Exterior exhaust hoods rim exhausts


The compressor can be driven by electric motors, gas or steam turbiaes, or internal combustion (usually diesel) engines. The compressor can also be a steam-driven ejector (Fig. 7b), which improves plant reUabiUty because of its simplicity and absence of moving parts, but also reduces its efficiency because an ejector is less efficient than a mechanical compressor. In all of the therm ally driven devices, turbiaes, engines, and the ejector mentioned hereia, the exhaust heat can be used for process efficiency improvement, or for desalination by an additional distillation plant. Figure 8 shows a flow diagram of the vertical-tube vapor compression process.  [c.246]

Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet,. should be relatively low with a uniform velocity but high enough so that it is not disturbed by the. surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker.  [c.966]

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The  [c.966]

Oil-firing systems use either pressurized steam, ak, or mechanical means of atomizing the fuel oil into small droplets as it is sprayed from the burner. Atomizing maximizes the surface area of fuel oil exposed to ak, ensuring thorough mixing of fuel and ak, and efficient combustion. Pressurized ak is introduced through primary, secondary, and sometimes tertiary ports in and around the burner. Forced-draft fans deflver the ak to the burner through windboxes, which are essentially flow-ways built into the walls of the furnace, exterior to the waterwaHs. Combustion ak is preheated via heat exchangers that extract heat from the boiler s hot exhaust gases. Induced-draft (ID) fans located in the exhaust duct near the stack may be appHed to provide further control of gas flow and heat transfer within the boiler. ID fans may also help compensate for the pressure drop that occurs across equipment located in the flue-gas stream, such as cyclone separators, fabric filters, or electrostatic precipitators (ESPs) systems used to capture flyash entrained in the exhaust gas.  [c.8]

In the Monsanto process (now called Lummus-UOP Classic process, as distinct from the SMART process to be described later), ethylbenzene, together with benzene and toluene, is separated from styrene in the first column. The overhead condensate from this column then goes to the benzene—toluene column and is redistilled to recover the benzene—toluene fraction in the overhead and ethylbenzene in the bottoms. It has been argued that energy is wasted in this scheme by distilling the benzene—toluene fraction twice, once together with ethylbenzene and once away from ethylbenzene. Actually, this extra energy consumption, which can be computed, is small. It has also been argued that less polymer is formed in having the styrene bypass the benzene—toluene column. Actually, any possible reduction of polymerization resulting from this arrangement, which has never been substantiated, is insignificant because polymerization of the styrene in solution with ethylbenzene through a small benzene—toluene column is negligible in comparison with polymerization of the high concentration styrene in the much larger ethylbenzene—styrene distillation column. Moreover, the higher vapor and Hquid loadings required by having to distill benzene and toluene together with ethylbenzene may slightly increase styrene polymerization. In any case, these effects are insignificantly small and every process owner continues to use its own design very likely for historical reasons. An optional scheme to utilize the latent heat of the ethylbenzene vapor in the column overhead to vaporize an ethylbenzene—water azeotrope for use as reactor feed is offered in conjunction with the Monsanto process. This scheme recovers less energy than the Fina-Badger scheme but does not require a compressor. However, the condensing temperature of the ethylbenzene vapor must be raised by having the ethylbenzene—styrene column operate at higher temperatures. The inhibitor usage and polymer formation increase substantially because the rate of polymerization increases rapidly with the temperature. Consequently, the quantity and viscosity of the residue are high. A wipe-film evaporator is then used downstream of the styrene finishing column to minimize yield losses. This scheme has also been used in the Far East. It has been reported that the increased polymerization has caused difficult operating problems and reduced operating flexibiUty. The economics of the optional energy recovery schemes are marginal at best in most situations insofar as the energy savings are negated by the incremental investment, extra inhibitor usage and yield losses, and reduced operating flexibiUty. Neither of them is used by the producers in the United States.  [c.483]

Catalytic Converter. The converter consists of a catalytic unit contained ia a metal canister which surrounds the fragile ceramic catalytic unit with a steel shell. The converter shell assembly is usually made from an iron/chrome Series 409 muffler-grade stainless steel that is resistant to internal and external oxidative corrosion. In between the steel shell and the exterior of the catalytic unit is a compliant layer that grips the catalytic unit with sufficient force to prevent movement of the catalytic unit within the canister, and which compensates for the differences ia thermal expansion between the catalyst and the metal shell. Several types of compliant layers are used, and all have spring-like properties under compression that provide the necessary gripping force at all exhaust temperatures. Cormgated knitted wire mesh was the first successfiil compliant layer. As of this writing, a material based on vermiculite, which expands upon appHcation of heat (about 300°C), is used, as is a wire mesh material wrapped several times around the catalytic unit. The compliant layer mounting system has proved to be durable for the life of the vehicle. The converter design, flow, and pressure drop characteristics are described ia the hterature (19—23).  [c.484]

