A: Jack Osborn, Airdusco, says:
What you’re experiencing is common for many centralized vacuum cleaning systems (CVCS). The systems that I’ve reviewed and audited have all had similar reasons for their constant plugging, poor performance, and high maintenance.
Since the publication of the NFPA 652: Standard on the Fundamentals of Combustible Dust (2016) and OSHA’s increased enforcement of the NFPA standards through their March 2008 National Emphasis Program (NEP) on combustible dusts, installing a CVCS has become a housekeeping requirement for facilities handling combustible dusts.
NFPA 652 defines a CVCS as a fixed-pipe system that uses a variable-volume negative-pressure (vacuum) airflow from remotely located hose connection stations to remove dust accumulations from surfaces. The CVCS then conveys the collected dusts to an air-material separator. A typical CVCS consists of hoses and tools, piping or tubing, a filter-receiver (basically a round dust collector), and a filter-receiver material discharge method, and a vacuum producing device, which can be a centrifugal positive-displacement (PD) exhauster.
A properly designed CVCS should provide years of reliable and consistently effective service. However, it’s been my experience that most CVCS are poorly designed for the actual application. The following provides examples of what are the typical problems to avoid.
Insufficient airflow for the vacuum hose. Often there’s stiff competition among suppliers for a project and one of the most cost-reducing methods for a proposal is to use lower airflows that will inherently reduce costs. Simply put, the effectiveness of a CVCS to vacuum material is based on the third power of the airflow velocity into the hose or tool. For example, if your 1.5-inch hose is designed for 75 scfm, the inlet velocity is about 6,100 fpm. This may sound terrific but bear in mind that the inlet velocity for 90 scfm is approximately 7,300 fpm. One would think the 6,100 fpm (75 scfm) hose would perform about 84 percent as well as the 7,300 fpm (90 scfm) hose. However, this isn’t the case. The efficiency is based on the third power of the ratio. When 6,100 is divided by 7,300 and taken to the third power, the result is an efficiency of only 58 percent. In reality, this means the 75 scfm airflow will take about twice as long to vacuum clean the same area as the 90 scfm airflow. This results in a significant difference in actual performance compared to the relatively small difference in airflow.
Multiple simultaneous operators. A problem can occur when a system is designed for only the maximum simultaneous operators and the design ignores that the system often only has one operator using the entire system. For example, let’s assume your system is designed for three simultaneous operators using a 2-inch hose at 160 scfm each. Also, let’s assume the system is designed to operate at 11-inch mercury (hg) vacuum. Since air expands under vacuum, the airflow volume for three simultaneous operators will be up to approximately 760 icfm at the vacuum blower inlet. In a 4-inch main line to the filter-receiver, this would result in a velocity more than 8,000 fpm. However, with only two simultaneous operators, this velocity drops to approximately 5,800 fpm. Unfortunately, with only one operator, the actual airflow (limited by the 2-inch hose and initial 2.5-inch tubing) in the 4-inch header (main piping to the receiver) is only in the 3,100 fpm range. This latter value will lead to eventual plugging of the 4-inch line. This is a common problem and is specifically not allowed for combustible dusts as per the NFPA combustible dust standards.
Improperly sized filter-receivers. Too often the system is designed for the standard conditions (scfm) versus the actual airflow (acfm) through the filter. This results an under-sized unit and will inherently reduce airflow. As an example, if the system is designed for 11-inch hg vacuum and the incoming airflow to the hoses is 90 scfm each with three simultaneous operators, the typical mistake is to design the filter receiver unit for 270 cfm instead of the expanded cfm due to vacuum, which in this case is approximately 430 acfm. A filter designed for 270 cfm would likely have only 54 square feet of filter (5-to-1), but in reality would have to handle approximately 430 acfm at an 8-to-1 ratio. This is excessive and would likely lead to failure.
Improper use of a vacuum blower selection. PD vacuum blowers are often preferred due to their performance characteristics. However, they’re limited to fewer simultaneous operators. Centrifugal CVCS units would be required for a higher number of simultaneous users. A PD vacuum blower inherently has a limited range in airflow. If a system is designed for three simultaneous operators at 430 acfm, but only one operator is on the system, the result would likely be 200 acfm or less. This would cause the relief valve of the PD blower to open as the volume is insufficient to prevent damage to the PD blower. A centrifugal vacuum blower (multistage) is designed for this situation with variable airflow volume. Conversely, a PD blower will tend to try to unplug a conveying line by increasing vacuum at approximately the same airflow. A centrifugal unit will back up the curve to increase differential pressure but at an inherently lower airflow. Each vacuum blower has its use, but it must be applied properly.
These are just some a few factors to consider when designing a CVCS. Work with a qualified supplier to design the most appropriate method for your cleaning needs.
Jack Osborn is the engineering manager at Airdusco.