Heat flux density

P = total pressure atm. q = heat-flux density, energy per time-area. 

Temperature and contaminant gradients along the room height and separation stability between the upper and lower zones are influenced by turbulent exchange between these zones. The heat flux density due to turbulent exchange can be determined as ... 

Lihou and Maund (1982) based their radiation limit on the work of Stoll and Chianta (1971). The average heat-flux density q- which will cause severe blistering... 

In addition to external conditioning processes and the need to provide internal chemical treatments to some or all steam-water circuits within the steam cycle, the scope of boiler water treatment includes, as mentioned earlier, the provision of suitable technical resources sufficient to control the steam-water chemistry within defined limits appropriate for the boiler plant under consideration. Because these steam-water control limits tend to narrow considerably with increase in boiler pressures (and heat-flux densities), suitable monitoring and control procedures may require implementing actions with knife-edge precision. 

Finally, although the basic development and nature of boiler water plant problems may be similar to those arising in other types of water systems (such as cooling water systems), the extremely high steam-water temperatures and heat-flux densities generally encountered impart a much higher level of intensity. This in turn creates the need for more highly focused and effective solutions to boiler plant problems. 

The tendency to form boiler waterside deposits is partly dependent on factors such as the solubility of the particular mineral species and the strength of physical adherence involved. As a general rule, the rate of deposition tends to increase with higher levels of BW dissolved solids. Also, the rate of deposition increases with increase in heat-flux density and with the inadequate dosage, inappropriate feeding, or otherwise usage of antisealants and other deposit control agents (DCAs). 

Under the same conditions of low heat-flux density, poor water chemistry, and gradual scaling circumstances, FT boilers may easily lay down from 1/16 to 1/4 inch of scale (6-12 mm) or more during a 12-month period. Tubes may become bridged by scales and sludges. Again, cleaning is required and the extra costs incurred for wasted fuel can never be recovered. 

The position is slightly different for LP steam-heating boilers, and a softener normally is not required if requirements for MU water do not exceed a minimum level of 5% or if heat-flux densities are low. 

Where LP steam boiler MU requirements are in excess of 5% or heat-flux densities are high but the source of MU water is a naturally low-hardness lean water (as in New York City, where total hardness is often below 20 ppm as CaC03), it is still advisable to install a water... 

If BW silica levels increase above 180 to 200 ppm, it may not, in fact, be possible to totally control silica deposition by water chemistry means alone. Also, as boiler pressures, temperatures, and heat-flux densities increases, so does the need for external silica removal equipment. 

This level of deposition cannot be tolerated in any modem boiler design, irrespective of where in the world a boiler operates or the type of duty required. Even limiting the maximum acceptable thickness of boiler waterside deposition to an eggshell thickness standard, as once proposed by many people in the industry, is not satisfactory today. Indeed, under modem higher heat-flux density conditions, operators of industrial WT boilers working to this standard would have to contend with frequent disruptions because of mptured tubes or worse, while FT boilers might eventually suffer from a collapsed furnace and need to be written off. 

Boiler metals used and heat-flux density requirements... 

In the BW market, many of the modem polymers have demonstrated their ability in controlling specific waterside deposition problems. Use of the materials has led not only to cleaner boilers but also to higher customer expectations and the all-round confidence to operate boilers at higher heat-flux densities than in the past (especially in the medium pressure ranges of, say, 250 to 900 psig). 

Strict limitations on the salinity content of BW become increasingly important as boiler pressure or heat-flux density increases. This is primarily effected by ... 

Although the risk of scale deposition and fouling in the boiler section is related to several factors such as the FW volume demands, boiler pressure, and heat flux density at various boiler surfaces, it is equally a function of the level of FW contaminants such as residual hardness, sulfates, silica, and iron. Thus, as a generality, the higher the quality of FW (reduced levels of contaminants), the lower the risk of deposition on boiler surfaces. 

Noselite (5Na20 3Al203 6Si02 2H20) is a hard, adherent scale found on boiler tubes and other areas of high heat-flux density. 

The formation of complex silicate scales takes place at high temperature (usually they are only found in boilers operating at over 300 psig) and high heat-flux density points in the boiler section. The presence of complex silicates such as analcite and acmite may indicate steam blanketing problems. 

Noselite 5Na203Al203-6Si02-2H20 Hard, adherent scale found on tubes and high heat-flux density areas. 

For simple HW and LP steam heating applications, employing perhaps a Scotch marine or firebox FT boiler, the water quality requirements are unlikely to be onerous, but as pressure ratings and heat-flux densities increase (and other factors such as special boiler designs and steam usage applications are taken into consideration) water quality becomes increasingly important. Thus, in general ... 

FW and BW qualities are related to boiler pressure and heat-flux density, such that increasingly higher pressure or highly rated boilers tend to require increasingly higher quality and higher purity FW. 

Boilers operating up to 40 bar (580 psig), but which have a local heat flux density greater than 300 kW/m2 should be treated as those in the column 41 to 60 bar. 

Heat flux density Heat recovery boiler, see Boiler, water tube, heat recovery 219... 

Surface energy (surface tension) Mass flux density Heat flux density... 

Heat flux density watt per square meter W/m2... 

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