Postgraduate textbook. Учебное пособие

Text 5. TYPES OF HEATING SYSTEMS

Central Heat. Furnaces

The majority of North American households depend on a central furnace to provide heat. A furnace works by blowing heated air through ducts that deliver the warm air to rooms throughout the house via air registers or grills. This type of heating system is called a ducted warm-air or forced warm-air distribution system. It can be powered by electricity, natural gas, or fuel oil.

Inside a gasor oil-fired furnace, the fuel is mixed with air and burned. The flames heat a metal heat exchanger where the heat is transferred to air. Air is pushed through the heat exchanger by the «air handler’s» furnace fan and then forced through the ductwork downstream of the heat exchanger. At the furnace, combustion products are vented out of the building through a flue pipe. Older «atmospheric» furnaces vented directly to the atmosphere, and wasted about 30 % of the fuel energy just to keep the exhaust hot enough to safely rise through the chimney. Current minimum-efficiency furnaces reduce this waste substantially by using an «inducer» fan to pull the exhaust gases through the heat exchanger and induce draft in the chimney. «Condensing» furnaces are designed to reclaim much of this escaping heat by cooling exhaust gases well below 140 °F, where water vapor in the exhaust condenses into water. This is the primary feature of a high-efficiency furnace (or boiler). These typically vent through a sidewall with a plastic pipe.

Heating system controls regulate when the various components of the heating system turn on and off. The most important control from your standpoint is the thermostat, which turns the system – or at least the distribution system – on and off to keep you comfortable. A typical forced air system will have a single thermostat. But, there are other internal controls in a heating system, such as «high limit» switches that are part of an invisible but critical set of safety controls.

The efficiency of a fossil-fuel furnace or boiler is a measure of the amount of useful heat produced per unit of input energy (fuel). Combustion efficiency is the simplest measure; it is just the system’s

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efficiency while it is running. Combustion efficiency is like the miles per gallon your car gets cruising along at 55 miles per hour on the highway.

In the U.S., furnace efficiency is regulated by minimum AFUE (Annual Fuel Utilization Efficiency). AFUE estimates seasonal efficiency, averaging peak and part-load situations. AFUE accounts for start-up, cool-down, and other operating losses that occur in real operating conditions, and includes an estimate of electricity used by the air handler, inducer fan, and controls. AFUE is like your car mileage between fill-ups, including both highway driving and stop-and-go traffic. The higher the AFUE, the more efficient the furnace or boiler.

Boilers

Boilers are special-purpose water heaters. While furnaces carry heat in warm air, boiler systems distribute the heat in hot water, which gives up heat as it passes through radiators or other devices in rooms throughout the house. The cooler water then returns to the boiler to be reheated. Hot water systems are often called hydronic systems. Residential boilers generally use natural gas or heating oil for fuel.

In steam boilers, which are much less common in homes today, the water is boiled and steam carries heat through the house, condensing to water in the radiators as it cools. Oil and natural gas are used.

Instead of a fan and duct system, a boiler uses a pump to circulate hot water through pipes to radiators. Some hot water systems circulate water through plastic tubing in the floor, a system called radiant floor heating Important boiler controls include thermostats, aquastats, and valves that regulate circulation and water temperature. Although the cost is not trivial, it is generally much easier to install «zone» thermostats and controls for individual rooms with a hydronic system than with forced air. Some controls are standard features in new boilers, while others can be added on to save energy.

As with furnaces, condensing gas-fired boilers are relatively common, and significantly more efficient than non-condensing boil-

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ers (unless very sophisticated controls are employed). Oil-fired condensing boilers are uncommon in the U.S. for several reasons related to lower latent heat potential, and potential for greater fouling with conventional fuel oil.

Heat Pumps

Heat pumps are just two-way air conditioners During the summer, an air conditioner works by moving heat from the relatively cool indoors to the relatively warm outside. In winter, the heat pump reverses this trick, scavenging heat from the cold outdoors with the help of an electrical system, and discharging that heat inside the house. Almost all heat pumps use forced warm-air delivery systems to move heated air throughout the house.

There are two relatively common types of heat pumps. Airsource heat pumps use the outside air as the heat source in winter and heat sink in summer. Ground-source (also called geothermal, GeoExchange, or GX) heat pumps get their heat from underground, where temperatures are more constant year-round. Air-source heat pumps are far more common than ground-source heat pumps because they are cheaper and easier to install. Ground-source heat pumps, however, are much more efficient, and are frequently chosen by consumers who plan to remain in the same house for a long time, or have a strong desire to live more sustainably.

Whereas an air-source heat pump is installed much like a central air conditioner, ground-source heat pumps require that a «loop» be buried in the ground, usually in long, shallow (3–6' deep) trenches or in one or more vertical boreholes. The particular method used will depend on the experience of the installer, the size of your lot, the subsoil, and the landscape. Alternatively, some systems draw in groundwater and pass it through the heat exchanger instead of using a refrigerant. The groundwater is then returned to the aquifer.

