6-4

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