The short answer: evaporation intensity determines how large the machine has to be, which drives capital cost. Specific heat consumption determines how much fuel you burn each year, which drives running cost. Both figures are often left vague in quotations, yet they are the main reason quotations from different suppliers differ. Compare these two lines before comparing machine prices.
1. Evaporation intensity: the basis for machine size
Evaporation intensity is the amount of water that can be evaporated per unit of heat transfer area (or per unit volume) per hour — typically kg water/(m²·h) or kg water/(m³·h).
It sets the key dimensions, in this chain:
- Water to evaporate (kg/h) ÷ evaporation intensity = required heat transfer area or volume
- Required area or volume → drum diameter × length, or drying duct size
A higher assumed intensity means a smaller machine and a lower price. That is why this number matters so much.
Volume-based figures are more common in practice. The ranges below give an order of magnitude only; the actual value must be based on material testing or trial data:
| Machine type | Volumetric evaporation intensity (order of magnitude) |
|---|---|
| Rotary drum dryer | Tens of kg water/(m³·h) |
| Flash airflow dryer | Tens to around a hundred kg water/(m³·h), depending on disintegration quality and inlet temperature |
Airflow types reach roughly one order of magnitude above drum dryers because the material is dispersed into suspended particles, giving an extremely large heat transfer surface per unit volume, and because a residence time measured in seconds allows a higher inlet air temperature. That is also why the equipment is compact.
2. Specific heat consumption: the basis for annual fuel cost
Specific heat consumption is the heat input required to evaporate 1 kg of water, in kJ/kg water. The physical floor is the latent heat of vaporisation of water at 100 °C, about 2257 kJ/kg.
Actual heat input is far higher, because most of the heat does not go into evaporation. It goes into:
- Heating the material itself (sensible heat)
- Heating the incoming air to process temperature
- Leaving with the exhaust air, which is usually the largest share
- System heat losses
In practice specific heat consumption usually falls in the 3000–4500 kJ/kg water range, corresponding to thermal efficiency of roughly:
- η = 2257 ÷ 3000 ≈ 75% (favourable conditions)
- η = 2257 ÷ 4500 ≈ 50% (poor conditions)
Convective (hot-air) drying is inherently at the lower end, because exhaust air inevitably carries away sensible heat. That is a process limitation rather than a sign of poor equipment. But between 3000 and 4500 the fuel bill differs by 50%, which is why this line has to be pinned down.
3. One question that tests whether the figure is real
If the assumed evaporation intensity in a quotation looks high, there are only two explanations: the supplier has substantial operating data for this specific material, or the machine has been sized too small. From the quotation alone you cannot tell — the difference usually shows up six months into operation.
So before signing, ask: "Which material and which operating plant is this evaporation intensity based on?"
A concrete answer — material type, throughput, operating history — is a good sign. An answer like "it's a rule of thumb" or "it always works out" deserves further checking. This is not suspicion: evaporation intensity depends heavily on stickiness, particle size, and whether the water is surface or bound. For the same machine type, a change of material can double or halve the value.
4. Three lines to compare first
| Item | Why it matters |
|---|---|
| Assumed evaporation intensity | Whether the machine size is reasonable |
| Specific heat consumption (kJ/kg water) | Running cost — the largest long-term expenditure |
| Design conditions: inlet temperature, initial and final moisture (state wet or dry basis) | Without conditions, the two figures above mean nothing |
The third line is easily overlooked. The same nominal "throughput" represents a completely different duty at 40% initial moisture than at 25%. Once these three lines are present, two quotations can be compared directly. Useful questions to ask: what is the evaporation load in your design? Based on your specific heat consumption, how much fuel would be consumed in a year? What initial moisture content does your stated throughput refer to?
5. Why the "cheaper machine" is often a trap
Consider an example. All figures below are illustrative assumptions used to show the order of magnitude; actual values are confirmed by material testing. Annual water to evaporate 2,000 t; Supplier A specific heat consumption 3200 kJ/kg water; Supplier B 4000 kJ/kg water; furnace efficiency 85%; natural gas lower heating value 35.6 MJ/m³; gas price 3.5 per m³ (illustrative).
- Supplier A annual gas: 2,000,000 kg × 3.2 MJ/kg ÷ 0.85 ÷ 35.6 MJ/m³ ≈ 211,500 m³
- Supplier B annual gas: 2,000,000 kg × 4.0 MJ/kg ÷ 0.85 ÷ 35.6 MJ/m³ ≈ 264,400 m³
- Difference: about 52,900 m³ per year, roughly 185,000 in currency at the illustrative gas price
On a timeline: the two machine prices may differ by a few tens of thousands, while the annual fuel bill differs by well over a hundred thousand. Within about two years the running difference has absorbed the price difference — in the opposite direction. And the supplier with the lower specific heat consumption usually quotes a higher machine price, because it built in more heat transfer area, better insulation and a more considered air path.
"A more expensive machine" and "a more expensive project" are two different things. Comparing machine prices alone is comparing a small part of the picture.
6. Turning these figures into acceptance criteria
- State a maximum specific heat consumption in the contract, for example a cap on gas consumed per tonne of water evaporated
- State the test conditions at the same time: which material, initial and final moisture, continuous running time
- Measure two things at acceptance: water evaporated (weighing in and out plus moisture determination) and fuel consumed
- Compare the result against the contract figure
The second item is the key one. Without agreed test conditions the measured result cannot be compared with anything — a small change in material moisture changes the outcome.
7. A real case
For a lithium carbonate rotary drying project in Guangdong, the customer received two proposals with a price difference of nearly 60,000. The difference was in the assumed evaporation intensity: one proposal used a higher figure with a shorter drum, the other a conservative figure with a longer drum. We suggested the customer ask the first supplier which commissioned plant the figure was based on. No specific plant could be named. The customer chose the longer drum, and the measured performance after commissioning was within the expected range.
That extra 60,000 was not paid for steel plate. It was paid for the validation data behind the evaporation intensity figure.
(The above is a record of on-site practice with a customer; specific figures are subject to sampling measurement and trial-run results.)
8. In summary
Choosing drying equipment is a decision about long-term running cost. The machine price is the ticket; specific heat consumption is the bill that follows. Treat it as the first line you compare, not the last.
Henan Jinyida Machinery Equipment Manufacturing Co., Ltd. is located at No.1 Nandi Industrial Zone, Gongyi City, Henan, China, and manufactures rotary drum dryers, electric heating dryers, batch rotary dryers and flash airflow dryers for powdery, granular and flake materials in the chemical, pharmaceutical, feed, food and inorganic substance industries. For a sizing calculation based on your material, send the material name, initial moisture, target moisture and throughput — WhatsApp +8615238028609. Actual evaporation intensity and specific heat consumption vary with material and operating conditions; official figures come from a trial run.
References and Standards
- JB/T 10278—2015 Flash dryers[S]. Beijing: China Machine Press, 2015. (in Chinese)
- JB/T 3266—2015 Rotary drum dryer[S]. Beijing: China Machine Press, 2015. (in Chinese)
- Mujumdar A S. Energy aspects in drying[M]//Mujumdar A S. Handbook of Industrial Drying. 4th ed. Boca Raton: CRC Press, 2014.
- Liu G W. Design Handbook of Drying Equipment[M]. Beijing: China Machine Press, 2009. (in Chinese)
- Liu X D, Li Z Y. Modern Drying Technology[M]. 3rd ed. Beijing: Chemical Industry Press, 2022. (in Chinese)