The heating medium decides your temperature ceiling, your jacket rating and your utility pipework. Here is how to choose before your enquiry goes out.
The medium is a temperature decision, not a preference
Choose the heating medium after you know two numbers: the hottest product temperature the material tolerates, and the surface temperature needed to reach it inside a workable cycle. Steam, hot water and thermal oil are answers to those two numbers plus the pressure the jacket or plate is rated to hold. Choosing by habit is how a project ends up with a boiler it cannot use or a jacket it cannot pressurise.
An indirect dryer moves heat through a wetted surface, so the medium never touches the product. That separation is useful, because it keeps a contaminated or flammable material away from the heat source, but it also means the medium is felt through only two things: the temperature it holds at the surface, and the area that surface offers. Get the medium wrong and the fix is rarely a controller setting. It is a different jacket rating, a new utility circuit, or a longer cycle the material may not survive.
Work from the product ceiling down to the utility
The product sets a ceiling, not a target. A material that discolours, melts or degrades above a certain temperature does not care how the heat arrives; the wetted surface has to stay under that limit, and the limit caps the medium.
Read the sequence in one direction. First the product ceiling, from a lab test, a specification or a trial batch. Second the surface temperature that reaches that ceiling with a usable driving force. Third the medium whose band covers that surface temperature. Fourth the jacket or plate rating that the medium demands, because steam and hot water do not make the same demand. A selection built in the other order, model first and medium later, tends to end with a derated jacket or a utility nobody budgeted for.
The temperature bands the product data actually states
Ruibo's product notes fix three bands for plate-heated dryers: hot water below 100 °C, steam from 100 to 150 °C, and thermal oil from 150 to 320 °C. Those bands, not a preference for one fluid, are what separate the options.
The bands meet at their edges, which is where most arguments start. A duty at 95 °C can be met by hot water or by steam at reduced pressure, and the two are not interchangeable in practice: one is a pump circuit, the other needs a pressure-reducing station and a trap set. Write the duty in degrees first, then let the band decide.
Heating medium bands as stated in the Ruibo product data
| Medium | Temperature band in the data | Where the band is stated | What it decides |
|---|---|---|---|
| Hot water | Below 100 °C | PLG continuous plate dryer, plate heating note | Lowest ceiling, suited to materials that cannot see a hot surface |
| Steam | 100 to 150 °C | PLG continuous plate dryer, plate heating note | Middle ceiling, with temperature set by pressure rather than by a set point |
| Thermal oil | 150 to 320 °C | PLG continuous plate dryer, plate heating note | High ceiling without high pressure inside the jacket |
| Steam, oven duty | 50 to 130 °C, 140 °C maximum, at 0.2 to 0.8 MPa | CT-C hot air circulating oven, heat source note | Steam can serve low-temperature duty when it is throttled |
Source: Ruibo product specification notes for the PLG continuous plate dryer and the CT-C hot air circulating oven.
Check what the jacket or plate is rated to hold
A temperature band means nothing if the wetted surface cannot hold the pressure that produces it. Saturated steam ties temperature to pressure, so the jacket rating is a temperature ceiling in disguise, while hot water and thermal oil separate the two.
This is the step that most often changes the answer. A jacket rated 0.3 MPa is an ordinary vacuum-dryer jacket, and it also fixes the hottest saturated steam that jacket can see. Oil at 200 °C asks the same jacket for almost no pressure at all, because the circuit is pumped rather than boiled. Before comparing media, ask for the design pressure of the surface you are buying: not the vessel, the jacket or the plate.
Pressure ratings on the heat-transfer surfaces in the Ruibo range
| Equipment | Surface that carries the medium | Rating stated in the data | Practical consequence |
|---|---|---|---|
| ZPG vacuum rake dryer | Jacket, half-pipe design on the larger models | Jacket design pressure 0.3 MPa | Steam duty is capped by the rating; check the saturation temperature at that pressure before specifying |
| PLG continuous plate dryer | Hollow drying plates | 0.4 MPa generally, 1.6 MPa maximum | Higher-pressure steam needs the uprated plate design, not the standard one |
| KJG hollow blade dryer | Hollow shaft and wedge blades | Steam, hot water or thermal oil heat sources | The medium is chosen for the material, not fixed by the model |
| CT-C hot air circulating oven | Steam coil or electric elements | Steam 0.2 to 0.8 MPa | Steam reaches 130 °C, electric and far infrared reach 300 °C |
Source: Ruibo spec tables and heat source notes for the ZPG, PLG, KJG and CT-C ranges.
Steam: the default above 100 °C, with one constraint
Steam suits duties above 100 °C, on a site that already runs a header, where the material tolerates a surface that cannot be set below the saturation temperature of the supply pressure.
