Hi Case study: industrial lubricant — Verderhus screw-channel pump from Verder.

Hi Case study: industrial lubricant — Verderhus screw-channel pump from Verder.

‘Industrial lubricant: Verderhus screw-channel pump’

Critical to all types of industrial processes, lubricants and oils require precise blending and gentle handling. When a UK manufacturer of lubricants approached Verder for an answer to its pumping problems, an ideal solution was found in the Verderhus screw-channel pump.

Hi Week's Free Resource: Hi Maintenance Free Linear Motion & Hi Linear Guides For The Next Generation Of Medical Machines.

Hi Week's Free Resource: Hi Maintenance Free Linear Motion & Hi Linear Guides For The Next Generation Of Medical Machines.

Hi Maintenance Free Linear Motion

Various takes on maintenance free linear guides and ways have been around for years now, but not all of them are created equal. Some focus on achieving the longest possible maintenance free intervals. Others focus on adding as little as possible to the size of the bearing's mechanical package. IKO's C-Lube linear guides have a unique design that balances both of these key objectives.

C-Lube linear guides provide maintenance free operation for 20,000 km or five years-a maintenance free term that often equates to the entire application life. For a more detailed overview of how C-Lube works to extend linear guide life, download this white paper.

Hi Download Now.

Hi Linear Guides For The Next Generation Of Medical Machines:

Not too long ago, the motion systems used in medical and lab automation equipment had technical requirements that were easy to satisfy. These lightly loaded applications generally required simple point-to-point moves with low to moderate positioning accuracy requirements.

However, medical motion systems have had to become more sophisticated in other respects to keep pace with unfolding trends and developments in the medical machine marketplace. For a more detailed examination of these trends and how linear guides have developed to meet them, download this white paper.

Hi Download Now.

Hi Membrane Technology and Industrial Applications: A Promising Future?

Hi Membrane Technology and Industrial Applications: A Promising Future?

Membrane technology like pervaporation (PV) and membrane filtration has been getting a lot of attention in the industry because of its energy efficiency and ability to break azeotropic systems.

The principle behind this technology is simple: the membrane behaves like a fixed filter that will allow water to pass through, while it catches suspended solids and other materials. Membranes are manufactured in a variety of configurations, such as hollow fiber, spiral, and tubular shapes. Each configuration offers a different degree of separation depending on the membrane process and the mixture to be separated.

PV in particular has been the only membrane process largely utilized for chemical purification over the last few decades with application in three primary areas: organophilic separation, removal of organic compounds from a dilute solution like water and dehydration of aqueous mixtures.

The process dates back to the late 1950s when researcher Robert Binning first took major research steps into PV commercialization. Binning reported on incorporating membrane pervaporation for dehydrating a ternary azeotrope (isopropanol-ethanol-water). This work marked the beginning of pervaporation research followed by others who covered topics like the separation of n-heptane and iso-octane and pyridine-water azeotrope separation, to name a few.

Membrane pervaporation as a clean technology for separating liquid mixtures was verified nearly 50 years ago, but commercial advancement did not occur due to the lack of market need.

Identifying Need:

Water treatment is one of the many accomplishments of membrane technology to date. Worldwide consumption of water management specialty chemicals is forecast to expand at a constant average rate of 3.2% per year and is forecast to reach $12.4 billion in 2015, according to IHS Chemical.

Leading companies like Dow Chemical and BASF Chemical invest heavily in membrane technology, as described by IHS Chemical Week in a 2012 cover story on water treatment. Dow was featured for its sustainability target to cut desalination costs by 35% by 2015 as a result of utilizing the advancements in membrane filtration technologies, such as ultra-filtration (UF) and reverse osmosis (RO).

Other than water treatment, membrane technologies are being used to meet the global demands of cleaner energy with applications in the refinery, petrochemical and natural gas industries.

The problem of the late 1960s, namely the lack of a market, ceased to exist by the 1980s, when separation processes were regarded as vital elements of profitability. Fifty years ago, PV was just starting out with industrial systems introduced by small companies like California-based MTR. Today, Sulzer Chemtech Ltd. is among the leading vendors to provide such systems along with Canzler LLC, Petro Sep Membrane Technologies Inc., Zenon Environmental and others.

Current Reality:

Recovering volatile organic compounds (VOCs) from aqueous solutions via PV has grown in importance over the past decade. Many efforts today focus on enhancing the potential of PV: new membrane materials are being created for specific VOC removal, more investment capital is being allocated to pilot-scale testing and the number of successful field trials is growing. While many membranes are being created on a lab scale, a relative few make it to commercialization. For a long time, the failure of many lab-scale developments to survive to industrial application was a matter of concern for researchers. Today, however, the situation has improved greatly.

“The gap between lab-scale developments and industrial application is becoming shorter and shorter,” says researcher Patricia Luis, a Materials & Process Engineering expert from Université catholique de Louvain, Belgium. "Industries are very interested in PV due to the low energy consumption in comparison with distillation. This is clear since the research departments of large companies are working on it. At the universities, we try to collaborate and do more 'crazy' research since we are not so limited in time and applications.”

In comparison with conventional separation methods, PV does not add to the air emissions problem and is increasingly used by industry to conform to regulations on chemical emission limits.

Advanced ways of separating VOCs from water are being created through the use of new membrane materials and techniques such as filling, grafting and coating to achieve high selectivity and high flux. However, according to Luis, permeability is one of the most important elements in assessing the potential of a particular membrane material.

