Why Does Pellet Mill Productivity Change?

You may not be aware of is but the productivity of a pellet mill does change depending on the material being processed. There are also several rules you can use to tell if a pellet mill manufacturer is been honest about the productivity per hour their presses can achieve.

Pellet Mill Productivity
Pellet mill productivity changes on the raw material

For instance for a ring die pellet mill it takes 1KW for every 10kg/h per hour of wood pellets. However for a flat die pellet mill due to the energy wasted during roller skip it actually takes more than 1KW to produce 10kgs per hour of wood pellets. The image to the right is of our Mini Pellet Mill which is based on the ring die design. Therefore for our single phase pellet mill setup using a 2.2Kw motor the press can achieve just over 20 kgs per hour for wood pellets. The three phase pellet mill setup uses a 3Kw electric motor and can produce up to 30kg/h of wood pellets.

If another pellet mill manufacturer claims a higher productivity per hour with a similar motor they have either found a way to bend the rules of physics or they are providing inaccurate information. However the above rule is only for wood pellets, the pellet mill will achieve a higher productivity per hour for lower density biomass such as grass pellets or animal feed pellets. It’s simply the case that these materials require less energy to compress and generally require less time in the die to achieve sufficient compression.

Typical materials for grass pellets include Switchgrass and Miscanthus. Over the next decade you should see more pellet fuel manufacturers offering these pellets instead of just wood pellets. Sawdust is becoming expensive to source and difficult to find as so many pellet plant operators are all competing for the same waste resource. We offer a range of pellet mills for sale, from our smallest unit the Mini Pellet Mill and our automated pellet lines.

What Is Flat Die Pellet Mill Roller Skip About?

The two main designs of pellet mill on the market today are the flat die and the ring die pellet mill. To be very general, most small pellet mills are designed on the flat die and most large pellet mills are designed on the ring die. The reasons for this are flat die machines are cheaper and easier to manufacturer, however the ring die is firmly established as the superior machine and therefore is used in the vast majority of pellet plants. However at PelHeat we are obviously one of the exceptions to this rule with the Mini Pellet Mill and our small pellet plants only using the ring die pellet mill design.

flat die pellet mill roller skip
Flat die pellet mill roller skip

However initially we purchased small flat die pellet mills from China for research and to see if we could modify them into a product we were happy to sell. This turned out not to be the case, one of disadvantages with the flat die presses is roller slip. The image to the right is a picture from one of the flat die pellet makers we have owned. I have added the light blue and red dots to explain the principle of roller slip. Roller slip (or skip) is due to the difference in speed between the inner and outer edge of the die. You will agree that as the die rotates the red dots will be going faster than the blue dots as they have to cover a greater distance in the same amount of time. If you look at the rollers you will see that it is the same diameter along the full contact with the die. Therefore the outer edge of the roller is trying to go faster than the inner edge of the roller. With a roller that is the same diameter across its entire length this is obviously impossible. Therefore this leads to the roller skipping and the clearance gap between the roller and die constantly changing.

For the best results you want a constant 1mm gap which a flat die pellet maker cannot provide. It also leads to increased roller and die wear and also increased electrical consumption. Ring die pellet machines do not suffer from roller skip/slip.

The Design Of The Flat Die Pellet Mill

As you may know from the other pages on our website, here at PelHeat we only offer pellet mills based on the ring die design. What you may not be aware of is that we have experimented with small flat die pellet mills which we imported from China. It was through these experiments that we came to realise we cannot offer these pellet press imports and feel comfortable that we are offering our customers a quality product that is up to the task. This is what led to the development of the Mini Pellet Mill.

Flat Die Pellet Mill
Flat die pellet mill from China

However besides the poor quality of these pellet machines, there is actually a fundamental issue with the design of this type of pellet maker. In the image you can see to the right the roller shaft is stationary and the die is driven through the shaft below. If you look at the die you will realise during rotation that the inner and outer edges of the die are turning at different speeds. Now look at the rollers, they are the same diameter consistently across their surface. Therefore the inner edge of the roller is trying to turn at a slower speed than the outer edge of the roller to match the speed of the rotating die. This is obviously impossible, and this creates an issue known as roller skip. This skipping action generates several issues which are detrimental to the process of making quality pellets.

