Physics - The Concept of Heat Transfer
Heat can be transferred from one substance to another, a phenomenon used for example during the transfer of heat. By this we understand such a transfer of heat between adjacent lying particles. Particles from the warmer part of a substance transfer their kinetic energy by colliding into particles located in the cooler area. The degree to which heat energy can be transferred depends on heat conductivity (the ability to transfer heat). For example, metals are good conductors of heat but wood is a very poor conductor of heat.
Another method of how heat can be transferred is through heat radiation. In this way, heat transfers without the need for other substances.
Yet another form of transferring heat is convection, which is accomplished through flowing gases or liquids.
An example of heat being transferred through flow or heat radiation is hot water heating. This type of heating is used to transfer energy generated within a boiler, heated with the use of coal or oil. By burning these fuels, energy is transferred to the water circulating within the boiler. The heated water has a smaller density, for which reason it rises, transferring its gained energy to a radiator or radiators. Heat energy is then further transferred within the radiators between the heated water and the air surrounding the radiator. The heated air surrounding the radiator in turn transfers its gained energy according to the principle of heat radiation: the heated air above the radiator rises and is replaced by cooler air drawn from below the radiator. By this process, the air within a closed environment gradually becomes warmer.
Heat transfer can be hastened and made more effective by the use of pumps. By the use of water pumps, heat energy is drawn from a certain medium with a lower temperature (heat source) and transferred to a medium with a higher temperature (heat storage). The principle of how a heat pump operates is similar to a refrigerator’s compressor, where the operating liquid medium is evaporated in the vaporiser due to the surrounding temperature (of the heat source) and then compressed in the compressor. During this process, both pressure and temperature increase, after which water is passed through the compressor and then circulated through the heating circuit. The working medium condenses and then resent to the vaporiser.
A heat pump is more effective the smaller the difference between the heat source and the heat storage. The disadvantage of a heat pump is that it absorbs heat from surrounding objects, as such cooling the environment in which it is located (such as for example a floor space).h
Solar Heat Exchangers - Heating Your Home With The Power Of The Sun
If you are interested in harnessing solar power for solar heat in your home, but are not quite in the position to be installing solar panels and a large solar heating system, you should consider using a solar heat exchanger. Solar heat exchangers are great for heating your home in many different capacities. They are also usually quite affordable.
Did you know that heat exchangers are used throughout your home already? They are commonly found in cars, refrigerators, and air conditioners and they regulate the heat that is generated and used in these items. They prevent over-heating and transfer the right amounts of heat to the right places. Solar heat exchangers are essentially the same, but they transfer the sun’s heat instead of heat generated by an appliance or engine. They usually use liquid and air to transfer heat to another area, such as your swimming pool or your hot water tank.
One of the most popular types of solar heat exchangers is a swimming pool heater. These are great for any swimming pool owner. As you may know, heating a swimming pool can be expensive and consumes quite a bit of energy. If you don’t want to give up your heated pool, but still want to be environmentally friendly, you should look into installing a solar heat exchanger.
Another type of solar heat exchanger is one that works to heat smaller areas, such as hot water heaters and sheds. These heat exchangers are great for those individuals that want to use solar heat, but do not want to spend a lot of money. These come in a variety of styles and sizes, so you can choose one that suits your needs.
Heated Fluid-Control Valve delivers precise flow rate
Featuring No-Drip[R] design, Model 2200-396-026 opens and closes its carbide needle and seat to start and stop material flow up to 5,000 psi. Its 60:1 air-to-fluid power ratio provides instantaneous on-off control. Double-air-operated through 1/4 in. NPT ports to ensure positive start and stop of flow, valve features 1/2 in. NPT fluid inlet and outlet ports for transfer, delivery, and dispensing of low- to high-viscosity, heated adhesives, sealants, and lubricants.PLYMOUTH, MI–Sealant Equipment & Engineering, Inc.’s new Model 2200-396-026 heated fluid-control valve is designed for precision control of low- to high-viscosity materials. It is used in a wide variety of applications for the transfer, delivery and dispensing of heated adhesives, sealants and lubricants The heated flow-control valve is typically used to start and stop the flow of material on the outlet of supply pumps, on the inlet and outlet of pipe manifolds and headers, on the inlet and outlet ports of metering assemblies and on the inlet of dispense hoses. It is also used as a high-flow dispense valve.
The unique No-Drip[R] high-flow valve design is ideal for controlling the flow of heated adhesives and sealants such as acrylics, epoxies, polyurethanes, silicones, lubricants and warm-melt materials requiring elevated temperatures. The valve opens and closes its carbide needle and seat to precisely start and stop material flow up to 5000 psi. The 60:1 air-to-fluid power ratio provides instantaneous on-off control of material flow. The heated flow-control valve is double-air-operated through 1/4″-NPT ports to ensure positive start and stop of material flow. The valve has 1/2″-NPT fluid inlet and outlet ports for high-volume flow of viscous materials. The valve includes two holes for easy mounting.
