Document MMGeRERk86XkMBLBbq0mQw2Qx

876 CHAPTER 57 1965 Guide And Dota.Book ings-Theyare available for.the smaller sizes of single-phase and ! polyphase motors up to. approximately 7M ;hp. Herr metically-eealed thermal protectors are used for protection of hermetic compressor motors of the same razes for mounting made the hermetic-shell. ' For-complete discussion1 of- motor protection, refer to the section Motor Control' at' the beginning of this chapter. ' ni Present-day electrical equipment is highly reliable, when control and-protection devices are kept clean and-dry; Silver contacts are universally..used and may-become black ened, but the ta-intsh does:not normally interfere with the current-carrying .capacity:; -Hie rough surface resulting from arcing is not harmful. In controllers, contacts should be re^ placed only when worn out;. . . Thermal 'and magnetic relays can be checked for tripping. by increasing-the; current through the heater, or-magnetic coil by a resistor connected across the motor terminals.iWhen the current is increased to approximately twice'the normal rating, the relay should trip in a few minutes. - For starting relays refer-to sections Domestic Refrigera- tion and Room Air Conditioners^ SMALL COMMERCIAL REFRIGERATION Practically all motors used, in small refrigeration applica tions are-below ,5 hp. Compressor motors are;usually her metically sealed. Motor control for self-contained as well as remote.condensing units depends upon type and size of, the installation.. Motors . * ; Single-phase, motors are _ most' common. Capacitor-start induction-run motors are used tip to hp, because of starting torque, requirements..' Capacitor-start capacitor-run motors are'used for larger 'sizes up to 6 hp, 'because;they provide high starting torque as well as full load efficiency and power factor: Perfo'nnance'data.aregiven in Table 4. ' * Shaded-pole and split-phasemotors are used for air moving ' devices, with split-phase motore.used in the higher horsepower ' range.*' 'J ' \\ -` Motor Control .. Automatic temperature or.pressure control equipment, de- signed for the special requirements of' commercial refrigera* tion, is almost'universally, used. Automatic controllers, for, motors up to 2 hp are similar.to those used for room air con^, ditioners. For larger motors and for most remotely mounted condensing units, magnetic contactors similar to those used on central air conditioners are employed. .. Motor Protection - . -r Protection equipment for gmaJl single-phase motors is: the same as that used in domestic refrigeration and for room air conditioners. Either thermal-protectors of'the open'type, located on'the shell 'of* the hermetically-sealed compressor; or sealed protectors,1 installed inside'the hermetic uuit'are used. When used. Overload relays-are part of tire magnetic controller and are'usually manual reset. Overload relays1 for hermetically-sealed motors should be quiebtrip units.*:i ~v' Because of their hermetic design, compressor motors need not be' serviced. Fan motors are accessible and should-'be occasionally serviced' in accordance with manufacturer's instructions. LARGE COMMERCIAL REFRIGERATION Compressors above 5 hp can be driven by electric motors, internal combustion engines and ste&m turbines. This chapter deals with motor-drive only. . * Motors ... .. . "' Most compressor motors used in large commercial refrig eration installations are alternating 3-phase current or directcurrent motors. Performance curves are given in Table'5. -`When single-phase current is available, split-phase or ca pacitor-start induction-run motors are used'for'air moving equipment''When single-phase-current is' not available, 3phase-'inotore are used: * Motor Control-and Protection - - Motor control .equipment used in large commercial refrig eration installations is essentially the same as that used for central air conditioners. . Motor, bearings must, be serviced in accordance .with the manufacturer's recommendations. Motor enclosures should be cleaned occasionally to provide adequate ventilation, On direct-current motors, .brushes and commutator must, be regularly checked for good electrical contact . m > ; CHAPTER 58 , : PIPE, TUBE, ; AND FITTINGS Pipe Materials, Types of Pipe, Commercial Pipe. D/mens/ons, Threading Practice, Pipe Fittings, Flange. Facings and-Gaskets, ' Valves, Welding, Expansion and Flexibility, Hangers and Supports, Application` MPORTANT