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American Society of Heating and Ventilating Engineers Guide, 1936
6 and 8. The recommended minimum inside dimensions and heights of chimneys for small and medium size installations are given in Table 1.
GENERAL EQUATION
The general draft equation for a steam producing plant may be stated as follows:
Dt -- hi = Af + Ab + /tBd + he + hBr + hv + ho + *E + hn
(12)
where
Pt = theoretical draft intensity created by pressure transformer, inches of water, hf = draft loss due to friction in pressure transformer, inches of water, hp = draft loss through the fuel bed, inches of water, hB = draft loss through the boiler and setting, inches of water, *Br = draft loss through the breeching, inches of water,
kv = draft loss due to velocity, inches of water, hBd = draft loss due to bends, inches of water, he = draft loss due to contraction of opening, inches of water, ho = draft loss due to enlargement of opening, inches of water, te = draft loss through the economizer, inches of water. Ar = draft loss through recuperators, regenerators, or air heaters, inches of water.
The left hand member of Equation 12 represents the total amount of available draft created by the pressure transformer, that is, the natural draft chimney, Venturi chimney, or fan, and is equal to ,the theoretical intensity less the internal losses incidental to operation. The right-hand member represents the sum of all of the various losses of draft throughout the entire boiler plant installation outside of the pressure transformer itself. The left hand member expresses the available intensity arid is analogous to the head developed by a centrifugal pump in a water works system, while the right hand member expresses the required draft in tensity and is analogous to the total dynamic head in a water works system. For a general circulation of gases
where
Pa = D,
Z?a = available draft intensity, inches of water.
Dr = required draft, inches of water.
'
n_ _
(13)
The draft loss through thefuel bed (hp), or the amount of draft required to effect a given or required rate of combustion, varies between wide limits and represents the greater portion of the required draft. In coal-fired installations, the draft loss through the fuel bed is dependent upon the following factors: (1) character and condition of the fuel, clean or dirty; (2) percentage of ash in the fuel; (3) volume pf interstices in the fuel bed, coarseness of fuel; (4) thickness of the fuel bed, rate of combustion; (5) type of grate or stoker used; (6) efficiency of combustion.
There is a certain intensity of draft with which the best results will be obtained for every kind of coal and rate of combustion. Fig. 7 gives the intensity of draft, or the vacuum in the combustion chamber required to bum various kinds of coal at various rates of combustion. Expressed in
Chapter 26--Chimneys and Draft Calculations
Table 1.
Recommended Minimum Chimney Sizes for Heating Boilers and Furnaces1
Warm Am Furnace Cafacitt
in Sq In. or Leader
Pipe
Steam Boiler Capacity Sq Ft or Radi
ation
Hot Water Heater Capacity Sq Ft or Radi
ation
Nominal Dimen sions or Fire Clay
Lining in Inches
Rectangular Flub
Actual Inside Dimensions
of Fire Clay Lining
io Inches
Actual Area
Sq Id.
Round Flub
IPnisnindye.
eTtuenrinogf
in Inches
Actual Area
Sq In.
Height in Ft
Above Grate
790 1000
590 690 900 900 1,100 1,700 1,940 2,130 2,480 3,150 4,300 4,600 5,000 5,570 5,580 6,980 7,270 8,700 9,380 10,150 10,470
973 1,140 1,490 1,490 . 1,820 2,800 3,200 3,520 4,090' 5,200 7,100 7,590 8,250 9,190 9,200 11,500 12,000 14,400 15,500 16,750 17,250
8^x13
13x13 8kxl8
13x18
18x18 20x20
20x24 24x24
7 xllk
nKxim 6%xl6}4
11)4 X16M
15Mxl5M
17K x 17^
17x21 21x21 24 x 24b
24 x 28b 28 x 28b
30 x 30b 28 x 32b
81
127 110
183
248 298
357 441 576
672 784
900 896
10
12 15
18 20
22 24
.27
79
113 177
254 314
380 452
573
35
40
45 50
55 60 65
This table is taken from the A.S.H.V.E. Code of Minimum Requirements for the Heating and Venti lation of Buildings (Edition of 1929).
bDimensions are for unlined rectangular flues.
other words, these curves represent the amount of draft required to force the necessary amount of air through the fuel bed in order to effect various rates of combustion. It will be noted that the amount of draft increases as the percentage.of volatile matter diminishes, being comparatively low for the lower grades of bituminous coals and highest for the high grades and small sizes of anthracites. Also, when the interstices of the coal are large and the particles are not well broken up, as with bituminous coals, much less draft is required than when the particles are small and are well' broken up, as with bituminous slack and the small sizes of anthracites. In general, the draft loss through the fuel bed increases as: (1) the per centage of volatile matter diminishes; (2) the percentage of fixed carbon increases; (3) the thickness of the bed increases; (4) the percentage of ash increases; (5) the volume of the interstices diminishes.
In making the preliminary assumptions for the draft loss through the fuel bed, due allowances should be made for a possible future change in the grade of fuel to be burned and also in the rate of combustion.- A value should be selected for this loss which will represent not only the highest rate of combustion which will be encountered, but also the grade of coal which has the greatest resistance through the fuel bed and which may be burned at a later date.
In powdered-fuel and oil-fired installations, there will be no draft loss
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