Does a speed controller on a 30 horsepower tea dryer blower save more than on a 25 horsepower one, and how do the two reports compare?

A speed controller on a 30 horsepower tea dryer blower saves 25,236 kilowatt hours a year in the Bureau of Energy Efficiency (BEE) Jorhat report, against 15,408 for a 25 horsepower blower, for almost the same price. The 30 horsepower blowers were measured working harder: their input power was 93% of the motor's rated output, against 68%.

What the controller does, in plain words

A tea dryer pushes hot air through the fermented leaf with a blower. Today the blower runs at one speed and a flap in the air pipe (a damper) is kept 20% to 40% shut to trim the flow. The motor still pulls nearly full power. It is like turning down the heat by half-closing a pipe valve while the pump runs flat out.

A variable frequency drive (VFD) is a controller that slows the motor itself. Both reports open the flap fully and run the blower 15% slower. The blower then draws less power for the same drying.

The two reports side by side

• 25 horsepower (hp) blowers, 7 units measured, average 12.69 kilowatts (kW) with a range of 7.50 to 18.24 kW. After the controller 8.41 kW. Saving 4.28 kW, or 15,408 kilowatt hours (kWh) a year. Cost 1.98 lakh rupees (Rs), where a lakh is 100,000, payback 1.72 years.
• 30 hp blowers, 8 units measured, average 20.76 kW with a range of 17.50 to 23.40 kW. After the controller 13.75 kW. Saving 7.01 kW, or 25,236 kWh a year. Cost Rs 2.09 lakh, payback 1.11 years.
• Both reports run the blower 12 hours a day for 300 days (3,600 hours) and cost power at Rs 7.45 a kWh, a mix of 70% grid at Rs 5.5 and 30% diesel generator at Rs 12.
• Both expect the saving to be 33.7% to 33.8% of the blower's power: 4.28 / 12.69 and 7.01 / 20.76.
• The 30 hp controller costs Rs 0.11 lakh more and saves 9,828 kWh more a year.

Why the bigger blower pays back faster

The controller price barely moves with motor size, but the saving moves with the power the blower actually draws. A 25 hp motor is about 18.65 kW and a 30 hp motor about 22.38 kW. The 25 hp blowers averaged 12.69 kW of input power, equal to 68% of the motor's rated output. The 30 hp blowers averaged 20.76 kW, equal to 93%. The reports' kilowatts are electricity going into the motor, while a horsepower rating is the push the shaft delivers, and motor losses sit in between. So the real shaft load is a little lower than these percentages, but the 30 hp blowers still work harder. That is our own arithmetic on the reports' numbers, but it explains the gap in payback.

So the rating on the nameplate is not the guide. Your own measured kilowatts are. One 25 hp unit in the report drew only 7.50 kW, and at that load the saving would be about 2.5 kW, not 4.28.

A slip in both reports, and what it does to the saving

Both reports say a 15% cut in speed lowers power by about a third and print the working as (0.15) cubed, or 33.75%. Cubing 0.15 gives 0.34%, so the printed working is a slip. The cube law the reports quote applies to the speed that remains: 0.85 x 0.85 x 0.85 is 0.614, so power should fall by 38.6%.

Both reports use 33.75%, which is lower than the cube law gives. Our reading is that the printed saving is on the cautious side, if the blower really drops to 85% of its speed and the flap is fully open. Fan laws are a guide, not a guarantee, so measure before and after.

The carbon dioxide line

• The reports show 15.86 tonnes of carbon dioxide (CO2) saved for 25 hp and 26.00 tonnes for 30 hp. Neither report prints a factor, but 15.86 / 15,408 and 26.00 / 25,236 both work out to about 1.03 kilograms per kWh.
• On the grid factor Senseible uses now for financial year (FY) 2025-26, 0.675 kilograms per kWh, the same savings are 10.4 and 17.0 tonnes if all of it came from the grid.
• Some of the report's power is diesel generator power, at a higher factor per kWh. If your blower often runs on the generator, the true CO2 saving is higher than the grid reading.

Both reports, line by line

• 25 hp: 12.69 minus 8.41 = 4.28 kW x 3,600 hours = 15,408 kWh. 15,408 x Rs 7.45 = Rs 1.15 lakh. 1.98 / 1.15 = 1.72 years
• 30 hp: 20.76 minus 13.75 = 7.01 kW x 3,600 hours = 25,236 kWh. 25,236 x Rs 7.45 = Rs 1.88 lakh. 2.09 / 1.88 = 1.11 years
• Input power against rating: 12.69 / 18.65 = 68% for 25 hp; 20.76 / 22.38 = 93% for 30 hp (shaft load is a little lower, as motor losses come off)
• Cube law instead of 33.75%, 30 hp: 20.76 x 0.386 = 8.01 kW x 3,600 = about 28,800 kWh a year
• Grid CO2 at 0.675: 25,236 x 0.675 = 17,034 kg, or 17.0 tonnes; 15,408 x 0.675 = 10,400 kg, or 10.4 tonnes

These are the reports' printed figures with our own arithmetic on top. The cube-law line is a reading of the fan law the reports quote, not a figure either report prints. Prices are the reports' 2010 prices.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records. The standard read is invoices and bills. Blower meters are a custom service, on request. Meanwhile a clamp meter before and after works, so keep the readings with the bills.

• Speed control on a tea dryer's hot air blower: the 25 hp case
• Tea dryer motors compared at 15, 18.5 and 22 kW
• Tea factory carbon footprint per kg of made tea
• Green loan readiness check

Sources: BEE and SAMEEEKSHA detailed project report: VFD in the hot air system of a tea drier with a 30 HP blower, Jorhat tea cluster, BEE and SAMEEEKSHA detailed project report: VFD in the hot air system of a tea drier with a 25 HP blower, Jorhat tea cluster, CEA CO2 Baseline Database for the Indian Power Sector, version 22.0 (FY 2025-26 grid factor)

Last reviewed 2026-11-07. Costs and paybacks are at the reports' 2010 prices. The input-versus-rating percentages and cube-law line are our own arithmetic on the reports' printed numbers. Re-check with measured blower kilowatts and a current quote.