Fluid Dynamics -- A Simple Key to the Mastery of Efficient Lautering
Understanding how the fluids in your mash tun move through the grain bed during sparging can help you maximize your extraction efficiency.
In last issue's "Stepping Up" column (1), Jim Busch discussed various aspects of lautering and how to achieve the highest extraction efficiency by manipulating the mash. In this article, we examine the same goal from a different angle; that is, how to achieve the greatest extraction by manipulating the fluid dynamics of the lauter system itself. We present the two basic types of lauter systems used by home brewers and explain how their designs govern the flow of wort through the grain bed and the degree of extract achieved.
In an optimum mash, all the available starch is converted to sugar. The optimal extraction varies depending on the malt, but it is generally about 35 points/lb/gal for a two-row barley base malt. In other words, 1 lb of malt that is crushed and mashed in 1 gal of water should yield a specific gravity of 1.035. Most brewers would not get this degree of extraction, but would get something closer to 1.031. The ratio of 31 to 35 represents an extract efficiency of 88%. The difference between ideal and actual extraction can be attributed to factors that influence the mash -- temperature and pH, for example -- but it can also be explained by looking at how well the grain bed is rinsed.
A unit volume of the grain bed contains a large number of grain particles and considerable adsorbed sugar. In an ideal world, the particles would all be the same size and have equal spacing between them, and all particles would be equally well-rinsed. In practice, of course, this isn't the case. Grain particles vary quite a bit in size, and this variation leads to regions of greater density within the grain bed. Fluids always flow by the path of least resistance, which leads to the problem of preferential flow through the grain bed. Some regions in the grain bed are completely rinsed, and others are left unrinsed. This phenomenon is called channeling.
The first step in solving the problem of a channeled grain bed is to supersaturate the bed by maintaining an inch of water over the grain. By keeping the grain bed supersaturated with water, the grain remains in a fluid state and is not subject to compaction by gravity. Each particle is free to move, and the liquid is free to move around it. Settling of the grain bed due to loss of fluidity is a major cause of stuck sparges.
The next principle for good lautering is uniform draining of the wort away from the grain particles. Notice that by fully wetting the grain bed, we have also set the conditions for complete draining, as long as the supersaturated condition is maintained.
The fluid dynamics of draining the grain bed is where we encounter the most variables. Supersaturation of the grain bed is easily maintained, so we can turn our attention to how flow rate and collection area/distribution affect our control of the lautering process. This article focuses on that issue.
WORT COLLECTION DEVICES
Figure 2c shows a simplified manifold consisting of a single pipe with slots cut halfway through, about 0.5 in. apart. One end is capped off so that all flow through the tube enters through the slots. Let's assume that the effective total area of the slots is greater than the internal diameter of the tubing. As long as the tube is flowing full, all the slots will contribute equally to the flow, and extraction will be uniform along the length of the tube.
The term flowing full defines a set of conditions in which the outflow velocity does not exceed inflow capability of the slots. The resistance of the valve at the outflow needs to meet or exceed the resistance to flow at the slots or holes of the intakes. As long as this condition is met, the outflow will draw equally from all points of the manifold. If this condition were not met, the manifold would draw preferentially from the openings nearest the outflow. For the rest of our discussion, we will assume that the manifold(s) are "flowing full," which in reality, is almost always the case.
Flow velocity: Figure 3 shows two configurations of manifold setup; Figure 3b is similar in design to Figure 3a except that it uses a longer length of tubing to collect from the same floor area of the lauter tun. (Notice also that the distance the water has to travel to reach the manifold is shorter than in 3a.) Let's assume that the manifold in Figure 3b is 15 in. longer, that the tube is cut with 2 slots per inch, and that the slots have a nominal area of 0.03 in.2 (My manifold tube is 0.5 in. i.d., the slots are the width of a hacksaw blade [0.035-0.40 in.]; the collection area of the slot is slot width X one-half the circumference of the tube.) This design therefore has an effective collection area of 15 X 2 X 0.03, or 0.9 in.2 more than the one in Figure 3a.
This difference may not seem like much, but what it means to the flow velocity at a given slot is significant. Dividing the flow rate by the total collection area allows us to compare the relative velocities at the collection points of various manifold configurations.
Flow rate calculations. Although the flow rate is easily determined by measuring the amount of fluid collected per minute, it is helpful for present purposes to translate that value into different units. Assuming that we are lautering at a flow rate of 1 qt/min, and that 1 ft3 of water equals approximately 7.5 gal (30 qt), the flow rate can be expressed as 1/30, or 0.033 ft3/min.
