Lesson 235 - Coalescers

The word coalescer (co-ah-less’-er) comes from the root ‘coalesce’, or ‘to bring together’.

Coalescing technology uses the physics of gravity separation to ‘bring together’ contaminant oils from the fluid being skimmed. Coalescers are not magical ‘recycling’ systems that can transform dead, dirty coolant into new fluid. What they can do is ‘refresh’ coolant by encouraging separation of tramp oil which would otherwise emulsify into the base fluid and make it go dead in the first place.

A coalescing tank provides a quiet, non-turbulent zone for this separation to take place. Its efficiency depends upon the amount of tramp oil that gravity separates within the amount of time the fluid is held in the coalescer. To determine the hold time of any coalescer, divide the coalescer’s tank capacity by its flow rate. For example, a 15 gallon tank with a flow rate of 1 gallon per minute will hold fluid for 15 minutes (15 ÷ 1 = 15).

We recommend taking a coolant sample to determine how much oil a given coalescer will remove from that batch of coolant. An empty water bottle works well. Dip the bottle into the sump, starting at the fluid surface, moving down to the bottom. Cap the bottle, shake it for a few seconds, and then let it settle, noting how long it takes. If the coalescer has a hold time of fifteen minutes, then the amount of oil in your sample that has naturally separated in fifteen minutes is the amount the coalescer will remove.

If more oil separates as the coolant sample settles beyond the coalescer hold time, the coalescer being considered will not remove it unless the sump itself is allowed to settle for this length of time before the coalescer begins processing the fluid. If you have deeply emulsified oils that take as long as a day to gravity separate we recommend sending the fluid to waste. Begin coalescing on a new charge of fluid to keep it fresher, improving its performance and extending its life.

Zebra Coalescing Technology


has a 15 gallon tank and is restricted to a flow of approximately 1 gallon per minute, regardless of what pump is used. This means it holds fluid for fifteen minutes.

Within a Zebra Coalescer there is plastic coalescing media which provide more oleophilic surface area than other brand systems. This media attracts small tramp oil particles which stick together, and once these particles get large enough, they float up and out of the base fluid. Although coalescing media accelerate the coalescing process, a sump sample is the best guide to determine whether any fluid batch can be serviced with the coalescer being considered.

A coalescer’s primary purpose is to remove tramp oil. Since coalescers draw oil and coolant from the fluid surface, only floating or suspended chips can be captured.

Zebra has some chip filtering abilities for the coalescer. This can be done using external cartridge filters that hang onto the outside of the unit.

Filtering chips means that there will be increased maintenance for the unit. If there are too many chips, the filter will clog and the unit will cease to operate. This can hurt the pump.

When is a coalescer the best choice?

There are a number of factors to consider when determining whether a coalescer will be the best option for any given application.

The Z17 is designed to be dedicated to a single sump, operating 24/7 to remove contaminant oil and circulate the fluid, further extending its life.

The Z17 helps with coolant monitoring. From the de-oiled surface within the coalescing tank, pH, hardness, and concentration can be monitored more effectively, since taking a sample from a dirty sump skews results.

Other considerations are whether a sump tends to accumulate biofilm or generally go rancid no matter how hard you try to prevent it. We see this in crowded tanks. We see this if the customer machines chemically aggressive material, such as cast iron.

Since a coalescer uses a pump to suction at the surface, it is not affected by biofilm or sludge formation as much as mechanical skimmers are.

A users preference is the last consideration. Some users like mechanical skimmers since it is easy to see how they work or when they need a little cleaning. Others like coalescers, because when they are set up correctly they can go a long time between overhauls.