This first issue is wasted energy, flat die pellet mills require more energy to operate than ring die presses because of the energy wasted in roller skip. The second issue is the accelerated wear of the roller and die. This not only means higher consumable parts costs it also costs in terms of production down time due to the more frequent roller and die changes.

What Is The Ideal Roller Gap To The Pellet Mill Die?

The above question may seem like a strange one to ask to some people who have previously used Chinese flat die pellet mills. There are several websites and forums that state that with Chinese flat die pellet presses you need to ‘clamp’ the rollers down against the die to ‘build up sufficient pressure to push the material through the die’. I need to make it clear straight away, that is terrible advice. While it maybe necessary on Chinese pellet makers to force the material through the die due to the poor condition of the die holes, on quality equipment you should never ‘clamp’ the rollers to the die.

pellet mill roller gap
Ring die pellet mill roller gap

Through such an approach you will damage the roller bearings, dramatically accelerate die and roller wear and reduce pellet quality. If you go and visit a professional large scale wood pellet manufacturing plant they will tell you that the die and rollers should never touch, there should be no metal to metal contact. However while these two components should never touch, that is not to say that the gap is irrelevant. You want to maintain a gap of 1mm to achieve the best balance of power consumption and pellet quality. If you set a gap above 1mm you will have issues with power consumption and potentially stall the motor. The greater the distance the more resistance, therefore above 1mm is only recommended for certain animal feeds and other low density materials.

The problem is, particularly on cheap pellet mill imports there is usually no method to set the distance of the die from the rollers. Therefore this usually means that you have to have metal to metal contact. Hence one of the reasons why it is practically impossible to produce quality pellets on small flat die pellet presses. On our high quality Mini Pellet Mill and small scale automated pellet plants we provide features to set the roller gap. We advise starting the die and then slowly adjusting the rolls to the die, as soon as you hear the contact just lift back slightly and then lock the rollers in position.

Why Are Ring Die Pellet Presses Superior?

Pellet presses are one term used to describe a pellet mill or pellet machine. Pellet presses come in various different sizes ranging from a small sized press such as the Mini Pellet Mill up to large scale commercial pellet plants. There are several different designs of pellet press including the flat die and the ring die designs.

The Flat Die Pellet Press Design Disadvantages

The flat die pellet press was the first design and it is the simplest design of pellet press. The majority of small pellet presses are based on the flat die design and they are manufactured in China. The advantage of the design are that they are very simple to produce and also very cheap to produce. However in terms of pellet quality and control of the process the majority of flat die pellet press are lacking the necessary features. Also the relationship between the rollers and die causes a skipping action. This skipping action means increased wear on consumables such as the roller and die and bearings. Skipping also generates increased friction which raises energy consumption of the pellet mill. These factors together mean that the flat die pellet press uses more energy to produce pellets.

The Ring Die Pellet Press Design Advantages

Pellet Press
Small ring die pellet press

The flat die pellet press was developed at the turn of the 20th century, then around the 1950’s the design was improved with the ring die. All pellet mills that PelHeat offer are based on ring die designs. The majority of pellet presses used in large scale commercial pellet manufacture use the ring die pellet press design. The main benefits being reduced electrical consumption and increased life of the main consumable parts such as the rollers and die. The Mini Pellet Mill is one of the very few small pellet presses to work on the ring die design. First material is entered into the hopper, the material needs to be in particles smaller than 6mm and have a moisture content below 15%. Material is fed through the hopper via an auger into a conditioner where water and vegetable oil can be added. The material is then metered into the pellet mill compression chamber. To learn more about the equipment we offer and our consultation service please click the links above.

