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Compressed air installations

The earlier parts of this work deals with the individual components of compressed air supply and treatment, dealing with the subject as a whole and focusing on the crucial points. However, an efficiently functioning compressed air installation is created only by combining all components to form a complete system. A simple compressed air installation consists of a compressor and air consuming equipment connected downstream by interconnecting piping.

This simple combination of compressor and air using equipment, cannot be considered suitable as far as compressed air quality is concerned. A ‘compressed air installation’ of this type may be adequate for the simplest works operations but, cannot satisfy the quality requirements of suitable compressed air.

If compressed air is dried by cooling only and not by adsorption, a further cooling of the air can occur in long and non-insulated pipes. The formation of condensate will then be the consequence. At temperatures below 0°C, this will lead to freezing of the compressed air lines. If the pipe cross-section is large, layers of ice will, in the first instance, form on the inner pipe walls. These will become progressively thicker and constrict the pipe cross-section. Smaller diameter tubing will freeze completely. As soon as thaw sets in, the layers of ice will become detached and may cause blockages, interrupting the compressed air supply.

In contrast to winter, hot summer days may lead to the compressed air being warmed in the piping. Rust particles, previously formed through the condensate, dry out and are conveyed to the air using equipment as fine dust, leading to potential malfunction of equipment.

In order to prevent the flooding of the distribution piping by condensate, condensate separators must be installed in the system at suitable points. In winter, additional trace heating of the separators is necessary. The piping must always be installed at a downward gradient towards the drainage points.

Because of high levels of rust and dirt, the separators should not be fitted with automatic draining devices and must be capable of manual operation. Unsuitable drains can cause leakage losses amounting to 5 - 10% of the total compressed air. In addition, this formation of condensate causes air lines to be corroded from the inside.

Such problems will not occur, if the compressed air is appropriately dried at the source. The economic advantages of effective compressed air drying are clear.

The largest benefit is derived from the elimination of the need for drainage, both in the main and the operational network.

Maintenance and servicing costs of the numerous drainage points in the compressed air system are drastically reduced. As a compressed air dewpoint of -25°C can be maintained throughout all seasons, small drying devices for instrument and control air become superfluous. Energy costs for heating the condensate separators during the winter period are also saved. Corrosion within the pipelines, icing up of working machinery and distribution network and malfunctions of the compressed air using equipment are eliminated.

If compressed air is used for conveying powder, fine materials or hygroscopic substances, a dryer is essential. Adsorption dryer installations need little maintenance and have a lower energy consumption compared with other methods of drying air.

It is also expedient to subdivide the types of contamination in the compressed air, using separators and dryers installed in accordance with the quality requirements. Purification should therefore be carried out in steps, from coarse to fine. Solids, water and oil in the droplet state, are taken out of the compressed air by means of filters\separators. Oil and water vapour are removed from the compressed air by means of absorbers and dryers respectively.

The question of which filters and dryers should be used, can be answered by “as much as necessary and as little as possible”. This generally valid answer is true, as each particular application must be assessed from other specific points of view. However, the ultimate decision as to what quality of compressed air must, be achieved, is that of the operator, who will judge this in relation to the application.

Installation examples

This section deals with the various options for installations including adsorption dryers. The examples mentioned represent a limited selection of typical installations found in industry.

These installation examples, serving as models, cover a large range of user problems.

The step by step recording of problems arising with individual installations are at the centre of these observations, not the basic scheme. By-pass lines were intentionally not drawn into the schemes. The valves shown in the layouts can be manually, electrically or pneumatically actuated.

Each application has its own, often concealed, problem arising from a specific requirement. This must always be considered so, for this reason these installation examples should be regarded as guides as to how to identify the problem. “We have always done it this way” is the surest way to overlook problems at the planning stage.

The logical starting point for a new installation is the specification by the user of the compressed air requirement for quality, quantity and pressure. Quality classes given in ISO 8573-1 will assist in selecting suitable air quality.

