WALTER WDF refrigeration dryers with integrated filters

Dew point +3 °C

WALTER WDF refrigeration dryers dry compressed air to a pressure dew point of +3 °C and filter it at the same time. A single cabinet houses a refrigeration dryer, a 1 µm pre-filter and a 0.01 µm fine filter, so you install one unit instead of three.

The range comprises 22 sizes from 23 to 5,850 m³/h (383 to 97,500 l/min at 7 bar), with a working pressure of 4 to 16 bar and R134a refrigerant. They are controlled by the Digi-Pro controller, which displays the dew point, reports faults and reminds you to replace the filters.

What the WDF dryer can do

• Dryer and two filters in one cabinet
• 1 µm pre-filter and 0.01 µm fine filter capture particles and oil aerosol
• Digi-Pro controller displays the dew point and signals alarms
• Filter replacement reminder
• Working pressure 4 to 16 bar
• Inlet air up to +60 °C, room temperature +4 to +50 °C
• Energy-saving mode and manual condensate draining with a button
• Heat exchanger with a maintenance-free centrifugal separator

Sizes and technical data

The code links to the dryer’s product page. The number in the code is the flow rate in m³/h at 7 bar. All sizes operate at up to 16 bar, with a pressure dew point of +3 °C and R134a refrigerant; the filter set is the designation of the spare elements for the given size.

The flow rate applies at a working pressure of 7 bar. The length is the width of the cabinet front, the width is its depth. Dimensions and weights are taken from the manufacturer’s manual, for WDF-3330 to WDF-5850 from the manufacturer’s website; the manufacturer does not yet state the power input or filter set for these sizes. The manufacturer’s dimensional drawing is on the product page of each size up to WDF-2775; the largest sizes, WDF-3330 to WDF-5850, are connected via DN100 flanges.

How to choose the size

The dryer must handle the full output of the compressor. The flow rate in the table applies at 7 bar, and the manufacturer does not specify correction factors for other pressures and temperatures for WDF dryers. Therefore, choose a size whose flow rate is at least equal to the compressor output, and check the operating conditions:

• working pressure 4 to 16 bar
• inlet air at most +60 °C (cool warmer air with an aftercooler first)
• room temperature +4 to +50 °C
• indoor installation only

Example: the compressor delivers 3,000 l/min. The WDF-155 with a flow rate of 2,583 l/min is not enough, the WDF-190 with 3,167 l/min is sufficient. If the inlet air or the room tends to be very warm, we will advise you on a size with some reserve.

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How the WDF dryer works

Humid compressed air enters the heat exchanger (E). In the first section, the air-to-air exchanger, it is pre-cooled by the cold dry air leaving the dryer. In the second section, the air-to-refrigerant exchanger, the refrigerant cools it to a dew point of +3 °C and the water vapour condenses. The centrifugal separator separates the water droplets and the condensate flows out through the drain (G).

On its way out, the dry air is warmed up again in the first section of the exchanger by the incoming air. This reduces the load on the refrigerant circuit, and no moisture condenses on the outlet piping. Downstream of the exchanger, the air passes through both built-in filters.

Refrigerant circuit

The R134a refrigerant is compressed by the refrigeration compressor (A), a reciprocating or scroll type depending on the size. The hot gas liquefies in the condenser (B), which is cooled by a fan with air from the room; the manufacturer also offers a water-cooled condenser. The liquid refrigerant passes through the filter (C), which traps moisture and contaminants, and in the expansion valve (D) its pressure and temperature drop. In the heat exchanger (E), it absorbs heat from the compressed air, evaporates and returns to the compressor.

The circuit is hermetic. The thermostatic expansion valve meters the refrigerant so that the temperature in the heat exchanger remains constant. From size WDF-210 upwards, the circuit has a hot gas bypass valve (F): when air demand drops, it maintains a constant evaporating pressure and the condensate in the exchanger does not freeze.

Modular heat exchanger

The exchanger has a monolithic modular design: a single body combines the air-to-air exchanger, the air-to-refrigerant exchanger and the centrifugal condensate separator, which requires no maintenance. Humid air enters from the side, dry air leaves at the top and the condensate drains through the bottom.

Built-in filters

Downstream of the exchanger, the dryer contains two in-line filters: the WFS pre-filter with a filtration grade of 1 µm and the WFC fine filter with 0.01 µm. They capture solid particles and oil aerosol, so you do not need to install a separate pre-filter and after-filter downstream of the dryer. You save space, joints in the compressed air network and installation work.

The controller reminds you to replace the filters with the FIL alarm. Always replace both elements at the same time so that the dryer does not consume unnecessary extra energy; if the alarm does not appear, replace them once a year. Spare elements are ordered as a set for the given size (Filter set column in the table), and it is worth keeping one in reserve.

Inside the dryer

Digi-Pro controller

The Digi-Pro controller digitally displays the pressure dew point and uses indicator lights to show the dryer status: operation, automatic draining, energy-saving mode, compressor standby and service. It reports the status of the unit, displays scheduled maintenance dates, has a timer and shows the temperature in °C and °F.

The energy-saving mode saves energy when no air is flowing through the dryer. Dryers from size WDF-930 upwards also have a main switch next to the controller.

Buttons

Alarms on the display

An alarm means that the dryer is not operating according to its settings. Contact us and we will arrange servicing.

