Anti -Corrosion Paints

Corrosion destroys metallic material fabricated structures with the time, we do not have exact treatment for it. Only thing that we can do is minimizing the corrosion. If it is not protected by using anti-corrosion technique, corrosion becomes worst enemy for metallic material structure. So engineers have huge responsibility to overcome corrosion. To do that there are lot of techniques, one of them is applying anti- corrosive paint on the surface of the metallic material structure. It is the most common method used in the paints and coating industries around the world. This anti-corrosive paints allow material to withstand maximum lifespan. Almost all the industries are being used anti corrosive paintings for various type of situations. Main industries that use anti corrosive paints can be pointed out by using below mentioned figure.

Figure No 01 : Places where Anti corrosive Paints are being Used

It is very important to select suitable protection technique for applications. It is a big responsibility that should be done by engineers. When considering anti-corrosive painting, there are lot of things to be considered. Those things can be mentioned as following.

• Material selecting (There are types of materials such as metallic coating and nonmetallic coating materials. Those two type have lot of coating types which have various characteristics, so it should be selected correct coating type by considering environmental impact and applications details. )
• Surface preparation (Surface should be cleaned properly. Oil, dust and other impurities can cause to fail the coating)
• Coating thickness ( Number of coating and thickness causes to have better protection against corrosion)

Procedure of applying anti-corrosive paint is very important to be considered. Main steps that must be done during a painting process can be mentioned as following.

• Surface Cleaning and Conditioning
• Solvent Cleaning
• Drying
• Applying Paints ( This can be done by applying more than one coating)

Although there are lot of classifications for anti-corrosive coatings, A wide variety of anti-corrosion coatings that  are available to match the performance requirements of a specific application can be mentioned as following.

Fluoropolymer - Resin/Lubricant Blends that Offer Excellent Corrosion Protection

Xylan - A Fluoropolymer that Can Extend Component Life

Molybdenum Disulfide - Friction Protection for High Pressure Loads

Epoxy, air dry - Cost-Effective, Corrosion Resistant Coating

Epoxy, thermal cure -   Excellent Impact Resistance, Plus Corrosion and Abrasion Resistance

Phenolic - Ideal in Low PH, High Temperature Environments

Phosphate - Ferrous Metal Coating for Anti Galling And Minor Corrosion Resistance

Polyurethane -High Gloss Topcoat for Epoxy And Inorganic Zinc

Inorganic Zinc - Corrosion and Weathering Protection for Steel

PTFE -  The Original Non-Stick Coating, Able To Withstand High Temperatures

PPS/Ryton - Thin Film Chemical Resistant Coating

FEP - PTFE characteristics with better abrasion resistance

PVDF/Dykor -  High Quality Coating Ideal For Chemical Processing Applications

ECTFE/Halar - Resistant to Most Chemicals Plus High Impact Strength

References

1. The Infection, Maintenance and Application of Marine Coating System, 3^rd Edition 2007, American Bureau of Shipping.
2. http://www.metcoat.com/corrosion-resistant-coatings.htm
3. http://www.corrosionpedia.com/2/1402/prevention/coatings/anti-corrosion-coatings-for-different-service-exposures

Air Conditioning System Designing Procedure

Air conditioning has become very important nowadays, because global warming and other environmental conditions have made uncomfortable atmosphere around us. So air conditioning system will be the very important part of a building in future.

As a mechanical engineers, we must know the process of designing air conditioning system. That design process has been linked with thermodynamics and also fluid dynamics. So engineers should have sound knowledge around those two field. 

Cooling load estimation is the first step to controlled air-conditioning. Air-conditioning is utilized to supply a controlled atmosphere to public buildings such as offices, halls, homes, and industries for the comfort of human being or for the proper performance of some industrial processes. When considering cooling load, it becomes very important to consider the way that building heat gain occur. It can be clearly illustrated as following.

Figure No 01 : Building Total Heat Gain Factors

Full air-conditioning implies that the purity, movement, temperature and relative humidity of the air be controlled within the limits imposed by the design specification. For any air conditioning system to perform satisfactorily, equipment of the proper capacity must be selected based on the instantaneous peak load requirements. The type of control used is dictated by the conditions to be maintained during peak and partial load. Undersized equipment will not provide the required conditions while a greatly oversized one will lead to operating problems such as “hunting”. Load estimating in air-conditioning system design has been carried out manually in many quarters in developing country .A lot of time and energy are wasted when estimating the cooling loads in complex and intricate buildings of modern time. But here manual calculation process is described

There are two steps to be done to install good air conditioning system, they are

• Estimating cooling load of the building
• Duct designing process

Estimating cooling load of the building

Cooling load estimation is a very important and complex task when it is done manually. There are three methods to estimate cooling load for a particular building, they are 

• Transfer Function Method (TFM) : This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet.
• Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF) : This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data.
• Total Equivalent Temperature Differential/Time-Averaging (TETD/TA) : This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method.