Each case is individually examined with regard to the quality class and the maximum residual values for solids, water and oil are prescribed. In order to achieve these values, the separator and dryers to be used need to be specified.

The adsorption dryer must always be installed with a pre-filter and a downstream filter. The pre-filtration and preparation at the inlet to the dryer is necessary in order to separate condensed moisture, oil aerosols and solid particles from the gas flow. The downstream filter at the outlet of the dryer protects the equipment connected downstream from desiccant dust.

Filters and dryers have very different construction, degree of separation and mode of operation. These differences were explained in the preceding chapters.

Possible installation schemes for adsorption dryers with heatless regeneration are explained in the following sections. It is not possible to reduce installation examples for adsorption dryers with heat regeneration, as these dryer systems call for a fundamentally different form of consideration due to the heat which has to be applied and extracted in the course of regeneration.

Receivers/adsorbers

Should the receiver be installed upstream or downstream from the dryer? While this question appears ambiguous both forms of installation are potentially correct, however, there must be a full evaluation of the installation requirements if there is any likelihood of high short term demands, installation of both upstream and downstream receivers should be considered.

Without considering marginal cases or abstract theories, the installation of receiver/dryer is operationally most practical. The precondition is that the dryer must be sized to the maximum compressed air consumption.

Installing an adsorption dryer (item 4) downstream from the receiver (item 2) is expedient if an even, continuous loading through the compressed air is assured. The dryer is then system sized to this even compressed air consumption.

A positive aspect of this arrangement is the additional cooling of the compressed air by the large receiver surface. Lowering the temperature accompanied by simultaneously reduced inlet humidity means theoretically that the dryer system fitted downstream can be reduced in size, at the same time achieving a saving in regeneration energy. With such an installation, sudden demand for compressed air has to be avoided in order to prevent an overload of the adsorption dryer. One also comes across plants with an intermittent compressed air consumption pattern. Sometimes the compressed air demand equals zero, i.e. there is no compressed air requirement. The adsorption dryer without dewpoint dependent switching with a rigid time cycle control will, however, permanently consume regeneration air during this low demand period.

There is a simple method of counteracting energy losses of this kind. The poor compatibility between zero load operation of the compressor and full load regeneration of the adsorption dryer is resolved by interlinking of the control system to the operational pattern.

One simple solution for an installation with adsorption dryers with heatless regeneration consists of linking compressor (item 1) and adsorption dryer electrically so that simultaneous running of both components is maintained. The electrical supply of the dryer is linked to the pressure monitor of the compressor as signal emitter. Both compressor and adsorption dryer are simultaneously switched on or off, depending on the demand. For this, the pressure monitor must be arranged in such a way that the pressure in the piping system is detected.

For an adsorption dryer with normally open inlet valves , the pressure monitor can be fitted to the compressor or to the receiver. For an adsorption dryer with normally closed inlet valves , the pressure monitor must be fitted downstream from the adsorption dryer, as otherwise, compressed air generation would be separated from the compressed air network through the closed dryer inlet valves in the power off condition.

However, all this linkage achieves is an adaptation of the regeneration air consumption in relation to compressor running periods. Adaptation to fluctuations in pressure or temperature cannot be achieved through this control method.

For small and medium installations, the above solution is practicable and expedient, as every operator of a compressor installation is capable of establishing this linkage without difficulty.

For adsorption dryers in the high performance range, a Dewpoint Dependent Switching system should, ideally, be incorporated in the original project plan, with a view to achieving an economic balance between regeneration energy and a wide range of load situations.

Duty/standby compressor, receiver

With only one compressor and dryer, uninterrupted long term compressed air supply is improbable. This risk is reduced through the installation of a second compressor. The below figure shows a typical installation with two compressors (item 1.0 and item 1.1) which are used in parallel. Compressor 1.0 is available as the base load, compressor 1.1 for covering peak loads.