“The separation factor is a point, but the permeability is more important in order to minimize the membrane area,” she says.

Trade-offs are being made between permeability and using filling as a technique. Although filled polymeric membranes demonstrate improved physical properties (such as increased stiffness or reduced creep to achieve thermal stability improvement, high voltage resistance or radiation shielding), they also have permeabilities that are much lower than conventional unfilled membranes. Consequently, these properties create barriers for oxygen, water and other solutes.

Other than permeability and the separation factor, membrane material also plays a role in the separation of a particular mixture. As a rule of thumb, membranes used for VOC separation are mostly hydrophobic, or water shedding, materials. Over the years, polymeric membranes such as polydimethylsiloxane (PDMS), poly (trimethylsilyl) propyne (PTMSP), and polyvinyl chloride have been used to separate VOCs from water. PDMS in particular has been extensively used due to its hydrophobic nature and rubbery composition.

Ceramic membranes, although a less-popular choice, also can be used in this field. Even so, says Luis, polymeric membranes receive more attention because there is more research involving them. That focus does not mean that ceramics are not potential membranes, she says. Pervaporative removal of VOCs namely, ethyl acetate (EtOAc) and methyl tertiary-butyl ether (MTBE) from water was possible using ceramic membrane - hydrophobized titania (TiO2) with a separation factor of 84% and 56%, respectively.

Future Prospects:

In the 1990s, researcher Peter Agre discovered water channel proteins (also known as aquaporins) that can make water delivery highly efficient. To build upon that discovery, other researchers have attempted to use biomimetic approaches while developing membranes to integrate biological elements or use concepts from biological systems.

Using engineered microporous support structures, this technology can be a more energy efficient alternative to conventional reverse osmosis and ultrafiltration membrane systems.

Due to the gift of selectivity and water permeability, biomimetic membranes may have the potential to achieve improved water permeability, reduced energy consumption and superior produced water quality. These characteristics, then, can be applied to ultra-pure water production, seawater desalination and water reuse.

Another possibility could be to apply biomimetic approaches to membrane fouling mitigation. Fouling has been a primary setback in membrane technology. Using concepts from biofilm, control and prevention in biological systems can be an important tool in efforts to prevent fouling.

However, this technology is in its infancy, with many obstacles in terms of lack of proficiency in understanding the relationship between functional molecules and matrix materials, the extent of attempts to synthesize biomimetic membranes, and the cost associated with producing large quantities of biomimetic materials.

With energy, water and environmental sustainability posing challenges around the world, membrane technology may experience wider use to address these challenges through additional research and process enhancements.

Hi Air dryers!

Hi Air dryers!

"Water in compressed air can damage production machinery, resulting in downtime and spoiled product."

When pneumatic components wear or become corroded as a result of moisture, they consume more compressed air — and lose energy efficiency. When this wear or corrosion becomes great enough, components must be repaired or replaced — increasing operating expense.

The cost of replacement parts, labor, standby inventory, and downtime can have a devastating effect on the plant's bottom line. Eliminating even one of them by drying a system's compressed air will offset the cost of installing and operating the equipment to do the job.

Types of dryers:

Dryers remove water vapor from the air, which lowers its dew point — the temperature to which air can be cooled before water vapor begins to condense. In broadest terms, there are four basic types of industrial compressed air dryers: deliquescent, regenerative desiccant, refrigeration, and membrane.'

Deliquescent dryers contain a chemical desiccant which absorbs moisture contained in the air, whether the moisture has already condensed or is still a vapor, Figure 1. The desiccant is consumed in the water-removal process and must be replenished periodically. The solution that must be drained from these dryers contains both liquid water and the deliquescent chemical, so disposal may be a problem. Local environmental regulations should be checked before disposal of this solution.

Deliquescent dryers reduce the dew point of the air 15° to 25° F below the inlet air temperature. If the incoming air has a dew point of 90° F, it will leave a deliquescent dryer with a dew point of about 65° F. Depending on operating conditions, some deliquescent dryers can produce dew points as low as 40° F; new deliquescent chemicals may produce even lower dew points. Two important points: desiccant level should not be allowed to fall below that recommended by the dryer manufacturer, and inlet temperature should be limited to 100° F or less to prevent excessive desiccant consumption.

Regenerative desiccant dryers remove water from air by adsorbing it on the surface of a microscopically porous desiccant, usually silica gel, activated alumina, or molecular sieve. The desiccant does not react chemically with the water, so it need not be replenished. However, it must be dried, or regenerated, periodically.

Heatless regenerative dryers use two identical chambers filled with desiccant. As wet air moves up through one chamber, a portion of the dry discharged air is diverted through the second chamber at close to atmospheric pressure, reactivating its desiccant. The moisture-laden purge air is vented to atmosphere. Some time later, air flow through the chambers is reversed.

Standard industry dewpoint ratings for these dryers is 40° F at pressure. By adjusting the flow rates and volume of purge air, 100° F pressure dewpoints can be achieved. These dryers, as with all desiccant dryers, should be supplied with oil-free air. Oil will greatly reduce the life expectancy of the desiccant.

