How solar thermal plant auxiliary boilers work
Concentrating solar thermal plants, using parabolic trough or tower collectors, need auxiliary fired heat for conditions the sun cannot cover: protecting heat transfer fluid or molten salt from freezing at night, raising start-up steam, warming the turbine and keeping the plant ready on cloudy days. This duty is met by a gas or oil-fired auxiliary boiler or a fired heater for the heat transfer fluid. These are usually packaged, pressurised furnace units with a forced draft burner, so there is no ID fan; the flue gas goes directly to a short stack. The plants sit in hot, dusty, often desert locations, and the auxiliary boiler may run for many hours one week and stand idle the next.
Where the FD fan sits
- Inlet filter
- FD fan
- Air damper
- Burner windbox
- Pressurised furnace
- Stack
The fan is mounted on the boiler skid or beside the burner and delivers air to the windbox. Because the furnace is pressurised and there is no ID fan, the FD fan pressure must cover the burner, furnace and the boiler or heater gas passes up to the stack, typically 250 to 500 mmWC. In desert sites the fan inlet is ducted through a filter or louvred hood to keep sand out, and the motor may sit in an enclosure for shade.
Air at the FD fan
| Air temperature at the fan | Ambient, up to 45 to 50°C in desert locations |
| Low ambient | 5°C or lower on winter nights, when freeze protection demands most firing |
| Dust | Fine sand and dust; inlet filtration needed |
| Standby | Long idle periods, then fast start-up |
| Pressure at the fan outlet | Typically 250 to 500 mmWC |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 solar thermal plant auxiliary boilers
| Blade form | Backward curved, for a stable rising curve and good burner control |
| Material of construction | Mild steel with external paint system suited to hot, sandy environments |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 on the boiler skid |
| Bearings and cooling | Sealed bearings and motor protected from dust ingress |
| Flow control | Variable speed drive or inlet damper linked to the burner modulation |
| Also specified | Inlet filter with differential pressure alarm, and fan sized at maximum ambient temperature |
What this FD fan has to cope with
Loss of capacity at high ambient
At 50°C air density is about 10% lower than at 20°C, so a fan sized at standard conditions cannot supply full combustion air on the hottest days. The fan is selected at the maximum site ambient temperature and altitude. Altitude adds to the effect at inland sites.
Sand ingress
Fine sand drawn in with the air erodes the impeller, wears bearings through their seals and blocks the burner air register. A filtered inlet, sealed bearings and regular filter changes keep the fan in condition. Filters sized for low face velocity hold more dust between changes and keep the pressure loss steady.
Starting after long standby
After weeks idle, bearings may lack lubricant film and dampers may stick. A periodic test run, with damper stroke checks, ensures the fan starts when freeze protection or start-up steam is needed. Space heaters in the motor prevent condensation in the windings during cold desert nights.
Energy and control
Auxiliary boilers in solar plants run intermittently, often at low load for freeze protection. A variable speed drive allows the fan to follow the burner at low firing without throttling and reduces start-up current. Because running hours are limited, efficiency is less important than reliability, and fan selection gives priority to robust construction.
Questions buyers ask
Does a solar thermal plant auxiliary boiler need an ID fan?
Normally no. These are packaged pressurised furnace boilers or heaters, and the FD fan overcomes the full gas side resistance up to the stack. An ID fan is needed only where heat recovery or long flues add resistance. The FD fan pressure therefore includes the stack.
How do you size an FD fan for high ambient temperature?
Calculate the air density at the maximum site ambient temperature and altitude, and select the fan to deliver the full air mass flow at that density. Sizing at standard conditions underestimates the fan size. The motor is then checked for the higher power at the coldest night ambient, when the air is densest.
What filtration does a combustion air fan need in a desert solar plant?
A filtered inlet with louvres or a weather hood and panel filters, sized for low velocity, protects the fan and burner. The differential pressure should be monitored and filters changed regularly. The inlet is placed away from roads and mirror washing areas, where dust is heaviest.
Information needed to quote an FD fan for solar thermal plant auxiliary 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.
- Site ambient. Maximum and minimum ambient temperature, altitude and dust conditions.
- Duty pattern. Expected running hours, number of starts and standby periods per year.
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.