RINSING, A COMMON SENSE APPROACH

Most of us haven't paid much attention to rinsing methods unless we have a problem with panel cleanliness. When surfactants from the cleaners get dragged down the line and create havoc with the process, we all get interested very quickly. The solution to this problem, as we all know, is to turn the water up and increase the agitation. More is better, right?

Wrong, at least partially. The problem is not the quantity of water used but the way it is used. Most of us have the rinses set up to agitate continuously, whether they are counterflowed or not. When a rack or basket enters the rinse, the dragout from the previous process or rinse tank mixes with the rinse water as it enters the tank. If you will go out to your line and watch a colored process solution being rinsed off in a tank, you will be able to see the process in action. As the rack enters, the solution quickly mixes in the top level of the rinse tank (the top inch of solution). Turn the mechanical and air agitation off to see this. I estimate that over 95% of the dragout is removed in the top inch of the water column as the rack/basket enters.

At present, we take the top 1" of water and quickly mix it with the rest of the tank during the agitation to remove the last 5 % of solution from the work. If you think about it, this could provide a way of reducing water use and increasing the efficiency of rinsing. What if we were to turn off the agitation when the work entered the rinse tank? We would have a stratified tank with most of the dragout in the top inch of the tank. If we could just remove the top inch of the water column, we could reduce the water usage by 95%. Well, maybe not quite 95%, but, in the real world, a lot.

The trick is to prevent the stratified solution from mixing and to remove it from the tank quickly so we can remove the remaining 5% of the dragout by agitation. The easiest and simplest method for removing the top inch of a water column is to add an inch or more to the bottom and let the top overflow. It really works and, after a colored process solution, you can see it work.

Test it: Using a colored process solution, place a rack after processing in a clean rinse tank. Make sure all agitation is turned off. You will see that most of the process solution mixes at the air-water interface of the rinse tank and it takes a few minutes for it to mix into the rest of the tank.

Now, after turning off all of the other rinses on the line to get the water pressure up (having water pressure problems are you?), turn the rinse on full bore. If you have a water sparger on the bottom, the whole surface seems to rise and the colored top layer quickly moves to the drain leaving a much cleaner tank. If you don't have a sparger, the water will rise in one area and push the top layer to the drain but with less efficiency.

Ok, so what's the best way to do this? Glad you asked. There are three ways to control rinses; conductivity, flow, and time. The conductivity controller insures cleanliness by running as much water as necessary to maintain the setpoint. Usually the setpoint has no relation to any predetermined standard of what is a clean rinse. Surfactants conduct very little so it is hard to determine if they are out of a tank by conductivity measurements. There is no way to turn the system on based on work entering the tank. They will probably turn off when work enters the tank, but you don't know that nor do you know how much of a lag there will be from the time you enter the rinse tank till they turn on (there will be a lag). Also, conductivity controls have two main disadvantages. First, they require very clean water to operated unless you do a regular preventive maintenance program which cleans the electrodes weekly. Second, you can reset the setpoint easily at the tank, so an operator who thinks the water is too dirty, can increasethe setting which will increase water use. It is very easy to set a conductivity controller so it is on all the time.

Flow controls piddle a little bit of water into a rinse tank continuously. They are not meant to be turned on and off. Unless you do every line in a shop, what happens is that any lines still open flow at full bore all the time, and you see very little change in water flow. One way to look at it is if you are using 30 gpm and you put 15 flow restrictors at 2 gpm on 15 rinses, you still flow 30 gpm. If you are at 45 gpm and do the same but leave two 1/2" lines uncontrolled, you will still be at 45 gpm.

Timers determine water flow based on line size, water pressure,and time. Once you put timers on, the first thing you notice is that your water pressure returns to normal. When this happens, flowing 12-15 gpm out of a 3/4" line is not unusual. Timers can be activated in many ways so that the flow can be configured for a predetermined rinsing program. For example, a pushbutton on the previous tank is pressed to activate the next rinse so that when the rack arrives, the water is rising and overflowing the tank. This will reduce the dragout remaining in the rinse tank and reduce the overall rinse water required to maintain process quality. The actual amount of water flowing through a tank can be determined and dragout rates and dilution ratios can be calculated and verified. You can combine timers with conductivity controllers to both control the starting time and the endpoint concentrations.

Each shop is different and the calculations vary with tank size, counterflow vs single flow rinsing and whether there is a dragout before the rinse. We will be glad to assist you in determining if timers and/or changes in agitation programs will improve your rinsing and save water. The REMCO ENGINEERING list of things to do to increase rinsing efficiency and reduce water usage:

1. Add dragouts after metal bearing rinses. (Batch treating will really help out your wastewater treatment system)
2. Use double or triple counterflow rinses.
3. Use timers to maximize water use efficiency.
4. Turn off mechanical and air agitation when work enters the tank. Agitate after the rinse timer turns off or on exiting the tank.

COMPARISON OF CONTROL METHODS

Timers with flow restrictors Water Timers

Operator cannot easily change setting
No maintenance required
Runs an exact volume of water with each initiation
Automatic turn off
Required expert installation (electrical)

Timers

Operator cannot easily change setting
No maintenance required
Runs a set volume of water each time
Required expert installation (electrical)
Automatic turn off

Flow restrictors

Easy to install
Operator controls flow (turns water on and off)
Inexpensive
Operator cannot change setting

Conductivity Conductivity Controllers

Required expert installation (electrical)
Weekly maintenance necessary
When rinsing with DI, may give a reading proportional to concentration
Operator can change setting
Correct set point difficult to determine
Requires DI water and aware operators to operate trouble free