Drying Equipment
Drum Dryer vs Spray Dryer: Which Suits Your Slurry?
Drum dryer or spray dryer? Choose by feed viscosity, tonnage and energy. Published figures from APV, BEE and Tang compared, with a selection checklist.
The short answer
Choose by the feed first, then by tonnage. A spray dryer needs a liquid it can atomise, and APV's dryer handbook gives the practical ceiling as a feed viscosity of about 250 to 300 centipoise. A drum dryer can take thicker feeds, including food pastes and cooked starch, that a spray dryer cannot turn into fine droplets without help. We make drum dryers, so read this with that in mind; where a spray dryer or another dryer is the better answer, this article says so.
The second test is scale, and there a spray dryer wins by a wide margin. The third is energy, and here the common claim that drum drying is the efficient option does not settle the question: the published efficiency ranges for the two types overlap, and the running cost depends on the machine, the duty and the feed solids as much as on the dryer type.
What each dryer actually does
A drum dryer spreads the feed as a thin film, typically 0.5 to 2 mm, on the outside of a hollow drum heated from inside by steam. The drum turns, the water boils off, and after about three quarters of a revolution a fixed knife scrapes the dried layer away. The product spends a few seconds to a few dozen seconds on the drum and often leaves below 5 percent moisture, wet basis. It comes off as a sheet that is then ground into flakes or powder. These figures are from the drum drying chapter by Tang, Feng and Shen in the Encyclopedia of Agricultural, Food, and Biological Engineering.
Single drum machines take their feed from a dip tray, applicator rolls, splashing or spray nozzles. Double drum machines feed into the nip between two drums turning towards each other, and the gap sets the film thickness. The right feeding method depends on the feed's solids, viscosity and how well it wets the drum, and Tang and colleagues note that the choice usually comes from previous experience or from tests with pilot units.
A spray dryer works the other way round. The feed is pumped to an atomiser, either a spinning wheel or a pressure nozzle, that breaks it into fine droplets inside a chamber of hot air. The droplets dry in flight and the powder is separated from the air, for example in a cyclone. Heat moves by convection from hot gas, not by conduction through a metal wall.
That difference, and the atomiser it depends on, explains much of what follows.
Start with the feed
The first question is whether a spray dryer can take your feed at all.
APV's dryer handbook gives "the limiting viscosity for good atomization" as 250 to 300 centipoise, as a rule of thumb, and elsewhere advises keeping the viscosity below 250 centipoise at the atomiser. When the viscosity is too high, it says, the liquid stops forming round droplets and comes out as large irregular particles and then strings, "like cotton candy". India's Bureau of Energy Efficiency (BEE) makes the same point from the other side in its best practice manual on dryers: for filter cakes, sludges and starch, "spray dryers would need very special pumping equipment to feed the atomiser."
A drum dryer has no atomiser to protect. Tang and colleagues list among its advantages that drum dryers "can dry very viscous foods, such as pastes and gelatinized or cooked starch, which cannot be easily dried with other methods." Viscous feeds are usually applied with feed rolls, and roll feeding is, in their words, "particularly effective for viscous and glutinous materials."
The drum is not the only option for thick material, though. APV rates film drums "good" for fluid feeds but only "fair" for pastes and dewatered cake, and its list of dryer options for high-viscosity materials names flash, spin flash, fluid bed, tray, tunnel, belt, rotary and several indirect dryers. BEE's guidance for pastes and sludges points to tray, tunnel and vacuum shelf dryers. The drum's strongest published case is food pastes, purées and cooked starch.
So the first test is a measurement, not a preference. Measure the viscosity of your feed at the temperature you would feed it. If it sits well under a few hundred centipoise at the solids you want to dry, both machines are candidates. If it is thicker, a spray dryer needs the feed heated, thinned or handled with special equipment, and thinning has a price.
The water you add is water you pay to evaporate
When a feed is too thick to atomise, one fix is to dilute it. APV warns that adding water "dramatically reduces the capacity of the spray dryer to produce powder." APV adds that heating the feed is a better way to lower its viscosity, and that it raises spray dryer capacity rather than cutting it. Dilution is the penalty case, not the only one.
