Forced draft installations

Steel chimneys have the advantages of lightness and strength, but since they are better conductors of heat, must be lined with brick for heights over 75 ft. except in forced-draft installations. They must be carefully inspected and painted from time to time, as they are subject to deterioration by corrosion. 

Induced-draft units are less likely to recirculate the hot exhaust air, since the exit air velocity is several times that of the forced-draft unit. Induced-draft design more readily permits the installation of the aircooled equipment above other mechanical equipment such as pipe racks or shell-and-tube exchangers. 

A boiler had been shut down for the repair of a forced draft fan. A blind was not installed in the fuel gas line, nor apparently was a double block and bleed in the fuel line utilized. Gas leaked into the firebox during the repair period and was not removed. A severe explosion occurred during the attempt to light of. 

Normal recirculation in average installations for forced draft may run 3-10% of total inlet air, and 1-8% for induced draft towers, all depending upon the location and wind conditions during any day or season. Some towers can be arranged to have less than 1% recirculation. If conditions are suspected of being conductive to recirculation, it should definitely be allowed for in design of the tower. Recirculation increases the wet bulb temperature of entering air, increases the total air required (and hence size of... 

Figure 10-175 illustrates the assembly of a typical forced draft unit with electric motor and gear drive. Note that walkways and access ladders are necessary to reach the exchanger connections where valves are usually installed. If... 

Figure 23.8 is a schematic illustration of such a unit. It is desirable for each boiler to have its own economizer. Where one economizer is installed to take the exhaust gases from more than one boiler special considerations must be taken into account. These will include gas-tight isolation dampers. Consideration must be made of flue-gas pressures at varying loads and maximum and minimum combined heat load to match economizer and a pumped feedwater ringmain. Economizers may be used for both forced-draft and induced-draft boilers, and in both cases, the pressure drop through the economizer must be taken into account when sizing the fans. 

Remember that outside influence (for example, new building work in the vicinity of the installation) can increase air-based pollution, such as cement dust entering the tower at air inlet levels or via the forced-draft fan. 

Where the inlet temperature of the process fluid is such that the outlet temperature from the convection section would be excessive, giving low thermal efficiency, this excess heat can be used to preheat the air to the furnace. Tubes would be installed above the process fluid section in the convection section. Forced draft operation would be needed to drive the air flow through the preheat section. 

Today, burner manufacturers have access to all kinds of designs for pilot lights, to fit all shapes and sizes of burners and burner ports and draft systems from the smallest breeze to gale-force winds—there is no longer any excuse to operate without pilot lights installed and working. 

Steam systems. An investment cost of 35 per lb/h of total steam generation capacity is recommended for preliminary estimates (costs as of 2001). This represents the total installed costs for gas- or oil-fired, forced-draft boilers operating at 250 to 300 psig, and all appurtenant items such as water treating, deaerating, feed pumps, yard piping for steam, and condensate. 

Obtain the engineering data for commercially available forced-draft fans and turn to the temperature and altitude correction-factor tables. Pick the appropriate correction factors from these tables for the prevailing temperature and altitude of the installation. Thus, in Table 6.30, select the correction factors for 90°F air and 5000-ft altitude. These correction factors are CT = 1.018 for 90 I air and CA = 1.095 for 5000-ft altitude. 

A letter sent by Topf to the SS New Construction Office of the concentration camp Mauthausen on November 1, 1940 contained the cost estimate for a coke-fired Topf double-muffle cremation oven with forced-air installation and for a Topf draft-enhancing installation , 132 The letter states 133... 

Also, the three forced-draft blowers initially installed in crematorium II of Birkenau worked with a pressure of 30 mm of water column, with a gas volume of 40,000 m3/h, each driven by a 380 Volt/15 HP engine. The standard forced-draft blowers installed at the oven of Gusen camp were also installed in the crematorium at Auschwitz with a gas volume of 4,000 mVh and an engine of 3 HP. The pressure difference it produced is not known, but it sure was not higher than 30 mm water column. 

The Ignis-Hiittenbau ovens in Terezin were by far the most modem and efficient of all those ever installed in German concentration camps. Their design had been inspired by the gas-fired Volckmann-Ludwig ovens. Additionally, they were equipped with a powerful forced-draft blower and an adjustable oil burner. We will later return to these special installations. 

Mechanical draft may be afforded by a steam jet, by blast fans (forced draft) discharging to the ashpit, or by suction fans (induced draft) placed between the boiler and the short stack. Steam jets, whether of the suction or pressure type, are wasteful of steam, though cheap to install. They will not give very high drafts, but tend to prevent clinkering. 

Boiler cost estimates include fittings, refractory, insulation and cladding for outdoor installation, internal piping, platforms and stairs, burner and flame safety equipment, economizer, force-draft fan plus drive and soot blowers. 

Increase convection heat recovery Install bigger impellers in forced draft fans and induced draft fans (PDF and IDF), bigger FDF/IDF fans and/or motors... 

The DTB crystallizer can also be operated as a conventional evaporator-crystallizer by incorporating a steam-heated heat exchanger in the clear liquor recycle line. In this case, however, an elutriating leg is not normally installed and the recycled hot liquor stream enters directly into the draft-tube. Detailed design calculations for a forced circulation evaporative crystallizer of the DTB type to produce 75ton/day of >200 pm urea crystals have been described by Bennett (1992). 

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