More information about fluid-control valves is available via a direct Web site link at http://www.sealantequipment.com/catalog/valves-page9.htm
Sealant Equipment, a leading manufacturer of precision meter-mix dispense systems and fluid-dispense valves, is registered to ISO 9001:2000. The company designs, manufactures and integrates precision-engineered systems and equipment to supply, proportion, control, meter, mix and dispense a wide variety of one-, two- and three-component adhesives, sealants, lubricants and other materials.
Visit www.sealantequipment.com or contact Sealant Equipment & Engineering, Plymouth, MI 48170; phone (734) 459-8600; fax (734) 459-8686; E-mail valves@sealantequipment.com.
Transport Phenomena and Unit Operations: A Combined Approach
TRANSPORT PHENOMENA AND UNIT OPERATIONS: A Combined Approach
R. G. Griskey, John Wiley & Sons, New York, NY, 448 pp., $99-95,2002
The idea of integrating transport phenomena and unit operations is a laudable one, inasmuch as these subjects are not mutually exclusive, as is too often assumed, but are indeed continuous and complimentary. This book addresses a broad range of topics within the general areas of fluid mechanics, heat transfer, mass transfer, staged operations and mechanical separations. It is aimed at the uninitiated student, and includes an impressive array of problems at the end of each of the fourteen chapters, as well as a large number of worked examples. However, the expectation that one book of moderate length can do adequate justice to such a broad range of topics is a bit overoptimistic. Due to the extensive breadth of coverage, the depth is quite limited, to the extent that developments, derivations and explanations of the origin and significance of many of the relations presented tend to be cursory or superficial and even, in some cases, misleading. There are also a number of errors or misprints that, hopefully, will be corrected in subsequent printings. The “transport” components of the book (i.e., transport coefficients and the microscopic conservation equations for molecular transport) comprise a relatively small part of the book, which is probably in suitable proportion to the fraction of practical systems amenable to analysis by these methods. The vast majority of the book is devoted to more-or-lessclassical unit-operations subjects. These include: incompressible flow in conduits; packed beds; mixing; conduction, convection, boiling, condensing and radiation heat transfer; heat exchangers; diffusion and convective mass transfer; equilibrium staged operations, including binary distillation (with a brief discussion of multicomponent distillation), packed absorption and extraction columns and leaching; filtration; centrifugation; sedimentation; and cyclone separations.
The material is mostly classic and the methods simplified based on graphical or empirical tools. Some of the material is current, but some is also outdated, with better or more accurate methods being available. For the beginning student or the non-chemical engineer, this book does provide a good introduction to a wide range of chemical engineering related topics, but should not be construed as the most complete or current treatment of the subjects covered.
Ron Darby, PhD, PE
Professor Emeritus of Chemical Engineering
Texas A&M Univ.
College Station, TX
Steam and condensate system rebuilds improve dryer control
Donohue mill improves steam and condensate systems, including stationary siphons and turbulence bars
WHEN PAPER MACHINE DRYER SECTIONS are designed for the best steam energy transfer to the sheet and efficient condensate removal, they are virtually set-and-forget operations. However, when they are reaching the limits of heat transfer or condensate removal capability, they can be difficult to control and can create many headaches for papermakers.
The Donohue newsprint mill in Thorold, Ont., faced such problems as the dryer sections on its No. 6 and No. 7 machines were limiting production and creating many operational difficulties.The 300 in. trim twin-wire Beloit machines, installed in 1981 and 1982, produce newsprint with basis weights from 45 gsm to 48.8 gsm (27.7 to 30 Ibs/3,000 ft). LIMITED CONTROL RANGE. Frank van Biesen, project manager at the Thorold mill, describes the situation before the steam and condensate systems underwent rebuilds in 1995 and 1996:
“Dryer pressures of 400 Kpa (58 psi) were just about at the maximum. On each machine, the existing thermocompressor was running wide open-out of control range-and we lacked the flexibility to control each dryer section individually. Operators found it difficult to change differential setpoints without dumping steam. To run the lighter weight grades (45 gsm) the operators had to shut down some dryers.’ The dryer sections were anything but set-and-forgetThe original steam and condensate systems for the 39 dryer sections on each machine were designed with single dry-end thermocompressor sections, which cascaded to the wet-end sections.A baby dryer was followed by three top uni-run dryers, which were individually pressure controlled.The bottom dryers of the two uni-run sections were not steam-heated, as the sheet is not in contact with the dryer surface. Both machines were equipped with dual rotating siphons up to dryer No. 23, with single rotating siphons in the remaining 16 dryers.
In addition to the lack of precise control, van Biesen reports the dryers were prone to frequent flooding, especially after repeated breaks. With the flooding problems and limited drying capacity, the mill looked for solutions to improve heat transfer to the sheet and to effectively remove condensate under all operating conditions, thereby improving machine efficiency.
PERFORMANCE STUDY, NEW SYSTEM DESIGN. To engineer an integrated solution to these problems, the mill chose Valmet-Enerdry of Thunder Bay, Ont. Valmet-Enerdry had previously conducted a dryer performance study from which it developed a longterm plan for improvements to the dryer sections. Based on this plan, the mill installed runability equipment such as blow boxes.
