Dust Problems with Lime Kiln ID Fans

Lime kilns play a vital role in pulp and paper mills as they convert calcium carbonate (a byproduct of the pulping process) to calcium oxide (lime). The lime returns back to the pulping process completing the loop utilized by mills employing the Kraft-pulping process. To facilitate this reaction, the calcium carbonate must be heated to temperatures over 1000°C to drive off carbon dioxide gas. Rotary style lime kilns are generally used as they represent the highest thermal efficiency presently available, given the production quantities required at a typical pulp mill. An ID fan is required to induce flow through the rotary kiln to facilitate the chemical reaction. This flow is normally heavily laden with dust (5-20gr/ft³), requiring scrubbing/cleaning equipment, which is often downstream of the ID fan.

The heavy dust load passing through the ID fan represents a major problem for mills, as the dust sticks to the fan blades. Over time this build-up accumulates and eventually a piece breaks off putting the rotor out of balance. Operators are forced to shutdown the entire lime kiln to clean the fan. Because lime kilns represent the completion of the pulping process cycle, this interrupts the entire mill operations. Unscheduled shutdowns are very time-consuming and have a significant impact on mill production.

There is a 12-24 hour time period required for kiln to cool down, before fan can be cleaned. Also, there is a 12-24 hour time required for the kiln to achieve stable production after start-up. This amounts to approximately 3-4 days of lost production for each unscheduled shutdown.

The frequency of unscheduled shutdowns range from every 2-3 weeks to 2-4 months. Some mills plan their scheduled shutdowns to coincide when operators know the fan will require cleaning, before vibration reaches an unacceptable threshold.

The inconvenience of unscheduled shutdowns (or planned shutdowns that revolve around the cleaning schedule of the fan) cannot be overstated. Yet, this ID fan build-up problem is endemic to the pulp and paper industry due to poorly designed fans. Often the suppliers of this poorly designed fan equipment will place the blame on the “system” (such as fuel type, amount of dust load, etc). However, the mill cannot modify the system to potentially reduce fan build-up, as market forces determine the fuel type or raw material quality.

These poorly designed fans are often straight bladed radial or radial tip blade types that represent 100+ year old fan technology. This design causes dust to impact at steep angles on rotor surfaces, causing build-up. These fans are also low-efficiency type; results from many site performance tests verify these fans operate in the 40-60% efficiency range.

We’ve successfully solved severe build-up problems in many lime kiln applications with our high-efficiency rotor upgrade approach. Good fan design involves ensuring dust impingement is minimized on rotor surfaces. In addition to solving build-up, we’ve been able to provide significant power savings, which is a great secondary benefit. Efficiency improvements of 20-30% are common. Project payback periods are very low, as we re-use existing housing, motor, bearings, and coupling.

The Static Pressure Cheat

Calculating Fan Performance

While it is not necessarily a poorly known fact, the “static pressure cheat” (SPC) is something that we should all remind ourselves of whenever considering the specifications of a fan’s performance.  The essential point to remember is that total pressure (Pt) is the measure to use in evaluating performance, not only static pressure (Ps).

In some cases, when an end user desires a fan for a specific purpose, they will understand that the fan they need has to have a certain static pressure delivery.  The thing to note about static pressure is that it is dependent on the cross-sectional area of the plane of measure.  That is to say that the duct size matters.  If we are comparing two points in a duct system that have the same area, there is no problem, however, fans have varying areas in terms of inlet and outlet flanges, which are the points at which mechanical performance is rated.

In the calculation of fan efficiency, we need to account for the total energy contributed to the gas (air stream) by the fan.  This means that we cannot use only part of the pressure equation above, but must rely on the complete pressure rise, or we have missed part of the whole picture.

Velocity pressure (Pv) is related to the velocity in the duct, so for a given volumetric flow rate, the Pv will change according to the size of ductwork the gas must flow through.  In the above equation for Pt, we can see that it is the sum of Pv and Ps that make up Pt.  So if losses are small, we can assume that as a gas volume flows through a duct, it’s energy state will convert between Ps and Pv to arrive at the same sum, Pt.

And here is where the “cheat” comes in.  If a fan’s performance is specified in terms of Ps, the fan designer can simply use a large diffuser at the outlet of the fan to bump up the static pressure.  That is, if we increase the cross-sectional area of the point of measure, we decrease the velocity, and increase the static pressure, making the fan look like it is delivering a higher performance.

static pressure cheat

To avoid this, fan performance should be expressed in terms of total pressure rise, and thus capture the complete energy developed by the fan when including flow rate.  This will enable the correct evaluation of fan efficiency and, for comparative purposes, negate the effects of differences between the cross-sectional areas of the points of measure.  Different fans with different inlet and outlet areas can now be compared and contrasted on an equal basis.

All the best in your fan specification developments!

If you would care to discuss this article or any details mentioned above, please contact Trevor Preis.

Fan Efficiency Explained

As costs continue to increase for operations, maintenance, and equipment replacement, the importance of efficiency in industrial processes cannot be overstated. The cost per unit of production continues to be the measure of decision makers to assess their best capital investment opportunities in maximizing returns. In this blog, we’ll also explore why fans may not be operating at peak efficiency.

It is with this in mind that we turn to the analysis of fan efficiency; its calculation, sources of error, and ultimately its use as a parameter for making decisions.

Calculating Total Fan Efficiency

Generally, the efficiency calculation takes the form:


Fan Efficiency

Sources of Error

We use the term “total fan efficiency” to differentiate our parameter from the pseudo-efficiency values of “static fan efficiency” and “fan static efficiency”. Of these terms, only total fan efficiency accounts for the total mechanical energy that the fan imparts to the gas stream.

Frequently fan curves will be published based on laboratory testing under ideal conditions (e.g. low pressure, fully developed flow). However, it is rarely the case that ideal conditions are found in the field. We employ field testing to account for conditions on site, including system characteristics that directly affect the efficiency of a given fan. Note that the total efficiency in the above equation takes into account the compressibility of the gas under the conditions of actual fan operation, which can affect the calculated efficiency value by up to 10%.

A Decision-Making Parameter

When evaluating your fan, the fan curve can be a useful tool to understand why the efficiency may be low at the given operating point.

fan efficiency

Figure 1 shows a typical fan curve (green) with two numerated system lines (dashed), and the rated efficiency curve.  The “S” denotes the selection point for the desired performance.  In this example, the fan design was developed for system line 1, while testing reveals that the actual system resistance curve is represented by line 2.  The selection point is at the peak efficiency of the design, however, under normal operating conditions, the fan efficiency is approximately 10-20% lower than design.

When deciding on a fan upgrade to improve efficiency, performance, or wear life, we  design the impeller to the actual system and system characteristics to maximize these benefits and will restore or exceed the original design fan efficiency.