Steady state behaviour and maintenance planning of a desulphurization system in a urea fertilizer plant

This paper presents the steady state behaviour and maintenance planning of the desulphurization process in the fertilizer industry. The process consists of four subsystems, A, B, C and D in series with three states; good, reduced and failed. One standby unit is provided for each pump. Taking constant failure and repair rates for each subsystem, mathematical modelling is done using the Chapman-Kolmogorov birth-death process. An expression for steady state availability is given. Based on the data available from a medium sized ammonia production process, the behaviour of each working unit in the process has been analysed. The computed results are discussed with the concerned plant personnel which is helpful to the management for implementing any future plan regarding design modification of the system/processes.     

An algebraic method for reliability calculations

This paper evaluates the reliability behaviour of a complex system consisting of three sub-systems G₁, G₂ and G₃ connected in parallel redundancy (1-out-of-3: G). These sub-systems are nothing but three power generators in a power house which are connected with switching devices. By the application of Boolean function technique and the theorem of summation of probabilities of compatible events, a symbolic expression for reliability of such a complex system has been evaluated by considering that failure times for various components of the complex system follow arbitrary distributions. In particular, reliability has been evaluated for exponential and Weibull distributions. Moreover, an important parameter, viz. MTTF, has also been computed for exponential failure rates of the units comprising the system. A numerical example, along with graphs, is discussed at the end to highlight the important results.     

Optimal control of a removable server in an M/Ek/1 queueing system with finite capacity

This paper deals with the economic behavior of a removable server in the N policy M/E_k/1 queueing system with finite capacity. Expressions for the probability mass functions of the number of customers in the system are derived and taken in closed-form. As special cases, the probability mass functions of the number of customers for the N policy M/M/1 queueing system, the ordinary M/K_k/1 queueing system, and the ordinary M/M/1 queueing system are obtained. The cost structure includes a holding cost per unit time spent in the system for each customer, costs per unit time for keeping the server on or off, a server start-up cost, a server shut-down cost, and fixed cost for every lost customer. Following the construction of the total expected cost per unit time, we determine the optimal operating policy at minimum cost.     

Reliability analysis of the naphtha fuel oil system in a thermal power plant

The paper discusses the performance and reliability analysis of the naptha fuel oil system in a thermal power plant. Reliability, the availability function (A_v) and the mean time to system failure (MTSF) of this system have been calculated. An example follows, assuming various values of failure and repair rate.     

On the Placement of Thermoelectric Generators in Automobiles

The placement of thermoelectric generators (TEGs) in vehicles is analyzed, taking into account the interaction of the TEG with the internal combustion engine (ICE). Alternative locations of the TEG directly in the ICE, on the exhaust pipe, and on the cooling system are considered. In all three cases there is a conflict between the two thermal machines, which reduces the total efficiency of the thermodynamic (ICE + TEG) system. It is shown that the cause of the conflict is the low efficiency of the TEG (η _TEG < 0.05) compared with that of the ICE (η _TEG < 0.4); this conflict increases with the net power W _e and decreases with increasing η _TEG. For this reason, attainable values of W _e, as well as waste heat recovery in cars by the TEG, are significantly limited. Also, some problems of finding materials for automotive TEGs and ways to suppress the parasitic Thomson effect in TEG legs are discussed.     

Reliability analysis and optimization applications of a two-unit standby redundant system with spare units

Consider a two-unit standby redundant system with two main units, one repair facility, and n spare units. When the main unit has failed and the other is under repair, a spare unit takes over the operation and if it fails, it is replaced by a new one until the repair of the failed unit is completed. The system fails when the last spare unit fails while one main unit is under repair and the other has failed. In this paper, we derive expressions for 1) the distribution function of the first time to system failure, 2) the probability that the total number of failed spare units during the time interval (0,t] is n and 3) the mean of the total number of failed spare units in (0,t] and its asymptotic behaviour. Introducing costs incurred for each failed main unit and each failed spare unit, the expected cost per unit of time of the system was also derived. Finally an optinmization problem is discussed in order to compare the expected cost of the system with both main units and spare units with that of spare units only, and particular cases are considered.     