Heat regenerative dryers also use two identical chambers, Figure 2. In this type, however, air flows through one chamber until its desiccant has adsorbed all the moisture it can hold. Then air flow is diverted to the second chamber. Internal heating elements or an external source of heat (steam or electricity) then dries the saturated desiccant in the first chamber. Because desiccant's adsorption capacity decreases as temperature increases, the dried desiccant bed must be cooled from the temperature it reaches during regeneration before it can be used again. The regeneration cycle in these dryers usually lasts several hours - 75% heating and 25% cooling.

Regenerative desiccant dryers can produce pressure dew points as low as 100° F. The type of desiccant used has a definite effect on the final dew point.

Refrigeration dryers condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants. These dryers produce dew points in a range from 35° to 50° F at system operating pressure.

Most 20-scfm and larger refrigeration dryers reheat the cooled air after it has been dried, usually by routing it through heat exchangers in contact with the hot incoming air. Reheating the cooled air prevents condensation from forming on the exterior of air lines downstream from the dryer and also precools incoming air.

Standard refrigeration dryers should not be used where ambient temperature can drop below 40° F because lower temperature can freeze condensate. This will block air passages and could damage the dryer's evaporator. Dryers may be equipped with heat tracing packages for operating in ambient temperatures as low as 50° F.

Refrigeration dryers should not be operated in conditions where the incoming air and ambient air heat load is 15 to 20% of the rating — a 100-scfm rated dryer (100° F inlet and ambient) can freeze up if operated at 20 scfm and 40° F.

Air dryers - continued:

Tube-in-tube refrigeration dryers, Figure 3, operate by cooling a mass of aluminum granules or bronze ribbon that in turn cools the compressed air. As the tube-to-tube refrigeration dryer cycles, a thermometer in the granule mass senses its temperature. As the temperature rises, a switch turns on the refrigeration unit. When the temperature drops to a cut-off point, refrigeration stops. These dryers are designed to produce dew points of 35° or 50° F.

Water-chiller refrigeration dryers, Figure 4, use a mass of water for cooling. An extra heat exchanger is needed to maintain chilled water flow through the condenser, as well as a water pump. Dew points can be between 40° and 50° F. Water-chiller dryers cycle as they operate.

Direct-expansion refrigeration dryers, Figure 5, use a refrigerant-to-air cooling process to produce pressure dew points of 35° F below standard operating conditions. (100° F temperature at compressor inlet, 100 psig, 100° F ambient - from the NFPA standard). No recovery period is necessary, so direct-expansion refrigeration dryers run continuously. The cost difference between cycling and continuous operation is difficult to calculate. The difference in electrical power consumption between cycling and non-cycling refrigerated dryers is outweighed by the value of continuous operation of the plant's pneumatic equipment.

Membrane-type dryers are gas-separation devices. They consist of miniature membrane tubes made of plastic materials compounded to allow water vapor to pass through when there is a vapor pressure differential. They work as your lungs do, venting water vapor each time you exhale.

Typically this membrane material is formed into bundles of thousands of individual fibers from one end of the dryer to the other. Water vapor escapes through the walls of the fiber to a sweep chamber from where it is continually vented to atmosphere as a gas. A fraction of the dried air is routed through the sweep chamber to continuously purge and exhaust moisture vapor.

Industrial-grade membranes can be used for years to dry air continuously. They respond spontaneously to any change in inlet conditions. They perform at temperatures between 40° and 150° F (ambient or inlet), and handle pressures from about 60 to 300 psig. They will deliver a consistent outlet dew-point reduction anywhere between these extremes. The inlet flow rate and pressure determine the outlet dew point suppression. In other words, membrane air dryers deliver a consistent level of drying protection that follows the rise or fall of the inlet dew point temperature, and can easily be sized to follow the ISA recommended 20° F pressure dew point suppression below ambient. Outlet pressure dew points can also be selected as low as 50° F. Flow capacities are relatively low, but modules can be installed in parallel for higher flows.

Prefilters mounted immediately upstream from the membrane dryer keep out liquids and solids to allow an almost unlimited service life. Because water vapor passes right through the membrane material, it does not accumulate there, so membranes do not become saturated and never need to be regenerated. Membranes have no moving parts to wear out. They are non-electric and suitable for most hazardous locations. They require no RF shielding or protection. They use no refrigerant gas or potentially dusty desiccants.

They make no noise. And, they can be mounted in any orientation. Their low-mass components are inherently vibration-resistant. Because they are static, inert devices, they never need service or adjustment and don't require monitoring devices. Made of plastic and aluminum, they do not rust or corrode and don't need painting. They have almost no pressurized volume, so most pressure code restrictions do not apply.

Note: membrane gas separators will remove other gases too. Some membrane-type compressed air dryers can reduce outlet oxygen concentrations (or not permeate oxygen at all). Consult the manufacturer to determine if membrane can be used for breathing air.

Dew point and its importance:

As already mentioned, wet air adds to plant operating expenses through the cost of:

• repair parts

•repair labor

•product damage, and

•production downtime.

The economic advantages of reducing or eliminating these detriments of moisture build a strong case for installing a dryer. Once the decision to install a dryer has been reached, two questions arise: how dry must the air be, and what type of dryer should be used?

The most important criterion in choosing an air dryer is the pressure dew point that it must produce. The required dew point of an air system determines how dry the air must be and to a great extent, which type of dryer to use. Dew point varies with pressure. For example: the dew point conversion chart, Figure 6, shows that air at atmospheric pressure with a dew point of 12° F has a pressure dew point of 35° F at 100 psig. Dryer manufacturers may specify the dew point that a particular model can attain at atmospheric pressure or at a typical system pressure, such as 100 psig. If performance is specified at atmospheric pressure, use a chart like Figure 6 to find what the minimum dew point will be at the system's operating pressure.