The catalytic unit is designed to provide enough surface area so that all exhaust gases contact the catalyst surfaces as they pass from the engine to the tailpipe (24—27). In order to function quickly after the engine is started, the catalytic unit must rapidly heat up to operating temperature. It therefore must possess good heat-exchange properties to extract the necessary heat from the exhaust gas. Once the minimum catalytic operation temperature is reached, the catalytic unit is designed to maximize transfer of the pollutants from the exhaust gas to the surface of the catalytic unit. Heat transfer and mass transfer are driven by temperature difference and concentration difference, respectively. At operating temperatures above 300 or 350°C, the catalytic reactions are so fast that only the exterior surfaces of the catalyst are utilized for the catalytic function (28).  [c.484]

Auxiliaiy Equipment If noxious gases, fumes, or dust are given off during the operation, dust- or fume-recovery equipment will be necessary in the exhaust-gas system. Wet scrubbers are employed for the recovery of valuable solvents from dryers. In order to minimize heat losses, thorough insulation of the compariment with brick, asbestos, or other insulating compounds is necessary. Modern fabricated dryer-compariment panels usually have 7.5 to 15 cm of blanket insulation placed between the internal and external sheet-metal walls. Doors and other access openings should be gasketed and tight. In the case of tray and truck equipment, it is usually desirable to have available extra trays and trucks so that they can be preloaded for rapid emptying and loading of the compariment between cycles. Air filters and gas dryers are occasionally employed on the inlet-air system for direct-heat units.  [c.1190]

Simple and regenerative cycle turbines differ in configuration, efficiency, and equipment cost (see Figures 7-19, 7-20, and 7-21). The regenerative turbine costs more because of the heat exchanger and extra ducting required to preheat the combustion air using the turbine exhaust gas. An increase in efficiency of about 6-7% results. Simple-cycle turbines normally have thermal efficiencies of 20-26% and regenerative  [c.293]

Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, power plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gains and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources.  [c.418]

This phenomenon is more common with large flow rates and large openings, such as use of a laboratory fume hood or a unidirectional horizontal flow field, than with small flow rates and small openings, such as exhaust hoods for welding, soldering, and grinding. Designing the system to place the worker beside the airflow path is generally recommended for all exterior hoods. When using partial enclosures with large openings to the surroundings, a person may also influence the flow field, e.g., by changing the flow into the hood. It is possible to counteract such wakes by using vertically directed supply air around the worker (see Sections 10.3.3 and 10.4.6).  [c.815]

An exterior hood is often a natural choice for an exhaust. It is usually easy to install, less expensive, and does not need any large changes in the outlay of the room or the process. Often it is possible to connect this type of hood to an existing exhaust duct system and when the flow rate is relatively small, the ex isting supply air system may be maintained without changes.  [c.818]

The first of these conditions is the most important factor when deciding to use an exterior hood. Exterior hoods are allowed when the demands on the exhaust, and the hazard of the contaminants, are moderate.  [c.819]

If the contaminated airflow rate that is to be exhausted, or the internal pressure, varies too much it could be advantageous to use an exhaust connection with a small distance between tube and duct, acting as an opening for additional air when contaminant flow rare is low. This could be in the form of a large exterior hood covering the outlet from the process and leaving only a very small opening gap for external air (thimble). See Fig. 10.40.  [c.878]

The low-momentum air, which is supplied from a relatively wide supply inlet, functions to transport contaminants to near the exterior hood. In addition, it functions to change the direction of contaminants toward the exterior hood when the direction of contaminated air is initially different from the exhaust direction. The momentum or velocity of the supply air to reach to the exterior hood will be sufficient when the motion of contaminants can be neglected. However, when the contaminated air exhibits significant motion or flows in a direction different from the exterior hood, the supply air velocity should have a sufficient momentum to control the contaminant flow.  [c.967]

The exterior hood could be placed beside, below, or above the contaminant source, and the direction of the supply air could be horizontal, vertical, or diagonal toward the exhaust inlet. The sources should be within the supply-airflow, and in some cases, a worker could also be located within the flow. When the worker is within the supply airflow, however, a region of a recirculating airflow, a wake region, can be created downstream. If the breathing zone of the worker and the contaminant source are within this wake region, high exposures may occur. Therefore, the relation between the direction of supply airflow and the position of the worker should always be considered, as the source should never be within the wake region (see Section 10.2.3).  [c.967]