The Limitations Of A PTO Pellet Mill

The pellet mill designs we offer here at PelHeat have a long history, for instance the pellet press designs used for the pellet plants actually date back to 1950’s animal feed plants. As this equipment has a large farmer client base the principle of a PTO pellet mill has been requested by our clients more than once and we have looked into the idea.

PTO Pellet Mill
Chinese PTO pellet mill

While technically simple the effect on the process and pellet quality do not meet the standards we demand of our products. The image you can see to the right is a typical example of a PTO pellet press imported from China. Typically for a pellet mill of that size you will able to produce 50 kg/s of wood pellets, up to 100 kg/h of animal feed and require a 30-40 HP tractor. We do appreciate from the customers perspective the principle setup may appear ideal. They already have a tractor so a separate diesel or electric motor is not required, reducing the cost of investment.

However there are limitations, and the first is of material input control. Many clients are not aware of how important the rate of feed into the pellet mill is on pellet quality and press productivity. On all our presses we have variable speed feed systems from a hopper. However with a PTO pellet mill setup there is only one source of power at one speed, therefore a variable speed control system is not possible. The second aspect is torque, while diesel engines do have a lot of torque compared to a gas/petrol unit they still cannot compare to an electric motor, particularly at variable speeds. There is also the question of running costs as a PTO setup is the most inefficient way to produce pellets.

Finally there are the serious safety concerns. Many people die each year from PTO related accidents, and with a pellet mill you are standing directly next to the shaft and many people choose not the fix the recommend protective covers. We do not wish to be responsible for any serious injuries or even deaths associated with using a PTO pellet mill, therefore this unfortunately something we cannot offer. However if you do decide to purchase a PTO pellet mill from say china and you need support to get the pellet mill operating you maybe interested in our five free guides and consultation service.

The Design Of The Ring Die Pellet Press

At PelHeat we only offer pellet mills and pellet plants that use the ring die design. There are several reasons why, but to boil it down the ring die pellet mill design is far more superior to the flat die pellet machine design. This is not just our opinion, if you go to visit a large scale wood pellet manufacturing plant today you will find that over 90% of those facilities will be using ring die presses.

Benefits Of The Ring Die Design

Ring Die Pellet Press
Ring die and rollers

One of the main reasons that this is a superior design to the flat die press is that it does not suffer from roller skip. Due to speed differences between the point of contact between the rollers and die, roller skip is a significant issue for flat die pellet machines. Ring die systems do not suffer from this issue as there is no difference in speed between the die and rollers at any position.

The benefits of eliminating roller skip are reduced power consumption, extended consumable parts life and improved pellet quality. There is only one feature that flat die pellet mill manufacturers claim that it is superior to ring die presses and that is material flow of low density materials such as straw and other grasses. While this can potentially be an issue for some ring die pellet mills, it is not the case for our equipment.

The 22kw and 30kw presses we use in our pellet plants are built on a vertical material flow design, the same as flat die machines. Therefore our systems also have the single benefit of flat die pellet makers and none of the other issues. Our pellet mills and pellet plants have been used to produce pellets for many different sectors and industries.

Previously we installed a miscanthus pellet plant for the production of fuel pellets. However one of the most common enquires are for small wood pellet plants. Not only do we offer testing services as well as the Mini Pellet Mill, we also conduct our own tests on the combustion results of various pellets. Our pellet boiler can cope with even high ash content fuels, this makes it an excellent pellet burner to see how some of these other fuel pellets burn, such as grass pellets. If you would like to learn more about the process please click on the consultation button on the right for five free guides.

The Challenge of a Single Phase Pellet Mill

Hi, I’m Chris. I started PelHeat.com back in 2007.

When it comes to making wood pellets, your choice of pellet mill is limited by your available source of power. For domestic properties and some commercial buildings, a single phase connection is used which has a voltage between 110v – 240v. For many commercial buildings, a three-phase connection is available which can carry a higher voltage of between 230v – 400v. When it comes to using a single phase pellet mill various limitations have to be acknowledged. These include the maximum power the pellet mill can consume.

