Steam Injection

by Charlie Scandrett, charlie@pressnet.com.au

posted to Homebrew Digest 1901, December 5, 1995

First, I'm a steam injection geek, I even boil (yes it can be done) with the stuff and I've ordered an electronically controlled "parabolic plug valve" to PID control it. Steam has its problems, it can be dangerous and needs to be understood very well.

Steam is a great carrier of heat and because it is at 100C to 200C in non contained systems, it can heat quickly without scorching. When superheated, it penetrates the fluid well beyond the boundary layer, and when a central manifold is used, it gives great convection and gelatinisation. (You still need to stir a grain mash for even temp, not to prevent scorching) It isolates the very high temperatures of the burners from your wort, while injecting heat without the usual boundary layer problems.

BASICS.
1 litre of water raised to 100C contains 419 kilo Joules of energy (sensible heat)and weighs 1 kg. Specific heat of water = 4.19 kJ/1C/litre, specific heat of steam = 2.16 kJ/1C/kg. 1 kg of steam at 1 atmosphere and 100C contains 419kJ (sensible) + 2257 kJ of heat of vapourisation (latent heat) and occupies 1,673 litres!

So, it gives off lots of heat when it condenses and expands *very* rapidly when vaporised.

Example
100 litres of water at 40C, to be raised to 60C quickly,(G.Fix's mash) needs (20C x 4.19 x 100) 8,380 kJ of energy. This means 3.4 kg of steam, or 3.4 litres of water vaporised and injected into the 100 litres.(makes 103.4 litres) If you needed to do it in 15 minutes, the steam generator would need to absorb (8,380kJ/900secs) 9.3 kiloWatts (32,000 BTU/hr) At a home setup efficiency of about 40%, you would need a 24 kW (82,000 BTU/hr) burner. I use a 22 litre pressure cooker as my primary steam boiler and a 40 kW burner for 85 litres.

Now there is WET steam and DRY steam, and SATURATED steam and SUPERHEATED steam. Wet and dry is easy, dry is 100% water *vapour*,a colourless gas, wet steam contains water droplets. (the stuff clouds are made of) Saturated steam is steam at the boiling point of water at that pressure. i.e. If the temp drops a tiny bit or the pressure goes up a tiny bit, it condenses. Saturated steam is just steam, it is at the point of condensation. All boiling chambers usually produce steam that is 98% saturated vapour and 2% water droplets, i.e. it is "wet" and "saturated". This is important to remember. The % dry is called the steam "quality".

Superheated steam is steam above the temperature of saturated steam for a given pressure. The STEAM TABLES will give the temperature of saturated steam for any given pressure, or vica versa. Thus 1 kilogram of superheated steam at 1 atmosphere and 150C has total heat = 419kJ (sensible) + 2257kJ (latent) + 105kJ (superheat)= "specific enthalpy" of the steam.

Example of Steam Tables 


        Pressure        Temp    Sensible        Latent  Enthalpy        Volume
        Atmos           oC      kJ/Kg           kJ/Kg   kJ/Kg           m3/kg
        1               100     419.04          2257    2676            1.673
        2               120.42  505.6           2201.1  2706.7          0.881
        3               133.69  562.2           2163.3  2725.5          0.623
        4               143.7   605.3           2133.4  2738.7          0.461

When we produce steam in a domestic pressure cooker (with the weighted safety relief valve operating normally please, don't touch it!) it is wet and saturated. Our mash tun and the piping's resistence to flow provide some back pressure, but essentially it is not a contained PRESSURE system and generally doesn't fall under the STEAM REGULATIONS. (These are very heavy, rightly so) However our system does cool the steam in the piping and manifold, so some condenses. This can lead to a dangerous condition called "water hammer" where condensation collects and blocks the fast flow of steam and builds up dangerous back pressure, causing further condensation. This can easly rupture equipment if you have tampered with your safety relief valve. Remember 3 kg of steam in 15 minutes is 4,800 litres of steam, that is 5 litres/second, don't stand in the way!

However superheated steam is well above condensation temperature for the low pessure of an open, uncontained system and ensures that high quality dry steam reaches your mash or kettle. Heat losses to piping are absorbed by the superheat.

Example
To "rolling boil" 100 litres of wort for 1.5 hours. In a normal vigorous boil you would expect to loose 15% of volume. Gee, this is exactly how much steam you need to generate to supply the necessary heat. Boil 15 litres of water to 15 kg of steam and inject. Because it condenses and then heats the water to boil the same ammount, in a perfect system there is no loss of volume. But because there are other heat losses, saturated 100C steam can't effectivly boil water at 100C. However 150C superheated steam would boil it and add a further 1620 kJ of energy, boiling off 2/3 a litre of wort in a perfectly insulated system. Conclusion, you can boil without scorching and get a good rolling boil with steam injection, but you can't reduce volume much. Result, smoother paler beers and a need to sparge to exact volume.

