12 M A R C H 2 0 0 4Water Conditioning & Purification

Summary: Water softener design and setupcan be a confusing thing to learn regardlessof the application. Here are basic guidelinesfor engineers and technicians to study to gaina fundamental understanding of this process.

The following is an introductoryarticle offering practical adviceon sizing and setting up water

softeners for residential, commercial andnon-critical industrial applications. Thereare significant variations and details notwithin the scope of this presentation butthat deserve further investigation suchas alternate methods of brining and re-generation cycle programming. Feelfree to surf the WC&P “archives” atwww.wcponline.com for more on thesesubjects. Meanwhile, read on.

STEP 1: Select the control valve youwant to use based on the service flowrequirements of the application—Thefirst objective is to select a valve thatoffers acceptable pressure drop from theinlet to the outlet of the softener at theservice flow required for the application.Use a maximum service flow pressuredrop guideline of 15 pounds per squareinch (psi)—1 bar—for the valve alone.

STEP 2: Determine volume of cat-ion resin required—The volume of cat-ion resin is generally determined byfour things:

a. The service flow rate,b. Hardness of the water,c. The volume of water to be

treated, andd. The number of times a day the

softener can be regenerated (i.e., valveswith flow meters allow multiple regen-eration per day). Usually three times isthe practical maximum number of re-generations per day because:

• It typically takes up to two hours

for regeneration, and• It takes up to three hours (depend-

ing on temperature) to make a saturatedsalt solution in the brine tank.

Learn this–Resin volume vs.service flow rate

The volume of cation resin requiredfor a given flow rate can be determinedtwo ways: 1) By the volume of resinwhich concerns contact time, and 2) bythe resin bed (the tank) cross section areathat concerns pressure drop in the resinbed. See Table 1 to determine the crosssection area of the most popular sizetanks.

By contact time—Use 5 gallons perminute per cubic foot (gpm/ft3), or 42cubic meters per hour per cubic meter(m3/h/m3), for continuous flow and 7.5gpm/ft3 (60 m3/h/m3) for peak flow. Toachieve low hardness leakage as re-quired in applications like boiler watertreatment, the flow rate should be lim-ited to 3 gpm/ft3.

By resin bed area—Use 10 gpm persquare foot (ft2), or 25 m3/h per squaremeter (m2), for continuous flow and 15gpm/ft2 (37 m3/hr/m 2) for peak flow.To assure low hardness leakage, limitthe area flow to 8 gpm/ft2.

For example, assume the serviceflow is 15 gpm—56.8 liters per minute(L/min). Based on the guideline above,the recommended resin volume is 15gpm/5 gpm/ft 3 = 3 ft 3. The cross sec-tion area guideline recommends a resinbed area of 15 gpm/10 gpm/ft2 = 1.5 ft2,which corresponds to a 16-inch ( ") tank(1.4 ft2) and a resin volume of 4.6 ft3.You have the choice of using a 16" tankwith 3 ft3 (84.9 L) or use a normal resinvolume of 4.6 ft 3 (130.2 L) for added ca-pacity to meet future expansion or in-crease in hardness.

Bear in mind, the 16" tank with 3 ft3

of resin produce a bed depth of just un-der 26 inches (65 cm), which creates the24" minimum resin bed depth require-ment of the resin manufacturer. Also beaware that if the service flow is too low,the water can “channel” through theresin bed only using a fraction of theresin’s capacity. Use a lower flow limit ofno less than 1 gpm/ft3 (8 m3/h/m3).

Learn this–Resin capacity vs.salt dose

The resin capacity for hardness, oramount of hardness that can be re-moved by cation resin, depends on howmuch salt is used (dosage) to regenerateeach liter or cubic foot of cation resin.The normal range is 6 pounds of salt percubic foot (lbs salt/ft3 or lbs/ft3) ofresin—about 100 grams salt per liter(g salt/L or g/L) of resin—to 15 lbs salt/ft3 (about 240 g salt/L) for standard meshresin. The capacity for each salt dosagecan be approximated in the following:

Low limit salt dosage—6 lbs salt/ft3

(about 100 g salt/L) resin gives hard-ness removal capacity of approximately21,000 grains/ft3—48 grams of hardnessas calcium carbonate (CaCO3) per liter[g(CaCO 3)/L] or 0.96 equivalents per li-ter (eq/L).

Medium salt dosage—9 lbs salt/ft3

(about 150 g salt/L) resin gives hard-ness removal capacity of approximately26,000 grains/ft 3—60 g(CaCO3)/L or1.19 eq/L.

High salt dosage—12 lbs/ft3 (about 200g salt/L) resin gives hardness removalcapacity of approximately 29,000 grains/ft3—66 g(CaCO3)/L or 1.33 eq/L).