Collection-area calculations. To calculate the collection area, simply multiply the area for each slot by the number of slots in the system. For example, the manifold in Figure 3a is 63 in. long, that 2 slots are cut per inch, and that the nominal area of each slot is 0.03 in.2 The collection area in that manifold is 63 X 2 X 0.03, or 3.78 in.2, or 0.0265 ft2. If the manifold in Figure 3b is 15 in. longer, its total collection area is 78 X 2 X 0.03, or 0.0325 ft2.
Comparing relative velocities. Now let's divide the flow rate by the total collection area to compare the relative velocities of wort at the collection points in the systems in Figure 3. Let's assume that we use a valve to hold the outflow rate constant at 1 qt/min (0.03 ft3/min) for both systems. For the manifold in Figure 3a, the velocity at each slot is 0.033/0.0263 = 1.25 ft/min. For the manifold in Figure 3b, the velocity is 0.033/0.0325 = 1.02 ft/min.
The more collection sites/area available, the slower and more evenly the sparge water can move through the grain to achieve a nominal outflow volume rate. By moving the water through the grain bed more slowly, the water can extract more sugar from each unit volume of grain, resulting in better extract efficiency per unit volume of water collected.
So what is the optimum outflow rate? This number really depends on the geometry of both the lauter tun and the lautering system. Dr. Narziss of Weihenstephan suggests the equivalent of 0.18 gal/min ft2 as an initial lautering rate (2). To use this number in your system, you need to multiply this value by the area of your lauter tun. I lauter in a Sankey keg, which has a diameter of 15.5 in. and a resulting area of 1.3 ft2. Multiplying this area by 0.18 yields a flow rate of 0.23 gal/min or about 1 qt/min. Because of the manifold geometry of my system, I find that I get better extraction from a slightly slower flow rate -- closer to 2 qt every 3 min. The reason for this difference can be found in the distribution of the collection sites across the floor of the lauter tun.
Example comparisons: Figure 4 better illustrates the differences in the flow rates and collection site distribution of the three common lautering systems. Figure 4a is a top and front view of a 1-ft3 lauter tun fitted with a false bottom (0.125-in. holes on 0.5-in. centers). Figure 4b shows the same tun equipped with a manifold (48-in long with 0.03-in2 slots, 2 slots/in.). Figure 4c shows a variation on the manifold -- a rolled up tube of screen that attaches to the drain pipe (6-in. long, 0.5-in. diameter screen tube). The lighter areas of the grain bed represent areas where most of the extraction is taking place.
The false bottom in Figure 4a naturally covers the entire area uniformly and shows that the nominal effective collection area is about 7 in.2 Using our nominal flow rate of 1 qt/min, this results in a flow velocity of 0.34 ft/min. The lauter tun in Figure 4b has an effective collection area of 3 in.2, and the screen manifold has a slightly larger area, 4.7 in.2, resulting in velocities of 0.80 ft/min and 0.51 ft/min, respectively. As you can see, the greater the total collection area, the lower the flow velocity at each opening. The flow velocity of the false bottom is less than half that of the manifold.
Collection Site Distribution: In Figure 4a, the false bottom collects equally from all areas of the floor of the grain bed. The manifold in Figure 4b also collects from a majority of the floor, but depending on the inlet flow rate dead zones will occur along the bottom and sides at the points farthest from the pipes. This effect is especially evident in Figure 4c, in which the localized collection site of the screen results in large dead zones along the sides of the tun.
The pressure difference is zero for a unit volume of water over on the side of the tun at the same level as the manifold. Therefore, the water that is drawn to the manifold will come from above where a pressure difference can cause flow. (By the way, the dead-zone effect beneath the false bottom, in the foundation water, is negligible because of the lack of flow resistance from the absence of grain.) A fluid will always flow along the path of least resistance, so if the flow rate demand is high, the flow will be preferentially drawn from directly above the manifold, causing channeling and resulting in very poor extractions. The smaller the effective collection area and the more localized it is, the slower the outflow rate has to be to obtain good extraction.
The principles to remember: All this detail can be summed up in three principles for efficient extraction. First, the higher the total collection area that feeds the flow, the slower the intake velocity at a particular site. Second, the slower the intake velocity at each site, the greater the volume above the site that will "feed" that site. Third, an increased volume feeding the collection system means fewer dead zones, less channeling, and better total extraction.
INTERPLAY OF AREA AND SPEED
|Issue 3.4 Table Of Contents|