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672 CHAPTER 25 1958 Guide changers to increase the coil utilization without undue risk of returning liquid refrigerant to the compressor. Where two or more evaporator coils are to be attached to a single con densing unit, and different evaporator temperatures are desired, a back pressure valve may be installed to limit the minimum evaporating tempera ture of the warmer coils. This valve also finds application where fluctua tion in the evaporator temperature prevents accurate control of air tem perature and humidity. Except in product precooling, the objective of air distribution is to ab sorb the heat load as it comes into the room while maintaining a desirable degree of ventilation-air movement through hygroscopic cargos that are not vapor-barrier encased. In cold storage practice, this generally requires the securing of as uniform an air distribution as possible to every part of the product zone, with employment of diffusion of air over the product and avoiding direct blasts against the product. These requirements must be recognized when selecting the unit coolers and their outlets and when selecting the locations for the unit coolers. It is important to note that the quantity of air in motion in the refrigerated space is not only that passing through the coils. A quantity of air many times in excess of the air- handled by the units is always set in motion by the induction effect of the moving cooled air. Therefore, in evaluating the velocity for any given area, the total air set in motion must be considered. It is a function of the type of outlet, the discharge air velocity, and the location of the unit relative to restrictive walls and product. Unit location is also important from the standpoint of occupant com fort, low-velocity outlets being preferred for floor type units. High-velo city outlets are acceptable in fur storage vaults, ice cream hardening rooms and other spaces where air motion is not an important factor. In general, unit air coolers should not be suspended in front of door openings, or close to them where moist warm air will be drawn directly into the unit each time the door is opened, thereby causing excessive frosting and loss of capacity. Better performance will be obtained by placing the unit air cooler in a location such that the air will be discharged toward the door. If the shape of the space is such that this location would result in excessive air velocity over the product, then the location of the unit air cooler should be changed so that the air is discharged parallel to the wall in which the door is located. REFERENCES 1 Prepared by a Joint Committee of the American Society of Refrigerating Engi neers, American Society op Heating and Ventilating Engineers, Refrigerating Machinery Association, National Electrical Manufacturers' Association, and Air Conditioning Manufacturers' Association (A.S.R.E. Circular No. 13-42).* 1 Standard Method of Rating and Testing Self-Contained Air Conditioning Units for Comfort Cooling prepared by a Joint Committee of the American Society of Re frigerating Engineers, American Society of Heating and Ventilating Engineers, Refrigerating Machinery Association, National Electrical Manufacturers' Association, and Air Conditioning Manufacturers' Association (A.S.R.E. Circular No. 16). * Proposed A.S.R.E. Standard Methods of Rating and Testing Forced-Circulatj?3 and Natural Convection Air Coolers for Refrigeration (A.S.R.E. Circular No. 2n-44J. 4 Air Conditioning Refrigerating Data Book--Applications, (1954 55) Chapter (American Society of Refrigerating Engineers). * ASRB has combined Circulars No. 13-42 and No. 16 in ASRB Standard No. 16-56, Methods Ratine and Teating Air Conditioners. CHAPTER 26 PIPE, FITTINGS, WELDING Pipe Materials, Types of Pipe, Commercial Pipe Dimensions, Threading Practice, Pipe Fittings, Flange Facings and Gaskets, Valves, Welding, Expansion and Flexibility, Hangers and Supports IMPORTANT considerations'in the selection and installation of pipe and fittings for heating, ventilating, and air conditioning are dealt with in this chapter. PIPE 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. Wrought-Steel Pipe. Because of its lower cost, the great bulk of wrought pipe used for heating and ventilating work at the present time is of wrought steel. The material used for steel pipe is a mild steel made by the acidbessemer, the open-hearth, or the electric-furnace process. Ordinary wrought-steel pipe is made either by shaping sheets of metal into cylindrieal form and welding the edges together, or by forming or drawing from a solid billet. The former is known as welded pipe, the latter as seamless pipe. Many types of welded pipe are available, although the smaller sizes most frequently used in heating and ventilating work are made by the lap-weld, resistance-weld, or butt-weld process. While the lap-weld and resistance-weld processes produce a better weld than the butt type, lapweld 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 pressure tubing below 2 in. Seamless steel pipe is frequently used for high pressure work or where pipe is desired for close coiling, cold bending, or other severe forming oprations. Its advantages are its greater strength which permits use of a inner wall and, in the small sizes, its freedom from the occasional tendency welded pipe to split at the weld when bent. mpT0UTs~AT-On' ^'Pe- Wrought-iron pipe may be identified by the spiral 1-nmayec* i*1*0 each length, either knurled into the metal or painted on ifaDDeaed F ^er bright color. Otherwise, there is little difference in the tion ofan<v wrou&ht i1"011 and steel pipe, although microscopic examinastructures TM6^ an<^ ^hed specimens will readily disclose different grain metali"^'err0WS There are now available several types of cast-ferrous nickel P1e mac*e a f?d grade of cast-iron with or without additions of > cnromium, or other alloy. This pipe is available in sizes from V/2 673