If you have ever asked a boiler supplier for "two fans" and got a quote back where one of them was roughly the same physical size as the other, the supplier was guessing. The FD fan (forced-draft, blowing air into the furnace) and the ID fan (induced-draft, pulling flue gas out) handle two completely different duties, and the ID fan is almost always the larger, more expensive and more carefully specified of the two.

This article is the engineering version of that observation. It is written for the buyer, EPC engineer or boiler-house operator who wants to know which fan does what, why the numbers on the data sheets look so different, and how to specify both correctly on the same RFQ.

1. The two-fan trick every boiler has

A solid-fuel or liquid-fuel boiler needs three things from its fans: combustion air, removal of the resulting flue gas, and a small controlled negative pressure in the furnace so the boiler does not "puff" hot gas into the room through every door and inspection port. The configuration that delivers all three is called balanced draft: one fan on the upstream (clean) side and one on the downstream (dirty) side, working against each other to keep the furnace at a small fixed negative pressure - typically -20 to -50 Pa relative to the boiler house.

Almost every package boiler, every industrial coal-fired boiler, every CFB, every waste-to-energy plant and  utility boilers use this pattern. The fans are not interchangeable: a backward-aerofoil fan that is  on the FD side will overheat, corrode, vibrate itself apart and shed its blades if you put it on the ID side of a coal-fired boiler. The two fans have to be specified to the duty, not to each other.

2. Where each fan sits in the gas path

The figure below is the simplified gas path of a fuel-fired boiler. The colour matters: blue is the clean combustion air being pushed in, orange is the hot flue gas being pulled out. Everything between the FD fan outlet and the ID fan inlet is at above-atmospheric (furnace and windbox) or below-atmospheric (everything after the boiler bank) pressure. The plot below the gas path is the gauge pressure along it.

Two practical points the figure does not show:

• Most plants actually have a third fan - the primary air (PA) fan - on a coal-fired boiler, blowing hot air into the pulveriser or under the bed. The PA fan duty is similar to the FD fan duty (clean air, but higher pressure - often 10 - 15 kPa on a PC boiler), and a Chinese-supplier RFQ will sometimes roll it into the same "boiler fan" line. It deserves its own line in the BOM.
• A secondary air (SA) fan shows up on some large boilers, blowing air over-bed or into the over-fire air ports. Same FD-fan family, smaller duty.

3. The 12 specification differences

If you send a supplier one RFQ that says "two boiler fans", the quote you get back will be wrong on at least four of the twelve lines below. The supplier does not know whether you are putting them on the right side, and the sales engineer will usually quote the more expensive (ID-fan) spec on both to be safe. That is a fast way to overpay.

Five of the twelve are worth dwelling on, because they are the ones that cause real failures in the field:

1. Density for power calculation. Use 1.18 - 1.20 kg/m³ for the FD fan, 0.60 - 0.90 kg/m³ for the ID fan. Using 1.2 for both (a common copy-paste error) undersizes the ID motor by about 30 %.
2. Volume flow. A typical 20 t/h gas-fired boiler has an FD fan moving ~17,000 m³/h and an ID fan moving ~32,000 m³/h, a ratio of 1.8 : 1. This is not the same fan with the same impeller trim.
3. Dust load. "Clean" FD-side air carries well under 1 mg/m³ of dust. "Clean" ID-side flue gas after a baghouse still carries 20 - 50 mg/m³. Without a dust collector, grate-fired flue gas carries 5 - 10 g/m³ - that is the impeller-blade lifetime on a clock.
4. Bearings. An ID fan with an uncooled bearing housing will cook its grease above about 130 °C. The ZIFENG Y5-47 uses a water-cooled integral bearing base for that reason. Smaller YN5-47 boilers (160 - 200 °C) get away with an air-cooled motor seat and a heat-insulating coupling spacer.
5. Shaft seal. On the FD side, a small leak is harmless. On the ID side the seal has to stop hot, possibly acidic, gas from leaking outandstop cold air from being sucked in (cold air dilutes the flue gas, ruins the draft setpoint, and condenses moisture into acid in the casing).

4. Why the ID fan moves ~1.8× the volume

Three numbers stack up: more mass, same stoichiometry, hotter gas.

1. More mass. Combustion adds about 11.6 Nm³ of wet flue gas for every 10.6 Nm³ of combustion air, because the hydrogen in the fuel carries its own volume as water vapour.
2. Excess air. Real burners do not run at stoichiometric. 5 - 15 % extra air is needed for complete combustion and acceptable CO / NOx. The ID fan sees that extra volume too.
3. Temperature. Combustion air enters the boiler at 25 °C. Flue gas leaves the air heater at 120 - 200 °C. The same mass of gas occupies about 1.5 - 1.7 × the volume at the higher temperature (the standard ideal-gas relation, which is exact enough for fan selection here).