Condensate drainage

The condensate is discharged by a solenoid valve controlled by a timer relay; in the default setting, it opens for 5 s every 5 minutes. You can also drain it manually with the MANUAL DRAIN button. The drain has a ball valve, so it is easy to disconnect; clean its filter every 6 months.

Refrigerant circuit protection

Single-phase refrigeration compressors are protected by a klixon, a thermal protector that switches the compressor off if it overheats. From size WDF-100 upwards, the circuit is monitored by a high pressure safety switch with manual reset (25 bar); sizes WDF-1388 to WDF-1800 also have a low pressure switch (1.6 bar).

The condenser fan is switched by a pressure switch according to the refrigerant pressure (9–12 bar), and the refrigerant temperature is monitored by a sensor (45 °C). The refrigerant filter traps moisture and contaminants that could damage the compressor. Larger sizes also have a liquid refrigerant receiver in the circuit.

Construction

The cabinet has removable covers, so the filters, heat exchanger and refrigerant circuit are accessible from the side during servicing. The air connections and condensate drain are on the rear or side panel; the exact position is shown in the drawing on the product page of each size.

The condenser is cooled by room air flowing through the grilles in the cabinet, so keep the space around them clear. The smallest sizes, WDF-23 to WDF-70, weigh 32 to 35 kg, the largest over 700 kg; a forklift with sufficiently long forks is suitable for transport.

Installation, operation and maintenance

Installation

• place the dryer on a level surface and leave at least 50 cm around it for servicing and free air circulation
• it is for indoor use only; the room temperature should be +4 to +45 °C, as the dryer itself gives off heat (the technical limit is +50 °C)
• there must be no chemicals nearby that attack copper and aluminium – ammonia, acids, alkalis
• the compressed air piping should have a diameter appropriate to the flow rate and pressure of the dryer
• keep the condensate drain free: do not reduce the hose below the connection diameter, and there must be no back pressure in it
• do not operate the dryer at a pressure higher than that stated on its rating plate

Operation

• during operation, keep an eye on the Digi-Pro controller – the dew point and any alarms
• do not switch the dryer off while air is flowing through it: first switch off the compressor or close the inlet and outlet, or divert the air through a bypass
• three-phase dryers have a crankcase heater on the refrigeration compressor; therefore, leave the main switch permanently on
• after a longer period of inactivity, let the dryer warm up for at least 4 hours before starting, and only then switch it on with the button
• discuss parallel connection with another dryer with us in advance

Maintenance

• keep the dryer clean and check the condensate discharge regularly
• clean the condenser once a month
• every 6 months, unscrew the drain access screw and rinse the condensate filter with water
• replace the filter elements after the FIL alarm, or once a year without an alarm – always both
• after maintenance, check the operating pressure, temperature and controller settings
• work on the refrigerant circuit must be carried out by a trained service technician; the dryer must be depressurised and disconnected from the power supply

Dew point and air purity

Compressed air carries water, oil and solid particles. The ISO 8573-1 standard specifies how much of them may remain, depending on the application – for an overview of the classes, see industrial filters according to ISO 8573-1. The water content is expressed as the pressure dew point: the temperature at which the air is saturated with moisture. When the air cools to the dew point or below, water condenses.

The WDF dryer achieves a dew point of +3 °C, i.e. water class 4. The manufacturer does not specify a purity class for solid particles and oil for the built-in filters; if you need to guarantee it, add filters according to ISO 8573-1 downstream of the dryer. For a dew point below 0 °C, for example in a compressed air network running outdoors in winter, you need a desiccant dryer.

WDF, a dryer with separate filters or a desiccant dryer?

A dryer with built-in filters is most often compared with a refrigeration dryer and two separate filters, for example an RDS dryer with filters of your own choice.

Where WDF dryers are used

• pneumatic tools and workshops
• car repair shops and paint shops
• CNC machines and engineering production
• industrial compressed air networks
• laboratories and metrology

Technical data of the WDF range

How do I choose the size of a WDF dryer?

By the compressor output: the flow rate of the dryer in the table (at 7 bar) should be at least equal to the compressor output. Check the working pressure of 4 to 16 bar, inlet air up to +60 °C and a room temperature of +4 to +50 °C.

Do I need additional filters for the WDF dryer?

The 1 µm pre-filter and 0.01 µm fine filter are built in. A separate filter is only useful for special requirements, for example activated carbon against odours and oil vapours.

How often are the filters replaced?

When the controller shows the FIL alarm, replace both elements at the same time. If the alarm does not appear, replace them once a year.

What dew point does the WDF dryer reach?

Pressure dew point +3 °C, i.e. water class 4 according to ISO 8573-1. For a dew point below 0 °C, you need a desiccant dryer.

Where can the dryer be installed?

Indoors only, on a level surface, with at least 50 cm of free space around it. The room temperature should be +4 to +45 °C (technical limit +50 °C), with enough air to cool the condenser.

How is the condensate drained?

Automatically by a solenoid valve with a timer relay (5 s every 5 minutes); manually with the MANUAL DRAIN button.

How much electricity does the dryer use?

The power input ranges from 0.34 kW for the smallest size to 4.74 kW for the WDF-2775; the value for each size is given in the table. The energy-saving mode saves energy when no air is flowing through the dryer.

The technical data of WDF dryers are based on the manual of the manufacturer WALTER (2021), the data of WDF-3330 to WDF-5850 on the manufacturer’s website (2025).

For other types of dryers, see compressed air dryers.