Superimposed over the existing structural shell of the building, which Wustemann stripped, patched, and varnished, the cross is defined by the planes of its ceiling and white epoxy floor (with radiant heating coils below), and by the running strip of cove lighting that snakes around its edges and beyond. The overall effect is a rich sense of spatial layering in a limited area.
The linear continuum of indirect light is fundamental to Wustemann’s concept. “[It] suggests depth and a horizon,” he explains. “There is no end to the space; it doesn’t stop. You will never see a light source in my projects, because there’s no horizon.” To emphasize the importance of this detail, he always keeps this discreet illumination turned on, but often dimmed.
Taking advantage of the new floor plan, Wustemann tucked a breakfast look and sleeping alcoves into the spaces that formed around the geometry of the cross. Then he created a kitchen along the axis adjacent to the living area, and a bath corridor across that, keeping all of the fixtures, fittings, and appliances hidden in cabinets when not in use. A section of counter lifts up to reveal a six-burner gas cooktop, and pocket doors open and fold back into the cabinetry when it’s time to access the refrigerator, freezer, oven, microwave, and storage. The architect also installed two lavatories behind a sliding door on the right arm of the bath axis. He housed the toilet in a cubicle, and a windowed shower nook at the far end. In the opposite direction, the left arm of the cross opens onto the master bedroom area.
The family can move various sliding and folding doors to define up to three bedrooms and isolate the bath corridor, but they prefer to keep them open. “The kids have foldable mattresses, so they can choose where they want to sleep,” says Wustemann. “They are ‘camping’ in the apartment.” One of their favorite places is a raised surface off the kitchen where a bathtub is hidden under removable panels.
This concept of loose, flexible living extends to the front of the apartment, where Wustemann installed new oak floors to define a habitable platform within the shell of the old structure. This expanse of wood continues up the walls to form a low wainscoting backed by a recess that accommodates runs of discreetly hidden fluorescent lights and radiators. The walls, like the ceilings with their exposed beams and typical Catalan rafters and vaulting, are stripped to the original material — wood, brick, stone, and plaster — and coated will a semigloss varnish to minimize dust and enhance the daylight filtering through the windows. During the renovation, Wustemann added steel beams to reinforce the ceiling and allowed the scraps and bits of different finishes and interventions to emerge. This includes the salvaged remnants of a plaster fresco, located behind the dining table.
Wustemann’s scheme is guided by a metaphor of urban space. In an earlier Lucerne loft, he used the idea of a glacier to create an interior “landscape” of ascending levels and descending light. Here, the unfinished walls, marked with time, are extensions of the Gothic Quarter, and the white cross and living platform are elements the family can appropriate freely. “It’s the aura of not finishing, keeping it urban and letting the process be visible that gives a feeling of freedom,” he says. This interpretation of domesticity as an improvised encampment amid historic remains offers an interesting insight into the life of a modern urban nomad, in which a family can commute between two different worlds and feel at home in both [Architectural Record, 2010, No. 9].
ELECTRICAL ENGINEERING
Text 3. DEFINITE PURPOSE MOTORS FOR HIGH PERFORMANCE DRIVES
By M.J. Melfi, R.T. Hart
General Considerations
The first task is to design a basic motor configuration which is matched to the general needs of adjustable frequency power and variable speed operation. Second, the design must be adaptable to match the specific needs of many different drive applications. Third, by relaxing inappropriate constraints associated with fixed frequency, fixed voltage, fixed speed applications the design can be tailored to meet the performance objectives by making typical design tradeoffs. Also, when the controller design is known, more subtle techniques which include the controller can be used. An example is the use of a lower than usual voltage at the low speed end of a region of constant horsepower, so that the flux level (hence, peak load capability) at the highest speeds can be maximized to produce sufficient torque without having to oversize the motor. Of course, this must be weighed against the increased current required of the controller at the low speed.
There are many design compromises that can be made within the motor to provide optimum performance for a given application. The following paragraphs will discuss issues that are commonly raised in discussions of variable frequency applications.