Where dilution is needed, the cost is arithmetic. The water a dryer must evaporate for each kilogram of dry product is (100 minus percent solids) divided by percent solids. APV's handbook uses this relation and shows how steep it is: in its example, a slurry fed at 30 percent solids needs about 6,360 kg/h of evaporation, and the same solids fed at 50 percent need about 2,700 kg/h.
Here is what dilution does to a paste. This is our own illustration, not a measured case. Suppose a product leaves upstream processing at 40 percent solids, and it has to be thinned to 25 percent before it will atomise.
At 40 percent solids, the dryer evaporates 1.5 kg of water per kg of product.
At 25 percent solids, it evaporates 3.0 kg of water per kg of product.
Thinning the feed doubles the evaporation load. A machine that can take the paste as it comes avoids that penalty. That is the real case for a drum dryer on a viscous feed: it skips the water you would have added, and on a feed that would need heavy dilution, that can outweigh the difference in dryer efficiency.
The same arithmetic works in the other direction. APV's handbook states that, in almost all drying applications, "the best energy saving technique is to pre-concentrate the product as far as possible prior to drying," and that removing water mechanically, by filtration or centrifuging, costs a small fraction of evaporating it. There is a limit. APV also notes that raising the solids raises the viscosity, which may exceed what a spray dryer can take, and that mechanical dewatering produces a paste or filter cake that cannot be spray dried. Concentrate up to the solids your chosen dryer can still accept: for a spray dryer, the viscosity limit; for a drum, a feed that still forms a good film.
Energy: the published ranges overlap
Drum drying is often described as one of the most energy efficient drying methods, and Tang and colleagues say exactly that. The published figures support it. Tang, Feng and Shen give 1.1 to 1.6 kg of steam per kg of water evaporated, which they equate to efficiencies of about 60 to 90 percent. APV's handbook gives 1.3 to 1.5 kg of steam per kg of water, or 65 to 75 percent.
India's Bureau of Energy Efficiency publishes a wider view. Its table of expected dryer efficiencies gives:
Drum dryers: 3.0 to 12.0 MJ per kg of water, efficiency 19 to 75 percent.
Pneumatic conveying and spray dryers: 3.5 to 8.0 MJ per kg of water, efficiency 28 to 64 percent.
Two-stage dryers: 3.3 to 6.0 MJ per kg of water, efficiency 38 to 68 percent.
Read together, the sources do not crown either type. APV's typical drum figure, 65 to 75 percent, is at the top of BEE's drum range and level with BEE's best spray dryers. APV itself works through a spray dryer example at 76 percent. BEE, for its part, calls spray drying "one of the most energy-consuming drying processes" and still gives it a range that overlaps the drum's. On efficiency alone, neither type wins by default.
Two cautions when you compare figures. First, the handbook drum figures are measured against the steam supplied to the drum, so boiler losses come on top; compare like with like before quoting either number. Second, BEE gives ranges, not causes. It names one source of heat loss for drum dryers, the surface, which we read as heat lost from the hot drum to its surroundings.
Tang and colleagues add operating effects on a drum: non-condensable gases collecting inside it, drum surface temperature varying by as much as 20 °C along its width, and a feed film that fluctuates in thickness and moisture. They tie these to uneven drying and lost throughput. We would also check them first on a drum whose energy figure looks poor.
Capacity: where the spray dryer wins
On throughput, the spray dryer is in a different class. APV gives a direct comparison: a typical film drum 1.2 m in diameter and 3 m long evaporates about 270 kg of water an hour, while a typical spray dryer evaporates about 10,000 kg. APV's conclusion: where tonnage production is required, the drum dryer is at a disadvantage. Tang and colleagues list "relatively low throughput compared to spray drying" among drum drying's disadvantages.
