96 MODERN GASWORKS PRACTICE
(3) Primary and secondary supplies botli heated at expense of producer. Regenerators.
(4) Primary an unheated. Secondary air heated by sandwiching between producer and waste gases. This is by far the commonest type.
(5) Primary air unheated. Secondary air heated by sandwiching between two streams of waste gases. In this way the producer is not deprived of heat.
(6) Primary air unheated. Waste gases sandwiched between two streams of secondary air.
(7) Primary and secondary air supplies both. heated by waste gases.
(8) Primary or secondary air initially heated by the extraction of sensible heat from the hot coke in the retort. That is to say, in reality the coke leaving the retort is quenched by the ingoing air supply. This arrangement is only practicable with. vertical retort systems of the continuous type.
With regard to the merits of the various systems, in the case of the ordinarv generator no use is made of the heat in the waste gases, and the secondary or prim-ary air is raised m temperature entirely at the expense of the producer. Com-pared with no initial heating of the air supply the system does, however, effeot some economy mtW the air appropriates part of that heat which would otherwise be lost by radiation. It is in connection with this point that so much diversity of opinion exists with. regard to regeneration by means of waste gases. Many authori-ties assert that to admit secondary air through channels adjacent to the producer ( igs. 27 and 49) is uneconomical, and for this reason provision is made for entirely enciTcling the air with waste gases as in Fig. 48. In support of this theory Brooke gives the following figures :—
Percentage Efficiency of Regenerator. Percentage of Total Heat in Secondary Air abstracted from Producer.
Outside producer Inside producer. Air flue sandwiched between waste gases Waste-gas flue between air supplies .... Air flue between side of producer and waste-gas flue Air flue by side of producer 100 90-95 80-90 70-80 6 5-10 10-20 20-30 94
Too much attention must not, however, be paid to this point; and it must be remembered that heat will mevitably pass outwards from the hot producer walls and cause loss by radiation. Thus, if the secondary-air flue is adjacent to the producer, it will undoubtedly account for a reduction in the loss from this source. In practical working the distinction between the two types (1 and 2) cannot be said to have a noticeable effect on fuel consumption.