The study concluded that dryer surface temperatures were low for the steam pressure applied. The steam heat transfer to the dryer shell was limiting the drying rate. Also, the existing system design contributed to unreliable condensate removal, steam waste, and loss of production.
To achieve the desired goals of a steam and condensate rebuild, all components of the system-thermocompressors, steam separators, valves, siphons, turbulence bars-must be sized and their interdependent characteristics must be evaluated so that they work hand-in-hand. Valmet-Enerdry provided this detailed design engineering for the system. The new design incorporated a Valmet steam and condensate system for each machine, with stationary siphons and turbulence bars from the Deublin Co. of Waukegan, Ill. With machine speeds approaching 4,000 fpm, the condensate layer is certainly in a rimming condition. The Deublin turbulence bars, which were installed on all dryers except the non-heated uni-run dryers, disrupt the laminar condensate layer. This induced turbulence improves heat transfer to the shell.
To support the system design objectives of improved condensate removal, the mill selected Model FS100 Deublin DeltaSint stationary siphons. With stationary siphons, condensate is effectively evacuated at low differential pressures since the pressure difference does not have to overcome the centrifugal force introduced by high-speed rotation. Consistent with the original design of each machine, steam is supplied from the drive side, with condensate removed from the tending side.
The Deublin stationary siphons are relatively new to North American mills, but they are prevalent in Europe and Asia. To evaluate their performance, mill personnel visited sites in Finland, where the siphons are installed on similar machines.
To ensure reliability, the siphons have been designed to avoid metal-to-metal contact with the dryer shell. The siphon pickup shoe is designed so that it “water skis” on the condensate layer. Life of the carbon seals in the siphon joint is extended by an end-face balanced design, which ensures low loading on the face of the seal.An inspection gap is incorporated in the design so that seal wear can be visibly monitored by maintenance staff. Replacement can be planned systematically.
Evaluating Dryers for New Services
This simple, reliable approach to rate dryers for continuous service combines simulation with the specific drying rate approach.
Drying is a complex operation that involves simultaneous mass and heat transfer to and from irregularly shaped particles (1). During this process, the paniculate matter undergoes primary transformation (e.g., loss of moisture upon exposure to heat) and secondary transformation (e.g., particle breakage, shrinkage, biochemical reaction and morphological change), the latter of which may alter the product’s bulk characteristics, quality and appearance, and cause problems in dryer performance.
For dryers with known mechanical features and operating parameters, “troubleshooting” means more than rinding and fixing a problem - it involves evaluating a dryer for “new process conditions.” The various troubleshooting guises include (2):
* How do we get this dryer, which has never worked properly, to perform as it should?
* Why does dryer performance deviate from the expected?
* What can be done to increase dryer capacity?
* How do we avoid problems during dryer scaleup?
* How do we evaluate a dryer for new services where operating parameters deviate from the design basis?
This article presents a straightforward, reliable procedure for rating continuous tray and plate dryers for new or unintended services. The continuous behavior of plate and tray dryers are predicted from tests that are carefully conducted in laboratory-scale, vendor-supplied batch simulators. Data are recorded on the drying behavior of solids inside the dryer, and are used in the specific drying rate (SDR) equation, which, in combination with other formulas, rates the dryer’s performance at the new feed conditions.
Continuous dryer configurations
Two types of continuous tray dryers are commercially available: the rotating-shelf tray dryer and the stationary-plate dryer with rotating conveyor arms (3). Both dryers are particularly useful when relatively long residence times are required to dry the product and when product containment is necessary. Both units are also relatively gentle in their tray-to-tray solids-conveying techniques. For moisture removal, tray dryers rely on convection heat transfer, while plate dryers use conduction heating (the plates are heated trays).
Krauss Maffei Corp. (Florence, KY; www.kraussmaffei.com) rolled out the plate dryer in the U.S. in late 1970, and is still the only company that manufactures a plate dryer. The tray dryer was introduced in the U.S. in 1930 by Wyssmont Corp. (Fort Lee, NJ; www.wyssmont.com), which marketed its product under the TurboTray brandname. This dryer’s widespread acceptance has made “TurboTray” nearly synonymous with continuous tray-drying technology. For this reason, the TurboTray dryer has also been selected for illustrative examples in this article. The general features of the Wyssmont’s TurboTray and Krauss Maffei’s plate dryer are compared in Table 1.
Krauss Maffei plate dryer. This indirectly heated dryer is vertically oriented and features horizontal plates, equipped with arms and plows that ease product transfer, mounted inside the housing. The plates are heated by hot water, steam or thermal oil to temperatures up to 320°C. Product enters through the top of the unit and is conveyed downward in a spiral fashion via a central-rotating shaft to which the plates are attached. Material movement is facilitated by the disc and doughnut arrangement of the plates (Figures 1 and 2). The even numbered plates have a larger diameter then the odd numbered plates. The entire operation is conducted under vacuum.
