Dryer Selection — Hopper Volume and Air Flow

How is this calculated?

Dryer size follows from two independent requirements. The hopper volume guarantees that the material stays in hot, dry air for the whole of the required drying time: the hourly consumption multiplied by the drying time is divided by the bulk density of the material (the poured density of the granules — not the solid density). The air flow, in turn, has to be high enough to carry the heat into the granules and take the moisture out; the industry rule of thumb for a connected system is roughly 1.9 m³/h of air per kilogram per hour.

The material determines the type of dryer: hygroscopic materials (PA, PC, PET, ABS, PBT, TPU and so on) hold the moisture inside the granule and can only be dried with a desiccant dryer (dry air, dew point −40 °C); PET, PEEK, PEI and LCP, which need drying above 150 °C, call for the high temperature (HT) class. Non-hygroscopic materials such as PP, PE and PS only carry surface moisture — a hot air dryer is enough there, and buying a desiccant dryer is money spent for nothing. Regrind lowers the bulk density, so the same consumption needs a larger hopper. Fillers such as glass fiber and talc work the other way round: the filler takes up no moisture but makes the granules heavier, and the poured density rises in proportion to the solid density (PA6 GF30 is about x1.2) — the same kilogram fits into a smaller hopper, while the drying temperature and time stay as the polymer requires.

Bulk density is not a fixed material property — treat the value in the table as a guide. Cylindrically cut granules and lens-shaped granules of the same polymer pack differently; as the particle size, the filler content and the supplier change, the value moves by 10-15%. Most published tables also confuse the solid density with the bulk density (the solid density of ABS is 1.06 kg/L while its bulk density is around 0.55) — which is why sources contradict each other.

Measuring your own granules takes two minutes and gives the most accurate result: fill a container of known volume (a 1 liter measure, for example) with free-flowing granules without compacting them, level the top off, and weigh it. Once the tare is subtracted, the remaining kilograms are your bulk density directly in kg/L. The standard method is DIN EN ISO 60 (the apparent density of material poured from a funnel). Enter the value you find in the "Bulk density" field above; the calculation then uses your measurement instead of the table value (since the granules you measured already contain the filler, the additive factor is not applied in that case).

The guide values in the table were produced as follows: bulk density = packing factor x solid density. The solid densities are taken from our 120-material table of processing data; the packing factor was measured from a dryer manufacturer's material table and verified against the physics of granule packing (a band of 0.52-0.65, average 0.56). The one exception is PVC: when it comes as a powder or dry blend, the factor is far lower.

If the material stays in the hopper far longer than it needs to, over-drying begins (PA yellows, TPU degrades); the recommendation therefore does not pick a hopper far above the requirement, and it shows the actual residence time.