Machines: efficiency 74 simple

Figure 16.33 shows a schematic of a simple gas turbine. The machine is essentially a rotary compressor mounted on the same shaft as a turbine. Air enters the compressor where it is compressed before entering a combustion chamber. Here the combustion of fuel increases its temperature. The mixture of air and combustion gases is expanded in the turbine. The input of energy to the combustion chamber allows enough power to be developed in the turbine to both drive the compressor and provide useful power. The performance of the machine is specified in terms of the power output, airflow rate through the machine, efficiency of conversion of heat to power and the temperature of the exhaust. Gas turbines are normally used only for relatively large-scale applications, and will be dealt with in more detail in Chapter 23. 

The progress in the calculation of highly correlated electronic wavefunc-tions is due both to the development of improved computational methods and to the rapidly increasing computing power available. In particular, the advent of vector computers has made it possible to perform much larger calculations than before in shorter times. In order to use such machines efficiently, it is essential to adjust the methods to the hardware available. Generally important is to remove all logic from the innermost loops and to perform as many simple vector or matrix operations as possible. 

As is apparent from Figure 5.12, at room temperature the amount of salt required to effect the phase separation and drive contraction by ion pairing is less than one-tenth that amount required when there are no negatively charged carboxylate (-COO") groups. This means a 10-fold increase in efficiency of the function of this salt-driven molecular machine. The simple replacement of Val by Glu has increased the... 

Numerous forms are available, but these are generally expensive. A simple, efficient but inexpensive shaking machine, which can easily be constructed by a competent... 

The next generation of gas turbine-based, combined-cycle power plants, under constmction in many parts of the world, is to feature net plant efficiencies in the 60% range based on LHV of fuel input. These faciUties, scheduled for start-up in the latter 1990s, are anchored by large gas turbines capable of simple-cycle efficiencies >40% LHV in some cases. To develop these machines, manufacturers have scaled up and improved upon designs that have already proved to be highly rehable. 

A 165-MW-class gas turbine/generator has been introduced by another manufacturer. This machine, also developed by scaling up a proven design, features a simple-cycle efficiency of 37.5% a turbine inlet temperature of 1235°C a pressure ratio of 30 1, up from 16 1 on the previous generation and an output of 165 MW for gas fuel firing under International Standards Organization (ISO) conditions (101 kPa, 15°C (14.7 psia, 59°F)). A combined-cycle facihty based around this machine could achieve efficiencies up to 58% or a heat rate of about 6209 kj/kWh (5885 Btu/kWh). 

At least two manufacturers have developed and installed machines rated to produce more than 210 MW of electricity in the simple-cycle mode. In both cases, the machines were designed and manufactured through cooperative ventures between two or more international gas turbine developers. One 50-Hz unit, first installed as a peaking power faciUty in France, is rated for a gross output of 212 MW and a net simple-cycle efficiency of 34.2% for natural-gas firing. When integrated into an enhanced three-pressure, combined-cycle with reheat, net plant efficiencies in excess of 54% reportedly can be achieved. 

Winnowing. Winnowing, often called cracking and fanning, is one of the most important operations in cocoa processing. It is a simple process that involves separating the nib, or kernel, from the inedible shell. Failure to remove shell results in lower quahty cocoa and chocolate products, more wear on nib grinding machines, and lower efficiency in all subsequent operations. 

Since fluid power is efficiently transmitted and controlled, it gives freedom in designing a machine. The need for gear, cam, and lever systems is eliminated. Fluid power systems can provide infinitely variable speed, force and direction control with simple, reliable elements. 

The oil separation, cooling and filtering for a twin-screwcompressor adds to the complexity of an otherwise simple machine. Some commercial screw compressors are available which have the oilhandling circuit built into the assembly, with a small loss of overall efficiency. 

The reciprocating compressor is a positive-displacement compressor. It is cheaper to purchase and install than a centrifugal compressor. Also—in theory—far more efficient (90 percent) than a centrifugal compressor (70 percent). Certainly, reciprocating compressors are more simple to understand and engineer than centrifugal machines. Best of all, they are not subject to surge. 

Focusing repair efforts to the source of the machine problems via vibration diagnoses so that repairs are effective and efficient. One service request for the vibration technicians involved a 200-H.P. motor-driven, induced-draft fan. An area maintenance crew considered sending the large motor to a repair shop because they were unable to get the system to operate smoothly. The vibration technician diagnosed the problem as a simple imbalance in the fan, which was corrected easily with balance weights. 

Heat pipes can easily be implemented inside sorption storage vessels. They are the most convenient thermal control devices for the solid and liquid sorption machines due to its flexibility, high thermal efficiency, cost-effectiveness, reliability, long operating life, simple manufacturing technology. 

Since all reversible heat engines working between the same two temperatures will have the same efficiencies, we can conclude that their efficiencies depend only upon the two temperatures between which they work. For further thermodynamic consideration it is, therefore, sufficient that we consider that type of reversible machine, which will lend itself to simple thermodynamic treatment. A machine employing Carnot s cycle is of such a type. 

---