The Analysis and Design of Rinsing Installations

Article From: Products Finishing,

Posted on: 7/1/2000

The technology and practice of rinsing seldom get the recognition they deserve, and yet rinsing is an indispensable operation in virtually every wet process.

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The technology and practice of rinsing seldom get the recognition they deserve, and yet rinsing is an indispensable operation in virtually every wet process. This stepchild in the process family gets little attention until a production crisis surfaces, or the cost of waste treatment is too high.

Rinsing has a sound technical base with a vital role in every successful process line. This presentation is primarily aimed at the process engineer, but there is also much here for the operator and supervisor who may want to gloss over the theory and focus on the applications. Although industrial wet processes are diverse in both chemistry and procedure, the principles and descriptions presented here apply universally to rinsing operations wherever they are used.

Typical Processes

Rinsing invariably follows such chemical processes as cleaning, plating, acid dipping, etching, etc. It consists of flushing the work with water by spray or immersion. Although water is the most common medium, there are others, such as fluoboric acid rinse prior to solder plating and sulfuric acid rinse prior to acid copper plating. Regardless of the medium, the same principles apply.

Rinsing processes fall into three categories: immersion, spray and a combination of the two. Immersion rinses can be classified as nonflowing and flowing, and these break down further into single or multi-tank installations. Selecting the right category will depend on a number of local factors: process solution to be removed, degree of rinsing desired, cost of water, waste treatment considerations, budget, etc.

Although there are rules of thumb by which to choose rinsing processes, it is better to use a more scientific method to validate the choice. In that way one can predict water consumption and waste treatment load with some accuracy, have a basis on which to select equipment and have a place to start when troubleshooting problems.

Since rinsing is necessary to remove the carryover of process solution from one tank to the next, let's first examine the concept of "dragin" and how it affects the selection of rinsing parameters.

Dragin can be thought of as that portion of a process solution that remains on the work and is transferred to the rinse tank from the process tank. Although it would be desirable to remove all of the process solution, in practice this is neither economical nor necessary. The goal is to remove enough contaminant so that the residual will not affect any of the following operations. This, then, allows us to define rinsing more specifically as the process of diluting dissolved chemicals on the part to a point where they do not affect product quality or contaminate subsequent process solutions.

To optimize rinsing and make it cost effective, first dragin volume must be reduced as much as possible. This is done by manipulating the factors that control it. An examination of the following equation for dragin will reveal those factors.

D = KA(lv/td)1/2 (equation 1)

Where D = dragin volume
K = a constant for the system
A = area of work
l = vertical length of work v = viscosity of process solution t = time of withdraw from tank
d = density of process solution

Work area

The factor most influencing dragin volume is the combined area of the work piece and rack. Double the area and you double the dragin volume.

Vertical length of work

Dragin will be reduced as the vertical length of the work is reduced. This suggests that it is beneficial to rack with the long dimension horizontal. Viscosity of process solution. Dragin will be reduced as the process solution viscosity is lowered, usually achieved by heating.

Time of withdraw

An increase in the time of withdraw from the process solution will reduce dragin by promoting draining.

Density of process solution

The higher the solution density, the more completely it will drain from the work. Conversely, raising the solution temperature will lower the density and promote dragin. As has already been seen, increasing temperature reduces dragin by lowering viscosity, and fortunately, this effect is substantially greater than the effect of lowered density.

Other aids

Several other things can be done to reduce dragin:

  • Hang the work as nearly vertical as possible and tilt the lower edge of panels and flat work slightly to promote draining;
  • Use wetting agents to lower the process solution surface tension;
  • Withdraw work through an air blow-off system;
  • Maintain good rack insulation to avoid solution entrapment;
  • Strip uninsulated plating racks regularly to avoid solution entrapment in rough or treed deposits; and
  • Install a drain rod across the tank as a resting-place for the rack while it is draining.

Nonflowing immersion rinsing

this rinsing operation consists of one or more tanks filled with a stagnant volume of water. As the work passes through, the process solution is diluted, most of it is removed and gradually the contaminant, usually a salt, builds up in the rinse water. As succeeding loads are processed, the amount of residual contamination progressively builds up until a maximum tolerance level is reached and the stagnant rinse must be dumped and refilled. The interval between dumps can be lengthened by adding a second rinse in series with the first.

