Solution Manual For Power Plant Engineering By P K Nag
Chiquita Mcnicholas
Productivity within industrial facilities is calculated through several key performance indicators (KPI). These key performance indexes include the overall equipment effectiveness (OEE) ratio, machine utilization, and throughput. Implementing automated material handling systems can optimize the majority of the KPIs used to track productivity, as the following examples will show.
The increased adoption rate of Industry 4.0 business models means the industrial sector continues to recognize the benefits of industrial automation and optimizing industrial processes. Automated material handling systems can enhance productivity in the following ways:
Shop floor equipment does not operate in a vacuum. Multiple interrelated operations occur to ensure the machine gets the tool heads, materials and information it is expected to work with. Inadequate material handling negatively affects the production flow and how quickly materials get to workstations, reducing machine utilization durations.
To eliminate the downtime caused by inadequate material flow or handling, automating material handling systems reduces human errors and eliminates the shop traffic that stalls material delivery. Getting materials and tools to the correct workstations ensure that equipment works optimally through their work cycles and operators have the tools required to produce components.
The total cost of production or warehousing includes the power expended to move materials across the shop floor and the energy material handling systems consume. For manual carts, power consumption is minimal; however, increased shop floor traffic can cause accidents or delivery delays, which adds to the total production cost. Forklifts are also responsible for most safety incidents on the shop floor. Furthermore, they consume a considerable amount of energy.
Automated material handling systems such as automated guided vehicles are industrial automation solutions that can carry heavy loads, unlike manual carts, and use less power compared to forklifts. Automated material handling systems also reduce the labor cost associated with manually moving materials across the shop floor. According to the Bureau of Labor Statistics, order pickers and material handling equipment operators are paid approximately $20 per hour. Despite the initial expenditure on purchasing an AGV, an AGV working three shifts will be more serviceable than paying 3 operators working full-time.
The concept of lean manufacturing is to reduce waste at every phase of the production cycle, which leads to increased throughput. The implementation of industrial automation to automate material handling leads to a more efficient material handling process. One example is the ability of automated material handling systems to accurately follow operational schedules and ensuring materials arrive on time.
Implementing a just-in-time material transportation process reduces queuing on the shop floor and the amount spent on waste. An optimized material handling process reduces waste in industries that handle raw materials with limited shelf lives. For example, the food and beverage industry rely on raw materials with short shelf lives that can reduce waste through an automated material handling system. The stability automation brings ensures proper handling, which limits scoffing or damage when handling sensitive materials.
Factory floor traffic caused by the transportation of materials and tools across the shop floor causes unplanned downtime as materials take longer to reach their destination. The implementation of an automated material handling system drastically reduces shop floor traffic as heavy equipment follows a defined pathway across the factory floor. Providing operators and equipment with the tools they work with on time leads to increased productivity.
The manufacturing industry is going through a transitional phase as older employees retire. The younger generation expected to fill the shoes of older employees does not find jobs in the industry attractive enough to build careers. This has led to a worker shortage that must be remedied for factories to improve productivity levels.
Industrial automation is a solution to reducing the effects of an aging workforce and shortages. Automating the material handling process eliminates the need for transport operators and frees up the time of the available operators. Operators can then spend the freed-up hours on other crucial activities on the factory floor.
Automated material handling systems support flexible manufacturing systems because of the scalability they offer. Expanding operations require a scalable material handling system to meet the new production capacity of the expanded facility. Automated material handling systems such as AGVs are easy to scale up, compared to the application of fixed material handling systems within the shop floor.
Adding to the problem is the fact that steam systems are typically not designed to eliminate air at startup or during operation. During shutdown of a steam system or its components, the system depressurizes, with the steam condensing and reducing in volume by as much as 1,600 times. This reduction in volume produces a vacuum in the steam system or steam components. Air is drawn into the steam system through steam components, such as air vents, valve packing, and flanges, and the air drawn in fills the vacuum. When energizing a steam system or steam heat transfer components, one of the first goals should be to vent the air out of the steam system or components.
Feedwater contains a small percentage of non-condensable gases in solution. When the boiler water changes state (liquid to vapor), the non-condensable gases are released and carried with the steam into the plant. Steam will release the latent energy to the process and condense down to condensate in the heat transfer area, but the non-condensable gases do not condense. These gases stay in the heat transfer component unless some method or action removes them.
When steam is shut down to the heat transfer unit or steam supply line for maintenance or process changes, the steam will condense and decrease in volume, which will open the vacuum breakers on the steam heat transfer unit, allowing air to flow into the system. On heat transfer units, it is extremely important to have functional vacuum breakers on the heat transfer units to enable the condensate to drain out of the unit by gravity. If vacuum breakers are not on the heat transfer unit, the vacuum will hold the condensate in the unit and cause another set of issues. A steam line that does not have vacuum breakers, will draw air in from components on the system (such as valve packing or flanges), and fill the void in the line.
The release of latent energy (change of state) to condensate in the steam components takes place on the heat transfer surface, which is where heat is being transferred due to the temperature difference (steam to the process). The steam component transfer is consuming the latent energy, and the steam is condensing to a liquid (condensate); the condensate is drained away by gravity, but the non-condensable gases and air remain.
The non-condensable gases form a stagnant film on the walls of the heat transfer surface, which creates a resistance. Heat energy transmitting through the heat transfer surface has to pass by conduction through these films of resistance. A film of air or non-condensable gases that is only one thousandth of an inch thick has the resistance of a three-inch wall of iron.
There are other steam system issues with the concentration of air and non-condensable gases. The buildup or volume of air and non-condensable gases in the heat transfer area is not constant. The thickness of a stagnant film of air can vary due to velocities, baffles, flow direction, metal finish, and other heat transfer internal designs. This factor can also lead to problems with uneven heating of products.
The air-venting device typically includes a thermostatic balance pressure bellows unit with a very low sub-cool. The device is able to sense air or non-condensable gases due to the resulting temperature suppression.
Steam traps are never considered to be primary air venting mechanisms due to the methods incorporated into their design to accomplish this task. Therefore, steam traps are always considered secondary air vent mechanisms. There are two methods to vent air in any steam trap: a leak path and a thermostatic mechanism.
The steam trap design has a leak path incorporated into the operational design. The steam trap leak path is very small to ensure no significant steam loss occurs during operation. Due to the small leak path, the steam trap is not able to provide sufficient air venting capabilities.
The other method is to use a thermostatic element inside of the steam trap that can offer a high capacity of venting air at startup due to the orifice size. In process applications, the preferred steam trap is a float and thermostatic steam trap, which incorporates a thermostatic air venting mechanism. Plants often employ one or more of the above items to remove the air from the steam system during startup or operation.
When steam lines are activated or started up, one of the main tasks is to remove the non-condensable gases. During startup, the drain valve off the steam line drip pocket is opened, venting air from the steam line and removing condensate. In some cases, a second manual air-venting valve is installed on top of the steam line to ensure removal of air from the system. See Figure 4, below.
A key factor in the location of air vents on process equipment is to understand the design of the unit. For example, a shell and tube heat exchanger has a port typically on the top of the shell for the placement of a vacuum breaker and air vent. The process steam side needs to have the air purged to ensure proper startup and temperature control. See Figure 5, below.
For process application steam traps, the preferred steam trap design is the float and thermostatic, which incorporates a thermostatic air vent mechanism. The steam trap thermostatic air vent mechanism becomes the secondary air-venting device.
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