Many processes generate contaminants in casting plants. Some practical applications of the low-momentum supply system in these plants will be introduced here. Figure 10.84 shows an example of the system applied to the process where molten metal is being poured from a low-frequency furnace to a ladle. The temperature of molten metal is about 1800 K and a highly buoyant plume containing metal fumes is formed above the tapping nozzle and the ladle. To control the plume, a supply inlet is placed horizontally from the center of the ladle by 2.5 m. and supply airflow is blown at a uniform velocity of 3.0 m toward the ladle. The direction of plume is turned toward the exhaust outlet by the supply airflow, and the flanged exterior hood exhausts the fume-containing plume. The dimensions and operating conditions of the system are indicated in the figure. The velocities of supply and exhaust airflow were determined using CFD simulations.  [c.967]

Figure 10.85 shows an example of the system applied to a shaking-out process in a casting plant. In this process, when molding sand around castings is shaken off, high concentrations of dust rise above the shakeout machine in a buoyant plume. To remove the dust, an exterior hood was placed beside the source and a supply inlet was placed on opposite side. Air is blown toward the exhaust outlet to change the direction of the dust toward the exterior hood. The temperature of castings is about 700 K, the  [c.967]

The final example is shown in Fig. 10.86. Several workers are breaking gates off of castings on the conveyor by hand. Much dust is generated by this operation and the dust rises due to buoyancy. To remove the dust, an exterior hood was placed beside the conveyor and a supply inlet was placed above the workers. The supply airflow is blown toward the breathing zone of the workers and the dust source. In this case, as the workers and the dust source are located within the supply airflow, the airflow functions to supply the workers with clean air and to transport the dust toward the exhaust inlet. The velocity of supply air is relatively low, 1.1 m s , and the exhaust velocity at the hood face is 2.75 m s . The dimensions of the system are indicated in the figure, and the depth of the device is 6.0 m (compare with Sections 10.3.3 and 10.4.6).  [c.968]

The inlet opening that supplies the low-momentum airflow should be sufficiently wide to cover the contaminant source and should face toward the inlet of exterior hood. The airflow functions to transport contaminants emitted within the flow to the exterior hood. The exhaust airflow created around the suction opening must exhaust all of the contaminants transported by the supplied airflow. From this point of view, the low-momentum  [c.968]

In the low-momentum supply system, the contaminants are emitted within the low -momentum airflow blown from the supply inlet and they are transported to near the exhaust opening. If the contaminants diffuse into the whole of the supply airflow, the exterior hood must exhaust the whole of the airflow. To diminish the exhaust flow rate, some methods to prevent the contaminants from diffusing into the whole of the airflow are required. One possible method is to supply the air as slowly as possible but with enough velocity to reach the exhaust outlet and to control the surrounding air motion. Another method is to blow supply air with uniform  [c.970]

Applying the flow ratio method to the low-momentum supply system, the required exhaust flow rate is often in excess of practical values. This is because the value of is given as the value at which all the supplied airflow should be exhausted by the exterior hood. In the low-momentum supply system, contaminant sources should usually be between the supply inlet and the exterior hood. The supply airflow is contaminated at the position of the sources and it flows to the exterior hood. Therefore, all of the airflow is not always contaminated. Unfortunately, a design method considering such cases (the diffusion of contaminants within the airflow) has not been established yet, and the appropriate exhaust flow rate has to be adjusted after the system is installed.  [c.972]

Contaminants captured by an exterior hood can cause exposure if allowed to enter the hood after passing through the operator s breathing zone. The capture efficiency describes the percentage of the generated contaminant that is captured directly by the hood. Occupational hygiene efficiency describes the effect of the use of the hood on the operator s exposure to the contaminant, The occupational hygiene efficiency is performed with the exterior hood working and not working. For example, many wood-processing machines produce large volumes of wood chips and dust and cannot operate without the exhaust hoods in use. With very toxic contaminants, it is essential to work with the hood exhaust operating at all times.  [c.1014]