A nonflowing rinse is seldom used with noncritical chemical processes because of the expense of downtime needed to change the solution. However, it is used as a dragout rinse after precious metal baths where the purpose is to reclaim expensive salts that can be recovered and reused. Another use is as a dragin rinse before common plating baths. This is a solution of the principal acid or base component of the plating bath, its purpose being to remove minor contamination and to replenish some of the plating bath chemistry by deliberate carryover.

In a nonflowing rinse, the contaminant builds up as it is removed from work. Finally, it reaches a point where the amount dragged out of the rinse into the next process solution approaches the tolerance limit of that solution and the rinse must be dumped and replaced.

To describe what is happening, we must assume that mixing of the dragin with the rinse is not only instantaneous but also complete. This is not a bad assumption for an agitated rinse because of the time interval between workloads. Then, if the concentration of contamination allowed on the part coming out of the rinse is fixed, and if the volume of dragin is measured or estimated, Figure 1 can be used to determine how many rack loads can be processed before the rinse has to be changed.

Figure 1

In Figure 1, z = Cn/Co where Cn is the contaminant concentration after n rack loads (number of rinsing operations) and Co is the contaminant concentration in the solution being dragged in. The number of rack loads is determined from the equation n = FV/D, where F is read from Figure 1, V is the rinse tank volume, and D is the dragin volume.

Rinsing effectiveness is improved not only by using nonflowing rinses in series, but also by adding spray rinses. As the work is removed from the rinse tank, it is withdrawn through a spray of fresh water that leaves very little contaminant to be carried to the next operation. The spray runoff drains back into the rinse tank. Sprays can be used in place of additional rinse tanks. The operation is especially attractive where sprays are installed over hot process solutions, such as gold or rhodium plating, and the spray runoff is used to replenish evaporation losses.

Stagnant rinses have their unique applications, but are labor intensive if used in high-volume operations. For these, flowing rinses are beneficial and preferred.

Flowing Immersion Rinsing

If rinse water is flowing instead of stagnant, the process can be run continuously without having to shut down for dumping and refilling. The rinse tank is fitted with a water inlet and an overflow outlet. Where more than one in-line rinse is used, fresh feed water can be supplied to each tank separately or the overflow from one tank can become the feed for the next.

The objective in analyzing a rinse installation is to determine the water flow rate and tank size. Flow is independent of tank size and is related to contamination level. Tank size is determined primarily by the expected size of the workload.

Single tank flow rinse

To analyze flow, it's assumed that there is complete dispersion of the dragin in the rinse water and that this happens quickly. The correlation between experimental and actual conditions is close enough to make this assumption.

For the rinse to be effective, the amount of contaminant (e.g., a salt) dragged in must equal the amount dragged out to prevent a buildup in the rinse. This equilibrium is expressed as salt weight in = salt weight out or, written another way, D x Cd = F x Cr

The equation tells us that the volume D of dragin having a salt concentration of Cd will require a volume F of rinse water to dilute the salt to concentration Cr. Now, if the dragin is periodic as in a continuous process, F then represents the rinse water flow rate. Since the other terms will be known, the flow rate can be calculated from the following:

F=D(Cd / Cr ) (equation 2)

Significance of terms

Let's take a closer look at Equation 2 and the significance of each term. F represents the effluent from the rinse tank. It can be used as the feed to another rinse or it can be discharged to the sewer. If discharged to the sewer or to the waste treatment system, this volume of water becomes a goal for conservation.

D represents the dragin volume to the rinse tank. It depends on the condition and area of the rack, the number of parts on the rack, properties of the process solution, and solution retention characteristics of the part (smoothness, number of cavities, type of material, etc.). It is also affected by the dwell, or drip, time of the rack over the process tank.

Cd represents the concentration of salts in the process tank and is fixed by the process. Cr is the concentration of salts in the rinse tank. It is determined by the amount of salt carryover permitted on the part, the salt concentration of the rinse feed water and the allowable salt concentration that can be discharged to the sewer or waste treatment system.

Flow volume vs. concentration change

Consider a flowing rinse that contains a known concentration of salt. And suppose that no additional salt is introduced into the rinse as it is purged by the flow. If the total effluent volume and salt concentration in the tank are simultaneously and periodically measured, the curve in Figure 1 will result.