The required dew point varies with each application. If preventing condensation in compressed air lines is the main concern, then the lowest ambient temperature to which air lines will be exposed will be the controlling factor. However, for some applications, dew point requirements will be more stringent, possibly as low as 100° F at line pressure. An example might be the air used for spraying a powdery substance. Even the slightest trace of moisture in such air could condense and cause particles to stick together.

If all the compressed air will be used inside a building where temperature is maintained at a stable level, then the required dew point can be fixed within a few degrees. But if some or all of the compressed air is subjected to outdoor temperature variations, the required dew point can change from day to day, or even hour to hour.

Do not be too aggressive by estimating an unjustifiable margin for error. Stating a dew point much lower than that actually required wastes money. A rule-of-thumb margin for error is about 20° F maximum.

Extremely low dew points may be required at only a few isolated locations. If this is the case, consider using individual small heatless regenerative dryers at locations requiring pressure dewpoints below 35° F. A less-expensive dryer to dry the air to less-stringent requirements can then be installed for the rest of the air system.

Evaluating flow capacity:

An air dryer not only must dry compressed air to the required dew point, but also must be able to handle the required air flow without causing excessive pressure drop. Flow capacity of a dryer depends on:

•operating pressure

•inlet air temperature

•ambient air or cooling water temperature, and

•required dew point.

When any of the above conditions changes, flow capacity of the dryer also changes. Dryer manufacturers can supply performance curves that show the relationship of their dryer's flow capacity to these four factors. Evaluating characteristics of the different types of dryers will help indicate which is best for a particular application. This is where cost finally can be considered. Purchase price of the dryer is only one factor to evaluate when choosing an air dryer. A deliquescent chemical dryer, for example, has a relatively low initial cost, but its chemical must be replaced periodically, adding to the operating cost. This cost is offset somewhat because the deliquescent chemical dryer requires no external power source.

Other dryer types may cost more initially, but have lower operating costs because they can run for long periods with little or no maintenance required. It should be clear, then, that cost analysis should be conducted based on manufacturers' specifications as they relate to an individual application's physical and economical requirements.

Installation and maintenance:

Location can affect how well an air dryer performs. The site for an air-cooled dryer should be well ventilated, so heat can be carried away, and readily accessible to aid maintenance. The maximum ambient temperature for a refrigerated dryer is about 100° to 120° F. Higher temperatures prevent the dryer from exchanging heat with its surroundings and keep it from operating properly. Dryers with water-cooled condensers can tolerate higher ambient temperature because they transfer heat to the cooling water instead of to the surrounding environment. Refrigerant dryers, whether air- or water-cooled, should not be exposed to ambient temperature below 32° F unless optional low-ambient-temperature controls are installed.

If a deliquescent dryer is used in a central compressed-air system, bypass piping should be installed around the dryer to maintain air supply whenever the dryer is taken off line to add desiccant. There should also be no set of operating conditions that permit system pressure to drop low enough to allow high, turbulent air flow through the dryer that might carry chemicals into system air lines. It is important to shut off the water in water-cooled aftercoolers when the air system is shut down. A leak in the aftercooler could flood the deliquescent dryer and fill downstream piping with desiccant, making all pneumatic components inoperable.

Refrigeration and deliquescent dryers should be drained regularly, depending on the volume of liquid accumulated. Most refrigeration dryers have automatic drains, at least as an option.

It should be noted that dryers remove water vapor, while filters remove liquid water. A good drying system always has a filter with an automatic drain installed upstream from the air dryer. Air dryers of all types are not stand-alone components. The cost of adequate prefilters, both particulate and oil coalescing, is a wise investment to protect the more expensive dryers. Postfilters are necessary for several reasons. For refrigerated dryers, a coalescing filter can catch any oil from a refrigerant leak. For deliquescent dryers, a particulate filter downstream will catch any carryover of the corrosive desiccant. For regenerative dryers, a 0.5-m postfilter is necessary to catch desiccant dust, which is common to all adsorptive desiccants.

Hi Air compressors.

Hi Air compressors.

"Although air compressor operating specifications may look the same on paper, their fundamental designs and controls can make major differences in how they perform."

Every compressed-air system begins with a compressor - the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle. It is important to remember thatcapacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity - even though many people seem to believe it will.

There are a number of basic air compressor designs - and variations of them - on the market today. They all fall into two general categories: positive displacement and dynamic. Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.

Reciprocating compressors:

Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase. Reciprocating units, commonly called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it. Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.

There are three basic selection decisions that must be made about reciprocating compressors:

• single- or double-acting operation,
• single- or multi-stage configuration, and
• air or water cooling.

In a single-acting piston compressor, the piston only compresses air in one direction of its stroke. In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.

A single-stage unit compresses air from inlet to discharge pressure in one operation. Amulti-stage unit compresses from inlet to discharge pressure in two or more operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage. This saves power and keeps the compressor's internal operating temperatures lower.

In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50% to 75% duty cycles, depending on the particular units and their application. In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water - more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.

Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi class air - the base pressure level in most industrial plants - providing the best efficiency per dollar cost with adequate reliability of internal working parts. For a reciprocating compressor to be categorized as continuous duty, it is generally agreed that it must be double acting and water cooled. Double-acting, water-cooled reciprocating compressors are offered in a variety of styles that combine efficient air compression with durability and reliability. However, they also are heavy and bulky, making them relatively expensive to install. They generally have more-significant unbalanced forces, which combines with their size to require a special foundation and support.

When they meet selection criteria such as capacity, weight, size, and price, single- and two-stage single-acting reciprocating units are a good choice - particularly in the 50- to 150-psig pressure ranges. (Three-stage reciprocating units are offered, but generally are used for pressures above 250 psig.)

Oil-cooled rotary-screw compressors:

The rotary-screw compressor is another positive-displacement machine. In an analogy with the reciprocating compressor, Figure 1, the male rotor is like a piston, pushing air along the female rotor, which is like the cylinder. The sealing strips are like piston rings, and air is compressed against the stationary end plate, which is like the bottom of the cylinder. This design has been around for about 50 years. However, until the mid 1970s, it was considered suitable only for engine-driven portables and small-horsepower electric-motor units because of low efficiency (the ratio of compressed-air delivery to power cost).

In the 1970s, development began on two-stage rotary screw compressors for pressures up to 250 psi. Rotor-profile development during the 1970s, 1980s, and early 1990s has led the oil-cooled rotary-screw design to become the significant choice in electric-motor-driven, lubricated, industrial air compressors, particularly in sizes from 20 to 300 hp.

Then, a significant breakthrough in air-end design occurred. The introduction of the unsymmetrical profile resulted in an efficiency improvement of approximately 15%. This improvement was significant enough to make the oil-cooled rotary-screw compressor competitive in the larger-horsepower sizes for continuous duty. It has almost the same efficiency as the single-stage double-acting units and smaller centrifugal compressors.

Two-stage rotary-screw compressors can approach and sometimes equal the full-load performance of two-stage reciprocating units in 100-psig class service. Today, two-stage oil-cooled rotary-screw compressors are frequently used in the 150- to 400-psia pressure range. They also are used for 100-psi service with significant power savings. Two stages offer advantages associated with lower compression ratio per stage. Reduced pressure differential across the rotors minimizes blow-by and significantly reduces thrust-bearing loads. (Obviously two-stage units require two air ends, which increase the initial cost.)

The unique characteristic of this compressor is that it is cooled by oil. Oil injected into the air stream absorbs the heat of compression while it is being generated. The heated oil then is taken to an air- or water-cooled heat exchanger for cooling. Because the cooling takes place right inside the compressor, the working parts are never subjected to extreme operating temperatures. The cooling oil never is cracked nor burnt. No matter what the load on the compressor is, there are no hot spots inside the airend. The resulting absence of wear produces trouble-free service and high efficiency. In other words, oil-cooled rotary-screw compressors can run at full load and full pressure -twenty-four hours a day, seven days a week. This compressor's useful life in operating hours and its maintenance cost per hour will be the same as under any other load condition.

Continuous duty:

The availability of continuous-duty air-cooled compressors (particularly in large sizes) offers a great deal of flexibility for installing them. Such compressors can be mounted on any surface that will support their static weight. In many facilities, great savings also are available in piping cost, compared to other types of systems. These compressors lend themselves to either the central- or departmental-compressor system concept. Units are available with electric motor and engine drives - on bases, on skids, on wheels, etc.

Compared to other types of continuous-duty air compressors, oil-cooled rotary-screw compressors offer a number of advantages:

• Oil cooling holds internal temperatures to an optimum level. As a result, discharge air is relatively cool -no more than about 180° F higher than ambient.
• Discharge air is clean - free from burned oil or carbon.
• The rotary design lends itself to higher speeds, particularly in the larger sizes. Consequently, larger flow capacity is available from compressors with physically smaller envelopes - providing significant savings on floor space and foundation requirements.
• Because of their compact size and inherent quiet-running characteristics, it is relatively easy to suppress noise. Electric-motor-driven models are commercially available rated from 75 to 85 dB at one meter per the CAGI Pneurop Test Code.
• Most models have fewer moving parts, and those parts run under more ideal conditions - resulting in lower temperatures and less vibration.
• Fewer parts make it easier to stock them for the rotary designs, and the machines are easier to work on.

In summary, oil-cooled rotary-screw compressors offer users a continuous-duty source of compressed air in a neat, compact package that has low initial cost, maximum flexibility of installation, and easy maintenance.

Non-lubricated rotary screw and lobe:

In addition to the non-lubricated reciprocating compressors that have become so common over the years, there are several versions of non-lubricated positive-displacement lobe or screw rotary compressors. These units are referred to as clearance-type compressors because the internal parts do not contact each other, so they require no lubrication in the compression chamber. Cooling is accomplished through the cylinder walls via water jackets.

The lobes or screws do not drive one another either; they are driven by some type of gear arrangement instead. This drive system also acts as a timing gear to maintain the rotor or lobe profile relationship accurately. Lubricant for the drive train must be confined to the bearing and gear area - and not allowed to get into the compression chamber.

In this basic design, there is a constant leakage rate for any fixed set of conditions. The critical internal clearances are between end covers and the rotor, between the rotor lobes, and between the rotor OD and the cylinder ID. These gaps, combined with no injected oil to help with sealing, are the main reasons why two stages are required for these units to produce acceptable efficiencies in 100-psi class applications.

