Document gE32BXGdaqV1BBrwyjD6kznkL

834 CHAPTER 52 possibility of escape of flue gases into the room; where1they are installed.-There are also limitations, since it is' necessary that the unit be installed on or adjacent .to:an outside wall. Current American Standards for approval of vented wall furnaces for installation in.,trailer coaches and.mobile homes require-that such heaters not take combustion, air from occupied.space, and that draft hood.relief and combustion air relief be in- the same pressure zone. The net result is that a sealed combustion system is the only practical solution.; ' -; , Current revisions to American Standards for Appliance Installation (Z21:30) amd most of the recently revised heating and installation codes exempt appliances'with sealed com bustion systemsfrom standard venting practices,' and allow 1965-Guide And DafaiBoolt them.to be installed'in accordance'with the approved mknu^ facturer's installation.instructions.'. ' '` V REFERENCES ' ' 1 / " B R Testing and Rating Code for Lino Pressure Cadi Iron Heating Boilers (Institute ol. Boiler and RjuiiAfaTM facturera,1June, 1958). -1 1 Testing and rating code for boiler-burner units (Engineering Standards Part II, Mechanical Contractors Association of Amer ica, 1956). : - i . ,, . * / =* B R Ratings far Cast-Iron-Boilers (Institute of Boiler and Radiator Manufacturers, May 10, 1955). , .... .. 4 Net load.recommendations for.heating boilers (Engineering Standards Part II,' Mechanical Contractors Association of Amer ica, 1956).' - : b.-.v l- ' . S'd,: -'.i'-i -c - V if:- ' . `.'.T'llAi.'' CHAPTER 53 . CHIMNEYS AND DRAFT Draff Definitions, Factors Affecting Natural Draff, Available.Draft, Required Draff; General Considerations, industrial Chimneys, Residential Chimneys, Short Chimneys, Gas Appliance Venting,- Available Draft for the Industrial Chimney, Determining Industrial Chimney Sizes; Chimney Sizes, for' Heeding arid Process Boilers, Determining Residential Chimney Sizes, Draft Requirements of Appliances, Recommendations of the National Board of Fire Underwriters DRAFT is the pressure'difference associated with" th$ given inlet temperature and flow rate, to. the ideal draft that movement of flue gases through a chimney. Natural would be observed if the same quantity of flue gases traversed draft is caused and measured by'the difference in weight of athe chimney without cooling and without friction. The column of flue gas within the chimney and a corresponding chimney efficiency may be calculated as follows: column of air of equal dimension outside the chimney. Nat ural draft is always negative and is expressed- in inches of water gage. Efficiency final measured draft ideal draft calculated from the inlet temperature - When the movement of air is supplied'by a fan, the draft Is Raid to be forced or induced: When the fan is located so as to push the flue gases through the chimney, the draft is forced. When the fan is located so as to draw the flue gases through the -combustion unit, the draft is induced. . - REQUIRED DRAFT ' Before the proper chimney for a particular installation can be RpW-tfri or designed, the required draft of the combustion unit must be known. The required draft is equal to the sum of all of the resistances to gas flow from the point at which FACTORS AFFECTING NATURAL DRAFT ; The theoretical natural draft is directly proportional to the height of the chimney, and is dependent upon the absolute mpRji temperature of the gases within the chimney. It is also directly proportional to the density of the air. outside-the chimney although the effect of this factor is minor. " , The formulas for theoretical natural.draft are based on rnpftn or average temperature which is one half of the sum of the entering and exit temperature. Since the difference in exit and entering temperatures, or the temperature gradient in the chimney, is due to the escape of heat through the walls of the chimney, the heat transfer characteristics of.the chimney con struction affect draft. Heat transfer is also affected by the rate of flow of the flue gases through.the chimney, so draft is also directly affected. combustion air enters the unit, up to and including the chim ney connection. This is based on the assumption that air for combustion is available without-restriction (such as would be caused by tightly enclosed building construction) at that point of entrance to -the unit- Particularly in residences, un restricted supply of combustion air must be assured. Fig. 1 presents information on the fuel-bed draft loss, for various kinds of coal burned-at different rates. The draft tenson required.in the firebox for oil firing or for stoker firing should be determined from the supplier of the oil burner or stoker, and the designer should be sure that the value selected is in compliance with all controlling laws or ordinances. In the. absence of more accurate data, the values presented in Table,1 may be used.1,1. ..Breechings or smoke pipes-should be designed without The flow of.flue gases through the chimney is retarded by friction which varies with .the. friction coefficient of .the chimney surface, and with the rate of flue-gas flow. The fno tion loss may be estimated by means.of one,of the formulas for duct8,.such as the Fanning equation, or it may be estimated .with sufficient accuracy by-the.method used for air ducts, using a roughness value of 0.01 (see Chapter 31). . . ..The height of,the chimney is measured from the point of en trance of the flue gases to the top. of the chimpey. ' available draft The available draft produced by any chimney is equal to the theoretical natural draft minus the friction-loss. Given the height, the heat transfer characteristics of the chimney con struction; the entering temperature of the flue gases and their volume, and the friction coefficient of the chimney and its dimensions, the available draft is readily calculable. - The efficiency of a chimney is defined.as .the ratio of the observed or available'''draft-produced- by-the chimney for a Comtwtna