The atmospheric air enters the compressor as a low pressure , high volume gas and leaves it as a high pressure and low volume gas . This is known as compressed air.
Frequently Asked Questions
Process air applications and other pneumatic applications need elevated pressure, since most of the valves and other pneumatic tools and equipments do not function at atmospheric air pressure.
Advantages of Compressed air.
- Versatile energy source
Widely used as a power, process and breathing medium.
CLEAN, DRY air is a mechanical mixture of approximately 78% by volume nitrogen and 21% oxygen, the remaining 1% being made up with minor quantities of some fourteen other gases. The composition of air remains substantially the same between sea level and an altitude of about 20 kilometers, but its density decreases with increasing altitude, and varies with pressure and temperature. At sea level, with a pressure of 1 bar and a temperature of 15°C, the density of air is 1,208 Kg/m3 (0.0765 lb/ft3). Thus 1 kg of air has a volume of 0.818 m3; or 1 lb of air a volume of 13.07 ft3.
- Rust and pipe scale deposits in steel pipes , causing leakages
- Increased resistance to flow
- Malfunctioning of pressure control equipment and instrumentation
- Corrosion in electro-magnetic valves
- Solidification of media in pneumatic conveying system
- Blistering or Orange peel effect in paint spraying
- Sluggish operation of cylinders , spool valves and other moving parts
Appropriate air quality gives optimal compressed air economy
- Low running costs
- Low maintenance costs
- Better product quality
Components in air sucked in by the compressor
elements added to the air by the compressor itself.
- Air borne humidity
- Dust particles
- Oil contributed by the compressor
- Gases, Vapours, Fumes.
A. Dew point is a temperature at which air is fully saturated and below which water will condense.
The water content in vapour form is independent of the air pressure but in a compressed air system, the pressure is an expression of the change in volume,thus , we express Dew Point both as Pressure Dew Point and Atmospheric Dew Point.
All atmospheric air contains a certain amount of water vapour which is mixed with other gases making up the air. This water vapor is drawn into the compressor with the incoming air.
Air at higher pressure cannot hold as much moisture as free air at the same temperature. Moisture would normally condense during compression but because the temperature of the air increases, which increases its ability to hold moisture in vapor form, there is no condensation. Cooling of compressed air is done prior to drying . This is done in after coolers which are heat exchangers that use either water or ambient air to cool the compressed air.
As the water and lubricant vapors within the compressed air cool, a significant amount condenses into liquid. The amount of condensation depends upon the temperature of the air when it leaves the after cooler. When the compressed air is cooled in the after cooler, the air is unable to hold all the moisture and it partly condenses out in the form of liquid water. This moist air needs to be dry before further use in application.
Compressed Air can be dried by two methods¬
Refrigeration :- A refrigeration dryer is based on the principle that when the temperature of the air is lowered, its ability to hold moisture is reduced. A refigeration dryer reduces the dewpoint of the air by reducing the temperature of the air, thus separating out the condensed moisture.
Sorption :- Solid sorption passes the air stream over a bed of granular desiccant which in its active state has a vapour pressure below the vapour pressure of air stream to be dried and therefore physically removes the moisture from the air stream.
Sorption can be classified into Absorption and Adsorption.
Absorption – Solid absorption systems use a desiccant that dissolves with the absorbed moisture. The desiccant cannot be regenerated.
Adsorption– Adsorption dryers operate by passing compressed air through a bed of desiccant material, to which the water vapour is adsorbed i. e. adheres to the surface.
These solids have a very great internal surface. The specific surface can be from 10,000 to 100,000 times greater than the external surface of the particles. These desiccants once saturated can be regenerated and reused.
Hot, moist air enters the refrigeration dryer through an air to air heat exchanger, where the temperature of the inlet air is reduced. This cooled air now passes through a refrigerant to air to air heat exchanger (evaporator), where the tempe¬rature of the air is reduced to between 2° to 5°C The cooling induces cendensation and the liquid water is removed in a liquid seprator. The dry air now goes out of the dryer through the air to air heat exchange where the temperature of the air is increased to slightly below the incoming air. This is a continuous process.
The limitation of this dryer is that the dewpoint cannot go below 2°C PDP. It is ideal for indoor plant air, for operation of pneumatic air tools, control and general instrumentation.