Starting Characteristics
Since adjustable frequency controllers typically accelerate a motor and load by slewing the motor voltage and frequency in such a way as to remain in a region of operation above “breakdown RPM”, the usual constraints of fixed voltage, fixed frequency starting and acceleration do not apply. Starting torque and current are no longer functions of the 1.0 per unit slip characteristics of the motor but are limited by the overload capability of the control. Thus, the controller can be matched to the motor in such a manner as to produce the appropriate starting torque based on a torque/amp ratio equal to that under full load conditions. By evaluating the drive as a motor and control “package”, the motor designer can take advantage of this to enhance the level of starting torque as well as overload torque per amp.
Peak Currents
In addition to the RMS current level, an important rating point for a transistor (typically used in adjustable frequency controllers) is the peak current capability. The high frequency transient current which results from the electronic switching of the control output voltage is inversely proportional to the leakage inductance of the motor. The leakage inductances can be increased by altering the design of the windings and the magnetic cores in the motor. The use of an electromagnetic design specifically for adjustable frequency power can significantly reduce the peak current required for a given level of power output. This will not only improve the reliability of the drive, but often can prevent costly over sizing of the AC controller and provide the most cost effective solution.
Motor Heating
One of the more obvious sources of increased stress on an induction motor insulation system is higher operating temperature when run on variable frequency controllers. The higher operating temperatures are the result of increased motor losses and often reduced heat transfer as well. As a result, many standard efficient, fixed frequency design motors will not achieve their nameplate rating when operated on an adjustable frequency control at 60 Hz while remaining within temperature limits. While these elevated temperatures may not lead to an immediate insulation failure they will result in a significantly shorter life. In most modern insulation systems, a 10 degree Celsius increase in operating temperature will result in a 50% reduction in expected life. This is one of the reasons why “High Efficient” designs, which have inherently greater thermal reserves, are often recommended for operation on adjustable frequency controls.
When an induction motor is run with voltage and current waveforms, the deviation from the ideal sinusodial waveshapes create additional losses without contributing to steady state torque production. The higher frequency components in the voltage waveform do not increase the fundamental air gap flux rotating at synchronous speed. They do, however, create secondary “hysteresis loops” in the magnetic steel, which along with high frequency eddy currents produce additional core losses and raise the effective saturation level in the lamination material. As another consequence of these higher frequency flux variations there are higher frequency currents induced in the rotor bars which generate additional losses. Appropriate electromagnetic design, including rotor bar shape can minimize these added losses.
Motor Cooling
As has been well documented in the literature, when AC motors are run across a wide speed range their heat transfer effectiveness will vary a great deal. Cooling fans whose rotation is directly supplied by the motor are subject to high windage losses and noise at high speeds. Modern AC controllers are capable of operating across a very wide frequency range, often up to several hundred hertz. While this provides great flexibility in the control, it places the motor cooling fan well above its fixed frequency design operating point which often leads to inefficient air flow and objectionable noise. In low speed operation the fan’s effectiveness falls off with the motor’s speed. In variable torque applications this reduction in cooling air often stays in balance with the reduction in motor losses as the load is reduced with speed. However, in constant torque applications the motor’s temperature limits will likely be exceeded. An independently powered blower can provide an essentially constant heat transfer rate. Although not a standard fixed frequency motor feature, depending on the load/speed profile required by the application, this can be a very effective choice and is often specified for high performance applications.
In addition to fan speed, the operating temperature of the motor is determined by how effectively the heat generated in the motor can be conducted to surfaces which are in contact with the cooling medium (generally air) and the ability to transfer this heat via convection to the cooling medium. In a conventional totally enclosed fan cooled motor the heat must be transferred from the laminated steel stator core to the cast iron frame and finally to the air. Since the fan is located opposite the drive end of the motor, there is generally greater air flow and heat transfer at one end of the motor than the other. Square laminated frame AC motors have been offered by a variety of manufacturers as a method to improve heat transfer. The laminated frame design eliminates the stator-to-frame interface and provides a more direct and effective heat transfer path to the cooling air while integral cooling ducts trap the air in contact with the frame along the motor’s length. This laminated frame construction has been common in variable speed DC motors for over twenty years.