considerations in the selection and installa I tion of pipe and fittings for heating, ventilating, and air - conditioning are dealt with in this chapter. ' iPIPE MATERIALS Use of corrosion-resistant materials for pipe, including special alloy steels, copper, and red brass,-has increased considerably during the past few years. The following- brief discussion indicates the variety of pipe materials and the types of pipe available. Wroughi-Steel. Pipe. Because of its'lower cost, the great bulk of-wrought:pipe used forheating and ventilating work . at the present time is of wrought steel. The material used 1 for steel pipe is a mild steel made by the aciddoessemer; the open-hearth,or the electric-furnace process; Ordinary wrought-steel pipe is made either by shaping sheets of metal ' into cylindrical form and welding the edges together, or. by forming or drawing from a solid billet. The former is known as tadded pipe; the latter as seamless pipe. Many types-of welded pipe are available,-,although the ctruJW most frequently used in heating and van*Hating work-are made-by. the lap-weld, resistance-weld, or butt weld process. While the lap-weld and resistance-weld proo- emes-produce a better weld than the butt type, lap-weld and . resistance-weld pipe are seldom manufactured 'in nominal pipe sizes less than 2 in. Seamless pipe can be obtained in the small sizes at a.somewhat higher cost; and is.listed as pres sure tubing below 2 in. - ; Seamless steel pipe is frequently used for high-pressure work or where pipe is desired for close coiling, oold bending, or other severe forming operations. Its advantages are' its . greater strength which permits use of a thinner wall and, in ' the small'sizes, its freedom from the.occasional -tendency of welded pipe to split at the,weld when bent. Wrought-Irim. Pipe. Wroiight-ironpipe may be identified by the'spiral;line marked into each length, either knurled into the metal or painted on it in red or other bright color. Otherwise, there is little, difference in the appearance of - wrought-iron and steel pipe; although' microscopic examina- Tte geaeral rezpcexibility tat this chapter 1 --* to TC lOA ReEricertot Pipin*. .tion of polished and etched specimens will readily disclose different grain structures. Cast-Ferrous Pipe. There are now available several types of cast-ferrous metal pipe made of a good grade of cast- iron with or without additions of nickel, chromium, or other alloy. This pipe is available in sizes from 1J in. to 6 in., and in standard lengths of 5 or 6 ft, with external and internal diameters closely approximating .those of-extra strong -wrought pipe. Cast-ferrous pipe may.be obtained coupled, beveled for welding, or with ends plain or grooved for the ;several types of couplings. It is easily cut and ^threaded as well as welded. The,fact that it'is readily welded.enables the manufacturers to supply the pipe in any lengths practicable for handling. : Alloy Metal Pipe. Both iron and steel pipe are available in the alloy class. In the case of iron pipe, when copper and molybdenum are; added, the material is known as alloy wrought-iron pipe.' By oommon custom, steel is considered to be an alloy when the marimnm range of alloying .elements-exceed certain limits, such as 1.65 percent manganese,* 0.60 silicon; or 0.60 copper. Also, a steel is an alloy if any of the following ele ments are specified in minimal quantities or.within limits: Al, B, Cr (up to 3.99), Co, Cb, Mo, Ni,:Ti, W, V, Zr, or any other element added- to obtain a certain alloying effect. Small quantities of certain elements are present in alloy steels, are considered as incidental, and may.,be present up to the following marimnm amounts: oopper--0.35 percent, nickel--0.25 percent, chromium--0.20 percent, and molyb denum--0.06 percent. ! Commercially, steel is considered stainless when chromium exceeds 3.99 percent, regardless'of the inclusion.of other al loying elements. - With tubing, an alloy becomes stainless when it contains a minimum of 10.5 percent chromium. . The alloy and stainless steels are used dor high tempera ture-piping and for highly corrosive fluids and gases. Copper Pipe and Fittings. Owing to inherent resistance to corrosion, copper and:red brass pipe have always'been used in heating, ventilating, refrigeration, and water supply instal lations, but the cost of standard dimensions for threaded con nections has been high. The introduction of fittings which 877