Frequently Asked Questions (FAQ)Our trade fair dates Blog News

Frequently asked questions about our products

This FAQ provides in-depth information on compressors, how they work, the differences between them and their typical applications.

For which operating temperatures are the devices suitable?

Our devices are designed as standard for a range of +5°C to +40°C. In addition, special versions are available for specific temperature ranges from -40°C up to a maximum of +55°C. For special operating conditions, we recommend contacting our customer advisors.

Advantages and disadvantages of oil-free and oil-lubricated compressors

In industrial production, companies must decide whether to use an oil-free or an oil-lubricated compressor. The choice depends on various factors.

Oil-free compressors can generate a maximum pressure of approx. 12 bar, which means lower performance compared to oil-lubricated compressors. Their power density is not sufficient, for example, to operate tools such as pneumatic impact wrenches. In some applications, oil-free compressors are indispensable, as even the smallest oil residues can render the end product unusable. Their advantages include low maintenance requirements, environmental friendliness, and quiet operation.

Advantages:

Lower acquisition costs
Minimal maintenance requirements
Clean compressed air without oil residues
Environmentally friendly
No hazardous waste in the form of oil residues

Disadvantages:

Operating pressure up to approx. 12 bar
Higher heat generation, which may reduce service life depending on designn
Lower speeds

Oil-lubricated compressors are used in industry when pressures above 12 bar are required. Maintenance requirements are significantly higher compared to oil-free systems. The entire system must be regularly checked for oil or air leaks, and defective parts must be replaced. To prevent contamination of the end product, the lubricant is removed using downstream components such as filters and oil separators, which leads to additional costs.

Advantages:

Long service life and low wear if maintenance intervals are respected
High operating pressure above 12 bar

Disadvantages:

Higher acquisition costs
Higher maintenance costs due to regular servicing and oil changes
Additional costs for accessories (e.g. oil filters, oil separators, hoses)
Safe disposal of used oil required
Higher environmental impact

What advantages do piston compressors have over screw compressors?

If both technologies are suitable for an application, it is worth comparing them carefully. Piston compressors are particularly strong when smaller to medium air volumes, higher pressures, fluctuating air demand or compact installation situations are required.

Lower purchase and service costs: Piston compressors are usually more cost-effective to purchase than screw compressors. They are simpler in design, easier to maintain and generally involve significantly lower service costs.
Low to high pressures: Piston compressors are suitable for a broad pressure range. They are particularly advantageous for smaller to medium air volumes and higher pressures. When operated at lower pressure, the load on the compressor is reduced. This has a positive effect on service life.
Advantages with fluctuating air demand: Piston compressors are robust under changing loads and are well suited to applications with frequent start-stop operation.
Compact design: Piston compressors can be integrated easily into devices, machines and customised systems – especially where installation space is limited.

Screw compressors, on the other hand, are mainly used where large, continuous air volumes and low to medium pressure are required. They are technically more complex and usually involve higher maintenance and service costs. For higher peak pressures, an additional piston compressor is often used to increase the pressure.

Can compressors generate pressures higher than the nominal pressure?

The nominal pressure provides a reference value for a piping system. According to DIN, EN, and ISO standards, it is specified using the designation PN (Pressure Nominal), followed by a dimensionless integer indicating the design pressure in bar at room temperature (20 °C).

Compressors from Dürr Technik can achieve pressures higher than the specified nominal pressure. However, the volume flow decreases at pressures above PN.

What is the maximum pressure at which the compressors can be operated?

The compressors can be operated up to the maximum allowable pressure (PS), which, depending on the series, can be up to 12 bar. In general, higher pressures reduce the service life.

Do the devices require maintenance?

Thanks to the use of oil-free technology, our compressors are virtually maintenance-free. We only recommend replacing the intake filters once a year.

Depending on the operating conditions, the cup seal and cylinder are subject to wear. Information on replacing these parts can be found in our operating manuals.

When should the carbon brushes on DC units be replaced?

The wear of the carbon brushes depends on the ambient temperature, the starting currents, and the switching frequency. As a rule, they last for thousands of operating hours.

What is inrush current?

Electric motors (both direct current and alternating current motors) draw higher currents when starting. This is because more power—and therefore more current—is required to accelerate the rotating mass to its rated speed than to maintain that speed. This higher current surge at switch-on is referred to as inrush current.

Diagram explaining ‘inrush current

Are three-phase motors suitable for operation with a frequency converter or traction inverter (so-called FC operation)?

Frequency converters/traction inverters are mainly used in the rail and vehicle sector. Not every motor is suitable for FC operation. Dürr Technik therefore offers a wide range of special units with reinforced motor winding insulation. If you have any questions, please contact our customer advisors.

When is an automatic condensate drain necessary?

