Pma

Axial End-over-end Circular rotation rotation rotation

Figure 2. Some common types of can rotation. Source: Ref. 12.

dicular to its own (Fig. 2). The axis of rotation is also located externally to the container. A positive movement of the headspace bubble is created throughout the rotating cycle. Because the headspace is moving along the length of the can and reverses direction every half a rotation, excellent mixing of the contents is achieved. In another rotating method of agitation (12) the cans are rotated in a circular path (Fig. 2) in such a way that the can orientation remains fixed. Agitation forces similar to those in EOE agitation are produced in this system.

The heating medium generally used is steam. Heating in direct flame with agitation of the cans is also used (14,15). The flame temperatures can be close to 2,000°C, and this high temperature is the primary contributor to rapid heat transfer.

Flow and Temperature Profiles

The complex nature of the agitation provided by the equipment discussed previously has generally defied any theoretical analysis to obtain detailed temperature and flow profiles. The temperature and flow patterns are affected by the headspace, fill of the container, solid-to-liquid ratio when solid particles are present, consistency of the liquid, and speed and type of agitation (11). There is only one theoretical study of liquid flow and heat transfer in a container that incorporates some agitations of the container similar to the sterilmatic-type retort discussed previously (16). The effects of container rotation about its own axis were considered to have the dominant effect on heat transfer and liquid flow over simultaneous rotation about an axis parallel to its own. Thus only container axial rotation was considered. Faster heating rates were observed in presence of rotation (centrifugally driven flows).

Experimental studies have measured temperature at several locations and correlated the heat transfer coefficient with other parameters. Nusselt-Prandtl correlations for obtaining heat transfer coefficient in axially rotated cans in a steam retort using water and silicone oil as model liquids have been provided (17). The temperature distribution in direct-flame heating of axially rotating cans was very uniform (15). Transient temperatures in an axially rotating can heated in a steam retort was shown to be quite uniform (Fig. 3). Temperature measurements in an endover-end rotating can were also similar (Fig. 4) and showed uniformity, which further improved when the direction of rotation was reversed at intervals. It has been noted that the heat transfer coefficient was much higher in EOE as compared to axial rotation (18). It was also noted that the mere presence of a minimal-size headspace in EOE rotation markedly increases the heat transfer coefficient, although its contribution becomes progressively smaller (Fig. 5).

Due to the complexities of a theoretical or an experimental study, and the presence of fairly uniform temperatures inside the agitated container, detailed spatial variations of temperature and velocities are often bypassed. Instead, the energy balance in equation 9 is based on T being the mean fluid temperature. Using this formulation, overall heat transfer coefficient, U, is made available from experiment often in the form of standard Nusselt-Prandtl

2 3 4 5 Time (min)

Figure 3. Temperature distribution at radial locations in an axially rotating horizontal can. Source: Ref. 18.

Figure 3. Temperature distribution at radial locations in an axially rotating horizontal can. Source: Ref. 18.

number correlations. The coefficient U depends on a number of processing parameters. For example, in EOE agitation, increasing the retort temperature increased the overall heat transfer coefficient, U, as shown in Figure 6, probably by lowering the viscosity of the fluid. Rotational speed also increased the heat transfer coefficient (Fig. 7), due to enhanced mixing. Figure 7 also shows the expected reduction in overall heat transfer coefficient for a more viscous fluid (oil).

The solution to equation 9 when starting from a uniform initial temperature of T0 is

T - Tm To - Tm which can also be written in the form loi j(To - TJ.

where j = 1 and/- = mcp/2.303UA. Thus a plot oflog((T -Tm)/(T„ — TJ) versus t would be a straight line charac-

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