Exhaust flow

Exhaust flow versus eompressor inlet temperature... 

Thermal destructive techniques have been widely used for many years to control some of these emissions. Thermal oxidizer sizes range from 100 SCFM up to 100,000 SCFM. Each industry has operations that dictate the exhaust flow that must be processed. 

Steam Turbine - A PR valve is required on the steam inlet to any steam turbine if the maximum steam supply pressure is greater than the design pressure of the casing inlet. The PR valve should be set at the casing inlet design pressure and sized such that overpressure of the casing is prevented, under conditions of wide open steam supply and normal exhaust flow. 

FIGURE 8.3 Model of a local recirculating system with a local exhaust hood, used for calculating the connection between contaminant concentrations, airflow rates, contamirtartt source strength, q , air cleaner efficiency, n and hood capture efficiency, a. is the concentration in the supply (outside) air c (equal to c h) is the concentration in the room Is the concentration in the returned air is the supply flow rate to the room equal to the exhaust flow rate, the recirculated flow rate (through the cleaner) is T is the time constant for the room and V is the room volume. 

The supply airflow rate should approximately equal the exhaust flow rate. A minor difference between supply and exhaust flow rates should nor disturb the exhaust, since exhaust systems usually are operated with higher pressure differences than supply systems. If the exhaust flow rate is higher than the supply, it could result in lower efficiency due to lower exhaust flow rates and cross-drafts (see Disturbances). If the exhaust flow rate is lower than the supply flow rate, there may be fewer problems with exhaust efficiency, but this could result in a supply airflow field different from the designed one and thus result in different kinds of disturbances. 

Certain operations require that the workspace be at a lower pressure than surrounding workspaces, e.g., radioisotope laboratories. In these cases, the exhaust flow rate should exceed the supply flow rate, but this excess should he within 10%. The additional resistance resulting from this imbalance should be considered in the design of the exhaust system, specifically in the selection of exhaust fans. 

This solution gives unequivocally the effective control range of both unflanged and flanged openings when the exhaust flow rate and velocity of the idealized cross-draft are known. The distance from the hood opening to the dividing streamline for a hood in uniform flow perpendicular to its axis is thus... 

This type of dependence of capture efficiency on the exhaust flow rate and cross-draft velocity has also been seen by Fletcher and Johnson who determined the capture efficiency of a flanged square exhaust hood in a cross flow. 

The original design was based on a control velocity recommended by dust control design manuals. The original design of 28 m s exhaust flow induced an inward velocity of about 0.5 m s" through the enclosure entrance and trolley shots. This was not sufficient to overcome plume trajectories aimed outward, or to overcome the effect of moderate wind levels. 

Mostly the use of a supply inlet as a local ventilation system presumes that the supply device (with air from outside the room) is located inside a large room, which also has an adequate exhaust airflow rate or has convenient ex-haust/transfer openings for the airflow. It is also necessary that the exhaust flow rate is maintained (or pressure difference kept). Otherwise the air supply could change in rate or direction. Instead of using air from a ventilation system, the supply air could be taken from the room (volume) it is situated in. In this case, the room must also have a supply and an exhaust flow rate. It is often necessary to clean the air before it is used in the supply inlet. 

The pressure difference is created by using a large difference between the supply and exhaust flow rates (in either direction). Higher pressure than the surroundings is used to prevent airborne transport into the room and lower pressure to prevent transport from the room. The airflow is then directed from the room with the (initial) higher pressure to the room with the lower pressure, and the transport of contaminants in the other direction is largely prevented. The pressure difference will disappear, for example, when a door is opened between the t o zones. When the pressure difference has disappeared, which could happen in fractions of a second, the preventive effect diminishes since, as has been pointed out earlier, it is impossible to have total isolation only by using air. 

These systems can be inside large halls and may have no fixed limits for their influence, except for some parts of the system (inlet device surface, etc.) They can also be situated inside small rooms, where walls, floors, and ceilings are the natural boundaries. The systems usually consist of one exhaust hood and one supply inlet, which interact. There are also special combinations, as two or more inlets and one exhaust hood, or one supply inlet and two or more exhausts. All of these combinations need careful design and an accurate relation between supply and exhaust flow rates and velocities. Some systems also need stable temperature conditions to function properly. All combinations are dependent on having a defined contaminant concentration in the inlet air. This usually implies clean supply air, but some systems may use recirculated air with or without cleaning. 

A number of workers at Pennsylvania State University examined the push-pull system and found good agreement between their numerical and experimental work. The computational algorithm SIMPLER was used to solve the flow in the two-dimensional push-pull system and it was concluded that for a tank 1.8 m long, the push jet must have an initial velocity of 3.8 m s, that the exhaust flow rate per unit width should be 0.495 m s", and that the ratio of the pull to push flow rates, q /qj, must be between 8.8 and 17.8. 

FIGURE 10.70 Simplified model of a wall jet combined with an exhaust flow. 

The assumptions that the exhaust flow has a negligible effect and that the offset jet can be treated as an equivalent wall jet were tested by Robinson and Ingham - and found to be reasonable over the majority of the surface of the tank, except close to the jet nozzle and exhaust hood. Far from the surface of the tank, the exhaust flow has a more noticeable effect. 

From the ACGIH recommendations, we can say that the system is operating safely if a fluid velocity greater than or equal to the capture velocity is induced across the whole of the tank surface, and the exhaust flow rate is sufficient to capture all the fluid in the jet. Since the maximum velocity at any... 

So far, since we have been treating the flow as being dominated by the jet, we have ignored the effects of the exhaust flow. Of course, the exhaust flow will increase the overall movement of the air, to a small extent far from the ex haust hood but quite significantly close to the hood. We shall discount these positive effects, and consider only the fact that the exhaust hood should remove all the fluid contained within the jet. This can be expressed as... 

Simple exhaust hoods have a very short effective range and the hood must be placed very close to the contaminant source to be efficient, which may interfere with technological processes. This lack of direction of the flow may result in the use of excessive exhaust flow rates with large source-to-hood distances and this may result in a large amount of wasted energy. 

FIGURE 10.79 Typical streamlines for the flow near the exhaust hood when there is (o) only suction. (b) some exhaust Bow, and (cl a large exhaust flow. (The flow is symmetrical about X = 0.) The shaded area represents the predicted effective capture region. 

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. 

FIGURE 10.84 Practical application to control highly buoyant plume. Supply airflow rate is 2.34 m s exhaust flow rate is 12.48 s". ... 

FIGURE 10.86 Practical application to remove dusts generated during handwork. Depth of the system is 6 m, supply airflow rate is 4.95 m s", and exhaust flow rate is 9.08 m s . ... 

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... 

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. 

In designing the push-pull hood, one always applies a safety factor, , resulting in the exhaust flow rate for design, which is expressed as the following ... 

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. 

For a constant exhaust flow rate, an increase in supply airflow provides better operator protection or production protection, but it also increases contaminant spread and risk of draft. Any decrease in supply airflow rate will result in a reduction of the design conditions of the operator or product protection. 

With a constant air supply flow rate, an increase in exhaust airflow will result in increased operating costs. Any decrease in exhaust flow rates will result in... 

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