Tubestill Heaters

Shell or batch stills are no longer used except when a single still may be used to rerun or distill several small stocks such as slops, special naphthas, petrolatum solutions, etc. 

Although all of the stills of Fig. 18-1 are useful for many services, there are certain features in which each excels or may be deficient. 

Flame Impingement, Too large a flame can cause impingement or overheating of certain tubes in nearly all of the stills, but tubes that are vulnerable are those just above the bridgewall in (c) down-convection, 

Hot Tubes. The rate of heat absorption tends to be high at the entrance to the convection section because heat is delivered by both radiation and convection [black tubes in types (o), (c), (d), (e), and 0 )]. The feed, because it is cooler, is sometimes introduced through these tubes (a screen bank), and in some instances steam is superheated or water is heated in the screen bank. Likewise, the tubes situated closest to the flame receive the largest amount of heat [black tubes in (d), (/), (p), (A), and (i)l. 

Oil Firing. Oil fuel tends to produce larger flames especially because larger burners are used. Stills (a), (6), (c), and (e) are best suited for oil firing although the larger sizes of the other stills encounter little difficulty. 

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