There are several different types of regenerative dryers but they all conform to the same basic concept. A regenerative dryer consists of two pressure vessels or towers filled with the desiccant. The desiccant in one bed is on stream, drying the air, while the desiccant in the other tower is being regenerated. The two towers are interlinked with switching valves so that when the desiccant in the drying tower is saturated, the valves switch the flow into the tower that has just been regenerated. This switching operation is automatic giving conti¬nuous drying.
With an adsorption dryer, very low dewpoint, down to -40°C or -60°C can be easily achieved. In special application where very dry air is required, only an adsorption dryer will do. Specific application areas are – operation of specialised instrumentation, electronic assemblies, pneumatic transportation of chemicals, climatic chambers and cryogenics.
Compressed air dryers fall into four general categories:
Deliquescent air dryers use an absorptive type chemical to provide 20°F to 25°F dew point suppression below the temperature of the compressed air entering the dryer. The moisture in the compressed air reacts with the absorptive material, which degrades to produce a liquid effluent, which is then drained from the dryer. We have to be careful as this effluent is typically corrosive and must be disposed of in accordance with local regulations.
Hot, moist air enters the refrigeration dryer through an air to air heat exchanger, where the temperature of the inlet air is reduced. This cooled air now passes through a refrigerant to air to air heat exchanger (evaporator), where the temperature of the air is reduced to between 2° to 5°C. the cooling induces condensation and the liquid water is removed in a liquid separator. The dry air now goes out of the dryer through the air to air heat exchange where the temperature of the air is increased to slightly below the incoming air. This is a continuous process.
The limitation of this dryer is that the dew point cannot go below 2°C Pressure dew point , it is ideal for indoor plant air, for operation of pneumatic air tools, control and general instrumentation.
Desiccant: There are several different types of Desiccant dryers but they all confirm to the same basic concept. A desiccant dryer consists of two pressure vessels or towers filled with the desiccant. The desiccant in one bed is on stream, drying the air, while the desiccant in the other tower is being regenerated. The two towers are interlinked with switching valves so that when the desiccant in the drying tower is saturated, the valve switches the flow into the tower that has just been regenerated. This switching operation is automatic giving continuous drying.
With an adsorption dryer, very low dew point, down to –40°C or –60°C can be easily achieved. In special application where very dry air is required, only an adsorption dryer will do. Specific application areas are-operation of specialized instrumentation, electronic assemblies, pneumatic transportation of chemicals, climatic chambers and cryogenics.
This type of dryer is relatively new to the market. It was developed using gas separation technology employed to generate nitrogen. The operation is simple. Compressed air, saturated with water vapor, flows into a membrane made up of bundle of hollow tubes. The compressed air “dwells” in the membrane where the water vapor permeates through the tube wall and vents to atmosphere, the dry air continues into the compressed air distribution system.
The typical features of this system are :
- No electricity required
- No refrigeration system
- Dew points to –40°F
- High initial cost
- No moving parts
1. Silica Gel
2. Activated Alumina
3. Molecular Sieve
- delair refrigeration dryers
- delair adsorption dryers
- Heatless type
- Heat regenerated (reduced purge)
- HOC type Compressed Air Dryer
- NLSF type Compressed Air Dryer
- Blower Reactivated type Air Dryer
Most compressed air applications fall into two categories :
i) General plant air
ii) for instrumentation, tools, etc.
a) For general plant air, dewpoints of 2°C to 4°C are considered adequate, well within the range of refrigeration dryers.
b) Refrigeration type dryers use less energy and hence are lower in first and operating costs.
c) Desiccant dryers, heatless type, use 10 to 1 51/o purge air for regeneration of desiccant i.e. 10 to 1 5%0 of compressor power (HP) is being used as energy. Operating costs for drying, thereby increasing the capital costs in installing extra capacity of compressor.
In heatless dryers because of the loss of large quantity of dryed compressed air as purge air, most customers prefer heat reactivated regenerative dryers. By heating the desiccant to desorb moisture, the need for purge air is reduced or eliminated, This is significant when dealing with large volumes of compressed air.
However heat reactivated dryers operate on a much longer time cycle than heatless dryers do. Typically the heated dryer uses a 6 hour cycle versus a 5 to 10 minute cycle for the heatless. This is necessary because of the time involved in heating up the desiccant during regeneration.
