Forever Fuels mainly uses pressurised tankers for delivering wood pellets. We only use unpressurised systems for the largest deliveries, where a marginal saving in cost is more important than the many disadvantages of unpressurised systems, because large boilers tend to be more robust and less vulnerable to quality problems.
Our fleet of 32 pressurised tankers (see below) represents somewhere between two-thirds and three-quarters of all the pressurised tankers used for wood pellet deliveries in the UK.
Pressurised tankers are the standard system used for delivering wood pellets in countries like Germany, Sweden and Austria, where they have many more years of experience and much larger markets for wood pellets than the UK.
Unpressurised systems (such as tipper-blowers) are not common in experienced pellet markets. They are mainly found in markets with less experience of wood pellets for heating, such as the UK and France, largely because secondhand 8-wheelers from the animal-feed industry require less upfront investment and can more easily be used for other purposes in summer.
There are a number of reasons why pressurised tankers are preferred in countries that know about these things. Click any of the items below to learn more.
The pellets encounter no moving parts during a delivery from a pressurised tanker.
In a tipper-blower, the wood pellets are fed in to the blowing line by a rotary valve, which will cause varying degrees of damage to the pellets depending on its age and condition. The pellets in the corners of the tipping body are typically fed to the rotary valve by augers (screws), which will also damage the pellets.
A more recent arrival in the UK is a French manufacturer called Transmanut. A number of inexperienced players have bought these systems, because they are marginally cheaper than proper tankers. Although they look somewhat like a tanker, they are not pressurised and have few of the advantages of a real tanker. They are arguably worse for the pellets than a tipper-blower. In a tipper-blower, gravity moves the pellets towards the rotary valve. In a Transmanut, a large screw (auger) moves the pellets from one end of the body to the other, to feed the rotary valve (which is still needed because the "tank" is not pressurised). An auger is likely to do more damage to the pellets than gravity.
Pressurised tankers have a wide array of controls to adjust the pressure, air-flow and product-feed rate during delivery to suit the conditions.
Pressurised tankers should give a better result under any conditions. But the differences may be small under ideal conditions. A tipper-blower operated by an expert delivering over a short distance (less than 10 metres, say) with few bends will only degrade the wood pellets marginally more than a pressurised tanker.
For less ideal delivery conditions, the differences are magnified. For longer deliveries, pressurised tankers can use higher pressures than are possible with unpressurised systems (e.g. tipper-blowers or Transmanuts) to achieve significantly less degradation during delivery.
It is still recommended to keep the delivery distance as short as possible. The ENplus guarantee of less than 4% delivered fines only applies where the blowing distance is under 30 metres, even if the wood pellets are delivered by pressurised tanker. And you should remember that multiple bends and gradients will increase the amount of degradation from all delivery vehicles. If you need a long blow or many bends, you should design your system to cope with higher fines, and clean your store more frequently.
But if there is no way to avoid a longer blow, pressurised tankers will significantly reduce the damage done to the wood pellets during delivery. For this reason, some operators of tipper-blowers will not blow more than 20 metres. Our pressurised tankers can blow upto 40 metres if there are no other impediments (e.g. bends), without completely disintegrating the pellets in the process.
There is a lot of confusion about speed (and pressure, see below). Speed might be thought of as the velocity at which the wood pellets travel in the pipe. Or it might be thought of as the length of time that it takes to deliver the wood pellets. Velocity (i.e. speed in the first sense) is a key factor in the stresses experienced by the wood pellets during delivery and therefore in the level of degradation.
Many people wrongly assume that the two types of speed are two sides of the same coin, on the basis that the pellets will get from the truck to the store in a shorter period of time if they are travelling faster. They therefore assume that a delivery in a shorter space of time equates to more damage to the pellets.
What this misses is that the time taken for delivery is determined by the flow rate (m3/s) not the velocity (m/s). If you blew the pellets into the silo one by one, it would take a very long time, even if the pellets were travelling at high speed. And they would be travelling at high speed, because all of the energy generated by the blower would be transmitted to each individual wood pellet. Conversely, if you can load the wood pellets thick in the pipe and move them quite slowly, you can deliver a lot of product in a short space of time, even though the wood pellets are travelling slowly.
The density of the wood pellets as they flow down the pipe is known as the "solids loading factor". Pressurised tankers can achieve higher solids loading factors than unpressurised systems. This means that they can achieve a higher flow-rate at lower velocity, and cause less degradation whilst delivering in a short space of time.
