Document GmX43EN11vZ2ezMgGEkrRNE9x

r --' Heating Ventilating Air Conditioning Guide 1938 2 f What are the principal differences between a radiator and a convector? A radiator, is commonly thought of as a commercial heating unit having a maximum amount of direct heating surface, whereas a convector is a heating device in which the extended or secondary surface may be several times that of the prime surface and which is specially designed to utilize to the fullest extent the convection principal of heating. The radiator ordinarily has vertical tubular chambers for the heating medium but most convectors have horizontal tubular chambers to which fins are attached so as to form vertical flues for the passage of air. While radiators are either exposed, enclosed, or shielded, convectors are concealed by means of a tight-fitting enclosure. Radiators are commonly made of cast iron but convectors may be made of a combination of metals, such as copper and brass, or copper and aluminum, as well as entirely of cast iron. 3 9 How did the term heating effect come into use? It has been found that a room requiring a radiator of a certain determined capacity could under certain conditions be properly heated, with less temperature gradient be tween floor and ceiling and with less steam condensation, by the same radiator or by one of a different design having the same commercially rated capacity. This resulted in the use of the term heating effect to apply to the useful heat output of a radiator, in the com fort zone of a room, as related to the total input to the radiator. 4 Is it necessary to make any allowance for the performance of-a convector because it is enclosed? No. The commercial ratings of convectors have been determined by testing the con vectors in proper, enclosures with grilles in place just as they should be installed for ordinary service. j* , 5 9 On what basis are the capacities of convectors published? Published ratings of convectors are expressed in equivalent square feet of direct cast iron radiation. Some manufacturers have increased their ratings by as much as 30 per cent to allow for a supposed improved heating effect. Tests indicate that the credit to be given heating effect'is, in all cases, probably less than 10. per cent, and in many cases negligible. ' 6 How are fins of convectors attached to the tubes or prime, surface? Tubes or a solid core may.be-forced ithrough .piercings in the. fins under pressure, or the tubes may be expanded into the holes through the fins. In addition a metallic bonding agent is sometimes used to insure permanent contact. 7 What is the procedure in selecting a convector when the required amount of radiation is known?- ' First the limiting factor or factors of the enclosure must be determined so the available size of the wall recess can be found. Manufacturers' catalogs show capacities of con vectors of each standard length and depth with varying enclosure heights. From these capacity tables, the proper convector of the required capacity can be selected for the available wall recess. If all three dimensions of the wall recess are insufficient to accom modate a convector of the required capacity, the available height and length can be maintained, but greater depth can be obtained by using a partially recessed enclosure. 8 Given a room to be heated to 80 F with outside temperature at'O F, assume the heat loss under these conditions to be 10,000 Btu per hour. ' Deter mine the size of the steam radiator to be installed. A square foot of radiation is equivalent to a heat emission of 240 Btu per hour under standard conditions of steam at one -pound gage pressure (215 F) and surrounding air at 70 F. With surrounding air at SO F, the heat emission from a radiator will be less. Under these conditions, the heat emission will not be 240 Btu per square foot of catalog rating per hour, but 240 Cs. Cs ( Is - ti Y:* \215 - 70/ /215 - 80 Y*` \215 - 70 / 0.912, and 240 C8 = 240-X'0.912 = 218.5 Btu. Therefore, the size of the radiator to be selected shall have a catalog rating of 10,000 divided by 218.5 or 45.8 sq ft. 274 A * j , ! i 1 j . Chapter 15 STEAM HEATING SYSTEMS Gravity and Mechanical Return, Gravity One-Pipe Air-Vent System, Gravity Two-Pipe Air-Vent System, One-Pipe Vapor System, Two-Pipe Vapor System, Atmospheric System, Vacuum System, Sub-Atmospheric System, Orifice System, Zone Control, Auxiliary Conditioning Unit, Condensation Return Pumps, Vacuum Pumps, Traps THE essential features of the common type of steam heating systems are described in this chapter. They may be classified according to the piping arrangement, the accessories used, the method of returning the con densate to the boiler, the method of expelling air from the system, or the type of control employed. Information concerning the design and layout of steam heating systems will be found in Chapter 16. GRAVITY AND MECHANICAL RETURN In gravity systems the condensate is returned to the boiler by gravity due to the static head of water in the return mains. The elevation of the boiler water line must consequently be sufficiently below the lowest heating units and steam main and dry return mains to permit the return of condensate by gravity. The water line difference1 must be sufficient to overcome the maximum pressure drop in the system and, when radiator and drip traps are used as in two-pipe vapor systems, the operating pressure of the boiler. The condensing return of'the radiation will increase the required water line difference and is especially important where the radiation is a type having a high condensing rate. This applies only to closed circuit systems, where the condensation is returned to the boiler. If the condensation is wasted, no water line difference is required, but other conditions are introduced which warrant the use of an appro priate mechanical system in preference to wasting the condensate. In mechanical systems the condensate flows to a.receiver and is then forced into the boiler against the boiler pressure. The lowest parts of the supply side of the system must be kept sufficiently above the waiter line .of the receiver to insure adequate drainageof water from the system, but the relative elevation of the boiler water line is unimportant in such cases . `The voter line difference is the distance between the water line of the boiler and,the level of the water in the dry or wet return main. (See Fig: 4.) ' . * ............. J-` 275 s'