Dew-point measurement: how dry is your compressed air?

What is pressure dew point?

Air always holds some water vapour. When it is compressed and then cooled, the vapour condenses into liquid water. The dew point is the temperature at which that condensation starts. In compressed air we use pressure dew point (PDP): the dew point measured at the actual line pressure. As long as the air never cools below its PDP anywhere in the system, no liquid water forms.

Atmospheric dew point is a different number. Air at 7 bar (101.5 psig) with a PDP of +3 °C has a much lower dew point once it is expanded to atmosphere. Instruments and dryer data sheets must state which one they give, and comparing a PDP with an atmospheric value is a common source of confusion.

Typical dew points and what they mean

The class you need comes from the process, the product standard or the customer specification. The dryer’s nameplate class is what it can achieve at rated conditions. Measurement tells you what it is actually doing.

Why the dryer display is not the full answer

Many dryers show a dew point or a coloured indicator on the controller. That is useful, but it has limits.

• Refrigerated dryers often display evaporator temperature, not a true measured dew point.
• Built-in sensors drift, become contaminated with oil or get damaged by liquid water.
• The sensor sits at the dryer outlet. Air can pick up moisture downstream through a bypass valve left open, a wet receiver, or condensate in poorly drained pipes.
• The display may be correct at light load and wrong at peak flow or on a hot afternoon.

An independent measurement with a calibrated instrument, at the point where air quality matters, is the proper check.

How we measure dew point correctly

1. Define the question. Is it dryer performance, air quality at a critical user, or compliance with a specified class? That decides where to measure.
2. Choose the sampling point. Dryer outlet for dryer performance; the point of use, after final filters, for product air.
3. Use a clean, short sample line of suitable material, ideally stainless steel or PTFE. Long plastic or rubber tubes absorb and release moisture and give wrong readings.
4. Control the sample flow so the sensor sees steady conditions, and check for leaks at fittings that would let ambient moisture in.
5. Allow stabilisation time. Dry readings, especially −40 °C and below, can take a long time to settle after the line has been exposed to humid air.
6. Record pressure at the sensor. If the sensor is at a lower pressure than the line, the reading must be converted back to line PDP.
7. Log through representative operation, including peak demand, high ambient and dryer regeneration cycles.
8. Interpret in context with flow, inlet temperature and dryer state.

Instrument types in plain terms

The instrument must cover the expected range. A sensor designed for refrigerated-dryer air will not give a reliable reading at −40 °C.

Why is my dryer not reaching its dew point?

When measurement shows a dew point worse than expected, the common causes are:

• Refrigerated dryers: flow above rating; inlet air too hot because the aftercooler is fouled or the compressor room is hot; dirty condenser; low refrigerant; failed drain so separated water re-enters the air; hot-gas bypass valve faults.
• Desiccant dryers: desiccant contaminated with oil or worn into dust; purge flow too low; faulty valves or controller timing; heaters failed on heated types; bypass open; inlet temperature or flow above design.
• Downstream causes: wet receivers after the dryer, open bypass valves, cross-connection with undried air, or undrained low points.

In Hyderabad and Andhra Pradesh summers, a refrigerated dryer can lose performance simply because the room is hot and the air entering it is far warmer than the dryer’s rated inlet. In the monsoon, inlet moisture load rises sharply. Both show clearly in a properly logged measurement.

Dew point and the rest of air quality

Dew point measures water vapour only. It says nothing about particles, oil aerosol, oil vapour or microbiological contamination. A complete ISO 8573-1 air-quality statement needs each contaminant measured by its own method. Dew point is often the first test because water causes the most visible problems – corroded pipes, rusty tools, spoiled paint finishes, swelling in pneumatic seals and microbial growth – but it is one part of the picture.

Where to place a permanent dew-point sensor

For critical systems, continuous monitoring is worth installing. The sensor should sit on a short side connection from the dryer outlet header, after the dryer’s own after-filter, with a small controlled bleed so the sensor sees fresh air rather than a dead pocket. Avoid locations where liquid water can reach the sensor, such as the bottom of a pipe or right after a drain. Connect the output to an alarm or the plant control system with a sensible limit, and log it so that trends – a slow rise in summer, or a step change after maintenance – are visible. For critical point-of-use air, a second sensor near the user confirms what the process actually receives.