Wyssmont tray dryer. An external view of the Wyssmont TurboTray dryer is shown in Figure 3. This dryer consists of a stack of rotating annular shelves (or trays) at the center of which turbo-type fans revolve to circulate the air over the trays. Wet material enters through the roof, falling onto the top shelf as the shelf rotates beneath the feed opening (Figures 4 and 5). After the tray nearly completes one revolution, the material is swept by a stationary wiper through radial slots onto the shelf below, where it is spread into a pile of uniform thickness by a stationary leveler. This action is repeated on each shelf; material transfer occurs approximately once per revolution. Material is discharged from the last shelf through the bottom of the dryer.
Applications. It is beyond the scope of this article to discuss the types of materials that can and cannot be dried in plate and tray dryers. Perry’s Chemical Engineers’ Handbook (3) describes tray-dryer performance data for several materials, including antioxidants, water-soluble polymers, antibiotics and petroleum coke. In addition, Perry’s presents plate-dryer performance data for three applications: plastic additives, pigments and foodstuff. Both dryers require free-flowing or friable feed. If the feed is doughy or sticky, the material may adhere to the tray, plate, plows or level arm, thereby impeding solids transportation and causing poor dryer performance. Lumpy material could bridge the slots in a tray dryer or lodge between the plows of a plate dryer. Pre-treatment of the feed is sometimes necessary to facilitate controlled solids transport inside the dryer.
Get smart about removing slag
More often than not, sootblowing is literally a shot in the dark. Clyde Bergemann’s solution to this problem: Control sootblowing operations intelligently, based on the outputs of real-time weight and heat-flux sensors and the calculations of a computer model.
Every coal plant operator knows that burning coal produces ash, which can melt into slag that can be difficult to remove. So difficult, in fact, that dynamite may be the only solution. Slag on boiler tubes has two undesirable effects. It always reduces the tubes’ heat-transfer capacity, and if it comes loose in big enough chunks and falls, it can damage or destroy tubes or structures below. In the first case, the consequences are reduced boiler efficiency and plant output and–potentially–catastrophic tube overheating. In the second, the result is typically a forced outage.
All coal-burning plants are susceptible to slagging, and different types of coal foster different kinds of slag formation. For example, the burning of low-sulfur Powder River Basin (PRB) coals produces ash whose low softening temperature can turn it to slag on hot convection-pass surfaces. At the other end of the spectrum is high-sulfur Eastern bituminous coal, whose high iron content significantly lowers the ash fusion temperature.
To clean their boilers, plants periodically sootblow their tubes and other heat-transfer surfaces with water, compressed air, or high-pressure steam. But determining the optimal cleaning frequency for each sootblower is nearly impossible in the absence of a real-time picture of how much slag exists, and where it is. For this reason, at many plants the operating frequency for each of the dozens of sootblowers is based on past positive results. But sootblowing too often is just as bad as sootblowing too infrequently. The former wastes money, labor, and steam, and the latter is just asking for trouble.
Two years ago, in the October 2003 issue of POWER, we reported that Atlanta-based Clyde Bergemann Inc. (CBI) had teamed up with Georgia Power Co. to fight slag at Plant Bowen. On that project, CBI installed real-time strain-gauge sensors on rods between the pendant heat exchange surfaces of the plant’s boiler to detect the increased weight of slag buildup. Since then, CBI has taken its “smart” sootblowing technology to a higher level, one that the company calls intelligent sootblowing (ISB). This article describes the results of a deployment of CBI’s new system–which features smart pressure and heat-flux sensors, smart water cannons and steam lances, and smart models and controls–at the coal-burning plant of an independent power producer in a midwestern U.S. state.
Slagbusters
Huiying Zhuang, a boiler process engineer at CBI, explains that the effectiveness of his company’s ISB line derives from several pieces of hardware and software working in concert. Figure 1 shows which pieces perform which functions, and where:
* SmartCannons (Figure 2) clean the entire furnace by spraying jets of water to the opposite or adjacent wall.
* SmartSensors (Figure 3) installed within the furnace’s waterwalls detect the local level of heat flux in real time. This information can be invaluable in determining where and when to sootblow, and which kind of sootblower to use.
* SmartGauges–strain gauges mounted on the support structure of the boiler’s pendants–quantify the extent of slag buildup.
* SmartLances–retractable sootblowers aimed at the superheater and reheater–use steam as their default cleaning agent. But they also can blend water with the steam when doing so would increase the lances’ effectiveness.
* The SmartModel constitutes the system’s “brain,” determining when and where cleaning is really needed.
* The SmartControls serve as a traffic cop, by taking information from the SmartModel and the sensors and gauges, and directing the operation of the cannons and lances.
Getting smart pays off
According to Zhuang, a plant can save big bucks by deploying an ISB system. He claims that an 800-MW unit might increase its profitability by one-third by boosting the efficiency of its boiler and reducing plant downtime. With such a dramatic gain in profits, a CBI system priced at $1.2 million might pay for itself in just six months, Zhuang said.
Zhuang went on to detail where those gains would be realized. Prefacing the following explanation with the remark that “fuel costs can represent 80% of a plant’s operating costs,” he said, “An ISB system can enable a plant to burn lower-quality, cheaper coal, saving millions of dollars annually.”
