Marine Engineering Study Materials

Good combustion is essential for the efficient running of the boiler. It gives the best possible heat release and minimum amount of deposits on the heating surfaces. Different types of boiler fuel oil atomizers are:

Simple Pressure Jet Atomizer

Figure below shows diagrammatically the principle of the simple pressure jet boiler fuel oil atomizers. Oil fuel at a pressure of not less than 8 bar is supplied to the tangentially arranged ports, wherein it falls in pressure, resulting in the oil swirling around at high velocity inside the chamber. The greater the pressure drop the greater would be the velocity, but generally supply oil pressure would not be greater than 25 bar, high pressures create pumping and sealing problems.

At outlet from the short sharp edged orifice (care must be taken not to damage the orifice during cleaning) a hollow expanding cone of fuel droplets is produced whose initial film thickness reduces as oil supply pressure increases.

The following features of the simple pressure jet boiler fuel oil atomizers should be noted.

1. Simple, inexpensive and robust.
2. No moving parts, hence no possibility of seizure.
3. Large range of droplet size for one pressure.
4. Turn down ratio is low, about 2.5 to 1. Turndown ratio is the ratio of the burner throughput maximum to minimum. If say the maximum throughput is 2500 kg/hr, then with a turn down ratio of 2.5 to 1 the minimum throughput will be 1000 kg/hr.
   Maximum throughput is about 3200 kg/hr
5. No alteration in air register arrangement for all outputs, this is a consequence of the low turndown ratio of this type of burner.
6. To vary throughput the pressure must be altered, this results in variation in droplet size, hence atomization and combustion will be affected.

Spill type of pressure jet atomizer

With this type of boiler fuel oil atomizers, throughput is varied by adjusting the amount of spill, i.e. the return flow from the burner.

Constant differential type atomizer

With the differential pressure between delivery and spill kept constant the pressure drop across the tangential ports is kept constant and the rotational velocity of the fuel in the swirl chamber will not alter. This will keep fuel cone angle and atomization constant.

The main disadvantage with these types of boiler fuel oil atomizers is the large quantity of hot oil being returned, it may prove difficult to keep control over oil temperature.

Plunger type of pressure jet atomizer

Figure below shows the principle of operation of this type of burner, as the oil supply pressure is increased the spring loaded plunger moves to uncover extra tangential oil entry holes. The pressure drop and hence the rotational velocity of the oil remains nearly constant.

Rotary or spinning cup jet boiler fuel oil atomizers

Figure below shows the arrangement in principle. It consists of a motor driven fan, metering pump and fuel cup.

 

The fuel cup which rotates at 70 to 100 rev/second is supplied with oil at low pressure (1.7 to 4.5 bar) from the metering pump which is used to control the throughput. Due to centrifugal force and opposite swirl of air to fuel the oil film leaving the cup is rapidly broken down into relatively even sized droplets.

Main features are:

1. High output possible, up to 3600 kg/h.
2. Low oil supply pressure.
3. At low throughputs atomization due to reduced oil film thickness is improved.
4. Wide turndown ratio, up to 20:1 possible.
5. Oil viscosity need only be reduced to 400 seconds Redwood number 1 for satisfactory operation.

 

References

“Steam Engineering Knowledge for Engineers” by Thomas D. Morton