Air heaters

axial continuous (barrel) furnaces in section 4.5, and rotary hearth furnaces in section 4.6.1. 

Some comments and warnings from chapter 3, sections 3.8.4 to 3.8.6 for batch-type furnaces operating in this temperature range may be applicable to continuous furnaces as well. 

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Chemical Regeneration. In most MHD system designs the gas exiting the toppiag cycle exhausts either iato a radiant boiler and is used to raise steam, or it exhausts iato a direct-fired air heater and is used to preheat the primary combustion air. An alternative use of the exhaust gas is for chemical regeneration, ia which the exhaust gases are used to process the fuel from its as-received form iato a more beaeftcial oae. Chemical regeaeratioa has beea proposed for use with aatural gas and oil as well as with coal (14) (see Gas, natural Petroleum). 

Fig. 13. Spray-dryer system designed for production of agglomerated food powders with instant properties (82) A, Hquid-feed system B, spray-dryer chamber C, drying air heater D, cyclones for fines recovery E, vibrofluidizer as afterdryer F, vibrofluidizer as aftercooler and G, fines return to drying...

Fig. 11. Two-truck tray dryer. A, air inlet duct B, air-exhaust duct with damper C, axial flow fan D, fan motor, 2—15 kW E, air heaters F, air-distribution plenum G, distribution slots and H, wheeled tmcks and trays. The arrows iadicate air and vapor flow pattern.

Enclosure (with venting air heaters 1 Divided solids feed (far end ). -... 

Steam is the usual heating medium, and a standard heater arrangement consists of a main heater before the circulating fan. When steam is not available or the diying load is small, electricalheat can be used. For temperatures above 450 K, products of combustion can be used, or indirect-fired air heaters. 

Auxiliary Equipment On direct-heat rotating equipment, a combustion chamber is required for high temperatures and finned steam coils are used for low temperatures. If contamination of the produc t with combustion gases is undesirable on direct-heat units, indirect gas- or oil-fired air heaters may be employed to achieve temperatures in excess of available steam. 

System includes finned air heaters, transition piece, dryer, drive, product collector, duct, and fan. Prices are for carhon steel construction and include entire dryer system (Novemher, 1994). 

Coal properties influence pulverizer capacity and the sizing of the air heater and other heat-recovery sections of a steam generator. Furnace size and heat-release rates are designed to control slagging characteristics. Consequently, heat-release rates in terms of the ratio of net heat input to plan area range from 4.4 MW/m" (1.4 X 10 Btii/[h ft ]) for severely slagging coals to 6.6 MW/m (2.1 X 10 Btii/[h ft ]) for low-slagging fuels. 

Storage of heat is a temporary operation since perfect thermal insulators are unknown thus, heat is absorbed in solids or liquids as sensible or latent heat to be released later at designated times and conditions. The collection and release of heat can be achieved in two modes on a batch basis, as in the checkerbrick regenerator for blast furnaces, or on a continuous basis, as in the Ljungstrom air heater. 

External Combustor (experimental). The heat exehanger used for an external-combustion gas turbine is a direct-fired air heater. The air heater s goal is to achieve high temperatures with a minimum pressure decrease. It consists of a rectangular box with a narrow convection section at the top. The outer casings of the heater consist of carbon steel lined with lightweight blanket material for insulation and heat re-radiation. 

Specification for dedicated liquefied petroleum gas appliances - Mobile and portable non-domestic forced convection direct fired air heaters. Superseded BS 4096 1967... 

Selective catalytic reduction (SCR) is cmrently the most developed and widely applied FGT technology. In the SCR process, ammonia is used as a reducing agent to convert NO, to nitrogen in the presence of a catalyst in a converter upstream of the air heater. The catalyst is usually a mixture of titanium dioxide, vanadium pentoxide, and hmgsten trioxide. SCR can remove 60-90% of NO, from flue gases. Unfortunately, the process is very expensive (US 40- 80/kilowatt), and the associated ammonia injection results in an ammonia slip stream in the exhaust. In addition, there are safety and environmental concerns associated with anhydrous ammonia storage. 

