What a drying test should measure, what sample to send, and how the results decide the model, the jacket medium and the cycle time.
Why a drying test comes before a quotation
A vacuum dryer is sized from the behaviour of one specific material, not from a catalogue table. Two filter cakes that look identical can behave completely differently: one crumbles and releases water quickly, the other smears across the heated wall and insulates itself. Until someone puts the actual material under vacuum and heat, any capacity figure is an assumption.
A small drying test removes most of that uncertainty for very little cost. It also protects both sides: the buyer gets a model that will actually hold the required batch, and the supplier is not asked to guarantee a duty nobody has measured.
What to send
Send two to five kilograms of the real wet material plus the six data points below. The sample decides whether the test means anything; the data points decide what the test has to prove.
- Two to five kilograms of the real wet material, taken after the plant's own dewatering step — not a lab-prepared substitute.
- The liquid being removed: water, a solvent, or a mixture, with the approximate solvent share if it is a mixture.
- Initial moisture content and the basis it was measured on (wet or dry basis) — this is the most common source of confusion.
- The target final moisture, and whether it is an average or a maximum permitted in any part of the batch.
- The maximum product temperature the material tolerates, and how long it may stay at that temperature.
- Any constraint that changes the design: explosion zone, solvent recovery, corrosion, abrasion, or a requirement for a specific discharge arrangement.
Test input record
| Parameter | Unit or basis | Why it changes the design |
|---|---|---|
| Wet feed mass | kg per batch | Sets the working volume |
| Initial moisture | %, wet or dry basis | A basis mix-up can double the water load |
| Target moisture | %, batch average or per-particle maximum | A per-particle maximum is the stricter duty |
| Product temperature limit | °C and minutes | Decides steam, hot water or oil jacket |
| Wet bulk density | kg/L | Converts batch mass into working volume |
Source: Ruibo dryer selection practice. No model is fixed before a test on the actual material.
What the test should measure
Measure four things: the drying curve, whether the material stays friable, the point where diffusion takes over, and the wet bulk density. Together they fix the agitator type, the working volume and the jacket medium.
- The drying curve: moisture against time under the intended vacuum and jacket temperature.
- Whether the material stays friable or turns into a paste that climbs the wall, because that decides whether a rake, a ribbon or a hollow blade is the right agitator.
- The point at which surface moisture stops dominating and internal diffusion takes over — that is where extra heat stops helping.
- The bulk density of the wet feed, which fixes the working volume for a given batch mass.
- Any foaming, crusting, dusting or agglomeration that would need a different charging or discharge method.
Reading the test result
| Observation during the test | What it points to | What it rules out |
|---|---|---|
| Cake crumbles, water leaves quickly | Rake or ribbon with high surface renewal | Feed that has to be pumped |
| Material smears and climbs the wall | Scraper blade close to the heated wall | A simple paddle or plough |
| Surface moisture stops falling early | Diffusion-limited; more heat will not help | Raising jacket temperature to cut the cycle |
| Foaming, dusting or sticking | Different charging, discharge or a cooling step | Standard gravity discharge |
Source: Ruibo dryer selection practice.
How the results change the model choice
The drying curve gives the cycle time for a given heat flux, and the cycle time together with the batch mass gives the working volume. Heat transfer area then has to be large enough to deliver that flux at the jacket temperature the product allows. If the material is thermally limited, the area grows faster than the volume and a larger model is needed for the same batch.
The same test decides the jacket medium. A product that tolerates 140 °C can use steam; one that must stay below 90 °C is normally served by hot water, because a steam jacket at that level would be throttled into an unstable regime.
When the test can be shortened
If the plant already dries the same material in a similar batch vacuum dryer, its logged cycle times and moisture results are better evidence than a new bench test. Send those records instead: batch mass, starting and finishing moisture, jacket temperature, vacuum level and cycle time. A test is still worthwhile when the material is new, when the target moisture is tighter than before, or when the existing unit is being replaced because it no longer meets the duty.
- How long does a drying test take?
- It depends on the material and the target moisture. The test runs until the drying curve flattens at the intended vacuum and jacket temperature, so a free-draining filter cake finishes sooner than a material that relies on internal diffusion.
- Can the test be run on a sample that has already dried out?
- No. Dried and re-wetted material does not behave like fresh cake: the particles have already shrunk and the surface has changed. The sample has to come from the plant's normal dewatering outlet.
- Is a pilot run necessary after the bench test?
- Not always, but it is the usual next step when the duty is large, when the material is thermally sensitive, or when the discharge behaviour cannot be judged from a small sample.