Document 5bB4NG0QBa7Moz7o05GaqOq94

American Society of Heating and Ventilating Engineers Guide, 1928 These appliances should always be vented, both from the standpoint of safety and also on account of the damage that can be done by the water vapor in the products of combustion being condensed within the room. Another variety of heater that falls, under the preceding head is the floor furnace. This is a convection type heater that is hung underneath the floor. It is generally installed with a duplex register having a warm air outlet and a cold air return, so that complete recirculation is secured. THE COMBUSTION OF GAS As has been previously mentioned, most gas appliances burn with a blue or Bunsen flame, although some room heaters use a luminous or yellow flame. Bunsen flames and luminous flames differ in the way in which the air necessary for combustion is supplied. Gas requires for complete combustion, air in the proportion of about 1 cu. ft. for each 110 to 120 B.t.u. of gross heat value. If gas is forced directly into the atmosphere through a small hole, the air required for combustion is drawn into the jet and the gas burns with a long yellow flame. As the holes i n such burners must be small in order to impart sufficient momentum to the gas jet to draw in enough air for combustion and to keep the flame from being too large to apply easily, the appliances using them cannot burn gas in large quantities. A luminous flame cannot be allowed to touch any cold surface as this will result in incomplete combustion with the formation of soot and carbon monoxide. Blue flame or Bunsen type burners are provided with an external mixer in which a. portion of the air (about 2 cu. ft. per cubic foot of gas) is mixed with the gas previous to ignition. This is called primary air. The flame issuing from the port of the burner has two distinct parts: a pale blue inner cone and a darker cone surrounding it. The heat of the flame issuing from the burner port draws currents of air past the flame and into it in sufficient quantity to cause complete combustion. This is known as secondary air. The flame should sit squarely on the port and should not have a yellow tip. A yellow tip indicates insufficient primary air, and is corrected by opening the adjustable air shutter. It should be noted here that the proportions of the orifice at the entrance to the burner tube, and the proportions of the burner ports must suit the heat content of the gas being burned. A burner propor tioned for natural gas is not suitable for water gas or coal gas, which are much bulkier in relation to their heat contents. Table 2 shows the heat values of a cubic foot of gas and air mixture, the gas being mixed with the amount' of air theoretically required to burn it. In practice excess air must be admitted to the fire in order to insure complete combustion. Table 2. Volume of Air Required for Combustion of Different Gases Gas B.t.u. per Cubic Foot Cu. Ft. Air to Burn Cu. Ft. Gas B.t.u. per Cu. Ft. op Mixture Mixed Coke Oven and Water Gas..... Producer Gas......................................... 1131 .490 525 140 10.70 4.18 4.05 1.13 96.8' 94.5 104.0 65.6 234 Chapter XIII--Heating with Gas . It will be noted that for any gas which is low in nitrogen (typified by natural, coke oven, and water gas) the heat content per cubic foot of air and gas mixture has no relation to the initial heat value of the gas. For gases high in nitrogen, such as producer and blast furnace gas, the heat value per cubic foot of mixture is much lower. HEAT VALUE AND EFFICIENCY A gas may be said to have two heat values; a gross or higher heat value and a net or lower heat value. The higher heat value is the entire heat that is liberated by the gas when it is burned completely. The complete combustion of a gas results in the formation of water vapor; the amount depending on the proportion of hydrogen or hydrocarbons in the gas. In order to utilize completely all of the heat of combustion of the gas it would be necessary to condense the water vapor in the products of com bustion; thus reclaiming its latent heat of vaporization, and then to cool down the water to the starting temperature. To condense any of the water vapor in the products of combustion it is necessary to cool them down to the dewpoint, which will always be below 212 deg. This is what is done in a calorimeter, but it is obviously impossible to do it in any commercial gas-burning appliance. Since it is not possible-to utilize all of the heat liberated by the gas in burning, the heat value is sometimes expressed in terms of the lower value; obtained by deducting from the higher value, the total heat of the water vapor down to the starting temperature. The lower heat value is always about 10 per cent less than the higher heat value. Although it is practically impossible to utilize the higher heat value in any house heating appliance, it is nevertheless customary to express boiler and furnace efficiencies in terms of this higher value. This gives a lower efficiency than one calculated from the lower heat value and is of course based on an unattainable standard, but it is a more accurate and consistent way of expressing efficiencies. An appliance may reclaim a little bit of the latent heat of the water vapor. If the test efficiency of such an appliance is calculated from the lower heat value, one is placed in the position of crediting the appliance with some heat that was not charged against it. Although quoted efficiencies are usually based on the higher heat value, care should always be taken to understand which standard guaranteed efficiencies are based on. For example.--Take an hypothetical gas having a gross heat value of 550 B.t.u. per cu. ft. and a net heat value of 500 B.t.tu. per cu. ft., burned in a steam boiler giving an evaporation of 465 lb. of water (from and at 212 deg.) per 1000 cu. ft. of gas burned: B.t.u. in steam Efficiency " B.t.u. in gas X 100 465 X 970.4 cu. ft. X heat value X 100 With Gross Value Efficiency -- 7 .* X 10O = 82.04 per cent ooU|UUU With Net Value Efficiency $ 451,235 500,000 X 100 = 90.25 per cent 235