A note on a multi-unit system with r repair facility

This paper deals with a system consisting of (n + m) identical units; n units are needed for the system to function and the remaining m units are warm standby supported by r repair facility. The online and standby units have different but constant failure rates; the repair time distribution for the standby unit is taken to be a constant and the distribution for the online unit is arbitrary. Functions describing the behaviour of all the other units when one unit is undergoing online repair are studied and an earlier result is recovered as a special case.     

Efficient Bipolar Blue AIEgens for High‐Performance Nondoped Blue OLEDs and Hybrid White OLEDs

Blue organic luminescent materials play a crucial role in full‐color display and white lighting but efficient ones meeting commercial demands are very rare. Herein, the design and synthesis of tailor‐made bipolar blue luminogens with an anthracene core and various functional groups are reported. The thermal stabilities, photophysical properties, electronic structures, electrochemical behaviors, carrier transport abilities, and electroluminescence performances are systematically investigated. The luminogen TPE‐TAPBI containing a tetraphenylethene moiety shows aggregation‐induced emission, while another luminogen TriPE‐TAPBI bearing a triphenylethene unit exhibits light aggregation‐caused quenching. In comparison with TriPE‐TAPBI, TPE‐TAPBI has stronger blue emission in neat film and functions more efficiently in nondoped organic light‐emitting diodes (OLEDs). High maxima current, power, and external quantum efficiencies of 7.21 cd A^?1, 6.78 lm W^?1, and 5.73%, respectively, are attained by the nondoped blue OLED of TPE‐TAPBI (CIE_x,y = 0.15, 0.16). Moreover, efficient two‐color hybrid warm white OLEDs (CIE_x,y = 0.457, 0.470) are achieved using TPE‐TAPBI neat film as the blue‐emitting component, which provide total current, power, external quantum efficiencies of up to 70.5 lm W^?1, 76.0 cd A^?1, and 28% at 1000 cd m^?2, respectively. These blue and white OLEDs are among the most efficient devices with similar colors in the literature.     

Influence of thermal contact resistance on thermal impedance of microelectronic structures

The thermal impedance Z_th(jω) has been calculated numerically for a silicon chip glued on a ceramic substrate. The non perfect thermal contact is taken into account by modelling the chip-substrate interface as a thermal contact resistance r_c. If Z_th is represented as a Nyquist plot, mainly two circular arcs are observed. The high frequency arc is found to be almost independent from r_c, whereas the low frequency part is largely influenced by r_c. The thermal resistance R_th = Z_th(jω = 0) increases linearly with r_c, as known from the literature. Additionally, our simulations have shown that similar conclusions can be drawn for the real and imaginary part of Z_th at a fixed frequency ω ≠ 0.     

Influence of samarium on the thermoelectric figure of merit of Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te alloys

The temperature and concentration dependences of the electrical (conductivity σ, the Hall coefficient R), thermoelectric (thermovoltage α), and thermal (thermal conductivity K _tot) characteristics of Sm _x Pb_{1 − x} Te alloys (x = 0, 0.02, 0.04, 0.08) are studied in the temperature range 100–500 K. Using the data for σ, α, and K _tot, the thermoelectric power α²σ, figure of merit Z, and efficiency δ are calculated. It is established that at room-temperature α²σ and Z peak at the hole concentration p ≈ 1.2 × 10¹⁸ cm⁻³.     

On-state voltage drop based derating/uprating on a MW converter to improve reliability

Power semiconductor devices are the most fragile components limiting reliability of power converters, where major stresses are temperature dependent parameters. Typically, the operating virtual maximum junction temperature is specified by a manufacturer for each individual device. The thermal system and operating load are designed based on this number. Online monitoring of an on-state collector–emitter voltage (υ_ce,on) and a junction temperature is necessary to ensure the design performance within a safe limit and also to make this method applicable for derating/uprating power. This paper presents the real time measurement of υ_ce,on and thereby the junction temperature estimation in high power converters. Knowing these parameters online, the maximum power capability can be detected. The operating maximum junction temperature is selected and thereby based on the operating condition the derating of power is shown for a megawatt (MW) converter. An experimental setup including online monitoring is described and the measurement of power derating is presented at the specified conditions.     

