A hollow paddle dryer is an indirect dryer in which the heating medium flows through the rotating parts and the jacket, so the heat reaches the product through large metal surfaces. The word “hollow” is the point of the design: more of the machine becomes heating surface, and the product dries without ever touching the steam, thermic fluid or hot water inside.
This guide explains what that means in practice, where hollow paddle dryers fit and what to check when you compare designs.
What “hollow” adds
In a standard jacketed dryer only the trough wall heats the product. In a hollow paddle design the rotating shafts carry the heating medium too, and in many designs the paddles do as well. That raises the heat transfer area for a given machine size, so a compact unit can do a lot of drying. AS Engineers’ paddle dryers pass the heating medium through hollow shafts and the jacket, with wedge-shaped paddles on two counter-rotating shafts.
What else defines the design
- Wedge-shaped, self-cleaning paddles. The wedge shape and the intermeshing shafts keep heated surfaces clear of build-up. The paddles run without metal-to-metal contact.
- Plug flow. Material moves from the feed end to the discharge end with little back-mixing, so the product leaves evenly dried.
- Enclosed operation. The machine is closed, so vapour can be routed to a condenser, scrubber or filter, and the dryer can run at atmospheric pressure, under vacuum or pressurised.
- Low off-gas volume. There is no large stream of hot air, so the exhaust treatment is small.
- Vertical shaft removal. Shafts can be lifted out vertically, which helps service and retrofitting.
The operating range AS Engineers states
| Parameter | Range |
|---|---|
| Steam heating | Up to 14.06 kg/cm² |
| Thermic fluid heating | Up to 400°C |
| Final dryness | Up to 99%, or held at any target moisture |
| Pressure | Atmospheric, vacuum or pressurised |
| Materials of construction | Carbon steel, SS304, SS316, duplex steel and other alloys |
| Configurations | Standard, dual zone and vacuum |
Where hollow paddle dryers work well
They suit wet, sticky and heat-sensitive materials: ETP, STP and chemical sludge, filter cake, pastes, slurries, pigments, starch, spent grain, polymer resins and a range of chemicals and minerals. The same machine can also heat, calcine, strip solvents, crystallise, react or cool, which is useful when one wet solid needs two steps.
Where to be careful
- Abrasive solids wear paddles and liners faster, so hard facing or a different dryer type may be better.
- Very heat-sensitive products may need vacuum operation or hot water as the medium.
- Easy, free-flowing powders that dry well in hot air can be cheaper to dry another way.
How to compare designs
Ask each supplier the same questions:
- How much heat transfer area does the machine have for my duty?
- What heating medium and temperature does the design assume?
- How does it deal with the sticky stage of my material?
- What is the peak torque of the drive, and how is it protected?
- How are shafts and seals serviced?
- Which parts touch the product, and what are they made of?
- Can I test my material before I buy?
Cheap quotations often hide differences in these answers, so compare design assumptions, not only price.
Test your material
AS Engineers runs pilot trials on a 25 kg/hr paddle dryer, at its works or at your site, for a nominal fee that is waived on order. Request a pilot trial.
Looking for a supplier? See the hollow paddle dryer manufacturer in India page. For the mechanism in detail, read how a paddle dryer works.
Related reading
- Paddle dryer manufacturer
- Steam vs Thermal Oil Paddle Dryer
- Thermal Oil Heated Paddle Dryer Guide
- Thermal Sludge Dryer vs Solar Sludge Dryer
- How a paddle dryer works
More from AS Engineers
See the full range on AS Engineers paddle dryer range at theasengineers.com.
- B.E. Mechanical · GTU (2018)
- PGDBA, Operations · Symbiosis
- With AS Engineers since 2019
- Writes on paddle dryers, sludge drying & blowers
Karan Dargode leads operations and environmental health & safety at AS Engineers, an Ahmedabad-based manufacturer with over 25 years of experience in centrifugal blowers, industrial fans, paddle dryers, sludge dryers, and air pollution control equipment. He joined AS Engineers in July 2019 and has spent over six years building operational systems that support the company's engineering and manufacturing work.
