Atmospheric Pressure

The Earth is 7,900 miles (12,715 kilometers) in diameter and is enveloped by a layer of gases about 60 miles (96.6 kilometers) thick which is called the atmosphere.  This mixture of gases is comprised of 78% nitrogen and 21% oxygen plus trace amounts of many other gases which collectively make up the atmospheric “air” that we all breathe.  

The Earth’s gravitational field holds the atmosphere so that it rotates in unison with the Earth and the atmospheric pressure exerted at any altitude is simply the sum of the weight of all the air molecules in a column above that point.  As altitude increases, air density decreases and there will be fewer molecules in the shorter column above the measurement point.  It is easy to see why atmospheric pressure decreases with increasing altitude.  At an altitude of 62 miles (100 kilometers) and beyond, atmospheric pressure approaches zero.  Even in deep outer space there are still a few gas molecules per cubic mile so a true absolute zero pressure is not achieved even though it is very close.

The International Standard Atmosphere (ISA) is defined as a mean atmospheric pressure of 29.92  inHg (760 mmHg) at 59°F (15°C) in dry air at sea level.  Other equivalent units are 14.72 psi, 1 bar and 101.3 kPa.  To complicate matters, the instrument used to measure atmospheric pressure is a barometer and atmospheric pressure is commonly called barometric pressure so the two terms can be used interchangeably.

In addition to altitude, atmospheric pressure is affected by air temperature, local weather conditions and other variables to a lesser extent.  The atmosphere is disturbed by weather systems which can cause either “high” or “low” pressure systems by increasing or decreasing the local atmospheric layer thickness.  What we usually hear from a weather forecaster is that the barometric pressure is “falling” and bringing in a storm, or, that the barometric pressure is “rising” so sunny days are forecast.


Vacuum

Vacuum is simply a pressure that is less than the surrounding atmospheric pressure.  Essentially it is a difference in pressure, or differential, that can be used to do work.  Since vacuum is by definition a negative pressure, the common terminology of high-vacuum and low-vacuum can be confusing. The preferred terminology is deep-vacuum or shallow-vacuum.  Both of which are relative to local atmospheric pressure.  The units of measure for positive pressure and vacuum pressure are the same but a minus sign (-) or the word “vacuum” signifies a negative pressure relative to atmosphere.   

A vacuum gauge has a calibrated mechanism that is referenced to local atmospheric pressure so the value displayed is the amount that the measured pressure is below atmospheric pressure.  This is convenient since the measured “gauge” vacuum level is the vacuum pressure differential that is available to do work and can thus be used directly for calculations of vacuum force which is directly proportional to vacuum pressure and the sealed area upon which it acts.



The relationship between atmospheric pressure, positive gauge pressure, sub-atmospheric pressure (vacuum) and absolute zero is shown in the previous drawing.  An absolute measurement is always positive because it is referenced from absolute zero.  A sub-atmospheric pressure line is shown where the absolute pressure is constant over a three-day period.  A sine curve represents the normal variation in atmospheric pressure that could occur over the same three-day period.  Vacuum pressure is measured from the atmospheric pressure curve down to the sub-atmospheric pressure line and it can be readily seen that the magnitude of available vacuum pressure is different for each of the three days.  In effect, the ability to do work (pressure differential), changes in accordance with the atmospheric (barometric) pressure.  This is why we recommend using a mid-range rather than a deep vacuum pressure when designing vacuum systems.

On Earth, a vacuum is not self-sustaining since seals leak and most materials are minutely permeable.  Over time, enough air molecules will be pulled through the material that the vacuum will be “lost” due to equalization with atmospheric pressure.  To maintain a vacuum for a long time period, a vacuum pump must periodically evacuate air molecules to maintain a desired vacuum pressure.  Depending on material permeability (porosity), continuous evacuation may be required to maintain a desired vacuum pressure.


Vacuum Flow

The performance of a vacuum pump is defined by its’ performance curve which is simply a plot of the vacuum flow rate that it is capable of producing at a particular vacuum pressure.  As vacuum pressure increases, it becomes more difficult to remove (pump out) additional air molecules, so vacuum flow rate decreases until it becomes zero at the deepest attainable vacuum pressure.  Vacuum flow rate will always be highest at atmospheric pressure (zero vacuum) where the pump is under no load.  Many pump manufacturers advertise the efficiency of their pumps with this misleading number.  In reality this specification is meaningless since force can’t be developed and work can’t be done unless vacuum pressure is being created.


Vacuum pressure determines the amount of force that can be developed to hold a work piece or to carry a load.  For a sealed system with no leakage, the two main concerns are; how much vacuum pressure is needed and how quickly can the system be evacuated to the required vacuum pressure?  Since the system is sealed, using a larger vacuum pump will reduce evacuation time but will not increase the system vacuum pressure since, given enough time, even a small vacuum pump will attain maximum vacuum pressure.  A larger vacuum pump will consume more energy without increasing the system load capacity so it is important to not over-specify vacuum pump capacity for a sealed system.