The solids loading factor that can be achieved depends on the installation. For longer blows or deliveries with more bends, the solids loading factor has to be lower. The delivery will take longer per tonne in that case, and there will be more degradation. For shorter blows with not too many bends, a high solids loading factor can be maintained, and delivery rates as fast as 2 minutes per tonne are not only achievable, but will also produce the best results. By contrast, unpressurised systems normally need around 5 - 7 minutes per tonne. That means we can do a better delivery whilst getting in and out of your way faster than the competition.
The reason that we can achieve high solids loading factors and low velocities is because pressurised tankers can operate at higher pressures. Another common misunderstanding is that high pressure means high velocity and therefore more damage.
This is a common fallacy held by installers, because they are used to dealing with liquids where this equation might be true. For gases (e.g. the air used for blowing the pellets), the opposite is the case.
When you compress a gas, its volume reduces in proportion to the pressure. If we deliver at 1 bar, we are delivering at roughly twice the level of atmospheric pressure. The volume of the air travelling down the pipes is therefore halved, and that means the velocity is halved (all other things equal).
If an unpressurised system such as a tipper-blower delivers at 0.4 bar, the volume of the air is only reduced by 29%. The air will be travelling 40% faster than if it were compressed to 1 bar.
The delivery air expands as it travels down the pipe, and reaches atmospheric pressure as it exits the pipes, whether it was pressurised to 0.4 bar or 1 bar at the start. So the difference in the average velocity in the pipes is less than the difference at the pickup point (i.e. maximum compression). But it is still a material difference.
If the difference were roughly the average between the pickup velocity and the exit velocity, the difference in the average velocity between 0.4 bar and 1 bar would be around 20%. The impact energy (e.g. the vibrations that damage the pellets when they bang into the walls of the pipes, bends and other obstacles) is proportionate to the square of the velocity, so a 20% difference in velocity equates to a 44% difference in the stresses experienced by the wood pellets.
Now add to this other factors influenced by the pressure:
1. Higher pressure allows us to achieve higher solids loading factors. As explained in the previous section, high solids loading factors mean a higher flow rate at a lower velocity.
2. A high solids loading factor has an additional benefit called the "Shielding Effect". When wood pellets travel thinly down a pipe, they rattle around and each pellet will impact with the side walls, bends and other obstacles multiple times. When wood pellet travel thickly down a pipe, they shield each other from the impacts. Only some of them hit the walls and bends. The rest jostle each other along, protecting each other from the worst impacts.
Unpressurised systems cannot use such high pressures as pressurised tankers, because the back pressure on the product being fed through the rotary valve reduces the product flow-rate. In pressurised tankers, the pressure pushing the wood pellets into the pipe is roughly the same as the pressure in the pipe, so higher pressures do not greatly affect the product feed-rate.
The net effect is that pressurised tankers can achieve higher solids loading factors and product flow rates at lower velocities and with a greater Shielding Effect. And that equates to less degradation in delivery. That will be a major reason why pressurised tankers are the standard system used for delivering wood pellets in experienced pellet-heating countries.
Our pressurised tankers are quite high. You should allow for around 4.3 metres clearance in your access. This may be marginally higher in transit than a tipper-blower. However, once on site, they do not need any additional headroom. Tipper-blowers may need upto 7 metres clearance at the parking space. If a tipper-blower driver is not careful, there is always a risk of catching an overhead cable.
The reading on a tipper-blower's weigh-loader can change by over a tonne after the truck has finished blowing. Two of our first delivery trucks were tipper-blowers. It was after witnessing the weight-reading spinning on one of these trucks as the truck sat there neither delivering nor moving, that we looked more closely into weigh-loaders that could be trusted to give uncorruptible readings.
What was required (by law, as well as by our customers) was weigh-loaders that were certified Legal for Trade by Trading Standards. The certification meant not only that the weigh-loaders had been calibrated and were capable of sufficiently accurate readings that any discrepancies would make a negligible difference to the value to the customer, but also that the system had been investigated and verified as reliable to maintain this level of accuracy under all conditions, and incapable of being skewed by operator intervention.
Many tipper-blower operators will try to confuse you that their calibrated weigh-loaders are effectively the same as Legal for Trade certification. That is not the case. Their truck can only be calibrated as accurate (a) at the time of calibration, and (b) when operated exactly as it was during calibration. There is no way for Trading Standards to verify that a tipper-blower will be operated in such a way that the weigh-loader will give an accurate reading. It is therefore not practical to certify weigh-loaders on tipper-blowers as Legal for Trade.
