Research - Bell Siphons

I have begun experimenting with bell siphons.  The premise and examples of the bell siphon can be gathered from any number of sites and illustrations.  Here, I am going to be working through, in stream-of-consciousness, the basic functionality and problems of the bell siphon.

The basic construction is as follows:

• Standpipe: the vertical portion of the siphon that extends from the base of the grow-bed to near the top of the bell.  The height of the standpipe dictates the highest point at which water will rise in the grow-bed.
• Bell: A pipe enclosed at one end, open-end-down over the standpipe.  Holes or notches are cut near the bottom to allow water to flood into the bell.  The height of the notches dictates the lowest point water will drain to.  The bell is usually sized such that it is within 1/2" of the top of the standpipe.  Its diameter must be suitable to the standpipe construction.
• Drain Pipe: The piping situated below the standpipe (and the grow-bed), which drains into either the fish tank or a sump, or in some cases other grow-beds.  Typical recommendations are for the drain pipe to contain at least two 90-degree angles, for back-pressure and outflow direction.

The "system" is the siphon and the container (grow-bed, tank, etc) it is situated in.  The system receives water at a fill rate that we will consider constant.  It drains at a rate determined by the size of the drain pipes.  The drain rate may, of course, be impacted by the size of the bell's intake holes, the size of the bell itself, the complexity of the drain plumbing, and other factors.  For sake of simplicity, we will assume the bell intakes are adequately sized to allow water through at a rate equal to or greater than the maximum flow rate of the drain pipes.

Let us consider system operation.  We'll consider four discrete phases:

1. Filling
2. Spillover
3. Drain
4. Siphon Break

Let's consider the four stages and discuss the potential problems in each.

Filling

The Filling stage is the simplest.  As long as there is no active siphon, this stage can proceed quite readily.  If the siphon-break from the previous iteration did not occur, then the filling stage generally cannot proceed.  More on that later.

It is recommended that the standpipe have an emergency drain hole, in case of pump failure.  The emergency drain hole is small, typically 1/8" in diameter.  This sizing is large enough to allow a reasonable rate of emergency drain, but small enough to not appreciably impact the fill rate.

This stage does not appear to have any other potential problems.

Spillover

The Spillover is the period between Filling and Drain.  Water has begun flooding into the standpipe, but the siphon has not yet started.

In some of my early testing, an insufficient fill rate would result in the siphon never starting.  I suspect a critical volume of water must accumulate in the standpipe or drain, in order to form the necessary suction.  Once suction is present, the air bubble at the top of the bell will be pulled down into the standpipe and proper siphoning action will commence.

Some outstanding questions:

• Does the fill rate alone determine whether or not the siphon will start, or does the geometry of the grow-bed factor in?  The reason this may be a question is that the length and width of the grow-bed translate into a rate of ascension, measured in height, for the water.
• What is the spillover rate, and can we calculate it based on the diameter of the standpipe?  If this rate can be calculated, it will quantify the spillover phase and we can ensure that the fill rate is sufficient.

Common solutions to the siphon-start problem:

• Introduce back-pressure via one or more of the following:
• Add 90-degree bends to the drain pipe - usually 2 are recommended.
• Restrict the diameter of the standpipe near its base.
• Flare the top of the standpipe, either by heating and shaping the PVC, or by adding a fitting such as a reducer coupling or union.

Those who introduce either a restriction of diameter into the standpipe, or flare the standpipe's top, both often hypothesize that this has the added benefit of introducing the Bernoulli Effect to the siphon.  Given the rates of flow and relative openness of the bell-and-standpipe assembly, this may or may not be true.  To be certain, using a sufficiently large reducer coupling seemed to benefit my test siphon, but more analysis is required before crediting the reducer exclusively.

I also started experimenting with a trap-style bend in the drain pipe.  I was not able to complete the trap, however, so water was left in the drain pipe after the siphon completed and air remained in the bell, causing the bell to float during refill.  It is likely that air was also being sucked in between the bell pipe and the cap, as I did not glue the pipe and cap together.  During one refill I was able to see air bubbles escaping from the bell.

Drain

Once draining has started, the two key factors are the fill rate and drain rate.  These two rates can be used calculate the time required to drain the system.  Effectively, it is the system working volume divided by the drain rate less the fill rate, or V / (D - F).  Mind your units.

