Which air dryer do you need? Start from the dew point
The question is not “which dryer is best”. It is “how dry must the air be at the point of use?” That is stated as a pressure dew point (PDP): the temperature at which water would start to condense inside the pipe at line pressure. ISO 8573-1 turns that into a water class, which is the right way to write it on a purchase order.
A refrigerated dryer cannot go much below +3 °C PDP, because the water would freeze in its evaporator. Anything drier needs a desiccant (adsorption) dryer. Buying more dryness than the process needs costs energy every hour the plant runs, so specify by user, not for the whole factory.
| Required PDP | ISO 8573-1 water class | Dryer type | Typical uses in Hyderabad and the region |
|---|---|---|---|
| +7 °C | Class 5 | Refrigerated (some designs) | General workshop air, tools, blowing, where pipes stay above about 10 °C |
| +3 °C | Class 4 | Refrigerated | Most factory air: textiles (spinning and weaving air jets), packaging machines, general food plant air, spray painting with a good filter set |
| −20 °C | Class 3 | Desiccant (heatless or heated) | Paint shops and powder coating with long or outdoor lines, pneumatic conveying of hygroscopic powders, air in air-conditioned rooms where lines are cold |
| −40 °C | Class 2 | Desiccant (heatless, heated-purge or blower-purge) | Pharma process and tablet coating air, instrument air, food and dairy product-contact air, electronics assembly, feed to some nitrogen generators |
| −70 °C | Class 1 | Desiccant with molecular sieve | Special pharma, laboratory and electronics duties; only where the user’s specification demands it |
Pharma units in the Hyderabad belt and food plants should take the dew point from their own quality documents, not from a supplier’s habit. For the theory of each type, read our guides to refrigerated air dryers and desiccant adsorption dryers.
Sizing for Hyderabad conditions: why a “350 cfm dryer” is often not 350 cfm
Dryer catalogue ratings are given at reference conditions. For refrigerated dryers these are commonly about 7 bar(g), 35 °C compressed-air inlet and 25 °C ambient. A Hyderabad compressor room in May is nothing like that. Hot air carries more water vapour, and a hot condenser rejects heat poorly, so the same dryer handles less air. Each maker publishes correction factors; the real capacity is the rated capacity multiplied by all of them.
Worked example: converting a catalogue rating to real capacity
(Illustrative.) A plant in an industrial area near Hyderabad runs a screw compressor delivering 10 m³/min (353 cfm) at 7.5 bar(g) (108.8 psig). The offered refrigerated dryer is rated 14 m³/min (494 cfm) at reference conditions, so on paper it has 40% spare. In summer the air leaves the aftercooler at 45 °C and the room reaches 40 °C. Typical factors from a maker’s table (confirm on the actual model’s data sheet):
| Condition | Site value | Typical factor |
|---|---|---|
| Compressed-air inlet temperature | 45 °C (reference 35 °C) | 0.72 |
| Ambient temperature | 40 °C (reference 25 °C) | 0.80 |
| Working pressure | 7.5 bar(g) (reference 7 bar(g)) | 1.03 |
| Combined factor | 0.72 × 0.80 × 1.03 | 0.593 |
- Real capacity = 14 × 0.593 = 8.31 m³/min (293 cfm), against a demand of 10 m³/min. The dryer is about 17% short exactly when the air is wettest.
- Minimum rated size for this duty = 10 ÷ 0.593 = 16.86 m³/min (595 cfm), so pick the next catalogue size up.
- Alternative: fix the room. If better ventilation and a clean aftercooler bring the inlet to about 38 °C (typical factor 0.89) and the ambient to about 33 °C (typical factor 0.88), the combined factor becomes 0.89 × 0.88 × 1.03 = 0.807, and the 14 m³/min dryer gives 14 × 0.807 = 11.29 m³/min, which is enough.
The second option is often cheaper and also helps the compressor. Our dryer correction factor worksheet runs the same arithmetic for your figures.
Desiccant dryers are corrected the same way, mainly for inlet temperature and pressure. Hot, wet inlet air shortens the bed’s working time. Above about 35–40 °C inlet, a refrigerated pre-dryer or a better aftercooler ahead of the desiccant dryer is often worth it.
