Central Vacuum Systems
Design and Sizing are Key Factors in Moving Material Effectively and Efficiently
Besides good grain cleaning and dust control, one of the key components that greatly helps in delivering effective and top-notch housekeeping results in a milling facility is a central vacuum system.
First, it’s important to keep in mind that a dust collection system is not the same as a central vacuum system. Typically, dust collection systems have larger air volumes and lower dust loading or handling capabilities, while a central vacuum system operates under lower air volumes but has much higher dust handling capabilities.
From the perspective of Kice, a central vacuum system is a custom-designed network of tubing, fittings, and elbows, which typically go back to a centrally located baghouse filter and vacuum device (e.g., high static fan, multi-stage blower, turbine, or vacuum air power unit).
The central vacuum system provides a high vacuum to the end of a hose with a variety of attachments for cleaning floors, walls, ducts, and around milling and other processing equipment. The system also moves bulk materials in the event of spillage, equipment cleanout, etc.
In any central vacuum system, there are two important factors that influence its performance. They include:
- Airflow, which is expressed in cubic feet per minute (cfm).
- The pressure, more commonly referenced as vacuum, which often is expressed in inches of mercury, inches of water column, or in pounds per square inch (psi).
Special instruments like a manometer or Bourdon gauge can measure this force and express it regarding inches of mercury or water. While the airflow measurement indicates how much air the central vacuum system is moving, the pressure or vacuum measurement indicates the system’s peak draw or force.
Striking a good balance between airflow and vacuum to optimize performance is why sizing and the design of the central vacuum system are so important. A central vacuum system generally will work well if designed and sized properly and has good, high velocity at the tool head, with enough velocity and suction force to pick up and transport the material. Also, a system will operate more efficiently if there are the correct number of users on the system, and if the system is sealed, grounded properly, and maintained properly. While central vacuum systems have become more commonplace, keeping them operating well and trouble-free can be challenging.
So, what if the system isn’t working properly and efficiently? The chief causes might be:
- Lack of suction force.
- Too many operators using a system simultaneously.
- Open points (snap caps become loose, loose couplings, or connection points that cause air leakage).
- Dirty filter bags or vacuum pump inlet filters.
- Improper design (e.g., too many sharp angles or elbows on the vacuum lines).
- Not grounded properly.
- Not letting free air in around the suction point.
- Trying to lift too much material beyond the system’s capability.
Essentially, there are three key things that can impact the efficiency and proper functioning of a central vacuum system. They are:
- Common resistance points
Sizing and Design Basics
Kice has done extensive testing on various tools (e.g., crevice tool, brush, floor tools), hoses (coiled and uncoiled), and the vacuum air power unit components, including filters, silencers, and check plates at various cfm rates and pressures.
Generally, a system is sized and designed to accommodate the farthest point of pickup and perhaps the worst-case scenario. Sizing a system properly needs to take into account what type of materials it will handle. For example, flour mills and general systems, which handle lighter products and powders in smaller amounts, may be sized to deliver 200 cfm at a vacuum pressure of eight inches of mercury (Hg) per operator.
In contrast, grain elevators, feed mills, and seed facilities, which typically handle heavier products, whole seeds, and larger amounts of denser products, might require 200 cfm at 10 inches of Hg vacuum per operator.
The system also should be sized for realistic conditions. For example, don’t go to extremes on sizing a system for too many operators that may lead to inefficiencies. Oversizing a system leads to excessive horsepower and unnecessary noise.
The distance between the vacuum lines isn’t always super-critical, but like with many mechanical situations, there are limitations. Generally, running multiple three-inch outer diameter (OD) lines as needed throughout the facility is sufficient. Another rule-of-thumb is that the number of pickup locations served by a single three-inch outer diameter header pipeline is not super-critical. However, the critical thing is the number of users on each three-inch OD line.
Kice recommends that a system should have a maximum of two operators/sweepers on a three-inch OD line. With one or two operators, the vacuum suction remains good; however, when more than two operators are using a dedicated line, the suction force will likely start to drop for all the operators.
