How condensing gas fired hot water boilers work
Condensing gas fired hot water boilers heat water for process washing, CIP systems, pasteurising lines and space heating in food plants. They burn natural gas or LPG in a premix or low NOx burner and cool the flue gas below its water dew point, about 55°C on natural gas, so latent heat is recovered. Return water at 30 to 50°C makes this possible. The flue gas leaves at about 40 to 70°C, and the slightly acidic condensate is drained and neutralised. Heat exchangers are stainless steel or aluminium alloy. The burner fan pushes the gas through the boiler and a sealed flue under positive pressure; an ID fan is not used.
Where the FD fan sits
- Air intake
- Gas venturi
- FD fan
- Premix burner
- Heat exchanger
- Condensate trap
- Flue
In premix units the fan sits ahead of the burner, with a venturi or gas valve at its inlet so that gas is drawn in proportion to the air. It delivers the premixed air and gas to a surface or cylindrical burner inside the heat exchanger. Fan pressure covers the burner, the closely spaced heat exchanger passages and the positive-pressure flue to atmosphere, typically 50 to 150 mmWC on larger units. Larger industrial units use conventional low NOx burners with a separate air fan instead.
Air at the FD fan
| Air temperature at the fan | Plant room air, 25 to 40°C |
| Pressure at the fan outlet | Typically 50 to 150 mmWC |
| Turndown | 5:1 to 10:1 on modulating premix burners |
| Medium | Air, or an air and gas premix in premix designs |
| Dust and humidity | Clean; steam and wash-down humidity in food plants |
| Density at the design temperature | About 1.15 kg/m³ at 35°C, 96% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for condensing gas fired hot water boilers
| Blade form | Backward curved, compact high-speed impeller on premix units |
| Material of construction | Aluminium casing and impeller on premix fans; mild steel on larger separate fans |
| Drive and arrangement | Direct mounted on an EC motor or VFD-driven motor |
| Bearings and cooling | Motor bearings |
| Flow control | Speed control by EC motor or VFD, with pneumatic air-gas ratio control |
| Also specified | Gas-tight casing for premix duty, speed or pressure feedback, inlet filter in humid wash-down areas |
What this FD fan has to cope with
Back pressure from the condensing heat exchanger
Condensing heat exchangers have narrow passages that foul with scale or with debris drawn in with dirty air, and a blocked condensate trap can back water into the gas path. Back pressure rises and the fan cannot reach full output. Pressure margin on the fan, trap maintenance and inlet filtration keep the boiler at its rated output.
Gas-tightness of premix fans
Where the fan handles a combustible air-gas mixture, any leak from the casing or shaft entry releases gas into the plant room. Gas-tight casings, a sealed shaft entry, a pre-purge with the gas valve closed, and leak checks at every service are part of the design and maintenance of premix fans.
Humid wash-down air
In food plants, hot water boilers often sit close to wash-down areas, and humid or steamy air reaches the fan. Moisture on the motor and electronics causes faults, and lint or cleaning chemicals deposit on the impeller. A ducted intake from a dry area and enclosures rated for humidity protect the fan and its drive.
Energy and control
Hot water demand in food plants peaks with cleaning cycles and is light between them. A burner fan with a wide speed range lets the boiler modulate down instead of cycling, which keeps return water cool and condensing efficiency high. Fan power is small next to fuel, but EC or VFD speed control still saves meaningfully over fixed-speed damper control during the long low-load hours.
Questions buyers ask
Does a condensing boiler fan handle gas?
In premix condensing boilers the fan often draws gas into its inlet with the air, so it handles an air-gas mixture. The casing is gas-tight and the control proves air flow before gas is admitted. In larger units with separate burners, the fan handles only air.
Why does a condensing gas fired hot water boiler need higher fan pressure?
The heat exchanger has narrow passages to cool the gas below its dew point, and the flue runs under positive pressure. Both add resistance, and condensate or scale can add more over time. The fan must cover this at full fire with some margin, or the boiler will not reach its rating.
Does a condensing gas fired hot water boiler need an ID fan?
No. The burner fan pushes the gas through the heat exchanger and flue under positive pressure, and the flue is sealed for that. An ID fan is considered only for unusual flue routes, or where several boilers share a common flue that needs fan assistance to stay under suction.
Information needed to quote an FD fan for condensing gas fired hot water boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burner type. Premix or conventional low NOx, and the burner rating.
- Flue route. Flue length and material, and whether boilers share a common flue.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.