398 



CHAPTER XIX 



8io, 8io, 795, 695, 840, 1,020, 1,080, 1,295, 1,245, 1.395. 1.635, 1,780 seconds ; 

 the last 31 cu. ft. of discharge took 1,500 seconds. These results are shown 

 as a graph in Fig. 239. E\ddently the rate of evaporation per sq. ft. is at 

 a maximum when graining, and the greatest evaporation occurs when all 

 the heating surface is first put into operation. The rate is so variable that 

 it is useless to speak of any mean rate of transmission of heat in a pan. 



This irregularity in the rate of evaporation may give trouble in the steam 

 generating department unless there are instalJed a number of pans sufficient 

 to equalize the load. The writer's opinion is that there should not be less 

 than four pans in order to obtain an approximately equal rate of steam con- 

 sumption at this station. 



Fig. 240 



Vacuum Pans. Standard Coil Pan. — The original pan of Howard is best 

 described as constructed of the caps of two spheres joined about their bases. 

 A double bottom, to which steam was admitted, was formed in the lower 

 cap, and this formed the only heating surface. The apparatus was very 

 shallow, and corresponded to the popular meaning of the word " pan." 

 The coil was introduced at an early date, the first patent drawing to show a 

 plurahty of coils with individual steam inlet and condensed water outlets 

 being that of Greenwood (878 of 1853.) 



A section through the modern " standard " coil pan appears in Fig. 240. 

 The heating surface is made up of helices as developed round an inverted 

 cone of flat angle. The coils are from three to five inches in diameter, 

 depending on the size of the pan. Each coil has its own steam entry, live 