However, when the work piece is porous (permeable) or the system otherwise leaks, the vacuum pump must produce enough vacuum flow rate to overcome the leakage and still attain the necessary vacuum pressure.  The pump must also have enough excess capacity to overcome possible future variations in work piece porosity – we have found corrugated board porosity variations of 4:1 among vendors supplying boxes to the same end user.  

System porosity flow increases directly with increased vacuum pressure while pump flow decreases with increased vacuum pressure in accordance with its’ performance characteristics.  As a result, doubling the vacuum pump capacity in a porous system will double the energy usage (air consumption) but will only cause a smaller incremental increase in vacuum pressure.  At deeper system vacuum pressures the diminishing-returns effect becomes more pronounced so this is another reason to design systems for proper operation at mid-vacuum pressure by simply increasing the effective area upon which the vacuum pressure acts.    

We offer free porosity evaluation and assistance with vacuum pump selection.  EDCO USA will do the calculations for you and help you select the correct pump for your application.


Vacuum Generators – Air-Powered Vacuum Pumps

A vacuum pump is a device that is capable of evacuating (removing) air molecules from a closed volume allowing a less-than-atmospheric pressure condition to be attained. Compressed air-powered vacuum pumps are also called vacuum generators and can be simple single-stage pumps (venturi) or more complex high-flow, multi-stage ejector designs. EDCO USA manufactures a range of single-stage and multi-stage vacuum pumps which helps us recommend the best pump for your application without bias.
 
Vacuum pumps are designed to evacuate a specific percentage of air molecules to attain a vacuum pressure that is dependent upon the available atmospheric pressure. For example: a pump that can attain an 80% vacuum will develop 23.9 inHg (760 mmHg) when barometric pressure is 29.9 inHg while the same pump will only develop 20.7 inHg (524 mmHg) at 4,000 feet above sea level where the local barometric pressure will be approximately 25.8 inHg (655 mmHg). Local weather conditions can also reduce vacuum pressure. When barometric pressure drops from 29.9 inHg to 28 inHg during a storm, vacuum pressure will also drop. It is important to realize that vacuum pressure fluctuations are a normal characteristic of all vacuum systems and are not necessarily caused by a problem with the vacuum pump.
 
To minimize the effect of vacuum pressure variations, we recommend that systems be designed for mid-range vacuum levels of 12 – 18 inHg (305 – 457 mmHg) that are consistently attainable no matter what the weather conditions may be.
 
Air-powered vacuum pumps are compact and lightweight allowing them to be mounted close to the point of vacuum usage which minimizes the internal volume of vacuum hose and tubing. Vacuum is produced immediately when compressed air flows into the pump eliminating the need to turn the pump on long before contacting a workpiece as is common with electro-mechanical pump systems.

Electro-Mechanical Vacuum Pumps

Premature wear will result from frequently starting and stopping an electro-mechanical vacuum pump. They are primarily suited for systems requiring constant vacuum flow. The pump must be powered continuously for this type of use. Most types of electro-mechanical vacuum pumps are also not suited for operating at maximum vacuum and zero flow conditions which causes poor lubrication and over-heating of the pumping mechanisms.
 
Electro-Mechanical vacuum pumps tend to be noisy, bulky, heavy, and hot. Because of this, they are usually mounted a considerable distance away from the point of vacuum use. In order to be used in a pick and place system, several additional components are often required. Additional components required include a motor starter, vacuum relief valve, exhaust muffler, large diameter vacuum hoses, three-way vacuum control valve, etc. Collectively, these components and the associated assembly labor add substantially to the installed cost of a vacuum system. On top of this, each additional component comes with the inherit risk for potential failure. Operating costs are also increased as electro-mechanical pumps are high-maintenance items and must be overhauled frequently.
 
Electro-mechanical pumps efficiently convert electrical power into vacuum flow and pressure. Because they must run continuously, they can’t take advantage of the system duty-cycle to reduce overall energy consumption. However, for systems requiring constant, large vacuum flows, they may be the best solution.

Duty Cycle and Energy Consumption

During a pick and place cycle, a vacuum source is turned on for the pick and remains on during the travel to the place location. Once the workpiece is in the correctly location to be placed, the vacuum source is turned off. Vacuum is not necessary for the travel back to the home position nor for the dwell time before the next pick is required. If vacuum is on for 1/4 of the full machine cycle, then the duty cycle is 25%. An air powered vacuum pump consumed compressed air only while it is creating vacuum. In this example, the average air consumption would be reduced to 25% of the cataloged pump air consumption rate whereas an electro-mechanical vacuum pump must run continuously and consumed energy 100% of the time.
 
