Save your energy
November 16, 2007
By Josef Karbassi
Reduce energy consumption on the plant floor with an optimized vacuum system
Efforts to improve efficiency over the past century have often focused on the reduction of waste, defined as processes and resources that represent direct costs but do not add any value.
The elimination of waste is at the core of the lean manufacturing management philosophy, which was famously implemented at Toyota with a focus on doing more with less.
The shift toward lean manufacturing, coupled with the need to comply with increasingly strict environmental regulations and growing expectations for corporate responsibility, is driving automotive manufacturers to run more than one car model on the same production line in the most energy-efficient manner.
Sheet metal stamping is one area of the automotive manufacturing segment in which many manufacturers are particularly focused on improving efficiency. This application, in which sheet metal is moved through a press or series of presses and formed into a panel or car part, uses a large number of vacuum components and demands a high amount of energy.
Additionally, the tooling that is used to handle the metal pieces, and thus the placement of the suction cups, requires different specifications each time a new vehicle model or car part is introduced. As a result of these frequent changes, sheet metal stamping applications require material handling solutions that are both flexible and energy efficient.
System design factors to consider
When evaluating the performance of sheet metal stamping applications, consider several design factors. First, analyze the physical positioning of the vacuum technology that powers the system. Traditionally, automotive manufacturers have relied on centralized vacuum solutions for the material handling operations between the presses. These systems are based on suction cups and air-driven single-stage ejectors with built-in control valves and vacuum switches.
In a typical centralized system, each lifting device uses two ejector units to improve flexibility and safety. The ejector unit also contains an air-saving feature that will turn off the ejector when the desired vacuum level is achieved to reduce air consumption.
On the surface, these systems run fairly well, except for the fact that the compact ejectors often switch on or off due to micro leakage in the system. But there is always room for improvement.
To effectively evaluate the overall efficiency of an existing vacuum system, it is helpful to focus on one station in the press line and begin by estimating the cost for compressed air per year in that station.
Evaluating an existing solution
The following data illustrates how an optimized vacuum system on a typical sheet metal stamping application–if it is designed appropriately–can reduce energy consumption by at least 95 per cent when compared to traditional vacuum system technology.
Table 1, below, illustrates a sample “vacuum audit” of a traditional dual-channel system with two compact ejectors.
Traditional dual-channel systems consume a significant amount of compressed air, which results in a considerable energy investment for the manufacturer. If the plant has 15 press lines, each with eight stations with similar vacuum handling systems, and each one of them has a dual compact vacuum pump installation, the total energy cost for compressed air to the ejectors would be: 15x8x507 = $85,401.11 (see Table 1).
An alternative
To best put this data into perspective, it is helpful to imagine a typical stamping plant scenario. For example, a production manager at a stamping plant for automotive parts may be tasked with having to find a way to lower the total cost for compressed air to ejectors/pumps in the plant below a level of $42,111 per year (< 50 per cent) and also improve response and release times. One way for the engineer to reduce energy consumption would be to switch from a traditional centralized vacuum system to a fully decentralized vacuum system, where the vacuum is generated independently for each suction cup at the point of use.
The main advantages with decentralized vacuum systems in stamping are:
• Lower energy consumption;
• Quicker response time;
• Faster release of object (break vacuum);
• Shorter time with vacuum pump “on” before the actual vacuum duty cycle;
• Short time for blow-off/release;
• Higher system reliability in case of leakage thanks to independent cups;
• No risk for false signals if vacuum switches are used;
• No risk for oversized vacuum pumps to compensate for pressure drops and extra volume, and;
• Reduced/eliminated risk for micro leakages from fittings and couplings that will have a bad effect on an air-saving function.
A key factor to consider when choosing a decentralized vacuum system is the quality of the air-saving features that are offered. To fully optimize a decentralized solution, you must use the right combination of air-saving multistage ejectors, vacuum valves and release valves.
Trimming PLC programming
An additional opportunity to cut costs during stamping applications lies in the opportunity to trim PLC programming so that it shortens the time the vacuum is on before the actual pickup. In a centralized system, the “on” period before the pickup needs to be longer to avoid a slow vacuum response.