In addition, increased boiler efficiency produces additional financial gains. As Zhuang explained, “A cleaner furnace means greater radiant heat absorption and a reduction in furnace exit gas temperature [FEGT]. It also means a lower attemperator flow, which reduces the cost of water treatment and the consumption of sootblowing steam.” Because a boiler with clean heat-transfer surfaces runs more efficiently, installing an ISB system can even enable a plant to sell, rather than buy, NOx emissions credits. What’s more, such a plant’s selective catalytic reduction system wouldn’t have to work as hard, which would reduce its O&M costs.
An Indoor Wood Burning Furnace May Mean a New Career
Are you looking for an immensely cheap way to heat your home? Every year, you get that bite cutting into your pocket book every year when the mercury on the thermometer dips low. This article is really to give you some information on the indoor wood furnace and the outside wood furnace. First of all, it may sound elementary but it’s useful to learn a bit about wood and some of its burning characteristics. A cord of wood stands 4 feet wide and 4 feet high and 8 feet long. In this measurement, there is an allowance made for air pockets, so you’re going to get about 85 ft.³ out of a cord of wood. One pound of wood on the average produces 7500 BTU’s of heat, regardless of the species.
One important consideration to remember is that dense heavy wood will deliver more heat per cord. You’ll want to take this into consideration when comparing prices for different kinds of wood. Each log contains moisture, and it takes about 1,000 BTU’s to evaporate the moisture from each pound. Dry wood produces 10 to 30% less creosote and is more usable to heat your home. It’s a good idea to burn drier wood for an indoor wood burning furnace. A good practice to get into is to cut or buy green wood in the early spring or late winter, dry it as quickly as you can by cutting it to length and stack it so the air can circulate through the pile. If you stack the wood outside, make sure you cover it from the weather and hold it for 18 months, or if you can, indoor storage is certainly more preferable. Here is a very interesting tip for you! If you fell your own trees for your furnace, cut them in the spring or summer and leave them “unlimbed” until the leaves wither, as they will draw moisture out of the wood. Cut the wood to the longest length possible to fit in your firebox, as the longer it is, the longer the fire will hold. Seasoned wood carries about 20% moisture content.
One type indoor wood furnace is made by several manufacturers and you may want to consider goodman furnace as one of your choices. Essentially, all you do is put unsplit logs into the firebox and ignite them with kindling and paper. The firebox is ceramic and as the fire grows, fresh air flows through the air intake manifold and fans the flames. After the gas is heated to a temperature of 2,000°F, it then flows out of the firebox and down the flame path toward the exhaust vent. This incredibly hot air moves toward the vent and its energy passes through a fluid flowing through an internal heat exchanger. This heat transfer fluid reaches 180°F before circulating to an external heat exchanger, usually mounted on the back of the furnace. The energy produced by the furnace is then passed to the home heating system. This heat exchanger is usually sold as an option.
To control the operation of the furnace, there are usually dual aquastats. One controls the damper on the air intake manifold by monitoring the temperature of the heat transfer fluid. When the desired temperature is reached, the damper closes, shutting off the flow of fresh air and thus extinguishing the fire. When your home cools off and more heat is needed, the damper opens and the furnace re-fires. Heat that is stored in the refractory walls of the firebox will support automatic re-firing for up to two days. The second aquastat is wired to your home heating system, and will continue to run for a short period of time after the furnace shuts down and will dissipate residual heat from the fire.
The decision as to whether an outside wood furnace or an indoor wood furnace would be more suitable for your application will depend upon your personal preference. As the name implies, the outdoor wood furnace sits outdoors, much like a utility building and is usually 50 to 100 feet away and may be safer than an indoor one. The outdoor furnace concept is simple, safe and effective. The outdoor application also removes the danger of a wood- stove fire in the house. If you’re considering a wood-fired heating system for your home, be sure to consult with the experts online. Also visit your local dealer and learn more about whether an indoor wood furnace or an outside wood furnace would be more suitable for you. Good luck!
Crack Down on Heat Exchanger Fouling
Heat exchangers are the unsung heroes of many industrial processes and as such they tend to be taken for granted - nobody likes paying for what is often seen to be unnecessary maintenance. Heat exchangers provide duty for so long, that when they start to drop in efficiency, it’s usually a gradual process that goes largely unnoticed - until their performance has deteriorated sufficiently to be a problem. Then it really is a problem - and one requiring urgent attention.
What aggravates the situation is the heat exchanger that has never been cleaned properly, coupled with the commercial need to keep it on-line. When the decision is made to carry out cleaning, often nobody knows what the performance of the exchanger is meant to be, either because the drawings have been lost, or no record of any improvement was made after the original cleaning.
When the exchanger finally is opened up to ascertain the extent of the fouling, it’s not surprising to find it is so severe that cleaning takes a lot longer than planned. Any benefit that might have been gained by a quick traditional clean is offset by the extended cleaning duration and costs - and, of course, lost production.