5. A worked example: 20 t/h gas-fired boiler

The chain below is what the heat balance of a 20 t/h gas-fired package boiler looks like when you write down every step instead of jumping from "burner rating" to "fan duty". The numbers are the actual numbers, not round ones, and they are the numbers you should be checking when a supplier sends you a fan selection report.

The audit points worth pulling out of the figure:

• The fan laws are exact here. For a given speed and a given system-curve shape, both the fan curve and the system resistance curve scale linearly with gas density. So if the system resistance at design is 4,500 Pa at 150 °C (ρ = 0.84), the same fan at the same speed in 25 °C air (ρ = 1.18) sees a system resistance of 4,500 × (1.18 / 0.84) = 6,320 Pa, and the operating point moves along the fan curve to deliver the same volumetric flow at the higher pressure.
• Same volume, 1.4× the power. Volumetric flow is unchanged (system-curve intersection in Q stays put), pressure rises by 1.4×, so shaft power rises by 1.4×: from 51.3 kW to 72.0 kW.
• The 75 kW motor is on the line. A 55 kW motor (sized on hot-duty power alone) trips on cold start. That is what the red banner in the figure is warning about.

6. The cold-start trap that picks the ID motor

This is the single  common cause of "the ID fan motor keeps tripping on  light-up" we see on new boilers.

On a cold boiler the casing, ductwork and dust collector are full of ambient air, not hot flue gas. The damper at the ID fan inlet is usually set closed to reduce the cold-start inrush, but as soon as the burner lights and the damper starts opening, the density of the gas in the fan drops from 1.18 kg/m³ to 0.84 kg/m³ over a few minutes. The fan power falls with it. The problem is thefirst 30 seconds, when the fan is seeing 1.18 kg/m³ air against a system curve sized for 0.84 kg/m³ gas.

Two rules that fix this:

1. Size the ID motor for the cold-start condition - which for a 150 °C design case means multiplying the hot-duty shaft power by 1.4 (for a 150 °C ΔT) or 1.7 (for a 250 °C ΔT). For our 20 t/h example the 51.3 kW hot-duty shaft power becomes 72 kW cold-start, and the next standard motor frame up is 75 kW.
2. Start the ID fan with the inlet damper closed and open it as the gas temperature rises. This is the line in the manual that the installer skips on the  commissioning and the operator skips on every cold start after that, until a winter morning when the motor trips and someone finally reads the page.

7. Which ZIFENG series goes where

Mapping the matrix back to our catalogue, and only to our catalogue. The figure is a decision tree, not a comparison - it tells you where to start the selection, not which is "best".

Three things that change the answer on both sides - the box at the bottom of the figure:

• Altitude. The fan moves the same mass, not the same volume. At 2,000 m above sea level the air density is about 18 % lower than at sea level, so for the same combustion-air mass you need about 18 % more volumetric flow. Get this wrong on the FD side and the burner runs fuel-rich; get it wrong on the ID side and the draft setpoint is unachievable. The ZIFENG selection sheet already adds about 9 % volume per 1,000 m, but you still have to tell us the altitude on the RFQ.
• Future retrofit. If the plant may add an FGD, a second dust collector, or a waste-heat boiler downstream in the next five years, say so on the  RFQ. Each of those adds 1.5 - 2.5 kPa, and it is much cheaper to oversize the ID fan impeller and the motor frame now than to retrofit later.
• Drive margins. Already covered in section 6, but worth restating: size the ID motor on the cold-start density, not on the hot-gas duty point.

8. Specification checklist for your next boiler RFQ

Twelve items to put on the RFQ. We can quote from a subset, but the more you give us the smaller the safety factor we have to add.

1. Boiler steam capacity (t/h) and steam pressure / temperature.
2. Fuel type and LHV. Excess-air target (typically 10 - 15 % for gas, 15 - 25 % for coal, 20 - 30 % for biomass / waste).
3. Altitude above sea level.
4. FD-side: combustion-air temperature (ambient, or preheated by a steam-coil air heater), inlet filter / silencer requirement, total pressure target at the burner inlet.
5. ID-side: flue-gas temperature at the ID fan inlet, dust load (mg/m³ or g/m³) at design and at the worst-case dirty-baghouse condition, total pressure target at the stack inlet including the dust collector ΔP and any planned FGD.
6. Acid-gas content (SO_x, HCl, moisture) and whether the gas is below the acid dew point at any operating condition. Drives material selection for the ID casing.
7. Drive type preference: direct-coupled, belt-driven, or with a fluid coupling. VFD or fixed-speed. Soft-starter or DOL.
8. Motor standard: IEC or NEMA. Insulation class, IP rating, efficiency class (IE2 / IE3 / IE4).
9. Certifications required: CE, CCC, EAC, ATEX zone, UL / CSA. 
10. Acoustic limit at 1 m (dB(A)).

Site constraints: ambient temperature range, humidity, available duct connections, foundation drawing.

1. Service factor: how dirty the gas is, how often the plant runs at part load, whether the boiler is base-load or two-shift.

9. Frequently asked questions