What is the function of the return line in atomizers?

[engnr_arsalan]

Hello All,

I have recently studied about oil atomization and have question regarding return line.

According to my understanding, oil pressure has influence on droplet size that is formed after atomization which is obvious , higher pressure means smaller droplet size as the oil is passing thru an orifice which makes the droplets due to sudden acceleration of fluid paticles overcoming the surface tension ( correct me if i am wrong) . Now the question is, my professor said that there may be a return line in atomizer that controls the amount of fuel (also oil pressure??) going into the combustion chamer/furnace and it does not influence the droplet size as much as it would if the pressure regulation would be in the supply line.

I am unable to understand this ...my thoughts on this are that if we have a return line in atomizer and there is a control valve that is controling the flow, when this valves open then pressure at the nozzle of atomizer should fall down and droplet size should increase. please correct me if am wrong..
a very simple diagram is attached for visualization..

 

[anorlunda]

Maybe it is like the return line in Diesel engines.

What kind of pump is supplying the pressure? Is the fluid compressible?

 

[engnr_arsalan]

its heavy fuel oil, I believe centriugal pump is used since fluctuations are not allowed in operation of a furnace.
Or may be a rotary P.D pump.

 

[Asymptotic]

Read all of "http://www.delavaninc.com/pdf/Fuel_Nozzles_for_Burners.PDF" by Delavan Manufacturing. Nearly everything about simplex nozzles applies to bypass nozzles.

my professor said that there may be a return line in atomizer that controls the amount of fuel (also oil pressure??) going into the combustion chamer/furnace and it does not influence the droplet size as much as it would if the pressure regulation would be in the supply line.

Page 9 doesn't go into much more detail, although it mirrors what your prof said.

 

[engnr_arsalan]

Thnx Asymptotic, the document u gave is very informative,however, I could not find what I am looking for.

 

[Asymptotic]

Thnx Asymptotic, the document u gave is very informative,however, I could not find what I am looking for.

The function of the return line (from page 9).

A bypass or return flow nozzle is identical in construction to a simplex nozzle with the exception that a return orifice is provided to "bleed" oil from the swirl chamber. The nozzle is so designed that with this bypass orifice completely closed the operation of the nozzle is essentially simplex.

When the bypass line is opened, however, with constant supply pressure, the energy of the fuel coming through the slots is available for atomization but part of the fuel is bled back through the bypass line to the tank.

With this type of nozzle it is possible to vary the discharge rate over a range as high as 10 to 1 with good atomization over the entire range and with constant supply pressure. For fuel burning purposes a more common "turndown ratio" is approximately 3 to 1. That is the limit of the air handling parts of most burners.

This strikes me as being roughly analogous to why both a pressure regulator and flow control valve(s) are used in pneumatic cylinder control circuits. In some cases, satisfactory cylinder rod extend and retract rates can be achieved solely through pressure adjustment. More typically, when air pressure is adjusted low enough to yield the desired speed it may no longer produce enough force to do the job (clamp a set of rolls together, etc.), but setting pressure high enough to produce the required force closes the rolls too rapidly, and results in excessive wear and tear on the machine.

The solution is to set pressure high enough to generate the required clamping force, and adjust speed with a flow control valve (or preferably a pair of valves; one for extend, and another for retract rate).

For a simplex atomizer, pressure can be set only so low before there isn't enough force to create properly formed droplets, and this marks the lower limit for fuel delivery. Controlling flow through the return line allows a wider fuel delivery range while still creating adequate nozzle pressure.

 

[JBA]

I don't know if this relates to diesel fuel systems; but, in some automotive constant injection fuel systems a fuel return valve is used to regulate the nozzle flow because it is easier to vary the nozzle flow rate, by reducing or increasing the nozzle pressure using a throttle controlled return valve than to do it by varying the pump flow rate. The pump is designed to deliver a constant fuel feed that is always greater than that required for engine operation and the varied pressure range to the nozzles is always high enough to insure good fuel atomization.

 

[engnr_arsalan]

For a simplex atomizer, pressure can be set only so low before there isn't enough force to create properly formed droplets, and this marks the lower limit for fuel delivery. Controlling flow through the return line allows a wider fuel delivery range while still creating adequate nozzle pressure.

Reference https://www.physicsforums.com/threads/what-is-the-function-of-the-return-line-in-atomizers.930903/

I don't know if this relates to diesel fuel systems; but, in some automotive constant injection fuel systems a fuel return valve is used to regulate the nozzle flow because it is easier to vary the nozzle flow rate, by reducing or increasing the nozzle pressure using a throttle controlled return valve than to do it by varying the pump flow rate. The pump is designed to deliver a constant fuel feed that is always greater than that required for engine operation and the varied pressure range to the nozzles is always high enough to insure good fuel atomization.

Reference https://www.physicsforums.com/threads/what-is-the-function-of-the-return-line-in-atomizers.930903/

This Makes sense to me.. Thanks a lot Asymptotic & JBA..

make me your disciple master.. :D