As described previously, there are two major aspects of flow within a high-efficiency cyclone to consider: a) the rotational flow or primary flow pattern (fig4) and b) the axial flow or secondary flow pattern (fig5).
2.1 Rotational Flow
The rotational, primary, flow pattern is used to impart greater inertia or centrifugal force on the particles than the gas molecules. The goal is to create an adequate centrifugal force field so that particles can not cross into the central core of the high-efficiency cyclone where the gas is travelling upward to exit the device. Another way to think of this is to consider the centrifugal field as a barrier or boundary much as we think of particle filtration where a particle can not pass through a media whose pore size is smaller than the particle. The factors that affect the centrifugal force placed upon a particle are described by the equation for centrifugal force:
Fc = Vt2/r • M where;
Fc is the centrifugal force, Vt is the tangential velocity, M is the particle mass, r is the radius of the circular path at the point of measurement
It is obvious from this equation that greater tangential velocity, greater mass, and a smaller radius of travel all result in greater centrifugal force. We can ignore the mass of the particle for this consideration since it is what it is. On the other hand, tangential velocity and the radius of the path of travel are both variables that we can utilize to affect cyclone collection efficiency.
We can increase tangential velocity by increasing the tangential component of the high-efficiency cyclone inlet velocity as well as by increasing the radial distance between the inlet and the inner core of the high-efficiency cyclone. In high-efficiency cyclone rotational flow, the tangential velocity increases from its initial tangential velocity as it moves to a tighter radius of travel due to a conservation of momentum (fig6). In the area where the velocity increases to conserve momentum, the flow is called a free vortex. At some point the viscous forces on the gas molecules prevent the increase in tangential velocity and the gas rotates as solid body or fixed or forced vortex. Therefore we can increase centrifugal force by not only increasing the inlet velocity but also by
- reducing the diameter of the outlet pipe
- reducing the high-efficiency cyclone diameter
- increasing the radial distance from the inlet to the center of the high-efficiency cyclone
It is important to understand though that not only do these rules apply to the particulate that is in the flow stream but to the gas itself. The resulting pressure gradient that is formed within the gas flow is the single biggest component of high-efficiency cyclone pressure drop. In other words, the greater the centrifugal force the smaller the particle we can collect but we pay for it in increased power consumption in the form of pressure drop.
2.2 Axial Flow
The axial, or secondary, flow pattern is downward along the outside of the high-efficiency cyclone assembly and upwards in the center. This flow pattern is critical to the performance of a high-efficiency cyclone since we pneumatically transport particles from the upper portion to a collection receiver or discharge point.
A common, but inaccurate, description of how a high-efficiency cyclone works often goes, “the particles are thrown towards the wall of the high-efficiency cyclone where they are slowed by friction and then fall into the receiver due to gravity.” This statement is almost completely wrong. While the particles do slow somewhat due to friction when they reach the wall of the high-efficiency cyclone, the flow patterns and forces placed upon the particles are many times greater than gravity. Often larger greater mass particles are transported to the receiver more slowly than small particles due to the particle’s inertia acting upon the sloped surface of the cyclone cone. Collected particles are pneumatically transported to the bottom of the assembly in a fraction of the time required for settling by gravity.
Our goal in high-efficiency cyclone design is to “land” the collected particles on the bottom surface of the receiver with the proper tangential and axial velocities so that it remains in the collection zone and is not re entrained in the escaping gas flow. If the axial velocities or angle of attack are too great, re- entrainment goes up. If they are too low, axial particle flow can become stagnant at some elevation in the high-efficiency cyclone. These particles will accumulate until the mass get large enough to disrupt the flow and they fall by gravity or they may be continuously re-entrained into the escaping gas flow. In either case, the probability of a plug occurring and unacceptable levels of cone erosion go up.
Since high-efficiency cyclones typically operate over a range of operating conditions as well as particle size, a dust receiver or vortex breaker is commonly used to reduce particle re-entrainment.