Reliability analysis of a powerloom plant with cold standby for its strategic unit

A powerloom plant consists of two units A and B(n) connected in series. Unit B(n) has n components in parallel redundancy that carry threads of different varieties/colours to unit A. Unit A is the strategic unit that weaves these threads together and produces a long piece of cloth of some standard size. Failure of unit A makes the entire system fail whereas the failure of k components out of n in unit B(n) results in the system working in a degraded state. The present paper deals with the reliability analysis of the powerloom plant with a cold standby for its strategic unit. The common cause failure and critical human errors have also been considered to avoid overestimation of the reliability of the system. Results of Dhillon [Int. J. cyst. Sci. 23, 1277–1287 (1992)] have been deduced as a particular case of our result.     

High Thermoelectric Power Factor Near Room Temperature in Full-Heusler Alloys

We present results on the electrical resistivity, Seebeck coefficient, and thermal conductivity for the Heusler alloys Fe₂VAl_1–x Si _x and Fe₂VAl_1–x Sn _x synthesized using standard arc-melting techniques. While alloys with x = 0 are p-type, upon substitution of Si or Sn for Al the alloys can be made n-type with optimized sample compositions exhibiting thermoelectric power factors in excess of that of bismuth telluride near room temperature. The lattice thermal conductivity κ _L of these alloys is too large to produce a high figure of merit; the prospects for and initial attempts at lowering κ _L are discussed.     

Improving the Power Factor and the Role of Impurity Bands

The thermoelectric figure of merit, Z, is proportional to the ratio of the power factor to the thermal conductivity. In the past, efforts to improve Z have largely been directed towards reduction of the thermal conductivity by lowering the lattice component, λ _L. This approach has been so successful that λ _L is now sometimes no larger than it is in a typical amorphous material. Any further improvement would require the development of thermoelectric materials with larger power factors. Here, we consider some of the ways in which the power factor might be enlarged. The carrier mobilities and density- of-states effective masses for different semiconductors are reviewed briefly, and the relevance of these properties to the power factor is discussed. It is shown that a semiconductor with the mobility and effective mass of electrons in silicon and with the minimum lattice conductivity might have a ZT value of about 6. Preferential scattering to improve the Seebeck coefficient is then considered. Finally, the effect on the power factor of a modification of the density of states through the introduction of impurity bands is calculated. It is found that such bands are not beneficial when they lie close to the edge of a main band. However, they significantly improve the power factor when they lie several kT from the band edge, and their effect can be enhanced by counterdoping.     

Low-Temperature Transport Properties of InxFeyCo4?ySb12

Interesting results for cobalt triantimonide partially filled with indium have encouraged us to explore skutterudites filled with higher indium fractions. For pure In _x Co₄Sb₁₂, the fraction of voids filled is limited to about x = 0.25. To enable the insertion of more indium atoms, charge compensation is necessary. In this work, we studied the skutterudite compound In _x Fe _y Co_4−y Sb₁₂ partially filled with indium, where iron substitution for cobalt was employed for charge compensation. Polycrystalline samples were prepared by direct reaction of constituents. Structural and chemical characterization were accomplished by x-ray diffraction and energy-dispersive x-ray spectroscopy. Electrical resistivity, thermoelectric power, and thermal conductivity were measured between 2 K and 350 K. The influence of indium and iron on the charge-carrier transport properties and thermal conductivity in In _x Fe _y Co_4−y Sb₁₂ compounds is presented and discussed.     

