Convective Mixers

See also Convective heat-transfer coefficient Convection mixers, 16 719 Convective heat-transfer coefficient increase in, 13 276 values of, 13 245t... 

Convection mixers use a different principle for blending. These mixers have an impeller. This class includes ribbon blenders, orbiting screw blenders, vertical and horizontal high-intensity mixers, as well as diffusion blenders with an intensifier bar. Scale-up considerations are similar to those for the tumble blenders. 

Convection Mixers Ribbon Blenders Aaron Paul O. Abbe Automatic Industry Machines Azo-Ruberg Custom Metal Craft Jaygo Kemutec Lowe Pneuvac Ross Vrieco-Nauta (Hosokawa)... 

The choice of mixers within the convective group is much wider. In most convective mixer designs an impeller operates within a static shell and groups... 

The Z-blade mixer is another twin trough mixer and is commonly used for pastes as well as powders (Figure 3.4). The polished Z-blades make this one of the easier convective mixers to clean between operations. 

Aeration of the mixture takes place over a period of 15 to 30 seconds and a similar period is allowed for the particles to re-settle. The length and frequency of the cycles can be varied by the control valve setting. The high capital cost of the unit compared with the equivalent capacity tumbler or convective mixer is compensated bythevery rapid mixing time. Typically, the IS minutes required to reach equilibrium mixture quality in the tumbler and convective mixers is reduced to 1 to 2 minutes in the Airmix, thus enabling a smaller capacity unit to be used. 

Low-speed convective mixers with complex impeller arrangements present cleaning problems and the whole class of convective mixers potentially have lubricated surfaces in contact with the mixture. Closed-circuit hopper mixers and fluidized mixers are not generally suitable if frequent changes product specification require a complete flushing of the circuit, and gas-circulated systems often have the problem of lubricant entrainment in the gas stream. 

Enclosed mixing systems such as the tumbler mixers cannot be readily adapted to continuous operation, whilst open systems such as the convective mixers and the fluidized mixers are generally favoured. The design criteria for a continuous mixer are quite different to those for a batch mixer. Depending on the control system used for the feed streams to the mixer it could be desirable that a substantial amount of backmixing occurs within the mixer. A fluidized bed could backmix most effectively whilst the performance of a ribbon blender would be a function of the ribbon geometry. 

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