Two quotations, the same capacity on paper, and very different fuel bills six months later. Here is how to spot the difference before you sign the order.
Two dryer quotations land on your desk. Both promise 500 kg of water removed every hour. One costs a little more. After commissioning, the fuel bills tell a different story.
The price on the quotation is paid once. The energy bill is paid every shift.
Most buyers compare price, capacity and delivery time. Very few compare the one number that decides running cost: the energy needed to remove each kilogram of water. There is no single “correct” figure for it, but there is a fair way to compare it.
Start With the Water, Not the Machine
Every dryer, whatever its shape, does one job: it turns liquid water into vapour.
At atmospheric pressure and around 100°C, evaporating 1 kg of water takes about 2,258 kJ of energy. That is roughly 539 kcal, or 0.63 kWh. It is a useful benchmark because it covers the evaporation alone, before any losses.
Three conversions will keep you out of trouble. One kcal is 4.19 kJ. One kWh is 3,600 kJ. One kWh is therefore about 860 kcal. Vendors quote in different units, so convert everything to one before comparing.
How Much Water Are You Actually Removing?
You cannot compare energy per kilogram of water until you know how many kilograms of water there are.
When moisture is quoted on a wet basis, the water removed is:
Water removed (kg) = wet weight × (initial moisture % − final moisture %) ÷ (100 − final moisture %)
A published grain-drying example shows it clearly. Take 1,000 kg of corn at 28% moisture and dry it to 14%. The water removed is 1,000 × 14 ÷ 86, which is 162.8 kg. Evaporating that water needs about 87,800 kcal even in a perfect world.
A warning on moisture basis. Plant records usually use wet basis, while the BEE dryer testing code works in kilograms of moisture per kilogram of bone-dry material. Mixing the two can give a substantially wrong evaporation load, and every calculation after it goes wrong too.
Why Real Dryers Use More Than the Minimum
The distance between the physics benchmark and your fuel bill is where the money goes.
A 2024 review published in Applied Food Research cites about 2,620 kJ/kg as the theoretical consumption of a properly designed and operated hot-air dryer. Real plants often use far more.
As a simple illustration, divide the 2,258 kJ/kg benchmark by an assumed efficiency. At 60% you get about 3,760 kJ/kg. At 50% it is about 4,520, and at 40% about 5,650. This is a simplified illustration only. A formal efficiency test also counts the heating of the liquid and the superheating of the vapour.
Published ranges show how wide the gap can be. Published Teaching references commonly quote 2,000 to 3,000 kJ per kg of water for steam-heated contact dryers and 4,000 to 10,000 kJ for conventional hot-air dryers. Actual consumption depends heavily on the material, operating conditions and design. A UK government-commissioned survey of industrial spray dryers, published in 2005, found figures from about 3 to 20 GJ per tonne of water, with an average of 4.87 GJ.
A worked example from the 2006 BEE dryer testing code makes it concrete. The sample dryer evaporated 1,182 kg of water an hour and took in 5,624,130 kJ an hour. That works out to about 4,760 kJ per kg, or 1.32 kWh. Under the code’s definition, 54.87% of the input counted as useful thermal energy, while 39.63% left with the exhaust. In that example, exhaust was the largest identified loss.
Turn Energy Into Rupees
A kilojoule means nothing to your accounts team. A rupee per kilogram of water does.
Fuel required per kg of water = thermal energy needed at the dryer ÷ (fuel calorific value × heater or combustion efficiency)
Fuel cost per kg of water = fuel required × fuel price
Then add auxiliary electricity for fans, conveyors and pumps as a separate line. The BEE code records electricity separately from the thermal heat balance, and you should too.
Here is an illustration only. Assume coal at 5,000 kcal/kg and 70% useful heater efficiency, so each kilogram of coal delivers 3,500 kcal. A hot-air dryer at 4,500 kJ/kg (about 1,075 kcal) needs roughly 0.31 kg of coal per kg of water. A dryer at 2,500 kJ/kg (about 600 kcal) needs about 0.17 kg. At 500 kg of water an hour over 4,000 hours a year, that gap is roughly 273 tonnes of coal. Use your own fuel quality and price, not these assumptions.
Electric drying needs the same care. One RF dryer manufacturer reports about 1.2 kg of water per kWh of RF power, or roughly 0.83 kWh per kg. That is a machine-specific figure, not an RF benchmark, and it leaves out fans and other auxiliaries. A kWh of electricity and a kWh of fuel heat rarely cost the same, so compare rupees per kg of water, not kWh.
Five Traps When Comparing Quotations
A fair comparison needs the same yardstick on both sides.
Capacity means different things. One vendor quotes kg per hour of wet feed, another of dry product, another of water evaporated. Ask for evaporation in kg per hour for the same feed and final moisture.
