Droplet separation

Which other parameters affect the selection and design of a droplet separator?

In addition to droplet size, there are a number of other important parameters to consider when selecting and designing a droplet separator. Our experienced staff and engineers are happy to help with calculations and to find the right solution.

Pressure drop

A process gas flow is generated by a fan or compressor, which creates a vacuum or outlet pressure. Pressure is energy, which is a cost and needs to be taken into account in order to find an optimal solution.

A droplet separator made from profiles is designed to withstand high gas flows without too much impact on pressure drop. On the other hand, a candle filter which consists of very small, tightly packed fibres has much greater impact on pressure drop.

Pressure drop is affected not only by the droplet separator design, but also by gas flow rate, liquid load, viscosity and amount of solid particles present in the gas stream. In general, the aim is to achieve the lowest possible pressure drop in relation to the highest possible separation efficiency.

Separation efficiency

Separation or removal efficiency targets may be set by statutory emission requirements or a need to reduce a certain amount of contaminants in order to avoid corrosion and other damage 'downstream' in the process. In some processes, droplet separators are used to reuse valuable substances and product.

Separation efficiency specifies the degree of liquid and particles the droplet separator removes from the gas flow. Separation efficiency is calculated using a number of variables, including type of droplet separator (mist eliminator), gas and liquid density, gas flow velocity and amount of liquid.

An increased gas flow rate leads to increased separation efficiency, but it also means an increase in pressure drop. It is important to consider the trade-off between level of separation efficiency and total operating cost.

Gas flow and process variations

One of the most important factors in designing and dimensioning a droplet separator is gas flow in relation to area (net), as each type of droplet separator has a maximum velocity allowed. Separation is optimised at the point of maximum velocity. If the maximum velocity is exceeded, droplets will follow the gas stream and flooding occurs.

Normally, a droplet separator (mist eliminator) is designed based on the maximum process flow. In the flow range below the maximum flow, droplet separator efficiency increases when the gas flow increases. At the same time, the pressure drop increases quadratically, which leads to increased operating costs, among other things. In cases where there are large process variations between maximum flow and nominal flow, or if the process very rarely operates at maximum flow, it is important to make a design compromise in order not to lose too much efficiency and to optimise operating costs. Thus, it is important to know and accurately specify the true process flow, not just the calculated process flow.

The following is specificed for a droplet separator at any given gas flow rate:

• efficiency - % of droplets ≥x µm separated from the gas stream
• limit drop size - the smallest drop size (x) separated from the gas stream
• pressure drop across the droplet separator

Operating temperature

It is important to take into account the operating process and its process variations in order to design the most efficient droplet separator. The operating temperature range usually determines which type of material to use for the droplet separator. 

Liquid load

Allowable liquid load by is defined by gas density, liquid surface tension and viscosity.

For vertical flow - allowed liquid load also depends on:

• Gas velocity - the higher the velocity (up to the maximum velocity), the lower the expected amount of liquid downstream
• Spacing between profiles/lamellas - allowed liquid load increases with decreasing profile/lamella spacing
• For very high liquid loads, a two-stage separator is recommended

For horizontal flow - allowed liquid load also depends on:

• Number of profiles/lamellas and distance between profiles/lamellas
• Height vs width of the droplet separator - at a given area, a wider droplet separator can handle more liquid
• To increase maximum liquid load, profiles with hooks can be used

Gas flow direction

Gas flow direction, i.e. vertical or horizontal, determines which type of mist eliminator to use and how it is installed.

Risk of clogging

Particles in the gas flow increased the risk of clogging. To reduce the risk, one option is to install a flushing system with a set flushing sequence. Another option is to install a system with pressure sensors that initiate a flushing sequence in the event of increased pressure drop across the droplet separator.

Available space for installation

Space available for installing the droplet separator can sometimes be critical. Are there limitations in width or height? Usually the droplet separator is manufactured in sections to be installed through a manhole, which also means that the sections can be removed for maintenance when needed.

For new installations, a well designed droplet separator can help reduce the diameter of the vessel or tower, leading to cost savings. If the tower dimensions are restricted by other column fittings, the active area of the droplet separator can be reduced to a rectangular or square area, optimising droplet separation efficiency, thus reducing costs.