Air dryer service and dew-point performance

Why a running dryer can still deliver wet air

Most dryer complaints start the same way: water in the lines, rusty drops at a tool, a spray gun spitting, or an instrument valve sticking, while the dryer panel shows a green light. The light usually means the dryer has power and no alarm. It does not mean the air leaving it has reached the required pressure dew point (PDP).

Pressure dew point is the temperature at which water vapour in the compressed air begins to condense at line pressure. If the air in your pipes ever cools below the PDP, water forms. That is why the dew point a dryer can hold, not the fact that it is running, is the real measure of its health.

Three conditions explain most “running but wet” cases:

• The dryer is overloaded. Actual flow, inlet temperature or ambient temperature is higher than the conditions the dryer was rated for, so its real capacity is lower than the nameplate figure.
• A component has degraded. A fouled condenser, a failed drain, worn valves, contaminated desiccant or a leaking purge path quietly reduce drying performance.
• Water is getting past the dryer. A bypass valve is open, a drain is blocked, or liquid water arrives at the inlet because the aftercooler or separator upstream is not working.

Service has to separate these three. Replacing parts without knowing which one applies often costs money without curing the water.

Dryer technologies and what each one needs

Each dryer type removes moisture differently, fails differently and is serviced differently. The table gives typical figures; exact values depend on the model and the manufacturer.

A refrigerated dryer cannot reach sub-zero dew points, because the water it removes would freeze in the heat exchanger. If your process needs instrument-quality or outdoor-pipe dryness, a desiccant dryer is normally required. Selecting the right class against ISO 8573-1 comes before any service decision.

Start with duty: is the dryer sized for today’s conditions?

Dryers are rated at reference conditions, commonly around 7 bar inlet pressure, 35 °C inlet temperature and 25 °C ambient for refrigerated units. Hyderabad and Secunderabad compressor rooms routinely run hotter than that in summer. Higher inlet temperature means more water vapour per cubic metre of air, and higher ambient means the refrigeration system rejects heat less effectively. Both reduce real capacity.

Before blaming a component, compare the dryer’s corrected capacity with the actual load:

1. Record the make, model, serial number and rated flow from the nameplate.
2. Measure or estimate the actual flow passing through the dryer, including any compressor added since installation.
3. Measure inlet pressure, inlet air temperature and room ambient during a hot, busy shift.
4. Apply the manufacturer’s correction factors for pressure, inlet temperature and ambient to get corrected capacity.
5. If actual flow exceeds corrected capacity, the dryer is undersized for present conditions, and no amount of servicing will fully cure it.

Room ventilation is often the easiest fix. A refrigerated dryer pulling in its own hot exhaust air, or a compressor discharging hot air straight onto the dryer condenser, can lose much of its margin. Our dryer correction factor worksheet helps with the arithmetic.

Refrigerated dryer service: what we check

A refrigerated dryer cools the air, condenses the water, separates it and drains it. Every stage has to work.

• Condenser and fan. Dust and oil mist block condenser fins quickly in industrial rooms. A blocked condenser raises condensing pressure and evaporator temperature, so the dew point climbs. Fan operation and fan-control settings are checked at the same time.
• Evaporator or dew-point temperature. The controller reading is compared with an independent measurement. A sensor that reads low can hide a warm evaporator.
• Refrigerant circuit. Suction and discharge conditions, signs of leaks at joints and the hot-gas bypass valve are checked. Refrigerant work is carried out only by competent technicians using correct recovery practice.
• Separator and drain. A drain that is stuck shut sends separated water straight downstream. A drain stuck open wastes air.
• Air-to-air heat exchanger. Fouling here raises pressure drop and evaporator load. Differential pressure across the dryer is measured at load.
• Icing. If the evaporator runs too cold, particularly at light load, ice can partially block air passages. Hot-gas bypass or cycling control settings are checked.
• Bypass valves. A bypass left partly open after earlier maintenance is one of the most common causes of wet air.

Desiccant dryer service: what we check

A desiccant dryer adsorbs water on one tower while the other regenerates, then switches. Performance depends on the media being clean and dry, and on the valves switching and sealing correctly.

• Prefilter. Oil aerosol coats desiccant beads and permanently reduces their capacity. A coalescing prefilter in poor condition is the most common reason desiccant fails early.
• Liquid water at the inlet. A desiccant dryer is designed for vapour, not slugs of water. A failed aftercooler drain or separator upstream can flood the bed.
• Valves. Inlet, outlet, purge, exhaust and check valves are checked for correct switching and leakage. A leaking exhaust valve wastes air and lets wet air bypass regeneration.
• Purge flow. Too little purge leaves the regenerating tower wet. Too much wastes compressed air you have paid to produce. Purge orifice condition and the purge setting are checked against the manufacturer’s recommendation. Our adsorption dryer purge consumption worksheet shows what purge costs.
• Repressurisation. If a tower switches before it has repressurised, the sudden flow causes bed movement, attrition and dusting.
• Heater and blower on heated and blower-purge units: heater current, thermostat function, regeneration temperature trend and cooling phase.
• Desiccant condition. Dust at the afterfilter, oil staining, channelling or rising dew point despite good valves all point to the media. Replacement is based on condition and dew-point trend, not just on a calendar date.
• Afterfilter. It captures desiccant dust before it reaches the process. A high differential pressure here is often the first visible sign of attrition.