The constraint is that steam temperature is a property of its pressure. Nobody dials 120 °C; the plant dials the pressure that saturates at 120 °C, and the surface temperature then follows the pressure actually held at the jacket. Condensate removal matters as much as supply, because a jacket half full of condensate is a jacket with a much smaller effective area, and the batch simply takes longer while the gauge still reads correctly.
Steam is also a poor fit below 100 °C. Sub-atmospheric steam is possible but uncommon in a plant, which is why materials with a low ceiling usually end up on hot water instead.
Hot water: tight control at a low ceiling
Hot water is the natural choice when the material ceiling sits below 100 °C, because the set point on the water is the surface temperature within a few degrees, with no steam table in between.
What you give up is driving force. A 70 °C jacket feeding a product that must stay near 60 °C leaves a narrow temperature difference, and the surface area has to carry a duty the difference no longer provides. That is a sizing question, not a reason to raise the temperature, since raising it defeats the reason hot water was chosen. Above 100 °C the circuit has to be pressurised, which turns a simple skid into a pressure-rated system.
Thermal oil: high temperature without a high-pressure jacket
Thermal oil carries 150 to 320 °C into a jacket that sees only circulation pressure, which is exactly why it appears on high-temperature duty in vessels that were never built for high-pressure steam.
The cost moves off the vessel and onto the fluid system. An oil circuit needs a circulation pump, an expansion vessel and a fluid whose condition is checked, because oil that has been overheated or oxidised stops doing its job quietly. Oil also transfers heat less aggressively than condensing steam, so a duty copied from a steam jacket usually needs more area in oil service. Treat oil as a system decision, not a fluid swap.
Putting the choice into one table
Use the observation on the left to reach a medium, and to see what that choice rules out before it is written into a specification.
From observation to medium to what it rules out
| What you observe | What it points to | What it rules out |
|---|---|---|
| The product discolours or degrades when the surface passes about 90 °C | Hot water circuit with a controlled set point | Atmospheric steam, which cannot run below 100 °C |
| The duty needs a 150 °C surface but the jacket is rated 0.3 MPa | Thermal oil, or an uprated jacket | Standard steam service on that jacket |
| The site already runs a steam header and the product tolerates 120 °C | Steam through a pressure-reducing station | A separate hot water skid and its pump |
| The material is heat sensitive but the cycle has to stay short | Hot water or oil with more heat-transfer area | Lowering the medium temperature and hoping the cycle shortens |
| Solvent has to be recovered from a closed vessel | Oil or hot water in a closed circuit, inert gas in the vessel | Open steam injection into the product |
Source: selection rules used in Ruibo dryer engineering; temperature bands taken from the PLG plate heating note.
What the enquiry has to state
Most disputes about heating media are missing-information disputes. Six items on the enquiry prevent nearly all of them.
- The maximum product temperature the material tolerates, and the test or specification it comes from.
- The surface temperature the cycle needs, and the batch time that makes it necessary.
- The design pressure of the jacket or plate on the model being quoted, in MPa, not just the word steam.
- The utilities actually available on site: steam header pressure, or the capacity of the hot water and oil circuits.
- Whether the vessel is closed-loop for solvent recovery, because that changes the medium circuit as well as the vapour line.
- Who supplies the control valve, the trap set or the oil skid, and where that responsibility ends.
Questions engineers ask about heating media
The same five questions come up in almost every selection review. The answers are short, and each one changes something in the specification.
- Can one dryer run on both steam and thermal oil?
- Only if the jacket is designed for the more demanding of the two duties. Design pressure is a single number on the nameplate, so a jacket built for low-pressure oil is not automatically suitable for steam under pressure. Dual-medium designs exist, but they are specified as such, not converted later.
- Why not use electric heating instead?
- Electric heating removes the boiler and reaches high temperatures directly; the CT-C oven covers 50 to 300 °C on electric elements and far infrared. The trade is running cost and electrical load, not capability, which is why electric suits smaller duties and pilot lines more often than large continuous ones.
- Does the medium change the drying time?
- Yes, through the surface temperature and therefore the driving force, but only if the area stays the same. Comparing two media on two different models tells you nothing, because the model carries its own heat-transfer area. Compare media at equal area, or compare area at equal surface temperature.
- What steam temperature does 0.3 MPa give?
- It is fixed by the saturation curve for the fluid, and the number depends on whether the rating is gauge or absolute pressure. Ask the supplier which basis the nameplate uses before converting anything, and check the resulting temperature against the product ceiling rather than assuming a round figure.
- Is thermal oil acceptable for food or pharmaceutical duty?
- The oil stays in a closed circuit and never contacts the product, so the question becomes one of product-contact materials, seals and the documentation the customer requires. Confirm those with the engineers before the medium is fixed, because changing it after the jacket is built is a pressure-vessel change, not an accessory change.
Where this leaves the selection
Decide the medium from the product ceiling and the surface rating, then choose the model that offers the area the duty needs. The dryer follows the medium; it does not choose it.