The Power restrictions of Single Phase

Single Phase Pellet Mill
A single phase mini pellet mill

If you only have access to a single phase electrical supply you are limited by the maximum power draw the pellet mill can demand. On a standard domestic two/three pin plug the maximum motor draw without tripping will be around 2.2kW. If there is a dedicated connection (similar to that used for a cooker), potentially the motor size could be increased to around 7kW.

However, if you are intending to make wood pellets this is not a lot of power to play with. Quality wood pellet production requires high compression, which in turn demands a high power input from the pellet mill. Without sufficient power, a single phase pellet mill can run into problems.

Potentially Increased Pellet Mill Die Blockages

Blocked pellet mill die
A blocked die on a single phase pellet mill can easily happen.

The number of Amps (power) a pellet mill draws depends on the type of material being processed. For instance, a single phase pellet mill can quite happily process an oily animal feed mix into low-density pellets. However, when it comes to processing wood pellets there are more intensive power demands. As wood has a higher material density and the pellets are also produced to a higher density this can lead to complications. As the single-phase pellet mill starts to process wood pellets the amps will increase. If there is a change with the raw material feed rate or the moisture content changes this could lead to issues.

As the pellet mill starts to work harder to push the wood through the die the motor amps rise. With a single phase pellet mill, it’s very easy to reach the limit of around 6 amps. After this point, the electrical connection will trip out. If this happens with the material left in the die it will start to cool down and harden. Depending on the condition of the die this could lead to a pellet mill die blockage.

Productivity per Hour Restrictions

As a roughly general rule when it comes to making wood pellets it requires 1kW of pellet mill power to produce 10kg/h of productivity. Therefore, on a single phase connection and motor power of 2.2kW, the productivity per hour will be 22kg/h. This is where you really have to weigh up the value of your time to the volume of wood pellets produced.

Wood Pellets
It takes 1kW of power to produce 10kg/h of wood pellets

A pellet mill is not a ‘set and forget’ operation. You need to be actively engaged, monitoring the raw material and pellet press at the same time. Therefore, what you really need to consider is if 22kgs of wood pellets are worth an hour of your time. There is also the matter of pellet mill maintenance to factor in. Furthermore, it’s not really an hour of your time, it’s even more. You also have to factor in the time taken to prepare the raw material ready for the pellet mill. That means reducing it to a suitable particle size and moisture content.

You can only run One Piece of Equipment at a Time

Single Phase Hammer Mill
Single phase hammer mill

As previously stated, a single phase connection means all available power is used by the pellet mill. Therefore, there is no surplus power available to run a hammer mill for particle size reduction. If there was suitably available power you could be using the hammer mill while the pellet mill does its job. As with single phase, there isn’t sufficient power, you have to use one piece of equipment at a time. This therefore obviously increases the total time it takes to produce a fairly small quantity of wood pellets.

There are small hammer mills on the market that use small diesel engines. This would obviously mean you could be using both the pellet mill and hammer mill at the same time. It should be noted that small Chinese diesel engines are not very efficient. Therefore the cost to process the material could be much higher than you would expect.

Is there a use for a Single Phase Pellet Mill?

From reading the above it should be clear that a single phase pellet mill is not a viable pellet production setup for volume. The cost per ton when you factor in the labour/time is not viable. However, there is still a viable use for single phase pellet mills.

Research and testing before investment in a pellet plant is probably the number one use for single phase pellet mills. Before a significant investment is made in a pellet plant a pilot project can be set up to produce small volumes of pellets. This can be a ‘proof of concept’ to help to secure funding. It can also be a useful setup to train future pellet plant operators.

The other exception of use for a single phase pellet mill is for producing BBQ wood pellets. As the profit margins are so high for BBQ wood pellets it can be viable to produce them on a single phase pellet mill.