Installing a duty/standby compressor, receiver or dryer will result in the dryer being subjected to a varied load of 0-50-100%.

In addition to the specified compressor performance of 0 - 50 - 100%, the receiver volume multiplied by pressure difference can, under certain circumstances, throw an additional load onto the adsorption dryer. This must be clearly identified and analysed from the outset in order to exclude undesired operating circumstances.

As adsorption dryers are designed for maximum moisture loads, deviating loads are, part loads, and thus smaller than 100%. A Dewpoint Dependent control system compensates for these part loads. To adapt the regeneration output of the adsorption dryer to the actual compressed air requirement calls for a continuous assessment of the load situation by means of the Dewpoint Dependent control system. A dewpoint meter adjustable with dewpoint sensor at the outlet of the adsorption dryer permanently registers the true pressure dewpoint of the compressed air.

The pre-set dewpoint is used as a signal for switching over the dryer from adsorption to regeneration. The loading time is inversely proportional to the part load. Working on the assumption that every part load operation is immediately followed by full load working, the regeneration period is set at a constant value corresponding to full load operation. Thus the regeneration air requirement will be inversely proportional to the load situation, in contrast to the regeneration air quantity which remains constant. The ratio of regeneration time to loading time determines the saving in regeneration energy.

When a Dewpoint Dependent control system is used, the adsorption dryer must be switched on throughout the entire operating time. Shutting off the compressed air from the system through the adsorption dryer is thus made impossible and continued operation guaranteed.

With adsorption dryers in the smaller ranges, the economic viability of employing load depending control systems has to be carefully scrutinised. For such cases, there is an alternative to a Dewpoint Dependent control system in the form of a direct linkage dryer/compressor, When setting up such an interdependence, the following must, be considered:

The adsorption dryer must be linked to the base load compressor. With standby or operation of both compressors sufficient control is achieved. However, 50 % load also means a 50% waste of regeneration energy as far as the adsorption dryer is concerned.

The compressors alternate as the base load. In this case, an OR-linkage for the dryer control must be installed. Also, when one compressor is running, 50 % of the regeneration energy of the adsorption dryer must be regarded as an unnecessary waste.

Duty/standby installation with cross-over

The installation of a duty/standby plant consists of two compressors and two adsorption dryers and offers optimum assurance of the compressed air supply. Such an installation can be utilised in different ways:

Depending on demand, compressor and adsorption dryers are switched on as required.

The duty machines (items 1 - 5) take over the entire compressed air provision, The standby machines (items 1.1 - 5.1) are available as reserve.

Receiver siting should be considered, therefore, not detailed here.

However, when considering this installation scheme, an automatic logic linkage between compressors and dryers provides a more effective solution. Both compressors (item 1.0 and item 1.1) and dryers (items 4 and 4.1) are connected in series and cross-over via one air receiver (item 2), but with interchangeably for automatic operation so that, should one unit fail, whether this be a compressor or a dryer, the stand-by unit takes over the supply of the compressed air.

So that the system automatically fulfils this task, both adsorption dryers must be equipped with a Dewpoint Dependent control system. Compressor 1 takes on the base load, compressor 1.1 is stand-by operation via a change-over switch,. The adsorption dryers are linked electrically via a programmable logic controller, which then signals the automatic operation.

During operation, the pressure dewpoint is measured at the outlet of dryer 4 and the limiting value used for the load governed fully automatic control of the dryer. As long as the dewpoint is lower than the set limiting value, the valve behind the downstream filter (item 5) remains open so that there is free flow towards the points of use. However, as soon as the dewpoint falls off and reaches the set switching point, the valve of the first line is closed and dryer 4.1 activated at the same time. The latter then takes over the drying of the compressed air.

