Contact heat transfer coefficient

In equation (3), p, Cp and A are thermophysical properties linked to the nature of the feedstock. Thus, the main parameters which govern the reactor design are the surface-to-particle contact heat transfer coefficient, and the bottom layer renewal eiUciency, fi. Since these two parameters are directly influenced by the reactor design, they allow a relationship to be established between the reactor design and the overall heat transfer throughout the bed of particles. 

The heat transfer coefficient in the batch rotative reactor has been determined both theoretically and experimentally. As indicated by the heat transfer model (Eq.1-3), the heat transfer coefficient between the reactor and the feedstock is a function of the thermophysical properties of the feedstock Cp, p and X, the contact heat transfer coefficient and the bottom layer renewal efficiency fi. When the feedstock material is known, the thermal properties and can be determined [5]. The heat transfer coefficient becomes a function of and t u. 

The temperature curve has a kink at the interface. The temperature gradient in the body with the lower thermal conductivity is larger, Fig. 2.3 a. Equation (2.21) is only valid if the two bodies are firmly joined. If this is not the case a contact resistance occurs, which results in a small temperature jump, Fig. 2.3 b. This resistance can be described by a contact heat transfer coefficient act. In place of equation (2.21),... 

To estimate the value of the contact heat transfer coefficient, hf, the following simplified equation is used (Schliinder, 1984) ... 

The ratio of the overall heat transfer coefficient of the wet bed to the contact heat transfer coefficient is determined from... 

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

Natural convection occurs when a solid surface is in contact with a fluid of different temperature from the surface. Density differences provide the body force required to move the flmd. Theoretical analyses of natural convection require the simultaneous solution of the coupled equations of motion and energy. Details of theoretical studies are available in several general references (Brown and Marco, Introduction to Heat Transfer, 3d ed., McGraw-HiU, New York, 1958 and Jakob, Heat Transfer, Wiley, New York, vol. 1, 1949 vol. 2, 1957) but have generally been applied successfully to the simple case of a vertical plate. Solution of the motion and energy equations gives temperature and velocity fields from which heat-transfer coefficients may be derived. The general type of equation obtained is the so-called Nusselt equation hL I L p gp At cjl... 

Sodium chloride, an ordinaiy salt (NaCT), is the least expensive per volume of any brine available. It can be used in contact with food and in open systems because of its low toxicity. Heat transfer coefficients are relatively high. However, its drawbacks are it has a relatively high freezing point and is highly corrosive (requires inhibitors thus must Be checked on a regular schedule). 

One manner in which size may be computed, for estimating purposes, is by employing a volumetric heat-transfer concept as used for rotary diyers. It it is assumed that contacting efficiency is in the same order as that provided by efficient lifters in a rotaiy dryer and that the velocity difference between gas and solids controls, Eq. (12-52) may be employed to estimate a volumetric heat-transfer coefficient. By assuming a duct diameter of 0.3 m (D) and a gas velocity of 23 m/s, if the solids velocity is taken as 80 percent of this speed, the velocity difference between the two would be 4.6 m/s. If the exit gas has a density of 1 kg/m, the relative mass flow rate of the gas G becomes 4.8 kg/(s m the volumetric heat-transfer coefficient is 2235 J/(m s K). This is not far different from many coefficients found in commercial installations however, it is usually not possible to predict accurately the acdual difference in velocity between gas and soRds. Furthermore, the coefficient is influenced by the sohds-to-gas loading and particle size, which control the total solids surface exposed to the gas. Therefore, the figure given is only an approximation. 

This type of exchanger usually provides relatively high heat transfer coefficients and does allow good cleaning by mechanically separating the plates, if back-flushing does not provide the needed cleanup. An excellent discussion on the performance and capabilities is presented by Carlson. To obtain a proper design for a specific application, it is necessary to contact the several manufacturers to obtain their recommendations, because the surfece area of these units is proprietary to the manufacturer. 

Figures 10-101, lO-lOJ, and 10-1 OK indicate the process flow patterns for single tube units and for multiple corrugated tubes in a single plain shell. These units are suitable for heating or cooling process fluids containing high pulp or fiber content or suspended particulates. The heat transfer coefficients are improved when compared to plain tubes as the turbulence improves the performance. The units can be arranged in multiple shells for parallel or series flow. The manufacturers should be contacted for details.

If the heat is being transmitted through a number of media in series, the overall heat transfer coefficient may be broken down into individual coefficients h each relating to a single medium. This is as shown in Figure 9.1. It is assumed that there is good contact between each pair of elements so that the temperature is the same on the two sides of each junction. 

For flow at a given rate, the only way to significantly increase the heat transfer coefficient is to reduce the channel size, whose optimum can be calculated assuming a practical limit on the available pressure. Recourse to multiple channels, instead of continuous coolant flow over the entire back substrate surface, enables one to multiply the substrate area by a factor (jp, representing the total surface area of the channel walls which are in contact with fluid. Single-row micro-channels etched dir-... 

It may be used, the relation of the time-averaged heat transfer coefficients on the top and bottom, as a criterion for determination of dryout. It was assumed that the relation he/hi < 1 indicates dryout, i.e., the surface superheat Tw -7f is greater than that, when the surface contacts single-phase water only (hg is the heat transfer at the bottom of the channel). This method can be applied to connect dryout with hydraulic conditions, if the value of he may be associated with intermittent flow parameters. 

This section is concerned with the UA xtiT — Text) term in the energy balance for a stirred tank. The usual and simplest case is heat transfer from a jacket. Then A xt refers to the inside surface area of the tank that is jacketed on the outside and in contact with the fluid on the inside. The temperature difference, T - Text, is between the bulk fluid in the tank and the heat transfer medium in the jacket. The overall heat transfer coefficient includes the usual contributions from wall resistance and jacket-side coefficient, but the inside coefficient is normally limiting. A correlation applicable to turbine, paddle, and propeller agitators is... 

The vial heat transfer coefficient is the sum of heat transfer coefficients for three parallel heat transfer mechanisms (1) direct conduction between glass and shelf surface at the few points of actual physical contact, Kc (2) radiation heat exchange, Kr, which has contributions from the shelf above the vial array to the top of the vials, Krt, and from the shelf upon which the vial is resting, Krb and (3) conduction via gas-surface collisions between the gas and the two surfaces, shelf and vial bottom, Kg ... 

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