Purge air cost: heatless versus heated and blower-purge
A heatless desiccant dryer regenerates its off-line tower with part of its own dried air. That purge air was compressed at full cost and is then blown to atmosphere. A heated-purge dryer uses a heater and much less purge. A blower-purge dryer uses ambient air from a blower plus a heater, with little or no compressed air. The purchase price rises in the same order; the running cost falls.
Worked example: annual running cost of three dryer types
(Illustrative.) A 10 m³/min (353 cfm) desiccant dryer for −40 °C PDP, running 6,000 hours a year. The compressor uses 6.5 kW per m³/min (typical for a modern screw at 7 bar(g); use your own measured figure). Tariff ₹8 per kWh (illustrative; use your own bill). Purge shares and heater or blower averages are typical values; use the maker’s data sheet.
| Dryer type | Purge air | Compressor power for purge | Heater or blower (average) | Total | kWh per year | Cost per year at ₹8 |
|---|---|---|---|---|---|---|
| Heatless (15% purge) | 1.5 m³/min | 1.5 × 6.5 = 9.75 kW | 0 | 9.75 kW | 58,500 | ₹4.68 lakh |
| Heated-purge (7% purge) | 0.7 m³/min | 0.7 × 6.5 = 4.55 kW | 3.0 kW | 7.55 kW | 45,300 | ₹3.62 lakh |
| Blower-purge (2% cooling purge) | 0.2 m³/min | 0.2 × 6.5 = 1.30 kW | 4.5 kW | 5.80 kW | 34,800 | ₹2.78 lakh |
Arithmetic for the heatless row: 9.75 kW × 6,000 h = 58,500 kWh; 58,500 × ₹8 = ₹4,68,000. The blower-purge dryer saves (9.75 − 5.80) × 6,000 × 8 = ₹1,89,600 a year against heatless in this example. Whether that pays back the higher price depends on hours run and your tariff.
Two practical points change the answer more than the dryer type:
- Purge is fixed, load is not. A heatless purge orifice is set for rated flow. At 50% average load, the purge is still 1.5 m³/min, so it is 30% of the air actually used. Dew-point dependent switching stretches the cycle at light load and cuts this waste.
- Purge must be supplied. If the compressor is already at full load, a heatless dryer needs a bigger compressor. Add the purge to the compressor sizing.
For dryers below about 5 m³/min (177 cfm), or plants running one shift, heatless is usually the sensible choice. For large, continuous duties, heated or blower-purge types deserve a costed comparison. Use our adsorption dryer purge consumption worksheet with your own figures.
Filters and drains: what goes before and after the dryer
A dryer is one stage of a treatment train. Oil and liquid water damage desiccant permanently and foul refrigerated heat exchangers, so the filters and drains matter as much as the dryer.
| Position | Typical grade | Why |
|---|---|---|
| Before a refrigerated dryer | General-purpose coalescing filter, about 1 µm (often with a water separator) | Removes bulk liquid and oil aerosol; keeps the heat exchanger clean |
| After a refrigerated dryer | High-efficiency coalescing, about 0.01 µm; activated carbon only if oil vapour or odour matters | Brings oil aerosol down to the class the users need |
| Before a desiccant dryer | General-purpose plus high-efficiency coalescing, about 0.01 µm | Protects the bed: oil coats desiccant and it never recovers |
| After a desiccant dryer | Particulate (dust) filter, about 1 µm | Catches desiccant dust before it reaches valves and products |
Change elements by differential pressure and by hours, whichever comes first. A loaded filter costs energy: every extra 1 bar the compressor must make adds about 7% power.
Drains are the usual weak point. Timer drains waste air if set too long and flood the system if set too short or blocked. Level-sensing, zero-air-loss drains on the aftercooler, receiver, filters and dryer separator are worth their price in this climate. Oily condensate should go to an oil-water separator, not the storm drain. Filter elements and drains are covered on our filter elements and drains page.
Monsoon load: how much water a compressor really makes
From June to September, intake air around Hyderabad often sits near 30 °C and 80–90% relative humidity. The compressor squeezes that water into the line. If any drain fails, it arrives at the users.
Worked example: water per hour in the monsoon
(Illustrative.) A 10 m³/min (353 cfm) compressor at 7.5 bar(g). Intake 30 °C, 85% RH. Hyderabad atmospheric pressure is about 0.950 bar(a), so line pressure is about 8.45 bar(a).