Noise
Operation of standard industrial AC induction motors on adjustable frequency power over a speed range often results in unacceptable sound power levels as well as an annoying tonal quality. While the actual sound power level has proven to be unpredictable due to the large number of possible motor and controller designs, the increase in sound level is typically in the range of 7 to 10 db. There has been some success in reducing these sound levels by pushing the variable frequency controller’s carrier frequency above the motor structure natural frequency spectral band. However, there are also motor design considerations which will improve sound levels.
One source of acoustic noise is the air noise caused by running shaft driven fans above their design speed to achieve a wider speed range. A separately powered, unidirectional, constant speed cooling fan will provide a consistent level of air noise independent of motor speed and eliminates annoying sound level changes as the motor accelerates and decelerates.
A second source is the magnetic noise from flux harmonics which are driving the magnetic core steel into a saturated condition. A well planned design will use lower than nominal flux levels with particular emphasis on avoiding localized regions of higher flux density or “pinch points”. Air gap length and rotor slot bridge thickness, which reduce saturation in localized areas are two contributing areas where additional reductions in sound power level can be achieved.
Conclusions
Providing high performance variable speed drives for maximum process productivity has always required complex engineering considerations. Rapid improvements in AC control technology, combined with the ready availability of standard fixed frequency AC motors has increased the number of possible solutions. However, a component approach will not lead to an optimal solution in many cases. In order to utilize the present (and next) generation of adjustable frequency controllers to meet application needs equal to or better than DC motors have in the past, a definite purpose AC motor is required. A square laminated-frame configuration with integral feet on the end brackets and adaptable electromagnetic designs is one approach that meets this objective [Textile, Fibre and Technical Conference, 2002].
MATERIALS ENGINEERING
Text 4. MANUFACTURING SOLUTIONS FOR CONCRETE PERFORMANCE
By L. Hills, F. Tang
Introduction
Concrete producers expect cement to remain versatile, maintaining a consistent and predictable performance with all types of concrete mixes. Concrete workability problems can be costly and affect concrete producers and users alike. Stiffening properties can arise from false setting or from stiffening due to aluminate control problems (also known as flash setting). Sulfate and aluminate characteristics are often the key to understanding the cause of these stiffening properties.
Premature stiffening of a mix can r also result from incompatibility among concrete components. The addition of fly ash will be discussed briefly in this paper, since fly ashes containing high amounts of aluminate or alkalies can affect the proper sulfate balance. Chemical admixtures can also disrupt control of the early aluminate hydration by the sulfates, but this topic is too broad to be covered in this paper. It should be remembered, however, that many cement parameters can play a role in such incompatibility, including grain size (clinker mineralogy) and amount of C3A; the content, chemical form and fineness of sulfate bearing phases; alkali and free lime contents; fineness and pre-hydration of cement.
Hydration reactions and concrete properties
Sulfate is added to cement to control aluminate hydration and to enhance C3S hydration, promoting improved strength development. The amount, form, and fineness of sulfate dictate its solubility and therefore its effect on aluminate hydration. This interaction between the sulfate and clinker phases is important to understand, as it influences concrete properties, such as workability, strength, setting time, drying/shrinkage, and expansion. The total SO3 content is a common QA/QC factor used at the cement plant. While that value provides some information, it does not reveal the entire picture of the available sulfate. The examples here are only a few situations that can affect how much sulfate is needed and what is available.
IVhat is ideal sulfate?
