Document 8m5gvz31BVGqDKXVKO82y52e

American Society of Heating and Ventilating Engineers Guide, 193^ The draft loss in the chimney stack is so dependent on conditions surroundi stack itself, that it seems desirable to consider the loss in terms of percentage of** draft rather than in specific terms of velocity. This treatment will be understood^ referring to formulae discussed later in this chapter. 00<` Forced Draft As the name implies, forced draft involves the use of a fan or blower f ? creating a pressure in the ash pit and forcing air through the fuel h^. The draft required where forced draft is employed is the sum of items xf 4 and 5; items 1 and 2 usually being taken care of by the forced drafJ apparatus. . __Draft and Chimneys Chatter fernenst uoafDllyramfteasured in inches of water, by which is meant that ytlie s'^tatk/he*SeUanidd^ooff^wwpa0ati't-ene*rrt ((iniinntiihnnecchhheeessa))teccoor,rrbrreeressppeoochnniddniignnggottrooctuhhiimee dnieffye,reantcwehi.ni.c. hprtehsei-suret*^ measured, and the pressure of the surroun- dj:in---g-- atmosphere. ijitaftisDe of draft is equivalent to 5.2 lb ' per square foot of pressure ' k some likelihood of a misunderstanding concerning what is ;?v<mH.!.HTTa<nhhauee--rfr-aeebcyitsudrresaros.fmtretehfrheokrueggiemhnuPaeorbubaoollyiilleetrro_ootrrhffeuurrbnnoaaiccleeer,. Low pressure heating boiler alsot'wh' epc-r-ow--m-,,-pu- W,lectne uunniitt ssoo tthhaatt Thefess of draft through the boiler would consist of items (1), (2) and (3) - under Draft, while manufacturers of power (high pressure) boilers refer 'to the furnace and boder as separate units. In this chapter when boiler * -inferred to, these three items are taken into account. Fig. 1. Draft Required at Different Rates of Combustion for Various Kinds of Coal Induced Draft With induced draft, the suction side of the fan is connected with the uptake or breeching of the boiler and the products of combustion are usually exhausted through a stub stack. When induced draftls employed the entire draft:required is generally taken care of by the induced draft apparatus. In the case of either forced or induced draft any part of the required draft may be taken care of by the chimney and the remainder by the apparatus, but the exact conditions to be met in this respect should be known and the chimney should be proportioned accordingly. The combination of natural and mechanical draft is frequently used, especially where loads are quite irregular, so that the plant may be run bn natural draft during light loads and on mechanical draft during peak loads, only. This combination also affords a factor of safety against breakdowns in the mechanical draft equipment, since at least a partial |oad may always be carried on natural, draft. 228 A^vtaTilaieblaevDairlabftle draft of a chimney is the theoretical draft after deducting the various losses back to the point of reference, such as at the base of th'-eThcheimlosnseeys. which must be deducted from the theoretical to obtain the available draft at this point are the velocity and friction losses in . the c2)h.Timhneelyo.ss due to velocity and friction of the flue gases in the chimney ; ^expressed by Formula 2 as a percentage'of the total draft. : CALCULATING CHIMNEY HEIGHTS AND AREAS ,Sf4t is desirable that practical chimney data be as free as possible from 1 alhpurely theoretical considerations which.are covered in most text books .1 oid this subject. Theoretical considerations have been eliminated from the following formula for calculating chimney areas and heights and it has'been found sufficiently accurate for conditions generally found in practice: H= Mda____ NW'p m HM == haecigohntstoafnstta(sceke aTbaobvlee t1h-efogrrvaatelueins fleoertv. arying con ditions). da = draft in inches ofwater available.at smoke pipe inlet to stack. N = a. constant (see Table 2 for values for varying conditions). W = weight of flue gases, pounds(per hour. = perimeter of flue in inches, o = area of flue in square inches. 229