Because electricity in a heat pump is used to move heat rather than to generate it, the heat pump can deliver more energy than it consumes. The ratio of delivered heating energy to consumed energy is called the coefficient of performance, or COP, with typical

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values ranging from 1.5 to 3.5. This is a «steady-state» measure and not directly comparable to the heating season performance factor (HSPF), a seasonal measure mandated for rating the heating efficiency of air-source heat pumps. Converting between the measures is not straightforward, but ground-source units are generally more efficient than air-source heat pumps.

Direct Heat. Gas-Fired Space Heaters

In some areas, gas-fired direct heating equipment is popular. This includes wall-mounted, free-standing, and floor furnaces, all characterized by their lack of ductwork and relatively small heat output. Because they lack ducts, they are most useful for warming a single room. If heating several rooms is required, either the doors between rooms must be left open or another heating method is necessary. Better models use «sealed combustion air» systems, with pipes installed through the wall to both provide combustion air and carry off the combustion products. These units can provide acceptable performance, particularly for cabins and other buildings where large temperature differences between bedrooms and main rooms are acceptable. The models can be fired with natural gas or propane, and some burn kerosene.

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Text 6. CIVIL ENGINEERING

Civil engineering is the oldest and one of the most highly respected of all the engineering disciplines. It is a traditional industry, by its nature. From the very beginning man has been a builder and his creative ability and skilful craftsmanship are what the modern civil engineering industry is founded on. Today these traditional skills are coupled with the entire modern technology and thinking available to enable civil engineers to carry out their work to the highest of standards.

The traditional background of the industry presents, however, some problems. First of all, it is often perceived as old fashioned and reluctant to fully come to terms with the business world today. But it is obvious that new ideas cannot be implemented until they have been tried, proved and tested. So, if new management techniques are not considered and adopted, there would be a loss in competitive advantage, a vital factor in a highly competitive industry.

That is why it is necessary to ascertain the importance and relevance of management training in the construction industry. People have been managing companies ever since companies came into existence. It is necessary to manage a company effectively and efficiently as well as to produce high quality goods. It is especially important nowadays when construction work is limited and construction companies have to cut margins. Therefore, the effective management of an organization has become an art form in itself.

Civil engineering is a business and its survival is in making a profit. And it is the success of the «team» that is important. The team could be made up of engineers, quantity surveyors, estimators, planners, QA managers; personnel and computer specialists; all form an integral and equally important part of the business. Engineers may not grow into managers, but they must be trained for this role. But it is a recognized fact that construction managers do not come into being by themselves. The training of graduate engineers in the construction industry at present consists of the undergraduate training within tertiary education, and postgraduate training within the indus-

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try itself. Although the number of hours devoted to teaching managements is increasing at educational establishments, graduates are still not adequately prepared for industry. Further training within the industry is varied and dependent on such factors as the needs of the company, size of the company, and current health of the industry.

Management and management training within the construction industry is varied and is dependent on the sector, consulting or contracting, within which an engineer operates. It is necessary to take into consideration the attitudes of civil engineers and their companies towards modern management practices.

Construction managers are moving rapidly to meet the challengers of a modern business world and are making full use of the tools available. Consultants are a little behind in their attitudes to modern management but are improving; they have not yet realized the full potential of management training.

The alteration of construction industry attitude towards management practice would appear to stem from the business market within which a company is going to operate.

Scan through the text and be ready to summarize it

The nature of the industry's products and services Construction activity is extremely diverse, ranging from simple housing developments to highly complex infrastructure projects. However, all types of construction project, regardless of size, have some common characteristics, which include the following: their unique, one-off nature: unlike other sectors, where prototypes can be tested before real production gets underway, construction projects tend to be one-off, unique organizations that are designed and constructed to meet a particular client's product and service needs.

This can lead to significant risks for people working on a project, which largely arise from learning-curve problems associated with new work activities and ever-changing workplace relationships. Their tendency to be awarded at short notice: many construction projects are awarded following a period of competitive tendering, where possibilities for thorough planning are often limited.

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Having been awarded a contract, a design consultancy or contractor has to mobilize a project team comprising an appropriate blend of skills and abilities to meet the project demands quickly. The resourcing function may need to respond to sudden changes in workload, as there can be no guarantee of how much work will be being undertaken at any particular time. Their reliance on a transient workforce: construction projects are, for the most part, constructed in situ.

Even with the increased use of offsite fabrication and the wider use of prefabricated components, the final product is normally assembled and completed in the required site location. This necessitates the employment of a transient workforce which can move from one project location to the next. This transience poses many problems for workers, such as longer working days, more expense in travelling to work and managing work-life balance issues, since their families may not be as mobile. Transience also arises within projects, since the composition of teams normally changes during different project stages, involving people from many organizations, backgrounds and locations.

Increasingly demanding clients: in recent years there has been a steady increase in the quality of service and product expected by clients procuring construction work. For example, in Australia it has been estimated that construction projects are being delivered in about half the time they were ten years ago.