Optimum sulfate can be discussed in terms of both strength development and setting properties. The amount of sulfate in solution needed to achieve the desired early hydration reactions in a cement paste depends on the properties of the clinker and cement (aluminate content and size, alkali aluminate content, particle size distribution), and the properties of sulfate (amount, form, particle size) present in the cement. In a concrete mix, chemical and mineral admixtures play a role in the reactions. The cement sulfate requirement for use in field concrete when admixture is used may be higher than for ASTM paste tests, often by 0.5 — 1.0% SO3. In order to determine the “ideal” sulfate conditions or analyse setting problems, diagnosing the reactions of a particular cement or cement/admixture combination is critical.
Diagnosis of reactions
There are several helpful tests to determine what is going on in the cement itself, and in the resulting paste:
• Thermal Analysis can be used to quantify the amount of gypsum and plaster in the cement. Methods include Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Differential Thermal Analysis (DTA).
• Particle size analysis combined with chemical extractions can determine the fineness of the gypsum particles. Microscopic techniques can also be used to identify large gypsum particles.
• The Conduction Calorimeter measures heat produced by a sample versus time; cement hydration reactions can be monitored starting upon introduction of water through a period of several days.
• Mini-slump cone tests performed on cement pastes determine early stiffening properties. Paste is prepared in a high shear blender using mixing speeds that closely simulate conditions during concrete mixing. The paste is consolidated into a mini-slump cone, and at 2 min the cone is lifted to allow the paste to slump. The remaining paste is further mixed, after which 5-, 15-, 30-, and 45-min tests are carried out. The workability, or flow property, of the cement paste is demonstrated in the size of the pat formed after slumping, i.e. the larger the area of pat, the more workable is the paste. Pat size at 5-min and 15-min tests, which are taken after the paste is remixed, has been shown to correlate better with flash or false set in the field compared to current ASTM methods (larger pat size after remixing indicates false set; smaller size indicates flash set).
Optimisation of sulfate
Establishing the reactions for a particular cement or concrete mix using the above tests will help identify where improvements to optimise sulfate can be made. Plant personnel are best able to determine the appropriate action from there. Here are some examples:
• If the ratio of gypsum to plaster is too low, the most feasible alternative at the plant is to decrease the mill temperature and thus minimise the formation of plaster. Another option is to use water spray to cool the mill and increase mill relative humidity. Increasing the ease of clinker grinding should decrease retention time and prevent excess gypsum dehydration. Another alternative may be the substitution of some of the gypsum with natural anhydrite.
• Cases of insufficient soluble sulfate may require several improvements. The total sulfate content may need to be increased, and/or the solubility of the sulfate present adjusted. Some plants using anhydrite as part of the gypsum source may need to minimise its addition due to its slow dissolution rate, and increase the amount of more soluble sulfate, such as plaster.
• In the case of coarse gypsum particles, a new source of softer- grinding gypsum may be needed. Or, if clinker is pre-ground before the finish mill, a method of pre-grinding the gypsum should help prevent gypsum particles from being too coarsely ground in the final product. Pre-grinding gypsum may be especially useful for plants operating finish mills equipped with high efficiency separators.
Conclusion
This paper has described some important considerations when it comes to sulfate. Specifics of sulfate additions may sometimes be overlooked, but the resulting stiffening properties of the cement usually are not. Total sulfate is a common measurement at the cement plant lab; however, it does not provide a complete picture of available sulfate. The balance of sulfate form and amount, with respect to reactive aluminate components in the cement and other materials, is an important relationship. Essential components of this relationship include: sulfate content, sulfate form, sulfate particle size, grain size and amount of clinker aluminate phase, and cement (and other cementitious material) alkali content. In concrete mixes with fly ash, the amount of C3A, or alkalies can affect the balance. Aside from rheological properties, conditions for controlling early stiffening are also linked to other performance characteristics, such as strength and durability, as have already been noted.
Flash set and poor strength development can result from insufficient sulfate, whether due to a cement with a low ratio of sulfate to aluminate content, a concrete mix with high C3A fly ash, or poor distribution of sulfate ions from large gypsum particles. Without sufficient sulfate ions in solution to control aluminate hydration, voluminous hexagonal aluminate hydrates will form, resulting in flash set and poor strength development.
