This is a big blog post and somewhat text heavy, but if you’re investing in a steriliser barrel and building it yourself it’s worth taking your time or you could make the mistakes I did when I first built mine. There is a full parts list at the end. Cheers for reading!
Steriliser barrels are a straight-forward piece of equipment to build. For more information about sterilising, pasteurising and everything in between you can read myblog on it, here. This should help clear up any misunderstandings about the process before you build one yourself. Atmospheric steriliser barrels use heat and time to kill most contaminants that will hide inside your substrate. It achieves this by using the heat transferred from the water at the bottom to permeate upwards and through the substrate.
Container
The most used and widely available containers are open-top barrels made from Stainless Steel, Mild Steel or Galvanized Steel. However, you aren’t limited to a barrel; any shape vessel will do provided it meets the requirements. Just remember, mild steel will rust so regular maintenance with a good quality red-oxide primer will help prolong the life of your barrel.
Tread with caution if you want to use plastic barrels. Since the temperature of the water will not generally exceed the barrels melting point, these have been considered by some as a cheaper alternative.
I don’t believe this is a long-term solution. Some of the problems are that over time the heating and cooling may cause the plastic to become brittle, the heating process softens the plastic allowing it to miss-shape and the heat may expand that plastic around the unions and clamps causing them to leak.
False Floor
At the bottom of the barrel you need a small water reservoir to cover the element. Because the element will boil the water into steam, there needs to be a way of regulating the water. This is done with a float valve.
To ensure the element, valve and water reservoir can operate, a false floor is used to load the bags onto, supporting the substrate. False bottoms need to be strong, to support the weight of the substrate (sometimes 200kg+) but also durable as it sits in boiling water most of its life.
These can made from rolled steel/aluminium with a plate on top. The plate doesn’t need to be perforated; the heat will transfer through and around the bags with steam. The ring is then notched to allow the element and float to pass through.
Heating Source
Gas or electric, here is my view. An electric element sits directly in the water, therefore, almost no energy is lost in the transfer between element and water. Compare this to a gas burner where you are wasting energy heating the air around the burner as well as the outside of the barrel.
Using an electrical element also gives you the option to use timers to control the heat cycle. Gas is simpler but requires babysitting the whole cycle since it has an open flame. I think in the long run, electricity is cheaper and safer, also greener too.
An alternative idea is using sauna steam generators. I can’t see an immediate issue with these, other than them not being designed to run for 18 hours in a row. You may also see designs where a separate boiler is utilised. This gives more space in the main chamber but only another layer of bags worth, so not a worthwhile investment for the added cost in my opinion.
Water Regulation
As the water is heated, it is boiled off into steam. Some of it will condense and re-join the reservoir in the bottom but a large part will be lost. To keep the reservoir topped up, you need to use a float valve. These float valves need to withstand high temperatures and mains water pressures.
The UK mains water pressure averages at 15psi. Cheaper float valves may let water leak around the seal and not function as intended at those pressures. Therefore, using a pressure regulator or a different way of regulating pressure will ensure your barrel doesn’t flood and overflow.
Barrels will run between 90c and 100c so plastic float balls will eventually fail in this environment. Brass or stainless float valve balls are more suitable but it’s important to ensure the internal mechanism of the float valve is suitable. Float valves are a simple mechanism but over time may allow water to seep around the seal, so keep in mind to regularly check the functionality of your valve to ensure it doesn’t overflow.
Overflow is less of a disaster than the barrel running dry. Fire is a risk if the barrel runs dry, but the more likely problem is your element burning out. If this happens the plastic bags may melt, sticking to everything. Avoid this by using a reliable water source. If you’re using water from a rain-fall catcher, make sure it’s free of moss and other debris that could clog up the water line.
The mains are usually connected to the barrel with a standard garden hose fitting. A more reliable method is to use a flexi hot water pipe hose as a stand-off, then attach a standard garden hose to that. This way you avoid the heat of the barrel expanding and warping the plastic of the garden hose.
Water Drain
It’s important to drain your barrel after each use, this prevents the build-up of limescale as well as keeping your barrel free from debris that are carried in with your bags. On mild steel and galvanized barrels this is also important because it prevents the build-up of rust during storage, storing these barrels inverted will help drain water from the seams.
You can drain your barrel by simply inverting it, however, the easier way is to attach a ball valve to the lowest point and then a hosepipe to channel the draining water away.