- Saturation vapour pressure at 30 °C is about 4.25 kPa; at 85% RH the vapour pressure is 3.61 kPa.
- Moisture content = 0.622 × 3.61 ÷ (95.0 − 3.61) = 0.0246 kg water per kg dry air.
- Dry-air density at intake ≈ 1.050 kg/m³, so 10 m³/min × 60 = 600 m³/h carries about 630 kg/h of dry air.
- Water entering = 630 × 0.0246 ≈ 15.5 kg/h.
- Air leaving a +3 °C PDP dryer at 8.45 bar(a) holds about 0.00056 kg/kg, or 0.35 kg/h as vapour.
- Water to be removed ≈ 15.5 − 0.35 = 15.1 kg/h, about 300 litres in a 20-hour day.
With the aftercooler outlet at 40 °C, roughly 12 kg/h of this condenses in the aftercooler and receiver and must leave through their drains. The dryer removes the remaining 3 kg/h or so. In a dry April afternoon (40 °C, 25% RH) the same compressor takes in about 7.7 kg/h. The monsoon roughly doubles the condensate, which is why drains that cope in summer fail in July.
Service schedule and common faults
The intervals below are typical for clean, well-ventilated rooms. Dusty sites, 24-hour running and hot rooms need shorter ones; the maker’s schedule is the minimum. Our guide to dryer service and dew-point performance explains how to measure dew point properly.
| Interval (typical) | Refrigerated dryer | Desiccant dryer |
|---|---|---|
| Daily | Dew-point indicator in the green; drain discharges when tested | Tower pressures and switching normal; drain working; no constant exhaust blowing |
| Monthly | Blow out or wash condenser fins (more often near cotton ginning, textile or dusty areas); check fan | Check filter differential pressures; listen for valve leakage at silencers |
| 3–6 months | Log inlet, outlet and ambient temperatures and PDP at full load; clean drain strainer | Verify PDP with a calibrated instrument; check purge setting and dew-point sensor |
| Yearly or 8,000 hours | Replace filter elements; check refrigerant pressures and electrical connections | Replace filter elements and silencers; service valve kits as the maker advises |
| 3–5 years (condition-based) | Heat exchanger and drain overhaul if pressure drop or PDP has drifted | Replace desiccant when PDP trend rises with valves, purge and pre-filter proven good |
| Fault | Likely cause | First check |
|---|---|---|
| Refrigerated: water downstream in summer only | Condenser fouled with dust or lint; hot room; dryer overloaded | Condenser fins, room temperature, inlet temperature against the rating |
| Refrigerated: high dew point, compressor running constantly | Low refrigerant (leak), failing refrigeration compressor, hot-gas bypass valve fault | Suction and discharge pressures by a qualified technician; leak test |
| Refrigerated: water slugs at users | Blocked or failed drain; bypass valve open | Test drain manually; check bypass |
| Refrigerated: trips on high pressure or overload | Dirty condenser, low supply voltage, fan failure | Voltage at the terminals under load; fan; condenser |
| Desiccant: constant air loss from exhaust | Inlet or exhaust valve leaking; seal worn; solenoid stuck | Exhaust of the on-line tower should be silent |
| Desiccant: white dust downstream | Desiccant crushing from fast repressurisation, over-flow or bed age | Repressurisation time; flow against rating; after-filter |
| Desiccant: PDP rising over months | Oil or liquid water in the bed; desiccant at end of life; purge too low | Pre-filter and drain; purge pressure; desiccant sample |
Refrigerated dryers also suffer on weak grids. A dryer refrigeration compressor running at low voltage overheats and trips; a voltage and phase monitor protects it. Spares for both types are listed on our dryer parts page.
Air dryer price: what drives it
The cost of an air dryer for a compressor depends mainly on:
- Corrected capacity, not the catalogue figure: a dryer sized for 45 °C costs more than one sized for 25 °C, and the cheaper one may not work in May.
- Dew point and type: refrigerated is the lowest first cost; heatless desiccant next; heated-purge and blower-purge higher, but with lower running cost.
- Controls: dew-point dependent switching, cycling or variable-speed refrigerated designs, and remote alarms add cost and save energy.
