Document 1g02m16oZM7Y200qxw9dm5KQo

Heating VentiIiAting Air Conditioning Guide 1938 system more slowly and remain in the radiator longer. The temperature drop through the radiator will be more than 20 F, and the difference in the weight of the water in the return and flow risers will be greater than that intended. The final result will be that the pressure head will become equal to the friction head at a value somewhere between ' 954 and 1381 milinches. Since the average water temperature in the radiator will be less than 190 F, the radiator should be larger .than the size given in Question 4. 7 # Should a hot water heating system be designed to embody small pipes or large pipes? As pipe sizes in gravity circulation heating are reduced, the friction head is increased and it is necessary to increase the temperature drop through radiators; this lowers the average temperature of the water in the radiators and necessitates an increase in thesize of the radiators, so whereas the cost of the pipe in a system is reduced, the cost of the radiators is increased. For each installation there is a definite pipe size which entails' maximum economy. As pipe sizes in forced circulation systems are reduced, friction heads are increased so a circulating pump of greater size or capacity is required. Thus, by decreasing the size of the piping, both the first cost of the circulating pump and the cost of its operation are* increased. There is a definite pipe size for every installation which is most economical.' For each installation of both types of systems there is a definite pipe size entailing maxi*' mum economy which can be determined by a series of comparative calculations. 8 What should be the size of the radiators for the elementary heating system of Question 3 in which the water enters the radiator with a temperature of 200 F and leaves with a temperature of 180 F? The average temperature of the water in the radiator is, approximately, 190 F. If test results are available for the particular radiators to be used, and for the tempera tures named, the size of the radiators should be selected from them. If no such test results are to be had, but if test results are available for. the type of radiator to be usedwhen it is supplied with 215 F steam and placed in a 70 F room, the required size may be determined by the following ratio; The required size is to the corresponding steam radiator size as (215 -- 70)1** is to (190 -- 70)1-*. This ratio works out to 1.281 Hence, the radiators should be 28 per cent larger under the conditions prescribed than are cor responding radiators under standard conditions. It is immaterial whether a radiator is filled with steam or with water, as long as the average temperature of its outer surface is the same in both cases. 354 Chapter 18 PIPE, FITTINGS, WELDING Pipe Material, Types of Pipe Used, Dimensions of Pipe Com mercially Available, Expansion and Flexibility of Pipe, Pipe Threads and Hangers, Types of Fittings, Welding as Applied to Erection of Piping, Valves, Corrosion of Piping IMPORTANT considerations in the selection and installation of pipe and fittings for heating, ventilating, and air conditioning work are dealt with in this chapter. PIPE MATERIALS Use of corrosion-resistant materials for pipe, including special alloy steels and irons, wrought-iron, copper and brass, has increased con siderably during the past few years. The recent development of copper, brass, and bronze fittings which can be assembled by soldering or sweating permits the use of thin-wall pipe and thereby has reduced the initial cost of such installation. The following brief discussion indicates the variety of pipe materials and the types of pipe available. Wrought-Steel Pipe. Because of its low price, 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 acid-bessemer, 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 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 or butt-weld process. While the lap-weld process produces a better weld than the butt type, lap-weld pipe is seldom manufactured in nominal pipe sizes less than 2 in. Seamless pipe can'be obtained in the small sizes at a somewhat higher cost. Seamless steel pipe is frequently used for high pressure work or where pipe is desired for close coiling, cold bending, or other forming operation. Its advantages are its somewhat 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-iron Pipe. Wrought-iron pipe is considered to be more corro sion-resisting than ordinary steel pipe and therefore its somewhat higher 355