Vertical Tube Coalescers (VTC)

This Vertical Tube Coalescer (“VTC”) technology is somewhat of a hybrid between a traditional coalescer and a secondary mesh pack. This design does not utilize “stoke’s law” (or the rise rate of oil) as its primary means to remove oil from the horizontal flow of wastewater. Rather, as the oil droplets drift by the polypropylene diamond-mesh tubes they are attracted to the tubes by adsorption (very much the same principal as the secondary mesh pack).

In theory the VTC coalescer should collect droplets of oil and agglomerate them (by coalescence) into larger droplets. The larger droplets are then to break free from the polypropylene matrix and move their way to the surface along the diamond-shaped coalescer. Solids are to simply fall down and out the vertical tubes.

These tubes are heat welded together and equipped with a handle to insert and remove the media from a tank. The idea is to tightly pack the entire volume of the tank with these bundles, and remove them for cleaning when necessary.

Below: a brief exerpt from a recent keynote presentation where lead design engineer Dave Goding was asked to compare the mainstream coalescing media on the market for use in oil water separators.

In Reality

The VTC coalescer’s diamond-mesh pattern that creates a fixed coalescer matrix is too “tight”. The openings for wastewater to flow are between 1/8” to 1/4” – which is way too small for typical industrial wastewater applications. The coalescer gets easily blinded by leaves, plastic(s), and solids that are 1/8” and larger. The fixed, tight matrix does attract oil well. However, at the same time the finer solids drift through the pack, they come into contact with the oil and simply cling to the oil particles—creating an oily sludge deep within the coalescer pack.

In practice, solids do not drift down and out of the pack’s tubes. They simply get caught in the coalescer as solids get caught in any filter. Once plugged up, the packs are almost impossible to clean completely. These packs need replacement more often than most other types of coalescers, and become an ongoing and expensive replacement part.

This is a disposable model, as there is no way to clean fully into the center of each bundle. Higher flow rates can have upwards of 20 of these tube packs that need to be pulled out and cleaned/replaced on a monthly basis.

The Secondary Coalescer is a Warning Signal: 

This treatment option is most often seen in the below ground cylindrical “simple tank” oil/water separator world. These packs are largely used in the storm water, petroleum marketing, and transportation industries. Below ground tank systems do have corrugated plastic plate coalescers, then they are followed up by mesh adsorption packs consisting of polypropylene mono filament fibers. In theory, the coalescer in the oil/water separator is the primary separation device and these mesh packs are supposed to allow the water to flow through while any of the small oil droplets coalesce into larger droplets and rise to the top. In most cases the plate coalescers are designed at too shallow an angle to get efficient oil separation, so the manufacturers add adsorption packs to “improve” removal efficiency and meet effluent standards.

View Mercer lead designer Dave Goding discuss this type of crutch practice by some oil/water separator manufacturers:

Relying on polishing mesh packs to “make up for” the shortcomings of an irregularly shaped plastic coalescer is not sound treatment. The design philosophy goes much like this: “Since we cannot meet 60 micron removal requirements with our oil/water separator at design at maximum flows, we will simply insert this high surface area mesh in the back and cover ourselves.”

Here is an important question to consider: If this were an effective design method to treat oily wastewater, why wouldn’t every oil water separator manufacturer simply fill the entire tank with this mesh and leave out the primary coalescer?

This is the worst fouling treatment in the marketplace. Originally created for treatment in mist eliminators, and a decent alternative for flock matter, this is by no means an effective alternative. Once these clog very quickly, the flow actually builds pressure back and makes these cartridges sag and rip very easily. They are not industrial duty and therefore present constant replacement costs over the life of the unit.

A Crutch for an Under Performing Coalescer--When insufficient coalescer design was recognized, especially in the below ground cylindrical market, manufacturers had to come up with a way to promise stated removal values. These “polishing” packs readily plug at their entrance points, and actually serve as a flow obstruction within the treatment.