Basic siphon physics (Bernoulli's equations) appear to adequately describe the bell siphon.  It is suggested that the length of the drain defines the siphon rate - the longer the drain, the faster the siphon.  I have yet to determine if this is actually the case for the bell siphon.  My calculations, nonetheless, appeared to predict the operation of the siphon.  I was able to determine the drain time to within 3 seconds of actual operation, and that error could be due to the fact that my timing and fill methods are very crude...as is my actual test apparatus.

More important than drain length appears to be pipe diameter.  For example, a 10 cm increase in pipe length adds - in my specific scenario - 126.7 cm3/sec of flow rate, whereas a 10cm increase in pipe diameter adds over 21,000 cm3/sec.  Of course, one would ideally not use a 10cm diameter pipe except in the most unique circumstances.

Siphon Break

At this final stage, the water level in the system has reached the top of the bell intakes, what we shall refer to as the low-mark, and air can now intrude into the bell.  Ideally, the air intake and fill rates are sufficiently balanced so as to break the siphon.  The remaining water in the bell and in the siphon discharge to the grow-bed and to the sump or fish tank, respectively, and the cycle continues.

This unfortunately appears to be fraught with problems, and there are a wide variety of solutions available.  Let us consider the problem in terms of stages:

1. The water reaches the low-mark, and air begins entering the bell and siphon.
2. The siphon action is perturbed, reducing the flow rate.
3. The fill rate remains unchanged, thus with the reduced drain flow rate the water rises above the low-mark.
4. The siphon recovers, and the level begins to drop.
5. Repeat.

As you can see, we have the makings for a never-ending siphon.  Some solutions:

• Reduce the fill rate - filling too fast means slow drains, and potentially unending siphoning.
• Enlarge the drain pipe - basically the other component of the equation for the previous solution.
• Add a "snorkel" - this is a vent pipe that defines the low-mark and injects air directly into the top of the bell.
• Add a snorkel with an inverted bell - this appears to be an improvement or a fix to the snorkel method.

The first two solutions are obvious and can be observed in calculation, so we will omit discussion.  The third and fourth solutions deserve comment.

The basic snorkel potentially encounters the same problem as the unchanged bell: the water reaches the bottom of the snorkel, air invades the bell, the siphon is perturbed, the rate diminishes, the fill recovers the lost water, and the siphon recovers.  Some have tried to prevent this by cutting the snorkel end at an angle.

The fourth solution basically appears to provide some buffering between the time the water reaches the bottom of the snorkel, and the time at which the snorkel begins taking in air.  It also ensures that water from the grow-bed cannot enter the snorkel once the snorkel begins taking air.

This video demonstrates the action of the cup+snorkel bell siphon.  The author of the video also utilizes a trap in the drain plumbing.  The theory behind the trap is that it forces the water level inside the bell to remain lower than the water level outside the bell.  Once the water starts to flood the standpipe, the bell is quickly flooded thanks to the additional pressure.  For this to function, one expects that the bell does not float much.

Further Discussion and Future Testing

The snorkel+cup bell appears to be a very promising siphon-break mechanism.  The necessity of the trap is undetermined.  One criticism I can see for the trap is that it would make predicting the high-mark water level of the grow-bed difficult.

For siphon start, the trap may help.  Of course, so might the reducer fitting.  Using both is probably excessive.  I would be curious to examine the function of a loop in the drain plumbing.  In theory, the loop would capture and cause a stable column of water to exit the siphon drain, power-starting the siphon effectively.  The most ideal operation would have the loop completely empty once the siphon in the bell is broken - in effect, a double siphon.  This would perhaps eliminate the standing water in the trap, and the reducer on the standpipe.  Eliminating the reducer would equate to reducing the diameter of the bell, which becomes important when one considers that in addition to the bell you must add a media-guard to the whole setup, thus taking up more grow-bed space.

One alternative to the bell siphon is the "U" siphon, which is constructed as a single pipe with two 45-degree bends, and two 90-degree bends, for the standpipe.  The low-mark is determined by a piece of downward-facing pipe.  This pipe enters a 45, which then proceeds into the two 90s, which exits into the second 45 and into the drain pipe.  The drain pipe and entry pipe are therefore parallel, and the combination of the 45s and 90s make for a skewed upside-down U shape.  The function is the same as for the bell: water spills over the top of the U, and a siphon is formed.  I could foresee this alternative having similar problems with siphon start and stop, but fewer remedies as the space available for adding hoses and such is very limited.