If that sounds like a nightmare scenario, bear in mind that this is the sort of situation specialist cleaning companies encounter every week. Cleaning is often carried out without any firm knowledge of how much of an improvement the cleaning will give and how long its effects will last. Having to make ‘finger in the wind’ predictions clearly is not a satisfactory way to plan maintenance.
One of the most popular and widely-employed heat exchanger configurations in industry, is the straight or hairpin shell-and-tube exchanger. With hundreds or thousands of small-bore tubes bundled together, the extent of quite modest scaling can involve major work to return the exchanger to anything near its commissioned performance. If the outside of the bundle is heavily scaled as well, the cleaning challenge rises by an order of magnitude.
There is potential to bring about a significant improvement in heat exchanger accessibility and ‘cleanability’, by working more closely with the people who design heat exchangers and fabricate industrial plants.
Better design would lead to improved cleaning - where improved means faster, cleaner and safer, possibly in-situ or even on-line and with better waste containment. It would then be easier and quicker to clean exchangers back to bare metal to return them to duty and their design performance faster.
Plants are generally specified and ordered on the basis of throughput, not accessibility and ease-of-cleaning. Suppliers are happy to comply with this and therefore tend to design heat exchangers with 30-40% excess capacity to ensure that they can continue to provide duty, even when quite extensively fouled. Heat exchangers the world over are currently designed and installed with a view to using one of three systems for cleaning: chemical, pressure jetting and/or mechanical and this approach has remained unchanged for over 50 years.
When it comes to maintenance, refineries - like most of industry - tend to compete on the same basis - a 21-day shutdown is decreed because it’s been done that way for maybe the last 20 years. The same cleaning methods are generally used slavishly, with high-pressure water as the cleaning medium.
Most companies look at their heat exchangers in isolation and simply try to extend their run-time, instead of having them designed or re-designed so they can be cleaned more regularly, but faster and better. BP’s Coryton refinery, for instance, managed to reduce cleaning time on one shell-and-tube heat exchanger from three days to three hours by applying a different approach to cleaning it.
If a plant is optimized for cleaning, almost full production can be maintained throughout the cleaning process. Relatively minor mechanical changes, such as adding isolating valves to heat exchangers, means that each exchanger, or bank of exchangers, can be taken down and cleaned while the others remain on-line. A redesign of the exchanger so that a header can be removed, means it can then be cleaned with a different system to the standard high-pressure water jetting, in a few hours instead of several days.
At Dow Corning’s silicone plant in Barry, south Wales, a tubular boiler and fire tube in the Energy Recovery Unit (ERU) required the removal of a 5mm layer of deposit in as short a time as possible to minimize lost production. Another obstacle was that the unit, which carries waste gases, takes 48 hours to cool and prepare - even with the introduction of a chilled nitrogen purge - before personnel can enter to clean it manually.
The solution involved developing a bespoke remote de-scaler, which was inserted through a small 50cm man-way. Once inside, the de-scaler expanded to fit the hot fire tube, while reaching the full length of the carbon steel tube. With cooling time and man entry eliminated, the shutdown was reduced from five days to three and there was a noticeable improvement in performance of the ERU when it came back on line.
Improved cleaning cycles also mean the rate of future fouling build-up is reduced, which in turn reduces the risk of tubes corroding as a result of the exchanger being open to the atmosphere longer for cleaning.
Heat exchange surfaces therefore remain smoother and provide better heat transfer. If and when the exchanger does foul up, it’s easier to clean next time around, using whichever system is preferred. This would represent a change of practice to what has been the norm since the 1980s, for instance, when what was then Mobil in the UK was one of the first refineries to decide that it would extend run-times by abandoning the annual clean and only clean every two years.
Today, typical service intervals have become stretched to three and even four years in some cases, but the apparent operational savings are actually a false economy. Shareholders are indeed happy, because they are getting longer run times, while competing refineries have little choice but to play the same game or lose millions during more frequent shutdowns. Four years down the line, however, the plant will have to come down for major cleaning and maintenance and it will experience a far higher capital replacement cost than ever before.
Mike Watson, Managing and Technical Director
Run by its founder and inventive visionary Mike Watson the company is supported by a wealth of hand selected department managers. With many years experience in developing engineered solutions to complex problems in industry, Mike’s belief is that convention should always be challenged in order to find a better way to achieve improved results. This “never say never” approach, led to him founding Tube Tech in the 1980s. Today, the company cleans the toughest cleaning projects the world can throw at it. Mike often says “If people say it can’t be done, its like a red rag to a bull to me. I will always find a solution”. Mike continues to invest in new technology development, leading the world in new cleaning methodology.
Molybdenum - Vital for Nuclear Reactors
Molybdenum plays a more vital role in the global nuclear renaissance than you might suspect. Without the silvery white metal, the world’s energy infrastructure would somewhat suffer. But, nuclear power plants would be set back at least two decades. The new high performance stainless steels (HPSS) contain as much as 7.5 percent molybdenum and can add more than three times the life to the world’s aging nuclear fleet condenser tubes.
