Document LJBbKDjnXdyZbk0v8zKVY93NQ

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 In powdered-fuel and oil-fired installations, there will be no draft lcwl through the fuel bed since there is none and, consequently, this facto,* becomes zero in the general draft equation. All other factors being! constant, the height of the chimney, in installations of this character be less than the height in coal-fired installations, and in the case of chanical draft installations the driving units need not be as large since the head against which the fan is to operate is not as great in the former a, in the latter. The draft loss through the boiler and setting (hf) also varies between wide limits and, in general, depends upon the following factors: (1) type 0( boiler, (2) size of boiler, (3) rate of operation, (4) arrangement of tubes (5) arrangement of baffles, (6) type of grate, (7) design of brickwork setting, (8) excess air admitted, and (9) location of entrance into breeching Curves showing the draft loss through the boiler are usually based on the load or quantity of gases passing through the boiler, expressed in terms of percentage of normal rate of operation. Owing to the great variety of boilers of different designs and the various schemes of baffling, it is impossible to group together a set of curves for the draft loss through the boiler which may even be used generally. It is therefore necessary to secure this information from the manufacturer of the particular type of boiler and baffle arrangement under consideration. ' When a boiler is installed ahd in operation, the draft loss depends upon the amount of gases flowing through it. This, in turn, depends upon the proportion of excess air admitted for combustion. Primarily, the amount of excess air is measured by the COt content; the less the amount of CO,, the greater the amount of excess air and hence the greater the draft loss. The loss of draft through the boiler will vary directly as the size of the boiler and the length of the gas passages within. The loss also varies as the number of tubes high, but not in a direct ratio inasmuch as the loss due to the reversal of flow at the ends of the baffles remains constant regardless of the height of the boiler. The arrangement of the tubes, whether the gases flow parallel to or at right angles to the tubes, has an appreciable effect on the loss. The arrangement of the baffles influences the draft loss greatly, the loss through a boiler with five passes being greater than the loss through one of three or four passes. A poor design and a rough condition of the brickwork will increase the loss greatly, whereas a proper design and a smooth condition will keep the loss at a minimum. The loss through the boiler will be less when the breeching entrance is located at or near the top of the boiler than when it is located at or near the bottom since the gases have a shorter distance to travel in the former instance. The draft loss through the breeching Qibi) may be found by applying the last term on the right, with the sign changed, of Equation 1 or 2 depending upon whether the breeching is cylindrical or rectangular and observing the following changes in the symbols: 7Y = absolute temperature of breeching gases, degrees Fahrenheit. f = coefficient of friction for the breeching. L -- length of breeching, feet. D -- diameter of cylindrical breeching, feet. x and y = sides of breeching, if rectangular, feet. 178 CHAPTER 8. CHIMNEYS AND DRAFT CALCULATIONS i - 1 ~ -- , It has been the general custom to lump off the intensity of the breeching loss at 0.10 in. of water per 100 ft of breeching length regardless of its size or shape or the amount and temperature of the gases flowing through it. This practice is hazardous and has no more foundation in fact than that of determining the friction head in a water works system without taking into consideration the size of the pipe or the amount of water flowing through it. When the length of the breeching is relatively short, any variation in any one of the factors in the equation will have no appreciable effect on the draft loss. However, when the breeching is relatively long the draft loss is affected greatly by the various factors, particularly by the siizzcTeevhaa.en.n~ddd-rssahhfaat pploeessaassduwweeetlloll aavsselbooyycittmyheef(hhw-yA)eiigshg(rtiivoefng-a-b-s-ye--s-thfleoweqinuga.tion , _ 0.000194W*TC V A'BaWc (15) wheAre = cross-section area at the top of the stack, square feet. The draft loss due to bends in the breeching (hm) is dependent upon the center line radius of curvature of the bends and the form of the crosssection. This loss is expressed in terms of the velocity head. (See Fig. 4, ChTahpetedrr3a1ft.)loss due to sudden contraction of an area (he) is given by the equation : he = 0.0001=-9-4--/-C--c-J--V-->-T--cA\B0WC (16) wheKrec = coefficient of sudden contraction based on the ratio of the areas of the smaller to the larger section = 0.5 ^ 1 -- ^ ^ da = area of the smaller section. - When the flue or passage through which the gases flow is suddenly contracted, a considerable portion of the static head in the larger section is converted into velocity head and a draft loss of some consequence, par ticularly in a short breeching, takes place. A sudden contraction should always be avoided where possible, At times, however, due to obstruc tions or limited head-room, it is necessaiy to alter the size of the breeching, but a sudden contraction may be avoided by gradually decreasing the areTahoevderraaft lloesnsgdthueotfosaevseudradlefneeetn.largement of an area (ho) is given by the equation: ho = -0--.-0-0--0--1--9-4--X--o--T--F--*-r-cAlBoWc (17) wheKre0 = coefficient of sudden enlargement based on A-j--t the ratio of the areas of the / As \2 smaller to the larger section =11 -- I When the flue or passage through which the gases flow is suddenly enlarged, a portion of the velocity head is converted into static head in the 1