VEGETABLE AND FRUIT DEHYDRATION 97 



tensity of flame and hence the dehydrator temperature can be con- 

 trolled by a manually operated globe valve placed in the fuel supply 

 line. This simple system, however, has many limitations. Tempera- 

 ture regulation is tedious and combustion efficiency is likely to be 

 poor. Close temperature regulation, however, can be accomplished 

 by substituting a modulating control valve for the manual control 

 valve. (See section on controls.) 



A basic rule, applicable with any fuel, is that combustion should 

 occur in a relatively high temperature zone with a minimum of chill- 

 ing from outside. If an open gas flame is exposed to an air stream 

 of relatively low temperature, the partially burned gases in the outer 

 surface of the flame may be chilled below the ignition point and swept 

 away, with a resulting decrease in the efficiency. Fuel oil, especially 

 the heavier grades, cannot be satisfactorily burned in an open flame, 

 because the chilling effect of the surrounding air currents will cool the 

 burning oil particles below their ignition point, and smoking will 

 result. 



A satisfactory way of burning either gas or oil is through the use 

 of a combustion tube. This device consists essentially of a refractory- 

 lined chamber of ample proportions to permit generation of the maxi- 

 mum heat required while the flame is confined to the combustion cham- 

 ber. In the design of the chamber and burner, care must be taken to 

 prevent direct impingement of flame on refractory lining, as this 

 might result in either damage to the refractory or, with oil, the forma- 

 tion of coke-cones, with consequent smoking. 



Figure 53 shows a direct-combustion heater suitable for use with 



^CYLINDRICAL STEEL SHELL- 



PRIMARY ZONE 



m\\s\\\\\\m\^3 



FIREBRICK LINING J FIREBRICK CHECKER WALL- 



Figuee 53. — Direct-combustion heater. 



either gas or oil. It will be noted that a refractory checker brick par- 

 tition divides the space into a primary and a secondary zone, and serves 

 as a screen to confine the radiant heat to the primary zone. When 

 operating, this zone reaches incandescent temperatures — a desirable 

 condition for gas combustion and an essential feature for complete, 

 smokeless oil combustion. The checker screen serves as a baffle to pre- 

 vent the escape of unburned oil droplets. Impingement of the drop- 

 lets on the incandescent screen will result in surface combustion. 



The secondary zone is an added precaution, desirable as a combustion 

 aid, not essential with gaseous fuels, but necessary as an assurance 

 against smoking when oil is used. Large unburned particles of oil 

 that escape from the primary zone will burn at an accelerated rate 

 when they come into contact with the high-velocity, high-temperature 

 gases flowing through the checker wall restrictions. The particle is 

 then given additional time in the secondary zone to burn completely 

 before coming in contact with the drying air. Without the secondary 

 zone, smoking might result. 



Because the products of combustion are to be mixed with the drying 

 air, any amount of air necessary for complete combustion in direct 



569Q74— 44 7 



