Negative pressures

This example of high and low pressure also shows the ambiguities of these tenns in science. All these pressures are essentially constant in tenns of tire range of pressures encountered in nature. From negative pressures in solids under tension (e.g., on the wall of flask confining a fluid), pressure in nature increases... 

It is, however, possible to calculate the tensile strength of a liquid by extrapolation of an equation of state for the fluid into the metastable region of negative pressure. Burgess and Everett in their comprehensive test of the tensile strength hypothesis, plot the theoretical curves of T /T against zjp, calculated from the equations of state of van der Waals, Guggenheim, and Berthelot (Fig. 3.24) (7], and are the critical temperature and critical... 

Plutonium solutions that have a low activity (<3.7 x 10 Bq (1 mCi) or 10 mg of Pu) and that do not produce aerosols can be handled safely by a trained radiochemist in a laboratory fume hood with face velocity 125—150 linear feet per minute (38—45 m/min). Larger amounts of solutions, solutions that may produce aerosols, and plutonium compounds that are not air-sensitive are handled in glove boxes that ate maintained at a slight negative pressure, ca 0.1 kPa (0.001 atm, more precisely measured as 1.0—1.2 cm (0.35—0.50 in.) differential pressure on a water column) with respect to the surrounding laboratory pressure (176,179—181). This air is exhausted through high efficiency particulate (HEPA) filters. 

Dilute Phase Conveying. Dilute conveying systems, sometimes called disperse conveying or stream conveying, operate as positive pressure systems at pressures up to 100 kPa (14.5 psig), or as negative pressure systems (vacuum conveying) at pressures up to —50 kPa (—500 mbar). 

Design the vents to prevent the development of damaging negative pressure from coohng of hot products or combustion. 

For example, in rotary vacuum dryers it is possible to prevent the formation of explosible dust-air mixtures by setting and monitoring a certain partial vacuum (negative pressure). This pressure value must be determined by experiment for each type of dust. With pressures of less than O.I bar, in general, hazardous effects of dust explosions need not be anticipated. If the vacuum system malfunctions, the partial vacuum must be released by inert gas and the instaUation shut down. 

Provide vapor-tight enclosure around filter and run at negative pressure with exhaust fans... 

Pressures less than atmospheric are recorded as negative pressures (-psi) on a simple pressure gauge. 

Technically speaking, negative pressures don t exist. Pressure is only a positive force and it is either present or absent. 

It is the energy in the liquid rec]uired to overcome the friction los.ses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump s curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. 

Physieal baniers, e.g. splashguards, sereens, or use of separate rooms. In speeial eases equipment enelosures and/or eomplete rooms may be maintained under a slight negative pressure. 

Total enclosure may be in the form of a room with grilles to facilitate air flow this functions as a hood and operates under a slight negative pressure with controls located externally. Entry is restricted and usually entails use of comprehensive personal protective equipment. Ancillary requirements may include air filters/scmbbers, atmospheric monitoring, decontamination procedures and a permit-to-work system (see page 417). 

Even when the building as a whole is maintained under positive pressure, there is always some location (for example, the outdoor air intake) that is under negative pressure relative to the outdoors. Entry of contaminants may be intermittent, occurring only when the wind blows from the direction of the pollutant source. The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing lAQ complaints in areas of the building that are distant from each other and from the source. 

Turbine contactors, which involve positive or negative pressure to the turbine. 

Breathing consists of the cyclic action of the lungs to inspire and expire atmospheric gases. Inspiration occurs when the diaphragm and intercostal muscles contract, generating a negative pressure in the pleura surrounding... 

Airflow in the vicinity of local exhausts (hoods) or general exhaust (due to negative pressure in the duct produced by mechanical systems)... 

The science of building aerodynamics considers the influence of wind forces over buildings and the associated mechanics of fluids these are complex in nature and are not considered here. It is sufficient to briefly consider Fig. 9.21, which shows how wind passing over a building produces a positive pressure on one side and a negative pressure on the other side. It is this pressure difference that produces airflow through openings. The combined wind and stack effects vary with the seasons. 

This merhod ot ventilation uses extractor fans. A fan will create a negative pressure within the space. With this method, a set flow rate can be achieved, as the fan will overcome the stack and wind effects hence, the system is not at the mercy of the weather. This system causes the inside of the building to be held at a negative pressure, so air will be drawn in from outside or from surrounding spaces that are at a high pressure. 

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