Inevitably, this requires a considerable commitment from those working in the industry, which tends to manifest itself in unsafe working practices, long working hours and increased levels of stress. A male-dominated culture: construction is one of the most male dominated industries in virtually every developed society. Men dominate both craft trades and professional and managerial positions within the sector.

This reliance on male employment leads to many challenges, such as skills shortages caused by recruiting from only a portion of the population, difficulties in the management of equal opportunities and workforce diversity, and considerable challenges in terms

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of creating an accommodating atmosphere in which individuals' diverse skills and competencies are fully utilized.

These challenges require construction companies to balance project requirements with competing organizational and individual employee expectations, priorities and needs. It is the industry's inability to manage these competing demands effectively which has caused many of the enduring problems which plague the industry today. Focusing on project and organizational requirements at the expense of human needs will result in employee dissatisfaction, reduced commitment, industrial conflict, increased turnover, more accidents, deprofessionalisation, recruiting problems and a continued poor public image.

Read and translate the article from the journal «Construction and Building Materials»:

Researchers at MIT are seeking to redesign concrete – the most widely used human-made material in the world – by following nature's blueprints. In a paper published online in the journal «Construction and Building Materials», the team contrasts cement paste – concrete's binding ingredient – with the structure and properties of natural materials such as bones, shells, and deep-sea sponges.

As the researchers observed, these biological materials are exceptionally strong and durable, thanks in part to their precise assembly of structures at multiple length scales, from the molecular to the macro, or visible, level. From their observations, the team, led by Oral Buyukozturk, a professor in MIT's Department of Civil and Environmental Engineering (CEE), proposed a new bioinspired, «bot- tom-up» approach for designing cement paste.

«These materials are assembled in a fascinating fashion, with simple constituents arranging in complex geometric configurations that are beautiful to observe», Buyukozturk says. «We want to see what kinds of micromechanisms exist within them that provide such superior properties, and how we can adopt a similar building-block- based approach for concrete». Ultimately, the team hopes to identify materials in nature that may be used as sustainable and longer-lasting

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alternatives to Portland cement, which requires a huge amount of energy to manufacture.

«If we can replace cement, partially or totally, with some other materials that may be readily and amply available in nature, we can meet our objectives for sustainability», Buyukozturk says.

Co-authors on the paper include lead author and graduate student Steven Palkovic, graduate student Dieter Brommer, research scientist Kunal Kupwade-Patil, CEE assistant professor Admir Masic, and CEE department head Markus Buehler, the McAfee Professor of Engineering.

«The merger of theory, computation, new synthesis, and characterization methods have enabled a paradigm shift that will likely change the way we produce this ubiquitous material, forever», Buehler says. «It could lead to more durable roads, bridges, structures, reduce the carbon and energy footprint, and even enable us to sequester carbon dioxide as the material is made. Implementing nanotechnology in concrete is one powerful example [of how] to scale up the power of nanoscience to solve grand engineering challenges».

From molecules to bridges. Today's concrete is a random assemblage of crushed rocks and stones, bound together by a cement paste. Concrete's strength and durability depends partly on its internal structure and configuration of pores. For example, the more porous the material, the more vulnerable it is to cracking. However, there are no techniques available to precisely control concrete's internal structure and overall properties.

«It's mostly guesswork», Buyukozturk says. «We want to change the culture and start controlling the material at the mesoscale».

As Buyukozturk describes it, the «mesoscale» represents the connection between microscale structures and macroscale properties. For instance, how does cement's microscopic arrangement affect the overall strength and durability of a tall building or a long bridge? Understanding this connection would help engineers identify features

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at various length scales that would improve concrete's overall performance.

«We're dealing with molecules on the one hand, and building a structure that's on the order of kilometers in length on the other», Buyukozturk says. «How do we connect the information we develop at the very small scale, to the information at the large scale? This is the riddle».

Building from the bottom, up. To start to understand this connection, he and his colleagues looked to biological materials such as bone, deep sea sponges, and nacre (an inner shell layer of mollusks), which have all been studied extensively for their mechanical and microscopic properties. They looked through the scientific literature for information on each biomaterial, and compared their structures and behavior, at the nano-, micro-, and macroscales, with that of cement paste.

They looked for connections between a material's structure and its mechanical properties. For instance, the researchers found that a deep sea sponge's onion-like structure of silica layers provides a mechanism for preventing cracks. Nacre has a «brick-and-mortar» arrangement of minerals that generates a strong bond between the mineral layers, making the material extremely tough.

«In this context, there is a wide range of multiscale characterization and computational modeling techniques that are well established for studying the complexities of biological and biomimetic materials, which can be easily translated into the cement community», says Masic.

Applying the information they learned from investigating biological materials, as well as knowledge they gathered on existing cement paste design tools, the team developed a general, bioinspired framework, or methodology, for engineers to design cement, «from the bottom up».

The framework is essentially a set of guidelines that engineers can follow, in order to determine how certain additives or ingredients of interest will impact cement's overall strength and durability. For

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