Spillback nozzles

The spillback nozzle is an innovative way to allow for variable flow rates through a nozzle whilst keeping the other fluid properties relatively stable. Particularly the droplet size of the spray can be maintained within fairly narrow parameters even though the flow rates may be varied by a factor of 10.
With normal hydraulic nozzles flow rate will depend upon the pressure drop across the nozzle. So at higher pressures we will get higher flow rates through a given nozzle and vice versa. This presents two problems:
1- Turn down rate is limited

Typically the relationship between flow and pressure is a root relationship so a 4 fold increase in pressure will result in a 2 fold increase in flow rate meaning that a 10 fold increase in flow would need a 100 fold increase in pressure. This being the case it follows that achieving a turn down rate (variance ) of a factor of 10 will need a pump that can vary the pressure by 100 fold. This clearly presents problems and is not very efficient.

2- Variable droplet size

The first issue (above) leads to a second problem. Changing the pressure drop across a nozzle does not only affect he flow rate through the nozzle, it will also affect other spray properties such as the mean droplet size. Higher pressure drops across a nozzle will result in smaller droplet sizes in the resulting spray. The very high pressure variance required for significant flow rate reduction means that the droplets formed by the turned down spray volumes will have much higher droplet sizes.

Typically we could expect the following from a standard hydraulic nozzle:

For a 4 fold increase in pressure we would get a 2 fold increase in flow rate but a 34% decrease in droplet size i.e. they are about 2/3 the size 
For a 9 fold increase in pressure we would get a 3 fold increase in flow rate and a 50% decrease in droplet size i.e. they about half the size
For a 100 fold increase in pressure we will get a 10 fold increase in flow rate and a 75 % reduction in droplet size i.e. they are 1/4 the size
This variable droplet size will have a very significant impact on how well a spray reacts, transfers heat or how quickly each droplet will evaporate and how long the droplets will remain in contact with the gas (residence time). This means that by changing the flow rate by simply turning up or lowering the pressure drop across the nozzle one could also:

- Overload mist eliminators 
- Reduce the efficiency of the quench spray
- Not achieve complete evaporation by a given point
- Fail to cool effectively
 

The solution The spill back nozzle seeks to solve the above problem. It works by having two fluid channels to the nozzle. One will deliver the spray and the other is a spill back channel that will return the fluid to the pump for re-circulation. The amount of fluid that is diverted away from the spray orifice can be varied by varying the pressure differential between the main fluid feed and the spill back channel. So if the two channels are of equal pressure then no fluid will be spilled back. As we decrease the spill back channels pressure fluid from the feed channel is diverted away from the nozzle resulting in a reduced nozzle flow rate whilst maintaining the pressure drop seen by the nozzle at a steady rate.

What this means is that the mean droplet size remains relatively stable even with highly varied flow rates. So the resulting spray will have similar cooling properties and residence time even when turned down by a factor of 10 from the maximum flow rate. 

Also because the spillback lance works on a pressure differential to cause the sill back effect its mean the pump itself can run at a steady pressure regardless of the desired flow from the nozzle. The only draw back is that the supply pump must work at the pressure that will supply the highest desired flow rate at all times and some of this work by the pump is "wasted" as it is spilled back and recirculated rather than being ejected through the nozzle. Nevertheless the advantages of the the spillback system far out weigh the pump work inefficiencies as it allows for controlled cooling of variable load gas flows.