This chapter describes the room air conditioning process, including the interaction of different flow elements room air distribution, heating and cooling methods, process sources, and disturbances. Air handling equipment, including toom air heaters, is discussed in the form of black boxes as far as possible. 

Direct air warming is achieved by means of air-heater batteries. 

Air heaters in industrial environments require corrosion-protective finishes that are capable of protecting the coil and case from damage by condensation, acid vapors, or aggressive chemicals, in the air or the primary medium. If air washers are used with coils placed after them, copper or other noncorroding metal tubes should be used. 

Furnace-type air heaters are manufactured from cast iron or steel, and cased in brickwork or steel. The cast-iron heaters are sectional, cemented and bolted together. The steel type are welded or riveted. It is essential that the joints are airtight, so that the cold air can pass over the heated surfaces of the furnace and flues without contamination by the flue gases. 

When selecting air-heater batteries, the following factors common to all types must be considered. 

Dual-circuit heat exchanger Combined air heater and air cooler battery, with independent pipework or ductwork circuits for the heating and cooling media. 

The nomenclature introduced by Hawthorne and Davis [4] is adopted and gas turbine cycles are referred to as follows CHT, CBT, CHTX, CBTX, where C denotes compressor H, air heater B, burner (combustion) T, turbine X, heat exchanger. R and I indicate reversible and irreversible. The subscripts U and C refer to uncooled and cooled turbines in a cycle, and subscripts 1,2, M indicate the number of cooling steps (one, two or multi-step cooling). Thus, for example, [CHT] C2 indicates an irreversible cooled simple cycle with two steps of turbine cooling. The subscript T is also used to indicate that the cooling air has been throttled from the compressor delivery pres.sure. 

Here, we first discuss whether such semi-closure (which is introduced so that CO2 separation can be undertaken more easily) is likely to lead to higher or lower thermal efficiency, and in this discussion it is helpful to consider recirculation in relation to an air standard cycle (see Fig. 8.4). Fig. 8.4a shows a closed air standard cycle with unit air flow Fig. 8.4b shows an open cycle similarly with unit air flow and an air heater rather than a combustion chamber. The cycles are identical in every respect except that in the former the turbine exhaust air from the turbine is cooled before it re-enters the compressor. In the latter, the turbine exhaust air is discharged to atmosphere and a fresh charge of air is taken in by the compressor. The quantities of heat supplied and the work output are the same for each of the two cycles, so that the thermal efficiencies are identical. 

Luft-erhartung, /. air hardening, -erhltzer, m. air heater, -emeuerung,/. ventilation, -er-scheimmg, /. atmospheric phenomenon, -er-scheinungslehre, /. meteorology, -fahrt, /. 

Ash handling Clinker grinder Clinker conveyor Econ. dust conveyor Air heater dust conveyor Rotary valves... 

This design procedure is detailed in the British Gas publication Flues for Commercial and Industrial Gas Fired Boilers and Air Heaters (lM/11). This publication addresses itself to ... 

Boilers and air heaters can be designed to operate in the condensing mode, that is, the outlet temperature of the products entering the flue can be at or close to the dewpoint. The usual design criteria for flues which attempt to avoid condensation are no longer applicable and it is necessary to incorporate additional features. These will include ... 

BS 5990, Specification for direct gas fired forced convection air heaters for space heating (60 kW to 2 MW input) (1981)... 

The steam/process air heater with multiple coils typifies the kind of application on which recovery of flash steam... 

Economizers for boilers have been available for over 150 years, almost as long as boilers themselves. For shell boilers, increasing efficiencies have made it increasingly difficult to justify the use of an economizer, the final decision being based in terms of payback period, which is also heavily dependent on fuel prices. Watertube boilers, on the other hand, need an economizer section in the gas passes in order to obtain satisfactory efficiency. For this reason, the economizer is integrated into the overall design, normally between the convective superheater and the air heater if fitted. 

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