False set can result from too much sulfate in the form of plaster. Since plaster goes into solution more quickly than gypsum, many calcium and sulfate ions are available to control aluminate reactions, therefore less C3A is reacted; the plaster will form crystals of secondary tabular gypsum particles, which interlock and cause false setting. False set is generally less of a problem than flash setting, as it can be overcome by continuing to mix concrete through the stiffening phase for a proper length of time. If a short mix cycle is used in the field and this setting problem occurs, water is often added to the concrete to attempt improve the workability; however, this “remedy” may reduce concrete strength and durability. Identifying the sulfate properties is a first step in preventing or resolving stiffening issues. Once the cause of cement behaviour is determined, proper manufacturing solutions can be implemented. Solutions involve finish mill temperature or humidity, type and grindability of sulfate added, and even clinker grindability [World Cement, July, 2004].
CHEMICAL ENGINEERING
Text 5. PROTECTING AGAINST WEAR
By A. Baulig
During the past 50 years wear protection engineering for the basic industries has definitely undergone changes. The available product range is today extremely diverse and capable of meeting the particular requirements of almost any application. The wide variety of available alloys and resulting hardness degrees for metallic materials has been extended, as has the family of mineral and ceramic materials.
In the mineral and ceramic sector, KALCRET hard compound was introduced on the market a few years ago. This wear protection material, suitable for moderate wear, offers the double benefit of having a large number of applications and a variety of installation methods. Since its introduction, KALCRET hard compound has performed well in applications in the cement and steel industries, in coal fired power plants and in other basic industries. It has been successfully applied for many purposes, particularly when combined with other protection materials in order to achieve an optimal solution from a technical and economic point of view. KALCRET hard compound is the general term for a group of cement bonded wear protection materials, the main components of which consist of inorganic materials of high strength and good wear resistance. These components can be varied to achieve specific characteristics for most applications.
Kalenborn Kalprotekt, Germany, uses bauxite and corundum as the aggregates for the production of this hard compound. The mixture contains at least 70% of these materials. The special packing density of the compound is achieved by a balanced particle size distribution of the individual components. It is not enough to simply use ground hard material. The particle size ranges from 0.1 to 4 mm and maximum attention is paid to adhere to specific particle shapes. In addition, the pores between the cement grains are filled with ultra-fine particles of micro and nanosilica. The result is a high final compressive strength of up to 190 MPa.
KALCRET is a wear protection material in a bag. The material is delivered unbonded, mixed in suitable mixers with a defined quantity of water and applied by different methods, such as trowelling, casting or spraying. To improve the structural strength, a specific portion of steel or other fibres is added. The type of fibres used depends on the particular thermal, chemical and corrosive stress. Steel fibres of appropriate quality are used in case of high temperatures. KALCRET reaches 75% of its high compressive strength after no more than eight hours. This is normally sufficient to restart plant operation. It is, therefore, a suitable material for repairs.
Concerning chemical and corrosive attack, this hard compound is stable against the effect of weak acids and alkalis, better than that of standard cements. However, it should be stressed that it cannot be used as the primary protection against acids and alkalis.
State-of-the-art spraying
In recent years, wear protection materials have had to meet a further requirement: quick workability. Due to shorter construction times for new plants and shorter periods available for repairs, it is important that wear protection linings are installed in a minimum amount of time. Whereas formerly 2 to 8 hours were needed per square meter for laying wear protection tile linings, current rates of 5 m2 and more per hour are feasible (rates applicable to each working team).
To achieve that goal requires the use of spraying technology. Having recognised these modern trends early, the company has developed KALCRET-S hard compound, which can be sprayed to line large surfaces in a minimum amount of time at rates of more than 5 m2/hour (at 25 mm hickness). Spraying can be carried out horizontally, vertically and can even be worked overhead, enabling the lining of complicated surfaces without difficulty. Investigations have shown that the properties of sprayed-on linings do not differ practically from those of comparable vibration compacted linings. The addition of steel or other fibres has positive effects on strength and structural stability. The fibres are added to the mixture during the spraying process.