Having studied about the various types of compressed Air Dryers, the first ques¬tion that comes to our mind is :¬
1. Degree of Dryness Required
2. First cost of the Dryer.
3. Operating cost of the Dryer.
4. Energy consumption of the Dryer
5. Pressure Drop through the Dryer.
6. Maintenance costs.
1. Degree of Dryness Required : Dry compressed air is a relative term. What may be considered dry air to one user is considered wet by another. Even if the term PDP
(pressure newpoint) is understood, it can often be difficult to apreciate the differences in dryness between one PDP and another.
A refrigeration dryer with PDP of 6°C will remove approximately 57% of the water contained in the air at 20°C. A refrigeration dryer with a PDP of 2°C would remove 67% of the water. In other words, a small change in PDP markes a change in the water content.
An adsorption dryer with a PDP of – 20°C would remove 95% of the water vapour contained in the compressed air at +20°C.
However, most people due to lack of understanding or proper guidance tend to overspecify their dewpoint requirement. A dewpoint of 2°C easily achievable by a refregeration dryer is the degree of dryness quite adequate for most common compressed air applications.
For applications requiring very dry air down to -20°C or -40°C only an adsorption dryer will do.
Hence the dew point requirement should be correctly specified to avoid overcapa¬city costs.
First cost and Operating costs : should be looked at in totality to arrive at the correct estimation of the dryer cost and optional compressed air economy.
It is essential to choose a dryer which is attractive not only in the short term i.e. the purchase price but also in the long term, i. e. running costs.
Comparision of Pressure drop : Pressure drop adds to the indirect energy cost of the dryer.
In the refrigeration dryer the design of the heat exchanger determine the pressure drop across the dryer.
Pressure drop across adsorption dryers is affected by filters, used to protect the desiccant from oil and dust. Improper design of filters can cause a significant pressure drop across the dryer installation.
Typical pressure drops for a good quality dryer are : Refrigeration dryer -.3 to.5 bar Adsorption dryer with filters. -.7 bar
Maintenance Costs : The cost of spares and frequency of replacement should be kept in mind while selecting a dryer.
Reliability : This is a factor commonly ignored. Dryer components should be of the highest quality and the performance and dewpoints certified.
All the above criteria should be evaluated against the design criteria and price to be able to choose the right dryer.
Refrigeration dryers consume very little energy for running the freon compressor and the condenser fan. Energy recovery equipment is incorporated in the dryer by way of an air-to-air heat exchanger where cooled dried air is passed through to be heated by the incoming warm compressed air.
In the heatless adsorption dryer purge air percentage directly adds to the energy costs in terms of extra compressor capacity bled off. Larger dryers use a combination of purge air and heaters to regenerate the desi¬ccant or use the heaters to heat the ambient air for regeneration.
The cyclic process of adsorption and regeneration is automatically controlled by cycle time. With dewpoint changeover the cycle becomes dewpoint controlled and the changeover is affected as soon as the dewpoint increases above the specified value.
Capacity and dewpoint are determined by the design of the dryer. When the design conditions are exceeded-ms, max. inlet temperature and minimum inlet pressure-the specified dewpoint is no longer obtained.
Dewpoint control runs the cycle on actual moisture load. Through extending the the cycle drying time regeneration occurs on fewer occasions and hence there is savings an energy consumption and operating cost.
The size of the total savings on a yearly basis easily amounts to approximately 29,000 kwh for a dryer capacity of 1000 m3Jhr.
The additional benefits are increase in deciccant life time and decrease in dryer maintenance.
The DBM heat regenerated series is truly a dryer for the nineties that fits the requirement and thinks for you. Tne most upto date design is completely controlled by PLC.
1. The PLC controls and checks the fully automatic cycle. Two displays present all information on the actual status per adsorber.
- drying -depressurizing, heating -cooling, pressurizing -stand by
- time (hours, minutes)
2. EPROM with cycle programme automatically stops cycle at failure. The failure is localized and displayed in LED indication on the PLC.
3. The pressure dewpoint is continuously and digitally indicated on the front panel in OC. A push button gives the option between time controlled or dewpoint controlled cycle.
4. Design is based upon a ‘failsafe’ cycle operation. This is realized by controlling the pressure, temperature and flow conditions during drying, switchover and regeneration by integrating these in the cycle control.
