Senseible

What does fan speed control save in a 16-trough tea factory compared with a 10-trough one, and does the 16-trough speed plan add up?

Slowing all 32 withering fans in a 16-trough tea factory saves 92,520 kilowatt hours a year in the Bureau of Energy Efficiency (BEE) Jorhat report, against 58,176 for 10 troughs. Each fan saves about the same. The 16-trough text and table describe two different speed plans, so check which one you would run.

The plain picture first

Withering is the first step in making tea. Fans blow air through freshly plucked leaf lying in long troughs, so the leaf goes limp. Today the fans run at full speed and someone moves a flap in the air pipe (a damper) by hand to cut the flow. That is like driving with the accelerator flat down and slowing the car with the brake.

A variable frequency drive (VFD) is a controller that slows the motor itself, so the fan simply spins slower and draws less power. A programmable logic controller (PLC) is a small box that holds the timetable of which speed to run when. One PLC cabinet runs all the drives.

The two reports side by side

Where the 16-trough report does not agree with itself

In plain words: the report's written plan and its table are two different timetables for the fans, and only the table gives the headline saving.

The 16-trough text says the fans should run 6 hours at full speed, 4 hours at 87.5% and 2 hours at 75%. The table that produces the 92,520 kWh uses a different plan: 1 hour at full speed, 3 hours at 90%, 4 hours at 85% and 4 hours at 75%. The 10-trough text and table both use the table plan.

On the text plan, with the report's own rule that a 10% cut in speed cuts power by 27%, the saving falls to about 48,500 kWh, not 92,520.

The rule itself is right at 10%: the report states the fan law that power follows the cube of speed, and 1 minus 0.9 x 0.9 x 0.9 is 27.1%. But the table stretches 27% per 10% in a straight line, which gives a 40.5% cut at 85% speed and 67.5% at 75%. The cube law gives 38.6% and 57.8%. On the table plan with the cube law the saving is about 84,000 kWh. That is our own reading, not the report's.

What this does to the payback

Before you sign for one

  1. Ask the supplier for the speed timetable in writing: hours at each speed, per day, for your leaf and weather.
  2. Log the hours your fans actually run at each damper position for two weeks, so you know your own starting point.
  3. Put a kWh meter on the withering fan feeder before the work, and read it for the same two weeks after, so the saving is measured and not assumed.

Three readings of the 16-trough plan, per fan per day

All figures are an example worked from the report's printed numbers. The cube-law and text-plan lines are our own arithmetic on those numbers, not figures the report prints.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records. The standard read is those bills, so a saving shows as fewer units. Fan hour logging or meters can be added as a custom service, on request. Meanwhile keep your own log of fan hours next to the bills.

Sources: BEE and SAMEEEKSHA detailed project report: automatic air flow variation in a withering system with 16 troughs, Jorhat tea cluster, BEE and SAMEEEKSHA detailed project report: automatic air flow variation in a withering system with 10 troughs, 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 cube-law and text-plan readings are our own arithmetic on the reports' printed numbers; re-check them against a supplier's written speed timetable.