Cooling air

2 Air Cooling. The thermal shield and the graphite. are cooled by air drawn through them from the Reactor Building. The amount of heat that, must be removed is.indicated in Chap. 4 the external air system to provide proper air flow is described in Chap. 8.  

The air enters at the top edges of the thermal shield through a system df 22 ducts which distributes it evenly to all four sides of the shield. It is drawn out from the space above the graphite through two large holes in opposite corners of the upper thermal shields. 

The major part of the air flows between the side plates of the thermal s,hield and then into the space between the bottom thermal shields. Holes in the upper plate of the bottom thermal shield distribute the air into the pebble zone and into the cooling holes of the permanent graphite. A small amount of this air goes outside the outer side plates of the thermal shield to help cool the concrete. By means of baffles in the inlet ducts some of the air is drawn between the two plates of the upper thermal shield and then through holes in the lower plate into the space above thegraphite. 

Since about 60% of the heat production in the graphite is in the pebble zone, the system has been designed to distribute the air in approximately this ratio between the pebble zone and the permanent graphite. In the pebble zone the air flows in the interstices between the pebbles, while in the gr.a ph.ite zone cooling is accomplished by allowing air to flow in the system of cooling holes illustrated in Fig. 2.7.A. 

Since graphite begins to oxidize (O.OOOS to 0.0009% per clay.) at about 570 F, this temperature was considered to be the maximum, allowable for 45,000-kw operation. With this limit it was found ) that the required cooling in the pebble zone could be obtained by an air flow of 840 Ib/min, which requires a pressure drop of 26 in. of water. This gives an average exit air temperature of 203°F from the pebble zone if the air is heated 15°F in the thermal shield to 90 F. To this air is added the 684 Ib/rnih flow through the permanent graphite and experimental facilities and the 174 Ib/min to cool the top 

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Separation of low-molecular-weight materials. Low-molecular-weight materials are distilled at high pressure to increase their condensing temperature and to allow, if possible, the use of cooling water or air cooling in the column condenser. Very low... 

Most refrigeration systems are essentially the same as the heat pump cycle shown in Fig. 6.37. Heat is absorbed at low temperature, servicing the process, and rejected at higher temperature either directly to ambient (cooling water or air cooling) or to heat recovery in the process. Heat transfer takes place essentially over latent heat profiles. Such cycles can be much more complex if more than one refrigeration level is involved. 

The maximum power is available with air cooling up to 45kV for insulation reasons, the tubes are downrated to about 20W at 65kV. 

Most of the advantages of MCB technology can be used to make small anode-grounded metal-ceramic X-ray tubes as well. These could be water- or air-cooled and reach power ranges up to 1 kW at voltages up to lOOkV. 

A gas—tungsten arc-welding system is more complex. In addition to the components of the shielded-metal arc system, provisions must be made for the inert gas supply and water or air cooling of the welding torch. GTAW systems may range from manual to automatic. 

Compounds having low vapor pressures at room temperature are treated in water-cooled or air-cooled condensers, but more volatile materials often requite two-stage condensation, usually water cooling followed by refrigeration. Minimising noncondensable gases reduces the need to cool to extremely low dew points. Partial condensation may suffice if the carrier gas can be recycled to the process. Condensation can be especially helpful for primary recovery before another method such as adsorption or gas incineration. Both surface condensers, often of the finned coil type, and direct-contact condensers are used. Direct-contact condensers usually atomize a cooled, recirculated, low vapor pressure Hquid such as water into the gas. The recycle hquid is often cooled in an external exchanger. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Air-Cooled Heat Exchangers for General Eefinery Service, API 661, American Petroleum Institute, Washington, D.C., Apr. 1992. 

Terephthahc acid (TA) or dimethyl terephthalate [120-61 -6] (DMT) reacts with ethyleae glycol (2G) to form bis(2-hydroxyethyl) terephthalate [959-26-2] (BHET) which is coadeasatioa polymerized to PET with the elimination of 2G. Moltea polymer is extmded through a die (spinneret) forming filaments that are solidified by air cooling. Combinations of stress, strain, and thermal treatments are appHed to the filaments to orient and crystallize the molecular chains. These steps develop the fiber properties required for specific uses. The two general physical forms of PET fibers are continuous filament and cut staple. 

Westinghouse Electric Corp. initiated a program to develop air-cooled PAFC stacks, containing cooling plates at six-ceU intervals. Full size 100-kW stacks (468 cells, 0.12-m electrode area) were built, and a module containing four of these stacks was tested. An air-cooled stack operated at 0.480 MPa yielded a cell voltage of 0.7 V at 267 m A /cm (187 mW/cm ). Demonstration of this technology is plarmed for a site in Norway. 

The high performance of modem spectrographs means that low power lasers can be used as excitation sources. These are typically 10—100-mW devices which are air-cooled and can be operated from 117-V a-c lines. In the green, the Ar" (514.5-nm) laser remains the most popular but is being challenged by the smaller and more efficient frequency doubled Nd YAG (532-nm). In the nir, diode lasers (784-nm) and diode-pumped alexandrite... 

Rapidly quenching to room temperature retains a maximum amount of alloying element (Cu) in soHd solution. The cooling rate required varies considerably with different alloys. For some alloys, air cooling is sufficiently rapid, whereas other alloys require water-quenching. After cooling, the alloy is in a relatively soft metastable condition referred to as the solution-treated condition. 

Fig. 17. Structuie of U-700 after piecipitation hardening temperature of 1168 C/4 h + 1079" C/4 h + 843 C/24 h + TGO C/IG h with air cooling from each temperature. A grain boundary with precipitated carbides is passing through the center of the electron micrograph. Matrix precipitates are y -Nij(TiAl).

The most dramatic evolution of a microwave power source is that of the cooker magnetron for microwave ovens (48). These magnetrons are air-cooled, weigh 1.2 kg, generate weU over 700 W at 2.45 GHz into a matched load, and exhibit a tube efficiency on the order of 70%. AppHcation is enhanced by the avaHabiHty of comparatively inexpensive microwave power and microwave oven hardware (53). The cost of these tubes has consistently dropped (11) since their introduction in the eady 1970s. As of this writing (ca 1995), cost is < 15/tube for large quantities. For small quantities the price is < 100/tube. 

Thermal Process. In the manufacture of phosphoric acid from elemental phosphoms, white (yellow) phosphoms is burned in excess air, the resulting phosphoms pentoxide is hydrated, heats of combustion and hydration are removed, and the phosphoric acid mist collected. Within limits, the concentration of the product acid is controlled by the quantity of water added and the cooling capabiUties. Various process schemes deal with the problems of high combustion-zone temperatures, the reactivity of hot phosphoms pentoxide, the corrosive nature of hot phosphoric acid, and the difficulty of collecting fine phosphoric acid mist. The principal process types (Fig. 3) include the wetted-waH, water-cooled, or air-cooled combustion chamber, depending on the method used to protect the combustion chamber wall. 

Fig. 3. Thermal phosphoric acid processes (a) wetted-waH combustion chamber (b) air-cooled combustion chamber (c) water-cooled combustion...

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