So, it’s important to design the system to accommodate the appropriate number of operators to avoid overtaxing the system’s capabilities. In sizing a system, a motor horsepower rating that will meet the vacuum relief setting established by the original equipment manufacturer is recommended. This will help minimize the risk of the motor overamping in a closed or clogged situation. When you get a clog in the system or extensive blockage, the vacuum relief valve is going to activate. The vacuum may be high, but the airflow through the relief valve will be minimal.
The end result is that the amps go up, which puts undue strain on the motor. Kice sizes for the release valve setting. In cases where motor overamping occurs, it might be due to the relief valve’s setting being a little too high or the motor’s horsepower a little too low.
For this reason, it’s important not to skimp on the motor’s horsepower rating to serve a given central vacuum system that meets the relief valve setting. Below are some typical horsepower requirements for a system serving a different number of operators.
- One operator 10-15 hp.
- Two operators 20-30 hp.
- Three operators 30-40 hp.
- Four operators 40-50 hp.
- Six operators (at the same time) 50-75 hp.
On the other hand, sizing for a four or six-operator system that then only uses one or two operators is wasteful. The vacuum relief valve is pulling in free air to keep the pump safe but uses extra horsepower.
Common Resistance Points
Besides sizing a central vacuum system properly to meet certain performance expectations, it’s also important to keep in mind some of the critical points in the system that may generate resistance and that potentially can reduce efficiency.
- Coiled hoses
- Overly long hoses that aren’t necessary to fulfill a given task
- Damaged hoses
- Damaged tool ends
- Clogged hoses
- Large, bulky piles of product that might choke the system
- Wet or moist product that can create undue resistance points in the system
- Open snap caps
- Coupling leaks
- Dirty filters
- Open relief valve
In laying out a central vacuum system, it’s important to avoid sharp twists or turns (e.g., mitered elbows) that will hinder the airflow and potentially lead to material buildup and blockage at these critical points.
Any back-to-back 90-degree angles will restrict the system’s pressure.
Planning for the Worst-Case Scenario
To help embrace what can impact a system’s efficiency, let’s run through some of the factors that come to bear. What follows is a basic accounting of how things add up when considering a worst-case scenario example with a system that is sized to provide 200 cfm at a pressure of eight inches of Hg. In the worst-case scenario, approximately 2.4 of those eight inches of Hg likely would be accounted for by the baghouse filter with dirty filter bags and the total resistance of the vacuum air power unit with a dirty inlet filter.
Another 2.1 inches of Hg might be accounted for from 25 feet of coiled hose and a crevice tool. So, already at this point more than half – or 4.5 inches of Hg – of a rated design of eight inches of Hg is being tied up due to dirty filter bags, dirty inlet filter, and a coiled-up hose equipped with a crevice tool (the worst-case scenario). Another 1.8 inches of Hg also could be attributed to the resistance created inside the three-inch tubing, elbows, and branch fittings used in a seven-story facility, and that is without moving any dust yet.
So, when adding it all up, out of a total of eight inches of Hg, approximately 6.3 inches of Hg is tied up under this worst-case scenario. This is why design, sizing, and keeping a central vacuum system free from obstructions is critical in moving the maximum amount of material effectively and efficiently.
Ways to Improve Performance
What can you do to improve performance:
- Take the time to calculate and understand the vacuum capabilities of the system’s design
- Remember that a central vacuum system is for cleaning; it’s not meant for large-scale conveying of product.
- Don’t forget that there will be more suction force closer to the filter.
- Limit operators to two on each three-inch OD header
- Reduce restrictions, including the use of more than 25 feet of hose(s)
- Check for leaks, open caps, and couplings regularly.
- Do not pull or kink
- Install a spring cap on each 45-degree elbow, so the hose does not start with a kink from the vertical or horizontal position when picking up piles of product.
- Allow easy access to air in and around the crevice tool.
- Verify grounding and continuity of the system.
- Perform regular maintenance of inlet filters and filter bags.
- Install branches correctly (e.g., either upward or horizontal, so branch legs do not fill up with material over time).
- Install cleanout ports at corners and long tubing runs.
- Have a good complement of tools like crevices (rigid and flexible), gulpers, and bushes to handle various tasks.
- Consider using static dissipative hose to mitigate static electricity build-up.