The ordinate can now be relabeled "Fraction contaminant removed" and the abscissa "Flow, in tank volumes." The curve shows that 90% of the contamination is removed with a flow of 2.3 tank volumes. Note that this depends only on the total volume of rinse water and not on any time period. A useful rule-of-thumb is that an average practical flow rate for a single tank will be 2.3 tank volumes per unit of time, say, per hour, and this will remove 90% of the contaminant from the rinse tank. For many operations, this is sufficient. Now let's apply the principles for single tank rinsing to the more economical and efficient multi-tank operation.

Multiple tank flow rinse

Multi-tank rinsing is characterized by successive rinses in two or three tanks. More than three tanks are usually not required, nor are they economical. The most effective arrangement is to introduce feed water to the last rinse tank and allow it to cascade from tank to tank in a direction opposite to the flow of work. This is called counterflow or countercurrent rinsing. The other arrangement is for each tank to have its own feed and discharge. This is called individual-feed rinsing and is not as efficient as counterflow.

Couterflow analysis.For counterflow rinsing, Equation 2 also applies. In this case, feed F and dragin D for each tank is constant; however, the impurity concentration differs. The feed to the last tank will be fresh water of acceptable, low-impurity content. The dragin will have the concentration of the next-to-the-last tank. The feed to the next-to-the-last tank will have the concentration of the last tank. Therefore, a similar description applies up to the first tank in the series. To arrive at a usable equation for calculating the flow rate, refer to these terms,

n = number of rinse tanks
Cp = impurity concentration for preceding tank
Cn = impurity concentration in last tank
Cn - 1 = impurity concentration in next-to-the last tank
D = Dp = Dn-1 = dragin volume
F = Fn = Fn-1 feed water volume

Then the flow through each tank is represented by the general form of Equation 2:

Fn = Dn-1 (Cn-1/Cn) and Fn / Dn-1 = Cn-1/Cn (equation 3)

Fn-1 = Dp (Cp/ Cn-1) and Fn-1 / Dp = Cp / Cn-1 (equation 4)

Taking the log of Equations 3 and 4, substituting Fn = Fn-1 and Dp = Dn-1, adding the equations, and taking the antilog, results in the following for a two-tank arrangement:

Fn = Dp (Cp/Cn)1/2

The general equation for n tanks becomes

Fn = Dp (Cp/Cn)1/n (equation 5)

Individual feed analysis

Applying the same definitions to the terms, it can be shown that the required rinse water volume (total for all tanks) is a multiple of the same number of tanks in counterflow. The general equation is

Fn = n[Dp(Cp/Cn)1/n] (equation 6)

For example, in a two-tank counterflow system, n = 2, and from Equation 6 the same degree of rinsing in a two-tank system with individual feeds will require twice as much water as in counterflow. A three-tank system will require three times as much water, etc.

Economy of multi-tank rinsing

Now let's summarize the benefits of multi-tank rinsing with an example borrowed from Durney. Assuming a dragin volume of 0.01 gpm (Dp equation 5) and a dilution ratio of the process solution of 1/1000 (Cp/Cn), the amount of rinse water (Fn ) required for a single tank will be 10 gallons. The following compares this to individual feed and counterflow setups using the same dragin and dilution ratio, clearly showing the economic advantage of multi-tank and/or counterflow installations.



No. of tanks
Fn gpm
Individual  
Single Feed Counterflow
1
10
2
0.62
3
0.30
2
0.31
3
0.10

The reductions in water consumption are indeed dramatic as long as one removes the work at the right time to avoid wasteful over rinsing.

Tank volume and flow rate

The amount of water required to provide a given degree of rinsing is independent of tank volume. Referring back to Equation 2, note that tank volume is not part of the equation, and the amount of rinse water, F, can be expressed as either total volume or flow rate. Tank size is determined by the minimum dimensions needed to accommodate the rack or basket.

Tank size influences flow rate to the extent that it determines how fast removed material is dispersed and how fast dragin and dragout reach equilibrium. The smaller the tank, the more rapid the dispersion, especially when facilitated by air agitation. Summary of rules governing flow. Several guidelines can be extracted from the above discussion.

