VEGETABLE AND FRUIT DEHYDRATION 101 



finished product, whereas low temperatures retard the drying. For 

 optimum conditions, the dehydrator should be operated at the highest 

 permissible temperature that can be controlled within close limits. 



Because of the small heat inertia of the open-flame method of 

 heating, the use of an off-on controller would be unsatisfactory. Up- 

 ward variations in flame intensities are immediately manifested by 

 increase in tunnel temperatures. With extreme care in operation, the 

 manually operated globe valve (fig. 55) can be opened to maintain a 

 low-fire condition, so that the resultant flame, without action from 

 the temperature controller, will produce a temperature within the 

 tunnel only a little lower than the set temperature. The off-on con- 

 troller may then be relied upon to admit periodically additional fuel 

 to bring the average tunnel temperature to the desired condition. 

 However, because of the low heat inertia of the heating system, even a 

 diminished fluctuation of fuel flow may permit the temperature to 

 fluctuate so widely as to scorch the onions at the dry end of the tunnel. 

 An effort can be made to improve this condition by adjusting the 

 manually operated globe valve to admit more fuel, so that the low-fire 

 temperature level within the tunnel is raised. The flow in the modu- 

 lating valve branch can be restricted, so that variations in flame in- 

 tensity will be minimized. However, constant vigilance will still be 

 necessary to prevent excessive tunnel temperatures during tunnel 

 charging and under other varying load conditions. 



When a modulating controller is placed on this system, the opening 

 of the modulating valve will be related to the heat demand. Thus a 

 comparatively even tunnel temperature will be realized. Obviously, 

 the globe valve can be used to desensitize the action of the controller. 

 However, care should be taken in its adjustment for low-fire condi- 

 tions so that the resulting temperature is not in excess of that desired 

 in the dehydrator under minimum-load conditions — that is, when part 

 of the tunnel is shut down for the changing of cars. 



As a second example, let us suppose that the same dehydrator em- 

 ploys a confined flame unit and a combustion tube similar to that 

 shown in figure 53 of the preceding section. The same close regula- 

 tion of temperature is required. Let us analyze this example and as- 

 sume that an off-on controller is used. The manually operated globe 

 valve shown in figure 55 is adjusted so that the low-fire temperature 

 level is approximately 10° F. below the set temperature of the con- 

 troller. A small drop in temperature below the set temperature will 

 open the motorized valve completely, and the burner will be under 

 high-fire conditions. The immediate reaction will be to store up heat 

 energy in the refractory lining of the combustion tube. There will 

 be a small time lag before full flame intensity is manifested by a maxi- 

 mum increase in heating effect on the drying air. Thus the large heat 

 inertia of the heating tube has a dampening effect on the fluctuations 

 of temperature produced by the off-on controller. Variations in tem- 

 perature regulation will, in all probability, be within acceptable limits. 

 Further dampening of the maximum amplitude produced by the off-on 

 controller can be obtained by a fixed throttling device placed in series 

 with the motor-operated valve. 



As a third example, let us assume that the same counterflow finish- 

 ing dehydrator used with onions is steam-heated. Close temperature 

 regulation is again essential. The problem is, again, to select a suit- 

 able type of controller. Admission of large quantities of steam to the 



