160 RECENT ADVANCE IN OIL BURNING. 
in the direction of larger units and the higher capacities made possible by the use 
of oil fuel. 
Referring to the matter of flexibility, I think I am justified in discussing another 
recent development which I believe marks a distinct advance in the oil-burning 
art. It is well known that the mechanical atomizer of the usual design has a 
decided limitation in range. Unlike the steam atomizer, it cannot be operated at 
low capacity if it is designed for a high one. The lower limit with the oil pressures 
ordinarily used is approximately one-half the upper. The reason for this is very 
simple. The burner depends for its atomizing effect on centrifugal force induced 
by giving the oil a rapid whirling motion in the central chamber of the tip. This 
whirling motion, in turn, is produced by delivering the oil to the central chamber 
through eccentric passages, or small channels substantially tangential to the walls 
of the chamber. All the oil which enters the chamber through these channels 
finds an exit through an orifice concentric with the axis of rotation. Thus as the 
capacity of the burner is reduced by the obvious method of reducing the oil pressure, 
there is a lower velocity through the channels and friction losses become more 
apparent, until finally the spraying effect is reduced till the oil issues from the tip 
in a solid stream or in a spray too coarse and uncertain to give the desired results. 
In burners of the usual type this point, as stated, is reached at about one-half the 
maximum capacity of the burner. 
In atomizers of the Thornycroft type, in which the size of the tangential 
channels may be reduced by an adjustment in the burner, the velocity through 
the channels is kept up to a large extent at low powers, and the range of these 
burners is much greater than in those of the usual type. But it has been found 
difficult in practice to make individual burner adjustment under service conditions, 
and this method has never become popular, even if it be assumed that a sufficiently 
wide range in capacity is secured thereby, which is questionable. The only alter- 
native, therefore, with burners of the ordinary design is to shut off some of the 
burners at low powers, or to change the size of tips, or to do both, which is not 
unusual. Both these methods are open to objection. 
Another and radically new plan, however, has been devised which promises 
very satisfactory results. This consists of maintaining the whirling motion in 
the central chamber of the tip undiminished whatever may be the capacity 
desired of the burner; in other words, instead of delivering through the orifice all 
the oil which enters the burner chamber, as is invariably done in all other mechanical 
atomizers, a part of the oil supply is diverted or by-passed from the central chamber 
and returned to the pump suction, and the actual effective capacity or amount of 
oil which is sprayed into the furnace depends merely on the proportion of the oil 
that is by-passed to the total amount entering the burner. Thus, as the amount 
of oil entering the central chamber through the tangential slots remains at all times 
at the maximum, the whirling of the oil and the atomizing effect remains also at 
a maximum, so that a perfect spray is secured whatever the capacity, at low powers 
as well as at the highest rate. By this method the range of the atomizer is very 
greatly increased, and, instead of being limited at the low powers to 50 per cent 
