Etiket Arşivleri: DESIGN AND ECONOMICS
FOOD ENGINEERING DESIGN AND ECONOMICS
GENERAL CONSIDERATIONS IN PLANT DESIGN
GENERAL CONSIDERATIONS IN A PLANT DESIGN
3.Plant operation and control
9.Health and safety
The geographical location of the final plant can have a strong influence on the success of an industrial enterprise.
The plant should be located where the minimum cost of production and distribution can be obtained.
The choice of the final site should first be based on a complete survey of the advantages and disadvantages of various geographical areas.
An approximate idea for the plant location should be obtained before a design project reaches the detailed estimate stage.
●1.1. Raw Materials
The source of raw materials is one of the most important factors influencing the selection because location near the raw materials source leads to reduction in transportation and storage charges.
Attention should be given to the;
●purchased price of the raw materials
●distance from the source of supply
●freight or transportation expenses
●availability and reliability of supply
●purity of raw materials
The location of markets or intermediate distribution centers affects the cost of product distribution and the time required for shipping.
It should be noted that markets are needed for by-products and end products as well as for major final products.
●1.3. Energy Availability
Power and steam requirements are high in most industrial plants and the fuel is ordinarily required to supply these utilities. Consequently, power and fuel can be combined as one major factor in the choice of a plant site.
In addition, the presence and cost of electricity is an important consideration for plant location. In industrial areas the cost, voltage and availability of electricity is different than in living areas.
Excessive humidity or extremes of hot or cold weather can have a serious effect on economic operation of a plant and these factors should be examined when selecting a plant site.
●1.5. Transportation Facilities
The common means of transportation used by major industrial concerns are roads, highways, railroads and water. For selection careful attention should be given to “freight rates”.
In food industry, raw materials and food products are in huge amounts and not very durable. Therefore, transportation should be done with a great care and should be fast.
●1.6. Water Supply
The process industries use large quantities of water for cooling, washing, steam generation, immobilized conveying and as a raw material. Therefore, the plant must be located where a dependable supply of water is available.
Water sources can be tab water, rivers, lakes, deep wells and artesian wells. If own sources are to be used the level of existing water, seasonal fluctuations, chemical, bacteriological content and cost for supply and purification must be considered.
●1.7. Waste Disposal
The site selected for a plant should have adequate capacity and facilities for correct waste disposal. In recent years many legal restrictions have been placed on the methods for disposing of waste materials from the process industries. In choosing a palnt site, the permissible tolerance levels for various methods of waste disposal should be considered carefully and attention should be given to potential requirements for additional waste treatment facilities.
●1.8. Labor Supply
The type and supply of labor available in the vicinity of a proposed plant site must be examined. Consideration should be given to prevailing pay rates, restrictions on number of hours worked per week, competing industries that can cause dissatisfaction or high turnover rates among the workers, the ethnic distribution and variations in the skill and intelligence of workers.
●1.9. Taxation and Legal Restrictions
Tax rates, health insurance rates and property tax rates do not change depending on position in our country. However, being a governmental policy some places are promoted for the development (as reduced tax and interest rates). In industrial regions permissions to be taken are important in cost and time delays. For the abroad enterprises local tax rates and promotions should be considered.
●1.10. Site Characteristics
The characteristics of the land at a proposed plant site should be examined carefully (topography and soil structure).
The cost of land is important as well as local building costs and living conditions. Future changes may make it desirable or necessary to expand the plant facilities.
The buildings that are constructed as a result detailed land analysis, soil analysis and structural calculations are very resistant to aging as well as natural disasters like earthquakes.
●1.11 Flood and Fire Protection
Before choosing a plant site, the regional history of natural events like floods or hurricanes should be examined.
Protection from losses by fire is another important factor for selection of plant location. In case of a major fire, assistance from outside fire departments should also be available as well as fire protection systems.
●1.12. Community Factors
The character and facilities of a comunity can have effects on the location of the plant. Cultural facilities as schools, shops, mosques, cafeterias, kindergartens, cinemas are important for a progressive community. If these facilities are not present it becomes for the plant as a necessity to provide such facilities.
