by Kelly E. Jones (Brewing Techniques)
I first learned about using steam for mash temperature control from HomeBrew Digest, a mailing list on the internet. Looking back, I am surprised the idea didn’t occur to me earlier. I recall doing a pilot-scale experiment on the use of steam for temperature control as part of a laboratory course in my chemical engineering curriculum. During this group experiment, one of my assigned tasks was to control the temperature of a 500-gal tank of an aqueous solution by adjusting the flow of steam through a pipe that emptied directly into the tank under the surface level of the liquid. The steam condensed almost immediately upon contact with the cooler liquid, thus transferring its heat to the liquid. To think I could have been replaced by an inexpensive automatic controller!
Before I delve into the nuts and bolts of steam injection, I want to emphasize the need for safety. Steam has a heat content many times that of boiling water, even when the two are at the same temperature. This heat content makes steam not only more useful but also more dangerous. Upon contact with the skin, boiling water quickly loses heat and drops in temperature; steam, on the other hand, stays at 100 °C until all of its energy has been transferred to the skin, at which point the steam becomes still-scalding 100 °C water. Steam can thus cause burns much more severe than those caused by boiling water and should be treated with due respect.
Note, too, that steam is actually a nearly invisible gas — the “steam” we see rising from an open kettle is actually condensed water vapor. Thus, make sure your system has no hidden pinholes or leaks that can emit unseen streams of scalding steam.
Given these caveats, though, the system described in this article is fairly safe. No high pressures are involved; it takes only about 0.5 psi to discharge the steam into the mash. Should the steam discharge tube become clogged, the pressure in the steam generator should get no higher than 15 psi before the pressure regulator begins to do its thing.
ADVANTAGES OF USING STEAM
Injecting steam directly into the mash tun offers many advantages, especially for home brewers mashing in picnic coolers or other nonmetal mash tuns. One of the chief drawbacks of nonmetal mash tuns is the difficulty of doing any type of stepped-temperature mash. It is possible, of course, to achieve temperature steps by adding measured amounts of boiling or very hot water or by decoctions. Adding hot water, however, also has the (usually) undesired result of thinning the mash, sometimes unacceptably so, and decoctions can be extremely time consuming. Home brewers who use picnic coolers therefore usually perform single-step infusion mashes. Steam provides a method for adding heat to such mash tuns and enables you to dough in at protein-rest temperature, raise the temperature to mash temperatures, and finally raise the temperature to mash-out, all without significantly thinning the mash.
Steam offers advantages even to those who mash in electrically or gas-fired kettles. Steam greatly reduces the danger of carmelization and virtually eliminates the possibility of scorching the mash because the steam temperature stays in the vicinity of 100 °C (the low pressure steam used in this setup is only a few degrees higher than 100 °C). Although stirring is still necessary to achieve even heat distribution, a momentary lapse in stirring will not result in a blackened mess (and ruined mash).
Steam may offer several advantages for commercial brewers as well. For those mashing in unheated tuns, direct steam injection might be a low-cost method of adding heating capability to the tuns for stepped mashes. This might be especially economical for those breweries that already use steam as a source of heat for the kettle. Furthermore, direct injection of steam offers certain advantages over steam jacketed tuns. The heat transfer rate will be higher, because of the absence of an intervening wall between the steam and the mash. Also, because the steam is injected directly into the bulk of the mash rather than at the periphery, the heat distribution will tend to be more even and require less stirring.
The power of steam is a consequence of the great affinity that water molecules have for one another. Because of this affinity, it takes a great deal of energy to rip the water molecules apart and create steam. Conversely, when these water molecules condense, a great deal of energy is given off as heat.
The heat capacity of water is roughly 4.2 kJ/(kg·K). That is, to raise the temperature of 1 kg of water by 1K (1 °C) requires 4.2 kJ of energy. 1 kJ is the amount of heat put out by a 1-kW heater in 1 s. By comparison, the heat of vaporization of water is 2260 kJ/kg. This means that 2260 kJ of energy are required to turn 1 kg of water into steam and that 1 kg of condensing steam releases 2260 kJ of energy.
When steam is injected into the mash, the steam will condense, giving up its energy to the mash. At that point, the condensed steam is converted to liquid water at a temperature of 100 °C. This hot water will further heat the mash; however, the heat added to the mash by condensation is so much greater than the heat added by the 100 °C water that the latter can be safely ignored for first-order calculations. Based on the ratio of the heat of vaporization to the heat capacity, for example, the heat content of steam is roughly 540 times that of water, on a per-degree basis. Put another way, 1 kg of steam is capable of raising the temperature of 540 kg of water by 1 K (1°C). As an example, suppose we have calculated the amount of heat necessary to raise a volume of mash to 73 °C. If we were to use boiling water at 100 °C, the drop in temperature of this water when added to the mash could be (100–73) or 27 °C. The quantity of boiling water required would thus be 540/27 or 20 times the quantity of the steam required to provide the same amount of heat.
As the steam condenses in the mash, it will of course thin the mash somewhat. However, this effect is close to negligible. Suppose, for example, that you dough in to a protein-rest temperature of 50 °C (122 °F), using (for a 5-gal brew length) 3.5 kg (7.7 lb) of malt and 7 kg (15.4 lb, or about 2 gal) of water. Simple thermodynamic calculations show that to raise this mash from 50 °C (protein-rest) to 76 °C (169 °F) (mash-out) requires about 0.4 kg (0.9 lb, or 14 oz) of water in the form of steam. This amount of water has only a very small effect on the thickness of the mash, certainly much less than would the addition of boiling water.