5. User friendly.
Designed and constructed totally according to industrial specification and requirement, this dryer outclassess other dryers in terms of controls and performance.
6.Value for Money
Delair dryers are a class above other dryers. They are better by design and provide good value for money. Delair scores over other dryers in selection of components and variables. A lot of stress is laid by engineering and design to select the best components and-adopting the right designs. The extra cost to Delair of these variables is related in’terms of payback periods which go a long way in certified performance, minimum operating costs and reduced maintenance costs.
1.Superficial Velocity through Desiccant affects or is affected by the choise of desiccant. vessel diameter and pressure drop. If the velocity of the air through the bed is very high it would fluidize the desiccant leading to attrition. The contact time would be too low to obtain a good dewpoint and pressure drop would be excessive.
The velocity of air is determined by the vessel diameter.
To attain the right velocity through desiccant Delair component costs goes up by 20% but the payback is in 3-4 months in terms of savings achieved in preventing attrition loss, vessel size and compressors H.P. lost.
2.Velocity through interconnect pipe work: High velocity may save first cost because of reduced pipe and valve sizing, but means higher pressure drop leading to compressor H.P. lost. Long pipe line also lead to compressor H.P. lost.
3. Purge : The % of purge air determine the operating cost and the compressor H.P. directly lost.
4.Choice of desiccant : The desiccant selection affects attrition, pressure drop, replacement costs and filter life. The Delair choice of desiccant adds 10 1 0% to the Delair cost but the payback period is one year in terms of savings in replacement cost and performance.
Valves : Type of valves and its sizing greatly affects the reliability of in right selection system, maintenance costs and its reliability. An extra cost of 50% is paid back in 6 months.
6. Vessel : Selecting and manufacturing vessels;to international codes adds 40% to the cost but the payback is immediate in terms of safety to human life.
7. Filters :
Selection of the right filters and their sizing greatly enhances filtration efficiency, replacement cost and achieving the quality of air. A 30% additional – cost in investing in the right type and size of filter is paid back in 6 month in terms of saving by in replacement costs, reductions in pressure drops and mainly deriving the right air quality.
Delair not only gives the complete range of dryers to optimize your selection.but gives the most reliable, efficient and dependable product based on 50 years of research, developments and experience in air and gas conditioning.
Delair is the leader in the field of air and gas drying. Their capability extends to drying of air, Oxygen, Nitrogen, natural gases and inert gases.
The Delair standard dryer is a basic unit enabling the desired conditions to be realized. In order to maximize the benefits from the investment and to extend control or to function more economically, Delair offers a number of control solution to make
it a more comprehensive system.
The Dewpoint Change Over System
A considerable Saving in the energy consumption of heat regenerated adsorption dryers is the most important incentive for applying dewpoint change over. Delair has designed several high pressure jobs over the years for various indus¬tries. Higher pressure handled have been in the range of 380 bar (g). Delair process equipment is now leading in the industry for their application and design capability.
At Delair, quality is regarded as an actual product. The Delair quality team is responsible for integral quality assurance. The quality of the product concept and the choice of components is tested. Quality standards apply to every important stage in the production process. A test department tests the newly arrived components. Welded joints are inspected using X-ray equipment. The end product are subjected to a functional test and the results compared with the applicable international standards.
Delair Provides the Total Solution
Delair has the experience, capability and the skill to produce the final product, that is, quality air.
Their design, engineering, production and installation capability provides the total solution in drying, filtration and separation technologies.
Contaminants in compressed air adversely affect all components of the air distribution system Wet, dirty compressed air costs money by:
Robbing the system of useful power
- Air lines rust and leak
- The power and efficiency of air tools decreases
Increasing maintenance and repair expenses:
- Lubricants wash out of air tools
- Solid particles abrade wear surfaces
Contributing to product rejects:
- The color/adherence of paint spraying is affected
- Product spoilage in food processing is promoted
With the proper compressed air treatment equipment, system power is maintained, operating expenses are reduced, and production quality is improved. The cost of using wet air far exceeds the cost of keeping it dry!
The amount of moisture air can retain is determined by its temperature and, to a lesser extent, its pressure. Any sufficient drop in temperature or increase in pressure will cause moisture to condense out of the air.
Relative humidity is the amount of moisture that is present in the air, compared to the total amount the air could hold at a certain temperature. For example: One cubic foot of air at 80°F, 27°C can hold about 12 grains of moisture.