During the early construction of nuclear power plants, steam condensers relied upon copper base alloys – brass and copper nickel – for heat transfer capabilities. These alloys have high coefficients of thermal conductivity required in steam generation to power nuclear reactor turbines. But copper-alloyed tubes were being replaced too quickly – with an average life of eight years – because of sulphide pitting. Hardest hit were those reactors using polluted seawater to cool their reactors.
Over the past 30 years ago, nuclear utilities slowly began turning to the super austenitic stainless steels as one way to make their nuclear reactors last longer. The addition of molybdenum, initially starting with percentage of less than four percent, helped increase the thermal conductivity lacking in nickel, iron or steel. At nuclear stations which replaced the copper alloys with HPSS condenser tubes, 57 percent rated the thermal performance good and all but one rated it normal. Molybdenum had helped overcome the thermal hurdle.
A large number of the 190 nuclear reactors, which now utilize HPSS condenser tubes, reported an average life in excess of 18 years. The longest stainless steel condenser installation has remained in service more than 26 years, according to a study done several years ago. According to a report published in 2000, more than 100 million feet of super-alloy stainless steel tubes have replaced the older, copper-alloy tubing.
Condensers are large heat exchangers used in nuclear power plants. Condensers have thousands of tubes horizontally mounted to condense and recover the steam passing through turbines. Each low-pressure turbine generally has a condenser, which also maintains a vacuum to optimize the turbine’s efficiency.
Water fouling deposits were cited as a major problem at many reactors, especially with condenser tubes where seawater or high-chloride brackish water was the coolant. Pitting corrosion, tube sheet crevice corrosion and galvanic corrosion put the tubes at risk for leakage. Plugging, mud, or detritus accumulating in condenser tubes reduce a power plant’s efficiency.
Utilities use cleaning systems with small, abrasive sponge-like balls to keep the tubes clean and test for tube defectives with probing devices. Tube thinning and corrosion create the opportunity for tube leakage. This can not be tolerated because chemicals such as sodium and chlorides find their way into the reactor vessel or steam generator.
Upgrading the steam condenser tubing to stainless steel also plays a vital role in the ‘power uprate’ program utilities have used to increase generating capacity for existing reactors as we recently discussed. The more advanced uprate program could add up to 20-percent capacity to existing U.S. nuclear reactors.
Different Molybdenum Alloys
There are several HPSS manufacturers for nuclear reactor condensers. The most prominent in the nuclear sector include Pennsylvania-based ATI Allegheny Ludlum and Finland’s Outokumpu. Each offers austenitic steels with chromium and nickel composition of between 20 and 25 percent for each alloy and a range of 6.2 to 7.5 percent molybdenum.
In a paper presented by Jan Olsson of Avesta Sheffield (before the company was acquired by Outokumpu), he highlighted the results of tests performed on the new super-austenitic stainless steel, 654 SMO®. Metals comprising this brand include 25-percent chromium, 22-percent nickel and 7.5-percent molybdenum. To increase pitting resistance, the manufacturers added up to 0.5-percent nitrogen and three-percent manganese (for make the nitrogen more soluble).
As with all pioneering developments – and remember that R & D breakthroughs have taken place over a two-decade-plus period, manufacturers have re-designed their metallurgical composition to find the most encouraging percentages of nickel, chromium, molybdenum and nitrogen. The earlier stainless steels relied on higher nickel content and lesser percentages of chromium and molybdenum.
At first, conventional austenitic grades, such as 316L, or high chromium-ferritic grades, were utilized. Pitting struck down widespread use of the 316L series and was replaced by higher alloy steels. For example, others, such as the 254 SMO® stainless steel, began aggressively replacing the copper alloy tubes and in some cases the 316L series. The 254 is comprised of 20-percent chromium, 18-percent nickel, 6.2-percent molybdenum and 0.20-percent nitrogen. It has also offered a high level of corrosion resistance at desalination plants without becoming cost-prohibitive.
The most significant breakthrough came after various stainless steels were tested at Scandinavian coastal reactors. In the Avesta paper, the failures of each lesser austenitic grade were checked off. Significant deficiencies included insufficient stress corrosion cracking resistance and resistance to natural seawater. Even titanium tubing was used as an interim measure because it increased total heat transfer by 17 percent, but the metal failed to stand up to high velocity steam and suffered ‘water droplet erosion.’
According to the study, “The only alloy fully resistant to all test conditions was 654 SMO®.” The results at nuclear power plants in Finland and Sweden, along the Baltic Sea, were astonishing! Four important conclusions about this super alloy were reached after the testing.
• Its corrosion resistance could cope with the hostile environments existing inside condenser tubes of desalination plants and power plants.
• Its corrosion resistance was good enough to cop with many other hostile brine and seawater environments.
• Its erosion resistance was advantageous where it was exposed to high velocity streams.
• There was no concern about its heat transfer characteristics.
Nuclear Consumption of Molybdenum
About 48 nuclear reactors are reportedly scheduled for construction by 2013. It may be possible that up to 100 could be constructed by 2020, depending upon political and financial climates. The largest number proceeding through the proposed, planned or construction phases will be located along coastal areas to service the most populated areas. The greatest numbers of new constructions are expected from China, India, Japan, Russia, South Korea and Japan (and possibly the United States).
