The largest loss that occurs when fuel is burned for steam generation is the so-called "sensible heat" carried away in the hot flue gas. The efficiency of a steam-generating unit provided with good fuel-burning equipment is a function of .the flue-gas temperature.

Theoretically, the minimum temperature to which the products of combustion may be cooled is the temperature of the heat-transfer surface with which they are last in contact. In the conventional boiler the theoretical minimum flue-gas temperature would be the saturation temperature of the water in the boiler tubes. The relative amount of boiler heat-transfer surface required to cool the products of combustion from 1500° F to lower temperatures is based on saturated water in the boiler tubes at 1000 psi. It will be noted that, as the temperature difference decreases, each increment of added surface becomes less effective and that the amount of surface required to cool the gases from 700° to 600° F is about 60 per cent of that required to cool the gases from 1500° to 700° F.

In general, it is not economical to install sufficient boiler surface to cool the gases to within less than 1500F of the saturation temperature of the water in the tubes, because sufficient heat cannot be transmitted to the tubes at such low temperature difference to pay for the cost of the boiler surface.

The gases must be cooled from the boiler exit-gas temperature to the flue-gas temperature required for high efficiency by means of heat exchangers supplied with fluids at temperatures less than the saturation temperature at the boiler pressure. This can be done in an air heater supplied with the air required for combustion at room temperature or in an economizer supplied with ,boiler feedwater at a temperature considerably below the saturation temperature, or both. In many installations, it is economical to install a small boiler and a large economizer and air heater and to deliver the gases to the economizer at temperatures as high as 900° F rather than to cool the gases to lower temperatures by a larger boiler.

In a typical economizer feedwater is supplied to the inlet header from which it flows through a number of parallel circuits of 2 in. o.d. tubes[11] of considerable length to the discharge header. If the inlet header is at the bottom so that the water rises as it flows from tube to tube, the hot gas nor­mally enters at the top and flows downward. Thus the coldest gas will be in contact with the coldest tubes, and it is possible to cool the gas to within 125° to 150° F of the temperature of the inlet water if sufficient surface is installed.

Since the economizer has water in the tube and a dry gas around the tube, the major resistance to heat transfer is on the gas side. In order to increase the surface exposed to the gas per linear foot of tube and thus increase the effectiveness of the tubular surface, the economizer has fins welded to the top and bottom of each tube. This increases the surface available for heat transfer from the gas without substantially increasing the pressure drop of the gas as it flows across the surface. The gas flows at right angles to the tubes, and the 2-in. finned tubes are staggered to promote effective scrubbing of the outside surface by the gas so as to improve the overall heat-transfer coefficient.

Where scale-free feedwater is available or acid cleaning of heat transfer surfaces is used to remove scale, the flanged return bends may be eliminated. The flow circuits then consist of continuous welded tubing between inlet and outlet headers.

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