2. Plant Layout
●After the process flow diagrams are completed and before detailed piping, structural and electrical design can begin, the layout of process units in a plant and the equipment within these process units must be planned. This layout can play an important part in determining construction and manufacturing costs and thus must be planned carefully with attention being given to future problems that may arise.
●“there is no ideal plant layout”
●A proper layout in each case will include arrangement of processing areas, storage areas and handling areas in efficient coordination and with regard to the following factors:
1.new site development or addition to previously developed site
2.type and quantity of products to be produced
3.type of process and product control
4.operational convenience and accessibility
5.economic distribution of utilities and services
6.type of buildings and building code requirements
7.health and safety considerations
8.waste disposal problems
10.space available and space required
11.roads and railroads
12.possible future expansions.
●Preparation of the layout;
1.First, elementary layouts are prepared which shows the fundamental relationships between storage space and operating equipment (process).
2.The next step requires consideration of the operational sequence and gives a primary layout based on; flow of materials, unit operations, storage, future expansion, administrative parts, laboratories, sampling, change rooms, training rooms, first aid , etc.
3.Finally, by analyzing all the factors that are involved in plant layout, a detailed recommendation can be presented and drawings and elevations, including isometric drawings of the piping systems can be prepared.
●While preparing the layout, three dimensional models are often made. The main advantage of three dimensional models is the possibility of observing the problems that may be missing in two-dimensional drawings. Three-dimensional models are also beneficial for orientation after the plant is completed.
+ “empty area”
●Considerable attention has to be given to auxiliary departments and this should be done before the manufacturing space has been planned in too much detail. Time clocks, restrooms, washrooms and toilets should be located so that they are convenient and accessible to workers entering and leaving the plant.
●General offices of the company should be located so as to provide ready access to the public and freedom from noise of the factory. In many cases it is desirable to keep them adjacent to working areas for closer supervision.
●Engineering and factory offices in particular should be located adjacent to production areas. Some companies separate these offices by a glass partition to isolate sound but still keep a closer touch with the manufacturing areas. This also improves a closer feeling of unity between factory operating personnel and the supervisory force.
●Layout design to minimize non-microbial contamination;
●There should be enough empty space for de-boxing or de-palletizing of raw materials and screening. Similarly space for storage of packaging materials should be present.
●Congestion in areas of open food production makes cleaning and maintenance difficult to achieve without putting food products and other equipments at risk.
●There should be separate storage rooms for finished products. If they have direct contact with raw material dirt and cross-infestation or odor tainting may take place.
●Insufficient space for maintenance operations will result in many problems. Work benches in open production areas should never be permitted. Fitters forced to work in cramped, dirty conditions will find it difficult to conform to the required hygiene standards when working in production areas. Lack of storage space for temporarily disused equipment often results in it being kept in production areas. Such equipment is frequently infested because it is not being cleaned before and during storage.
●If equipment cleaning centers are located too far from production areas, there will be temptation to neglect cleaning schedules. An equal temptation will be for cleaning materials including concentrated detergents. The quantities of foodstuffs tainted by detergents should not be under-estimated.
●There should be enough equipped smoking/snack rooms adjacent to production areas. Otherwise, smoking near machinery or food consumption in unauthorized parts of the factory may take place.
●There should be short and direct routes for waste removal.
●Sometimes returned goods are collected in plant. Such goods are often infested and/or in a state of decomposition. Therefore, they must be isolated from all raw material and production areas.
●There should be;
●Adequate surface drainage to avoid ponding
●Provision of a surface that can be easily cleaned
●A correct sitting and construction of waste-collection areas
●A good housekeeping controls in nearby buildings
●A control for weed growth
●A correct sitting and control of effluent treatment
●Control of stocks of surplus equipment (wood, empty containers, etc.) which are often kept because “it might come in useful”.
