Aaberg exhaust hoods


The mathematical model presented in the previous section has been developed under the assumptions that the flow induced by an Aaberg exhaust hood is inviscid and potential and that turbulent effects have been limited to the flow in the jet. However, the typical experimental operating conditions of an Aaberg exhaust hood lead to Reynolds numbers in the order of 10 to 10. Thus the fluid flow in the jet and in the region surrounding the exhaust inlet are very likely to be turbulent. However, in the region of practical interest, i.e., the region of the flow where there is likely to be large amounts of contaminant, the airflow created by the Aaberg exhaust hood is a convergent flow and therefore in this region we expect a low level of turbulence.  [c.964]

Mathematical Model for an Axisymmetric Aaberg Exhaust Hood  [c.964]

G. R. Hunt. The fluid mechanics of the Aaberg exhaust hood. Ph.D, thesis, University of Leeds, 1994.  [c.1010]

It should be noted that when there is no jet reinforcement of the flow, i.e., the exhaust hood is used in its conventional mode, then in the two-dimensional form of the Aaberg principle the fluid flow velocity due to the exhaust decays approximately inversely proportionally to the distance from the exhaust opening. However, for three-dimensional exhaust hoods the fluid velocity outside the hood decays approximately inversely as the square of the distance from the exhaust hood. Thus in the three-dimensional conventional hood operating conditions the hood has to be placed much closer to the contaminant in order to exhaust the contaminant than is the situation for the two-dimensional hood (see section on Basic Exhaust Openings). Thus for ease of operation it is even more vital to develop hoods with a larger range of operation in the three-dimensional situation in comparison with two-dimensional hoods.  [c.961]

Supplementary fired cycle The recovery of heat from gas turbine exhaust by the generation of steam has, as its basic limiting factor, the approach temperature at the pinch point (see Figure 15.11). The principal pinch point occurs at the cold end of the evaporator and consequently the combination of the steam-generating pressure and the magnitude of the approach determine the amount of steam which can actually be generated for a given gas turbine. In single-pressure systems, the only heat which can be recovered below the evaporator pinch point is to the feedwater, and this, in turn, now determines the final stack temperature. If, however, a small amount of fuel is used to supplement the gas turbine exhaust heat, then while the pinch point  [c.182]

Combined Cycle. Combined cycles use a steam turbine system as a bottoming cycle for a combustion turbine. Exhaust gases leave the combustion turbine at 510—593°C (950—1100°F). This heat is wasted in the simple combustion turbine cycle. However, it can be used to boil water in a heat recovery steam generator (HRSG). This process lowers the temperature of much of the rejected heat to that leaving the steam turbine. However, to prevent HRSG corrosion, the stack gas temperature is nominally 150°C (302°F), causing additional heat rejection to the environment. Because the combustion turbine does not need preheated air, there is no air heater in combined cycles. The economizer of the HRSG can extract as much heat as possible from the combustion gas. Thus the feedwater heating of conventional utiHty cycles is not needed. There is usually only a single direct-contact heater to remove oxygen from the feedwater. Combined cycle power plants have achieved 58% and newer designs are expected to approach 60% thermal efficiency (32).  [c.367]

When the partial pressures of the radicals become high, their homogeneous recombination reactions become fast, the heat evolution exceeds heat losses, and the temperature rise accelerates the consumption of any remaining fuel to produce more radicals. Around the maximum temperature, recombination reactions exhaust the radical supply and the heat evolution rate may not compensate for radiation losses. Thus the final approach to thermodynamic equiUbrium by recombination of OH, H, and O, at concentrations still many times the equiUbrium value, is often observed to occur over many milliseconds after the maximum temperature is attained, especially in the products of combustion at relatively low (<2000 K) temperatures.  [c.516]

Centrifugal Discharge. In centrifugal discharge elevators, the buckets are spaced apart on the chain or belt. Material is scooped from the boot and then discharged by centrifugal force as the buckets approach and pass over the head pulley or sprocket. Speed of the chain or belt is critical to proper discharge of the material. The critical speed has been defined as the speed at the point where the centrifugal force at the center of mass of the material in the bucket is equal to the gravitational force (30). The best operating point for an elevator is where the centrifugal force is two-thirds the gravitational force. The effects of bucket geometry and speed on material discharge have also been reported (31,32). Changes in exit trajectory of the material leaving the bucket as a result of material friction on the bucket wall and the change in center of gravity of the mass as it sHdes along the wall have been investigated.  [c.159]

