Switch considerations

Fuel switch. The choice of fuel used in furnaces and steam boilers has a major effect on the gaseous utility waste from products of combustion. For example, a switch from coal to natural gas in a steam boiler can lead to a reduction in carbon dioxide emissions of typically 40 percent for the same heat released. This results from the lower carbon content of natural gas. In addition, it is likely that a switch from coal to natural gas also will lead to a considerable reduction in both SO, and NO, emissions, as we shall discuss later. 

Data for the production and sales of maleic anhydride and fumaric acid ia the United States between 1979 and 1992 are shown ia Table 5. Production of maleic anhydride during this time grew - 2% on average per year. Production of fumaric acid has declined during the same period as customers have switched to the less cosdy maleic anhydride when possible. All production of maleic anhydride in the United States in 1992 was from butane-based plants which used fixed-bed reactor technology as shown in Table 6. The number of fumaric acid producers has been reduced considerably since the early 1980s with only two producers left in the United States in 1992 as shown in Table 6. Pfizer shut down its fumaric acid plant at the end of 1993. However, Bartek of Canada will start up an expanded fumaric acid faciUty to supply the North American market for both their own and Huntsman s requirements. 

Sihcone contamination has been impHcated as a cause of failure in telephone switching systems and other devices that contain relay switch contacts (507). Analysis of airborne particulates near telephone switching stations showed the presence of siUcones at these locations. Where the indoor use of sihcones is intentionally minimised, outdoor levels were found to be higher than inside concentrations (508). Samples of particulates taken at two New Jersey office buildings revealed sihcone levels that were considerably higher indoors than outdoors. In these cases, indoor sihcone aerosols are beheved to be generated primarily by photocopiers, which use sihcone fuser oils. 

The aimual production value of small, sealed nickel—cadmium cells is over 1.2 biUion. However, environmental considerations relating to cadmium are necessitating changes in the fabrication techniques, as well as recovery of failed cells. Battery system designers are switching to nickel —metal hydride (MH) cells for some appHcations, typically in "AA"-si2e cells, to increase capacity in the same volume and avoid the use of cadmium. 

Owing to governmental regulations, considerable research has been expended to develop systems suitable for substitution of solvent-based systems, particularly for automobile and container appHcations. In the switch from solvent-based to waterborne systems, epoxies are successfully bridging the gap largely by adaptation of conventional resins. 

Ethanol s use as a chemical iatemiediate (Table 8) suffered considerably from its replacement ia the production of acetaldehyde, butyraldehyde, acetic acid, and ethyUiexanol. The switch from the ethanol route to those products has depressed demand for ethanol by more than 300 x 10 L (80 x 10 gal) siace 1970. This decrease reflects newer technologies for the manufacture of acetaldehyde and acetic acid, which is the largest use for acetaldehyde, by direct routes usiag ethylene, butane (173), and methanol. Oxo processes (qv) such as Union Carbide s Low Pressure Oxo process for the production of butanol and ethyUiexanol have totaUy replaced the processes based on acetaldehyde. For example, U.S. consumption of ethanol for acetaldehyde manufacture declined steadily from 50% ia 1962 to 37% ia 1964 and none ia 1990. Butadiene was made from ethanol on a large scale duriag World War II, but this route is no longer competitive with butadiene derived from petroleum operations. 

In certain cases, where the motor is too large and it is idle for a long period before it is installed and electrically connected (the space heaters are therefore OFF) even these heaters may not be sufficient to absorb moisture which might have condensed deep in the slots, unless, of course, the heaters are kept ON for a considerably long period. In such cases, it is advisable to heat the motor by methods 1 and 2 in addition to using the built-in heaters, until the insulation level of the windings reaches the required level. Once the motor has been installed, these space heaters, when provided in the windings, are switched ON automatically as soon as the motor is idle, and thus eliminate deep moisture condensation. 

On 66 kV networks the MVAr loading is normally high and therefore one praeliee is to install MVAr melers and adopt a manual. switching during variation of MVAr beyond the permissible level, purely as a cost consideration. 

The distance between the structure and fixed impressed current anodes is an important factor. The number of anodes has to be small so the anodes need to be relatively large, which will result in too negative a potential if the distance is not sufficiently great. A minimum distance of 1.5 m is prescribed [1-3], but this involves considerable construction effort due to the effects of heavy seas. Besides the so-called restriction on impressed current installations, there is the requirement that the corrosion protection be switched off when diving work is being carried out [14]. This regulation is not justifiable. Work on the underwater region of production platforms takes place continuously, as far as the weather allows if the protection must be switched off each time, the impressed current protection becomes very limited. 

Since the object to be protected represents a cell consisting of active and passive steel, considerable IR errors in the cell current must be expected in measuring the off potential. The considerations in Section 3.3.1 with reference to Eqs. (3-27) and (3-28) are relevant here. Since upon switching off the protection current, 7, the nearby cathodes lead to anodic polarization of a region at risk from corrosion, the cell currents and 7, have opposite signs. It follows from Eqs. (3-27) and (3-28) that the 77 -free potential must be more negative than the off potential. Therefore, there is greater certainty of the potential criterion in Eq. (2-39). 

Nonuniform current and potential distribution is usually to be expected with uncoated objects to be protected. The distribution can be considerably improved by coatings (see Section 20.1.3). In enamelled tanks, the current and potential distribution of cathodic protection is very good. By arranging the anode centrally, IR errors from equalizing currents in the switching-off phase can be ignored. The anode potential in the switching-off phase can be evaluated from the information... 

Id. For flyback-mode converters it is a good idea to select a power switch average current rating of about 1.5 times the maximum average input current of the supply. Another consideration is the loss. By overspecifying the current the /2i DS(on) loss (conduction loss) can be reduced with very little penalty on cost and input capacitance. 

The current-mode controlled forward converter has one additional consideration there is a double pole at one-half the operating switching frequency. The compensation bandwidth normally does not go this high, but it may cause problems if the closed-loop gain is not sufficiently low enough to attenuate its effects. Its influence on the control-to-output characteristic can be seen in Figure B-14. 

Controlling high frequency noise generation and radiation is the blackest of the black box art in switching power supply and product-system design. It is a subject that warrants a book all to itself and it is the final area that will interfere with the release of your product into the market. This appendix cannot adequately cover the subject, but will overview the major considerations involved with product design. 

Balogh Laszlo, Practical Considerations for MQSFET Gate Drive Techniques in High Speed, Switch-mode Applications, Seminar APEC99. March 1999. 

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