  1. The removal of contamination from the rinse tank depends on the volume of water, not on the time of flow or the tank size.
  2. Multiple rinses use less water than single rinses to do the same job.
  3. Counterflow rinses use less water than the same number with individual feeds.
  4. Two rinses with individual feeds will use twice as much water as two countercurrent rinses. Likewise, three rinses with individual feeds will use three times as much water as three countercurrent rinses.

Although immersion rinses can be designed to be very efficient, they can be further improved by combining with integral or separate spray rinses.

Spray rinsing

Spray rinsing is very efficient for removing dragin materials and can be particularly effective on flat work. The continuous impingement of fresh water on the surface does not rely on dilution but forcefully blasts away the contaminants. To do the same job, spray rinsing will use less water than immersion rinsing. This fact leads to attractive possibilities for reducing the load on waste treatment systems.

Flow analysis

The water consumption of a spray rinse will be governed by the on-time of the sprays. The on-time can be determined by an analysis of the rinse runoff from the work at various intervals after the onset of spraying. For example, a conductivity cell can be placed in a trap below the rinse tank to monitor the change in conductivity during rinsing, or samples collected periodically can be checked for conductivity. The on-time will be that time needed to reduce the runoff conductivity to the desired level.

Mohler describes a mathematical treatment for determining the volume of spray rinse water required, and uses examples to show how this volume is between that required for a single tank flowing rinse and a two-tank countercurrent rinse. Some practical work done by Beyer and Siniscalachi shows that the already efficient spray rinsing operation can be made more efficient by some simple methods. Beyer found that a periodic spray stream saved about 70% in water consumption. During one rinse treatment, he used three spray periods of 1.5 seconds with an interval of two to five seconds between. Another test was to withdraw the work from the process tank through air knives. This lowered the dragout 30 to 40% of what it was before. When combined with periodic spray rinsing, an air blow-off saved another 7% of the amount needed for immersion rinsing. Additionally, there would be a reduction in effluent needing waste treatment and an elimination of the drip time over the process tank. The air knives can be mounted either on the process tank or on the hoist and be activated by a switch as the rack is withdrawn. Beyer also experimented with installing a drip pan on the hoist under the rack. As the rack finished lifting through the air knives, the pan would swing under it and then swing away as the hoist reached the next tank. The principal benefit of the pan was the improved cleanliness of the line and a reduction in maintenance by eliminating dripping. The pan was fitted with a valve and the collected drippings were occasionally drained and discarded.

Applications

Spray rinsing lends itself to a variety of applications and installations.

  1. As an individual operation, an open spray tank can be sequenced with an immersion rinse or as a substitute for it.
  2. Sprays can be mounted over an immersion rinse tank so that the work can be withdrawn through them. It has been estimated that they will be equivalent to an extra one-half counterflow rinse.
  3. Sprays over process tanks can be used for pre-rinsing, thus conserving process chemicals and providing makeup water. For a warm or hot process solution, this may be the only makeup needed.
  4. Fogging nozzles over precious metal tanks can be used as a dragout rinse. Fog condenses on the work, forming droplets that drip back into the tank.
  5. An inline spray rinse can be fed by water pumped from a reservoir tank. The reservoir is lower than the spray tank and is gravity fed by the spray runoff. Cleanliness of the water in the reservoir is controlled by a continuous or controlled flow as described for immersion rinsing. Although this system uses as much water, it is useful where a spray is preferred.
  6. By careful selection, a strategically located, single spray rinse (maybe two) can serve a number or process operations. This arrangement is especially adaptable to an automatic in-line operation. For a manually operated line, a centrally located operation amidst a circle or semicircle of process tanks provides the same effectiveness. Either arrangement will shorten the line and save space by eliminating rinse tanks, reducing capital outlay, conserving water and reducing the load on the waste treatment system.

Whether one uses spray or immersion rinsing, or a combination of the two, it is wise to consider suitable process controls to avoid excessive water consumption.

Process Controls

The regulation of rinse water flow is generally accomplished by two means, solution conductivity or mechanical devices. Conductivity meters. Conductivity devices control flow automatically and usually pay for themselves through savings in water and waste treatment costs. The conductivity cell senses the ionic level in the tank. The controller reacts if the ionic concentration is above a preset value, sending and electrical signal to open a solenoid valve, allowing rinse water to flow. When flow has lowered the ionic level to the preset value, the solenoid closes. In this mode, sensing delays often result in no flow when there is work in the tank or flow when there is no work in the tank.