Because these are rotary units, they enjoy all the advantages of rotaries over similar-sized non-lubricated reciprocating units:

• compact size,
• smooth delivery of cool air,
• ease of installation, and
• simple (but critical) maintenance

They also have some disadvantages, depending on the specific type of compressor and its duty cycle:

• more sensitive to dirty inlet air,
• lower efficiency - resulting in higher power cost, and
• any repair work is more sophisticated and requires specialized training, which the user may not have nor want to have. This means repair work will probably have to be performed by the distributor or the manufacturer.

Sliding-vane rotary compressors:

Oil-cooled sliding-vane compressors, Figure 2, operate as other positive-displacement compressors do by trapping a charge of intake air - in this case, between the vanes. As the eccentric rotor turns, the vanes are forced into the rotor slots, shrinking the size of the cell holding the trapped air. The air is compressed to full discharge pressure when it reaches the outlet port. The heat of compression is removed by cooling oil sprayed right into the air while it is being compressed. The same oil helps with sealing the vane tips.

For decades, oil-cooled, sliding-vane rotary compressors have been popular for continuous-duty applications. Their design has a number of unique characteristics:

• light weight - yet continuous rating,
• integrated and compact configuration,
• efficient production of compressed air at relatively low rotary speeds,
• smooth operation with little vibration,
• extremely quiet operation,
• coolest possible discharge air, and
• few wearing parts, making the machine easy and economical to repair.

However, the oil-cooled rotary-vane design in its single-stage configuration is limited in capacity. Bending stress applied to the vanes is the problem. The speed, size, and weight of the vanes must be limited for the machine to be durable. Because of this, oil-cooled rotary-vane compressors generally are applied only in a size range between 2 and 100 hp.

Lubricated or lube-free?

Two fundamental groups of compressor types are lubricated and lube-free. Lubricated compressors use oil to reduce friction between moving parts. As a result, some oil is entrained in the air being compressed. The entrained oil must be removed from or tolerated by the downstream system.

Lube-free compressors use no oil in the airend, and thus add no oil to the compressed air they produce.

Power and efficiency:

Brake horsepower is the input power required at the compressor input shaft for a specific speed, capacity, and pressure condition.

Motor or engine horsepower is the nominal rating of the prime mover.

The service factor is the additional power built into an electric motor above its nominal rating - expressed as percent. Within the service factor, the brake horsepower driving an air compressor can be higher than the motor's nominal horsepower.

The power efficiency of a compressor is the ratio of the air delivered by the compressor and its input electrical requirements. Efficiency usually is expressed as brake horsepower per 100 cfm of delivered air.

Water-cooled rotary screws:

Another version of oil-free rotary-screw compressors is a single-stage design that uses water injection to cool and seal the rotors during compression. The bearings and drive gears are lubricated with oil and sealed from the compression chamber. These units serve a selected market and are a special design. In some applications, care must be taken to avoid the build-up of bacteria in the water.

Dynamic air compressors:

Dynamic, or centrifugal compressors, Figure 3, are dissimilar to the positive-displacement machines already discussed because they raise the pressure of air by converting the energy of its velocity into pressure. First, rapidly rotating impellers (similar to fans) accelerate the air. Then, the fast flowing air passes through a diffuser section that converts its velocity head into pressure by directing it into a volute.

Because the centrifugal is a mass flow compressor, it has a limited stable operating range. This has a large effect on economic operation or bhp/100 cfm delivered at part load. Minimum turn-down capacities for centrifugals may vary from 20% to 30% of full load, depending on impeller design, number of stages, etc.

There are limits to the pressure rise that can be achieved in a single stage by a centrifugal compressor - due to both physical and economical restraints - so two- to four-stage units are built that incorporate one to three water-cooled intercoolers. Cooling the air between stages reduces the power required to compress the air further, resulting in more efficient operation. Intercooling actually may permit the desired compression to be accomplished in fewer stages.

The centrifugal compressor is definitely a continuous-duty unit because its service life is unaffected by full-load operation. However, it is also a relatively sensitive machine because it operates at high speeds - often as high as 50,000 rpm. Ambient factors which affect flow are altitude, inlet air temperature, and the relative humidity of inlet air. The operating life of this type of unit is primarily determined by the amount of entrained liquids and solids carried into the unit at the inlet - and the quality of the cooling water. As in all machinery, correct installation and maintenance is critical to the efficient production of compressed air and reaching a satisfactory operating life.

When a facility requires a continuous-duty, high-volume (2,000 to 25,000 cfm) supply of non-lubricated air, the centrifugal compressor is one of the best choices. In fact, it is the only choice in sizes above 1,000 hp. Whether or not it fits the installation best is another question to be answered after analyzing the job conditions. In any event, when correctly applied, installed, and maintained, a centrifugal compressor offers a reliable, continuous source of compressed air.

Advantages and disadvantages:

After reviewing the comments on air compressors in this article, one conclusion is fairly obvious: each design has advantages and disadvantages which must be matched to a specific application. The table on this page summarizes a number of selection factors for the most common basic designs. Other factors, such as air quality and installation requirements, are difficult to quantify. The unavoidable cost factor - initial, operating, and maintenance - is noted with them in the following text.

Double-acting reciprocating - Advantages: highest efficiency, longest service life, field serviceability. Disadvantages: highest initial cost, high installation cost, high maintenance cost.