For strictly manual cooling load calculation method, the most practical to use is the CLTD/CLF method as described in the 1997 ASHRAE Fundamentals. This method, although not optimum, will yield the most conservative results based on peak load values to be used in sizing equipment. It should be noted that the results obtained from using the CLTD/CLF method depend largely on the characteristics of the space being considered and how they vary from the model used to generate the CLTD/CLF data shown on the various tables.

Considerations and Assumptions

• Design cooling load takes is done by making lot of assumptions some of them can be mentioned as following.
• Weather conditions are selected from a long-term statistical database. The conditions will not necessary represent any actual year, but are representative of the location of the building. ASHRAE has tabulated such data.
• The solar loads on the building are assumed to be those that would occur on a clear day in the month chosen for the calculations.
• All building equipment and appliances are considered to be operating at a reasonably representative capacity.
• Latent as well as sensible loads are considered.
• The latent heat gain is assumed to become cooling load instantly, whereas the sensible heat gain is partially delayed depending on the characteristics of the conditioned space. According to the ASHRAE regulations, the sensible heat gain from people is assumed 30% convection (instant cooling load) and 70% radiative (delayed portion).
• The ventilation rates are either assumed on air changes or based on maximum occupancy expected.
• Design dry blub of 2.5% in summer and 97.5% in winter for Chittagong city is considered. 2.5% means that temperature exceeds the value (330C) only 2.5% of all the hours in summer months. Windows are externally shaded.

As above mentioned manner Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF) is the most accurate and most widely used method. Following this method it is very easy to find cooling load for any space of a particular building. After estimating cooling load, next step is duct designing.

Air flow rates should be calculated for each and every space of the building. It is done by  using estimated cooling load values. To do that  psychrometric chart can be used.

In here both sensible and latent heat transfer take place. For sensible heat transfer, the driving potential is the temperature differential. For the latent heat transfer the driving potential is partial pressure difference or the corresponding specific humidity difference. By considering energy balance for latent or sensible heat transfer we can find mass flow rate for particular building space (room). By following same method all the required mass flow rates can be calculated. After that air flow rates can be calculated

Duct designing process

Duct designing is the most critical task in air conditioning system designing. Engineers have responsibility to design very effective duct system by considering several factors. The chief requirements of an air conditioning duct system can be mentioned as following manner.

• It should convey specified rates of air flow to prescribed locations
• It should be economical in combined initial cost, fan operating cost and cost of building space
• It should not transmit or generate objectionable noise

Although engineers have achieved major requirements of duct designing, those things will not be enough. Because to install very effective duct system, design engineers must consider following general rules for duct design

• Air should be conveyed as directly as possible to save space, power and material
• Sudden changes in directions should be avoided. When not possible to avoid sudden changes, turning vanes should be used to reduce pressure loss
• Diverging sections should be gradual. Angle of divergence ≤ 20o
• Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4
• Air velocities should be within permissible limits to reduce noise and vibration
• Duct material should be as smooth as possible to reduce frictional losses

Duct systems can be classified as following manner. According to that classification, we have to select suitable air flowing velocity for the system.

• Low pressure systems : Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)
• Medium pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 15 cmH2 O (g)
• High pressure systems: Velocity > 10 m/s, static pressure 15<p ≤ 25 cm H2O (g)

When considering air velocity, engineers have to consider lot of factors. Because they have to select optimum velocity for the system otherwise lot of problems can occur. There are air velocity standard table for that, so referring those tables air velocity can be calculated. Recommended air velocities depend mainly on the application and the noise criteria. Typical recommended velocities are

• Residences: 3 m/s to 5 m/s
• Theatres: 4 to 6.5 m/s
• Restaurants: 7.5 m/s to 10 m/s

When we have high velocities in the ducts results in

• Smaller ducts and hence, lower initial cost and lower space requirement
• Higher pressure drop and hence larger fan power consumption
• Increased noise and hence a need for noise attenuation

Duct system design become very difficult when we have huge buildings. So engineers should identify the lot of parameters for that. Generally at the time of designing an air conditioning duct system, the required airflow rates are known from load calculations. The location of fans and air outlets are fixed initially. The duct layout is then made taking into account the space available and ease of construction. In principle, required amount of air can be conveyed through the air conditioning ducts by a number of combinations. However, for a given system, only one set results in the optimum design. Hence, it is essential to identify the relevant design parameters and then optimize the design.

