Neutral pressure plane

Tightened building shell to minimize the amount of air needed to pressurize the basement and to lower the neutral pressure plane. 

The hottest gas within a furnace (or any enclosed chamber) rises to the top, creating a higher pressure at the furnace s higher elevations and a lower pressure at the furnace s lower elevations. (This is stack effect within the furnace.) The zero gauge-pressure plane or neutral pressure plane is the locus of points where the pressure inside the furnace is the same as the atmospheric pressure outside the furnace at the same elevation. The neutral or zero plane is the boundary between + and — pressures within the furnace. If there are leaks through the furnace walls, furnace gases will leak outward from the space above the neutral plane and air will leak inward to the space below the neutral plane. (See fig. 6.13.)... 

In most industrial heat-processing furnaces, it is desirable to have the entire furnace chamber at a positive pressure with an automatic furnace control system having a setpoint of 0.02 in. wc (0.5 mm) at the elevation of the lowest part of the load(s) or better yet, at an elevation just below the lowest leak. To keep out tramp air inleakage, raise the furnace pressure enough to drive the neutral pressure plane below the furnace bottom, in a liquid bath furnace, below the liquid surface level. 

Fig. 6.13. Effects of furnace temperature and input on the level of the neutral pressure plane elevation shown on six sectional elevation views of a furnace with no furnace pressure control. If there were any gas flow in the furnace, the neutral pressure plane would be more like a wrinkled sheet than a plane. The top three show the effect of temperature with no change in input. The bottom three show the effect of input rate with no change in furnace temperature.

The elevation of the pressure-sensing tap does not necessarily have to be at the elevation desired for the neutral pressure plane. The most desirable height for the zero pressure plane may be at a point that turns out to be bad for good measurement, for example, below the hearth, at a level where scale might plug the pressure tap, or in a place where liquid metal may splash into the tap. In such cases, a very workable solution is to locate the sensor tap at a convenient higher position and then adjust the controller s setpoint in accordance with the correction for the rise in pressure for the chosen higher elevation from table 6.2. (See example 6.2.)... 

The next choice would be to locate the tap in the wall opposite the burners, but equally spaced between the burner centerlines and elevated 2 feet above the hearth. The setpoint of the furnace pressure control will have to be biased to correct for the difference in elevation between the pressure tap and the desired level of the neutral pressure plane (at the hearth). Interpolating from table 6.2, the setpoint bias should be 0.0118 in. X 2 feet of elevation = 0.0236, or say 0.025 or 0.03 in. wc to allow for expected wear on the car seals. 

Q4. Is the neutral pressure plane (or zero pressure plane ) really a plane ... 

N or N2 = nitrogen = an inert gas, comprising about 80% of air and a large part of poc, unless using oxygen enrichment, net heating value = nhv = lower heating value, Ihv. See Ihv. neutral pressure plane = zero pressure plane = balanced pressure line (invisible), or level at which the pressure inside a furnace is exactly equal to the pressure outside the furnace at the same elevation. Usually not really a plane, but an invisible surface rumpled by burner jet and draft effects. See sec. 6.6.1. nm /h = normal cubic meters per hour, a unit of volumetric flow rate, equal to 37.9 scfh. nm is standardized at 0 C, 760 mm Hg, dry air or gas. A standard ft is defined at 60 F, 30 Hg, saturated air or gas. normal air = European near-equivalent of U.S. standard air , see also). 

When there is a difference between inside and outside temperatures, there is some point between the top and bottom openings where the internal and external pressures are equal. This location is the neutral pressure plane. An overpressure from a fire inside a building will move the neutral pressure plane downward. If the temperature outside a building is less than inside, a vent opening can move the neutral pressure plane upward. 

The location of the neutral pressure plane can influence the distribution and buildup of smoke in a tall building. However, the movement of heat and smoke in tall buildings is not fully understood. It is difficult to model. If air enters at the bottom near a fire, the fire can become more intense, adding smoke. External winds can change the patterns of movement. On a hot summer day, often the external air temperature is higher than the air-conditioned interior air. This reduces or eliminates the stack effect. 

