Intermittent operation

Regenerative heat exchangers of both the fixed-bed and moving-bed types (67) have been considered for MHD use. The more recent efforts have focused on the fixed-bed type (68), which operates intermittently through recycling. A complete preheater subsystem for a plant requites several regenerators with switch-over valves to deflver a continuous supply of preheated air. The outlet temperature of the air then varies between a maximum and a minimum value during the preheat cycle. 

Spinning-cup atomisers are used ia some plants to provide finer atomisation, allowiag smaller burner chambers and easier turndown, but with the burden of added rotating equipment. Rotary kiln burners were once popular to bum lower quaHty sulfur, but few are stiU ia operation. Spray burners can be operated intermittently and used at higher rates than rotary burners. 

When the process is operating normally, no alarms should be triggered. Within the electric utihty industry, this design objective is known as darkboard. Application of darkboard is especially important in batch plants, where much of the process equipment is operated intermittently. 

Equipment in which water flow is slow or intermittent is subject to deposition and associated corrosion. Hence, service water-system components that operate intermittently frequently suffer attack. Deposits accumulate in narrow orifices, screens, long horizontal pipe runs, sumps, or at regions of constricted flow. 

Failures of this type occurred every 6 to 9 years. The well was operated intermittently, since several available wells are operated on a rotating basis. It is probable that most of the graphitic corrosion occurred during idle times actual metal loss occurred during operation of the pump. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

On normally operating equipment, because the demand is continuous, all failures can be considered as functions of time but for equipment that is operated intermittently, the relative proportion of demand- and time-related failures in combination with the frequency of demands and the type of maintenance and inspection program will have an impact on its total failure rate. This point is best illustrated by an example ... 

The number and severity of failures experienced by the equipment under study must be related to the operations of the facility. It would be inappropriate to assign the same operating histories to a continuously operating system and a system that operates intermittently. The number of hours in different operating modes (for example, 100% production versus shutdown) affect failure rate calculation and service description for taxonomy definition. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

The manufacturer must know if the suction side of the pump is associated with v acuum equipment, or is to lift the liquid. This can make a difference as to the type of impeller suction opening he provides. If the system operates intermittently it should be noted. A piping diagram is often helpful in obtaining full benefit of the manufacturer s special knowledge. 

A steam generator designed to operate intermittently to meet peak demands for steam and electricity. 

The choice of a corrosion inhibitor as an additive in antifreezing agents is also dependent on the mode of operation. For instance, cars are operated intermittently. Here the corrosion inhibitors must also protect the system when it is idle. Film-forming silicates can protect the system while idle. This is especially true of aluminum parts, which are introduced in cars for the sake of weight reduction. But silicones can react with ethylene glycol to form crosslinked polymers. These gels may clog lines. 

Batch processes are designed to operate intermittently. Some, or all, the process units being frequently shut down and started up. 

Anon., CISHC Chem. Safety Summ., 1984, 55(218), 34-35 Diethylamine fumes from a reactor were usually absorbed in a glass scrubber through which sulfuric acid was circulated, but an unresolved fault in the level sensor caused the acid circulation pump to operate intermittently. While the pump was not running, amine fumes condensed in the dip pipe, forming a sohd crust (of the sulfate) which allowed a quantity of condensed amine to accumulate out of contact with the acid. When the pump was restarted, the neutralisation exotherm was sufficient to shatter the scrubber and distort the mesh guard around it. 

Continous mode of operation Intermittent batch operation... 

A fuel cell utilizing UDMH as fuel and N02 as oxidant is reported. Operated intermittently over a 3-month period with degradation, it consistently produced a power density of 40mw/ cm2 (40w/ft2). The cell consists of a sandwich of Zr acid phosphate in a polyvinylidene fluoride (PVF) binder and diffuse-catalyst layers of Pt black, Zr acid phosphate and PVF. Pt screens are used as current collectors] 3) G.R. Eske-lund et al, Chemical-Mechanical Mine , PATR 3724 (1968) [A mine feasibility study is reported in which the hypergolic system UDMH—... 

Figure 19.13. Parametric cycle operating intermittently in five periods valves that are open each time are identified with their flow rates Vif and the low and high temperature levels are identified with an asterisk in the proper column.

Since the fully accelerated TMP will have a high compression ratio for mass fragments with M > 40 (see Section 3.3.2), this may be of little consequence if the TMP runs continuously. If, however, the TMP is operated intermittently, contamination could be a problem. 

Subjects 1 and 2 live in an air-conditioned apartment of 5 rooms (5088 cubic feet), in which air-conditioning equipment operated intermittently during the period of observation. In effect, it is one big room since the doors to the two bedrooms are rarely closed and there are no doors between the living room, dining room, and kitchen. Five Vaporizers were installed in this area and changed at the frequency indicated above. 

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