Dew point in hot and humid conditions

Warm air holds far more moisture than cool air. On a humid monsoon day in coastal Andhra Pradesh, or on a hot afternoon in a poorly ventilated Hyderabad compressor room, the water load reaching the dryer can be several times higher than on a cool, dry morning. A refrigerated dryer sized for nominal conditions may then be working above its capacity, and its dew point drifts upward. Correction factors for inlet temperature, ambient temperature and pressure, published by dryer makers, show how much real capacity falls. Measuring dew point during these worst-case conditions, not only on a mild day, is the honest test of a treatment system.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka, Secunderabad, measures pressure dew point as part of dryer service and compressed-air audits for plants across Telangana and Andhra Pradesh, with project support elsewhere in India where feasible. As an independent multi-brand specialist, we work on refrigerated and desiccant dryers and compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan and many other makes.

• We check dew point at the dryer outlet and at critical points of use.
• We diagnose dryers that are not achieving their dew point and repair or service them.
• We supply desiccant, filter elements, drains and dryer spare parts identified from the nameplate.
• We recommend correctly sized dryers, new or rental, where demand has outgrown the existing unit.
• We link dew-point findings to wider air-quality and energy actions.

What to send us

• Dryer make, model and type, with a nameplate photo.
• Compressor capacity and working pressure.
• The dew point or air-quality class you need, and why (process, customer specification, standard).
• Where the wet air problem shows up and when.
• Compressor room temperature and whether there is a bypass line.

Frequently asked questions

What dew point does a refrigerated air dryer give?

A refrigerated dryer typically delivers a pressure dew point of about +3 °C, with many data sheets quoting +3 to +7 °C at rated conditions. That corresponds to ISO 8573-1 water class 4 or 5. Performance falls if inlet temperature, ambient temperature or flow exceeds the dryer’s rating, which is common in hot Indian summers. If you need drier air, for example for outdoor lines or instrument air, a desiccant dryer is needed.

What is the difference between pressure dew point and atmospheric dew point?

Pressure dew point is the condensation temperature at the actual line pressure, which is what matters inside the pipework. Atmospheric dew point is the same air expanded to atmospheric pressure, and is always lower. A +3 °C pressure dew point at 7 bar is a much lower figure at atmosphere. Always check which value an instrument or data sheet states before comparing results.

Why is there still water in my air lines after the dryer?

Possible causes include a dryer running above its rated flow or with hot inlet air, a failed condensate drain, an open bypass valve, wet receivers or undrained pipes after the dryer, or a dryer fault such as low refrigerant or contaminated desiccant. Measuring dew point at the dryer outlet and further downstream shows whether the dryer itself or the distribution is to blame.

How often should dew point be checked?

Critical air systems such as instrument, pharmaceutical or food air should have continuous dew-point monitoring with alarms, plus periodic checks with an independent calibrated instrument. General plant air can be checked at each dryer service and whenever water problems appear. Sensors themselves need calibration at the interval their maker recommends, because drift gives false confidence.

Which dew point do I need for instrument air?

Instrument air is commonly specified with a pressure dew point well below the lowest temperature the pipework will ever see, and many plants use around −40 °C from a desiccant dryer. The correct value comes from your plant specification, the instrument makers and any applicable standard. We help match the dryer and measurement to that requirement rather than assuming one number.

Can I rent a dryer while mine is being repaired?

Often, yes. Where a dryer fails or a plant needs extra treatment capacity during a shutdown or validation period, temporary refrigerated or desiccant dryers can be arranged with a compressor package. Tell us the flow, pressure and dew point needed, and the site conditions. We check dew point at commissioning so you know the temporary air meets your requirement before production restarts.

Related pages

• Compressed-air quality measurement
• Dryer service and dew-point performance
• Refrigerated air dryers
• Desiccant adsorption dryers
• Air dryer spare parts
• Compressed air audit and measurement hub