In a counterflow system, the sensor should be installed in the tank in which rinse water is introduced. This insures that the work gets its final rinse in a controlled tank. The table can be used to anticipate the conductivity of solutions for which controls are being considered. It lists the conductivity of common process solutions at several concentrations. A rule-of-thumb is that the final rinse should contain no more than one gram per liter of process solution. This corresponds in the table to approximately 1,000 ppm.

Table 1. Conductivity of Various Process Solutions, Micromhos Concentration ppm
Material
Nickel plating
Tin-lead fluoborate plating
Light-duty cleaner
Copper sulfate plating
Copper cyanide strike
Heavy-duty cleaner
Sodium hydroxide
Sulfuric acid
Hydrochloric acid
10
60
10
110
60
260
170
400
600
700
1,000
530
590
1,540
1,630
2,540
2,960
5,800
6,300
11,000
10,000
3,900
6,400
13,000
4,000
10,000
27,000
53,000
48,000
103,000

Flow restrictors

If conductivity control is not used, flow should be controlled by some type of restrictor. The simplest is an in-line control valve that is manually adjusted. To avoid readjusting it from day to day, a shutoff valve should be installed upstream from the control valve. For a sensitive adjustment, the control valve should be a globe valve, while the shutoff can be any suitable type.

For tamper-proof operation, an in-line fixed orifice restrictor should be used. These are inexpensive commercial devices that can be purchased in a wide range of flow rates. Flow meters and weirs. A flow meter installed in a rinse water feed line can give quite accurate control once the desired flow rate had been determined. Their advantage is that flow is adjustable over a considerable range, so that one meter will handle sizable variations. A weir is a type of flow meter but with much less accuracy. It can be visualized as a V-notched dam across the outlet of the final rinse tank. Rinse water flows through the V into the sewer. The water level in the V is calibrated so that flow can be read directly.

Tank Design

Flow principles. The design of every rinse tank should follow several guidelines.

  1. Water may enter at the top or bottom of the tank.
  2. Flow is diagonally through the tank from inlet to outlet to promote mixing.
  3. The rinsing solution or the work should be agitated.
  4. Outflow is over a dam, or skimmer, positioned horizontally across the tank.
  5. Outflow through a standpipe or sidewall outlet is undesirable.
  6. Each tank should have a bottom drain (except precious metal dragout rinses). The drain valve should be located at the lowest point of a sloping bottom or in a trough built into the bottom. Precious metal rinses are excepted because of the possibility of draining the tank by mistake.

Inlet design. Feed water is preferably brought into the tank through a sparger extending across the bottom of the tank. The sparger is a perforated plastic pipe with drilled holes spaced 0.5 - 1 inch apart along its entire length. Its advantage is in providing a flow pattern across the entire width of the tank. The sparger should be located on the same side as the outlet. By directing the inlet stream across the bottom of the tank, it will prevent sediment from collecting and will produce a skimming action across the solution surface as it flows toward the outlet.

The next, and perhaps most used, method is to bring feed water into the tank through a single pipe located on the side of the tank opposite the outlet and extend it to the bottom of the tank. This doesn't distribute the flow as evenly as the sparger, but, in conjunction with an overflow dam, it can do an adequate job.

The least desirable method is to bring feed water in with a hose and risk the possibility of its interfering with the insertion or withdrawal of the rack or basket. Outlet design. The most effective and desirable method of handling outflow is to have a dam overflow across the entire width of the tank.

The dam should be installed level so that the entire solution surface will move over it to produce a skimming action. Skimming is important to prevent dead areas on the solution surface. Areas of little or no movement tend to accumulate foam or floating residues that can be picked up by the work. The dam is one wall of a small trough that collects the outflow and drains it to the sewer.

The least effective outlets are the standpipe and sidewall opening. Dead areas on the solution surface are guaranteed, and floating residues will collect.

Agitation

For effective rinsing there should be some relative motion between the work and solution. This can be obtained by agitating either the work or the solution. Mechanical agitation of racks or baskets can be readily provided on automatic or hand-operated lines with motor-driven, oscillating work rods. These are particularly effective for getting circulation through holes and recesses.