For estimating purposes for direct-heat drying applications, it can be assumed that the average exit-gas temperature leaving the sohds bed wih approach the final solids discharge temperature on an ordi-naiy unit carrying a 5- to 15-cm-deep bed. Calculation of the heat load and selec tion of an inlet-air temperature and superficial velocity (Table 12-32) will then permit approximate sizing, provided an approximation of the minimum required retention time can be made.  [c.1224]

The economizer in the system is used to heat the water close to its saturation point. If they are not carefully designed, economizers can generate steam, thus blocking the flow. To prevent this from occurring the feed-water at the outlet is slightly subcooled. The difference between the saturation temperature and the water temperature at the economizer exit is known as the approach temperature. The approach temperature is kept as small as possible between 10-20 °F (5.5-11 °C). To prevent steaming in the evaporator it is also useful to install a feedwater control valve downstream of the economizer, which keeps the pressure high, and steaming is prevented. Proper routing of the tubes to the drum also prevents blockage if it occurs in the economizer.  [c.91]

Fire tube boilers are widely used to recover energy from waste gas streams commonly found in chemical plants, refineries, and power plants. Typical examples are exhaust gases from gas turbines and diesel engines, and effluents from sulfuric acid, nitric acid, and hydrogen plants. Generally, they are used for low-pressure steam generation. Typical arrangement of a fire tube boiler is shown in Figure 1. Sizing of waste heat boilers is quite an involved procedure. However, using the method described here one can estimate the performance of the boiler at various load conditions, in addition to designing the heat transfer surface for a given duty. Several advantages are claimed for this approach, as seen below.  [c.150]

The temperature is approximately 20°F below the 265°F temperature limit. The sections differ by less than 1 F. This is probably just luck because that good a balance is not really necessary. Also, it should be noted that to maintain simplicity the additional factors were ignored, such as the 10°F temperature pickup in the return stream due to internal wall heat transfer. Also, nozzle pressure drops for the exit and return were not used. Balance piston leakage was not used as it was in Example 5-3. When all the factors are used, the pressures for each section would undoubtedly need additional adjustment as would the efficiency. However, for the actual compression process, the values are quite realistic, and for doing an estimate, this simpler approach may be quite adequate,  [c.183]

During the normal Mode of Operation of the system the process air enters the RTO System Fan and passes through the Inlet Diverter Valve where the process air is forced into the bottom of the left ceramic heat transfer bed. As the process air rises through the ceramic heat transfer bed, the temperature of the process stream will rise. The tops of the beds are controlled to a temperature of 1,500 F. The bottoms of the beds will vary depending upon the temperature of the air that is coming in. If it is assumed that the process air is at ambient conditions or 70 F, then as the air enters the bottom of the bed, the bottom of the bed will approach the inlet air temperature of 70 F. The entering air is heated and the media is cooled. As the air exits the ceramic media it will approach 1500 F. The process air then enters the second bed at 1,500 F and now the ceramic media recovers the heat from the air, and increases in temperature. At a fixed time interval (usually 4 to 5 minutes), or based on thermocouple control, the diverter valves switch and the process air is directed to enter the bed on the right and exits the bed on the left. Prior to valve switching the air heated the right bed and now this bed is being cooled. The cooling starts at the bottom and continues upward because the media is hot and the energy is transferred. The process air then goes through the purification chamber and exits through the second bed. When the valves are switched, whatever organics had not been destroyed prior to the flow being reversed are then exhausted out of the stack. In addition, the rapidity of switching or closure of the valves is critical to minimize the bypass of unoxidized organics. If the emissions versus time were plotted, the graph would reflect a very low exhaust concentration level, but whenever the diverter valve switches an organic pulse occurs in the exhaust stream. Since the valves shift every four minutes these pulses reduce the overall destruction efficiency of the organics. Several methods of processing the pulse exist in order to achieve higher destruction efficiencies.  [c.484]

