Stack draft

Increase stack draft by increasing height or by use of jets. 

The frictional pressure loss in the stack must be added to the loss in the heater when estimating the stack draft required. This can be calculated using the usual methods for pressure loss in circular conduits, see Section 12.8. The mass velocity in the stack will be around 1.5 to 2 kg/m2. These values can be used to determine the cross-section needed. 

In engineering terminology, the stack draft often is expressed in pounds per second, since this quantity is invariable for different atmospheric pressures and effluent densities. The effluent velocity (V2) varies considerably with such values, however, for any fixed discharge rate and stack diameter. 

Read the stack draft per 100 feet from Figure 1-16. Calculate the draft the stack must produce and the required height. If additional height is required for fume dispersal, the stack diameter z ay be reduced. 

Fig. 7.1. Diagrams showing the cause of stack draft by anaiogy with a U-tube manometer. Soiid iines represent a duct or stack of hot gas dashed iines represent an adjacent coiumn of coid air. The weii, or short, fat ieg of the far right manometer, has a cross section so many times iarger than the ieft ieg that the change in eievation of the right ieg can be ignored.

It is possible to calculate the dimensions of ports and flues so that the resistance of ports and flues will be balanced by the draft (suction) plus furnace pressure. However, good practice in automatic furnace pressure control usually necessitates a stack damper that always takes a minimal pressure drop. Therefore, the real balance is stack draft -I- furnace pressure = AP furnace exit orifice + AP stack skin friction -I- A P damper. Tables 7.2 and 13 from Prof. Trinks fifth edition list information for a few specific cases that illustrate points mentioned earlier and equations 7.3,7.4, and 7.5 below. 

Once the reactor has been placed in operation there will always be active particles within the air passages. To prevent these active particles from entering the Reactor Building, a basic requirement of the reactor air system is that there always be a flow of air in the proper direction. Upon failure of the main exhaust blowers and the auxiliary fans, this flow will he induced by whatever stack draft is available. 

After the reactor has been in operation long enough so that the reactor, reactor air system, and stack are hot, there is a stack draft of about 0.5 in. of water available. This is based on air temperatures of 200°F in the stack and 90°F. atmospheric. I.t is estimated that this, draft.would pr.oduce a flow of 2000 to 3000. cfm through the reactor in..the event of failure of all blowers... 

Also, because of the use of positive displacement blowers, it was necessary to provide h by-pass around all the blowers and fans so that the stack draft will be available to provide a flow of air through the reactor air system. The by-pass is provided by a valve between the plenum chamber ahead of the fans and blowers and the exit duct leading to the stack. This valve will be operated manually.. ... 

Efficient heater operation requires that excess air entering the convection section be minimized, whieh is indicated by a very small negative pressure at the convection section inlet. To achieve this, it should have a well balanced draft pressure profile between the firebox and stack. The hot gas pushes so that the pressure is always greatest at the firewall while the stack draft pulls. When this draft is eorreetly balanced, the pressure at the bridge waU should be around 0.1-0.2 WG (water gauge). Too mueh draft allows cold air leakage into the fired box resulting in wasted fuel. 

Stack draft The magnitude of the draft measured at the inlet to the stack. 

Stack Draft. The draft that stacks or chimneys will produce is discussed in all boiler and combustion textbooks. A draft of 0.5 to 0.7 in. water is adequate for nearly all refinery stills. The draft at the burner... 

Example 14-8. Effect of Heat Losses on Stack Draft. Figure 14-6 is used as follows ... 

The area through which heat is transferred may be considered as the outside area of the tubes because the gas film coefficient controls. The gas velocity can seldom exceed 12 ft per sec in stills that are dependent on a stack for draft. A stack draft of 0.6 in. of water will cause a velocity of about 15 ft per sec, but in the now popular long-tube stills (Fig. 18-ld, h h velocity ranges downward to 4 ft per sec. In recirculation... 

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