Cool comfort for super-heated elements

Hot gasses are everywhere in the process industries and one of the quickest and most efficient ways of cooling them is to inject water directly into the gas stream.

As the droplets mingle with the gas and the cooling water evaporates, latent heat is required to drive that phase change, reducing the overall temperature. It sounds simple, but there are a number of potential pitfalls.

“The key to efficient cooling is often maintaining the droplet size of the spray,” says Ivan Zytynski, of Bete UK.

“A small droplet size will result in a bigger surface area per volume of fluid and thus a great interaction with the gas. However, smaller droplets tend to get swept along in a gas flow more rapidly and so may have a reduced residence time, meaning less time to exchange heat. This means that any variation in droplet size may negatively impact the effectiveness of the cooling system.”

The key to efficient cooling is often maintaining the droplet size of the spray

Ivan Zytynski, Bete UK

Generating the required droplet size means striking a balance between surface tension and drag, as Lechler’s Chris Roberts explains: “The droplet size depends on the surface tension of the liquid and the drag forces acting on it. Drag shears the droplets and breaks them into small droplets. For bigger droplet sizes drag is higher and the surface tension cannot prevent the droplets from disintegrating into smaller droplets.

“But as the droplet sizes become smaller, drag has less effect and the surface tension keeps the droplet from disintegrating. Droplet size can also be decreased by providing additional external forces to overcome the surface tension. For a given spray volume it requires more energy to produce smaller droplets than larger ones.”

In some systems, the added shear needed to atomise the water is provided by pushing the fluid through an orifice under pressure. In others, a secondary fluid such as compressed air is used to help complete the process.

Whatever the system, balancing all the parameters underpinning nozzle choice and droplet size is one thing when dealing with a constant cooling load and a steady flow of water. However, it presents a whole other challenge when the load is variable.

Size matters

“In many gas cooling applications the amount of gas needing to be cooled and the temperature of the gas may vary. At first glance it may seem a simple enough problem to solve; simply increase the flow from the nozzles by increasing pressure to compensate for any higher gas flows or additional cooling required,” says Zytynski.

“The problem with this approach is that flow rate is not the only fluid property changed by increasing the pressure drop across the nozzle. Most importantly, droplet size is also likely to change and this will drastically affect cooling.”

Since the pressure drop across the nozzle is critical in determining the droplet size; increasing the water pressure/flow generates smaller droplets.

This increases the risk of the gas stream sweeping smaller droplets through the system before they evaporate. Conversely, decreasing the pressure drop for a lower load will increase the droplet size and bigger droplets may not have time to evaporate completely.

Whatever the reason, incomplete evaporation can cause a range of practical problems in addition to heat transfer issues. For instance, one of Lechler’s key areas of expertise is sprays for cooling towers in cement plants. Here, incomplete evaporation from a poorly designed system would result in a slurry of cement dust and water accumulating at the base of the tower, where it’s baked hard by hot exhaust gas.

There are a range of possible solutions to suit variable cooling loads in different specialist applications. Take desuperheaters, for example, which often inject cooling water to transform superheated steam into saturated steam. 

Spirax Sarco’s Andrew Prew says: “We have different technologies for different turndown ratios. For applications needing a 10:1 turndown, we would recommend a venturi-type desuperheater, which modulates the water flowrate. For more variable loads we can achieve up to 50:1 with variable area systems.” These bring more nozzle orifices into play as the load increases, effectively increasing the overall flow area for the water.

There are a range of possible solutions to suit variable cooling loads in different specialist applications

In other applications, spill-back lances are a popular option. These recirculate a variable proportion of the cooling water back up the lance in order to adjust the output.

For instance, Bete’s spill-back lance controls the proportion of fluid that is ‘spilled back’ by varying the pressure differential between the spill-back channel and the feed channel. “If the pressure of the main inlet fluid is equal to the pressure on the spill-back channel, no fluid will be spilled back. By lowering the pressure of the spill-back channel a proportion of the feed spray will be diverted back away from the nozzle. The important thing is that the pressure differential across the nozzle orifice remains unchanged and so the spray characteristics, including droplet size, will also not change,” says Zytynski.

According to Lechler, another advantage of spill-back systems is that they are cheaper to run than nozzle systems that rely on compressed air to help atomise the water.

This is the key to their popularity in cooling towers, says Roberts: “From a cost efficiency standpoint, the spill-back system is often less expensive to operate and requires less capital to purchase, as it does not need expensive compressors. Spill-back systems have been successful all over the world where energy concerns have been an issue.”