Feed and final moisture differ. A dryer taking 40% moisture down to 5% does a very different job from one taking 20% down to 8%. Reducing final moisture further can take disproportionately more energy and residence time, especially in the falling-rate period.
Design figures are not plant figures. Nameplate numbers are promises. Site measurements are results. Ask what test method and system boundary produced the number.
Thermal and electrical energy get mixed. A dryer with a low heat figure can still carry a heavy fan load. Convert both to rupees and add them.
Efficiency claims are not comparable. Efficiency numbers only compare when the test method, boundary and energy basis are the same. A percentage without those details tells you very little.
Measure Your Own Dryer
The most honest number is the one measured in your own plant.
The BEE method is simple in outline. Measure the moisture of the material entering and leaving the dryer, using sealed samples so that the sample neither loses nor picks up moisture. Weigh the dried product or measure its flow. Record the thermal input from the steam meter or from fuel consumed, and read electricity from separate meters. For a batch dryer, cover the whole batch, because drying rate and energy input change through the cycle. For a continuous dryer, run it under stable conditions for a few hours and repeat the test.
Accuracy matters. The BEE worked example showed an uncertainty of 7.4% on the calculated efficiency. Improving steam flow, material flow and moisture measurement reduced it far more than improving air-temperature measurement did. Start with a reliable moisture test and a reliable scale.
Where the Savings Usually Hide
Before replacing a dryer, check what the current one is wasting.
The BEE code points to a familiar list. Remove water mechanically before it reaches the dryer, using a centrifuge or filter press where the material allows. Stop steam leaks and check traps. Seal doors to prevent air leaks. Keep filters and fans clean. Improve insulation. Watch exhaust temperature and humidity, and consider heat recovery.
Drying stage matters too. In the constant-rate stage, higher temperature and airflow speed up drying. In the falling-rate stage, the drying rate is limited by moisture moving out from inside the material, so extra heat does not help in the same way. Controls that follow the drying curve waste less energy than a dryer that runs flat out throughout.
The Number Worth Asking For
Next time you compare dryers, ask for evaporation in kg per hour, the feed and final moisture behind it, and the energy input in one common unit. Then convert it to rupees per kg of water. Kerone Engineering Solutions Ltd. also looks at evaporation load, feed and final moisture, and energy source together when it considers dryer selection, which is the same logic used above. What is your dryer really spending to remove one kilogram of water?
Frequently Asked Questions
1. What is specific energy consumption (SEC) in a dryer?
It is the energy a dryer uses per kilogram of water evaporated. The BEE dryer code expresses it as kg of steam, kJ or kg of fuel per kg of evaporation. It lets you compare dryers of different sizes on the same basis.
2. What is a good SEC for an industrial dryer?
There is no single figure. It depends on the dryer type, material, feed moisture and operation. Published ranges are wide, from 2,000 to 3,000 kJ/kg for some contact dryers to 4,000 to 10,000 kJ/kg for hot-air dryers. Track your own figure over time.
3. How do I convert kcal, kJ and kWh?
One kcal equals 4.19 kJ. One kWh equals 3,600 kJ, or about 860 kcal. Convert every quotation to one unit before comparing them.
4. What is the difference between wet-basis and dry-basis moisture?
Wet basis is water as a share of the total wet weight. Dry basis is water per kilogram of bone-dry material. Mixing the two gives a substantially wrong evaporation load.
5. How do I calculate the water removed by my dryer?
On a wet basis, multiply the wet weight by (initial % − final %) and divide by (100 − final %). Drying 1,000 kg from 28% to 14% removes 162.8 kg of water.
6. Does a higher drying temperature always save energy?
No. Higher temperature and airflow help in the constant-rate stage. In the falling-rate stage, internal moisture movement limits the rate, so more heat does not help in the same way.
7. Should I compare thermal and electrical energy together?
Record them separately, as the BEE code does. Then convert each to rupees using your fuel price and electricity tariff, and add them for a full cost per kg of water.
8. Is electric or RF drying cheaper than fuel-fired drying?
It depends on your tariff, material and machine. One RF manufacturer reports about 1.2 kg of water per kWh, but that is a machine-specific figure. Compare rupees per kg of water, including fans and auxiliaries.
9. How can I measure my dryer’s SEC in the plant?
Measure inlet and outlet moisture, the dried product weight or flow, and the thermal and electrical energy used. For batch dryers, cover a full batch. For continuous dryers, run under stable conditions and repeat the test.
10. Why do suppliers quote different efficiencies for similar dryers?
Efficiency depends on the test method, what is included inside the system boundary, and the energy basis. Two numbers are only comparable when all three match. Always ask how a figure was measured.