The company’s spraying technology has been specially developed for working this particular product. This includes the specific spraying system. It provides for continuous and homogeneous delivery of the K.ALCRET-S compound up to the spray nozzle. Water dosage and injection result in an optimal moisture level in the hard compound. The minimum working distance from the surface to be protected is 800 mm. The delivery distance of the sprayed compound from the spraying machine to the point of spraying is up to 100 m horizontally. The material can be delivered vertically more than 40 m. Consequently, in most cases the hard compound can be fed into the spraying system on the ground, which means simplified material handling and a reliable material flow, while the compound is applied directly to the spot to be protected. The spraying machine can be controlled from the point of spraying, allowing the nozzle operator flexibility in the application.
Fit for high application temperatures
KALCRET wear protection material can be used within a broad temperature range up to 1200 °C. Two different materials are offered for this range: the basic KALCRET-N, suitable for temperatures up to 400 °C, and high-temperature KALCRET-T, which is capable of withstanding temperatures up to 1200 °C. Operating temperatures beyond 50 °C require selective provision for expansion joints. Expansion joint dimensioning has to be precisely calculated and adapted to each specific application. Temperatures beyond 100 °C require observing for special heat-up curves. For initial start-up, the lined equipment is gradually heated to 150 °C after maintaining ambient temperature for eight hours. After a holding time of approximately 12 hours, the lined equipment is then heated to the desired final temperature. Maintaining the heat-up curve is necessary to permit first the bonding and then evaporation of the existing water without detrimental effects to the structure of the hard compound. The procedure is based on experimental values drawn from other cement-bonded materials.
Heating the equipment after extended shutdowns should follow a relatively low start up to 150 °C and can then be continued quickly. The heat-up rate after short interruptions of operation, as well as during normal operation, should be a maximum of 300 °C per hour.
Depending on the particular application, additional insulation may be placed between the lining and the steel casing. Heat losses are reduced and low cost unalloyed structural steels may be used thanks to a lining system characterised by an optimised temperature gradient. The combination of insulation material and the company’s compound permits, for instance, a temperature difference up to 620 °C at a wall thickness of 55 mm. This means in practice that with an internal process temperature of 700 °C, the outer steel casing has a temperature of no more than 80 °C. This is possible at minimised dimensions and relatively low weight [World Cement, September, 2006].
MECHANICAL ENGINEERING
Text 6. PLASTIC GEAR PAY-OFF: ELIMINATING NOISE, WEIGHT AND WEAR PROVES VALUABLE IN 2012
By M. Jaster
The only plastic gear applications mentioned in Darle Dudley’s Handbook of Practical Gear Design — originally published in 1954 — involved items like toy trains, film projectors and cash registers. Thanks to energy efficient manufacturing as well as a desire to cut down on costs, the plastic gear has significantly evolved. Opportunities readily available to plastic gear manufacturers today include automotive, business and printing machines, lawn and garden equipment and medical applications — and business is booming. “I can’t speak about other segments of gear manufacturing, but plastic molded gears still seem to be the focus in the industry for improved performance and cost savings,” says Rod Kleiss, president of Kleiss Gears, Inc., located in Grantsburg, Wisconsin. “We are stretched to keep up with demand.” Andrew Ulrich from Thermotech, located in Hopkins, Minnesota adds, “Though we are not acutely aware of how the machined gear market is doing, we can say the molded gear market is strong and growing.” “Especially from a custom gear/gear tooth perspective,” adds Bruce A. Billmeyer, president/owner, Plastic Powe rDrive Products, LLC, located in Elk Grove Village, Illinois. “Although a portion of the U.S. molded gear market does come from foreign sources, the innovation still resides here in the United States. This innovation comes in the form of materials, gear combinations with other components and gear assembly techniques.”