How to pick a compressed air dryer for Gulf heat and humidity: pressure dew point, refrigerated vs heatless and heated desiccant, correction factors at 45°C, purge losses, drains and condensate.
For general plant air in the UAE and Saudi Arabia, a refrigerated dryer giving a pressure dew point of about +3 °C is usually enough, provided it is sized for Gulf inlet and ambient temperatures, not catalogue conditions. Choose a desiccant dryer when air lines pass through cold or air-conditioned spaces, or when the process needs a dew point below freezing. Compare air dryer suppliers in the UAE on corrected capacity, purge loss and service.
This guide is for maintenance engineers, plant managers and buyers choosing or replacing the dryer behind a compressor. It goes deeper on one question than our guide to industrial air compressors in the UAE and Saudi Arabia, which covers compressor types, sizing, brands and the rules in each country. Here we cover dew point, dryer types, heat derating, purge losses, filters, drains and condensate disposal, with a worked sizing example. Buyers in the Kingdom can go to air dryer suppliers in Saudi Arabia.
All atmospheric air carries water vapour, and compressing it concentrates that water. Atlas Copco states that warmer air holds more water vapour than cooler air, and gives a worked example: a compressor delivering 200 l/s at 7 bar, drawing in air at 20 °C and 80% relative humidity, releases about 10 litres of water per hour. A Gulf plant room in summer is far warmer than 20 °C, so the water load on the system is heavier.
Some of that water drops out in the compressor's aftercooler. Atlas Copco says an integrated aftercooler can remove up to 70% of the moisture. The rest leaves the aftercooler as saturated air, and it will condense wherever the air cools further downstream: in pipework, valves, cylinders, instruments and spray guns. The dryer's job is to lower the pressure dew point so that this does not happen anywhere in your system.
Specify the dryer by pressure dew point (PDP), the temperature at which water vapour in the compressed air starts to condense at line pressure. Lower is drier. Do not confuse it with atmospheric dew point: Atlas Copco's example is that a PDP of +2 °C at 7 bar corresponds to about -23 °C at atmospheric pressure.
ISO 8573-1 turns dew point into a water class. The classes most buyers meet are Class 4 (+3 °C or lower), Class 2 (-40 °C or lower) and Class 1 (-70 °C or lower); the main guide has the full table. Write the required class into the RFQ, then let the dryer type follow from it.
A practical rule for choosing the PDP: it must be below the lowest temperature any pipe or component will reach. In the Gulf that is rarely outdoor air. It is the chilled room, the air-conditioned control room or packing hall, the instrument line running past an air handling unit, or the line laid in a cold store. If any line drops below +3 °C, or sits in a space cooled close to that, a refrigerated dryer is the wrong choice for that line.
| Dryer type | Typical pressure dew point | Compressed air used for regeneration | Where it fits |
|---|---|---|---|
| Refrigerated (non-cycling, cycling or VSD) | +2 to +10 °C; many models rated +3 °C | None | General plant air in warm pipework; workshops, packaging, general manufacturing |
| Heatless desiccant | -40 °C, down to -70 °C on some models | About 15 to 20% of rated flow | Low running hours, backup duty, remote or hazardous sites, high inlet temperatures |
| Externally heated desiccant | -40 °C | About 8% (Sullair average) | Medium and large flows where purge loss matters |
| Heated blower purge desiccant | -40 °C | About 2%, or zero on zero-purge designs | Large flows running continuously |
| Heat of compression (rotary drum) | -25 to -45 °C depending on model | Zero by design | Behind oil-free screw or centrifugal compressors |
Purge figures are the makers' own: Atlas Copco gives 16 to 20% of rated flow for heatless dryers, and Sullair's brochure gives averages of 15% heatless, 8% externally heated and 2% heated blower purge.
A refrigerated dryer cools the compressed air in a closed refrigeration circuit so that water condenses and is separated, then reheats the dry air, usually against the incoming warm air, so that the outlet pipe does not sweat. Atlas Copco gives the usable dew point range as +2 to +10 °C, and its FX range is rated as low as +3 °C. They cannot go much lower because the condensed water would freeze inside the heat exchanger.
