30 terms
Entries
Resources · Terminology
ESP terminology glossary
Thirty terms a plant or procurement engineer meets in precipitator, bag filter and boiler specifications, each defined in one to three sentences with the governing numbers: resistivity limits in Ω·cm, SCA in s/m, emission bases in mg/Nm³ at stated O₂. Definitions match the usage across the rest of this site.
10⁴–10¹¹ Ω·cm
ESP workable resistivity window
75.7 s/m
Design-basis SCA
mg/Nm³ @ 6 % O₂ dry
Emission basis used on this site
01 — How to use it
Terms defined the way they are used in specifications
Every definition carries its numbers, because in this field the number is the meaning.
This glossary is part of Arrow Energy's open engineering resources and uses the same figures as the rest of the site — the sizing arithmetic behind them is worked through in the ESP efficiency calculators, and the equipment itself on the technology pages. One convention applies throughout: dust concentrations are stated in mg/Nm³, dry, at a stated reference O₂ — 6 % O₂ for the solid-fuel boilers on this site.
Why do dust concentrations need an oxygen reference?
Because dilution is free and collection is not. Adding excess air lowers the measured mg/Nm³ without removing any dust, so regulators and honest engineers correct every reading to a fixed O₂ level using the factor (21 − O₂,ref) ÷ (21 − O₂,measured). Two figures at different references are different quantities and must never be compared directly.
| Statement | Comparable? | Remark |
|---|---|---|
| 24 mg/Nm³ @ 6 % O₂, dry | Yes | Complete basis: unit, O₂ reference, moisture condition |
| 24 mg/Nm³, O₂ unstated | No | Could be diluted; the figure is unverifiable |
| Dust in ppm | No | ppm is a gas-phase unit; particulate is always mass per volume |
02 — A to λ
The 30 terms
Alphabetical; cross-references link to the page where each term does its work.
- Acid dewpoint
- The temperature at which sulphuric acid and water vapour condense from flue gas — 115–140 °C for sulphur-bearing fuels, lower for low-sulphur biomass. Cold-end metal must stay above it at all loads; heat-recovery design works stack temperature toward the dewpoint margin, never through it.
- Air-to-cloth ratio
- Bag filter filtration velocity: gas volume flow divided by installed media area, in m/min. Biomass fly-ash designs run about 0.8–1.2 m/min net; going faster raises pressure drop and drives dust into the fabric depth instead of the surface cake.
- Approach temperature
- The margin between economizer water outlet temperature and drum saturation temperature, typically held at 20–40 °C so the economizer does not steam at low load.
- Back-corona
- Electrical breakdown of a collected dust layer whose resistivity exceeds about 10¹¹ Ω·cm. The layer emits positive ions that neutralise the charge on incoming particles; collection efficiency can fall sharply until power levels, rapping or gas conditioning restore the layer.
- Can velocity
- The upward gas velocity in the open area between bags in a baghouse. It is held conservatively low — near or below about 1 m/s — so that dust pulsed off the bags can fall to the hopper against the rising gas instead of redepositing.
- Collecting electrode
- The earthed plate onto which an ESP collects dust, roll-formed with profiles that create quiescent zones sheltering the dust layer; modern plate spacing is typically 400 mm. Total plate area is the A in SCA — 3,888 m² in the site design basis.
- Corona onset
- The voltage at which localised ionisation begins around the discharge electrode, set by emitter radius, gas density and temperature. An ESP field operates above onset and below spark-over; the working window between the two is what the controller manages.
- Corona power
- The DC power absorbed by the fields — secondary voltage times corona current. The site design basis absorbs 177 kW across four fields; a mid-size ESP is on the order of 250 kW.
- Cut diameter (d₅₀)
- The particle diameter a collector captures with 50 % probability. The design-basis ESP-train inlet has d₅₀ = 21 µm; mechanical cyclones weaken rapidly below about 20 µm, which makes the case for electrostatic augmentation.
- Deutsch-Anderson equation
- η = 1 − exp(−w·A/Q): ESP collection efficiency from migration velocity w, plate area A and gas flow Q. Exact only for uniform flow and monodisperse dust — practical sizing uses the Matts-Öhnfeldt form with exponent k ≈ 0.5.
- Discharge electrode
- The high-voltage emitter — spiked frame, rigid mast or weighted wire — whose corona charges the dust. Emitter geometry sets corona onset and current distribution; a broken one can short a whole field to earth.
- Economizer
- A tube bank that transfers flue-gas heat to boiler feedwater. Retrofits typically recover 3.1–4.0 percentage points of boiler efficiency; each ~20 °C of stack-temperature reduction is worth about 1 point.
- Electrocyclone
- The Electrocyclone is the proprietary hybrid particulate collector of Arrow Energy Co., Ltd., combining cyclonic separation with electrostatic augmentation in one casing — 85–95 % stage collection at service temperatures to 400 °C, removing ~90 % of particulate load ahead of a final collector with near-zero moving-part maintenance.
- Emission basis
- The complete statement a dust figure needs to mean anything: mg/Nm³, dry or wet, at a stated reference O₂ (6 % O₂ dry on this site). Correction between references uses (21 − O₂,ref) ÷ (21 − O₂,measured).
