Making sense of ESPs
What do ESPs do?
ESPs (Electrostatic Precipitators) are designed to remove fine particles from air. These can be dust or smoke, or wet particulates such as grease and oil found in kitchen ventilation applications. Gaseous odour molecules known as VOCs (Volatile Organic Compounds) pass straight through ESPs and need to be treated by other means such as ozone injection, in-line UV-C or activated carbon.
How do ESPs work?
ESPs work by ionising particulate matter and trapping these charged particles onto collector plates. A number of factors contribute to their efficiency:
- The high voltage of the ioniser and collector cells. This should be upwards of 12kV and 6kV respectively for maximum efficiency;
- Depth of collector cells – the deeper the better to ensure that the charged particles are trapped efficiently;
- Smooth air flow through the ESP. It is important that there is laminar air flow through the unit. This allows for air to be equally distributed through the ESP. If the air flow is turbulent, for example if the unit is placed too close to a bend, or if there is no diffuser plate in the equipment, this will dramatically reduce efficiency.
In a Plasma Clean ESP, after passing through a diffuser plate and pre-filter, contaminated air passes across robust stainless-steel spiked ioniser blades. The charged particles then pass into a collector cell made up of a series of equally spaced parallel plates, and are then collected.
Understanding the particulate collection efficiency of ESPs
This is an area where there can be a lot of confusion. Particulate collection efficiency depends on the flow rate and particle size. Generally speaking, efficiency at low flow rates and large particles is high. This reduces as the flow rate increases and the particle size decreases.
This is why it is important to check the stated flow rate and particulate size against manufacturers claims of efficiency. If there is a stated efficiency of, say 99.9%, always check what flow rate and particle size this was tested at.
It is extremely important to keep within flow rate capabilities to maintain efficiency and performance. Always remember multiple units can be joined together for increased volume flow rate or efficiency.
What other factors are important when choosing an ESP?
Pressure drop and grease handling are other important factors to consider when choosing an ESP.
Plasma Clean ESPs have a pressure drop of 90Pa per pass even when dirty. This means that a smaller, lower power fan could be an option at the design phase and will certainly result in lower running costs.
Grease handling is key. The grease which is collected by the ESP must be kept in check otherwise it is a fire risk. Wherever possible there should be provision to drain the grease and oil. Where this is not possible, routine cleaning is required. A large grease collection tray presents a fire risk and should be avoided.
Maintenance is important:
The grease and smoke collected by the ESP would otherwise find its way into the downstream duct work causing a fire risk and requiring TR19 Grease duct cleaning. By employing an ESP early on in the extraction system, downstream duct cleaning is dramatically reduced. This is particularly important when the duct runs through a building and access is not possible.
The ioniser and collector cells require a cleaning regime which normally involves exchanging the spent cells with clean ones. The ioniser and collector cell voltages should also be checked to ensure the equipment is working at its highest efficiency – if not, ask why.
On average for kitchen ventilation applications, this takes place every 3 months depending on the level of grease and smoke in the cooking process.
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