They are cheap to buy and simple to maintain. The control type matters for running cost:
A desiccant dryer has two towers filled with an adsorbent. Wet air flows through one tower while the other is regenerated, then the towers swap. The regeneration method decides most of the running cost.
On life-cycle cost, Atlas Copco states that over ten years a heated blower purge dryer has roughly a 40% total cost of ownership advantage over a heatless dryer, and that lost purge air accounts for 80% of that difference. On a plant running around the clock, the cheaper heatless dryer is often the more expensive one to own. Two features reduce the loss further: dew point dependent switching, which only changes towers when the desiccant is actually loaded (Atlas Copco claims savings of up to 90% from it), and a dryer pressure drop low enough not to force a higher compressor setpoint; Atlas Copco targets below 0.2 bar for most of its desiccant models.
Plants running oil-free screw or centrifugal compressors have another option. Atlas Copco's MD, MDG and ND rotary drum dryers use the heat generated in compression to regenerate the desiccant, with zero purge by design. The maker lists dew points of -25 °C for the MD, -40 °C and lower for the MDG and to -45 °C for the ND, and says the MDG's only electrical load is a 120 W drum motor. If you are buying an oil-free compressor and need a sub-zero dew point, ask whether a heat of compression dryer can be packaged with it.
This is where most dryer problems in the region start. Dryer capacity is published at reference conditions, and those conditions are cooler than a Gulf compressor room in summer. As one example, Hertz Kompressoren's HRD refrigerated dryer datasheet states capacity to ISO 7183 at 7 bar, 35 °C inlet and 25 °C ambient. Above those temperatures the dryer has to remove more water with less cooling margin, so its real capacity falls.
The same datasheet publishes the correction factors, which show how fast the capacity drops:
| Condition | Value | Correction factor |
|---|---|---|
| Inlet air temperature | 35 °C (reference) | 1.00 |
| Inlet air temperature | 40 °C | 0.92 |
| Inlet air temperature | 45 °C | 0.78 |
| Inlet air temperature | 50 °C | 0.65 |
| Ambient temperature | 25 °C (reference) | 1.00 |
| Ambient temperature | 35 °C | 0.93 |
| Ambient temperature | 40 °C | 0.84 |
| Ambient temperature | 50 °C | 0.70 |
| Working pressure | 7 bar (reference) | 1.00 |
| Working pressure | 6 bar | 0.94 |
The datasheet's sizing method divides the compressor flow by each factor. A worked example for a summer case:
At 50 °C inlet and 50 °C ambient the same 10 m3/min needs a dryer rated at about 22 m3/min, more than twice the compressor's output. Factors differ between makers and models, so treat these figures as an illustration of scale and ask every bidder for their own correction table and the dryer's maximum rated inlet and ambient temperatures. The HRD range, for instance, is rated for up to 60 °C inlet and 50 °C ambient; a dryer with lower limits may trip or pass wet air in August.
Three site measures reduce the derating: duct the compressor's hot cooling air out of the room so the dryer is not breathing it; keep the aftercooler and the dryer's condenser clean, because dust on the fins raises temperatures; and put the dryer where it draws the coolest air in the room. Desiccant dryers have inlet temperature limits too, so ask for them.
The dryer handles water vapour. Particles and oil need filters, and the dryer needs protecting from them. A typical train, as shown in maker datasheets such as Hertz's, is:
Size filters for the full compressor flow at the actual pressure, and ask for the pressure drop at that flow, because every filter adds to the pressure the compressor has to make. See compressed air filter suppliers in the UAE and the wider compressed air treatment category.
A dryer that cannot get rid of its water is not drying anything. Drains are fitted at the aftercooler, receiver, dryer and filters, and the type decides how much compressed air leaks away with the water. A Compressed Air Challenge article on Compressed Air Best Practices calls timer drains a quick fix: they may not open long enough to clear all the condensate, or stay open and waste air. Its worked example shows a quarter-inch timer drain open 10 seconds every 3 minutes at 100 psi wasting about 3.6 cfm on average. The same article treats float drains as no-waste (with the float replaced yearly) and electronic level-sensing drains as more reliable still. On humid coastal sites, where condensate volumes change sharply between seasons, level-sensing drains are the safer specification.