If it is holding all 12 grains, it is said to be at 100% relative humidity or saturated. If that same cubic foot of air is holding only 6 grains of moisture, it is at 50% relative humidity.
Saturation point is the point at which air is holding all the water vapor it can (100% relative humidity). If we added more water vapor to saturated air, the water vapor would condense into a liquid.
Remember that a drop in temperature or increase in pressure will cause moisture to condense out of air that is saturated. Condensation is the changing of water vapor into a liquid.
Dew point is the temperature at which water vapor in the air condenses into a liquid.
Pressure dew point is more meaningful because it indicates, at a given pressure, the temperature at which condensate forms in the compressed air lines.
Contaminants in compressed air adversely affect all components of the air distribution system Water vapor and other contaminants in the ambient air enter the compressor intake. One cubic foot of air at atmospheric pressure contains:
- Liquids – Water droplets
- Solids – Dust and pipe scale
- Gases – Oil and water vapors
Once atmospheric air passes through a compressor operating at 100 psig; 7 bar, the air is compressed to 1/8th its previous volume, yet still contains the same amount of contaminants.
The increase in pressure would normally cause moisture to condense out of the air, however, during the compression process, the temperature of the air rises due to frictional heat, increasing its ability to hold water vapor.
As air leaves the compressor and travels throughout the system, it begins to cool. Once the air temperature drops below the pressure dew point temperature, water droplets begin to condense.
Condensation is the changing of water vapor into a liquid.
It is necessary to remove the moisture and contaminants from the air system in order to lower the dew point and avoid operating problems, costly maintenance, and repair expenses.
Important Guideline: For every 20°F, 11°C drop in compressed air temperature, the moisture holding capacity of air is reduced by 50%.
CoAftercoolers are a good first step. They lower the compressed air temperature to safe, usable levels and remove up to 70% of the water vapor; however, the air is still saturated. A further drop in temperature will cause additional condensation to occur in downstream air lines.
Water vapor and other contaminants in the ambient air enter the compressor intake.
Air dryers remove the water vapor and lower the dew point temperature of compressed air. This prevents liquid water from forming downstream, but does not eliminate all the entrained contaminants.
Separators and filters remove the liquid water, and solid and gaseous contaminants that adversely affect the air system.
Drains discharge the accumulated water and liquid contaminants from various points throughout the air system.
Condensation is the changing of water vapor into a liquid.
Dust and dirt that accumulate on the condenser coil of an air-cooled refrigerated dryer will hamper the dryer’s ability to produce a consistent outlet dew point, and ultimately to function at all. The condenser coil should be cleaned a minimum of once a month to prevent this from happening.
When selecting a compressed air dryer, consider these factors and then determine the most cost-effective system suitable for the application.
Compressed air dryers vary in relation to their dew point, initial cost, and ongoing maintenance requirements.
1.Choose a dew point temperature below the lowest ambient temperature to which your compressed air system will be exposed.
Consider air lines:
- Located in front of open doors or windows
- Throughout air conditioned or unheated areas
- Running underground or between buildings
- 2.Determine which dryers will produce the required dew point.
3.Consider initial and operating costs. The lower the dew point, the more expensive the dryer is to purchase and operate
Once the appropriate drying system has been selected, determine the actual conditions under which the dryer will be operating.
1.Flow capacity (scfm, Nm3/hr, Nm3/min, l/sec)
2.Pressure dew point (°F, °C)
3.Inlet air pressure (psig, bar, kg/cm2)
4.Inlet air temperature (°F, °C)
5.Ambient or cooling water temperature (°F,°C)
6.Maximum allowable pressure drop (psi, bar, kg/cm2)
In North America, in accordance with CAGI ADF100, conditions for rating compressed air dryers are: 100 psig (7 bar), 100°F (38°C) saturated inlet, 85°F (25°C) cooling water, 100°F (38°C) ambient air and a maximum 5 psi (0.35 bar) pressure drop.
In Europe, and much of the rest of the world, the ISO 7183 standard for rating compressed air dryers is used: 7 bar (100 psi), 35°C (95°F) inlet, 25°C (77°F) ambient air and a maximum 0.35 bar (5 psi) pressure drop.When operating at other than rated conditions, adjustments to the dryer’s rated flow capacity must be made.