A STEAM-INJECTION SYSTEM FOR HOME BREWERS
Construction: To build my steam heat system, I started with an old aluminum pressure cooker, the type with a weight on top to control the pressure. These can usually be found at garage sales or flea markets for $ 10–15. My cooker also has a metal disk safety release, which will rupture if the pressure or temperature in the cooker gets too high.
It is important when building a steam generator not to remove or otherwise alter any existing pressure-regulating or safety devices. Thus, I installed a new port in the lid of the cooker for steam removal. I drilled an 11/16-in. hole and tapped it with ⅛-32 NPT threads. This allowed me to install a ⅛-in. MPT to a ¼-in. tubing Swagelock connector. When I’m not using my cooker as a steam generator, I close off this fitting with a plug so that the cooker is still usable for its original purpose.
On brewing day, the steam generator sits on a 2100-W electric burner (again, purchased at a flea market) next to my mash tun. I normally mash in a rectangular picnic cooler, big enough for 10-gal brewlength. A copper tube extends from the swage fitting to a ball valve, and from the valve extends another piece of copper tubing that also serves as the steam bubbler (Figure 1). The bubbler is made by drilling a series of 1/16-in. holes, spaced ½-in. apart, in the last 12 in. of tubing. The end of this tubing is pinched shut. The bubbler end sits near the bottom of the mash tun, just above my slotted copper sparge manifold. I considered injecting the steam through the sparge manifold but decided against it because of the effect the heat would have on the plastic mash tun.
Application: To use my system, I fill the pressure cooker about three-quarters full with clean water. Although only a few cups of water are used to heat the mash, it is generally better to start with more water than is needed rather than risk running short. I then seal the lid, attach the copper “out” tube, and turn on the electric burner. At this point, I leave the regulating weight off the cooker.
Meanwhile, I dough in the mash at protein-rest temperature (or some other temperature, depending on the type of mash schedule I am using). By the time I am ready to raise the mash temperature, the steam generator is ready, as evidenced by the stream of steam emitted from the top of the cooker. This warm-up period also serves to purge any air from the cooker, which eliminates any worry about hot-side aeration effects from bubbling hot air through the mash. At this point, I replace the regulating weight onto the top of the cooker and open the valve to begin steam heating. The steam can be heard “bumping” inside the mash, and I stir frequently to keep the mash at a uniform temperature. Generally I can raise the temperature of the mash 1 °C/min with no problems.
On the subject of heating rate, remember that in thermodynamics there is no such thing as a free lunch. The steam is not really a source of energy; it is merely a medium for transferring heat from the primary heating source (electric or gas burner) to the mash. If the primary heat source does not have enough “oomph” to generate sufficient quantities of steam, the rate of temperature rise of the mash using injection of steam will not be any greater than that is the heat source had been used to heat the mashtun directly.
When I want to stop the heating process to provide a temperature rest, I simply turn off the burner and close the valve. It is best to remove the regulator weight from the cooker; otherwise, condensing steam can cause a vacuum to develop inside the cooker. This vacuum can suck the wort out of the mash tun and into the cooker if the valve is opened. If the length of tubing between the cooker and the mash tun is significant, wrap the tube with insulative tape or foam to prevent excessive heat loss.
If my mash tun didn’t have to do double duty as a picnic cooler, I would drill a hole through the side near the bottom to bring the steam directly in. As it is, I bring the tube in from the top and use a piece of Styrofoam (with a hole cut out for the steam tube) as a lid. If I were to use this system with a metal mash tun (such as the stainless steel keg mash tuns with a false bottom used by many home brewers), I would simply bring the steam in through the drain tube, introducing it through the sparge manifold or under the false bottom. Regardless of the method used, the steam generator needs to be higher than the liquid level in the mash tun — or the steam tube needs to be looped so that it reaches a point higher than the liquid level in the mash tun — to prevent the mash from draining into the pressure cooker.
Another option: A possible modification to this setup would be to use some sort of heat exchanger for the steam, such as a coil of copper tubing, inside the mash instead of discharging the steam directly into the mash. Although this modification would prevent the addition of any water to the mash, it has two major disadvantages. First, heat transfer will be slower because of the wall of copper tubing between the steam and the mash; the wall presents a thermal resistance that is not present when direct injection is used. Second, a relatively large area will need to be supplied for heat exchange because of the relatively low temperature of the steam (compared with a burner element or gas flame, for example).* Discharging steam directly into the mash provides steam bubbles that disperse to form a surface area large enough to accomplish the heat transfer. What can be accomplished with 12 in. of bubbler tubing for direct injection would probably require many feet of tubing for a heat-exchanger design. Because the steam’s contribution of water to the mash is so minimal, I see no significant advantage to using this heat-exchanger modification.
*Heat transfer is proportional to the temperature difference times the heat transfer area.
ANYONE CAN DO IT
A steam heat system can be added to virtually any experienced home brewer’s setup. The cost of materials, assuming you have some basic scrounging skills, should be less than $ 50. The construction time is only 1–2 h. The flexibility this setup adds to your brewing system will be well worth the time and effort.
At the pub and microbrewery scale, direct injection of steam is in its experimental stage. My earlier experience with temperature control experiments using a 500-gal tank suggests that direct injection of steam may soon become a tested and accepted method for professional brewers as well.
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