TECHNOLOGY FOCUS

CENTRAL SYSTEMS

PART 2 -  FILTRATION or WHY HAS MY COOLANT CHANGED?

by Dom Ruggeri

MAY 2002:

As I once said, “The coolant is the life blood of the machines”; and as such it should be kept clean.  However, this can be taken to an extreme.  Such was the case at one account I visited back in the early nineties.

I received a customer complaint the coolant was changing after it passed through this customer’s filtration system.  This particular filtration system was a Henry Wedge Wire.  This type of system passed the dirty coolant through a wire mesh screen.  The pore size was 25 microns, this screen, would stop rocks, trees, and perhaps the occasional iron block, but not much else.  Yet many shops have this type of filtration set up and use it with great success.  Here’s why; as the metal fines pass through the screen they form a filter cake, effectively reducing the filter pore size such that the screen becomes a very effective filter.  A scraper is passed over the screen to remove the metal fines and any trapped dirt.  The waste metal is then transferred to the recycle area to be hauled away and reused and the process begins again.  The scraper can be on a timer or a vacuum system.  Either way works well.  If the filtration system is functioning properly it should not hurt the coolant.

The salesman and I entered the account and surveyed the central system.  It was 25,000 gallons using a chlorinated soluble oil, at a concentration of 5%.  This soluble oil yielded a milky white emulsion, however once it passed through the filtration system the coolant became translucent.  Obviously there was a problem.  I began with the standard concentration tests; alkalinity, refractometer, and of course acid split.  The results were as follows:

1. Concentration by Alkalinity = 5.30%

2. Concentration by Refractometer = 0.75%

3. Concentration By Acid Split = 0.50%

As you can see from the above something was very wrong.  We began to focus on the filtration system, checking each and every parameter.  All was within specification, but we noticed a white material on the wire mesh.  I questioned the engineer about it, and he explained that it was a pre-coat that was metered into the dirty coolant.  Its intended function was to increase the effectiveness of the filtration.  We discovered that it was actually chopped, surface-activated paper.  This paper was, in fact, reducing the pore size of the wedge wire, thus creating a more effective filter.  However, when the scraper passed across the mesh it left behind a fair portion of this pre-coat.  As the pre-coat built up, the pore size decreased to the point where the coolant could not pass through.  The activated paper would actually adsorb the oil and emulsifiers, literally breaking the emulsion.  The customer explained that this pre-coat was necessary due to the pore size of the Wedge Wire.  I couldn’t argue with that, however they must have gotten their dosage recommendation from Johnny Bravo (cartoon network) as they were adding 5 times what the supplier recommended for that size system.  The metering device was timed wrong, but the pre-coat supplier, selling lots of pre-coat, chose not to inform the customer of this problem.  After a painstaking process of adjustments to the metering device, the pre-coat and my coolant coexisted.

The coolant must be kept clean.  Dirty coolant can cause no end of trouble in a central system.  Certainly there are many types of filtration systems, all designed to clean the coolant.  The media used to filter the coolant defines a filtration system such as:

1. Paper Type:

This media comes in a variety of pore sizes and compositions from paper to polypropylene.  Matching the pore size to the emulsion particle size is as important as maintaining the mechanics of the system.  Many of these media are coated to enhance filtration and yield a cleaner coolant.  Make sure the coating is compatible with your coolant.  Failure to do this can cause you many problems down the road.  These types of filtration systems are generally vacuum driven.  The dirty coolant is pulled through the paper using a vacuum.  The vacuum increases as the paper becomes covered with layers of metal fines and other debris.  When the vacuum reaches a preset level, the paper indexes and a fresh paper surface is exposed.

2. Wire Type:

This media comes in a few different shapes and sizes from a wire mesh screen, to a drum configuration and, a Star configuration.  The pore size can range from 25 microns; (will stop rocks and trees) down to 1 micron.  The advantage of these systems is a durable filter media and less consumables.  These types of filtration systems operate using fluid flow rates or a vacuum system.

3. Flow Rate:

This system measures the fluid flow rate through the filter and when the fluid flow slows to a preset level a blade scrapes the metal fines and other debris from the wire and conveys it off to be recycled.

4. Vacuum Systems:

This system uses vacuum to pull the coolant through the filter as layers of metal fines build up on the filter the vacuum increases.  When the vacuum reaches a preset level the blade comes in and scrapes these fines and other debris from the screen conveying them to the recycle area.  Depending on the configuration of the filtration system, compressed air may be used to blow off the metal fines and take them away.

5. Weir System:

This is a cascading type system.  It functions by allowing the coolant to cascade over a series of descending weirs.  The heavier materials will not come across these weirs, but will settle to the bottom of the tank where it will be conveyed to the recycle area.

I do hope this gives you some idea of how a filtration system works, an over simplification, perhaps, but volumes have been written on this subject.  As always, if I can be of any help feel free to e-mail me at the magazine

Good Luck

Dom