The simplest and most cost-effective way to remove condensate from the tank is manual draining. However, this is often neglected. As a result, the compressed air tank fills up with condensate, reducing the available tank volume for storing air. This ultimately leads to more frequent switching cycles and unnecessarily shortens the service life of the unit. An automatic drain is therefore recommended if regular manual draining cannot be ensured. All our stations with the “K” option (e.g. HA-160K) or with the DürrTronic electronic control (e.g. WA-062C1) are equipped with such an automatic condensate drain.

Why does condensate form and how is it created?

Condensate inevitably forms during any compressed air generation. It results from the moisture contained in the intake air. During compression, and the associated increase in temperature, this moisture initially exists as vapor. After compression, only a fraction of the original volume remains, leading to oversaturation of the air. As the air cools, the moisture then condenses and forms liquid condensate.

How does the automatic condensate drain work?

Each time the compressor is switched on, and additionally at regular intervals preset at the factory, the condensate drain valve opens.

Why is a dryer necessary?

At the outlet of the compressor, the compressed air is 100% saturated. The task of a compressed air dryer is to reduce the moisture contained in the compressed air to a defined residual level so that no condensate can form.

The membrane drying system from Dürr Technik fully automatically delivers dry and clean air in accordance with ISO 8573-1. The dried air is stored directly in the air receiver.

Why is a start-up unloading system required and what are the advantages of an automatic start-up unloading system?

Compressors generally do not start against pressure. For this reason, we offer mechanical unloading valves and timer-controlled solenoid start-up valves for our stations. Each time the compressor switches off, the system is therefore vented.

In the case of short power interruptions or brief manual switching off and on during compressor start-up, pressure may still remain in the line with a mechanical unloading system. As a result, the compressor may fail to start against the pressure and the motor can overheat. In such cases, the on/off switch on the pressure switch must be operated.

Dürr Technik offers a convenient automatic start-up unloading system. This is available in stations with the “A” option (e.g. HA-160AK) or with the DürrTronic electronic control system (e.g. WA-062C1).

What does compressed air quality according to DIN EN ISO 8573-1 mean?

The ISO 8573-1:2010 standard classifies the quality or purity of compressed air. The classification is based on defining a maximum content of contaminants permitted in the air. These contaminants include particles, water, and oil. Each class specifies a defined maximum value for these three elements. The classes range from 0 to 9, with an additional class X.

The required purity decreases as the class number increases.

	Solids / Dust*
	Max. number of particles per metre³ of particles with d (μm)						Humidity ** pressure dew point /	Total oil content
class	≤ 0,1	0,1 < d ≤ 0,5	0,5 < d ≤ 1,0	1,0 < d ≤ 5,0	μm	mg/m³	x=Water content in g/m³ liquid	mg/m³
0	Specified according to the application and better than 1
1	-	100	1	0	-	-	≤ -70°C	≤ 0,01
2	-	100.000	1.000	10	-	-	≤ -40°C	≤ 0,1
3	-	-	10.000	500	-	-	≤ -20°C	≤ 1,0
4	-	-	-	1.000	-	-	≤ +3°C	≤ 5,0
5	-	-	-	20.000	-	-	≤ +7°C	-
6	-	-	-	-	≤ 5	≤ 5	≤ +10°C	-
7	-	-	-	-	≤ 40	≤ 10	x ≤ 0,5	-
8	-	-	-	-	-	-	0,5 ≤ x ≤ 5,0	-
9	-	-	-	-	-	-	5,0 ≤ x ≤ 10,0	-

* measured in accordance with ISO 8573-4, 8573-2 and 8573-5; reference conditions: 1 bar absolute, 20°C, 0% relative humidity
** measured in accordance with ISO 8573-3; reference conditions: 7 bar operating pressure, 20°C

Which filters are used in the units?

Compressor units must be equipped with intake filters. For various models, we offer a selection of filters. Filter fineness of 2 or 3 microns provides excellent protection for the compressors against dirt particles.

Our membrane drying systems are equipped as standard with sintered filters and a second filter with 3 µm. An optional fine filter with 0.01 µm is also available. Further information can be found in the diagram below.

In addition, we offer activated carbon filters for odor removal for the SICOLAB models.

Diagram illustrating filterable particles

Which compressed air classes does Dürr Technik offer?

In terms of particle size, we achieve Class 1 by using our fine filter with 0.01 µm. This filter is optionally available for the drying stations.

By using our membrane dryer, we achieve up to Class 3 for water content, depending on the compressor delivery rate (pressure dew point -20°C at 7 bar).

With our oil-free compressors, we achieve Class 1 for oil content as standard.

Where can I find additional data on the individual devices?

A gas generator produces various gases such as nitrogen, zero air or hydrogen directly on site. Depending on the device, the required gas is generated either from ambient air or from deionized water – hygienic, on-demand and without gas cylinders. In laboratories, gas generators are used to supply equipment such as LC-MS, GC, spectrometers or analytical systems with reliable, high-purity gases.