Conventional jackets

P). Otherwise the baffles should be located iaside the cod helix. A conventional jacket consists of a vessel outside the main vessel with a gap for the flow of heat-transfer fluid. Half-pipe jackets are usefld for high pressures up to 4 MPa (600 psi). They are better for Hquid than for vapor service fluids and can be easdy 2oned. Dimple jackets are suitable for larger vessels and process conditions up to 2 MPa (300 psi) and 370°C. Internal cods can be either hehcal or baffle cods (Fig. 34). 

The pitch of the coils and the area covered can be selected to provide the heat transfer area required. Standard pipe sizes from 60 imn to 120 mm outside diameter area are often used. Half-pipe construction can produce a jacket capable of withstanding a higher pressure than conventional jacket design. 

Dimpled jackets are similar to the conventional jackets but are constructed of thinner plates. The jacket is strengthened by a regular pattern of hemispherical dimples pressed into the plate and welded to the vessel wall, Figure 2.12b. 

Figure 7.3 compares calculated overall heat-transfer coefficients for several reactor heat exchangers, using water for both the jacket and reactor fluid. The figure shows that the highest heat-transfer coefficient is obtained with internal coils and the lowest with the simple jacket (called the conventional jacket in Figure 7.3) without a spiral baffle or agitation. It is assumed that the flow rate for the internal coil is the coil flow rate and not the jacket flow rate, as plotted in Figure 7.3. Heat-transfer coefficients for the half-pipe coil, agitated, and baffled jackets are conparable.

Small tubes are commonly employed where the reaction is rapid and/or the heat of reaction must be removed rapidly. The two conventional types of tubular reactors are (1) coils immersed in a constant-temperature bath and (2) a jacketed pipeline in which the inner tube is designed to withstand the reaction pressure. A modification of the conventional jacketed-pipe reactor can be used where it is desirable to minimize the thickness of the inner tube in order to reduce the area required for heat transfer. An example of this type of equipment is the liquid-phase heat exchanger of the Bureau of Mines, in which the outside pipe has an outside diameter of 4 2 ill- wall thickness of 1.005 in. The outside diameter of the inner tube is in., but the wall thickness is only 0.16 in. The worldng... 

For conventional jacketted kettle. t For flat stainless steel panels. 

Used widi steam or coofing. Liquid flow velocities are low and the flow is poorly distributed. Natural convection equations are suitable and cooling coefficients have low values. Conventional jackets are best applied to small vessels or high pressure applications, where flie vessel internal pressure is twice the jacket pressure as a minimum. The conventional jacket is the most common... 

This design is a variation of the conventional jacket. The internal, spiral baffle allows for high flow velocities to be reached. Some clearance must be allowed for between the baffles and the inside of the jacket, to allow for assembly, fabrication and tolerances. Although this clearance may be small, the total leakage area per baffle turn may be substantial when compared with the cross sectional flow area of the baffle passage. Thus, the actual velocity may only be a fraction of the calculated value. To compensate for this leakage around the baffles it is recommended that a 10% allowance be applied to either the total area required or total heat required. 

This design provides high velocity and turbulence within the jacket. This in turn will result in an unusually high film coefficient. The half pipe coil is recommended for high temperature and all liquid applications. It is better than conventional jackets because the pressure drop can be carefully controlled and calculated. It is not however practical for small vessels, less than 500 gallons. Because there are no limitations to the number of inlet and outlet connections, this type of jacketing can be divided into multiple zones for maximum flexibility and efficiency. 

Pressure drop in conventional jackets without spiral baffles and dimple jackets are deliberately excluded from this procedure. Other sources should be consulted for these applications. 

For design purposes, the half pipe coil and conventional jackets with spiral baffles are treated as coils. This procedure converts the shape of the passageway into an equivalent diameter pipe coil. 

For either a dimple jacket design or a half pipe jacket, the pressure drop will be higher flian that of an equivalent conventional jacket, due to flie increased turbulence. 

When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed kettle. This may be a batch or continuous operation. The rate of heat transfer is generally lower than for other types of evaporators and only a limited heat transfer surface is available. The kettles may or may not be agitated. Jackets may be of several types conventional jackets (formed with another cylinder concentric to the vessel), dimpled jackets, patterned plate jackets, and half-pipe coil jackets. (See Figure 11-1.) This variety provides a great deal of flexibility in the choice of heat transfer medium. 

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