| Air Dryer |  |
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| Features | Specifications & Performance | Selection Tips | Installation Tips |
| Why Dry Air? Compressed air has various application in industries. There is dire consequences of using wet air. Scale and Rust deposits in steel pipes, resistance to flow, malfunctioning of process control instruments, corrosion in electromagnetic valves and pneumatic system, blistering and peeling effect on spray painted surface etc. These affects the quality of product and can lead to exorbitant maintenance expenditure. |
A Pressure Swing Adsorption based air dryer works on the principle of water removal through selective adsorption. The adsorption dryer draws out water vapour from the compressed air this is done by attraction and adhesion of water vapour molecules in a gaseous or liquid state to the surface of the adsorbent material. The adsorbent can be regenerated as many times by removing the adsorbed water. A typical air dryer unit can consist of adsorber vessels with molecular sieves.
These days air dryers are coming with designs that are more efficient, compact and rugged than earlier air dryer systems based on conventional PSA methodology. One of the added features has been that unlike earlier conventional systems, the new range of air dryer do not necessitate the requirement of additional support equipment. Also the new systems offer applications, where earlier on-site/on-board dry air generation was almost impossible.
The
unique designs of the advance technology PSA air dryer module has virtually
eliminated many of the components and interconnections that were a
common feature of conventional PSA systems. Though the number of
adsorption beds may vary but the basic operation remains the same. In
the new type air dryer just a single rotary distribution valve is
continuously rotated about at lower speeds by a motor. Operating in a
sequential fashion, the flow of compressed air is directed to a group of
sieve beds (adsorption), similarly another set of beds are purged into
the atmosphere through the valve (called desorption). While the
remaining beds are interconnected through the valve to equalize pressure
as the sieve beds transit sequentially between the processes of
adsorption and desorption. These new industrial air dryers employing small
amplitude pressure swings generated by the multi sieve beds eliminates
loud noise pulses, need for pressure regulators, and reduce wear and
tear of compressor.To conclude, the PSA process is ideally for relatively small capacities and requiring mediate dew points. The investment costs are low but there is a disadvantage too and that is of a purge loss of around 15 to 30 % (depending on the operating conditions and application). The cycle time of the PSA process is about a few minutes.
The following table tries to compare the Features of New PSA Systems of Air Dryers with the conventional ones:
| Conventional PSA Air Dryer | New PSA System Air Dryer | |
| Pneumatic Connections | 40-60 | 2 |
| Electrical Connections | 20-30 | 2 |
| Solenoid Valves | 2-8 | nil |
| Control Electronics | Required | Not Required |
| Inlet Air Pressure (PSIG) | 90 | 18-35 |
| Pressure Reducing Regulator | Required | Not Required |
| Maintenance Required | Required | Not Required |
| External Product Tank | Required | Not Required |
| Filter with Drain | Required | Not Required |
The table given below gives specifications and performance for a few models of Air Dryers:
| Power Supply | Minimum Input Air Requirements | Performance Data | Weight | ||
| Flow | Pressure | Flow -60 °F / -51 °C |
Dry Air Delivery Pressure | ||
| 120 / 220 V 50 / 60 Hz |
2.7 SCFM 76 SLPM |
25 PSIG 1.72 bar |
0.3 SCFM 20 SCFH 9.4 LPM 0.6 m3 / hr 36 lb / day |
5 psi | 9.6 lbs 4.4 kg |
| 120 / 220 V 50 / 60 Hz |
3.1 SCFM 98 SLPM |
25 PSIG 1.72 bar |
1 SCFM 60 SCFH 28.3 LPM 1.7 m3 / hr 108 lb / day |
5 psi | 14.7 lbs 46.7 kg |
Types of Air Dryer: A few important points in the operation of Air Dryers:
- Ideal Compressor System: A very important point is the
fact that lower the temperature of air at dryer inlet, consequently
smaller is to be the size of Air Drying Unit.
If the outlet air temperature from the after cooler is 400°C, Moisture content is 8000 ppm. At 450°C compressed air has 10,500 ppm. While at 500°C moisture content is 13500 ppm. which is dried by air dryer. This temperature and moisture content needs to be taken into account. - Precool Factor: Before air dryer is applied it is essential to bring down the If compressed air temperature is more than 400°C, it is essential to cool it to 400°C. This cooling can be done in a shell and tube type heat exchanger using the medium of cooling water. This cooling makes some moisture to condense out. This helps in keeping the moisture load low on air dryer. Consequently a better Dew-point of air.
- Oil filters: Oil present is hazardous for the dessicant air dryer. Therefore if the compressor is oil lubricated, then an oil filter before the air dryer is a must.
- Selection Of Dew Point: By Dew point we can measure the moisture content in Air. It is specifically the temperature at which moisture of the air starts to condense. That is, lower the Dew Point, higher the dryness of air. For most of industrial uses Dew point of around (-) 1000°C is satisfactory. However, in the industrial usages, air dryers are rated for (-) 400°C Dew point. It is to be noted that lower the Dew point, more would be the operating cost. Hence, the importance of proper Dew point selection.
- It is very important to select the correction factor for minimum
pressure (CFP) to the inlet of dryer (Allowances must be given for
system pressure losses when determining minimum operating pressure).
- The next important thing is to Select the proper correction
factor for maximum temperature (CFT) to inlet of dryer. As given in
the table:
Maximum Temperature to Inlet of Dryer ( CFP ) -C 25 35 40 45 50 -F 77 95 104 113 122 Heatless Dryers (CFT) 1.0 1.0 0.97 0.88 0.73 Heat Regenerative Dryers (CFT) 1.1 1.0 0.76 0.58 0.45 - Thirdly it is the Calculation of dryer capacity required. This is
appended below:
Inlet flow requirement/CFP x CFT= Minimum Dryer capacity requirements.
A table is given for selecting dryer model using dryer capacity requirement, it has to be ensured that the dryer model selected is equal to or greater than the dryer capacity requirement.Correction Factor Minimum Pressure to Inlet of Dryer (CFP) bar g psi g 0.63 4 58 0.75 5 73 0.88 6 87 1.0 7 102 1.13 8 116 1.25 9 131 1.38 10 145 1.5 11 160 1.63 12 174 1.75 13 189
Heatless type air dryers works on the principle of Pressure Swing Adsorption. Generally the air dryers are self-contained units, and has a skid type of design. All the accessories are provided with the skid. Following points are to be remembered while installing an air dryer:
- A well ventilated room.
- Protection from rain, moisture and dust. In fact the compressor room is the best location.
- The air to be provided to the dryer must be sufficiently cool. Ideally from 25 to 45 degree centigrade. That is the reason why an after cooler is a must.
- Compressor must be checked if it is in good condition.
- The air dryer should be free from any kind of transmission vibration.
- The air dryer should be mounted at vertical level.
Apart from the general tips on installation of air dryers, there are two ways for installing an air dryer. They are the following:
- Treatment of air with the Dryer succeeding the receiver:
This particular type of installation is suggested when the usage of
air is not that much, while consumption of air is maximum equalling
to the delivery rate of the compressor. Here the load of the dryer
remains constant.
- Treatment of air with the dryer preceding the receiver:
This particular type of installation is applied when the use of the
air is not constant. The consumption here is instantaneous that can
be either less or more than the rate of delivery from the
compressor.