It is necessary for automatic continuous operation that dryer 4 be regenerated. To achieve this, this adsorption dryer is regenerated at a predetermined interval until the pressure dewpoint is reached. Once the pre-set pressure dewpoint is reached at dryer 4, this line switches over to standby and becomes available again for operation if required. Regeneration takes place in parallel to the operation of dryer 4.1. Of course, other solutions may be selected as well. However, alternatives either call for more electro-technical resources or are limited in their degree of automatic response.

Altering the control system to other operating conditions presents no problem. The increasing demand for compressed air for production, is covered by the parallel operation of both lines with automatic adaptation to a load in the range of 0 - 100%.

Adsorber/receiver

Installation of adsorber/receiver is the second variant. With the equipment installed in this order, adsorption dryer size solely matches the compressor output so that an accurately specified load situation can be allocated to the dryer. With this installation overloading, the adsorption dryer through sudden high compressed air demand from the compressed air receiver is not possible, provided the design is correct.

Piston compressors compress the air in a pulsating manner and may have a more significant effect on the adsorption dryer. These pulsations do not exert a damaging influence on the adsorption dryer if the volume between compressor and dryer corresponds to 50 times the final stage of the compressor, or if pulsations are removed by pulsation dampers between compressor and dryer.

To install the adsorption dryer upstream of the receiver is possible but irregular, as varying demand for compressed air is to be expected. While irregular compressed air demand is not always totally evened out by a large compressed air receiver, it is balanced between suitable limits.

Arguments presented for this type of installation is the reduced corrosion of the receiver, as the compressed air enters the system in a dry state once it leaves the dryer, no further moisture is precipitated, so elaborate condensate traps are not necessary.

Varying load situations are possible for the operation of the adsorption dryer, i.e. continuous operation as well as partial loading, caused either by pressure and/or temperature fluctuations at the inlet of the dryer. In the unfavourable situation where, no compressed air is drawn off but there is a steady consumption of regeneration air.

As shown , the aim should be to achieve optimum performance from no load operation to full load regeneration between the adsorption dryer and the compressor.

To this end, heatless adsorption dryers are electrically linked to the pressure monitor of the compressor system, thus forming a matched running unit. However, with this adaptation, the dryer running time is dependent on the compressor running time. Other criteria, such as temperature variations and/or pressure fluctuations, cannot be catered for by this linkage. This solution is expedient for small to medium installations and can often be arranged on site.

With this linkage, the following is worth considering as a possible variant:

The compressor pressure monitor emits the unload signal for compressor/dryer when the upper switching point is reached. The purge valves of the adsorption dryer close when this status is reached, to block the escape of regeneration air into the atmosphere. The main valves remain open allowing the pressure build up at the receiver to feedback to the compressor pressure monitor.

Pressure equalisation inside the adsorber will take place by the regeneration air repressurising the empty vessel. After pressure equalisation inside the adsorption dryer, the pressure may be lower than the minimum operating set at the compressor sensor non-return valves. The non-return valves in the outlet of the adsorption dryer prevents full pressure equalisation. In the worst case, the system is restarted.

The pressure level after pressure equalisation, depends on the volumes of both adsorbers, this forms a criterion for checking and, if necessary, correcting the pressure difference set at the pressure monitor. The pressure drop inside the adsorption dryer is compensated for by an additional compressed air return feed, This return feed by-passes the non-return valves of the adsorption dryer. The direction of flow is controlled by an non-return valve (item 8). A leak proof valve (item 9) prevents dewpoint degradation in the course of normal operation.

Alternatively, the pressure monitor is not fitted to the compressor but to the receiver. In this case, the pressure drop inside the adsorber can be ignored (excepting that it is not too high to cause the safety valve to blow off). With this arrangement, direct coupling of compressor and adsorption dryer cause no difficulty, and the return feed becomes unnecessary.

