Furnace pressure control

Fig. 1.18 Car-hearth heat treat furnace with piers for better exposure of bottom side of loads. The spaces between the piers can be used for enhanced heating with small high-velocity burners. (See chap. 7.) Automatic furnace pressure control allows roof flues without nonuniformity problems and without high fuel cost.

Figure 3.5 shows a 40 ft (12.2 m) long car-hearth in a 17.5 ft (5.3 m) high fiber-lined furnace with high-velocity burners at top and between the piers. Automatic furnace pressure control makes it possible to use top flues. Drilled square air manifolds shoot curtains of air across the flue exits as throttleable air curtain dampers for furnace pressure control. 

Bottom flueing is preferred, but in-the-wall vertical flues have been found too costly, and they pull a harmful negative pressure at the hearth level. With top firing, the best arrangement is hearth-level flues with automatic furnace pressure (damper) control. If fired with top and bottom burners, use of a roof flue with automatic furnace pressure control is suggested. The flue location should be determined to enhance the design circulation pattern. (See chap. 7.)... 

Use an automatic furnace pressure control with the setpoint at -t-0.02" wc (0.05 mm water gauge) to prevent air inflow... 

Install a minimum of two fixed baffles and one movable baffle. Provide a furnace pressure control system if the present control is inadequate. 

Better furnace pressure control to minimize leaks and nonuniformities... 

Sensible heat carried out of the furnace by the furnace gases (poc) is often the largest loss from high-temperature furnaces and kilns. It is evaluated by the available heat charts mentioned in section 5.1 100% — %available heat = %heat carried out through the flue. It can be reduced by careful air/fuel ratio control, use of oxy-fuel firing, and good furnace pressure control. 

Furnace Pressure Control. This type of control prevents excessive outleakage of unburned air, unburned fuel, poc, and pic (products of incomplete combustion) before they have had time to transfer heat to the loads. Chapter 7 of reference 51 describes how a variety of furnace pressure control systems work and how to evaluate the savings from their use. Furnace pressure control also prevents unnecessary infiltration (inleakage) of unwanted tramp air, which is excessive excess air. 

The tramp excess air also will absorb some heat from the load or furnace, and carry that heat out the flue. The cold excess air tends to creep across the hearth and up the flue without helping to burn fuel or circulate heat. For this reason, industrial furnace engineers advocate holding a slightly positive furnace pressure (+0.02"wc, +0.51 mm H2O) at the level of the lowest possible leak. (See Furnace Pressure Control in pt 7 of reference 52.)... 

For metallurgical reasons, some rotary hearth furnaces are divided into sections by radial baffles. Rotary furnaces designed to heat rounds for seamless tube mills have some very special problems (1) furnace pressure control, (2) air/fuel ratio... 

Furnace Pressure Control. Extraction of load pieces may be as frequent as one to four pieces per minute therefore, door maintenance is difficult, with the result that discharge doors are often left open. These doors may be very large to accommodate a peel bar mechanism, so leaving a door open permits a large quantity of furnace gas to escape and results in loss of heat and furnace pressure. This problem, combined with the two-way combustion gas flow of a rotary hearth furnace, necessitates three baffles. This solution is described in the following paragraph. 

If the furnace is fired only with conventional (type A) burners or with long-flame (type F or G) burners (fig. 6.2), in its outer wall, the recommended positioning usually puts loads where they can benefit most from the radiation and convection characteristics of those flames. This combination plus two more baffles (to control gas movement and allow effective furnace pressure control, and reinstating the firing of zone 1 almost to the charge door) raised the furnace capacity (figure 6.7). 

Fig. 6.13. Effects of furnace temperature and input on the level of the neutral pressure plane elevation shown on six sectional elevation views of a furnace with no furnace pressure control. If there were any gas flow in the furnace, the neutral pressure plane would be more like a wrinkled sheet than a plane. The top three show the effect of temperature with no change in input. The bottom three show the effect of input rate with no change in furnace temperature.

The next choice would be to locate the tap in the wall opposite the burners, but equally spaced between the burner centerlines and elevated 2 feet above the hearth. The setpoint of the furnace pressure control will have to be biased to correct for the difference in elevation between the pressure tap and the desired level of the neutral pressure plane (at the hearth). Interpolating from table 6.2, the setpoint bias should be 0.0118 in. X 2 feet of elevation = 0.0236, or say 0.025 or 0.03 in. wc to allow for expected wear on the car seals. 

How should an engineer select situation 1, 2, 3, 4, or 5 (i.e., automatic furnace pressure control setpoint) for the pressure sensor location And is the pressure sensor located properly for the process ... 

Encourage operators to be proud of the prime condition of their furnaces. Keep an automatic furnace pressure controller, and its sensing lines, clean and in good operating condition. 

Section 6.6.2 gives recommended details and locations of furnace pressure control sensors and their compensating (room pressure) taps. 

TABLE 7.1 Elevation bias corrections for furnace pressure control setpoint when the furnace pressure sensor is above desired control level... 

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. 

Fig. 7.5. Back-wall-fired in-and-out furnace. Stacks without bottom openings (without barometric dampers) must have automatic furnace pressure control.

---