Existing reactors along coastal areas in Asian countries presently breaks down as follows: Japan (57), South Korea (26), China and Taiwan (19) and India (11). Because these are the most prone to seawater or brackish corrosion, they are also the likely candidates for upgrading existing condenser tubing to high alloy stainless steel. And their new reactors are likely going to be constructed along their coasts, requiring the super austenitic grades. As an aside, of the previously mentioned 190 nuclear power plants which had replaced their condensers with HPSS, 45 percent used fresh water as coolant. Those plants chose the high alloy steel as a ‘fail-safe’ measure to prevent interrupted service or a potential reactor incident.
The United Nations estimates that two-thirds of the planet’s population will be living with water stress by 2025. Global freshwater scarcity may demand the use of brackish or seawater as nuclear reactor coolant. To prevent the accompanying corrosion, the higher-percentage molybdenum alloy, specifically the 654 SMO®, could emerge as the condenser tubing material of choice. Either the 254 SMO® or the 654 would be utilized in desalination plants required to overcome water shortages in the hardest hit areas: North Africa, the Middle East and West Asia.
Typically, nuclear power plant condenser tubing requires approximately 520,000 feet of stainless steel. According to the International Molybdenum Association (IMOA), larger reactors could utilize up to one million feet of stainless steel. With the higher molybdenum grades found in the super alloys, new nuclear reactors could require tens of thousands of metric tons of molybdenum.
By comparison, nuclear waste containers proposed for the Yucca Mountain nuclear waste repository were forecast to consume about 15,000 metric tons of moly. While this project may or may not proceed as planned to the construction phase, the Nuclear Energy Institute (NEI) has proposed regionalized storage of spent fuel.
Should comparably designed storage canisters be utilized to ‘temporarily’ contain the nuclear waste, it is likely molybdenum will play a key role. According to the U.S. Government’s Energy Citation Database, as published by the Department of Energy’s Office of Scientific and Technical Information, “Alloys with combined chromium plus molybdenum contents greater than 30 percent were the most resistant to general and local attack.” This was the conclusion reached after corrosion scouring tests were performed on stainless steel and nickel-based alloys to immobilize high-level, radioactive waste.
Another aspect where high-percentage molybdenum stainless steel would double up is with the expansion of nuclear desalination plants. In the past, and in our publication, “Investing in the Great Uranium Bull Market,” we have discussed the rise of nuclear desalination across those coastal areas, requiring far more freshwater than can possibly be transported through other means. The World Nuclear Association (WNA) has reported of numerous such desalination projects in progress.
Will The Energy Bull Have Sufficient Moly?
From nearly every energy project – oil, gas, coal and nuclear, and for water, molybdenum demand will continue increasing. Super austenitic grades demand a higher moly content to combat corrosion and provide reliability of service. Of course, there will be substitution in the face of future supply shortfalls. In some instances, there are reports the Russians have substituted vanadium for molybdenum in some of their oil and gas pipelines to conserve on moly consumption. ATI Allegheny Ludlum has argued for the substitution of two-percent manganese for every percent of nickel, but in the lower grade austenitic groups which do not demand the corrosion resistance of energy projects. While reviewing the anticipated new projects from the molybdenum mining sector, we foresee the high probability of supply inadequacy. Aside from China Moly’s Sandaozhuang molybdenum mine, which the company hopes could produce 28,000 tonnes of molybdenum concentrate this year and perhaps grow by another 17 percent the following year, there is a paucity of new molybdenum projects coming fully online before 2009.
Based upon China’s voracious appetite for molybdenum – one research firm estimated compounded annual growth rate over the previous five years at 17 percent, whatever excess moly production comes from China Moly’s mining efforts could very well be domestically consumed.
Future North American molybdenum producers may need to ramp up their projects to meet the growing demand. During 2006, demand grew above the historical norm of four percent; most of the consumption came from China. This is unlikely to stagnate or decrease, and could interfere with North American and European consumption of molybdenum.
Only one company is scheduled to commence molybdenum mining in 2007, Roca Mines. Because the company is limited to a small-mining permit, anticipated production could not exceed three million pounds. By late 2008, or early 2009, Adanac Molybdenum hopes to commence its start-up efforts to reach eight-figure moly production. Later, Blue Pearl Mining hopes to commence high-grade molybdenum mining at the Davidson deposit in British Columbia. Around this time, the Climax molybdenum mine could re-open and begin production in Colorado. Moly Mines hopes to begin production at the company’s Spinifex project. Possibly, before the decade ends, Idaho General might commence operations in Nevada. Perhaps before those 48 nuclear reactors come online, US Energy’s Mt. Emmons deposit may be mined in Colorado.
Many of these projects are subject to environmental permitting and/or financing, putting any material amount of forecasted supply in jeopardy. And this comes at a time when some experts believe byproduct molybdenum production at copper mines could be constrained. There are many conditional requirements which do not necessarily guarantee a reliable supply from the new breed of primary moly producers. We have witnessed comparable obstacles in the uranium sector, which has since been accompanied by a hyperbolic price rally in this metal.