How much electricity does a VFD on boiler FD and ID fans save in a small textile unit?
About a quarter of the fan's power, if the fan is big and throttled. BEE's Solapur report fits a drive to an ID fan drawing 9.16 kW with its damper 30 to 40% open: 8,250 kWh saved a year, Rs 0.20 lakh at Rs 2.42 a kWh. Cost Rs 0.32 lakh, payback 1.6 years, not the cover's 1.9.
Why a damper wastes power
About 30 boilers and 10 thermic fluid heaters in the cluster have ID fans pulling flue gas and FD fans pushing combustion air, controlled by hand-set dampers. A fan with its damper half shut still spins at full speed. Fan power falls with the cube of speed, so slowing the motor instead of choking the air cuts power far more than the flow. The report assumes 25%, a little under the cube law's 27% for a 10% slower fan.
Two fans, two answers
- Unit 1: ID fan drawing 9.16 kW, damper 30 to 40% open, 12 hours a day for 300 days = 33,000 kWh a year. A 25% cut saves 8,250 kWh and Rs 0.20 lakh. Payback 0.32 / 0.20 = 1.6 years.
- Unit 2: ID fan drawing 2.2 kW, damper 40 to 50% open = 7,920 kWh a year. A 25% cut saves 1,980 kWh, about Rs 4,800 at the report's tariff. The same Rs 0.32 lakh drive would take over 6 years to pay back.
- The report lists the cluster's ID fans as 3 HP, which fits unit 2 but not a fan drawing 9.16 kW. Check your motor's nameplate.
What it costs and how to fit it
- Drive, panel, switches and cabling Rs 0.30 lakh, fitting and interest Rs 0.02 lakh, Rs 0.32 lakh for one fan.
- Two to three hours to install. Leave the damper fully open and let the drive set the draught.
- Set a minimum speed with your boiler operator so the furnace never goes positive or loses draught.
The fuel is a separate question
These boilers burn wood and other biomass. The drive saves only electricity. Good draught control can also trim excess air and fuel, but the report does not count that. Wood CO2 is biogenic and is reported apart from fossil emissions.
The 9.16 kW ID fan, one year
- 9.16 kW x 12 h x 300 days = about 33,000 kWh
- Saved at 25%: 8,250 kWh
- CO2: 8,250 x 0.675 = 5.6 t
- The report's 7 t uses an older grid factor of about 0.85
CO2 at CEA's FY 2025-26 grid factor of 0.675 kg per kWh.
What Senseible does here
Senseible's calculators turn kWh into CO2 on the current grid factor, and it reads monthly electricity bills into records with the bill attached. It has no factor for wood, so keep the tonnes burnt in your own log.
- Carbon calculators
- Boiler excess air and fuel saving
- Solapur textile unit boiler CO2
- VFD on a yarn doubling machine
Sources: BEE, Detailed project report: variable frequency drives for FD and ID fans, Solapur textile cluster (SAMEEEKSHA), CEA CO2 Baseline Database, version 22.0
Last reviewed 2026-11-01. Report figures are from a BEE SME programme project report; costs and paybacks are at the report year's prices. Check today's tariff and a current drive quote.