Oil-flooded, single-stage rotary screw - Advantages: low initial cost, low maintenance cost, packaged design. Disadvantage: low efficiency.

Oil-flooded, two-stage rotary screw - Advantages: higher efficiency, simple packaged design, same low maintenance cost. Disadvantage: higher initial cost.

Oil-free rotary screw - Advantages: high-quality air, moderate efficiency, simple packaged design. Disadvantage: higher initial cost.

Centrifugal - Advantages: the only type available above 600 hp, high-quality air, moderate efficiency, longer service life than other rotaries. Disadvantages: higher initial cost, must be water cooled, air flow is sensitive to changes in ambient conditions.

Importance of capacity controls:

Many compressed-air conservation program on the demand side target such issues as:

• identifying and repairing air leaks,
• eliminating open blowing,
• fixing malfunctioning condensate drains, and
• managing all potential inappropriate uses.

When these programs are completed successfully, often it is found that the facility consumes less compressed air for production, but electrical energy consumption does not go down proportionally. The reason: without appropriate capacity controls operating correctly on compressors, it is impossible to effectively translate lower air use into lower electrical energy input.

When working effectively, compressor-unloading controls should:

• match air supply to demand when needed,
• eliminate or minimize system overpressure,
• maintain the necessary minimum acceptable operating system pressure,
• reduce the input power cost to the optimum point proportional to the air flow demand, and
• turn off unneeded air compressors and bring them back on when required.

Regardless of the type of air compressor, the operating principles of capacity controls can be grouped into several basic categories. (Note that some will only perform on certain types of compressors.) Here are descriptions of these categories with some of the pros and cons of each.

Automatic start-stop control - This control simply starts and stops the electric motor or driver automatically. It can operate any type of compressor. A pressure switch usually accomplishes this function, shutting off the motor at the upper pressure limit, restarting it at the minimum system pressure.

Pro: the air compressor runs at its two most efficient modes, fully loaded and off.

Con: most AC electric motors can survive only a finite number of starts over a given time frame, primarily due to heat build up. This limits the application of automatic start-stop controls - particularly for motors larger than 10 to 25 hp.

Con: the compressor must run above minimum system pressure to hold that pressure.

Con: the system must have adequate air-storage capacity to perform satisfactorily.

Continuous-run controls (step type) - With these controls, the driver or electric motor runs continuously while the air compressor is unloaded in some manner to match supply to demand. System pressure usually commands the unloading arrangement. Continuous-run controls can be categorized as step or modulating type.

The most common is the two-step control which holds the compressor inlet either fully open or fully shut. Over the complete operational band, the compressor runs fully loaded (or at full flow) from the preset minimum pressure (or load point) to the preset maximum pressure (or no-load point). At the latter, the control shuts off air flow completely. The unit then runs at no flow and full idle until system pressure falls back to the load point. The control then goes immediately to full-flow capacity. A pressure switch typically actuates the two-step control, which can be either the primary control or part of a dual-control system on virtually every type of air compressor. (Some reciprocating compressors can be fitted with 3- and 5-step controls.)

Pro: the compressor runs at its two most efficient modes - full load and full idle - which results in the lowest possible input power cost. Full idle at lowest input power is accomplished almost immediately, except in the case of lubricated or lubricant-cooled rotary-screw compressors.

Con: both correct piping and adequate air storage are necessary to allow enough idle time over the operational pressure band to generate any significant energy savings.

Con: when two-step controls are misapplied, not only is there little or no power cost savings, but short cycling (i.e.: 20 sec. on/ 20 sec. off) can damage the equipment and shorten the life of normal wearing parts.

Con: too much backpressure in the interconnecting system can cause short cycling or ineffective unloading.

Con: at 85% to 95% loads, step controls consume some extra power because they have to compress at full capacity to a higher pressure just to hold a lower design system pressures.

Continuous-run controls (modulating) - These controls match supply to demand very accurately all along the operating band pressure range. Most incorporate some type of regulator, which in effect converts the operating pressure control band into a proportional band. If system pressure fluctuates as little as 1 psi, the modulating control immediately decreases or increases flow proportionally, depending on the signal. (This control generally is installed only on lubricant-cooled rotary-screw and centrifugal compressors.)

Pro: the minimum set system pressure draws the most power. As system demand falls, pressure rises, flow cuts back, and power usage also falls. This results in a savings at higher demand (and is the opposite of 2-step unloading where the power draw actually increases as system demand falls).

Pro: more efficient at high loads.

Pro: holds a relatively steady pressure when demand is stable, and responds responds quickly to any change.

Pro: does not depend on storage capacity to operate effectively.

Con: is generally more inefficient at lower loads.

Con: too much backpressure in the interconnecting piping can force multiple units into running on part load, when one or more could be shut off.

Controls for rotary screws:

Industry's most commonly used air compressor in sizes above 30 hp today is the lubricant-cooled rotary-screw compressor. A significant number (80% to 85%) of these compressors use some form of modulating control as the primary unloading control or as the upper-range portion of a dual control. Two types of these controls for oil-injected rotary-screw compressors are throttled inlet and variable displacement.

In a throttled inlet control, the compressor's inlet valve is opened or closed to match supply to demand as sensed by a pressure regulator. The inlet valve modulates continuously and responds immediately in to any change in the sensed system pressure. In effect, flow capacity is controlled by restricting air intake. The control holds a constant system pressure with minimal valve movement at any given steady system demand.