Other differences between THF and oxetane polymerisations are noteworthy. The added ring strain in the four-membered ring oxetane results in much faster polymerisation rates and higher heats of polymerisation than are observed for THF polymerisation. At near ambient temperature and moderately high initiator concentrations, high conversions to PTHF usually require hours to achieve, whereas most oxetane polymerisations are near 100% conversion in seconds or minutes. Typical heat of polymerization values, Aid, are 84.6, 80.9, 67.9, and 25 kj/mol (20.2—6 kcal/mol) for BCMO, OX, DMOX, and THF, respectively (6,282). Nominally, the polymerisation of oxetanes is reversible. However, the formation of a four-membered ring under the usual polymerisation conditions is energetically so disfavored that the depolymerization reaction can be ignored the formation of cycHc oligomers becomes much more significant in oxetane polymerizations. The amount of cycHc oligomer formed depends on the particular oxetane, the polymerization conditions, and the initiating system, polymerization temperature, and solvent (327—331). Super acids and their esters have been found to be of Htde value in oxetane polymerizations. The equiUbrium between ester and the oxonium ion in a BCMO polymerization, for example, is far toward the ester form, which is virtually unreactive to the cycHc ether and does not participate in the propagation reaction. Moreover, the active oxonium ion, when formed, collapses immediately to the corresponding ester (332). Thus a suitable mechanism for rapid polymer formation with this group of initiators does not exist for BCMO, and probably not for other oxetanes.  [c.369]

A large number of diverse solvents are used in exterior and interior coatings in plants for manufacturing both three- and two-piece cans. Most of the organic solvents are found in the cure-oven exhausts at concentrations of 2—16% of the lower explosive limit (LEL). The oven exhaust volumes are usually 1—35 mr /s. When burned, these concentrations of combustibles provide an exotherm of 30 to 220°C. The heat that is released is used for preheating the incoming effluent and/or heating the cure oven by recycling the hot, cleaned gases to the supply blowers or by heating makeup air by heat exchange. A few plants use the heat of the cleaned exhaust to produce hot water for the two-piece can line washers, hot air for dry-off ovens, or building space heating. For example, one large can company utilizes the heat energy contained in the stream, leaving some of their catalytic fume abaters to supply all the heat energy required by the oven s heating zones, which have no burners. The fuel energy suppUed to the catalytic fume abater is less than would be needed to heat the oven if the solvent fumes were exhausted directiy to the atmosphere without use of the fume abater. The exhaust rate of the oven is adjusted to maintain a  [c.514]

Phthalates prepared from alcohols with about eight carbon atoms are by far the most important class and probably constitute about 75% of plasticisers used. There are a number of materials which are very similar in their effect on PVC compounds but for economic reasons di-iso-octyl phthaiate (DIOP), di-2-ethylhexyl phthaiate (DEHP or DOP) and the phthaiate ester of the C7-C9 oxo-alcohol, often known unofficially as dialphanyl phthaiate (DAP), are most commonly used. (The term dialphanyl arises from the ICI trade name for the C7-C9 alcohols-Alphanol 79.) As mentioned in the previous paragraph these materials give the best all-round plasticising properties. DIOP has somewhat less odour whilst DAP has the greatest heat stability. Because of its slightly lower plasticising efficiency, an economically desirable feature when the volume cost of a plasticiser is less than that of polymer, dinonyl phthaiate (DNP) may also be an economic proposition. Its gelation rate with PVC is marginally less than with DIOP, DAP and DEHP.  [c.331]

The retention chamber and heat transfer chamber are fabricated of reinforced carbon steel exterior and ceramic fiber lining. The thickness of the ceramic fiber lining is based on the required destruction temperature of the organics cind the desired outside shell temperature. The ceramic heat exchange media can be of various types including ceramic saddles, tye pacs, or structured packing. The ceramic structured packing is a recent development in the industry reflecting lower pressure drops for equivalent heat transfer. A reinforced carbon steel structure is provided to support the loads of the oxidizer chambers and the structured packing support grid, and the wind and/or earthquake loads. The packing support grid is fabricated of stainless steel and is designed to support the structured packing. If organic particulates in the process exhaust builds up on the "cold" surfaces at the bottom of the oxidizer, the process must be shut down and a volatilization of these organics or a "bakeout" is required. When bake-out is activated, the flow diverter valves will stay in one position until the exhaust air temperature from the outlet bed reaches 850°F. At this temperature, most organic oils will volatilize, as in a self cleaning oven. When the first outlet bed reaches 850 F, the flow diverter valves will switch and will stay in position until the outlet temperature of the second bed reaches 850°F.  [c.485]

There are many possible ways to classify local ventilation systems. When local ventilation is used to describe exhaust hoods only, one classification is hoods that totally surround the contaminant source (enclosing hoods), hoods that partially surround the contaminant source (partially enclosing hoods), and hoods where the contaminant source is outside the hood (exterior hoods). A similar classification is used here for the exhaust hoods. Since local ventila tion, as described in this chapter, includes more than exhaust hoods, the following three main categories are used exhaust hoods, supply inlets, and combinations of exhaust hoods and supply inlets. (See Fig. 10.1.)  [c.812]

