Hot-side and cold-side ESPs

Hot-side and cold-side describe where an electrostatic precipitator sits relative to the boiler air heater. A hot-side ESP is installed upstream of the air heater and treats hotter, lower-density gas. A cold-side ESP is installed downstream of the air heater and treats cooler gas closer to stack temperature.

Electrical and process trade-offs

Ash resistivity changes strongly with temperature, moisture and flue-gas chemistry. Very high resistivity can cause back-corona, poor particle charging and unstable electrical fields. Historically, hot-side ESPs were used on some low-sulphur western coals because the hotter ash had more favourable resistivity than it would have after the air heater.

The disadvantage is size. Hot gas has lower density, so the volumetric flow through a hot-side ESP is larger for the same mass flow. The casing, fields, bus sections, rappers and hoppers therefore become bigger. Hot-side units also need materials, expansion joints and insulation suitable for sustained high-temperature service.

Cold-side ESPs are more common in new and retrofit projects because they are smaller, easier to integrate with downstream equipment and can use flue-gas conditioning when resistivity is a problem. Their main risks are acid dew point corrosion, ammonium-bisulphate fouling after SCR systems, and sticky ash deposits when the air heater outlet temperature is pushed too low.

Cleaning implications

Both arrangements need reliable plate, electrode and hopper cleaning. Hot-side ESP deposits may be drier but the hardware is exposed to thermal expansion, oxidising conditions and high roof temperatures. Cold-side ESP deposits may be more corrosive or sticky, especially near the cold end or after ammonia slip from SCR.

Sonic horns can support both designs by preventing ash from consolidating on collecting plates, hoppers, turning vanes and inlet distribution screens. Hot-side service may require Inconel 625 or 718, remote-mounted drivers, insulation details and careful penetration sealing. Cold-side service can often use 316 stainless, but the design must consider acid condensate, washdown water and enclosure ratings for solenoids and controllers.

Operating symptoms

Operators see cleaning problems as rising opacity, increased spark rate, uneven field power, hopper high-level alarms, rapper overloads or dust carryover during load changes. A hot-side or cold-side label alone does not determine the cleaning method; ash chemistry, gas distribution, hopper behaviour and access for maintenance matter more.

Design and diagnostic detail

Hot-side and cold-side labels are shorthand for a much wider design envelope. The useful variables are gas temperature, sulphur trioxide level, moisture, ash resistivity, inlet velocity, collecting-electrode spacing, hopper temperature and the location of the air heater. Hot-side units avoid some cold-end acid problems but expose internals, rappers, insulators and penetrations to high thermal movement. Cold-side units are easier to maintain thermally, but they are more vulnerable to acid dew point corrosion, ammonium-bisulphate deposits and sticky ash after low-temperature operation.

Failure analysis should separate electrical limitation from mechanical fouling. High spark rate, low secondary current, back-corona and rapper timing problems point in different directions. Hopper build-up can create re-entrainment even when the fields appear electrically healthy. Inlet screens and turning vanes may also become dust shelves, causing maldistribution that looks like poor collection efficiency downstream.

Acoustic cleaning is most useful when it is treated as part of the ESP cleaning sequence. Horn firing should not simply release a heavy dust wave into a full hopper. The rapper programme, hopper evacuation and acoustic cycle need to work together so dislodged ash leaves the casing instead of becoming a new source of opacity spikes.

Related terms

• Electrostatic precipitator
• Resistivity (fly-ash)
• Back-corona
• Air heater