Pro: smooth, non-cycling control of system pressure is easier on the power train and most other components.

Pro: is relatively efficient at loads from 60% to 100%.

Pro: will not short cycle, regardless of storage capacity and or piping.

Pro: simple to operate and maintain.

Pro: usually results in lower lubricant carryover in lubricated units.

Con: relatively inefficient at loads below 60%.

Con: backpressure must be overcome in order to reach full capacity.

Con: instant response may make the machine back down and unload, even when flow is needed for the base load.

Con: sensitivity and rapid reaction make correct piping and backpressure control necessary for optimum operation. (Note: this is true for all types of unloading controls).

Variable-displacement controls:

These controls for rotary-screw compressors all match output to demand by modifying or controlling the effective length of the rotor compression volume. The inlet pressure remains the same throughout the turn down, and the compression ratio stays relatively stable. This method of reducing flow without increasing compression ratios has a power advantage over modulating and/or 2-step controls in the operating range from 50% to full load.

The two most common of these unloading controls are the spiral-cut high lead valve and the poppet valve. Both methods open or close selected ports in the compressor cylinder, thus changing the seal-off points. These ports are located at the start of the compression cycle where pressure is very low. Opening them even a small amount prevents compression from occurring until the rotor tip passes the cylinder bore casing that separates the ports. This effectively reduces the trapped volume of air to be compressed and consequently the horsepower needed to compress it.

Pro: very efficient part-load performance from 50% to 100%.

Pro: maintains set pressure at minimum system pressure. Pro: very responsive.

Con: at higher loads, some units lose efficiency due to increased leakage.

Con: the mechanism is complex.

Con: still must run 2-step or modulation in lower operating range.

Variable-speed drives:

Variable-speed drives (VSDs) control the speed of the prime mover. In theory, the performance unloading curve for compressors powered by VSDs is very attractive. Depending on the type of compressor, model, conditions, etc., unloading can be almost optimal in the range from 50% or 60% to 90% of load - i.e.: 75% power could produce close to 75% flow. Variable-speed turbines and engines have proved effective for years on all types of compressors. These drives maintain system pressure at the minimum set point and will modulate back as soon as the sensed system pressure raises.

In the world of electric motors, the most commonly applied VSD has been the variable-frequency driver (VFD) - usually as a retrofit or part of a special package. VFDs convert 60-Hz alternating current to direct current, and then reconvert it to AC at the frequency required to turn the motor at the desired speed. This conversion usually consumes about 2% to 4% more energy, and therefore VFDs are less efficient at full load than other types of controls.

Many VFDs have been installed successfully on lubricant-cooled rotary-screw compressor packages over the years, but there are some areas of concern that have limited their economies relative to cost and overall performance - particularly in retrofits. First, the design of some rotary-screw compressors causes efficiency to drop at less than full-load speed. Second, changing speeds can produce harmonic amplification problems that were not considered at the original design speed. Third, the motor itself may have efficiency problems at the low end of the speed range, possibly because of inadequate heat rejection and cooling capacity. Compressors with air ends designed specifically for VFDs will eliminate or minimize many of these potential problems.

Switched-reluctance VSDs:

Another type of VSD being offered is the switched-reluctance system. This electrical control converts standard 3-phase AC power into 2-phase DC. The rectified AC voltage is passed to a bank of capacitors where it is increased to 600-V DC and stored. The bank then supplies the power required by each phase of a brushless motor, eliminating surge currents in the main power supply. The brushless motor has the inherent ability to survive an unlimited number of starts and stops per hour because the absence of inrush current surges keeps its operating temperature low.

The true application for any compressor with a VSD should be as a trim machine, not as the plant air system's base-load unit.

Where to put it:

Industrial air compressors are rugged machines that will perform under adverse conditions, but it always is advisable to provide proper operating conditions to maximize reliability at minimum operating cost. Traditionally, compressors have been located in separate rooms to isolate their noise. Such locations are almost mandatory today to meet OSHA requirements. However, it still is important that the compressor room have an adequate foundation (particularly for reciprocating machines) as well as ample space so that the machine is easily accessible for inspection and maintenance. Stairways and catwalks can assist these procedures on larger compressors.

The compressor room ideally should be clean and dry. Auxiliary equipment, piping, and wiring should be arranged so that it does not interfere with routine inspections. Instruments should be located within easy view of operators.

Partial summary of air compressor selection factors - 100 psig service
Type	Capacity in scfm	Horsepower	Cooling medium	Lubrication
Reciprocating	<1 to 3,018	<1 to 600	<100 hp - Air >75 hp - Water	For some models
Single-stage, lubricated rotary	14 to 3,000	5 to 700	Air or water	Yes
Two-stage, lubricated rotary	560 to 3,100	100 to 600	Air or water	Yes
Dry rotary	75 to 4,200	40 to 900	Air or water	No
Centrifugal	400 to 25,000	125 to 6,000	Water only	No

Applicability of air compressor unloading controls
Type of control	Lubricant-cooled rotary screw	Oil-free rotary screw	Reciprocating (single-acting)	Reciprocating (double-acting)	Centrifugal
Automatic start-stop	Yes	Yes	Yes	Yes	Yes
Two step Three and five step	Yes No	Yes No	Yes No	Yes Yes	Yes (dual) No
Throttled inlet Variable displacement	Yes Yes	No No	No No	No No	Yes N/A
Variable speed	Yes	No	No	No	No

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