Document Zn8rgKp3MGrgbEVxezbybjjMY

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 consequence, particularly in short breechings, takes, place. A sudden enlargement in a breeching may be avoided by gradually increasing the area over a length of several feet. In large masonry chimneys, the area of the flue at the region of the breeching entrance is considerably larger than the.area of the breeching at the chimney, and a sudden enlargement exists. The draft loss through the economizer (Ae) should be obtained from the manufacturer but for general purposes it may be computed from: 6.6WlNTc Ae = 10a (18) where Wn = pounds of gases flowing per hour per linear foot of pipe in each economizer 1 section. N = number of economizer sections. An economizer' in a steam plant affects the draft in two ways, (1) it offers a resistance to the flow of gases, and (2) it lowers the average chimney gas temperature, thereby decreasing the available intensity. In the case of a natural draft installation, both of these factors result in a relative increase, in the height of the chimney and, in the case of a large plant, they may add as much as 20 or 30 ft to the height. The decrease in the temperature of the gases after they have passed through the economizer has an extremely important'effect on the performance of a natural draft chimney and also upon the performance of a fan. GENERAL CONSIDERATIONS FOR DOMESTIC CHIMNEYS The draft of domestic chimneys may be subject to a variety of influences . not usually encountered in power chimneys4. Horizontal- winds have an aspirating effect as they cross the chimney and are an aid to draft. However, surrounding objects, such as trees or other buildings, may affect the direction of the wind at the chimney top and may even direct it down the chimney, tending to reduce the draft or even to cause-it to be negative. Although the chimney should extend well above the highest part of the roof, it is impracticable to carry it much beyond this point. It is also important to consider the.source of the air supply for proper combustion. Usually the boiler or furnace is located in the basement: When the furnace room has windows or doors opening to the outside on two or more sides of the house,- the leakage of air will be sufficient for combustion, even though the windows and doors may be shut. However, if the leakage is not sufficient to prevent an appreciable drop of pressure in the furnace room below that of the air outside, the chimney draft will be reduced by the difference between the atmospheric pressure outside.and that inside the boiler room. In case the boiler room is fairly tight and is open to the outside on only one side of the house, then the draft will be affected in windy weather even with windows or doors open. If the wind is blowing toward the boiler room the draft will be increased, but if. blowing in the opposite direction the draft may be seriously decreased. ^Chimneys and Draft (Chapter 32 in Winter Air Conditioning, by S. Konzo, published by National Warm Air Heating and Air Conditioning Association; 1939). 202 CHAPTER 9. CHIMNEYS AND DRAFT CALCULATIONS It is not to be assumed that increasing the cross-section area of a chimney will always effect a cure for poor draft. The opposite result may be experienced because of the cooling effect of the larger area. ' This reduces the theoretical draft and the velocity of the gases, and affords a greater opportunity for counter currents in the chimney. The only practical remedy for a chimney with bad draft, when the chimney is of the proper size and is affected by conditions beyond the control, is to resort to mechanical draft. This can usually be done at small expense if only operated when necessary. Two experimental chimneys of 9 x 9 and 9 x 13 in. nominal size were arranged to operate at 15, 20, 25, 30 and 35 ft heights and were tested at several rates of gas flow and inlet temperature to simulate residential performance. Temperature gradients throughout the height of each chimney were reported, together with draft conditions and friction losses5. CHIMNEYS FOR CAS HEATING The burning of gas differs from the burning of coal in that the force which supplies the air for combustion comes largely from the pressure of the gas in the supply pipe, whereas air is supplied to a bed of burning coal by die force of the chimney draft. If, with a coal-burning boiler, the draft is poor, or if the chimney is stopped, the fire is smothered and the combustion rate reduced. In a gas boiler or furnace such a condition would interfere with the combustion of the gas, but the gas would con tinue to pass to the burners and the resulting incomplete combustion would produce a dangerous condition. In order to prevent incomplete combustion from insufficient draft, all gas-fired boilers and furnaces should have a back-draft diverter in the flue connection to the chimney. A study of a typical back-draft diverter shows that partial or complete chimney stoppage will merely cause some of the products of combustion to be vented out into the boiler room, but will not interfere with com bustion. Other functions of the back-draft diverter are to protect, the burner and pilot from the effects of down-drafts, and to neutralize the, effects of variable chimney drafts, thus maintaining the appliance efficiency at a substantially constant value. As is the case with the complete combustion of almost all fuels, the Table 2. Minimum Round Chimney Diameters for Gas Appliances (Inches) Chiicnbt Fsbt 100 Gab Consumption in Thousands or Btu pes Houb 200 300 400 500 750 1000 1500 2000 20 4.S0 5.70 6.60 7.30 8.00 9.40 10.50 12.35 13.85 40 4.25 5.50 6.40 7.10 7.80 9.15 10.25 12.10 13.55 60 4.10 5.35 6.20 6.90 7.60 8.90 10.00 11.85 13.25 80 4.00 - 5.20 6.00 6.70 7.35 8.65 9.75 11.50 12.85 100 3.90 5.00 5.90 6.50 7.20 8.40 9.40 11.00 12.40 `Observed Performance of Some Experimental Chimneys, by Ri S. Dill. P. R. Achenbach and J. T. Duck (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning,'April. 1942. p. 252). 203