VALUES OF HEATING SURFACES.
are similarly placed, in relation to the uptake,as with high boilers, the action of the flameis therefore much the same in each instance.The exception in the present case refersespecially to the tubes below the level of thecrowns of the fire boxes. It is understoodthat, to render these tubes effective, a givenamount of heated gases must enter them.Now the situation of the uptake in the presentexample prompts the idea that the tubes nearthe base line of the shell cannot receive muchof the proceed from the grate, other thansmoke and soot. The top rows of tubes alwaysreceive the maximum volume, due to theirsituation; and the action of the flame, whenin the uptake, is common to other arrange-ments, which have been already dwelt on.
With reference to the effect produced, abrief allusion will not be out of place. Theproceed from the grate affects the crowns andside surfaces of the fire boxes similar to thatwith other arrangements.
The hack of the combustion chamber, forminga continuous curve with the bottom of the firebox from the grate, the effect of the flameon that surface is thus rendered of higher valuethan with a vertical action.
The crown surface is equally effective as thatof the fire boxes. The sides or ends of thecombustion chamber are of increased value asagents of evaporation proportionately to othersurfaces.
The effective area of the tube plate is ofproportionate value, relative to the positionof the tubes. The total length of the plate inquestion multiplied by the width above the fireboxes, and the areas of the respective tubessubtracted = the surface worthy of most con-
143
sideration for the present purpose. The remain-ing area exposed is of the least value, and consi-dered to he one half of the former. The clusterof tubes above the fire boxes is of the ordinaryvalue, while those at the side are of the sameratio as the tube plate at that locality. Havingthus far rendered certain the values of thesurfaces in question, the following classifica-tion of ordinary ratios is deduced :—
TUBES.
Cluster above the fire boxes ...
2-976
Cluster at the side of the fire box
1-488
FIRE BOX.
Crown...
...Total surface...
1-000
Sides ...
...Above grate...
•500
COMBUSTION CHAMBER.
Crown...
...Total surface...
1-000
Tube plate
Surface above fire boxes...
•875
>>
Surface at side of fire boxes
•437
Back plate
Surface in front of furnaces
•720
Ends ...
Surface above grates ...
•250
DIRECT RETURN TUBULAR ARRANGEMENT.
To render the arrangement of any portion ofmechanism or structure conceivable by illustra-tion, without mistake as to its purport, threeviews at least are requisite. It is for this pur-pose the sectional elevations, Fig. 23—page144—are introduced. In page 115, a sectionalplan is given, in common with the principle ofthe arrangement now under notice. In thatcase two fire boxes are on each side of thetubes, while only one is shown at present.The boilers in each instance are nearly similarin design, but of different power and capacity.The roof of the boiler in the prior example isentirely flat, whereas in this instance a givenportion ol the top is raised to form a dome,