The Home Steam Generator

I strongly recommend a pressure cooker as the base chamber, even though this is not a pressure system. If you do something silly, you are left heating a standards approved pressure vessel, with a working safety relief valve. The SS tubing will take pressures way beyond your capacity to generate. Your plumbing skills will suffer some loss of face, not much else!

How to superheat it?

First make a little pentagon out of firebricks to support your pressure cooker and contain your burner, leaving ample vents at the bottom. Install a SS outlet in the side of the vessel and lead a SS tube (no copper, preferably hard SS) under the vessel and make a flat spiral of it across the bottom, exiting down at the center. This is a bit like a hypnotic spiral or lolipop. The idea is to lead the steam back through the flame and superheat it and exit down through the flame to your tun manifold inlet. The wort/mash level should be below the spiral. The temperature of the flame is typically 1200C so superheating the steam is not very difficult.( The high temperature of the flame is why we are remote heating in the first place) Monitor the exit temperature and reduce windings if too hot. Remember steam behaves like a gas and expands when superheated, you cannot put a valve on this system without becoming a pressure boiler.

How do I control it?

Remember I said the system has a little back pressure from the tun fliud depth and the resistance to rapid flow of steam? Use this. Install another outlet on your cooker and put a valve and a larger pipe exiting into a open drum with water in the bottom, this is a heat sink.(outlet NOT under water) This valve releases the back pressure and lets your saturated steam out before the superheater, thus providing a path of less resistance away from your wort/mash. This is not a contained system because the other outlet is still unvalved, thus open. Of course the gas controls give further adjustment, but not as quick a response.

Remember to becareful about constriction of steam flow. If the back pressure is too high and you open the control valve suddenly, you drop the pressure in the vessel instantly, superheating the steam inside. This superheat will vaporise more steam, causing some tendency to boilover out your control valve. Start at 1/2 inch basic, and scale up with bigger volumes.

What sort of manifold?

Your sparge manifold would do, or a spiral at the centre of your kettle. Lots of holes equal to 150% of the cross sectional area of your supply line. This can be copper as it is not direct heated and never dry.

What about scale and "crud"?

Commercial boilers produce this because they *continously* inject water and increase the concentration of minerals by evaporation. They try to control it by "blowing down" some of the concentrated water before adding more fresh. As you *batch* boil, you can flush the system in between uses. If your water is too hard it will foam and cause "priming", basicllly water in your steam lines. You will still normally produce some slight "carry over" as you are producing initially 98% steam, 2% water droplets. However this can be negligible if you use softened or distilled water. Scale is basically a back to nature movement. Water contains minerals dissolved from rocks in its catchment area. When you boil the water off and add heat, you essentially metamorphise....rocks! Scale encrustations can be removed by freezing the metal to break the bond or chemically.

An important startup step, boil your water with the relief valve open for a few minutes to expell O2 (and Cl). This will otherwise corrode your system and HSA your wort.

How big a pressure cooker do I need?

If boiling, to boil 100 litres for 90 minutes needs about 15% wort volume in kg's steam. A pressure cooker of 20% of wort volume would do even this demanding job. For mashing only, an even smaller one would do.

How big a burner?

The example in the first part gives you a tool for calculating your scale. Remember the hottest part of the flame is the tip of it.

What if my plumbing leaks?

Don't look for leaks with anything but a rolled up newspaper. (this is the professional method since Stevenson) If they are high pressure they will shred the headlines and not your hand! If they are low pressure they will condense and cook the paper and not your hand! The connections in this system should be all threaded gas fittings or metal compression seals. No household type plastic ones of threadseal(unless teflon).

What are the effects of steam?

Many of the harsher flavours of beer come from overheating of the boundary layer between a hot metal wall and your wort. Steam injection doesn't have a heating boundary at all.

This problem could be in your RIMS unit, mash tun or kettle. Such flavours and colouring are often desirable, some people throw white hot rocks in their wort to get these reactions! However, these Maillard (browning) and Caramel reactions not easy to control at this interface (scorching) and I think are best controlled is seperate decoctions. Steam injected into another pressure cooker of decoction is my preferred method. A RIMS unit of a steam injection manifold inside the recirculating pipe is an idea of mine. If the pump was below the injection point (to avoid cavitation), the pump speed would be irrelevant to scorching, allowing very light recirculation. Just a recirculating pump with a normal mash injection manifold would give exactly the same result.

If steam is injected into a unstirred settled mash from beneath the grain bed, it temporarily heats the bed to 80c -> 90C before dissapating into the thinner parts. This gelatinises the starch globules well without a decoction of the colour increase of a decoction. Rapid and complete conversion is enhanced.

In conclusion, steam is powerful and precise, a gentle heating method but dangerous without proper understanding. I urge you to get a competent engineer to check your constructions if you harness it's power.

Charlie (Brisbane, Australia)