Generator-field control employs an individual generator for each elevator, with the voltage applied to a dc driving-machine motor adjusted by varying the strength and direction of the generator field. Multivoltage control impresses successively on the armature of the drivingmachine motor various fixed voltages, such as those that might be obtained from multicommutator generators common to a group of elevators. Rheostatic control varies the resistance or reactance of the armature or the field circuit of the driving-machine motor. Single-speed, alternating-current control governs a driving-machine induction motor that runs at a specified speed. Two-speed alternating-current control governs a two-speed driving-machine induction motor, with motor windings connected to obtain various numbers of poles. Dispatching Drive. A device that operates a signal in a car to indicate when the car should leave a designated floor or to actuate the cars starting mechanism when the car is at a designated floor. Driving Machine. See Machine. Emergency Stop Switch. A car-located device that, when operated manually, causes the car to be stopped by disconnecting electric power from the drivingmachine

Copper 2xxx Manganese 3xxx Silicon 4xxx Magnesium 5xxx Magnesium and silicon 6xxx Zinc 7xxx Other elements 8xxx Unused series 9xxx alloys to indicate the compositions and the tempers of the various metals. Wrought alloys are designated by a four-digit index system. 1xxx is for 99.00% aluminum minimum. The last two digits indicate the minimum aluminum percentage. The second digit represents impurity limits. (EC is a special designation for electrical conductors.) 2xxx to 8xxx represent alloy groups in which the first number indicates the principal alloying constituent, and the last two digits are identifying numbers in the group. The second digit indicates modification of the basic alloy. The alloy groups are listed in Table 4.20. For cast alloys, a similar designation system is used. The first two digits identify the alloy or its purity. The last digit, preceded by a decimal point, indicates the form of the material; for example, casting or ingot. Casting alloys may be sand or permanent-mold alloys. Among the wrought alloys, the letter F, O, H, W, and T indicate various basic temper designations. These letters in turn may be followed by numerals to indicate various degrees of treatment. Temper designations are summarized in Table 4.21. The structural alloys general employed in building fall in the 2xxx, 5xxx, and 6xxx categories. Architectural alloys often used include 3xxx, 5xxx, and 6xxx

Total internal sensible load 9,697 Fresh air (sensible load), 77  1.08  15 1,250 Total sensible load 10,947 Occupants (latent load), 5  255 1,275 Fresh air (latent load), 77  0.67(99
77) 1,140 Total latent load 2,415 Total load 13,362 TABLE 13.16 Cooling-Load Analysis for Building in Fig. 13.3 (p. 13.43) Space Tons Flow with ducted fresh air, ft3 /min Flow without ducted fresh air, ft3 /min First-floor store 7.25 2842 3052 First-floor office 0.35 137 164 Second-floor office No. 1 3.50 1737 1887 Second-floor office No. 2 1.13 499 563 Cooling and supply-air requirements for the building are summarized in Table Figure 13.29 shows the basic air-conditioning cycle of the direct-expansion type. The compressor takes refrigerant gas at a relatively low pressure and compresses it to a higher pressure. The hot gas is passed to a condenser where heat is removed and the refrigerant liquefied. The liquid is then piped to the cooling coil of the airhandling unit and allowed to expand to a lower pressure (suction pressure). The liquid vaporizes or is boiled off by the relatively warm air passing over the coil. FIGURE 13.29 Direct-expansion air-conditioning cycle. The compressor pulls away the vaporized refrigerant to maintain the required low coil pressure with its accompanying low temperature. Chilled-Water Refrigeration Cycle. In some systems, water is chilled by the refrigerant and circulated to units in or near spaces to be cooled (Fig. 13.30), where air is cooled by the water. In water-cooled and belt-driven air-conditioning compressors formerly used, motor winding heat was usually dissipated into the atmosphere outside the conditioned space (usually into the compressor rooms). For average air-conditioning service 1 hp could produce about 1 ton of cooling. When sealed compressor-motor units came into use, the motor windings were arranged to give off their heat to the refrigerant suction gas. This heat was therefore added to the cooling load of the compressor. The result was that 1 hp could produce only about 0.85 ton of cooling. Because of water shortages, many communities restrict the direct use of city water for condensing purposes. As an alternative, water can be cooled with cooling towers and recirculated. But for smaller systems, cooling towers have some inherent disadvantages. As a result, air-cooled condensers have become common for smalland medium-sized air-conditioning systems. Because of the higher head pressures resulting from air-cooled condensers, each horsepower of compressor-motor will produce only about 0.65 ton of cooling. Because of these developments, airconditioning equipment manufacturers rate their equipment in Btu per hour output and kilowatts required for the rated output, at conditions standardized by the industry, instead of in horsepower. FIGURE 13.30 Chilled-water air-conditioning cycle. The air-distribution system is the critical part of an air-conditioning system. If insufficient air is circulated, proper cooling cannot be done. On the other hand, handling large quantities of air is expensive in both initial cost and operation. The amount of cool air required increases rapidly the closer its temperature is brought to the desired room temperature. If, for example, we wish to maintain 80
F DB (dry-bulb) in a room and we introduce air at 60
F, the colder air when warming up to 80
F will absorb an amount of sensible heat equal to qs . According to Eq. (13.30), qs  1.08Q1(80
60)  21.6Q1, where Q1 is the required airflow in cubic feet per minute. From Eq. (13.30), it can be seen also that, if we introduce air at 70
F, with a temperature rise of 10
F instead of 20
F, qs  10.8Q2, and we shall have to handle twice as much air to do the same amount of sensible cooling. From a psychrometric chart, the dew point of a room at 80
F DB and 50% relative humidity is found to be 59
F. If the air leaving the air-conditioning unit is 59
F or less, the duct will sweat and will require insulation. Even if we spend the money to insulate the supply duct, the supply grilles may sweat and drip. Therefore, theoretically, to be safe, the air leaving the air-conditioning unit should be 60
F or