Condensate from an oil-injected compressor carries oil. Atlas Copco's oil-water separator literature says an oil-water separator removes the oil before the water enters the sewer, and that condensate treatment is a legal requirement in most countries. Ask your free zone, municipality or industrial city authority what applies to discharge at your site.
Size the separator for Gulf conditions as well. Atlas Copco's OSC leaflet rates capacity at 25 °C inlet air, 60% relative humidity and 12 running hours a day, and publishes correction factors of 0.50 at 35 °C ambient, 0.30 at 40 °C, 0.74 at 80% humidity and 0.5 for 24-hour running. A separator picked from the headline capacity can be badly undersized for a humid site running around the clock.
Most compressor makers sell dryers, and specialist treatment makers sell them too. Two examples from makers' own literature:
Atlas Copco offers refrigerated dryers (the FX and FD ranges, including FD VSD), twin-tower desiccant dryers including the BD+ heated blower purge and BDZP zero-purge ranges, and the MD, MDG and ND heat of compression drum dryers for oil-free compressors. It also makes OSC oil-water separators. It suits plants that want the compressor, dryer and treatment from one service organisation.
Sullair lists desiccant dryers in four series: SAV heated blower purge (500 to 7,500 cfm), SAH externally heated (500 to 3,000 cfm), SA heatless (55 to 3,000 cfm) and DMD modular (3 to 240 cfm), all rated at -40 °C dew point. Its website also lists compressed air dryers, filters, separators and drains among its product categories.
To get quotes against this list, post your air dryer requirement and suppliers in the UAE and Saudi Arabia can respond. If the dryer is part of a new compressor package, the main screw air compressor category lists the suppliers who package both.
More in this series: Screw vs Piston Air Compressors, Portable Diesel Air Compressors and Compressor Rental in the UAE and Saudi Arabia, How to Size an Air Compressor, Oil-Free Air Compressors for Food, Pharma and Electronics Plants.
Almost always for industrial use. Compressing air concentrates its water vapour, and Atlas Copco's example of a 200 l/s compressor at 20 °C and 80% humidity releases about 10 litres of water per hour. An aftercooler removes up to about 70% of it; the rest condenses in pipes and tools unless a dryer lowers the pressure dew point below the coldest point in the system.
Neither is better in general. A refrigerated dryer gives a pressure dew point of about +2 to +10 °C at low cost and suits warm plant pipework. A desiccant dryer reaches -40 °C or lower and is needed for instrument air, lines through cold or air-conditioned spaces, and many food, pharmaceutical and electronics uses. Choose from the dew point you need.
Atlas Copco gives a range of +2 to +10 °C, and many models, including its FX range, are rated as low as +3 °C, which matches ISO 8573-1 water Class 4. Refrigerated dryers cannot go much below that because condensed water would freeze in the heat exchanger. If you need a sub-zero dew point, use a desiccant dryer.
It depends on the regeneration method. Atlas Copco puts heatless dryer purge at about 16 to 20% of rated flow, and Sullair gives averages of 15% heatless, 8% externally heated and 2% heated blower purge. Zero-purge and heat of compression designs use no compressed air for regeneration. On continuous duty, purge loss usually outweighs the price difference.
It cuts capacity. Ratings are given at reference conditions such as 35 °C inlet and 25 °C ambient. One maker's table derates a refrigerated dryer to 0.78 at 45 °C inlet and 0.84 at 40 °C ambient, so a 10 m3/min compressor needs a dryer rated around 15 m3/min. Ask every bidder for their correction factors and maximum temperatures.
Avoid timer drains except as a stop-gap: a Compressed Air Challenge example shows one wasting about 3.6 cfm of compressed air on average. Float drains waste little if the float is replaced yearly, and electronic level-sensing drains are more reliable still. Route oily condensate from oil-injected compressors through an oil-water separator sized for your humidity and running hours.
Featured image: Photo: PEO ACWA, CC BY 2.0, via Wikimedia Commons
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