Cation Exchange, Part 3: Back to the Basics
By Gary Battenberg
In Part 2, we looked at the basic components of a water softener, which included those components that comprise a typical, standard water softener. This was followed by a description of the backwash cycle, which is the first cycle of the regeneration sequence for a down-flow type of softener. This description clarified the importance of sufficient flow and pressure required for thorough expansion and fluidizing of the resin bed in preparation of the brine cycle, the second step in the sequence. In addition to expanding the resin bed, the backwash flushes away accumulated sediment (including any fractured resin beads or fines) from the top of the bed, as well as smaller particulates trapped within the resin bed. We stressed the importance of proper handling and transport of the softener to prevent a Transverse Distributor Effect, as well as consideration for the various types of sediment filtration and testing to determine what type of sediment filtration may be required to provide suitable feed stock to the softener in order to maintain a good baseline softener health index. In this installment, we will look at the next step in the regeneration sequence, the brine and rinse cycles.
Brine and rinse, down flow
After the backwash cycle completes, the resin bed is ready for brine injection. It is important to note here that a typical backwash cycle is 10 minutes but may be as much as 15 to 20 minutes, relative to the amount of sediment in the feedwater to the softener. It is easy to determine the required length of a backwash cycle by observing the clarity of the water at the termination of the drain line. Where sediment is present in the feedwater, the first few minutes of the backwash cycle will generally present dirty (turbid) water that will gradually clear up by the end of the backwash cycle. If the water is not clear at the end of 10 minutes, let the backwash cycle continue until the water to drain is clear and increase the backwash time by the number of additional minutes required to fully clean the resin bed. Then adjust the time on the control valve timer or program settings.
The brine is drawn from the brine tank into the softener via an injector assembly that is properly sized for the square-foot area of media tank, also referred to as the diameter of the resin tank. In order to inject the brine at the proper flowrate the injector must be sized to permit a specific brine draw rate of between 0.20 and 0.40 gpm (0.9 and 1.5 L/min) per cubic foot. The injector is comprised of two parts that include a throat and nozzle. The throat is typically a conical-shaped tube that is open on the inlet end and is restricted on the outlet end. The water pressure is reduced at the outlet but the velocity is very high. This high velocity creates a partial vacuum (to atmosphere) at the outlet of the throat, drawing the brine from the brine tank, which is then mixed with the high-velocity stream within the injector housing, continuing onward to the resin bed where the exchange of calcium and magnesium ions for sodium ions begins.
If you remember elementary science class, you may remember the Venturi Effect experiment where a drinking water straw was inserted into a glass of water. A second drinking water straw was used to blow air across the top of the straw in the water glass, which in turn, drew the water up the straw and created a water spray. Taking a deep breath and expelling through the straw, the student could empty the water glass after several repetitions. By slowing down and speeding up the air velocity, one could control the draw rate of water from the glass. A very elementary explanation, but I believe you get the picture and it quite possibly brings back memories of dousing your classmates during the experiment. For a proper brine and rinse cycle, this fundamental concept is more precisely controlled with a balanced throat and nozzle assembly specific to the media tank diameter.
The best hardness recovery results are obtained where brine-draw flows are maintained between 0.20 and 0.40 gpm per cubic foot. The typical residential softener will draw brine at 0.25 gpm (0.95 L/min) per cubic foot. Therefore, a specific volume of water is required at the throat to draw 0.25 gpm from the brine tank. The typical motive flowrate required to create sufficient vacuum for a 0.25-gpm brine draw is 0.40-0.60 gpm (1.5-2.3 L/min) per cubic foot. To understand this, it is important to know the weight (specific gravity) difference between water and brine. Typical saturated brine is achieved when salt is dissolved in water to a concentration of ~26 percent; one gallon of water will dissolve 2.6 pounds (1.2 kg) of salt. One gallon of water weighs 8.4 pounds (3.9 kg) and has a specific gravity of 1.0. One gallon of saturated brine weighs 10.0 pounds (4.6 kg) and has a specific gravity of 1.2. Therefore, 0.25 gpm of 26-percent concentrated brine requires 0.40 gpm of motive water flow to draw the heavier brine from the brine tank, for a total brine/rinse flow of 0.65 gpm (2.4 L/min).
