In the Bureau of Energy Efficiency Vapi hot air generator report, which efficiency and wood figures can a dye unit trust?

A hot air generator is the wood-fired heater that dries wet dye powder. The Bureau of Energy Efficiency (BEE) Vapi report tests one old unit at 27% efficiency and proposes a 53% one, saving 150 tonnes of wood a year for rupees (Rs) 6.78 lakh (678,000 rupees). Payback is 1.52 years, but several printed figures disagree.

What the machine does, in plain words

After filtering, dye powder is still wet. A hot air generator burns wood to heat air, and a fan blows that hot air over trays of wet powder until it dries. Think of a hairdryer for chemicals, with a log fire where the heating coil would be.

Efficiency is the share of the wood's heat that ends up in the air going to the trays. Think of carrying water in a leaky bucket: efficiency is how much reaches the tank instead of dripping out on the way. A low figure means you buy wood that mostly heats the chimney and the room.

The report is a 2010 detailed project report (DPR), a costed plan for one upgrade, written by the Bureau of Energy Efficiency (BEE) for dye and chemical units in Vapi, Gujarat.

What it measured and what it proposes

• Old unit, tested at one Vapi plant: 450 kilograms of wood burnt in 3 hours, each kilogram holding 3,800 kilocalories (kcal) of heat. That is 1,710,000 kcal in. The hot air carried 457,459 kcal out. 457,459 / 1,710,000 is about 27%.
• New unit proposed: a three-pass wood-fired generator with a fan that pulls the draught, rated 150,000 kcal an hour and burning 75 kilograms of wood an hour. 75 x 3,800 = 285,000 kcal in, so 150,000 / 285,000 is about 53%.
• Saving: wood falls from 305 to 155 tonnes a year, so 150 tonnes. Power use rises by 14,000 kilowatt hours (kWh) a year for the extra fan and blower.
• Money: wood at Rs 3,500 a tonne, power at Rs 5.7 a kWh, a net saving of Rs 4.45 lakh a year (a lakh is 100,000) against a cost of Rs 6.78 lakh. Payback is how long the saving takes to repay the cost, like counting how many months of saved bus fares pay for a bicycle: 6.78 / 4.45 = 1.52 years.
• Basis: 6 hours a day for 350 days, 2,100 hours a year, and 7 days of shutdown to fit it.

Where the report disagrees with itself

• Old efficiency: the summary says 25 to 30%, section 1.3.1 says 20 to 37%, and the annexure tests one unit at 27%. Only the 27% has a calculation behind it.
• Wood for the typical unit: Tables 1.1 and 1.2 show 505 tonnes (505,000 kilograms for 603,000 kilograms of dye). Section 3.1.1 and the benefit table start the saving from 305 tonnes. Our check: the tested unit burns 150 kilograms an hour, and 150 x 2,100 hours = 315 tonnes, close to 305 and far from 505.
• Unit names: the text names Swapnil Organics for the 603 tonne unit, while Table 1.2 gives the same 505,000 and 603,000 to Nandosal Chem Industries. The Vapi 400 kilogram boiler report gives Swapnil Organics 80,000 kilograms of production.
• Equipment cost: the text and Table 4.1 say Rs 5.40 lakh. Annexure 5 says Rs 5.30 lakh, and 5.30 + 0.30 + 0.35 + 0.73 adds to Rs 6.68 lakh, not the 6.78 printed. The 5.40 version adds to 6.78.
• Heat out: 13,875 kilograms of air x 140 degrees of rise x 0.24 = 466,200 kcal, against 457,459 printed. Either way the efficiency rounds to 27%.
• Some headings still say pumps: Annexure 1 is titled efficiency of the existing pumps. Our reading is that the report was built from a pump template, so check each number before you borrow it.

Before you use it for your own unit

• Weigh the wood you burn in the generator for a full week and divide by the hours it runs. That is your baseline, not 305 tonnes.
• Measure the air going in and coming out, as the report does, before you accept 27% for your unit. The report's own 20 to 37% range shows units differ.
• Ask the maker to put the efficiency at your trays and load in writing.
• Redo the payback at today's wood and power prices.
• Our reading: the 198 tonne carbon dioxide line counts wood saved as a cut. How that treatment sits with the greenhouse gas rules is on the Vapi reports page linked below.

The report's saving rebuilt from its own inputs

• Old unit: 450 kg in 3 hours = 150 kg an hour. 150 x 2,100 hours = 315 tonnes (305 printed)
• New unit: 150,000 / 0.53 = about 283,000 kcal in. 283,000 / 3,800 = about 74.5 kg an hour (75 printed). 75 x 2,100 = 157.5 tonnes (155 printed)
• Wood saved: 305 minus 155 = 150 tonnes x Rs 3,500 = Rs 5.25 lakh
• Extra power: 14,000 kWh x Rs 5.7 = Rs 0.80 lakh. Net saving Rs 5.25 minus 0.80 = Rs 4.45 lakh
• Payback: 6.78 / 4.45 = 1.52 years

All inputs are the report's own, at 2010 prices. The 315 and 157.5 tonne lines are our check on the 305 and 155 printed.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so a year of wood or briquette purchases is on file to test the report's baseline against. Wood weighed at the dryer and hot air readings are taken on site; sensors can be set up with the Senseible team on request.

• Green loan readiness check
• Vapi reports: wood counted as a cut or zero
• Efficient wood hot air generator saving
• Boiler efficiency: direct and indirect method

Sources: BEE, Detailed project report: hot air generator, Vapi dyes and chemicals cluster (2010, SAMEEEKSHA), BEE, Detailed project report: energy efficient boiler 400 kg/hr, Vapi dyes and chemicals cluster (SAMEEEKSHA)

Last reviewed 2026-11-07. Figures are from the report named above, written in 2010; costs, fuel prices and paybacks are at that year's prices. Check today's prices and a current maker's quote before you rely on the payback.