Product Manager – Pneumatic Actuators
Festo Corp. • Hauppauge, N.Y.
Vacuum generators come in an extensive array of types, sizes, designs and efficiencies to suit widely ranging applications. And while maximizing performance for a given task is important, anyone using a vacuum system should also take a close look at overall costs before specifying essential components.
Given that energy is a valuable and, at times, expensive resource, the cost of running a suitable vacuum system should play an important role in any design. For circuits based on vacuum ejectors, the amount of energy necessary to operate a pneumatic vacuum ejector with compressed air cannot be overlooked. Always remember one golden rule at all times: air is expensive.
With electric-motor-driven vacuum pumps, on the other hand, users can measure and assess energy costs much more easily based on prevailing electricity prices and electrical power consumption. Let’s take a closer look at these two basic types of vacuum generators, and how they compare on the bottom line.
Vacuum ejectors basically generate vacuum using pneumatically driven nozzles without moving parts. They produce high vacuum at relatively low flow rates.
A classic ejector consists of a jet nozzle (also called a Laval or venturi nozzle) and, depending on the design, at least one receiver nozzle. Compressed air enters the ejector and a narrowing of the jet nozzle accelerates the flowing air to up to five times the speed of sound. The ejector has a short gap between the jet-nozzle exit and the entry to the receiver nozzle. Here, expanded compressed air from the jet nozzle creates a suction effect at the gap which, in turn, creates a vacuum at the output vacuum port.
Vacuum ejectors come in two basic versions, single and multi-stage. A single-stage ejector includes a jet nozzle and only one receiver nozzle. A multi-stage ejector also includes a jet nozzle. However, in line with the first receiver nozzle are additional nozzle stages, each of which has a larger diameter in proportion to the falling air pressure. Air drawn in from the first chamber, combined with compressed air from the jet nozzle, is thus used as a propulsion jet for the other chambers. In both versions, air exiting the receiver nozzle is generally discharged via a silencer or directly to the atmosphere.
Among their benefits, vacuum ejectors are compact, lightweight, relatively inexpensive and they respond quickly, with fast start and stop times. They are wear-resistant and essentially maintenance-free, although suppliers recommend operating them with dry and filtered compressed air. And they can mount in any position and experience no heat build-up in operation.
Another advantage is that vacuum ejectors consume energy only as needed. Compressed air to run the ejector is only required during suction and workpiece handling, and remains switched off during discharge and return when no vacuum is required.
Many ejectors also have a built-in energy-saving function that ensures compressed air is only consumed during vacuum generation. After attaining the necessary vacuum level, the ejector is switched off. Vacuum is maintained and monitored using valves and switches. A typical energy-saving function includes a 2/2-way valve, pressure switch and a non-return valve.
In general, multi-stage ejectors can generate pressures up to approximately 85% vacuum and have higher suction flow rates compared with single-stage ejectors. Multi-stage ejectors have, on average, a much lower level of air consumption and thus consume less energy than single-stage ejectors. However, their evacuation time is higher, which in some cases can reduce the energy-saving benefits over single-stage units.
On the downside, vacuum ejectors do not produce extremely high suction rates. Festo ejectors, for example, generate suction rates that are relatively limited at approximately 16 m3/hr. Beyond that, higher compressed air consumption per cubic meter of vacuum increases energy costs dramatically, although designs with energy-saving functions can compensate for this to a certain degree.
Mechanical vacuum pumps generally fall into one of two different types: positive-displacement and dynamic/kinetic. Displacement vacuum pumps essentially operate as compressors with the intake below atmospheric pressure and the output at atmospheric pressure. They draw in a fairly constant volume of air, which is mechanically shut off, expanded, and then ejected. The main feature of vacuum pumps of this type is that they can achieve a high vacuum with low flow rates. Types include reciprocating piston, rotary vane, diaphragm and rotary screw. They are often suited for precision industrial applications.
With kinetic vacuum pumps, gas particles are forced to flow in the delivery direction by applying additional force during evacuation. Rotary blowers, for example, operate according to the impulse principle: a rotating impeller transfers kinetic energy by impacting air molecules. In operation, air is drawn in and compressed on the suction side by the impeller blades.
These vacuum pumps generate a relatively low vacuum, but at high flow rates (high suction capacity). They are usually suited for handling extremely porous materials, such as clamping cardboard boxes, and where large suction rates per unit of time are important.
Among the advantages, typical positive-displacement industrial pumps generate up to about 98% vacuum—beyond the capability of ejectors. (Some specialized designs attain extremely high vacuum levels above 99%.) And blowers can offer high suction rates well beyond 1,000 m3/hr.
However, electromechanical vacuum pumps are almost always in continuous operation with vacuum requirements regulated by valves. This means that electricity consumption and, consequently, energy costs can be quite high. They also have high initial costs and ongoing maintenance expenses. Finally, compared to ejectors, they are larger, heavier, and tend to have more-restricted mounting orientations.
Comparing energy costs
In some cases, the preferred type of vacuum generator for an application is fairly obvious—such as a vacuum blower when the task requires low vacuum and high flow. But in many settings, the choice is not clear-cut. Here, it pays to compare the energy and operating costs of likely candidates.
