energy, which is fed back to the network. Accurate braking control as well as energy recovery are the main reasons for using this mode of operation both in industry and in traction.
Electrical drives play an important role in energy saving when machines such as pumps, compressors, and fans are controlled by speed variation as an alternative to other dissipative methods (see Chaps.10,11,12,13).
In addition, electrical drives are among the main components of industrial automation systems and allow improvements in production quality and quantity, thus generally determining a decrease in specific energy consumption per unit of production. Interfacing with the plant centralized control, diagnostics and monitor- ing are other factors that make electrical variable-speed drives attractive for indus- trial applications.
7.4.3 Application Problems
Electrical variable-speed drives may be a source of thermal, mechanical, and electrical malfunctioning which must be investigated carefully in order to guarantee a safe operation of the drive itself and of the electric network.
The power factor at the drive feeding node changes according to the operating mode: generally, the lower the actual load, the lower the power factor. That depends on the internal structure of the power converter and on the current and voltage harmonics, but it can be assumed as a general rule. A control system is usually required to keep the power factor value within the accepted range.
Derating of electrical machines, when fed by power converters, is necessary because of the increase in losses due to the harmonics and because of the reduction in efficiency of cooling systems at low speed. A derating of 10–15 % depends on the harmonics; higher derating, up to 50 %, is due to thermal phenomena caused by the reduced speed range. In any case, an independent cooling system is installed, generally a fan driven by an independent electric motor.
7.5.1 Joule Losses Principle
The operating principle refers to the Joule losses (power¼resistancecurrent2).
In metal treatment applications, two situations may occur: (1) the current is fed from the line and flows through a resistive winding; (2) the current is fed from the line and flows through an inductive winding, thus determining induced voltage and current inside the metal to be heated, which is generally inside the inductor winding.
Because of the losses in the power plant producing electric energy from fuels or other combustibles, electric heating does not seem convenient from the primary energy consumption point of view (roughly 6,600–
10,500 kJ/kWh or 6,256–9,952 Btu/kWh are required from a utility power plant, whereas 1 kWh delivered to the end user yields 3,600 kJ or 3,412 Btu as heat).
Heating from combustibles thus consumes less primary energy and is often cheaper, except for electric energy’s special low rates (night, holiday, others) charged by utilities.
Production requirements, in terms of constant quality and quantity, may impose the use of electric heating. Notice that end-user efficiency of heating from combustibles (typical values 50–60 %) is lower than that of electric heating (the ratio between the two efficiencies is roughly 0.7), because of the unavoidable flue gas losses due to combustion, but that does not modify the considerations already made.
Efficiency can be increased by recovering heat from high-temperature flue gases to preheat combustion air (see also Chap.15).
Boilers producing steam or hot water by using the Joule losses principle have been discussed in Sect.6.5.
7.5.2 Electromagnetic Wave Heating
An alternative method of heating is electromagnetic wave heating. This subjects the material under treatment to electric waves ranging from a few Hz to 1 GHz in frequency for radiofrequency systems and from 1 to 300 GHz for microwave systems. Typical values are 13–27 MHz for the former and 2.5 GHz for the latter.
Advantages of these systems are the possibility of heating the material from the core to the surfaces instead of vice versa like in traditional heating systems based on fuel combustion, the reduction of local pollution, the easy control of the energy flow, and the reduction of the heating time to 1/3–1/20.
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These systems are used in sterilization, defreezing, cooking, and drying.
Industrial drying applications are quite common in textile, food processing, and other industries; in addition, both domestic and industrial cooking increasingly employ these systems.
Notice that these systems alone do not permit a significant energy saving in drying because the specific consumption, the energy consumed per kg of evaporated water, is roughly equal to or higher than the consumption in the case of traditional drying based on combustion.
Typical values are 1.2–1.3 kWh/kg of evaporated water, which are simi- lar to those obtained when fuels are used for standard drying; of course, specific consumption as primary energy needed by utility power plants is higher than with combustion-based dryers.
These systems are very often installed as a second step after thermal or mechan- ical, e.g., centrifugal, drying, to remove water from the core of the material. In this way, the quality of production is improved by action on a phase of the process in which traditional systems would require higher energy consumption to achieve similar results.
The size of single modules generally ranges from 1 to 10 kW output for microwaves and from 20 to 100 kW for radiofrequency systems. Modules are assembled for higher power.
7.5.3 Heat Pumps
A heat pump is a device that operates cyclically to transform low-temperature energy from a source (air or water) into high-temperature energy by the application of external work, mainly mechanical. The prime mover is either an electric motor or a fuel engine. Energy can be saved depending on the low-temperature source available and on the requirement of the end users at high temperature (see Sect.12.9).
7.5.4 Mechanical Recompression and Thermocompression of Steam
This system increases the pressure and thus the temperature of saturated steam by using the mechanical energy of compression instead of thermal energy from a boiler. It is used to make small pressure changes, particularly when rejected steam from process (for instance liquid concentration or distillation) is available for reuse in the same process equipment.
Typical applications are in dairy industries, food processing, and chemical industries.
In mechanical vapor recompression systems, typical values are 20–30 kWh for recompressing 1 t of steam with a temperature increase of 6–8 K orC(11–15F). Greater increases will require higher energy consumption.
For mechanical recompression in concentration plants the basic lay- out is shown in Fig.7.8: typical values are 25–35 kWh of electric energy plant consumption for evaporating 1 t of water.
Lower values can be achieved if mechanical compression is combined with multiple-effect evaporation plants. Ifnis the number of effects, the amount of evaporated water referred to the steam consumption is n0.95nkg water/kg steam.
If the compressor is driven by a turbine or a reciprocating engine with heat recovery (see cogeneration in Chap. 9), operating costs can be reduced.
An alternative to mechanical recompression is thermocompression (see Fig.7.8) where steam pressure (and thus temperature) increases by means of an ejector (see Sect.11.1). In this case, high-pressure steam from boilers is used to transport the rejected steam into the ejector, thus converting the velocity of the mixture to pressure in a diffuser.
The higher the driving steam pressure in proportion to the suction pressure of the rejected steam, the lower the driving steam flow rate will be.
Typical operating parameters of thermocompression are 0.6–1.2 MPa (87–174 psi) for the driving steam pressure, 0.05–0.02 MPa (7.25–2.9 psi) for the suction pressure, 1.5–2 for the compression ratio of the ejector (between sections 1 and 2 in Fig. 7.8; 15–20 K or C or 27–36F of temperature increase), and 0.5–1 for the ratio between the driving steam flow rate and the rejected steam in the ejector. End-user energy consumption is ten times greater than with mechanical recompression systems.
This system can be used also in the first stage of a multiple-effect evaporation plant; in this case, the plant requires less steam or fewer effects than a pure multiple-effect evaporation plant with the same evaporation rates, but still more than plants with mechanical evaporation.
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Fig. 7.8 Operating principles of MVR (mechanical vapor recompression) and TVR (thermal vapor recompression)