Maximum limit salt dosage—15 lbssalt/ft3 (about 250 g salt/L) resin giveshardness removal capacity of approxi-mately 31,000 grains/ft3—71 g(CaCO 3)/L or 1.42 eq/L.

Notice that even though the salt

Learn to Do It Right the First Time:

By Robert Slovak

Guidelinesfor Designing

Water Softeners

13Water Conditioning & PurificationM A R C H 2 0 0 4

dosage more than doubles, the hardnessremoval capacity doesn’t. The higherthe salt dosage, the lower the salt effi-ciency.

Bear in mind, the above capacity val-ues are only approximate. Consult thespecifications from your resin supplierfor more accurate capacity specifications.

Learn this–Resin volume vs.volume of treated water

To determine the volume of resinby volume of water treated and hard-ness, start by estimating the maximumtotal grains (or kilograms) of hardnessthat must be removed each day.

For example, assume the water hasa hardness—as (CaCO3)—of 13 grains/gallon [222 milligrams per liter (mg/L),calculated by multiplying 13 grains by17.1]—and that 20,000 gallons (75.8 m3)of water must be treated. The total hard-ness is 13 grains/gallon × 20,000 gallons= 260,000 grains—16.9 kilograms (kg)—of hardness per day.

At the medium salt dose, each cubic

foot of resin has a capacity of 26,000grains (each liter of resin can remove0.060 kg hardness). If you are limited toonly one regeneration per day (as witha timer-controlled valve), then a volumeof 260,000/26,000 = 10 ft3 (about 283 L)of resin is required. This typically re-quires a 24" × 72" tank.

Consider multiple regenerations—It’simportant to understand that multipledaily softener regenerations can reducesystem size and cost. If the application issuited for multiple regenerations per day[you need an alternating multiple-tanksystem or a cistern (reservoir) that cansupply treated water for the customerduring regeneration], then a more costeffective softener system can be de-signed. This is an especially good strat-egy where cisterns are commonly usedto store treated water before the pointsof use (as in Latin America). Considerusing three regenerations per day(roughly every eight hours for continu-ous flow) for the example above. Then,to meet the capacity requirement, only

one-third the amount of resin is re-quired or 3 ft 3 (85 L) for this example.

But let’s check if this reduced vol-ume of resin can meet the service flowrate requirement of 15 gpm. Referringto “Resin volume vs. service flow rate”section, 15 gpm (57 L/min or 1 pm) ser-vice flow requires at least 3 ft3 of resin.The flow rate is compatible with theresin volume required for the hardnesscapacity. Now, instead of a 24" tank with9 ft3 of resin, the system can be reducedto:

1. A twin-metered alternating soft-ener using 16" tanks with 3 ft3 of resin(each regenerates three times every twodays—about every 16 hours) where con-tinuous direct flow of softened water isrequired

2. A single-metered softener usinga 16” tank with 3 ft3 of resin (which re-generates three times per day) where acistern of treated water is available tosupply the customer with a continuoussupply of softened water.

Table 1. Water softener sizing and performance chart

Backwash 2

Resin @60F & 77F Service Peak Regen. System Regen. System Regen. System Regen. SystemTank Tank vol.1 @15C &25C flow flow salt capacity3 salt capacity3 salt capacity3 salt capacity3

size area ft3 gpm60

/ gpm77

gpm gpm lbs grains lbs grains lbs grains lbs grainsD” x H” ft2 (m 2) (liters) (lpm

15 / lpm

25) (lpm) (lpm) (kg) (kg) (kg) (kg) (kg) (kg) (kg) (kg)

6 x 35 0.19 0.4 1.0/1.3 2.0 3.0 2.4 8,400 3.6 10,400 4.8 11,600 6.0 12,400(0.018) (10) (3.6/4.9) (7.6) (11.4) (1.0) (0.48) (1.5) (0.60) (2.0) (0.66) (2.5) (0.71)

7 x 44 0.27 0.6 1.3/1.8 3.0 4.5 3.6 12,600 5.4 15,600 7.2 17,400 9.0 18,600(0.025) (15) (5.1/6.8) (11) (16.5) (1.5) (0.72) (2.3) (0.90) (3.0) (0.99) (3.8) (1.07)

8 x 44 0.34 0.8 1.7/2.3 4.0 6.0 4.8 16,800 7.3 20.800 9.6 23,200 12.0 24,800(0.032) (20) (6.5/8.7) (15) (22.5) (2.0) (0.96) (3.0) (1.20) (4.0) (1.32) (5.0) (1.42)

9 x 48 0.44 1 2.2/3.0 5.0 7.5 6.0 21,000 9.0 26,000 1 1 29,000 15.0 31,000(0.041) (25) (8.3/11.2) (19) (28.5) (2.5) (1.20) (3.8) (1.50) (5.0) (1.65) (6.3) (1.78)