- Field (electrical)
- An independently energised ESP section in the direction of gas flow, with its own TR set and controller. The design basis uses four in series; losing one (n−1) moves the outlet from 24 to 57 mg/Nm³ — which is why fields are counted, not lumped.
- Fly ash
- The particulate carried out of a boiler by flue gas. Bagasse fly ash is high in unburned char and silica-rich — abrasive to tubes and fans — which shapes both collector selection and wear design.
- Gas distribution (ICAC EP-7)
- The uniformity of gas velocity across an ESP's treatment cross-section, specified per ICAC standard EP-7: standard deviation of velocity σ ≤ 15 % of mean, verified by traverse. Poor distribution wastes SCA in overloaded lanes and is corrected with distribution screens.
- Isokinetic sampling
- Extracting a dust sample at exactly the local duct velocity, so inertia neither over- nor under-samples any particle size. It is the basis of every credible mg/Nm³ compliance measurement.
- Lower heating value (LHV)
- Fuel energy excluding the latent heat of its water vapour — the honest basis for wet fuels. Bagasse at ~50 % moisture as fired has an LHV of about 7.2–7.5 MJ/kg.
- Migration velocity (w)
- The effective drift velocity of charged particles toward the collecting plates, in cm/s. Back-calculated from the design basis: 4.5 cm/s; biomass fly ash typically falls around 4–15 cm/s depending on resistivity and particle size.
- MIGI rapper
- Magnetic-impulse gravity-impact rapper: a solenoid coil lifts a plunger that gravity then drops onto the electrode system. Intensity and repetition rate are electrically adjustable per field, allowing outlet fields to rap gently and rarely.
- Normal cubic metre (Nm³)
- A cubic metre of gas referred to 0 °C and 101.325 kPa. Emission concentrations are stated per Nm³ dry so that temperature, pressure and moisture cannot move the number.
- Opacity
- The percentage of light attenuated across the stack — a continuous, cheap surrogate for dust. It correlates with mg/Nm³ only through a site-specific calibration against isokinetic measurement, and the correlation shifts with particle size.
- PRDS
- Pressure-reducing and desuperheating station: a control valve drops steam pressure and spray-water injection trims temperature, delivering process steam at the conditions the consumer needs rather than what the boiler produces.
- Rapping
- Periodic mechanical impact that dislodges the collected dust layer so it falls to the hoppers in coherent sheets. Timing is a balance: too seldom thickens the layer and invites back-corona; too often re-entrains dust straight into the gas.
- Resistivity
- The electrical resistivity of the collected dust layer, in Ω·cm. ESPs work well between roughly 10⁴ and 10¹¹ Ω·cm; above the window lies back-corona, below it re-entrainment. Temperature, fuel and moisture all shift it.
- SCA (specific collecting area)
- Collecting-plate area divided by gas flow, A/Q, in s/m — the strongest single sizing lever an ESP has. The site design basis uses 75.7 s/m for 96.67 % collection across four fields.
- Sneakage
- Gas that bypasses the energised zones through hoppers and roof spaces, carrying its dust uncollected. Even a few percent of sneakage caps achievable efficiency regardless of SCA, which is why baffling matters more the lower the target outlet.
- TR set (transformer rectifier)
- The unit converting mains AC to the high-voltage DC an ESP field runs on — typically rated 72 kV secondary for 300 mm gas passages and 90–110 kV for 400 mm. One TR set energises one field, preserving the n−1 logic.
- λ (excess-air ratio)
- Actual combustion air divided by the stoichiometric requirement. Biomass units often run high λ; trimming toward λ ≈ 1.15 is typically worth 0.7–0.8 percentage points of boiler efficiency, provided CO and unburned carbon are watched.
FAQ
Engineering questions, answered
What does mg/Nm³ at 6 % O₂ dry actually mean?
Milligrams of dust per normal cubic metre of dry flue gas — a normal cubic metre being gas at 0 °C and 101.325 kPa — mathematically corrected to a 6 % oxygen content. The O₂ correction stops dilution air from flattering the number: without it, doubling excess air would halve the reading while emitting exactly the same dust.
What is the difference between SCA and migration velocity?
SCA (specific collecting area, s/m) is geometry you buy: collecting-plate area divided by gas flow, 75.7 s/m in the design basis. Migration velocity (w, cm/s) is what the dust does: its effective drift speed toward the plates, about 4.5 cm/s back-calculated on bagasse fly ash. Efficiency follows from their product via η = 1 − exp(−w·SCA).
What resistivity range does an electrostatic precipitator need?
Roughly 10⁴ to 10¹¹ Ω·cm. Above 10¹¹ Ω·cm the collected layer breaks down electrically — back-corona — and collection can fall sharply; below 10⁴ Ω·cm particles lose their charge on contact and re-entrain. Fuel, temperature and moisture all move resistivity, which is why a fuel change justifies re-measuring it.
Send us your plant data
Fuel, boiler capacity, gas flow, current emission and the limit you must meet. An Arrow engineer replies with a technical assessment basis — not a brochure.