Document N821zgoDXQ1319QM1ndKra0y

.408 CHAPTER 19 1949. Guide and area, be of permanently tight construction and should be as smooth internally as practicable. It should be remembered that mechanically fired devices, oil burners and stokers, are equipped with blowers so that, with these devices, ,the chimney is not required to overcome the resistance of a fuel bed. Nevertheless, a draft in the fire box, of about 0.03 in. of water is considered desirable so that any small openings in the fire box or flue passages will result in leak age of air inward, and not leakage of combustion products outward. This is not to be taken to condone leaks.in fire boxes. Such leaks adversely affect, plant efficiency. OBSERVED TEST PERFORMANCE The observed performances of some brick chimneys' are given in Tables 1 and 2. The tests on which these data are based were made at various outside temperatures as shown and, to make them comparable among themselves, the observed drafts were corrected to 32 F, 1 atmosphere pressure, by the formula: Os = Oi -f- St -- Si (12) where Si = computed static (theoretical) draft, experimental conditions. St = computed static (theoretical) draft, standard conditions. Oi = observed draft, experimental conditions. Os = observed draft corrected to standard conditions. It will be noted that the observed draft exceeded the computed static draft during some observations on the shorter chimneys. This is inainly attributed to the draft producing effect of the hot gases immediately above the chimney. By means of a manometer it was found that a measurable draft existed in this.gas column for some distance above the chimney top. However, the temperatures in the chimney were measured with unshielded thermocouples and the actual gas temperatures may have been higher for this reason than the observed temperatures on which the computations of draft were based. The tests were made in calm weather, Tests were made at the National Bureau of Standards to find the draft produced by round, metal smoke pipe set in vertical position to act as chimneys. Curves are presented in Fig. 5 showing the computed static or theoretical draft for short chimneys for various heights and temperatures. For this purpose, the density of chimney gases was assumed to be the same as -that of air at the same condition, since the error thus introduced was not considered important in this case. The results of the tests showed that the following procedures would yield the available draft for 6-in. flue pipe used as a chimney within 10 per cent for the range shown and for fuel ^burning rates from about one-quarter to three-quarters of a gallon of oil per hour. Using the temperature at the smoke collar of the heater, find the static draft corresponding to the available chimney height. Then: 1. If the chimney is bare, multiply the static draft by 0.76 to find the available draft. 2. If the chimney is insulated with 1 in. of air-cell material with a 1-in. air space, multiply the static draft by 0.85 to find the available draft. Chimneys' and Draft Calculations 409 3. If the chimney is insulated with 1 in. of air-cell material with a'l-ih: air space, open at top and bottom for ventilation, multiply the static draft by 0.81.to find the available draft.. . . 4. If the chimney is insulated with 1 in. of air-cell material and has a 1-in.- air space closed at top and bottom to prevent ventilation, multiply the static draft by 0.85 to find the available draft.' The use of the 1-in. air-cell asbestos insulation in the tests discussed is not to be construed as an approval of such insulation in all cases in regard to fire resistance. Several laboratories are working on the fire resistance JFig7 5. Computed Static Draft for Short Chimneys aspects of the problem but definite rules are not yet' available. For coalor oil-burning devices, a bare smoke pipe is probably safe if kept 2 ft or more from any woodwork and the better the pipe is insulated, or the lower its temperature, the nearer it can be placed to Combustible materials! DRAFT REQUIREMENTS OF DOMESTIC APPLIANCES . Typical, flue-gas temperatures and drafts required at rated output for several kinds, of domestic heating appliances4 are contained in Table ,3. i: CHIMNEYS FOR GAS HEATING Heating appliances designed to bum gas as well as appliances converted to gas burning, except those equipped with power type burnera and exr cepting conversion burner installations in' excess of 400,000 Btu per hour input in'large'steelBoilers, are always equipped with a draft hood attached to' the5 flue outlet of the appliance. This draft hood, is required/if tie appliance is to meet the approval requirements of'the American' Gasi `As- ..X