The weigh-loaders on our pressurised tankers are not just accurate to 20kg. There is no way for the operator to skew the reading significantly. There is no moving body to affect the reading. The weigh-loaders are tested to give the same reading at different gradients front-to-back, and left-to-right. The weigh-loaders have a cut-off so that they refuse to give a reading if the truck is on such a steep gradient that it would affect the reading.
When you get a print-out from one of our weigh-loaders, you know that is the quantity that was delivered. You cannot know that with a tipper-blower. Indeed, we have delivered to customers who have been surprised that the capacity of their store is smaller than they thought. We have filled their store and given them an accurate weight measurement. But they had been charged for significantly more by their previous supplier. The discrepancy could not be explained by differences in bulk density, as ours were on the high end of normal (650 - 670 kg/m3), and the other supplier's pellets would have had to be higher than any softwood pellets ever seen to account for the discrepancy. It was hard to avoid the conclusion that they had been over-charged.
When you get a cheap price from an operator that uses tipper-blowers, you should bear this in mind. If you save £10/tonne by going with the cheaper supplier, but that suppliers bills you for half a tonne extra on a 5 tonne load, you are actually £10/tonne worse off than if you had paid £10/tonne more and been billed accurately. And that is quite apart from all the other quality benefits of tankers, which would justify a £10/tonne premium anyway.
Pressurised tankers are built on an aluminium cylinder. The aluminium is to minimise the weight. The cylinder is the best shape to contain the pressure. The outlets consist of a series of cones or "pots" beneath the cylinder, to create the angled floors needed so the pellets will flow out without any mechanical intervention.
The result is a lot of space beneath the sloping floors. Tankers may look like they have vertical sidewalls, but that is simply a decorative panel concealing the storage areas.
The area immediately below the sloping floor, highlighted with a red box in the photo, is ideal for holding a series of tubes to contain the delivery pipes. This makes it easy for us to carry 40 metres of pipe on our 6- and 8-wheelers.
Further down, highlighted in blue, is a large box that contains the specialist suction fan and the associated equipment - wide-bore flexible pipe, "sock" (i.e. filter), connectors, long electrical extension lead, etc.
Tipper-blowers are slab-sided. Finding space for the pipes and the suction equipment is difficult. That is another reason why tipper-blowers often cannot blow as far as tankers - they will probably not be carrying as much pipe. And it is why they often are not equipped with a specialist suction fan. Fans were not normally required for the agricultural purposes for which most of the wood pellet tippers were previously used. If they are present at all, they are often small units (such as converted hoovers) carried in the cab.
Small fans will not be able to remove as much air as the blower blows into the store, resulting in pressurisation of the store during delivery.
Loading a wood pellet delivery truck is a dusty business. As the wood pellets fall into the truck a cloud of dust is generated and thrown into the air.
Tankers are loaded through hatches. The loading chute can be sized to fit snugly into the hatch. As the pellets land inside the tank, there is little opportunity for the dust to escape, maintaining a pleasant and safe working environment, and preventing pollution of the neighbours.
We go a step further in our depots. They are equipped with "bellows" that fit tightly onto the hatches. The bellows consist of an inner tube through which the wood pellets flow, and an outer tube from which the air is extracted and filtered, to safely and cleanly remove the dust generated during loading.
Tippers and Transmanuts are loaded by peeling back the "sheet" (i.e. cover) and dropping the wood pellets into the body. The body is wide open. There is no way to control the dust. It will billow out, affecting people working around it, and contaminating neighbouring properties. And it will settle and accumulate on any flat surfaces, until it creates an explosion risk, like the wood-flour deposits that caused the catastrophe at Bosley Mill.
Pressurised tankers have to be tightly-sealed, or they could not be pressurised. The only points of entry are a series of loading hatches at the top, which are sealed shut and capable of withstanding pressures of 1 bar during delivery. There is negligible risk that moisture will get into the wood pellets in transit.
Tippers and Transmanuts have only a fabric cover to keep the rain out of the body. A tear in the fabric, or a badly-fitted sheet can allow rain to get into the wood pellets while the truck is driving to site.
Wood pellets disintegrate on contact with moisture. The reference sample used by ENplus-accredited wood pellet suppliers to determine whether a load was in spec is taken at the time of loading the truck. The pellets may well have been in spec at that point, but yet have disintegrated from moisture-contact by the time they get to you. But you will have a hard time proving non-compliance, because the reference sample is in spec.
Forever Fuels has the vast majority of the pressurised tankers dedicated to wood pellet deliveries in the UK. We have fleets of tankers in each region of mainland Britain. If you want a tanker delivery, we are the only wood pellet supplier who can guarantee it across the country.