When dynamic simulation is used for process equipment and process safety design, it is necessary to ensure the model s assumptions are conservative. For example, if dynamic simulation is used to calculate the pressure rise in a heat exchanger after a tube rupture, the highest calculated pressure may be used as the design pressure. If all the assumptions are conservative, the actual heat exchanger pressure will not exceed the design pressure during a tube rupture. Despite this conservative approach, equipment design conditions calculated by dynamic simulation are often much less severe than the conditions determined by conventional calculation methods. This often leads to considerable cost savings. Dynamic simulation software should support the addition of user-written code for specialized equipment and control system models. For example, an unusual fractionator tray design or a correlation for an off-design heat transfer coefficient may have to be programmed into a user-written model. Dynamic simulation of "first-of-a-kind" plants often requires developing a dynamic model for a new equipment item. A control system vendor s DCS algorithm may also need to be programmed into a custom PID controller model. Users may need to add their own fluid property systems to increase computational efficiency and handle unusual systems. "Black box" models are too restrictive to provide realistic models for most dynamic simulation problems.  [c.46]

For a new process plant, calculations can be carried out using the heat release and plume flow rate equations outlined in Table 13.16 from a paper by Bender. For the theory to he valid, the hood must he more than two source diameters (or widths for line sources) above the source, and the temperature difference must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests. - If exhaust volume requirements must he established accurately, small scale modeling can he used to augment and calibrate the analytical approach.  [c.1269]

Fume velocity can be measured at the roof truss level by using several propeller-type anemometers mounted on a grid. The output from the anemometers can be connected to a recorder located at the operating floor level. Experience has shown that six to eight anemometers are usually sufficient to give a good description of the plume velocity. This approach has the advantage of obtaining both a velocity profile and an average velocity for the plume. The combined information of velocity distribution and observed plume size provides the necessary design parameters to ensure a satisfactory performance for the designed hood. Figure 13.29 is a plot of average plume flow rates measured at roof truss level as a function of time for a typical tapping operation on an electric steelmaking furnace. To carry out such velocity measurements as a standard method is not recommended because fume and dust tend to harm the propeller hearings, making the anemometer inoperative after a number of tests.  [c.1269]

For the rule-of-thumb approach, the control velocities required through A and Ai to prevent fume emissions from the hood are obtained from standard references. - The exhaust required to control emissions from the opening A is greater when the pot is empty. The short-circuiting of flow Q2 is not a serious problem as long as the clearance A2 is small. These rule-of-thumb estimates can be improved by applying the three well-established fluid mechanics equations governing conservation of mass, energy, and momentum to the fume collection process.- These equations simply state that flows in must equal flows out and that all flow forces (pressures) must be balanced at all times. Unfortunately, a high degree of inaccuracy still exists in this approach because of the large number of assumptions regarding the expected flow field in the hood. For industrial applications in which complex hoods are needed, it  [c.1277]

There are of course also disadvantages in this approach these are essentially the same as the advantages The seam method automatically includes the effect of different reaction energies, since a more exothermic reaction will move the TS toward the reactant and produce a lower activation energy (Section 15.5). This, however, requires that the force field is able to calculate relative energies of the reactant and product, i.e. the ability to convert steric energies to heat of fonnation. As mentioned in Section 2.2.9, there are only a few force fields which have been parameterized for this. In practice this is not a major problem. Usually the reaction energy for a prototypical example of the reaction of interest can be obtained from experimental data or estimated. Using the normal force field assumption of transferability of heat of formation parameters, the difference in reaction energy is thus equal to the difference in steric energy. Only the reaction energy for a single reaction of the given type therefore needs to be estimated, and relative activation energies are not sensitive to the exact value used.  [c.49]

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature.  [c.182]

The easiest approach to a solution is likely to be that of altering the environment or to reduce the residual stress levels in the component by appropriate heat treatment providing the latter does not induce deleterious changes in microstructure (sensitisation). However, the aggressive environment may have arisen from a concentration mechanism, and in such cases small changes in the bulk environment are unlikely to be an eflfective solution to the problem. Care should be taken with welding procedures, or new welding procedures should be devised. An example of the latter is the heatsink welding that has been successfully applied to 304 tubing in BWR cooling circuits.  [c.1223]

The design of the axisymmetrical Aaberg exhaust hood is very similar to a traditional flanged hood. However, it is fitted with a flange through which air can be ejected radially from a narrow slot (see Fig. 10.77). The dramatic effect of the blowing jet on the hood s overall airflow can be explained as follows due to the friction developed at the radial jet/air interface an entrainment flow develops which, under the correct conditions, has the property of removing the clean air from in front of the hood (the recycled flow) as well as enhancing and concentrating the exhaust s suction in a zone along the hood s longitudinal axis (the efficient flow). The flow in front of the exhaust opening is now directional and the process is capable of creating a larger fluid flow toward the exhaust opening at greater distances along the axis of the exhaust hood. Further, although replacement air should still be supplied, the Aaberg exhaust works with sig-  [c.956]