The run with the highest pressure drop is called as the index run. From load and psychrometric calculations the required supply airflow rates to each conditioned space are known. From the building layout and the location of the supply fan, the length of each duct run is known. The purpose of the duct design is to select suitable

dimensions of duct for each run and then to select a fan, which can provide the required supply airflow rate to each conditioned zone.

Due to the several issues involved, the design of an air conditioning duct system in large buildings could be a sophisticated operation requiring the use of Computer Aided Design (CAD) software. However, the following methods are most commonly used to design duct systems

• Velocity method
• Equal Friction Method
• Static Regain method

Very effective and easy method to design duct systems is equal friction method, because other to methods can be a result to have large pressure drops. So brief introduction about equal friction method as follows.

In this method the frictional pressure drop per unit length in the main and branch ducts (Δpf/L) are kept same. After that we can follow the procedure, because we have already calculated air flow rates and air velocity for the application.

• Select a suitable frictional pressure drop per unit length (Δpf/L) so that the combined initial and running costs are minimized.
• Then the equivalent diameter of the main duct is obtained from the selected value of (Δpf/L) and the airflow rate airflow rate in the main duct.
• Since the frictional pressure drop per unit length is same for all the duct runs, the equivalent diameters of the other duct runs, can be found by considering frictional pressure drop equation.
• If the ducts are rectangular, then the two sides of the rectangular duct of each run are obtained from the equivalent diameter of that run and by fixing aspect ratio as explained earlier. Thus the dimensions of the all the duct runs can be obtained. The velocity of air through each duct is obtained from the volumetric flow rate and the cross-sectional area.
• Next from the dimensions of the ducts in each run, the total frictional pressure drop of that run is obtained by multiplying the frictional pressure drop per unit length and the length.
• Dynamic pressure losses in each duct run are obtained based on the type of bends or fittings used in that run.
• The total pressure drop in each duct run is obtained by summing up the frictional and dynamic losses of that run.
• The fan is selected to suit the index run with the highest pressure loss. Dampers are installed in all the duct runs to balance the total pressure loss.

After following above mentioned steps, duct system design process will be finished. Then engineers should implement installations process according to the design data.

Figure No 02 : Air Conditioning Systems Layout and Types

References

1. ASHRAE, (1997). ASHRAE Fundamentals Handbook, American Society of Heating, Air-Conditioning and Refrigeration Publishing Service.
2. ASHRAE, (1989). ASHRAE Fundamentals Handbook, American Society of Heating, Air-Conditioning and Refrigeration Publishing Service.
3. C.P Arora, Refrigeration and Air Conditioning, Second Edition.

Lean Manufacturing

Cost of the production has become big enemy for manufacturing companies, Lean manufacturing is a business model and collection of tactical methods that emphasize eliminating non-value added activities (waste) while delivering quality products on time at least cost with greater efficiency. Lean implementation is rapidly expanding throughout diverse manufacturing and service sectors such as aerospace, automotive, electronics, furniture production, and health care as a core business strategy to create a competitive advantage. Toyota is the very famous automobile company which has developed Lean manufacturing system. So they are experts in Lean productions.

Lean manufacturing system is a collection of lot of manufacturing supporting concepts. Few of those concepts are 5S concept, Kaizen, Kanban, PDCA ( Plan, Do, Check, Act ), KPI ( key performance indicator), Total Productive Maintenance (TPM) etc.

Lean manufacturing is based on finding efficiencies and removing wasteful steps that don't add value to the end product. There's no need to reduce quality with lean manufacturing the cuts are a result of finding better, more efficient ways of accomplishing the same tasks.

To find the efficiencies, lean manufacturing adopts a customer value focus, asking "What is the customer willing to pay for?" Customers want value, and they'll pay only if you can meet their needs. They shouldn't pay for defects, or for the extra cost of having large inventories. In other words, they shouldn't pay for your waste.

Waste is anything that doesn't add value to the end product. In Lean Manufacturing, mainly  there are seven  categories of waste , it can be demonstrated briefly  as following manner.