10 x 54 0.54 1.3 2.7/3.7 6.5 9.8 7.8 27,300 11.7 33,800 15.6 37,700 19.5 40,300(0.051) (35) (10.4/13.9) (30) (45.0) (3.5) (1.68) (5.3) (2.10) (7.0) (2.31) (8.8) (2.49)

12 x 52 0.78 2.0 3.9/5.3 10.0 (15) 12.0 42,000 18.0 52,000 24.0 58,000 30.0 62,000(0.073) (55) (14.8/19.9) (38) (57) (5.5) (2.64) (8.3) (3.30) (11.0) (3.63) (13.8) (3.91)

13 x 54 0.91 2.2 4.6/6.1 11.0 16.5 13.2 46,200 19.8 57,000 26.4 63,800 33.0 68,200(0.085) (60) (17.3/23.2) (44) (66) (6.0) (2.88) (9.8) (3.60) (13.0) (3.96) (16.3) (4.26)

14 x 65 1.07 3.0 5.4/7.2 15.0 22.5 18.0 63,000 27.0 78,000 36.0 87,000 45.0 93,000(0.100) (85) (20.3/27.3) (57) (85.5) (8.5) (4.08) (15.0) (5.10) (20.0) (5.61) (25.0) (6.04)

16 x 65 1.41 4.0 7.0/9.5 20.0 3 0 2 4 84,000 36.0 104.000 48.0 116,000 60.0 124,000(0.131) (115) (26.6/35.8) (76) (114) (11.5) (5.52) (19.5) (6.90) (26.0) (7.59) (32.5) (8.17)

18 x 65 1.76 5.0 8.8/11.8 25.0 37.5 3 0 105,000 45.0 130,000 60.0 145,000 75.0 155,000(0.164) (140) (33.3/44.8) (95) (143) (14.0) (6.72) (24.8) (8.40) (33.0) (9.24) (41.3) (9.94)

21 x 62 2.40 6.5 10.8/14.6 32.5 48.8 3 9 136,500 58.5 169,000 78.0 188,500 97.5 201,500(0.223) (185) (41.0/55.1) (123) (185) (18.5) (8.88) (30.0) (11.10) (40.0) (12.21) (50.0) (13.14)

1. The listed resin volumes are nominal values and are subject to change based on user preference and experience.2. The water temperature determines the backwash flow rate. Colder water temperature requires less backwash flow. Refer to water viscosity charts for other temperatures.3. These are approximate capacities based on the resin volume. Small inconsistencies may result from rounding off and unit conversions.SOURCE: AquaCorp. Ltda., Rio de Janeiro, Brazil (aquacorp@aquacorp.com.br)

Minimum salt Medium salt High salt Maximum saltdosage dosage dosage dosage6 lbs/ft3 9 lbs/ft3 12 lbs/ft 3 approx. 15 lbs/ft3

(100 g/L) (150 g/L) (200 g/L) (250 g/L)

14 M A R C H 2 0 0 4Water Conditioning & Purification

STEP 3: Determine the tank size—Generally, the tank is filled to ½ to 2/3of the total tank volume. The depth ofcation resin should be at least 24” (0.6m) in smaller tanks and 30” (0.75 m) inlarger tanks. The tank should be tallenough so the resin can expand to atleast 50 percent of its normal depth intothe void space. Refer to the accompany-ing chart for standard tank sizes.

STEP 4: Determine the backwashflow rate—The backwash segment ofthe regeneration removes dirt and de-bris from the resin and is determined bytwo things:

a. Diameter of the tank—Refer to theaccompanying design chart (see Table 1 )for cross section area.

b. Temperature of the water—This de-termines the water viscosity as an in-verse relationship. As water tempera-ture decreases, it becomes more viscous(thicker) and will cause greater expan-sion of the resin bed during backwash.

Learn this–Backwash flowrate vs. water temperature

This requirement of correct softenerdesign may be the most overlooked fac-tor, especially in warm water regionssuch as Latin America. The flow must besufficient to expand the bed at least 50percent. Use the following guidelinesbut, for exact backwash flow require-ments, refer to the backwash require-ments vs. water viscosity chart fromyour resin supplier.

• 4.2 gpm/ft2 (10.3 m3/h/m2) back-wash flow at 10°C (50°F)

• 5.0 gpm/ft2 (12.3 m3/h/m2) back-wash flow at 15°C (59°F)

• 5.5 gpm/ft2 (13.5 m 3/m2) back-wash flow at 20°C (68°F)

• 6.7 gpm/ft2 (16.4 m3/h/m2) back-wash flow at 25°C (77°F)

STEP 5: Determine the brine injec-tor size—The injector creates suction forthe salt solution. During regeneration,water flows through the injector ven-turi and creates a vacuum to pull saltbrine from the salt tank. The injector isprimarily determined by the diameterof the tank. The proper injector selec-tion is found in the manual of the con-trol valve you’re using.