3. Plant Operation and control
●In the design of an industrial plant, the methods which will be used for plant operation and control help for the determination of many of the design variables.
●It should be remembered that maintenance work will be necessary to keep the installed equipment and facilities in good operating condition.
Instruments are used in an industrial plant to measure process variables such as; temperature, pressure, density, viscosity, specific heat, conductivity, pH, humidity, liquid level, flow rate, chemical composition, moisture content, etc. By use of instruments having varying degrees of complexity, the values of these variables can be recorded continuously and controlled within narrow limits.
Automatic control is widely used with resulting savings in labor combined with improved ease and efficiency of operations. (which overcomes the added expense for instrumentation) This control is achieved through the use of high speed computers.
Maintenance work includes; repairs, equipment upgrading, testing, field adjustment, etc. Many of the problems involved in maintenance are caused by the original design and layout of the plant and the equipment.
In most cases the design engineer is concious only of first costs and fails to recognize that maintenance costs can easily overcome the advantages of a cheap initial installation.
Sufficient space for maintenance work on equipment and facilities must be provided in the plant layout and the engineer needs to consider maintenance requirements when making decisions on equipment.
●Utilities are the process supplements of an industrial plant as power, steam or water.
●The most common sources of energy are oil, gas, coal and nuclear energy. The decreasing availability of some sources will necessitate the use of alternative forms of energy.
●In production industry the required power is primarily in the form of electricity, other sources are steam engines, internal-combustion engines and hydraulic turbines.
●When a design engineer is setting up the specifications for a new plant, a decision must be based on whether to use purchased power or producing its own power. (if both exist continuous operation is achieved)
●The quantity of steam used in a process varies depending on thermal and mechanical requirements and should be generated from whatever fuel is cheapest.
●Water for industrial purposes can be obtained from the plant’s own sources or a municipal supply. If the demands for water are large, it is more economical for the plant to provide its own water sources. Such a supply may be obtained from drilled wells, rivers, lakes, dammed streams or other supplies.
●Before a company agrees to go ahead with a new project, it must ensure itself of a sufficient supply of water for all industrial, sanitary and safety demands both present and future.
●The value of an abundance of good water supplies is reflected in the selling price of plant locations. Treatment of water significantly increase the operational cost for a plant. Increased cost of water processing necessitates maximum yields for the use of processed water. In general, high cost for both processing and disposal of water lead to minimum amount of utilization for water.
5. Structural Design
●For a successful structural design, it is necessary to know the characteristics of the soil at a given plant site. The allowable bearing pressure varies for different types of soils and the soil should be checked at the surface and at various depths to determine the bearing characteristics.
●The purpose of foundation is to distribute the load so that excessive or damaging settling will not occur. (footing with plain concrete; foundation walls with reinforced concrete)
●Although cost is important for the selection of materials of construction, resistance to adverse effects and flexibility of construction for future changes and expansions are also important. Therefore, corrosive effects of the process, cost of construction, possible future changes and climacteric effects should be considered together.
●Floor design is worth considerable thought and high initial investment. Concrete floors are used extensively in the process industries and covering materials should be used for making the floor resistant to heat and chemical attacks and more important in food industry ease of cleaning and sanitizing. The disruption caused by defective floors may be very costly and piecemeal repair is difficult to achieve. Many floor defects arise from poor planning and preparation and not from the actual finishing material. Floorings which do not contain high-odor materials should be used.
●The number of partition walls should be kept to the minimum and used only to isolate operations which otherwise lead to the spread of contaminants. Too many separate rooms complicate inspection, control and prevents the achievement of uniform standards.
●Correct floor drainage is important in the control of insect infestation and odors. Covered channels present major cleaning problems when compared to open wall junction channels.
●Similarly, it is unnecessary and undesirable for equipment to drain directly on to the floor. This spreads dirt into inaccessible areas and is uncomfortable for operatives.
●Water based cleaning systems are very much restricted in dry production areas such as in bakeries because of lack of drainage. This requires the use of high pressure systems usually applied in inefficient manual methods which demand a greater level of supervision.
