CARBON FIBER RECYCLER: OVEN HEATING SYSTEM
ANALYSIS
The first set of analyses takes the previous oven system’s dimensions and specifications for calculations to determine the anticipated outcome that the former project members designed. The latter half of the analyses use similar engineering methods to predict what adding and replacing oven parts will produce. A variety of engineering principles such as Laws of Thermodynamics, Heat Transfer processes, and Basic Electricity were applied to dissect and pin-point the potential errors and failures occurring with the previous JCATI carbon fiber recycler and showcases the changes that were made to achieve successful recycling through pyrolysis.
Design Requirements
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The Oven Heating System must be powered with 110 - 240VAC.
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The oven temperature must maintain 500°C ± 5 °C during the 30-minute pyrolysis process.
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The oven must reach target temperature within 15 minutes.
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The cartridge heater mount must support a 1.25-lb. load.
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The oven’s outer surface must not exceed 45 °C during pyrolysis.
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The system must not add more than 3 feet to the existing envelop (3 x 3 x 4.5 feet).
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The pyrolysis process must remove 90% of resin from the composite material.
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The purging gas must fill the entire oven in 15 minutes during preheating.
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The oven must have a minimum 1 ft x 1 ft means of entry to access the heating system for maintenance.
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The oven must have legs to support a 250-lb load.
Analysis 1
Oven Dimensions for Heat Production
The objective of the first analysis was to determine the maximum oven dimensions that the heating element should be able to supply. The maximum volume of air allowed for the system to rise to pyrolysis temperatures in 15 minutes, with a power supply of 208 volts and heating element at 2000 watts, was found to be 5.4 cubic meters. This means a length extension up to 20.8 meters is possible for target temperatures fulfilling design requirements 2 and 3.
Analysis 2
Reaction Forces on Cartridge Heater
In the second analysis, the reaction forces at each end of the heating cartridge due to the force of gravity were determined. This correlates to design requirement 4 - the cartridge heater mount must support a 1.25-pound load. Designing the rod to be fixed at both ends allows better heat distribution as opposed to having supports closer to the center. Each end of the 10-inch cartridge heater reacts with a 0.625-pound force upward in the vertical axis.
Analysis 3
Electrical Schematic
The objective of analysis 3 was to provide an electrical schematic that will wire the oven heating system. This schematic is helpful for future investigation of the circuit and to confirm the components involved in the electrical part of this system were compatible. It is also related to design criteria 1 – the entire system will be powered with 110 – 240V AC. The components included in the schematic are the power source, switch & light, fuses, two Solid State Relays, two heating elements, a temperature control panel, fan motor, and thermocouple.
Analysis 4
Oven Outer Surface Temperature
The design criteria that analysis 4 aimed to meet was an outer oven surface temperature criteria of 45°C, requirement 5. Using heat transfer principals, it was determined that both convection and conduction were involved in this heating system. After analyzing with an internal temperature of 500°C, the external surface temperature resulted in 28.06°C.
Analysis 5
Forced Convection Power
Analysis 5 worked to solve for the power needed to supply forced convection by a fan. This analysis aims to support the idea to add a fan to the heating system for a more efficient pyrolysis process while attempting to stay within the constraints of design requirement 1 - the Oven Heating System must be powered with 110 - 240VAC. The same Heat Transfer properties and relations from Analysis 4 were used with the heat transfer coefficient inside the oven being increased to 1000 W/m2K. The power was determined to be 150 W, which is less than 1 watt greater than if the system was ran with natural convection (no fan). Since the power addition is minimal, this means the power source should be able to supply this fan on top of the other components.
Analysis 6
Volume Flow Rate
The objective of the sixth analysis was to determine the volume flow rate of argon gas which fulfills design requirement 6 - the system must not exceed dimensions 3 x 3 x 4.5 feet. Therefore, the argon gas flow rate must fully occupy this space in the necessary time. Using the equation for volume flow rate, to fill an unoccupied oven space of 10.5 ft3, the gas would need to flow at 42 ft3 per hour (cfh) to meet criteria. This information is important in determining the required pressure to release the purging gas at in order to fill the oven at the same time that it reaches target temperatures.
Analysis 7
Pressure at Hose Inlet
The intent of this analysis was to determine the pressure needed for the purging gas flow rate from the gas hose to match the flow rate required to fill the oven within the desired preheating time limit. This analysis also aimed to meet design criteria 8 which requires the argon purging gas to replace all fluid from the oven. Using Bernoulli’s equation, the design parameter for this analysis resulted in a required pressure of 302.8 pascals from the 3/16” gas hose to the hose fitting.
Analysis 8
Pressure Difference
Analysis 8 was intended to determine the pressure difference of the purging gas to the pipe nipple in which transports the gas into the oven heating system from the hose fitting. This analysis also aimed to meet design criteria 8 which requires the argon purging gas to replace all fluid from the oven. By using Bernoulli’s equation once again, the design parameter for this analysis resulted in a pressure decrease of 21.05 pascals at the pipe nipple. Therefore, the pressure entering the oven where pyrolysis will occur is 282 pascals. This criterion is important to ensure pyrolysis system factors are prepared within 15 minutes of start-up in the heating system.
Analysis 9
Diameter of Hinge Pin
The purpose of Analysis 9 was to determine the diameter of the hinge pin that secures the oven back door. The analysis shows how Mechanics of Materials methods utilize shear stress properties on the hinge to solve for the pin’s area. The design parameter produced from this analysis based on a shear force of 7.2 lbs. is a minimum hinge pin diameter of 0.025 inches. This relates to design requirement 9 - the oven must have a 1 ft x 1ft minimum means of entry to access the heating system for maintenance. The oven back door design has dimensions slightly greater than 2 ft x 1.5 ft which meet the design criteria. Determining the pin diameter ensures that the purchased hinge will be able to hold a door of that size and weight.
Analysis 10
Maximum Deflection on Door
All parts related to the oven back door must be analyzed in-depth for verification of compatibility. Analysis 10 serves to determine the deflection of the oven door based on the maximum release force that the door latch can handle. This also relates to design requirement 9. By using beam deflection formulas from Mechanics of Materials methods, the maximum deflection resulted in 0.0000032 inches. Since the design parameter is extremely small, this means the door will withstand the pulling force with almost no deformation. This means there is no need for a safety factor greater than 1.
Analysis 11
Reaction Forces on Fan Motor
The objective of Analysis 11 was to determine the reaction forces on the fan motor. The fan motor is compatible with a power source of 230VAC which meets design requirement 1. Since the motor will sit adjacent to and outside of the oven side wall, it will need to be properly supported. The fan-motor kit includes H-type mounting brackets. This analysis shows how free body diagrams and equilibrium equations can be used to find the weight that each mounting bracket will need to hold to support the motor. The design parameters produced reaction forces in the horizontal axis of 0.625 pounds and 1.0 pound in the vertical axis. Since these weights are relatively light, and the brackets come with the motor, it is with confidence that they should be able to support the weight.
Analysis 12
Friction Force on Oven Legs
The final analysis served to determine the pushing force that each oven leg can withstand to maintain a static body. Design criteria 10 requires the oven weight to be at a maximum of 250-lbs. This calculation analyzes the oven weight from a range of 100-200 pounds. Using Statics properties, the friction force resulted in a range of 18.5 – 37 pounds. Assuming a lighter oven weight allows a safety factor of 2.5. Therefore, the oven should be able to combat 18.5 pounds of force at minimum before slipping.