THERMAL DESIGN CONSIDERATIONS

Definitions Substrate Design Guidelines Thermal Design Considerations Adhesives Solder Paste and Joint Formation... 

Electrical and Thermal Design Considerations 2.4.1 Electrical Design Tools... 

Design considerations and costs of the catalyst, hardware, and a fume control system are direcdy proportional to the oven exhaust volume. The size of the catalyst bed often ranges from 1.0 m at 0°C and 101 kPa per 1000 m /min of exhaust, to 2 m for 1000 m /min of exhaust. Catalyst performance at a number of can plant installations has been enhanced by proper maintenance. Annual analytical measurements show reduction of solvent hydrocarbons to be in excess of 90% for 3—6 years, the equivalent of 12,000 to 30,000 operating hours. When propane was the only available fuel, the catalyst cost was recovered by fuel savings (vs thermal incineration prior to the catalyst retrofit) in two to three months. In numerous cases the fuel savings paid for the catalyst in 6 to 12 months. 

In design considerations for Thermonized process lines, temperatures may be determined by the Stagnation Method. The calculations involved in this method are based on static conditions where process fluid flow is not present, and are independent of the viscosity, density and thermal conductivity of the process fluid. The process temperature may be calculated from the following relationship ... 

In addition to flow, thermal, and bed arrangements, an important design consideration is the amount of catalyst required (W), and its possible distribution over two or more stages. This is a measure of the size of the reactor. The depth (L) and diameter (D) of each stage must also be determined. In addition to the usual tools provided by kinetics, and material and energy balances, we must take into account matters peculiar to individual particles, collections of particles, and fluid-particle interactions, as well as any matters peculiar to the nature of the reaction, such as reversibility. Process design aspects of catalytic reactors are described by Lywood (1996). 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

Irrespective of whether an instrument employs fixed cells or batch cells, some design considerations are common to all HSDSC instruments. To measure accurately small powers, it is necessary to ensure good baseline stability throughout the course of an experiment. This is usually achieved by maintaining very accurate control of the calorimetric block temperature and ensuring that the properties of the sample and reference cells, such as geometry, cell volume, local environment, thermal conductivity and conduction pathways, and heating rate, are identical. 

Bell-Delaware Method. Pressure drop and heat transfer calculations (the step 6 of the above thermal design procedure) constitute the key part of design. Tubeside calculations are straightforward and should be executed using available correlations for internal forced convection. The shellside calculations, however, must take into consideration the effect of various leakage streams (A and E streams in Fig. 17.30) and bypass streams (C and F streams in Fig. 17.30) in addition to the main crossflow stream B through the tube bundle. Several methods have been in use over the years, but the most accurate method in the open literature is the above mentioned Bell-Delaware method. This approach is based primarily on limited experimental data. The set of correlations discussed next constitutes the core of the Bell-Delaware method. 

The thermal design and some aspects of the mechanical design of a shell-and-tube heat exchanger are empirically based, as discussed above. However, there are many criteria for mechanical selection [5], many experience-based criteria that can avoid or minimize operating problems [155], and other design considerations such as identification of thermodynamic irreversibilities [15, 110], thermoeconomic considerations [111], system optimization, and process integration [112], In industrial applications, thermoeconomic optimization should be... 

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