• Overproduction – Are you producing more than consumers demand?
• Waiting – How much lag time is there between production steps?
• Inventory (work in progress) – Are your supply levels and work in progress inventories too high?
• Transportation – Do you move materials efficiently?
• Over-processing – Do you work on the product too many times, or otherwise work inefficiently?
• Motion – Do people and equipment move between tasks efficiently?
• Defects – How much time do you spend finding and fixing production mistakes?

Figure No 01 : Seven type of waste

If we can minimize above mentioned seven waste, it will be a great win for our company. Because all value added unwanted expenses can be saved and it will be a big amount of money for large manufacturing factories. Expenses for implementing lean manufacturing system is very less, only thing that need is a good knowledge about the process. If manufactures can plan their process according to the lean system, they can see out- come of the system very quickly. Lean system cannot be implanted without top managers approval, so people who are responsible for the production process should make proposal to top people by mentioning advantages of the Lean system for the company growth.

When considering above mentioned facts, It can be understood that mechanical engineers have responsibility to implement lean manufacturing system very effectively in their working place. Because they are the people who have lot of experience in manufacturing and production processes. Most of the directors, general managers, factory engineers etc are mechanical engineers, so they always responsible for company profits and stability of their market. Implementation of lean system to all  production processes definitely make higher profits. So mechanical engineers must have  better understanding  about above mentioned things

Tidal Energy

Tidal energy is produced by the surge of ocean waters during the rise and fall of tides. Tidal energy is a renewable source of energy. During the 20th century, engineers developed ways to use tidal movement to generate electricity in areas where there is a significant tidal range the difference in area between high tide and low tide. All methods use special generators to convert tidal energy into electricity.

Tidal energy production is still in its infancy. The amount of power produced so far has been small. There are very few commercial-sized tidal power plants operating in the world. The first was located in La Rance, France. The largest facility is the Sihwa Lake Tidal Power Station in South Korea. The United States has no tidal plants and only a few sites where tidal energy could be produced at a reasonable price. China, France, England, Canada, and Russia have much more potential to use this type of energy.

In the United States, there are legal concerns about underwater land ownership and environmental  impact. Investors are not enthusiastic about tidal energy because there is not a strong guarantee that it will make money or benefit consumers. Engineers are working to improve the technology of tidal energy generators to increase the amount of energy they produce, to decrease their impact on the environment, and to find a way to earn a profit for energy companies.

Figure No 01 : Various design types for Tidal energy extraction 

Tidal Energy Generating Methods

There are currently three different ways to get tidal energy. They are tidal streams, barrages, and tidal lagoons. Brief introduction about above three methods can be mentioned as following manner

Tidal stream

For most tidal energy generators, turbines are placed in tidal streams. A tidal stream is a fast-flowing body of water created by tides. A turbine is a machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is much more dense than air, tidal energy is more powerful than wind energy. Unlike wind, tides are predictable and stable. Where tidal generators are used, they produce a steady, reliable stream of electricity.

Placing turbines in tidal streams is complex, because the machines are large and disrupt the tide they are trying to harness. The environmental impact could be severe, depending on the size of the turbine and the site of the tidal stream. Turbines are most effective in shallow water. This produces more energy and allows ships to navigate around the turbines. A tidal generator's turbine blades also turn slowly, which helps marine life avoid getting caught in the system.

The world's first tidal power station was constructed in 2007 at Strangford Lough in Northern Ireland. The turbines are placed in a narrow strait between the Strangford Lough inletand the Irish Sea. The tide can move at 4 meters (13 feet) per second across the strait.

Figure No 02 : Tidal stream concept 

Barrage

Another type of tidal energy generator uses a large dam called a barrage. With a barrage, water can spill over the top or through turbines in the dam because the dam is low. Barrages can be constructed across tidal rivers, bays, and estuaries.

Turbines inside the barrage harness the power of tides the same way a river dam harnesses the power of a river. The barrage gates are open as the tide rises. At high tide, the barrage gates close, creating a pool, or tidal lagoon. The water is then released through the barrage's turbines, creating energy at a rate that can be controlled by engineers.

The environmental impact of a barrage system can be quite significant. The land in the tidal range is completely disrupted. The change in water level in the tidal lagoon might harm plant and animal life. The salinity inside the tidal lagoon lowers, which changes the organisms that are able to live there. As with dams across rivers, fish are blocked into or out of the tidal lagoon. Turbines move quickly in barrages, and marine animals can be caught in the blades. With their food source limited, birds might find different places to migrate.

A barrage is a much more expensive tidal energy generator than a single turbine. Although there are no fuel costs, barrages involve more construction and more machines. Unlike single turbines, barrages also require constant supervision to adjust power output.