An optimized decentralized vacuum can offer cost savings of $677.52 per year if implemented at one station in a typical press line, which translates to a 95 per cent reduction in energy consumption and an annual savings of $80,853.12 to the plant (see Table 2, below).
Additionally, an optimized decentralized vacuum system provides a 65 per cent faster vacuum response and 86 per cent faster release. This can shorten the cycle time in the press line by 0.22 seconds, so the new cycle time would be 5.78 seconds, which will lead to10.38 strokes/minute. Moreover, while the original system would produce 600 parts/hour, the new system will allow for the production of 622 parts/hour–an enormous opportunity to increase revenue.
Josef Karbassi is the business unit manager, automotive industry, at PIAB.
Traditional dual-channel systems
General conditions
Number of suction cups:………………………………………….. 8
Suction cup volume:…………………………………………0.24 lit
(30 cm3 per suction cup)
Total system volume: …………………………………………0.8 lit
(incl. hoses, fittings and manifolds)
Machine operating hours per year:……………………. 6,000
Cycle time…………………………………………………………..6 sec
(10 strokes per minute)………………………………………………
Vacuum duty cycle……………………………………………… 3 sec
Blow duty cycle……………………………………………….. 0.2 sec
Vacuum “on” before actual pick………………………. 0.4 sec
Cost to produce 1m3 of compressed air:…………….. $0.02
(average value)
Vacuum pumps
Number of compact vacuum pumps:………………………… 2
Feed pressure:………………………………………………..0.5 MPa
Nozzle diameter, vacuum:………………………………..2.5 mm
Air consumption, vacuum:………………………….660 Nl/min
(both pumps continuously)
Air consumption, blow-off………………………….450 Nl/min
(both pumps continuously)
Air Saving system:………………………………………………….Yes
No. of recoveries per vac. duty cycle:……………….. 2 times
(due to micro leakage in the system)
Vacuum pump “on” during recovery……………….0.15 sec
Result
Performance, Air-Consumption and Cost
Vacuum response time to -60 kPa:…………………0,230 sec
Actual release time:………………………………………0,085 sec
Total air consumption per year:………… 42,228 m3 of air
Cost per year for one station…………………………..$711.68
If the plant has 15 press lines, each with eight stations with similar vacuum handling systems, and each one of them has a dual compact vacuum pump installation, the total energy cost for compressed air to the ejectors will be 15x8x507 = $85,401.11
An optimized decentralized vacuum system
General conditions with a decentralized system
Number of suction cups:………………………………………….8
Suction cup volume:………………………………… 0.03 lit/cup
Total system volume:……………………………………. 0.24 lit
(30 cm3 per suction cup)
Machine operating hours per year:……………………6,000
Cycle time…………………………………………………………6 sec
(10 strokes per minute)…………………………………………….
Vacuum duty cycle…………………………………………….3 sec
Blow duty cycle…………………………………………………0 sec
(Atm quick release)
Vacuum “on” before actual pick……………………..0.1 sec
(PLC trim)
Cost to produce 1m3 of compressed air:…………….$0.02
(average value)
Vacuum pumps
Number of COAX-Vacustat units:…………………………….8
Feed pressure:…………………………………………… 0.32 MPa
Air consumption, vacuum:…………………………211 Nl/min
(all units continuously)
Air consumption, blow-off……………………………0 Nl/min
(Atm Quick Release)
Air Saving system:……………………………………………….Yes
No. of recoveries per vac. duty cycle:…………………….0 *
Vacuum pump “on” during recovery……not applicable
* The micro leakage in the vacuum check valve is not big enough to reduce the vacuum level beyond the built-in hysterisis during the vacuum duty cycle of 3 seconds.
Result
Performance, Air-Consumption and Cost
Vacuum response time to -60 kPa:……………….0,081 sec
Actual release time:…………………………………….0,012 sec
Total air consumption per year:…………. 2,281 m3 of air
Cost per year for one station…………………………..$38.11
If the new solution is implemented in the entire plant, the cost for compressed air-to-air vacuum units will be: 15x8x27 = $4,573.26