Steam Vent
These barrels aren’t designed to be pressurised nor do they need to be using this process, therefore you’ll need a way for the steam to escape in a controlled manner. One method is to simply drill an exhaust port in the lid. This will work fine outside, but if you’re using your steriliser indoors you may want to control the flow of steam.
By using a ball valve threaded into the bung hole of the barrel lid, you can attach a length of high temperature hosing to direct the steam. The ball valve also allows you to close the barrel to minimise heat loss as the temperature rises, making it more efficient.
Steam Lid Spacer
This is an important feature often overlooked when steriliser barrels are built. During the cycle, bags may inflate even if unsealed. If a bag inflates it could block the hole your steam escapes through potentially pressurising the barrel making it unsafe. A small spacer at the top of the barrel prevents this by leaving a safe margin around the barrel. This way, if a bag inflates it would need to cover the entire perimeter of the barrel to cause a blockage, so there is no single point of failure.
Optional Extras
Insulation
To make the barrel more thermally efficient, you can use a wide variety of insulative materials. These include neoprene sheeting, radiator reflection foil, Kingspan insulation and rockwool. These can be attached in a wide variety of ways but the most common is to simply wrap with your chosen material and tape with aluminium tape.
Temperature controller
If you’re using an electric element, you can make use of an electronic temperature controller. These work with the same principle as an oven thermostat only to a much tighter tolerance value. Some models can also be programmed as a timer. Once a certain temperature is reached, the barrel can be heated and held at that temperature for x number of hours before it shuts the element off.
Dolly
Once full, the barrels are heavy so moving them requires them to be unloaded. You can use a dolly to make moving the barrel around your workspace much easier. I put a sheet of wood at the bottom of my dolly and top it up with sand. This way I bed my barrel into the dolly and the sand supports the barrel floor better.
How To Build A Barrel Steriliser
Step 1 – The Barrel Body
Decide where you want to put components. I try to bunch them all together so the unit can be kept tidy when in use. As you can see here, I have the drain, element, and float all within the same 1/4 wedge of the barrel.
There are 2 main ways to mount an electrical element to a barrel. Tri clamp/hygienic fittings or threaded elements. Depending on which element you buy, this will determine what you need. The threaded elements are great, they usually come with a washer and nut in a kit. Tri clamp ferrules require a bulkhead attached the barrel first. This can be either welded or weldless. Weldless ferrules are threaded and lock down with a nut and welded require welding to the barrel.
For my barrel I am using a 1.5” tri-clamp and a 3500w element and attaching it with a weldless bulkhead. I’ve always used threaded elements but they’re often more expensive than wiring your own, don’t commonly go higher than 2400w and don’t allow you to easily upgrade/change your element. Take note that a UK standard spur can only deliver around 3000w safely and not for a prolonged time. A more robust option is an ABB 3 pin plug which are rated for 16amps (3500w/240v = 14.5amps). As with all electrical work, consult a trained professional if you’re unsure of what to do.
Before you decide where the element and drain will go on the barrel, take note that the bottom of some barrel rims aren’t the bottom of the barrel. On this barrel, the floor of the barrel is around 10 – 15mm from the bottom of the rim. So, it’s easier to measure from the inside out and not the outside in to get the correct height.
I’m using a pre-notched false bottom (Eventually available in theshop). To ensure my components will align correctly when installed on the barrel.
Therefore, I needed to work out how to correctly space the components on the barrel to align with the notches. A simple task I initially thought, but being awful at maths, I enlisted the help of my mate, Gabrielle. He measured the distance between the false bottom notches and then, using maths, calculated the spacing I would need to install my components at based on the circumference of the barrel I was using. He then made a spreadsheet and saved me potentially ruining a £450 barrel. I may have overlooked an easier method, but nothing is easier than getting your friends to help! Cheers Gab!
Once the spacing was calculated I needed to work out where to drill the holes in relation to the bottom of the barrel. For this I made a stencil by taping two piece of paper together landscape. This step is also very important since an incorrect height clearance may prevent the ball valves float from reaching the top of it’s travel, consequently never shutting off the flow of water and filling your barrel.
Rather than using the actual barrel bottom as my datum for the element and drain, I use the 5mm tolerance as the bottom. If you tried to install the nut, on any element, flush to the bottom of the barrel, you wouldn’t be able to tighten it up, since the corners of the hex would foul against the bottom and unless you’re a professional sheet metal worker, it’s good practice to give yourself a bigger margin of error. It’s worth noting the barrel bottoms sometimes come concave, another reason to have a good tolerance value.
