Point of use

Chemical Conversion. Except for control of nitrogen impurity levels, the same chemical conversion methods used for nitrogen purification at low flow rates can also be used for argon purification. Although used less commonly for argon purification than for nitrogen purification, these chemical conversion methods are appHed in point-of-use purifiers located close to where the gas is consumed. 

Purification of specialty gases can be divided into two areas purification done by the gas suppHer on a bulk scale prior to filling the cylinder or other dehvery container, and purification carried out by the consumer on a point-of-use scale generally just prior to use. 

Chemical conversion processes can also be used for moisture and oxygen removal. These tend to be the same ones developed for the smaller point-of-use purifiers. Consequently there is Httle economy of scale and they are seldom able to be regenerated. 

Point-of-Use Purification. For the user of cylinder quantities of reactive specialty gases, there are only a limited number of ways to remove impurities and obtain high purity. Specialized point-of-use purifiers have been developed that purify small streams of many important reactive gases. Whereas these point-of-use purifiers cannot remove all important impurities, they are usually effective for removing the contamination added by the users gas distribution system, mostly air and moisture. 

One important class of point-of-use processes utilizes a porous polymer containing reactive metals. Variations in the metal and polymer chemistry are made to optimize the process for different gas appHcations. This is an active area of development and purifiers are available for most of the principal specialty gases. 

Acetone can be handled safely if common sense precautions are taken. It should be used in a weU-ventilated area, and because of its low flash point, ignition sources should be absent. Flame will travel from an ignition source along vapor flows on floors or bench tops to the point of use. Sinks should be rinsed with water while acetone is being used to clean glassware, to prevent the accumulation of vapors. If prolonged or repeated skin contact with acetone could occur, impermeable protective equipment such as gloves and aprons should be worn. 

Ra.don Sepa.ra.tion, Owing to its short half-life, radon is normally prepared close to the point of use in laboratory-scale apparatus. Radium salts are dissolved in water and the evolved gases periodically collected. The gas that contains radon, hydrogen, and oxygen is cooled to condense the radon, and the gaseous hydrogen and oxygen are pumped away. 

Iron ores of different characteristics and compositions can be blended to a more uniform composition. This can be accompHshed during handling operations involved in transporting ore to its point of use, or through special blending faciUties, such as stacking and reclaiming. 

Shipments of concrete pipe in 1990 had a value of > 1.6 billion dollars. Much of this piping was for storm and sanitary sewers, culverts, and irrigation piping. Pressure piping sales were close to 300 million. There are a large number of manufacturers of concrete pipe since much of the pipe is produced reasonably close to the point of use. 

These appHcations require withdrawal of water from a source and subsequent treatment and conveyance to the point of use. Water is also used without being withdrawn from a source, eg, for navigation, recreation, wild and aquatic life propagation, hydroelectric-power generation, and waste assimilation and transport. The principal types of withdrawal uses and their average rates are given in Table 2. Some of these withdrawal rates represent multiple uses of the same water along main rivers in metropoHtan and industrialized areas. 

Ozone. Ozone is an allotropic form of oxygen, O. Because it is an unstable gas, it must be generated at the point of use. Ozone is an effective, clean oxidizing agent possessing powerful antibacterial and antiviral properties. 

In all cases, chlorine dioxide is produced at the point of use either from sodium chlorite or sodium chlorate. Production volume can be accurately estimated from total sodium chlorate consumption for chemical pulp bleaching because this use accounts for greater than 95% of all chlorine dioxide production. 

Clays vary in price from only a few doUars per ton for common clay to > 0.25/kg for some of the specialty surface treated clays. For clays that are used in large quantities such as kaolins for paper coatings, transportation to the point of use may be the primary cost component. 

Except where otherwise noted here or in the text, either consistent system of units (SI or U.S. customary) may he used. Only SI units may he used for electrical quantities, since no comparable electrical units exist in the U.S. customary system. When special units are used, they are noted at the point of use. 

Coal analyses are reported on severalbases, and it is customaiy to select the basis best suited to the application. The as-received basis represents the weight percentage of each constituent in the sample as received in the laboratoiy. The sample itself may be coal as fired, as mined, or as prepared for a particular use. The moisture-free (diy) basis is generally the most useful basis because performance calculations can be easily corrected for the ac tual moisture content at the point of use. The dry, ash-free basis is frequently used to approximate... 

The fuel value of most solid wastes is usually sufficient to enable self-supporting combustion, leaving only the incombustible residue and redlicing the volume of waste eventually consigned to sanitaiy landfills to only 10 to 15 percent of the original volume. The heat released by the combustion of waste can be recovered and utilized, although the cost of the recoveiy equipment or the distance to a suitable point of use for the heat may make its recoveiy economically infeasible. 