- Filters and drains included: compare like with like.
- Pressure and materials: higher-pressure designs and stainless or food-grade construction cost more.
How Reddy Compressors helps
- Supplies refrigerated, heatless and heated desiccant dryers, matched to the compressor and the process, as an independent multi-brand supplier since 2002.
- Services dryers of all major makes, including under AMC, with 20+ service engineers and technicians.
- Supplies dryer spares, filter elements, drains, valve kits and desiccant, identified from nameplate data.
- Covers all of Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad; supplies across India.
- Rents compressors from 50 to 2,000 cfm if your plant needs temporary air during dryer work.
What to send us
- Photos of the compressor and dryer nameplates (or the compressor model and its flow in cfm or m³/min).
- Working pressure and the dew point your process needs, or what the air is used for.
- Summer compressor-room temperature and the air temperature leaving the aftercooler, if known.
- Existing filters and drains, with photos.
- For a fault: what you see (water, dust, air loss, trips) and when it happens.
- Hours run per day and your electricity tariff, if you want a running-cost comparison.
Frequently asked questions
Which air dryer is right for my compressor in Hyderabad?
For most factory air, a refrigerated dryer giving about +3 °C pressure dew point is enough. Choose a desiccant dryer when the process needs −20 °C or −40 °C, for example pharma, instrument air, powder conveying or cold lines. Size the dryer on your summer conditions, not on the compressor’s cfm alone: at 45 °C inlet and ambient, a dryer can lose 30–40% or more of its catalogue capacity. Send us the compressor details and air use and we will propose a type and size.
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What is the difference between a refrigerated and a heatless air dryer?
A refrigerated dryer cools the air to about +3 °C so water condenses and drains away; it uses electricity for a small refrigeration system. A heatless desiccant dryer passes air through beads that adsorb water, reaching −40 °C or lower, and regenerates them with a share of its own dried air, typically around 15% of rated flow. Refrigerated is cheaper to buy and run; heatless gives much drier air but costs more in compressed air.
What is the price of an air dryer for a compressor?
It depends on corrected capacity, dew point, type of regeneration, controls, and whether filters and drains are included. A refrigerated dryer costs least; heated-purge and blower-purge desiccant dryers cost most to buy but least to run. Compare quotes at the same corrected capacity and the same scope. Tell us your compressor, pressure, dew point and summer temperatures, and we will quote options for the same duty.
Are you compressed air dryer suppliers in Hyderabad for all makes?
Yes. Reddy Compressors is an independent multi-brand supplier in Secunderabad, operating since 2002, with tie-ups with all major brands. We supply refrigerated and desiccant dryers and their spares, and we match them to any make of compressor. We are not an authorised dealer of any single brand, which lets us propose what fits the duty.
Do you do air dryer service and repair at site?
Yes. Our engineers service refrigerated and desiccant dryers at site across Telangana and Andhra Pradesh: condenser cleaning, drain and filter work, refrigerant-circuit checks by qualified technicians, valve and solenoid repair, purge setting, desiccant replacement and dew-point measurement. Dryers can be included in a compressor AMC. Send the nameplate photo and the symptom on WhatsApp so the engineer brings the right parts.
Where can I get dryer spare parts in Hyderabad?
We supply dryer spare parts for all major makes: drains, filter elements, valve kits, solenoids, silencers, desiccant, fans, controllers and sensors, plus refrigeration parts. The quickest way to the right part is a clear photo of the dryer nameplate and of the old part with any numbers on it. Some parts look alike across models but differ in rating, so we confirm before supply.
How often should desiccant be changed in an air dryer?
There is no fixed rule. With clean, dry, oil-free inlet air, activated alumina often lasts three to five years. Oil carry-over or liquid water can ruin it in weeks. Change it when the dew point keeps rising even though the valves switch properly, the purge is correct and the pre-filter and drain are working, or when it is visibly oily or dusting.
Why does my dryer pass water only in the monsoon?
Because the compressor takes in roughly twice as much water in humid weather. A 10 m³/min compressor can make about 15 kg of condensate an hour in the monsoon. Drains that cope in dry months then overflow, and water bypasses the separator. Test every drain, check the bypass valve and confirm the dryer’s corrected capacity at your inlet temperature.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.