However, for large heatless adsorption dryers the adaptation to varying operating conditions can be achieved only by a Dewpoint Dependent control system. Using a governed control system with pre-set automatic regeneration, for long standby periods, one has to supply the regeneration air requirement of the adsorption dryer via a return feed line. Regeneration by means of dry air from the receiver, provided this has a sufficiently large volume, can be assured without compressor restart.

Duplicate compressor, dryer

In line with the installations described, the following applies to differing load situations :

One compressor (item 1) is available for the duty output, the second compressor (item 1.1) as standby.

Both compressors remain in standby, with the variants:
Two-compressor operation 100 % dryer load
One-compressor operation 50 % dryer load
Compressor standby 0 % dryer load

The varying running periods of the compressors call for an adaptation of the adsorption dryer (item 4), to maintain economic performance. In order to match the regeneration performance with the load, the loading fro 0 - 50 - 100% must be coped with.

This is achieved in practice by means of a modified Dewpoint Dependent control of the dryer system. Deviating load ranges are always part loads. A load governed control system caters for this part load and adapts the dryer running= periods in an effective manner.

Full load operation can follow immediately after a part load use. For this reason, the length of the regeneration period has to be set in such a way that continuous drying is possible when changing over from part load to full load operation without a dewpoint peak (dewpoint collapse) during continuous operation.

Regeneration time and regeneration air quantity remain constant. In the partial load range, the loading time will be inversely proportional to the load, i.e. with 50 % load, the loading time will be made longer by the factor 2. Regeneration air requirement is likewise inversely proportional to the loading. The ratio of regeneration time to loading time determines the regeneration air requirement and economic utilisation of the adsorption dryer.

Using a load governed control system, the power supply to the dryer must be ensured at zero loading, i.e. the dryer remains open allowing the pressure monitor in the compressor to trigger the pre-selected lower switch-on and upper switch-off point as limiting values for operation.

As an alternative to load depending control, it is possible to link the compressors directly to the dryer. Dryer/compressor simultaneous running have been discussed in section 10.4.2. As varying load situations are unpredictable, the following assumption applies:

Compressor 1 supplies points of use for brief periods only, remaining off-load for long periods, compressor 1.1 is required as a standby.

These conditions, long periods of standby and short periods of operation have a detrimental effect upon the pressure dewpoint.

Long standby periods call for additional regeneration phases. With connection of the dryer/compressor, automatic regeneration can be simply achieved. Two additional time relays are included in the control system. Relay 1 has the maximum period of availability and relay 2 the time of regeneration. Setting these relays is based on the acceptance of a pressure dewpoint peak. The return feed for regeneration air from the receiver has to be provided with this installation scheme also.

Duplicate installation in parallel

The operation of two compressors and two dryers, operated in parallel, and can be regarded as ideal.

Line 1 (item 1 - 5) takes on the base load and
Line 2 (item 1.1 - 5.1) is for stand-by operation

As desired, duty periods are allocated to the compressors via a base load change-over switch. Should one line fail, the second line takes over the total supply.

A further development of dryer control is called for if the system is meant to run fully automatically. Both dryers are fitted with a load depending control system and linked to the compressors.

Line 1 operates, the dewpoint at the outlet of the dryer is better than the set point. The valve after the downstream filter (item 5) is opened to the take-off point. If the dewpoint rises and reaches the set point, line 2 becomes automatically activated. Depending on the loading, each line can take on 50% or line 2 takes on the entire load when required.

Regeneration of line 1 is timed and automatic. . External regeneration lines (item 6 and item 6.1) connect the dryers of line 1 and line 2. As required, regeneration air is taken to the dryer to be purged.

Stand-by adsorber

Two compressors (item 1 and item 1.1) and two dryers (item 4 and item 4.1) upstream of a receiver (item 2) are cross linked to each other and should be suitable for automatic operation as, if one unit fails, whether compressor or dryer, the parallel unit takes on the total supply of the compressed air. The system operates automatically without manual switching, both dryers are fitted with a load governed control system. Here, too, compressor 1 takes on the base load, compressor 1.1 is meant for stand-by operation. The compressors operate as selected via a base load change-over switch.

