Document aXEk8JgqVVOm1ERqOoXQJ6QN

fjj' 184' ' - ,Chapter 9 _________________ 1945 Guide - I ----- measurable-draft-existed-in -this gas-column-for;some- distance-above the--4 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 a vertical position to act as chimneys.. Curves are presented in Fig. 5 showing the computed static 4! I* I "or theoretical draft for short chimneys for various heights and ternperatures. For this purpose, the density of chimney gases was assumed I l Effective Chimney Temperahjre-Degrees F Fig. 5. Computed Static Draft for Short Chimneys 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 }-in. air space, multiply the static draft by 0.85 to find the available draft. 3. If the chimney is insulated with 1 in. of air-cell material with a 1-in. 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 Tin. 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 . fi the available draft. ii Chimneys and Draft Calculations' 188 --The use of- the-l-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 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 a couple of feet 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 devices or appliances4 are contained in Table 4. Table 4. Drafts Required by Typical Domestic Heating Devices or Appliances Device Space Heater, Oil Burning, Pot Burner................................ Draft, Inches Water 0.06 to 0.08 0.06 0.06b 0.06 0.03a 0.05a or less 0.04b 0.06b Stack Temperature Deg F 1000 860 900 860 400 900 Draft In fire-box. bFor chestnut sized anthracite. CHIMNEYS FOR GAS HEATING Heating appliances designed to burn gas as well as appliances converted to gas burning, except those equipped with power type burners, are always equipped with a draft hood attached to the flue outlet of the appliance. This draft hood is required if .the appliance is to meet the approval require ments of the American Gas Association and the American Standards Association and is essential for safe operation. It is designed to prevent excessive chimney draft which would lower appliance efficiency, to prevent a blocked flue or a down draft in the chimney from impairing combustion, to provide a relief opening for the products of combustion during down draft or blocked flue conditions, and to prevent spillage of the products of combustion to the space surrounding the appliance if there is a chimney draft equivalent to that provided by a 3-ft chimney. As the draft hood is designed without moving parts, the relief opening is always open and consequently sonle air is drawn into the chimney. While the air drawn in lowers the gas temperature in the chimney, it also' 'lowers the dew-point of the gases and tends to prevent condensation of moisture. The products of complete combustion of gas are water vapor (HtO) and carbon dioxide (CO,) with a trace of sulphur trioxide (S03). Sulphur usually burns to the trioxide in the presence of an iron catalyst. The ,4National Bureau of Standards Commercial Standards: CS101-43 Oil-Burning Space Heaters Equipped With Vaporizing Pot-Type Burners, CS75-42 Automatic Mechanical Oil Burners Designed for Domestic Installations. CS(E)104-43 Warm Air Furnaces Equipped With Vaporizing Pot-Type Burners; and Trade Standards: TS3536a Solid-Fuel-Burning Forced Air Furnaces, TS3518 Oil-Burning Floor Furnaces Equip ped With Vaporizing Pot-Type Burners.