Applications. One of the most demanding appHcations for zinconia ceramics is in automotive engine parts, particulady for the diesel engine (130). AppHcations attempt to exploit its low thermal conductivity and/or the wear-resistance characteristics. One approach utilizes ceramic liners or inserts (eg, piston crowns, head face plates, and piston liners) attached to metal engine components. PSZ is a favored material for this approach, not only because it has low thermal conductivity and is a good insulator, but more importantiy, because its high thermal expansion coefficient is close to that of cast iron. This compatibihty faciHtates attachment and reduces the possibiHty of failure during engine cycling. Other engine appHcations for zinconia include components which are limited by wear, particularly in the valve train,such as cams, cam foUowers, tappets, and exhaust valves.  [c.325]

Fastness. The fastness of a colored pigment defines its inherent abiUty to withstand the chemical and physical influences to which it is exposed during and subsequent to its incorporation into a pigmented system. Fastness describes the characteristics of a pigment in terms of its color stabiUty in a pigmented system upon exposure to light, weather, heat, solvents, or various chemical agents. Ideally, a pigment should be insoluble and chemically and photochemically inert. Only a few organic pigments approach such perfection. Fastness properties which ate of practical significance are those observed for pigments incorporated into a coating or resin sysem under the exact conditions of incorporation and use.  [c.23]

Heat Kecope Steam Generators. Heat recovery steam generators, a special class of boilers where essentially all heat transfer takes place convectively, are used to extract energy from hot gas streams. One of the principal uses is extraction of heat from the exhaust gases of a combustion turbine. Turbine exhaust gas is typically 540°C (1004°F). Carehil design of the HRSG allows the gas leaving the HRSG to approach the feedwater temperature within 28°C (50°F). High heat-transfer rates in modem HRSGs are assisted by finned tubing.  [c.359]

Here, is the mole fraction of O2 in the products at temperature T, and the rate is given in ppm/ms. The exponential implies a large effective activation energy of 570 kJ /mol, the sum of that for the O—N2 reaction and half the dissociation energy of O2. In typical hydrocarbon—ak flames, the rate of NO formation by the thermal mechanism can be shown to be about 8 ppm/ms, or in a 10 ms residence time the thermal NO would be about 80 ppm. If preheating the mixture were to raise the gas temperature by 100 K, the rate of NO production would be nearly tripled, making the NO concentration unacceptable. Conversely, the rate can be reduced by the same amount by a 100 K reduction in temperature by precooling or heat abstraction from the flame itself, or by dilution of the mixture with excess ak, steam, or other inert gas such as reckculated, relatively cool exhaust gas. Control of thermal NO thus involves reduction of the maximum attainable temperature, or the residence time at high temperature, or both. Such measures, however, always entail some compromise in stabiUty and control, and possibly also in the efficiency of the combustion process. The afterburning of CO tends to be quenched by rapid temperature reduction, and the resulting increase in the emission of CO must be balanced against the desked NO reduction. Heat abstraction or cooling of the flame always occurs to some extent by radiation from the highly luminous flames produced by pulverked-coal or oil combustion, as is typical in boilers and similar furnaces. When heat is rejected, eg, by a boiler fluid, and not returned or recuperated to the unbumed mixture, the maximum temperature and thermal NO formation will be reduced. An extension of this effect has been appHed to achieve low NO emissions in some furnaces and boilers in which combustion occurs in a very rich, relatively low temperature primary stage, followed by heat abstraction by convection as well as radiation to reduce the gas enthalpy (two-stage combustion). Secondary products and any excess ak are then introduced to complete the combustion and, owing to the previous heat transfer, the maximum temperature attainable in that stage will never approach the adiabatic flame temperature. Much soot, which is responsible for the radiative heat loss, may be present in the rich primary flame products. To avoid smoke from such two-stage processes, care must be taken to assure its oxidation in the second stage. In practice [O] / [O] is seldom unity as assumed. Though [O] is decreased in the burned gas, its average value may be several times [O] and NO formation may be correspondingly higher than predicted from the [O]. Similarly, the very high radical concentration, eg, [O], in the reaction 2one of a flame often leads to almost instantaneous NO production, even though the temperature is still relatively low and the residence time is relatively short. Other fast reactions involving transient flame species producing N atoms, for example,  [c.529]