Measuring dew point correctly

A dryer service is only as good as the dew-point reading behind it. Common mistakes produce misleading figures:

• Measuring at atmospheric pressure but reporting it as a pressure dew point. Atmospheric dew point is much lower than PDP for the same air.
• Using a sample line that is too long, made of the wrong material or not purged, so the sensor reads the tube rather than the air.
• Taking a single reading at light load early in the morning instead of during a hot, fully loaded shift.
• Trusting a built-in dew-point display that has never been checked against a reference instrument.

We measure at the dryer outlet and, where useful, at the point of use, and record pressure, temperature and load at the same moment. A trend is more useful than a single number. Our dew-point measurement page explains the method in more detail.

Pressure drop: the hidden energy cost of air treatment

Every dryer and filter takes some pressure from the system. When that loss rises, operators often raise the compressor setpoint to compensate, and the compressor uses more power for every hour it runs. As a rough rule, each additional 1 bar of generation pressure increases compressor power by about 7%.

Service should record the pressure drop across the whole treatment train, not just the dryer:

A practical service and lifecycle plan

1. Baseline. Record dryer identity, duty, PDP at load, pressure drop and drain function after commissioning or after a full service.
2. Routine checks. At short intervals, operators check the controller display, alarms, drain discharge and visible water at points of use.
3. Planned service. At intervals set by the manufacturer and the site conditions, technicians clean condensers, change filter elements, test drains, inspect valves and measure dew point.
4. Condition-based work. Desiccant replacement, valve overhaul and refrigeration repairs are triggered by measured trends rather than by guesswork.
5. Review. Once a year, compare actual flow and temperature with the dryer’s corrected capacity. Plant expansion often makes a dryer that was correctly sized five years ago too small today.
6. Repair or replace decision. When an old dryer needs a major refrigeration repair or repeated media changes, compare that cost with a correctly sized new or refurbished unit.

Symptoms and likely causes

How Reddy Compressors helps

We service refrigerated and desiccant dryers alongside the compressors that feed them, across the brands commonly found in Indian plants, including Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic and others. We are an independent multi-brand specialist based in Tarnaka, Secunderabad, serving industry since 2002. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spare parts across India.

A typical visit covers duty and sizing review, dew-point and pressure-drop measurement, condenser, drain, valve and filter checks, and a written note of what we found and what needs a decision. Where parts are required, we identify them from the dryer’s make, model, serial number and nameplate, and confirm fit before quoting. Dryer care can be included in a compressor AMC, and we can also carry out wider air audits when the problem is bigger than one dryer.

What to send us

• Dryer make, model and serial number, with a photo of the nameplate
• Dryer type (refrigerated, heatless, heated or blower-purge, membrane)
• Compressor make, model and approximate running pressure
• What you are seeing: water, alarms, high pressure drop, dust or high air use
• The dew point or air quality your process needs, if known
• Photos of the dryer installation, drains and filters

Frequently asked questions

Why is my air dryer not removing water?

The most common causes are an open or leaking bypass valve, a drain that has stopped discharging, a dryer overloaded by higher flow or hot room conditions, and a fouled refrigerated condenser. On desiccant dryers, oil-contaminated media or failed switching valves are frequent causes. Measure the outlet pressure dew point at full load first, then check drains and bypass before replacing any parts.

What dew point does a refrigerated air dryer give?

A refrigerated dryer typically delivers a pressure dew point of around +3 °C when working within its rated conditions. It cannot go much lower because the condensed water would freeze inside the heat exchanger. If your pipes run outdoors in cold conditions, or your process needs instrument-grade dryness, a desiccant dryer with a typical −40 °C dew point is usually needed.

How often should a compressed air dryer be serviced?

Follow the manufacturer’s schedule as a minimum, then adjust for your site. Dusty, hot compressor rooms need more frequent condenser cleaning. Filter elements are usually changed on a running-hours or differential-pressure basis, and drains should be checked on every visit. Desiccant replacement should be based on measured dew-point trend and media condition rather than on a fixed date alone.

When should desiccant in an air dryer be replaced?

Replace desiccant when the dew point keeps rising even though valves, purge flow, prefilter and heater are working correctly, or when the media is visibly oil-stained, broken or dusting. Changing beads without fixing the cause, such as a failed prefilter or liquid water at the inlet, usually means the new desiccant fails early too. Always use the media type and quantity the dryer was designed for.

Why is there high pressure drop across my air dryer?

High pressure drop usually comes from loaded prefilter or afterfilter elements, fouling in a refrigerated dryer’s heat exchanger, icing at light load, or desiccant beds that have packed or broken down. It matters because operators then raise compressor pressure to compensate, which increases power use on every running hour. Measure the drop across each component separately to locate the restriction.

Is my desiccant dryer wasting compressed air?

It may be. Heatless dryers use a portion of dried air to regenerate the off-line tower, and leaking purge or exhaust valves, oversized purge settings or no dew-point-dependent switching can waste more than intended. If air blows continuously from the exhaust silencer or the compressor runs harder than expected, have the valves, purge setting and cycle control checked.

Related pages

• Refrigerated air dryers: selection and sizing
• Desiccant and adsorption air dryers
• Air dryer reliability and failure causes
• Condensate drain reliability and air loss
• Air dryer spare parts