Operational Aspects of Oil Mist Eliminators

What are the issues facing the user?

On equipment like turbo-compressors, turbines, and clutches, high-speed rotating shafts enter and exit oil-filled reservoirs through shaft seals in the reservoir housing. These seals use compressed air blown against the shaft to keep oil from leaking out of the housing. During operation, the pressure inside the reservoir increases as a result of the high-speed rotation of gears and shafts and the expansion of air from heat.

If the force exerted by the pressure inside the housing becomes greater than the force exerted by the compressed air against the shaft seals then the oil begins to leak from the reservoir housing. This situation creates adverse environmental conditions and increases the total consumption of reservoir oil over a given time period.

To combat this problem, a pair of cooling fans is commonly used to create a positive pressure in the turbine room in which the clutch housing is located. Users have reported that with both cooling fans running at high efficiency enough positive pressure is created inside the turbine room to offset the positive pressure inside the clutch housing and eliminate the leakage. Users also report that when the fans lose some efficiency, fail, do not run simultaneously, or don’t run at all due to the cooling requirements inside the turbine room that the leakage resumes and can be quite severe.

What is the function of a BAE (Blower Assisted Oil Mist Eliminator) unit?

A BAE creates and maintains a continuous vacuum level inside an oil-filled reservoir to stop oil leakage through the shaft seals by reducing pressure build-up inside the reservoir housing.

With the vacuum extractor connected directly to the clutch’s oil reservoir (normally outside the turbine room for ease of installation and maintenance) a constant negative pressure can be maintained inside the clutch housing. When set at the correct level this constant negative pressure will prevent oil leakage under any operating conditions encountered inside the turbine room.

As a function of the vacuum extractor, oil mist laden air is pulled out of the clutch oil reservoir; this oil laden air cannot be exhausted directly to the atmosphere. The air must be passed through a filter system to remove the oil from the air. The clean air can then be exhausted to atmosphere and the oil, which will be collected in the filter housing, can be drained back into the reservoir. This is a continuous process that takes place whenever the vacuum extractor pump is turned on.

In order to achieve different vacuum levels for different clutch and reservoir configurations or applications the capacity of the vacuum extractor pump must be adjustable. Also as the filtration system matures it will cause increasing backpressure which will have to be overcome by the vacuum pump in order to maintain the constant negative pressure level in the clutch housing.

To accomplish this, a variable frequency drive may be used to control the motor speed on the vacuum pump. This gives the user the ability to adjust the negative pressure level in their system to find the level that works best for them and allows them to adjust the speed to compensate for a maturing filtration system.

Testing has been performed to determine a recommended level for the negative pressure but all systems are configured differently and the best operating conditions for each system should be determined on-site. A negative pressure level of 12-nches Water Column in the clutch housing has solved the majority of the user’s problems. This level has stopped all oil leakage and has not affected the performance of the clutch or any other systems working with the clutch or the clutch oil reservoir.

There are some important aspects to consider when designing a vacuum extraction system including the size (flow rating) of the filter element, size (flow rating) of the vacuum pump, elevation of filter housing relative to the oil reservoir, and the configuration of all the key components to maximize their performance. There may be some additional considerations for an installation regarding the environment and the mounting of the equipment.

We have learned the most efficient and effective means of designing and manufacturing vacuum extractors for different applications through experience, research and development.

What is the typical design?

A BAE unit consists of a filter housing (constructed of carbon or stainless steel); a coalescing element a centrifugal or side-channel blower, and a drain line. A method for controlling the vacuum level is also designed with the system. The vacuum control method can vary from a manual bleed filter and valve to a completely automatic, variable frequency drive which controls vacuum level based on the back pressure in the system.

Typical BAE Applications:

Thus far the following application issues are known:

Application Type - Current Design Issues

In these cases a piece of equipment has been designed with inadequate shaft seals; they do not produce enough force to keep the oil from leaking out of the housing. You might not want to go through major disassembly and reassembly of the equipment to have the correct shaft seals installed; they would prefer a bolt-on solution, which the BAE provides.

Application Type - Cost Reduction Issues

In these cases you may be concerned about operating costs, which can be assessed in several different ways:

1. The actual dollar value cost of the compressed air required to operate a shaft seal so that it functions properly is more expensive then the electricity that it takes to run an efficient electric motor on a blower.
2. The overall performance cost, which occurs when the compressed air produced by a system is “robbed” to operate the shaft seals increases the costs of operating the system. To help minimize costs, these customers are interested in reducing the amount of compressed air used in the shaft seals as much as possible. Creating vacuum in the oil reservoir reduces the amount of compressed air “robbed” from the system.

Sizing a BAE Application:

Step 1:
Obtain Requirements

Of several parameters listed, you will need to specify, the operating temperature, the desired under-pressure in the reservoir, the filter efficiency and the inlet connection size. The additional data, while helpful, is less critical to sizing the application.

As with any system, adding the optional, additional equipment will increase the cost of the system- so, in price sensitive applications, consider removing the optional equipment first.

	Main Parameters	Sample Specifications
	a) Flow	80 cubic meters/hr
	b) Operating Temperature	58 degrees Celcius
	c) Desired under-pressure in reservoir	6 mBar
	d) Filter efficiency	99.98% @ 0.3 microns
	Connections
	a) Inlet	DN100/PN10 Flange
	Additional Equipment
	a) Vacuum gauge - reservoir	Please include
	b) Vacuum gauge - filter housing	No, thanks
	c) Vacuum control - manual or automatic	Please include manual system
	Electric Motor Specifications
	a) Voltage	230/400 VAC
	b) Frequency	50 Hz
	c) # of phases	3
	d) Thermal protection	F, class B
	e) Motor protection	IP 54/55

Step 2:
Sizing the Blower and the Motor

Once you have specified the electrical parameters for the blower and motor, the blower and motor can be selected based on the desired under-pressure requirements. A centrifugal blower will be used in applications requiring less than 12.45-milibar and a regenerative blower will be selected for applications requiring more than 12.45-milibar. The outlet size will be dictated by the blower selection along with several operating parameters; such as: power and Revolutions per minute.

In all cases, you may specify the blower type which is used on systems you purchase.

Step 3:
Select the Coalescing Element

After confirming that the stated operating temperature and filter efficiency are within our products’ capabilities, we look at the flow rate and desired under-pressure to determine what size filter element best works for this application.

During this process, estimated pressure drop curves are calculated and may be shared with you upon request.

Step 4:
Find Correct Blower

After we have calculated the pressure drop of the selected filter element, we can use the anticipated pressure drop reading to help size the blower using performance curves supplied by the manufacturer.