Whenever an adequate supply of compressed air is available, especially if the system has an intermittent vacuum requirement or duty cycle, it’s best to consider an air powered vacuum pump.


Air Powered Vacuum Pump Control Methods

On / Off

Air powered vacuum pumps can be simply controlled by a single air valve. When air is supplied to the pump, vacuum is supplied to the system. When the air supply is stopped, atmospheric air is drawn into the vacuum system through the pump exhaust to dissipate vacuum and release the workpiece. A three-way valve mounted close to the vacuum pump is recommended for fast operation.

Blow-Off

A compressed air assist will provide a faster workpiece release for high-speed systems. A stored-volume, automatic blow-off is commonly used for small systems and consists of a volume chamber that is charged with the same air supply that operates the vacuum pump. When the three-way air supply valve is turned off, a brief pressurized air pulse from the chamber is directed into the vacuum system and the workpiece is quickly released. For larger systems or systems requiring more control, an air valve can be connected to the vacuum system via a release check valve that prevents loss of vacuum through the blow-off air valve. The blow-off pulse duration is controlled by how long the blow-off air valve is left on. During the blow-off mode, a flow path exists from the vacuum system to atmosphere via the pump exhaust port. It is normal for air to escape this way. This also means that no significant positive pressure can be developed in the vacuum system. Long restrictive tubing lengths to vacuum cups may cause workpiece release delays.
 

Energy Saving

For sealed vacuum systems, a non-return vacuum check valve can be added to prevent back-flow from the vacuum pump exhaust when the pump air supply is stopped. This allows the vacuum pump to be cycled on until a desired vacuum pressure is achieved and then turned off to conserve energy (compressed air). A vacuum switch senses when vacuum pressure has decreased and cycles the pump on to restore the vacuum pressure. A separate vacuum volume chamber can be added to decrease the “leak-down” rate but proper energy saving system operation still entirely depends on maintaining a sealed system. If the system will handle a porous workpiece, do not use an energy saving control.


Vacuum Cups

Vacuum cups are sometimes called suction cups. This is not entirely accurate as the cups cannot create suction on their own. A vacuum cup requires a vacuum source to grip a workpiece. Most vacuum cups are round because that is a strong shape that resist collapse under vacuum pressure and load force is efficiently distributed through the cup walls to the fitting. A circular shape also provides the greatest area for the space it occupies. Industrial vacuum cups usually employ a metal fitting for mounting the vacuum cup and for connecting a vacuum source to allow the inner volume to be evacuated.
 
Vacuum cups are made of rubber and included a flared lip to form a flexible seal against a workpiece allowing the cup to be evacuated with a vacuum pump. Several cups can be connected to a central pump or a small vacuum pump could be used for each cup. When the vacuum cup is evacuated, an attraction force is developed that holds the cup to the surface of the workpiece. For a vertical cup axis, this attraction force is the lifting capacity. If the load is perpendicular to the cup axis (shear load), the attraction force must be multiplied by the appropriate coefficient of friction to determine an allowable shear load. In either case, an additional factor-of-safety must be applied for prudent design. When rapid movement occurs in automation systems, a designer must consider the combined magnitude of both lifting and shear loads when selecting components.
 
Depending on the contours of the workpiece, the allowable vacuum cup diameter may be limited. Multiple vacuum cups may be required to increase the total area and achieve a desired load capacity plus a generous factor-of-safety. We do not recommend increasing the required vacuum level to make a system work. Instead, increase the number or size of vacuum cups to increase the total effective area making it large enough for proper system design. Vacuum cups are relatively inexpensive making additional vacuum cups a cheap insurance against potential system failure.
 
The vacuum force equation F = P x A (force = Pressure times Area) is difficult to apply to rubber vacuum cups because the cups are approximately sized according to the outer lip diameter. This can be misleading as it is much larger than the actual effective diameter that the vacuum pressure acts upon. A rubber vacuum cup also changes shape under load. Because of this, the effective area varies somewhat depending on the vacuum level inside the vacuum cup. It is more expedient to use the rated force at a specific vacuum pressure from the vacuum cup specification tables.
 
The force equation can be useful for vacuum “clamps” where a cavity with a seal formed around the perimeter is used to hold flat workpieces such as wood or stone. The area with the seal can be calculate with some degree of accuracy. Equation units must be consistent with each other. Vacuum pressure must be converted to the appropriate unit of measurement.
 
Three points define a plane. For good stability, use three or more vacuum cups spaced apart as far as possible. Start with the largest cup size that can be reliably placed on the workpiece then increase the number of vacuum cups until a suitable factor of safety is achieved. For handling boxes and other containers, apply the vacuum cups in corners and near the outer vertical walls. Remember, the box contents sit on the box bottom so the weight load is transferred to the box top via the side walls.

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