Total enclosures

Evaluation of the AS" s that charac terize an enclosure involves solution of a system of radiation balances on the surfaces. If the assumption is made that all the zones of the enclosure a re gray and emit and reflec t diffusely, then the direct-exchange area ij, as evaluated for the black-siirface pair A and Aj, applies to emission and reflections between them. If at a surface the total leaving-flnx density, emitted phis reflected, is denoted by W (and called by some the radiosity and by others the exitance), radiation balances take the form ... 

If an enclosure may be divided into several radiant-heat sources or sinks Ai, A2, etc, and the rest of the enclosure (reradiating refractoiy surface) may be lumped together as A at a uniform temperature Tr, then the total interchange area for zone pairs in the black system is given by... 

Sin e-Gas-Zone/Two-Surface-Zone Systems An enclosure consisting of but one isothermal gas zone and two gray surface zones can, properly specified, model so many industrially important radiation problems as to merit detailed presentation. One can evaluate the total radiation flux between any two of the three zones, including multiple reflec tion at all surfaces. 

HEAT AND MASS TRANSFER TABLE 5-10 Total-Exchange Areas for Four Arrangements of Two-Zone-Surface Enclosures of a Gray Gas... 

Material characteristics, both chemical and physical, should be considered, especially flowabihty. Abrasiveness, friability, and lump size are also important. Chemical effects (e.g., the effect of oil on rubber or of acids on metal) may dictate the structural materials out of which conveyor components are fabricated. Moisture or oxidation effects from exposure to the atmosphere may be harmful to the material being conveyed and require total enclosure of the conveyor or even an artificial atmosphere. Obviously, certain types of conveyors lend themselves to such special requirements better than others. 

The test equipment is the same as described for degree of protection I. But the enclosure under lest is tilted up to an angle of 15° in respect of its normal operating position successively, in iw o planes at right angles (to cover all four sides). The total duration of the test will be 10 minutes (2.5 minutes each side). The lesl results should be the same as for degree of protection I. 

The design criteria and construction details of this system are totally different from those of a non-isolaled phase bus system. This type of enclosure is therefore dealt separately in Chapter 31. 

Vk, Vk2 and Vk (W. when considering the outdoor part) can thus be determined. The total heal, Vk, so generated can be naturally dissipated through the enclosure by radiation and convection. If natural cooling is not adequate, forced cooling can be adopted through forced air or water. But precautions must be taken to ensure that the system is protected from absorbing dirt, dust or moisture from the atmosphere. 

For a near-total shielding of the field produced by the main cotiductors (i.e. for - /. to be very low), it is essential to have the thickness of the enclosure is netir to 5p as possible. But this m iy prove to be a costly proposition. In addition, ti higher induced current in the enclosure will also tnean higher losses. This hits been established by computing the cost of the enclosure and capitalizing the cost of losses for minimum losses in the eticlosure... 

Optimization factor This is a function of the cost of enclosure for different thickness. /. the cost of the cooling system (if cooling is considered necesstiry) and the capitalized cost of the losses at different thickness /. A curve as shown in Figure 31.13. rather sitnilar to that in Figure 31.12. ciin be established theoretically between the total cost of the IPB system versus /. 

The above two curves will help optitnize the thick-nes.s, /, of the enclosure foi a total minimum cost of the system. Enclosure losses may not be lowest at this thickness as shown in Figure 31.12, but they wotild maintain the tetnperature rise of the enclosure within limits. The magnetic field in the space, being already very low, would require no other measure. Moreover, it small field in the space may ciiusc only a small amount of heat in nearby structures, which may be... 

Figure 31.13 Optimization curve between thickness of enclosure and the total cost of the IPB system...

Slits are left ( 50 mm wide) to facilitate heat dissipation from the inside surface of the conductors [enclosures are totally closed] Note... 

In fact, it is the solar effect that is causing the maximum heat. The factors considered for the solar effect are also highly conservative. Nevertheless, a canopy over the outdoor part is advisable in the above case. This will ensure the same size of enclosure for the outdoor as well as the indoor parts and also eliminate the requirement for a thicker enclosure or a forced cooling arrangement. Now there will be no direct solar radiation over the bus system and the total solar effect can be eliminated, except for substituting the indoor ambient temperature of 48 C with the maximum outdoor temperature for the outdoor part of the bus system. 

For LT bus systems The length of the test piece will be a minimum 6 m as in lEC 60439-2 with at least one Joint. The Joints must be both in the conductor and in the enclosure in each phase. If the total length of the bus section is less than specified, the entire length of the bus system in the fully assembled form will then be tested. 

If the enclosure and covers not to be touched during normal operation the temperature rise limit ean be considered to be 40 C. For the elass of insulation, not supposed to exeeed a total temperature of 90 C during normal operation. 

The philosophy of any EMI shield is to encourage eddy currents to flow within the surfaces, thus dissipating the noise energy. Also, the assembled enclosure should act as a gaussian enclosure where there is good electrical conduction totally around the enclosure. So removable hatches and enclosure members need very good electrical connections around their peripheries. RF gasketing is sometimes used in particularly troublesome cases. 

Total enclosure of the process and handling systems unless not reasonably practicable. 

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). 

Gravity settlement Total enclosure Wet scrubbing Absorption tower ... 

Absorption Destruction Direct recycling Dispersion Total enclosure Gas Wet scrubbing... 

Cooling tower fan motors must be properly selected for long life and trouble free operation. In addition, it is important to match the motor s enclosure to the application. Motors in draw through applications that are mounted directly in the air stream, for example, experience tough duty and must be of the totally enclosed type. Motors are categorized as follows ... 

TEAO - Totally Enclosed Air Over where the motor has large cooling fins and depends on the cooling tower air stream for air movement. The motor shaft protrudes from the enclosure at one place only. 

Calculate total loads (heat, humidity, contaminants) from different subprocesses and environment (building) to ventilated enclosure (zones). 

The more enclosed a process is, the easier it is to keep a low concentration in the workroom. It is usually necessary for the workers or for some equipment to have physical contact with the process, w hich could make it difficult to use complete enclosures. If it is possible to enclose the contaminant source and the tool, a total enclosure is recommended, especially if the workers only need to access the process during pauses in operation. Total enclosures may also be necessary for processes that generate highly toxic contaminants. Where total enclosures are not practicable, partial enclosures may be used. F xterior hoods are the least effective exhaust hood. 

The classification of hoods into exterior hoods and enclosures could sometimes make it difficult to specify a hood. This classification is only an attempt to describe the hoods. Enclosures can be separated into partial and total enclosures partial enclosures have an opening to the surroundings big enough to use for work, and total enclosures do not. Both have the contaminant source inside a physical volume and for some of these hoods this volume is large enough for some workers to work inside. See Fig. 10.39. 

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