As noted above, actual performance and efficiency of individual vacuum generators can vary widely by type and manufacturer. Engineers can narrow the options by doing a bit of math. Here are some straightforward calculations to help compare the economics of vacuum generators of similar performance.
In this example, we compare annual energy costs for vacuum ejectors with and without an air-saving function, and for an electrically driven vacuum pump of similar performance. Assume that:
• Electricity price is $0.11/kWh.
• To generate compressed air from atmospheric air, taking into account electricity price and all costs such as material, depreciation, and labor, plan on costs of approximately $0.022 per 1 m3 (35.3 ft3 ) volume at 7 bar (100 psi) supply pressure. These costs apply at pressure range up to about 10 bar. At higher pressures up to 20 bar, the cost for compressed air can easily double.
• Supply pressure for the ejectors is 6 bar.
• Energy used to compressed air (1 m3 at 7 bar) is 0.095 kWh/m3.
In addition, let’s assume the following criteria for purchase and maintenance costs.
• Initial cost of a vacuum pump is $786.
• Initial cost of an ejector is $371.
• Annual maintenance costs for the vacuum pump is $337.
(Note that referencing the initial cost of an ejector in the example would apply to a vacuum generator like the Festo OVEM, which has an integrated control valve to turn the ejector and vacuum on/off. It also has a vacuum pressure sensor and may or may not have the “air-saving” feature. Festo also offers very compact, low-cost generators (VN family) controlled with an external valve. The cost of a VN is approximately $35 (plus a control valve and, possibly, a sensor). Thus, costs will vary between stand-alone generators and generators with integrated valves/sensors.)
Consider a typical handling operation where the equipment runs 250 days/year and 16 hours/day. Each operating cycle takes 5 seconds and individual work steps include evacuation, workpiece pick-up, transport, vacuum discharge, unloading and return to the start position to begin the next cycle. The amount of time required for each step depends on the vacuum generator.
An ejector with an air-saving function consumes air (energy) only while picking up the workpiece, and this requires 0.5 seconds.
An ejector without an air-saving function consumes compressed air during pick-up and transport of the workpiece. This requires 2 seconds.
The vacuum pump consumes energy for the entire operation cycle, as the pump does not normally switch off. Run time per cycle is 5 seconds.
Compare the energy costs for these vacuum generators as follows:
Calculate the number of products (units) per year from total running time (sec)/time per operation cycle (sec) = 250 x 16 x 3,600/5 = 2,880,000 units.
Determine running time per year by the number of units × the run time for an ejector per unit. For an ejector without an air-saving function it is 2,880,000 units x 2 sec = 5,760,000 sec or 96,000 min. For an ejector with an air-saving function, run time is 1,440,000 sec or 24,000 min.
Assume air consumption at p = 6 bar is 505 l/min. Calculate air consumption per year from running time per year/air consumption = 96,000 min/505 l/min = 48,480 m3 for the ejector without an air-saving function. With the air-saving function, it is 12,120 m3.
From this information, calculate energy costs per year based on air consumption (without air saving) x price per cubic meter for compressed air = 48,480 m3 x $0.022/m3 = $1,066.56. Likewise, for the ejector with an energy saving function the annual cost is $266.64.
This shows that an ejector with an air-saving function can cut air consumption by 75% or, in this example, by more than 36,000 cubic meters of compressed air per year. That equates to a savings of $800 per year.
Vacuum pumps driven by electric motors require similar attention. Run time per year = operating hours per day x operating days per year = 16 x 250 = 4,000 hours. Assume a 1-hp electric motor consumes 0.55 kW/hr. Energy consumption per year = running time per year x energy consumption per hour = 4,000 hr x 0.55 kW = 2,200 kWh. Energy costs per year = energy consumption per year x costs per kWh = 2,200 kWh x $0.11 = $242.
Finally, let’s compare overall costs for the vacuum ejectors and vacuum pump. Remember that a vacuum system includes investment, maintenance and energy costs. Investment costs are initial, one-off costs, while maintenance and energy are annual costs.
A typical vacuum pump has an investment cost of $786, annual maintenance cost of $337 (after 4,000 to 6,000 hours of operation) and energy costs of $242. In contrast, an ejector without an air-saving function has an investment cost of $371, energy cost of $1,066 but no maintenance costs. Likewise, the ejector with an air-saving function has an up-front cost of $371, energy cost of $266 and, again, no maintenance costs.
A direct cost comparison shows that the vacuum pump has the lowest energy costs, closely followed by the ejector with an air-saving function. The ejector without the air-saving function has considerably higher energy costs than the other vacuum systems. If we also take maintenance and investment costs into account, this reduces the advantage that the vacuum pump has over the other systems due to its low energy costs.
The example shows that in many applications, ejectors more than justify their existence. The high investment costs for vacuum pumps as well as annual maintenance costs associated with their continuous use and wearing parts confirm this conclusion. While ejectors may use more energy, their simple design keeps initial costs and maintenance costs to a minimum.