In practice there are many different combinations, such as two exhaust hoods close to each other or two or more air curtains placed around a horizontal (or v ertical) source or a hood that is partly an exterior hood and partly an enclosure.  [c.812]

The exhaust opening should be placed as close as possible to the source. This normally means a distance less than one exhaust opening diameter between source and exhaust opening. If the distance is larger both opening and flow rate must be increased so much that it may be infeasible to use an exterior hood. For sources with one principal generation direction, the distance could be larger wuth only slightly less efficiency. One example is a canopy Itood over a hot bath.  [c.820]

Since exterior hoods are unshielded, nearly all disturbances can dramatically change the performance of the hood. Changes in direction of contaminant generation could result in contaminant spread outside the exhaust a variable generation rate could result in intermittent contaminant spread directly to the surroundings when the source volume rate becomes larger than the exhaust flow rate tools moving around or rotating could either disturb the flow field or redirect the contaminants or both cross-drafts from moving persons or vehicles, or leakage through door openings or w all cracks could temporarily or permanently disturb the exhaust flow field and result in spreading of the contaminant in the room.- A moving source demands a moving exhaust or a very large opening to ensure that the hood is as close as possible to the source. An exhaust can be moved virtually, i.e., the openings are not moved, but instead the exhaust flow rate is connected to different openings or parts of one opening (at different locations) depending on where the moving source is at that moment. I his is a way of minimizing the exhau,st flow rate without diminishing the efficiency. I his type of hood puts high demands on the control system for the exhaust. The misplacing of a baffle in the exhaust opening (to diminish the necessary flow rate) could also result in contaminants not being captured.  [c.822]

Partial enclosures are a compromise between containment and access. Most people misunderstand the function of partial enclosures. It is not possible to completely separate the interior from the surroundings with partial enclosures. Complete separation is only possible with total enclosures. The function of a partial enclosure is as dependent on the flow rate, the flow field, the working procedures, the contaminant generation process, etc. as is the function of exterior hoods. The advantage with a partial enclosure is that the physical walls diminish the possibilities for the contaminants to escape from the hood to the surroundings. Thus these hoods could be used when relatively high demands are put on the contaminant concentration outside the hood. Some of the most commonly used enclosures, such as fume cupboards and booths, are described. Many variations of these exist, e.g., enclosure of the complete process, and some of these are described here.  [c.878]

Since the low-momentum supply system should enhance the efficiency of an exterior hood by supplying low-momentum airflow to a source, the system can be applied to practically any sources where an exterior hood can be used. In particular, it is effective to apply the system when an exterior hood cannot be placed close to a source or the exhaust direction is different from the initial contaminant release direction.  [c.967]

An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method. How-evec, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow.  [c.971]

Kehlhofer then suggests that more heat can be extracted from the exhaust gases, even if there is a high limiting value of Tb (imposed by use of fuel oil with a high sulphur content). It is thermodynamically better to do this without regenerative feed heating, which leads to less work output from the steam turbine. For a single pre.s.sure system with a pre-heating loop, the extra heat is extracted from the exhaust gases by steam raised in a low pressure evaporator in the loop (as shown in Fig. 7.8, after Wunsch [11]). The evaporation temperature will be set by the sulphuric acid dewpoint (and feed water entry temperature Tb 130°C). The irreversibility involved in raising the feed water to temperature Tb is split between that arising from the heat transfer from gas to the evaporation (pre-heater) loop and that in the deaerator/feed heater. It is shown in Ref. [1] that the total irreversibility is just the same as that which would have occurred if the water had been heated from condenser temperature entirely in the HRSG. Thus, the simple method of calculation described at the beginning of Section 7.5.1 (with no feed water heating and Tb T.f) is valid.  [c.122]

In shell boilers with a working pressure of between 7 and 17 bar the temperature of the mass of water in the boiler is typically in the range of 170-210°C. Allowing for, say a temperature difference of 30-50°C between the exhaust gases and the water temperature, the boiler exit gas temperature cannot be practically reduced beneath about 200-260°C, dependent on the operating pressure. It becomes necessary therefore to modify the process on principles to achieve further heat utilization and recovery. In the case of economizers conducting the feedwater supply via an economizer wherein the exhaust gas passes over tubes carrying the feedwater does this. The feed-water, normally at temperatures between 30° and 100°C, represents a further cooling medium for the exhaust gases and provides the potential for the extra heat utilization. This is shown in Figure 25.1.  [c.386]


See pages that mention the term Exterior exhaust hoods rim exhausts : [c.967]    [c.971]    [c.842]   
Industrial ventilation design guidebook (2001) -- [ c.848 , c.849 , c.850 , c.851 ]