Drum capacity scales with heated surface area. Tang and colleagues give production rates of 5 to 50 kg of product per hour per square metre of drum, depending on the product and its moisture, so a larger duty means more or larger drums. For a single product made in large tonnages from a feed that atomises well, a spray dryer is the usual answer, and we would not argue otherwise. BEE describes spray drying as essential to the production of dairy and food product powders.
The drum's advantage runs the other way. Tang and colleagues describe drum dryers as flexible and suitable for multiple products in small quantities, and easy to operate and maintain. For a plant that makes several products, or runs campaigns rather than continuous tonnage, that flexibility counts.
Product form and heat
The two machines make different products from the same feed.
A spray dryer produces a powder of droplet-sized particles. APV notes that its ability "to produce a uniform powder at relatively low temperatures" is why it is probably the most common powder dryer, and it is the natural choice when the customer specifies a free-flowing spherical particle in a size range. Spray dryers fitted with fluid beds also agglomerate powders so that they reconstitute easily, which is how many instant powders are made.
A drum dryer produces a sheet that is ground into flakes or powder. Tang and colleagues note that drum-dried products have good porosity, and so rehydrate well, because the water leaves by boiling.
Heat treatment differs too. On a drum, the product touches a steam-heated surface, and Tang and colleagues warn of possible scorching that gives a cooked flavour and off colour. For heat-sensitive products, the drums can be enclosed in a vacuum chamber to lower the drying temperature, though that equipment is more expensive and is usually reserved for high-value products.
When a drum dryer is the wrong answer
A drum dryer is not a universal machine. Tang and colleagues list its limits, and they are worth checking against your feed before anything else:
Products that do not form a good film on the drum surface are not suitable.
High-sugar products can be hard to scrape off, because they turn soft and rubbery at drum temperature. A jet of cold air just before the knife, to harden the film, is one fix.
Salty or otherwise corrosive feeds are listed as unsuitable, because they pit the drum surface.
Changing the drum surface is expensive, because it needs precision machining.
If your feed may fail one of these tests, a pilot trial is the way to find out, and it is better found before a purchase than after.
A quick way to choose
Work through these in order. The first clear answer usually decides it.
Is the feed a melt that needs to solidify, not a liquid that needs to dry? Then you need a drum that is cooled rather than heated: a drum flaker.
Can the feed be atomised at the solids you want to dry, if necessary after heating? If not, and it is a food paste, purée or cooked starch, a drum dryer is a strong candidate. For filter cakes and sludges, flash and other paste dryers are also options.
Would you have to dilute it to spray it? Work out the extra evaporation with the formula above. That extra water is a running cost a drum dryer avoids; weigh it against the throughput question below.
Is the duty tonnage of one product? If the feed atomises well and the volume is large, a spray dryer is the usual answer.
Does the customer specify a spherical, free-flowing powder? That points to a spray dryer.
Will it be sold as flakes? That points to a drum dryer. Fast rehydration can come from either: drum products are porous, and spray dryers agglomerate powders for instant use.
Is the feed corrosive? The literature counts that against a drum. Is it sticky when hot, or a poor film former? Test it on a pilot drum before committing.
Whatever the answer, take out as much water as you can before the dryer, up to the solids the dryer can still accept. Water removed by a filter or centrifuge is far cheaper than water evaporated. If the duty starts with a dilute slurry, a rotary vacuum drum filter may be the first step, with the cake then going to a dryer suited to cake.
About Sharpenn
We design and manufacture drum dryers and drum flakers in India, alongside filtration equipment. Across all our product lines, around 1,000 units are installed worldwide in 460+ installations. We manufacture drum driers and have exported them to France, Bangladesh and Saudi Arabia. Our export to France was a CE-marked drum flaker for cosmetic waxes.
Send us your feed
If you are choosing between drying routes, the most useful thing you can send us is a description of the feed: solids content, viscosity at feed temperature, target final moisture, the throughput you need, any heat sensitivity, and anything corrosive or sticky in it. We will tell you whether a drum dryer suits it, and say so if it does not. Contact our engineers.
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