The tidal power plant at the Rance River estuary in Brittany, France, uses a barrage. It was built in 1966 and is still functioning. The plant uses two sources of energy, tidal energy from the English Channel and river current energy from the Rance River. The barrage has led to an increased level of silt in the habitat. Native aquatic plants suffocate in silt, and a flatfish called plaice is now extinct in the area. Other organisms, such as cuttlefish, a relative of squids, now thrive in the Rance estuary. Cuttlefish prefer cloudy, silt ecosystems. 

Figure No 03 : Barrage 

Tidal Lagoon

The final type of tidal energy generator involves the construction of tidal lagoons. A tidal lagoon is a body of ocean water that is partly enclosed by a natural or manmade barrier. Tidal lagoons might also be estuaries and have freshwater emptying into them.  A tidal energy generator using tidal lagoons would function much like a barrage. Unlike barrages, however, tidal lagoons can be constructed along the natural coastline. A tidal lagoon power plant could also generate continuous power. The turbines work as the lagoon is filling and emptying.

The environmental impact of tidal lagoons is minimal. The lagoons can be constructed with natural materials like rock. They would appear as a low breakwater (sea wall) at low tide, and be submerged at high tide. Animals could swim around the structure, and smaller organisms could swim inside it. Large predators like sharks would not be able to penetrate the lagoon, so smaller fish would probably thrive. Birds would likely flock to the area.

But the energy output from generators using tidal lagoons is likely to be low. There are no functioning examples yet. China is constructing a tidal lagoon power plant at the Yalu River, near its border with North Korea. A private company is also planning a small tidal lagoon power plant in Swansea Bay, Wales.

Figure No 04 : Tidal lagoon 

Tidal energy potential Potential

Worldwide potential for wave and tidal power is enormous. So this will be a next energy supplying resource for the world.

Figure No 05 : Tidal energy potential in the World 

Environmental Impacts

Unlike fossil-fueled power plants, wave and tidal energy facilities generate electricity without producing any pollutant emissions or greenhouse gases. Since the sea wave and tidal energy facilities are currently being deployed, the full environmental impacts of wave and tidal power remain uncertain but are projected to be small. Concerns include impacts on marine ecosystems and fisheries. Environmental impact studies are currently underway and several pilot and commercial projects are undergoing environmental monitoring. The East River tidal turbine pilot project includes a $1.5 million sonar system to monitor impacts on fish populations. Careful siting should minimize impacts on marine ecosystems, fishing and other coastal economic activities. Wave and tidal facilities also have little or no visual impact, as they are either submerged or do not rise very far above the waterline.

Advantages of Tidal Energy

• It is an inexhaustible source of energy.
• Tidal energy is environment friendly energy and doesn't produce greenhouse gases.
• As 71% of Earth’s surface is covered by water, there is scope to generate this energy on large scale.
• We can predict the rise and fall of tides as they follow cyclic fashion.
• Efficiency of tidal power is far greater as compared to coal, solar or wind energy. Its efficiency is around 80%.
• Although cost of construction of tidal power is high but maintenance costs are relatively low.
• Tidal Energy doesn’t require any kind of fuel to run.
• The life of tidal energy power plant is very long.
• The energy density of tidal energy is relatively higher than other renewable energy sources.
• Tides are totally predictable, enabling us to calculate when we can generate more, and at times when the generation is low, shift the load to some other source of electricity generation.
• Offshore turbines and vertical-axis turbines are not extremely expensive to build and do not have a large environmental impact. 

Disadvantages of Tidal Energy

• Cost of construction of tidal power plant is high.
• There are very few ideal locations for construction of plant and they too are localized to coastal regions only.
• Intensity of sea waves is unpredictable and there can be damage to power generation units.
• Influences aquatic life adversely and can disrupt migration of fish.
• The actual generation is for a short period of time. The tides only happen twice a day so electricity can be produced only for that time.
• Frozen sea, low or weak tides, straight shorelines, low tidal rise or fall are some of the obstructions.
• This technology is still not cost effective and more technological advancements are required to make it commercially viable.
• Usually the places where tidal energy is produced are far away from the places where it is consumed. This transmission is expensive and difficult. 
• Many birds rely on the tide uncovering the mud flats so that they can feed. Fish can’t migrate, unless “fish ladders” are installed
• There are only a few suitable sites for tidal barrages 

Tidal Energy is thus a clean source of energy and does no t require much land or other resources as in harnessing energy from other sources. However, the energy generated is not much as high and low tides occur only twice a day and continuous energy production is not possible.