On the line parallel to the heating line is where I installed my drain and inlet. Since the ball valve outlet is going to be the approximate water line, I measured up approximately 50mm from the top of the element and then half the width of the valve to ensure I had the correct water coverage. I then made sure that, at that spacing, the float would be able to move up fully without going beyond the spacer line. I forgot to get photos of this process, so here are some rubbish drawings (none are to scale)
Then, using the same principle, I used the right angle drain valve, with the bottom aligned to the 5mm tolerance and made a mark through the centre point. Remember, a lot of barrels are concave, though they will flatten out over time with the substrate weight it will make installation a bit of a pig. Drainage won’t ever be perfect without installing a bottom drain, but you’ll get the majority out with this method.
Then with all your components marked out. Find the centre points and punch 3 small holes in the middle of them using a screwdriver or centre punch.
Next put on a headtorch and climb inside your barrel with your stencil and a marker pen. Rest the bottom of the paper on the bottom of the barrel and push it against the barrel radius. Using the pen, mark the holes onto the inside of the barrel through your stencil. Avoid making any holes in or close to the vertical barrel seam weld. The paper may drop into the seam around the edge of the barrel (if yours has one) you can just tape it in place level with the bottom.
Walk away, have a cup of tea, and come back. Now look at what you’ve done again and make sure you’re happy with the spacing, measurements, diameter, and clearances. You may have been able to do this method with a tape measure. But I find it easier to visualise when it’s in front of my eyes. Finally, align the bits in the barrel roughly and make sure they pass through the middle(ish) of the notches.
Once you’re happy, centre punch and drill the 3 holes with a small pilot drill from the inside out.
You can use many methods to drill a hole this big, you can chain drill it out and clean it all up with a file, use a hole saw, step drill or a knockout punch. Hole saws on rounded thin metal aren’t the best, they tend to tear the metal, jump chatter, leaving the hole 1 or 2mm bigger than the stated size and often not where you piloted. Chain drilling is very messy but will work in a pinch. Step drills are good but easy to drill oversized so proceed gently. Though expensive a Q-Max knockout punch is the cleanest way to make a perfect hole.
From the outside in, drill a hole the diameter of the Q-max cutter’s bolt. Unscrew the cutting portion and pass the bolt through the hole and reattach the cutting portion on the inside of the barrel. Tighten the bolt down and draw the cutter through the barrel until it falls out. Repeat this process for each hole using the correct size cutter and you should be left with 3 accurate, bur free holes in your barrel – Hopefully in the right position. If you’re using mild/galvanized steel, now would be the time to add some red oxide primer on the cut holes rims to reduce rust.
Smear a light film of vaseline on the O-ring and place it sitting in the recess of the body then pass the threaded portion through the hole.
Tighten the nut down by hand then nip up with stilsons, Knipex or large spanner. Don’t over tighten the nut as it may damage the O-ring. It doesn’t require a great amount of torque for it to be watertight. As you can see from the photo below, if we hadn’t included the safety margin, the concave bottom would have made it almost impossible to do the nut up.
There are several ways to install a drain valve. I simply used ½ BSP Stainless Steel hardware that I had lying around. However, NPT and BSP are similar enough in pitch to use interchangeable in low pressure environments. The difference is in the thread angle, therefore, use PTFE tape to keep the joint watertight.
BSPT (British Standard Pipe Tapered) and BSPP (British Standard Pipe Parallel) is something to consider. Tapered threads get fatter as they tighten up whereas parallel will run the whole thread without tightening down until they hit a stop or bottom out. BSPT is mostly what I used with the exception of the stainless steel running nipple in this drain.
To install the float valve, remove the nut and one of the washers. Pass the threaded portion through the inside of the barrel and align it so the outlet is pointing down. Pass the second washer back over the outside thread and tighten up with the locking washer.
Step 2 – The Element
This is what I did, i’m not an electrician and would advise you to seek a qualified professional before undertaking any electrical work. This element requires wiring. To be on the safe side I’m using 4mm 3 core H07RN-F Rubber Cable. Firstly, I secure a cable gland to the element back-shell and pass the cable through, don’t tighten the cable into the gland yet. Then strip back some of the external insulation.