●The buildings are usually with flat roofs and special combinations are outer coating in order to reduce effect of seasonal conditions. The interior coating of the roofs is also important since water droplets from condensed vapor on roofs may cause to contamination.
●In conventional buildings ceilings are covered in a network of pipes, lights, cable trays, heaters and channels (ducting). (overhead)
●Maintenance work overhead may cause direct contamination to product or contamination of equipment and subsequently to product. This work should take place in out of work hours and production lines are covered but in practice due to emergencies this cannot be met.
●For powdered materials a difficulty in cleaning occurs when such materials settle on overheads. This cause subsequent mouse or insect activity which in turn endangers production lines.
●Overheads may also function as roosting for small birds where droppings on equipment and raw materials are completely undesirable.
●These problems can be prevented by providing a separate service floor which will house much of the channels (ducting), pipes etc. The result is a clear ceiling which is easy to clean and the removal of many activities, which could cause contamination to the separate service area. Further it does not restrict maintenance work to outside of working hours. This system is also flexible and allows direct feed to be maintained after production line layout changes.
●In the operation of a process plant adequate storage facilities for raw materials, intermediate products, final products, recycle materials, off-grade materials, fuels, cleaning agents, packaging materials and other items.
●Storage of raw materials permits operation of the process plant regardless of temporary supply of delivery difficulties.
●Storage of intermediate products may be necessary during plant shutdown for emergency repairs. this is not practical for food systems since they are sensitive to contamination)
●Storage of products makes it possible to supply the customer even during a plant difficulty or unforeseen shutdown.
●An additional need for adequate storage is encountered when it is necessary to meet seasonal fluctuations.
●Depending on the physical and chemical properties of the materials storage conditions should be determined.
●Bulk storage of liquids is generally handled by closed spherical or cylindrical tanks to prevent escape of volatiles and minimize contamination.
●High-pressure gas is stored in spherical or horizontal cylindrical pressure vessels.
●Solid products and raw materials are either stored in air-tight tanks with sloping floors or in outdoor bins or mounds. Solid products are mostly packed directly on retail packages.
●in all storage areas;
●there must be a control unit (for controls and recording)
●there should be necessary warnings (on doors, walls, instruments etc.)
●ambient or cooled temperatures should be selected depending on the properties of raw material and the products
●depending on amount of material required storage area should be determined
●if long periods of storage are required coding systems should be used to supply equal storage time for all materials.
●if cold storage is to be used, to supply economic utilization of energy, storage area can be divided into parts so that for each loading or unloading temperature of whole area is not raised. For this purpose, rooms may be formed by plastic curtains. by this way input and output of materials could be done without energy losses.
●in storage areas raw materials and products should be stored separately.
●a constant temperature in storage area should be supplied with suitable wall clearance and stack height.
●storage should be made on transportation means to prevent contact with basement.
●there must be enough place to be walked for control purposes. these controls are especially important for ambient temperature since spoilage may take place more often.
7. Materials Handling
●Materials handling equipment used for transportation of solids, liquids and gases are divided into two main groups as continuous or batch.
●Liquids and gases are handled by means of pumps and blowers in pipes and ducts and in containers such as drums or cylinders. (hydraulic conveying)
●Solids may be handled by conveyors, elevators, lift trucks and pneumatic systems.
§Gravity or manually powered conveyors
§Powered conveyors as; roller conveyors, belt conveyor, slat conveyors, chain conveyors, vibratory conveyors, magnetic conveyors, screw conveyors and flight conveyors.
§Package elevators and bulk elevators
§Trucks with low and high vertical lifts
§Pneumatic equipment where air is used to reduce solid-solid friction, either ‘air-cushion’ principle or solid fluidization or solid suspension.
●The selection of materials handling equipment depends upon the cost and the work to be done.