STEP 6: Determine the regenerationfunction times—The regeneration cycleconsists of a sequence of functions thatcorrespond to control valve “positions.”For most customers who only have toremove hardness, the following se-quence of regeneration functions is rec-ommended:

Backwash–Downflow Brine/Slow Rinse–Fast Rinse–Salt Tank Refill–Return to Ser-vice

Learn this–Softenerregeneration function times

Backwash function—Eight to 15 min-utes is recommended depending on theamount of particulate matter (sediment)in the water supply and water scarcity.If iron is present, use the upper range.The backwash flow is regulated by acontrol orifice that restricts the drainflow to the specified flow rate.

Downflow brine function—It’s impor-tant to understand that the downflowbrine function is only the first part ofthe complete “downflow brine/slowrinse” task. The control valve remainsin the same position for the entire func-tion. During this first part, saturated saltbrine solution is sucked (“educted” or“drawn”) from the brine tank, mixeswith the venturi injector water stream(to a concentration of about 10 to 13 per-cent), and flows through the resin bedto regenerate it (or chemically displacethe hardness removed). The NaCl con-tent of saturated brine is 2.7 lbs/gallon(300 g/L). The brine function time is de-termined by:

1. The amount of salt required toregenerate the resin bed (refer to accom-panying design chart)

2. The flow rate that the injectoreducts brine into the resin bed (consultthe control valve manual for the thisspecification)

For example, assume you havespecified the size of the softener to be a16” × 65” tank with 4.6 ft3 (130 L) ofresin. You decide to choose the mediumsalt setting of 9 lbs/ft3 (150 g/L), refer-ring to the accompanying design chart.The amount of salt required is 42.9 lbs(19.5 kg) and the amount of salt brinecontaining this is 15.9 gallons (60 L) satu-rated brine. The control valve manualsays that the injector for a 16” diametertank educts the saturated salt brine at arate of 0.7 gpm (2.6 L/min). Therefore,the downflow brine time required is 22.7minutes.

Slow rinse function—As statedabove, the slow rinse cycle is actually anextension of the “downflow brine/slowrinse” function”—but with no salt brinebeing educted. The continued water flowthrough the injector provides this slowrinse. The recommended guideline is tohave the entire downflow brine and slowrinse cycle be three times as long as thebrine cycle.

For example, consider the instanceabove. The brine time is approximately

23 minutes. The total brine and slowrinse cycle should be 3 × 23 minutes = 69minutes.

Fast rinse function—The fast rinseflows through the backwash flow con-trol so its flow rate is the same. For mostapplications, the fast rinse time shouldbe 6 to 12 minutes depending on the vol-ume of resin and the application. Forinstance, most domestic and commer-cial applications can use the lower rangewhile reverse osmosis (RO) and deion-ization (DI) pretreatment and boilerfeed and industrial process applicationsshould have longer rinse times.

Salt tank refill function—During thiscycle, a controlled stream of water flowsinto the salt tank to make up a saturatedbrine solution for the next regeneration.The salt tank refill time should be basedon the fact that 1 gallon of fresh waterdissolves approximately 3 lbs (1 L dis-solves 360 g) of salt.

For example, consider the situationabove. The amount of salt required forthe medium salt setting is 42.9 lbs. (19.5kg). The manual says the salt tank refillflow rate is 0.5 gpm (2 L/min). The vol-ume of water required to dissolve 42.9lbs of salt is therefore 14.3 gallons (54.1L) and the time is 28.6 minutes.

ConclusionThe softener design guidelines out-

lined in this article and accompanyingchart are intended to provide a base ofinformation for general purpose soft-ening applications such as domestic use,commercial and institutional washingand laundry, non-critical industrial pro-cesses, low pressure boilers, coolingtowers, and RO and DI pretreatment.More demanding applications like high-pressure boilers, medical autoclaves andwater sources containing problem con-taminants like iron and manganese re-quire additional special considerations.Thus, you should consult your otherprofessional sources for specific appli-cation guidelines.

About the author Robert Slovak is president of Aroman Inc.,

a water treatment consulting company inIncline Village, Nev., and currently servesas a water consultant and manufacturer’srepresentative in Brazil for products im-ported by AquaCorp LTDA in Rio de Janeiro.He is also a member of the technical advisorycommittee of Agua Latinoamérica, a sisterpublication to WC&P, and a former memberof a similar WC&P committee. He and hisbrother, Jack, were founders of Water Fac-tory Systems, now a division of CUNO Inc.Slovak can be reached by email:robtslovak@aol.com