To illustrate the power of the transfer matrix approach and, at the same time, the effect of the dimension M, we calculate the heat of adsorption for a simple adsorbate on a square lattice with nearest neighbor repulsion [29]. In Fig. 2(a) we show Qiso 0, T) for a set of reduced temperatures, T/Vi , obtained (a) in the (analytical) quasichemical approximation, Eq. (16), (b) from the M = 4 transfer matrix, Eq. (37), and (c) for M = 8, for which the results are essentially exact. The drop at half coverage has been discussed earlier. The significant new features emerging from the semi-infinite (M x oo) lattice are (/) the sharpening of the plateaus at lowest T, and ( ) the appearance of local maxima and minima around half coverage, most pronounced for temperatures less than the ordering temperature of the c 2 x 2) structure at Tc/Vxn 0.57. This latter feature has been experimentally observed and quantitatively explained for CO on (hexagonal) Ru(OOOl) [30]. It cannot be reproduced in any finite cluster calculations because they treat the statistics of lattices of such small size that they miss much of the variation of entropy that occurs upon ordering. The correlators and comparison TPD spectra, corresponding to the panels in Fig. 2, can be found elsewhere [29].  [c.453]

Thermochemical recuperation (TCR), also known as chemical recuperation, has been under evaluation for several years as a promising approach to increase power generation efficiencies. In a TCR power plant, a portion of the stack exliaust gas is removed from the stack, compressed, mixed with natural gas fuel, heated with exliaust heat from the gas turbine, and mixed with the air compressor discharge as it enters the combustor. As the mixture of natural gas and flue gas is heated by the gas turbine exhaust, a chemical reaction occurs between the methane in the fuel and the carbon dioxide and water in the flue gas. If this reaction occurs ill the presence of a nickel-based catalyst, hydrogen and carbon monoxide arc produced. For complete conversion of the methane, the effective fuel heating value is increased. Therefore, the natural gas/fltte gas mixture absorbs heat thermally and chemically, resulting in a larger potential recuperation of exhaust energy than could be obtained by conventional recuperation, which recovers energy by heat alone. In fact, tvith full conversion of the natural gas fuel to hydrogen and carbon monoxide, up to twice the energy recuperated by the standard recuperative cycle may he recovered.  [c.1176]

In parallel operation (sensible heat transfer), fluids A and B (Figure 10-30) flow in the same direction along the length of travel. They enter at the same general position in the exchanger, and their temperatures rise and fall respectively as they approach the outlet of the unit and as their temperatures approach each other as a limit. In this case the outlet temperature, tg, of fluid B, Figure 10-30, cannot exceed the outlet temperature, Tg, of fluid A, as was the case for counterflow. In general, parallel flow is not as efficient in the use of available surface area as counterflow.  [c.56]

Laboratory tests used in the development of inhibitors can be of various types and are often associated with a particular laboratory. Thus, in one case simple test specimens, either alone or as bimetallic couples, are immersed in inhibited solutions in a relatively simple apparatus, as illustrated in Fig. 19.34. Sometimes the test may involve heat transfer, and a simple test arrangement is shown in Fig. 19.35. Tests of these types have been described in the literatureHowever, national standards also exist for this type of test approach. BSl and ASTM documents describe laboratory test procedures and in some cases provide recommended pass or fail criteria (BS 5117 Part 2 Section 2.2 1985 BS 6580 1985 ASTM 01384 1987). Laboratory testing may involve a recirculating rig test in which the intention is to assess the performance of an inhibited coolant in the simulated flow conditions of an engine cooling system. Although test procedures have been developed (BS 5177 Part 2 Section 2.3 1985 ASTM 02570 1985), problems of reproducibility and repeatability exist, and it is difficult to quote numerical pass or fail criteria.  [c.1083]


See pages that mention the term Aaberg exhaust hoods : [c.959]    [c.960]    [c.507]    [c.957]    [c.330]    [c.2346]    [c.106]    [c.190]    [c.66]    [c.277]   
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Industrial ventilation design guidebook  -> Aaberg exhaust hoods


Industrial ventilation design guidebook (2001) -- [ c.0 ]