The COP s of specific air conditioners will vary widely with both manufacturer and application. Electricity utility efficiencies will also differ between countries. However, the reason for the economic interest in heat-driven cycles remains clear. Given that primary fuels can cost the consumer approximately 25% of the cost of electricity and that electricity frequently costs more at times of peak demand, there is justification for considering alternative systems. The use of a primary fuel at the point of use can also reduce COj and other emissions. 

Gas storage cabinets are designed to contain one to four gas cylinders. The cylinders are connected to a gas distribution system which is also contained in the cabinet. Very sophisticated systems are available from cabinet manufacturers. These may include automatic or semiautomatic change-over capabilities, fire sprinklers, purging systems, and gas detection systems which may include alarms and automatic shutoff. Point-of-use scrubbers may also be incorporated into the design, depending on the gas being used (see Fig. 10.43). 

Local ventilation The transportation of air into or from a space near its point of use. 

Screw conveyor A conveyor that uses an Archimedes screw to convey granular material from a hopper to point of use. 

Preserving product while the product is under your control may be addressed by your handling and packaging provisions, but in-process preservation may also be necessary to protect finishes from deterioration during further processing. Such measures need to be specified in the work instructions for particular products. For products that start to deteriorate when the packaging seal is broken, the supplier s responsibility extends beyond delivery to the point of use. In such cases markings need to be applied to the containers to warn the consumers of the risks. 

When possible, air from cylinders or a dedicated system should be used instead of general-purpose compressed air. If the latter has to be used, it should be tested at the point of use immediately before use, every time. 

Whatever the method of production, the product, NG. is still hazardous, and it is now being replaced by safer explosives. Increasingly, quanies now use explosives prepared at the point of use by mixing two nonexplosive ingredients. 

This is the horsepower consumed by the cylinders and does not contain any losses in transmitting the power from the driver to the point of use, such as belts or gears. It does contain 95% mechanical efficiency for the cylinder itself. 

Electric power is almost always transmitted as three-phase AC current. In domestic use, current is often distributed from a substation at 13,200, 6,600, or 2,300 V, which is stepped down by a transformer close to the point of use to 600, 480, and 240 V for three-phase current for commercial power and 240 and 120 V for single-phase, three-wire current for household power and lights. If DC current is required, synchronous converters or rectifiers are used to convert the AC supply to DC. 

Normal supply pressure to industrial and commercial users is 21 mbar. Allowing for pressure losses in the system, at least 17.5 mbar should be available at the point of use. Higher pressures can often be supplied by agreement where available. There may be process advantages in having a higher pressure. 

The NTS is used for the conveyance of gas from the terminals to regions sited in different parts of the country. Regional transmission systems are used to convey gas across each region. For local conveyance of gas to the final points of use, the distribution system is used. 

Gas pipework in a user s premises serves the function of transporting the gas from the meter to the point of use in a safe way and without incurring an avoidable pressure loss. For low-pressure installations, the permitted pressure loss is only 1 mbar from the meter to the plant manual isolating valve at maximum flow rate. The pipework must be sized adequately to allow for this. Boosters are sometimes used to overcome pressure losses, but the use of a booster should never be considered a satisfactory substitute for correct design of pipe sizes. Where gas is available at higher pressures it may be permissible to tolerate pressure losses of more than 1 mbar. 

It is essential when designing the pipe layout for gas distribution that unavoidable pressure losses are not incurred. For low-pressure gas, the pressure available at the meter inlet will be only 21 mbar, and the allowable pressure loss to the point of use only 1 mbar, although higher pressures may be available in some circumstances. If such a low-pressure loss is not to be exceeded it is essential that the pipework be sized correctly. It is preferable to oversize pipework rather than undersize, particularly as this allows... 

With steam generated at or close to the boiler design pressure it is inevitable that some of the steam-using equipment will have to be supplied at a lower pressure. In some cases the plant items themselves have only been designed to withstand a relatively low pressure. Sometimes a reaction will only proceed when the steam is at a temperature below a certain level or an unwanted reaction will occur above a certain level. For these and similar reasons, steam often is distributed at a relatively high pressure which must then be lowered, close to the point of use. Pressure-reduction stations incorporating pressure-reducing valves are fitted to perform this function. 

The problem of lamps being left on when not required could be solved by provision of automatic switching, of which there are a number of systems available. These can be time switch controlled with manual override at the point of use. 

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