●Factors that must be considered in selecting such equipment include:
1.Chemical and physical nature of material being handled
2.Type and distance of movement of material
3.Quantity of material moved per unit time
4.Nature of feed and discharge from materials handling equipment
5.Continuous and intermittent nature of materials handling
Depending on movement of raw materials and products outside of the plant, some type of receiving and shipping facilities must be provided in the design of the plant. In those facilities cleaning and sanitation units should also be present.
Safety considerations should involve unsafe conditions (insufficient working space, inadequate aisle space, inadequate guarding of running machinery, defective equipment, inadequate lighting and ventilation, unsafe design or construction of equipment and bad floor surfaces) and unsafe acts (unsafe loading and stacking, disregard of traffic signals, carrying out repairs and adjustments on the run, operating without authority, working at unsafe speeds, using incorrect equipment, exceeding the capacity of equipment, failing to use protective clothing and practical joking)
8. Waste Disposal
●Waste from an industrial plant in form of gas, liquid or solid cause to pollution. In order to control this pollution several factors should be evaluated as;
●pollution source (pollutants and the total volume dispersed)
●properties of pollution emissions
●design of the collection and transfer systems
othe size of the equipment is directly related to the volume being treated, therefore exhaust volume should be reduced to decrease equipment cost. If a reduction in pollution source can be obtained process or raw materials can be changed.
●selection of the control device
oselection of the most appropriate control device requires consideration of the pollutant being handled and the features of the control device.
●dispersion of the exhaust to meet applicable regulations.
i. Air Pollution Abatement: Air pollution control equipments can be classified into two major categories, those suitable for removing particulates and those associated with removing gaseous pollutants.
To obtain the greatest efficiency in particulate removal, particular attention must be given to particle diameter and the air velocity.
●Large diameter particles can be removed with low energy devices such as settling chambers, cyclones and spray chambers.
●Intermediate particles can be removed with impingement separators or low energy wet collectors.
●Submicron particles must be removed with high energy units such as bag filters or electrostatic precipitators.
●Noxious Gas removal
Gaseous pollutants can be removed from air streams either by absorption, adsorption, condensation or incineration.
●Condensation, is a method for removing a solvent vapor from air or other gas if the concentration of the solvent in the gas is high and the solvent is worth of recovery. Since condensation can not remove all of the solvent, it can only be used to reduce the solvent concentration in the carrier gas.
●Gas liquid absorption processes are normally carried out in vertical, countercurrent flow through packed, plate or spray towers. For absorption of gaseous streams good liquid-gas contact is essential, therefore proper equipment selection is important.
●Adsorption is generally carried out in large, horizontal fixed beds often equipped with blowers, condensers, separators and controls. Dry adsorbents like activated carbon and molecular sieves are used in removing final traces of objectionable gaseous pollutants.
●Incineration is the simplest way when polluting gas has no value and combustible. There are two methods in common use direct flame and catalytic oxidations.
ii. Water pollution Abatement: since waste liquid may contain dissolved gases or solids or it may be in a form of slurry, physical, chemical or biological treatment methods can be used.
●The problems of handling a liquid waste effluent are considerably more complex than those of handling a waste gas or solid effluent.
●For applicable situations, recovery for reuse or sale should be investigated.
●If product recovery is not capable of solving the problem, the design engineer should decide which treatment, process or combination of processes will give the best performance.
The first step in any waste water treatment process is to remove large floating or suspended particles. The size of solids is wide and several separation methods are used. A common method involves the separation of coarse material using screens. (bar screens, vibrating screens, rotary drum screens etc) Screen apertures range from 25 mm down to micrometer sizes depending on the application.
This is usually followed by sedimentation or gravity settling. Sedimentation takes place in large open ponds if sufficient land area is available. Otherwise gravity sedimentation tanks are used. Entering to those cylindrical vessels the liquid stream slowly rises to the top of the tank to be removed via an overflow launder as a clarified liquid stream. Denser solids settle to the bottom as a thick sludge underflow. Slow speed scraper blades help compact the sludge and drive it to the center off-take pipe for removal. Usually residence times in these units are insufficient for anaerobic decomposition to occur.