From Figure 11.3, under free convection, there will be a height in the vent at which the flow is zero, N this is called the neutral plane. The pressure difference across the vent from inside (i) to outside (o) can be expressed above the neutral plane as... 

The neutral plane (point N in Figure 7.10) is the point at which the subtended angle is 9 = Off, and can be found in several ways. A precise method is to balance the entry and exit forces pressures i.e., Pe = Px and solve for H at the neutral plane, Hn, which can then be substituted into Eq. (7.7) to solve for Off. Alternatively, there are empirical relations for Off. Consider the cold-rolling of an aluminum strip that is rolled from 4 mm to 3.3 mm in thickness with a roller 500 mm in diameter. The coefficient of friction is 0.06. 

As discussed in Section 8.2 (Fig. 8.3), interparticle and wall friction, force dissipation from particle to particle, and sliding under shear cause differences in density distribution in a compact that is produced in a die with one-sided compression by one of the punches. To obtain a somewhat more uniform structure, compaction can be carried out by both punches. If both move at the same rate and for the same stroke length, thus exerting identical forces, and assuming uniform filling of the die, a mirror image of the density distributions that were shown in Fig. 8.3 (Section 8.2) develops along a neutral plane which, under those conditions, is located in the middle of the compact. Machines that operate in this manner may be identified as presses with double pressure (see also below). 

Particularly in complex parts, it is also necessary to control the position of the neutral plane, the low density zone that is approximately perpendicular to the direction of pressing. This is achieved by the relative motions of the tooling members (Fig. 8.99). It is also important to understand that, particularly under pressure, particles will not move from one level or position in the developing structure of a part to another one. As a consequence, if parts are pressed that feature more than one level, separate pressing forces must be applied simultaneously for each level. As a result, neutral planes will exist for each part level (Fig. 8.100). 

We integrate the force balance equation in the volume of a prism of the same section S but smaller height i, < L, summing over all ionic and neutral species /, For central forces in the average over the surface S the pair interactions are cancelled out exactly. At the plane L these forces will contribute to the bulk pressure P. The statement of dynamic balance is that... 

Let us consider a plane sandwich cell into which the cathode injects electrons that negatively charge neutral molecules. Due to an excess negative charge density Q(z) near the cathode, a force QE is directed to the anode and tries to shift the charged layer of a liquid to the right (Fig. 29a). Since the cell is sealed and the liquid is incompressible, a circular convective flow occurs in order to reduce the internal pressure (Fig. 29b, c). This case resembles the well-known Benard problem in the thermoconvection of liquids. 

When a Morton neuroma is suspected on clinical grounds, we select a dorsal approach to examine the web spaces because the skin (and especially the stratus corneus of the epidermis) over the dorsum of the foot is thinner than that of the sole and the attenuation of the US beam is less. The patient lies supine or seated on the bed with the knee extended and the ankle in a neutral position. The intermetatarsal spaces should be examined in sagittal planes applying firm pressure with the transducer on the dorsal aspect of the foot while exerting finger pressure in the web spaces from the plantar surface (Fig. 17.50). The thumb of the hand not holding the probe works well for this purpose. The examiner should invite... 

If the principal curvatures are both concave and thus negative, or if, one being concave and the other convex, and the first is in excess, the second term of the formula is negative, and, consequently, the total pressure at the point considered is lower than that of a plane surface. In this case, the shape of equilibrium has finite and negative mean curvature. Moreover, since the pressure P always finds itself neutralized as I remarked ( 18 and 20), one can look at the action of the surface layer as constituting a suction. 

F1g. 13.12a,b. Trapping of neutral atoms in a standing light wave, (a) In-duced light pressure force, normalized to the spontaneous force =2hKr as a function of the particle velocity v. (b) One-dimensional oscillation of a trapped particle around the minimum of the potential energy in a plane standing wave... 

Nonetheless, this attraction can be overcome, at least in part, by swelling, and under some conditions completely, resulting in layer separation. It should be realized that a neutralizing plane of cations, e.g., 1.47 x 10 Na+ ions per //m, corresponds to a Na concentration amounting to a supersaturated 7.63 M, causing an osmotic pressure II = 190 Kg cm . At the edges of the smectite layers, where all the hydrated Na" " ions are enclosed in the 0.32 nm wide... 

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