Archive for the ‘Mechanical Engineering’ Category

Introduction

Direct fired heaters are used extensively in the oil refining and chemical process industries. In many instances, those heaters are relied on to heat hydrocarbon liquids or gases to temperatures of 900 to 1500°F or higher. At those temperatures, the stationary fluid film on the inside tube surfaces is subject to thermal decomposition, resulting in the deposition of coke at that location. The coke layer behaves as an insulating barrier between the process fluid and the tube wall, gradually increasing in thickness; this causes the tube wall to reach higher and higher temperatures to transfer the required heat input to the fluid. With the passage of a sufficient amount of time, the tube wall temperature reaches the design temperature, and tube damage occurs if operation at design throughput is continued. At that point, the heater must be shut down and the coke deposits removed. This is accomplished mechanically by “pigging,” the process of passing a cleaning plug driven by fluid pressure through the tubes by using compressed air–driven rotary cutting tools passed through the tubes by an armored compressed air cable, or by controlled burning of the coke at a high temperature—about 1300°F—in an oxygen-rich atmosphere. The interval between the start-up with clean tubes and the shutdown for decoking is termed the run length.

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What does the characteristic frequency of a filter polynomial really represent? This article provides an intuitive insight into this question with a rigorous graphical and mathematical treatment.

Electronic filter design is a well-established subject. Nevertheless, there are certain conceptual stumbling blocks that the student—and even the experienced designer—sometimes encounter when dealing with filter polynomials (transfer functions). In this article we deal with one such concept that underlies the design of a filter that is described by a polynomial transfer function, whether implemented in analog or digital form, active or passive— the so-called characteristic frequency of the filter.

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Introduction

In this article I will explain the nature of blast forces and their effects on buildings, in a straightforward manner. I will also clarify the difference between blast resistance and blast resilience. Various measures to improve blast resilience in buildings will be described, with the intent that an experienced architect or structural engineer can make informed decisions in the early stages of planning and design.

The premise is consideration of blast loadings in the context of terrorist threats.

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Abstract

Electrorheological fluids (ERFs) have been described as highly engineered variable-impedance fluids that possess rheological and electrical properties that demonstrate dramatic nonlinear change with an applied electric field.  ERF is a two-phase composite colloidal suspension of electrically polarizable particles dispersed in insulating media.  This article describes an application of low-voltage electrochemical impedance spectroscopy (EIS) to comparative characterization of four different ERFs and a prediction of their performance in ER-activated commercial devices.

The ERF impedance response is interpreted in the context of a classical Debye relaxation model.  EIS data analysis allows the determination of the electrical characteristics of polarizable particles and base fluid, an investigation of the preferred conduction mechanisms in the fluid, an evaluation of the potential for chemical agglomeration, and modification of the electrical properties of the ERF to achieve better fluid performance.  The ERF performance improves significantly for highly concentrated suspensions of well-dispersed, small, closely packed particles.  The recent introduction of nanoparticles-based ERF has created an opportunity for further development of fluids with transformationally superior performance.

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This article attempts to determine the energy and exergy losses in a combined hydrogen generation and locomotive transportation system that uses hydrogen fuel (Figure 1). Hydrogen generated by a thermochemical copper-chlorine cycle and stored onboard the locomotive provides power to a prime mover via proton exchange membrane (PEM) fuel cells. The thermodynamic irreversibility of a fuel cell increases as the power rating increases. A problem arises with frequent stops and high acceleration, which increase the hydrogen consumption. Locomotive manufacturers have an interest in the appropriate match of fuel cell type and size, as needed for feasible designs for commercialization. Read More »

Problem Description

A flotation cell is essentially an agitation-aeration machine. A series of sub- and micro-processes are realized in mechanical flotation machines (sub-aeration type or with external air-blow). Among these the following are included: suspension of the particles in the pulp; reagent mixing; input and dispersion of air into fine bubbles; particle-bubble collisions and attachment only of the hydrophobic particles on the bubbles; formation of particle-bubble aggregates; and raise of the aggregates to the froth. It must also be pointed out that all these sub- and micro-processes take place under high turbulent conditions (Reynolds number 2.4×10⁴ – 7×10⁶). Consequently, turbulence is an important process-determining factor in the operation of flotation machines. Read More »

We present analytical and simulation models for autonomous vehicle storage and retrieval systems (AVS/RSs). AVS/RS represents a new material handling system (MHS) based on autonomous vehicle (AV) technology. It is automated and used for unit load (UL) storage systems. This new technology has already been used in Europe. We model a particular AVS/RS analytically as semi-open queuing network (SOQN). After developing the SOQN model of the system, we solve it by using an approximate analytical model (AAM) and the matrix geometric method (MGM). We compare the two approaches’ results based on the simulation results. We also perform simulation-based performance analysis of the system. First, we complete 133 simulation experiments to find out near-optimum values for number of AVs and lifts in the system. Second, we study a performance comparison analysis for AVS/RS and its alternative system – automated storage and retrieval system (AS/RS). We implement this study for a company that utilizes AVS/RS in France. Read More »

Description of Case

In many industrial applications, such as the flow of oil and natural gas in flow lines and wellbores, knowledge of two-phase pressure drop and nonboiling two-phase heat transfer is required. As a result of the complex nature of two-phase gas-liquid flow, heat transfer data and applicable correlations for nonboiling two-phase flow in various pipe inclinations are not readily available. The hydrodynamic and thermal conditions of nonboiling two-phase flow are dependent on the interaction between the two phases. In most situations encountered by practicing engineers, direct heat transfer measurements for two-phase flow are extremely difficult to perform. It is for this reason that correlating heat transfer and pressure drop can provide useful information. Read More »

Introduction

The introduction of a new problem can come from many sources. Usually it comes from someone in authority communicating the need for an improvement in a manufacturing process or the solution of a long-term problem in a particular technical area. The problem addressed in this article arrived in the form of a challenge from a national laboratory: Can you solve the problem of inserting an intervenous (IV) needle in a critically injured patient at night? This issue had been an active area of research but seemed intractable to the many approaches attempted. An obvious approach was to use a light source such as a flashlight to highlight the vasculature of the patient so that the emergency medical technician (EMT) might see and therefore access a patient’s vein. However, that approach had been tried and shown not to be feasible by researchers with a great deal of experience in the field of biomedical optics.¹ Fortunately, that work was unknown to us, and so we plunged ahead with the only optical equipment available: a pair of military night vision goggles as the detector and a television remote control as the light source. To our surprise, we were able to see veins when the remote control was pointed at our forearms. Although the contrast in these images was low, the method showed promise, and so we experimented with other arrangements of the source and the detector. We quickly discovered that it was possible to transmit the light across our forearms, hands, and feet with much improved images, such as those shown in Figures 1 and 2. Read More »

Introduction

The need to reduce the cost of the enclosure material for direct fired heaters or power boilers prompted this study, whose purpose was to determine whether significant cost savings could be achieved if the structural enclosures of such heaters employed steel-reinforced concrete construction rather than steel plate and structural steel construction. The article indicates the manner in which mechanical and thermal stresses in such internally heated structures can be determined and provides a proposed design for structures of this type. Read More »