cwi,
Let say that I have two setups with the same flow rate.
System one had a trombone, swan neck, whatever. It has a 1/2" ball valve that is wide open feeding a pathway no smaller than 1/2 in diameter. The flow-rate is limited by the hight of the arch at the top of the said device.
System two has outflow that is reduced only buy reducing the diameter of the exit value thus reducing the diameter of the exit.
The second system would have liquid traveling at a greater speed. Does this effect grain compaction? I would imagine not, especially with a false bottom. But I just had to ask.
Your setup and question is a little puzzling. If I had to guess, you have a false bottom, and use a valve to control lautering rates. If you are looking for some solid ground to stand on to support this approach, you may be out of luck. It's only "pros" may be cost and simplicity, and perhaps that these other approaches are overkill. However, I would say that a prerequisite to claiming these devices are overkill is personally experiencing a stuck mash.
The short answer is that the velocity of the wort at the valve restriction has no direct impact on the compaction of the grain bed up-valve. This is not because a false bottom is used, or even particularly mitigated by it. In your scenario, given identical flow rates, the wort within the bed will have the exact same flow rate in both cases. If there is an elevation gain post valve, once past the "pooling point", the velocity in the hose will the same in both cases also. However, things are not that simple in the real world. One cannot assume that the grain bed remains static throughout lautering. In fact, it is almost guaranteed not to remain static.
The long answer gets very complicated, especially since I will provide some analysis and background info on what I am basing my positions on.
Caveats: This should be basically correct, but I may have missed something; not correctly reasoned some detail; or not adequately explained some part. The effect of a pump was not considered in this analysis, mainly due to them only exacerbating the situation and not mitigating any of the issues. A pump simply increases the maximum negative relative pressure (suction) that can occur at the bottom of the bed. The swan neck is a type of weir with a riser pipe drain. There should be some papers online, written by real scientists, if you google it.
Here is a first crack at it:
What causes compaction is the force of the moving wort (throughput) on the restriction/resistance within the bed, and/or dewatering of the grain bed. As flow increases, the downward force on the bed increases. A special (bad) case is when the restriction in the upper part of the bed is higher than in the lower part of the bed . In this case, if the outflow exceeds the recharge rate through the upper bed, a localized negative pressure differential (relative vacuum) is created by the lower bed that exerts a pulling force on the upper bed. Dewatering is a worst case scenario of the above where air infiltrates the lower bed, causing the lower bed to run dry. At that point, the force of the entire mass of the upper bed (grain and wort) is exerted on the lower bed, instead of the much smaller force of the bouyed upper grain bed (grain less the mass of the water it displaces).
What the swan necks and pressure gauge/valve combos can do that a cracked valve cannot is to automatically regulate the compaction force applied to a bed due to throughput. This is achieved by guaranteeing that the reduced pressure at the outflow does not go below a set pressure level.
With the swan neck, the flow is determined by controlling the pressure. A fixed pressure set point is established based on the height differential of the tun level and neck apex (tube diameter is a factor as well, more on that later). Physics takes care of controlling/maintaining the pressure. This both guarantees the bed can't dewater, and also regulates flow, in a sense, as a side effect. If the bed becomes more restrictive, the pressure at the apex drops, and the flow decreases. This brings the pressure back to the set point. The flow is variable according to how restrictive, or not, the bed becomes, but the pressure exerted on/through the grain bed remains the same.
With a gauge and a valve, the pressure is determined by controlling the flow. The pressure gauge provides an indicator of the force being applied to the bed, and also if the bed is becoming dewatered. A set point is determined (just as with a swan neck), and is controlled by increasing or decreasing flow directly. What a pressure gauge can do that the swan neck can't, is maximize flow by having a larger flow capacity available that does not adversely affect control of the system.
This is due to the swan neck having a flow limit of sorts built in, on purpose. The diameter of the tubing is part of the pressure set point equation. As flow increases, the pressure at the apex also increases, reducing the differential and limiting flow. Bigger tubing can allow for more flow while maintaining the same pressure set point, but also makes establishing the pressure set point more problematic, since the differential range will be smaller, among other things.
With a valve only, the above systems can be mimiced, but the only feedback indicator available is the rate and change in outflow, whose variability decreases as the valve restriction decreases. This is not a very precise method, and may actually be made worse by a false bottom, since the buffer created by the dead space below the false bottom can hide a dewatering event before any noticeable (by a human) reduction in flow is observed. The worst case is the dead space running dry. This is an issue with manifolds and bazookas also, but a bazooka/manifold provides more immediate feedback since there is less buffering of the output, which allows faster detection. However, a false bottom does a better job of spreading the flow over the entire bottom of the bed. This helps prevent localized compaction from occurring which can lead to a clogged manifold/screen.
Setting a valve to an initial flow that seems "'just 'bout right" and then walking away is the simplest method, but provides virtually no protection against a compacted grain bed.
Decide how much chance you think you have of getting a stuck mash; how much you want to avoid one; and pick your poison.
) Similarly, some may challenge the validity of what you say; look at that as an opportunity to share your knowledge and experience in more detail; if they still disagree (or are unwilling to take your advice, for whatever reason), just let it go.
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