电气监控系统在电厂升压站控制改造中应用与分析

针对某火电厂220 kV升压站控制室与#1机组为同一控制室这一特殊工况,解决#1机组实施监控系统改造,将#1机组原来的电气控制进入改造后的监控系统系统,将升压站的电气控制也接入改造后的监控系统.通过改用先进硬件设备,加强逻辑组态的严密性,重点处理好分散处理单元不间断供电以及输电线路继电保护重合闸充、放电逻辑时序配合等关键技术,确保升压站电气控制接入监控系统过程中及运行期间的平稳、安全、可靠.     

Effect of Ionic Radius and Resultant Two-Dimensionality of Phonons on Thermal Conductivity in M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> (M = Li,Na, K) by Perturbed Molecular Dynamics

Phonon thermal conductivity calculations for Li _x CoO₂, Na _x CoO₂, and K _x CoO₂ (x = 1, 0.5) have been carried out by perturbed molecular dynamics to clarify the dependence of thermal conductivity on alkali-metal vacancy concentration in these materials. While thermal conductivity decreased for all compounds upon introduction of alkali-metal vacancies, the magnitude of the decrease is strongly dependent on the size of the alkali-metal ion. Further numerical analyses using fictitious physical parameters reveal that, with increasing ionic radius, the two-dimensionality of the phonons in the CoO₂ layers, which are responsible for overall thermal conductivity, is enhanced, resulting in lower thermal conductivity in vacancy-free compounds as well as ineffectiveness of alkali-metal vacancies in lowering thermal conductivity. In contrast, for systems with smaller alkali-metal ionic radius, even though higher thermal conductivity is predicted when no vacancies are present, vacancies are quite effective in significantly lowering thermal conductivity by modifying phonon states in the CoO₂ layers, more so than in systems with larger alkali-metal vacancies.     

Engineering Thermal Conductivity for Balancing Between Reliability and Performance of Bulk Thermoelectric Generators

The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W $^?1, but it raises the thermomechanical vulnerability issue. By considering a balance between the thermoelectric performance and thermomechanical reliability issues, it is discussed how to improve device reliability of thermoelectric generators and the engineering thermal conductivity of thermoelectric materials.     

Adaptive orthogonal frequency division multiplexing channel capacity employing diversity in cellular fading environments with symmetric scattering distributions

Gaussian and hyperbolic angle‐of‐arrival probability density functions are used to derive channel capacity of orthogonal frequency division multiplexing transmission employing diversity techniques and adaptive policies in cellular wireless fading environments. The intercarrier interference (ICI) power is quantified and given as a function of Doppler shift f_d, symbol duration T_s, frequency correction ζ and propagation ratio τ. Two scattering distributions, which have been shown to closely fit experimental empirical data, are examined in this paper: (i) Gaussian and (ii) hyperbolic. A new signal‐to‐interference‐and‐noise ratio probability density function is derived as a function of the ICI power using diversity techniques and adaptive policies. From that, effects of f_dT_s, ζ and τ on channel capacity can be discussed. The main contribution of this work is to model ICI as a function of f_d and symbol duration T_s. Two diversity techniques are considered: (i) maximal ratio combining and (ii) selective combining. Three adaptive policies are studied: (i) optimal rate adaptation, (ii) optimal rate and power adaptation and (iii) channel inversion with fixed rate. Closed‐form expressions and bounds on various channel capacity with orthogonal frequency division multiplexing transmission under different scenarios are derived. Copyright © 2012 John Wiley & Sons, Ltd.     

Fabrication and properties of In2O3/CdS/CuInSe2 heterostructures

Heterostructures are prepared by the vacuum thermal evaporation of CdS〈In〉 onto heated p-CuInSe₂ substrates. An In₂O₃ layer is deposited on the wide-gap component of each structure by magnetron sputtering. The photosensitivity of the heterostructures in ambient light and in linearly polarized radiation is investigated. The photosensitivity of higher-quality structures attains 80 mA/W at T=300 K. The heterostructures exhibit induced photopleochroism, and the laws governing its angular and spectral dependences are discussed. It is concluded that the prepared heterostructures have possible applications as narrowly selective photoanalyzers of linearly polarized radiation. Fiz. Tekh. Poluprovodn. 33, 801–804 (July 1999)