2.6 pounds (1.2kg) of salt x 0.25 gallon per minute (0.95 L/min) = 0.65 gallon per minute (2.4 L/min) combined flow
At 0.25 gpm, it will take four minutes to draw one gallon (3.8 L) of brine from the brine tank. Therefore, it will be easy to determine that portion of the brine/rinse cycle. The brine and rinse cycle is sequential until the available brine is drawn from the brine tank, at which point the air check at the bottom of the draw tube actuates to prevent the entrance of air into the softener and ending the brine draw. At this point, the motive water flow continues at 0.40 gpm, starting the (displacement) rinse portion of this regeneration sequence. Remember, the injector creates vacuum and since the brine tank is exposed to atmosphere, it is very important to have an air-check assembly either in the brine tank or, as some control valves are equipped, with a sight-glass air-check assembly. Both air-check types feature a float ball that remains buoyant during the brine draw. When all of the available brine is drawn from the tank, the float ball descends and when it reaches the bottom of the submerged air check in a brine tank or a sight glass, the ball seats on the meniscus. There it is held tightly by the vacuum created by the injector assembly to maintain a bubble-tight seal, thereby preventing the entrance of air into the softener.
This portion of the brine/rinse cycle is equally important in order to achieve a highly efficient hardness recovery during regeneration. The typical ratio of brine draw to rinse time is 1/3 to 2/3 where the final length of time is selected to provide a sufficient volume of displacement rinse water to carry out the exchanged hardness minerals and the chloride portion of the salt (sodium chloride). Assuming a six-pound (2.8-kg) salt setting for regeneration of a one-cubic-foot (28.3-L) softener, one gallon of brine will contain 2.6 pounds (1.2 kg) of salt (sodium chloride); the amount of brine solution available for regeneration is 2.3 gallons (8.7 L). At 0.25 gpm, it will take nine minutes to draw the brine volume from the brine tank. Therefore, the displacement rinse portion of the cycle is 18 minutes, for a total brine/rinse cycle of 27 minutes.
It is very important at this point to clarify how critical it is that the brine draw time must be maintained within the 1/3 of the total brine/rinse cycle. If the brine draws too slowly, the brine solution will be diluted and a partial regeneration results. In most cases, there will be a salty water event at the kitchen or bathroom tap that will most assuredly generate a call from a frustrated customer recounting how a salt water rinse after tooth brushing or salty coffee is not how they prefer to start their day. Simply put, if the 1/3 and 2/3 cycles are reversed, there is a serious hydraulic problem with the softener and the water quality will be very poor. Hint: A good installer will identify this hydraulic problem upon startup and commissioning of the softener, taking the necessary steps to correct a problem before leaving the job site. Drawing the brine at the right rate and within the proper time frame will ensure a concentrated brine strength evenly distributed through the resin bed, resulting in an even regeneration of the entire resin bed and yielding a consistent soft-water service run between regenerations.
When the brine injection has ended, the resin bed is bathed in a concentrated brine solution and, in order to lengthen the contact time of the salt with the resin, a slow rinse will continue to displace the brine slowly, hence the term displacement rinse. The displacement rinse volume is the same 0.40 gpm as the motive flow used to draw the 0.25-gpm brine solution. The 2/3 time (18-minute) displacement rinse is generally sufficient to displace most of the brine to approximately five-percent saturated brine (approximately one-percent salt). Once the displacement rinse is completed, the fast-rinse cycle can be initiated.
About the author
Gary Battenberg is a Technical Support and Systems Design Specialist with the Fluid System Connectors Division of Parker Hannifin Corporation in Otsego, MI. He has 34 years of experience in the fields of domestic, commercial, industrial, high-purity and sterile water treatment processes. Battenberg has worked in the areas of sales, service, design and manufacturing of water treatment systems and processes utilizing filtration, ion exchange, UV sterilization, reverse osmosis and ozone technologies. He may be reached by phone at (269) 692-6632 or by email, gary,email@example.com