Once passed through I cut away insulation and expose the 3 strands and crimped them with ring terminals, attached them to the right poles and earth. Then, keeping the cable tight, screw the back-shell on, avoid twisting the cable with the tightening motion. Finally, tighten up the glands nut to secure the cable. I then cut right length of cable from the reel and terminated a 3 pin ABB plug.
To secure the element, take the 1.5” tri clamp washer and place it in the recess of the ferrule. Pass the element through the hole and make sure the element is correctly mated to the element and ferrule.
Pass the clamp over the two flanges and tighten down.
Then finally, pass the clamp over the two flanges and tighten down.
Step 3 – The Lid
I had my spacer laser cut. To install it I first find the centre of the barrel lid and drill a 6mm hole. I then secured the centre hole to the lid.
Using the remaining holes as a guide, match drill through the lid. I pinned each hole with a screw and nut as you drill them, this helps prevent you from miss-match drilling/drift.
My barrel has 2 bung holes. 2” and ¾” BSP. In the ¾” BSP hole I install a reducing bush to bring it down to ½” BSP. In the 2” BSP I install a 2” – ¾” reducing bush and thread the valve handle and tighten down. In the ½” BSP I temporarily slaved an analogue thermostat while I was building my PID circuit. In the 3/4” BSP I installed a ball-valve to give some control over outflow and the option of installing an exhaust hose.
Step 4 – The False Bottom
A false bottom has 2 parts, the frame, and the plate. To make a false bottom, first measure the top of your float valve balls travel from the bottom of the barrel.
To make the frame take a strip of metal the same width as the height of your float valve’s travel with a safety margin. Remember, if the float valve doesn’t reach the very top of its travel it won’t shut the water flow off. Roll that strip of metal into a circle and secure it to itself with rivets or bolts. Notch some holes out to make room for you float valve and element to pass unobstructed. Next, make a metal disc that almost reaches the edge of the barrel. If using mild steel, treat the components with red oxide primer before using.
I had mine laser cut, rolled, and welded by a fabricator. Coming soon to the store is a notched and un-notched false bottom, keep an eye on the shopwww.archersmushrooms.co.uk/shopto get yourself a kit.
Accessories
I made a post about how to build a PID controller. You can read it here https://archersmushrooms.co.uk/how-to-build-a-mushroom-steriliser-temperature-controller/ . You can use an Inkbird Brewing Temp controller, however, just make sure the wattage/amps of your element won’t overpower the controller..
As far as insulation goes the choice is broad. I’ve chosen to use some leftover neoprene sheeting. I cut it to the right circumference and height, knocked out 3 holes for the inlet, outlet and element and secured it to the barrel with contact adhesive along one strip.
As I explained above, these float valves can’t handle UK mains pressure. You have 2 choices to get around this, those are, a gravity fed system. So long as the water level remains above the barrels waterline it will flow through the valve. The second option, which is what I did, was to add a PRV (pressure reducing valve) in-line with the hose. I used a hot water flexi pipe to go from the barrel to the PRV and then from the PRV I attached a standard hosepipe fitting. You then simply adjust the pressure with an allen key in the top of the valve.
This isn’t a complicated project, it just requires some planning to make sure you have the right bits and you don’t miss-drill your holes. If you have any questions don’t hesitate to drop me a message and i’ll do my best to help.
These blogs take a while to make, if it helped you out you, feel free to buy me a beer!
Barrel –https://bit.ly/3babCvY
Tri Clamp Bulkhead –https://bit.ly/33Dgm9f
1.5” Tri clamp Element –https://amzn.to/3et7OYt
PID Controller –https://amzn.to/3twCcWa
1/2” Type K Thermocouple –https://amzn.to/3vVPiy5
Tri Clamp Ferrule & Gasket –https://amzn.to/3uGQhBG
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Hosepipe End –https://amzn.to/3uAbSvW
Hole cutter for Weldless Bulkhead –https://amzn.to/3esTTSa
Hole cutter for float valve and drain –https://amzn.to/3hbyej9
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Float Valve –https://ebay.us/SMqzVO
Pressure Reducing Valve –https://amzn.to/3fhuWIX
O-rings x 2 –https://bit.ly/3eqK8nx
Recessed backnuts x 2 –https://bit.ly/3tqFxGh
65mm Running Nipple –https://bit.ly/2PZrdad
Elbow –https://bit.ly/3f6J6wp
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