For an installation of this type, i.e. cross-over connection of compressor and adsorption dryer, fitting a load depending control to each adsorption dryer is the
correct and optimum solution.

With dewpoint sensing at the outlet of each adsorption dryer, the output signal ensures quality remains constant for all load situations. Adaptation to varying load levels takes place automatically.

From the multitude of possibilities, three fundamental rules emerge and these must always be followed.

Rule 1:
With a Dewpoint Dependent control system, automatic operation for each dryer can be achieved, as the dewpoint is utilised as the control signal.

Rule 2:
If a load governed control system is used ie. compressor and adsorption dryers are linked up for simultaneous running, free passage of the pressure to the pressure monitor must be ensured, irrespective of how the dryers are installed.

Rule 3:
If the adsorption dryer is mounted upstream of the receiver, a return feed of compressed air for pressure equalisation has always to be considered.

Refrigeration dryer/adsorber

If an adsorption dryer is installed downstream of a refrigeration dryer, the criteria changes. With this layout, pre-dried compressed air is fed to the adsorption dryer. The refrigeration dryer removes part of the moisture from the compressed air. As a rule, the pressure dewpoint at the outlet of the refrigeration dryer is between 2 - 5°C. The compressed air at the inlet to the adsorption dryer is no longer 100% saturated, typically 15 - 30%. Formula 10.4.8.1 is used for determining the relative humidity.

Refrigeration dryer/part-flow adsorber

Using an adsorption dryer in a branch line for the purpose of drying a portion of the overall flow which has been pre-dried by the refrigeration dryer.

In the preceding examples, installation of an adsorption dryer into a branch line
is subjected to basic consideration regarding:

• Moisture load
• Selection of the drying medium
• Refrigeration dryer operation
• Automatic control
• Adsorption dryer size

Automatic control of the adsorption dryer operation in relation to the pressure dewpoint of the refrigeration dryer is achieved if the adsorption dryer is equipped with Dewpoint Dependent control. The variable cycle times adapted to the current operating state at any time, justify such a control system even for smaller adsorption dryers.

The refrigeration dryer fault signal is not always available to the adsorption dryer as signal for switching over the cycle times. Distances between the centrally installed refrigeration dryer and the decentralised mounting of the adsorption dryer can, in practice, be very far apart and thus practically exclude direct electrical connection.

Adsorber/part-flow adsorber

A rare application is one where compressed air is further dried after already having been dried to a low pressure dewpoint. This occurs when, downstream of an adsorption dryer, a proportion of the total compressed air must have an even lower pressure dewpoint

Compressed air with an inlet temperature of about ti = 35°C into the adsorption dryer (item 4) has a humidity level of hi = 39.9 g/m³. At the outlet from the dryer, a pressure dewpoint of Pdp = -25°C is achieved. At this pressure dewpoint, the remaining humidity level is to about ho = 0.55 g/m³.

The moisture content at the inlet to the adsorption dryer (item 4.1) in the branched-off air flow is less than 1%. Such a low relative humidity at the inlet of the adsorber calls for a completely new way of reviewing at the design criteria of the dryer.

Having selected a suitable drying medium and looking at dimensional criteria, this type of application always calls for a check whether flow takes place in an intermittent turbulent state and the dwell time in the adsorber is sufficient.

These considerations may lead to an adsorber which is larger than in a standard installation. Dwell time depends on the pressure drop of inlet to outlet of the air dryer.

However, in extreme cases the intermittent flow in the turbulent flow, adequate dwell time, or high pressure drop, cannot be resolved. Under these circumstances, reliable after-drying is questionable.

Regeneration has be to looked at from the same points of view. Logically, the quantity of regeneration air must be higher.

For these part flow dryers, time and load depending control systems are used. These must be set with a safety margin and also fitted with a time cut-off.