Industrial Utility Efficiency

Industrial Dust Collection Vacuum System Audits

Industrial vacuum systems are a challenge to optimize. They have more distribution system variables to balance than a compressed air system does. Vacuum systems conveying particulate are sensitive to velocity. If the velocity is too high, pressure drop results. If it’s too low, particulate doesn’t stay in suspension, and there can be compliance and safety problems. For instance, when conveying wood or other explosive dust, dropping below “critical” velocity allows dust to accumulate in the bottom of the duct, creating an explosive hazard. Entire plants have burned to the ground, and lives have been lost due to these types of incidents. According to OSHA1, wood is not the only explosive material in dust form:

Any combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosible ... The force from such an explosion can cause employee deaths, injuries, and destruction of entire buildings. For example, 3 workers were killed in a 2010 titanium dust explosion in West Virginia, and 14 workers were killed in a 2008 sugar dust explosion in Georgia ... A wide variety of materials that can be explosible in dust form exist in many industries. Examples of these materials include: food (e.g., candy, sugar, spice, starch, flour, feed), grain, tobacco, plastics, wood, paper, pulp, rubber, pesticides, pharmaceuticals, dyes, coal, metals (e.g., aluminum, chromium, iron, magnesium, and zinc).

The magic number in dust collection safety is the minimum transport velocity. It varies for each type of particulate. For wood dust, we typically use 4000 fpm. Table 1 offers a comprehensive list2 of minimum conveyance velocities for various products.

Table 1: Typical Minimum Conveyance Velocities

 Product Approx. Weight Suction Pressure to Pick Up Carrying Velocities (lb/ft3) (in H2O) (m/s) (ft/min) Flour 17 - 30 3500 - 6500 Grain dust 10 - 15 2000 - 3000 Sawdust and Shavings, Light 12 2.5 10 - 17 2000 - 3500 Sawdust and Shavings, Heavy 17 - 23 3500 - 4500 Shavings, Light 9 2.5 22 3500 Shavings, Heavy 24 3 20 4000

The Energy Efficiency Challenge

Plants will rightly always have a higher priority for compliance and safety than energy efficiency. To gain efficiency in an industrial dust collection vacuum system via controls, fan speed has to be dropped sometimes. Power is proportional to the cube of speed, all other things being equal (30 percent speed reduction can deliver over 60 percent kW savings). Even if vacuum is kept constant, power is directly proportional to speed (30 percent speed reduction results in 30 percent kW savings). However, this can be seen as a risk that isn’t worth it — sometimes for good reasons, sometimes for perceived reasons. Thus, optimizing dust collection systems for energy efficiency alone is not recommended. In our experience, the “hit ratio” for these kinds of audits is low. We recommend the following process:

1. Perform a system audit.
1. Measure all branch flows, fan speeds and static pressures before and after major components.
2. Add up branch flows and determine if they balance with fan flows. If not, the difference is false load or leakage.
3. Determine if flows and velocities are above minimum required levels for safety, air quality and process control.
4. Determine if vacuum is sufficient to develop needed flow at pick-up points.
5. Determine if pick-up points are properly designed to create velocity at the right place and do the work needed.
6. Determine production needs. This can be simply adding up all individual uses and multiplying by a conservative duty cycle. It can also be done by data logging all machine centers to determine what flow is really needed.

2. If the system is not compliant or does not meet production needs, adjust flow in the most cost-effective manner. This is the “baseline” project. This can actually require more total flow — not less, thereby increasing energy requirements. It can also increase vacuum at the fan inlet, increasing energy consumption. If you are lucky, it can reduce energy consumption if there is more vacuum than needed. The improved system becomes the “baseline” for the energy efficiency effort. In most utility energy efficiency programs, the baseline can be the improved system, not the as-is system. This increases the claimed energy savings and incentives for a project. The improved system might include some of the following changes:

1. Fan speed change
2. Addition of manual balancing dampers
3. Fan motor size increase
4. Addition of blast gates, spark sensing and other safety controls

I realize the plant wants one project — not a baseline and then an energy efficiency project.  But for the purposes of getting energy efficiency incentives, you might need to develop both on paper to determine the incremental benefit of the efficiency dollars. A skilled energy auditor can navigate the line between baseline and energy efficiency, and provide an incremental analysis for the energy efficiency project.

3. Develop energy efficiency opportunities to improve on the baseline. Typical opportunities include:

1. Excessive velocity in any particular branch or subsystem
2. False load, that is, air demand at a location that has no production — either part or all of the time
3. Low-efficiency fan
4. High pressure drop in cyclone, filter or ducting
5. Less than optimal ducting, requiring excessive flows in areas that don’t need it
6. Machine centers that are rarely on but have flow all the time

4. Develop an energy-efficient design that can reduce velocity, flow or vacuum, and still be compliant. Opportunities include:

1. Changing fixed fan speed
2. Changing fan to higher efficiency unit
3. Changing filter to lower pressure drop unit and reducing speed of fan accordingly
4. Implementing “on-demand” controls that match the fan flow to the actual demand in real time

This article will focus on the last method, an “on-demand” system. Recent technological innovations have come to the market that control demands and fan speeds in an optimal manner to meet production, remain compliant and reduce energy. Capable plants, vacuum pump suppliers, consultants and integrators can design and install their own version of an on-demand system.

A Generic On-Demand System Description

• If not already done, split the system into subsystems that have similar hours and vacuum levels. This could require significant ducting beyond the “baseline” project.
• Modify ducting to balance systems within each set for design flows, required minimum velocities and a common static pressure. Use balancing dampers to avoid ducting replacement where possible. However, dampers are an energy loss, so avoid them where possible and practical.
• Reconfigure fan locations if needed (match fans best to subsystems).
• Install variable speed or inlet guide vane control on fans.
• Install gates on each major line.
• Install demand sensors on each machine center, like current transducers (CTs) or motion sensors. In sandblasting booths, we recommend compressed air flow meters. They should indicate that production is needed at that location.
• Install on-demand controls to determine when each major branch needs flow (first machine center on) or can be shut off (last machine center off). Controls must maintain proper velocity at each major branch and vacuum level.
• Control miscellaneous transfer fans optimally.

A Fictional Audit at a Secondary Wood Products Manufacturer

We will create a fictional project that is a composite of several that we have seen. A secondary wood products manufacturer has two major areas: planing/sanding and cutting. Total flow is currently 47,000 cfm and requires a 200-hp motor to be 90 percent loaded to 141 kW. It runs 20 hours per day, 6 days a week. This costs over $52,000 a year in electricity at$0.06/kWh. Most of the system has excessive velocity, except the cutting area’s main branch line and the planer line. It is dusty in the planer area, and OSHA has flagged it for potential explosive hazard.

Baseline Project

An energy auditor has recommended reducing fan speed and changing sheaves to drop the velocities in the system, but did not realize that the planer area was not compliant. The plant engineer rejected the idea. After further velocity measurements, the plant engineer determined that the cutting area’s main line needs an increase in velocity to avoid being sub-critical.

Conclusions

In conclusion, on-demand controls in industrial dust collection vacuum systems can save significant energy. Implemented properly, they are a robust, common sense design that will be able to adapt to the constantly changing production needs of the plant and still optimize for compliance and energy efficiency. But the system has to be audited first, and a baseline project should be developed to address the safety compliance issues and provide a comparison for energy savings and incentives calculations.

For more information, contact Tim Dugan, P.E., President, Compression Engineering Corporation, tel: (503) 520-0700, or visit www.comp-eng.com.