Final thoughts on Single Phase Pellet Mills

Many people start off looking into purchasing a pellet mill to make wood pellets from home. However, before such a decision is made it’s very important to learn as much about the process as possible. Making good quality wood pellets is a challenge, and even more so using a low powered single phase pellet mill. To make the process easier to deal with its recommend to consider a low inclusion pellet binder. This will aid the process and partly address the issue of power limitations with a single phase pellet mill.

Before you purchase a single phase pellet mill it is a good idea to have a chat with a local electrician who will advise on the best way proceed. A dedicated connection for the pellet mill will probably be their recommendation.

I hope you found the above useful. Remember, before you make any purchase make sure you have researched the process of making pellets thoroughly. With this website, I’m trying to produce as many helpful hints and tips from my experience over the years.

Is a Used Pellet Mill worth the Risk?

Hi, I’m Chris. I started PelHeat.com back in 2007.

When it comes to making wood pellets, looking for a used pellet mill may be an option to reduce your initial investment. However, there are potential risks you need to consider before such a purchase. Also before you consider purchasing any equipment I would encourage you to read my posts on how to make wood pellets. You will learn about the best raw materials to make wood pellets from and the perfect moisture content.

Why is the Used Pellet Mill available for Sale?

Used Pellet Mill
Used pellet mill from China

The fact is many of the used pellet presses for sale on eBay are where the individual has purchased a Chinese pellet mill and experienced issues. Now those issues could be down to user error due to a lack of information/experience on the process of producing pellets or the issue could be with the press itself. Either way, it will lead to the item going up on eBay with comments such as ‘barely used’.

If the issue was operator error you could potentially pick up a very cheap pellet maker that with the right help could produce wood pellets. However, it could have also been operator error that actually led to an issue with the press. The rollers may have been clamped down again the die, large pieces of raw material used in the machine etc. They may have found frequent pellet mill die blockages to be too frustrating. The risk is that in some cases there may be damaged bearings, rollers or die. In other cases the pellet mill casting could be broken, there is an element of risk to any used pellet mill purchase.

Are you actually Saving any Money with a Used Pellet Mill?

Flat Die Pellet Mill
Most used pellet mills available are flat die machines made in China.

Let’s presume you are considering purchasing a used/second-hand flat die pellet mill. Odds are that the machine was produced in China. However, there are a lot of resellers on the Internet making the process of making wood pellets appear much easier than it really is in order to sell their pellet mills. So an individual purchases the pellet mill only to realise the process is a lot more complicated than they thought. They try to get a refund which is refused so they try and shift the pellet mill on eBay.

They know they won’t be able to sell the pellet mill for what they purchased it for so try and get between 50% and 75% of the original purchase price. However, the fact is many of these resellers are marking up the price of the Chinese pellet mills by 200-300%. Therefore a second hand (potentially damaged) pellet mill on eBay may still be significantly overpriced.

Purchasing Directly from China

The reality is to is often a better deal to purchase a new pellet mill directly from China than to purchase a used pellet mill from eBay. You can use an import agent however they are going to charge you quite a significant service charger for purchasing a single press. So there are two options.

Asking to Purchase a ‘Sample’ Machine

Common Chinese Pellet Mill
A typical example of Chinese pellet mill

So the best place to get in contact with Chinese pellet mill manufacturers is Alibaba which is an online Business to Business market place. Simply search for pellet mill and various companies will appear. You can contact the companies and explain to them you are potentially interested in starting a business selling their products. You request to purchase a single unit as a test sample. Some will refuse but some will agree to the deal. You can then use the Escrow service offered by Alibaba. They then hold the money on the purchaser’s behalf and only release the payment to the seller if you are happy with the purchase.

Purchasing a Pellet Mill through AliExpress

So AliExpress is basically the retail side of Alibaba, instead of being business to business its sales to consumers. You may not be able to secure quite such a good deal as purchasing a ‘test sample’. However, you will be able to get a much better deal than purchasing a pellet mill from a reseller adding a 200-300% markup. You can also pick up spare parts of the pellet mills at a fairly reasonable price as well.