Another type of agitation is rapid movement of the rinsing solution past the work. Sometimes the natural flow of rinse water through the tank is enough, or it can be enhanced by pumping through a recirculating system. Because of low flow rates through countercurrent systems, solution movement is usually not enough to be effective.

Multi-chamber construction. There are two types of multi-chamber installations. The first consists of two or three individual tanks installed in line and plumbed one to another using the methods described previously. The outlet of one tank becomes the feed for the next, and solution flow is opposite to the direction that the work travels down the line.

The other type consists of a single, long tank that is suitably partitioned to form chambers that are in effect separate rinse tanks. Flow is diagonally through the first chamber, into a space that separates the first and second chambers, and then into the second chamber. There are two flow paths possible, each providing the same rinsing effectiveness.

In the first flow path, the feed enters the bottom of the first tank, flows diagonally through the tank and overflows the top of a dam. Then it travels down the space between chambers and enters the second chamber under the partition.

In the other flow path, the feed enters at the top of the first tank, flows diagonally through the tank and enters the space under the partition. It then flows up the space and overflows a dam into the next chamber.

In order to get gravity flow, the solution level in the downstream tank must be lower than the one before. It is critical to determine that there is sufficient depth to fully immerse the rack or basket. This primarily affects the automatic lines because the vertical hoist travel is fixed. One means of maintaining the same level in a series of tanks is to pump the solution from tank to tank.

Another condition occurs when a bulky load is put into one of the tanks and displacement causes the solution level to rise. A single tank should be equipped with an outlet trough of sufficient volume to catch the overflow and enough tank freeboard to insure that the rising solution will not spill over the sides. Of course, slowly immersing the load would reduce this effect, but in practice, this usually does not happen.

In counterflow tanks, the rising level will cause a backflow into the double-wall space and possibly into the clean tanks. If the space between the chambers is filled with the same solution as the clean tank, backflow would cause little or no pollution. Materials and linings. Tanks should be made from materials or have linings that resist the long-term effects of water contact. To a lesser extent, they may have to afford some resistance to chemical attack by process solution dragin. If the rinse is hot, this will impose yet another special consideration. Your tank vendor can be quite helpful with his recommendations, especially if the basic tank material is one that must be chosen to facilitate the use of special techniques of applying the liner.

Because of low cost, good chemical resistance and ease of fabrication, one of the most popular liners is polyvinyl chloride. Other choices are polyethylene, polypropylene, fluorocarbons, and neoprene. Metal liners are used where there are special process conditions or environments to accommodate.

Sprays

When sprays are used, there is a choice between perforated tubes and nozzles. Drilled PVC tubing manifolded along opposite sides of the tank is an inexpensive means of constructing sprays. However, the resulting spray pattern is frequently not uniform, and there is poor control over the volume of water used. Spray nozzles can provide good pattern control and will get the same degree of rinsing with less water.

Nozzles are available that spray in cone or fan patterns. Coverage and control are better, and water consumption is less than with drilled PVC tubing. The spray pattern must cover the tank width to insure complete rinsing of both the rack and work. The installation can be readily fabricated from PVC pipe that has been drilled and threaded to receive nozzles. Cone nozzles emit a cone-shaped sheet of water and spray a round pattern; this is good where flooding of the work is desired. Fan nozzles emit a flat, fan-shaped spray and are useful for washing down the work and blasting through holes and into cavities. To minimize splashing during wash-down, fan nozzles should be inclined slightly downward. For effective flushing of holes, sprays on opposite sides of the work should be staggered so the holes are not sprayed at the same time.

Optimizing rinsing operations. Not only can water consumption be optimized by cascading two or three tanks, but often rinses can be piped to an operation elsewhere in the line. For example, the spent rinse from one acid dip might be used as a rinse for another acid dip. Gang rinsing can suggest many possibilities for optimizing a line. Bear in mind that the condition of the work surface at every point in the line is the criterion. It is of the utmost importance to avoid a rinse that will produce a precipitate when brought into contact with the process solution, or one that will in some way poison the work surface. In one application of gang rinsing, remember also that the flow rate will be determined by the individual rinse with the highest flow requirements.




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