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Low-pressure humidifiers

- Continuous-flow water spray humidifiers -

What continuous-flow water spray humidifiers have in common is that the water quantity atomised within the airflow is only slightly greater than the water flow required for humidification. The small percentage of unevaporated water is not recycled and re-atomised, but is channelled to the waste water system as discharge. The humidifier’s injection system is therefore constantly operated by means of fresh water according to the continuous flow principle. A distinction is drawn between high-pressure, medium-pressure and low-pressure humidifiers based on atomisation pressure.

Low-pressure humidifiers

Pressure range: up to 5 bar
Pump type: no separate pump, system pressure from demineralised water mains
Nozzle type: swirl nozzle without lock function

Equipment variant

Water meter for determining the water loss percentage
Application areas for continuous-flow water spray humidifiers, implemented as low-pressure humidifiers

Application areas for continuous-flow water spray humidifiers, implemented as low-pressure humidifiers

  • Especially economical in the case of low humidification (0.0 to 0.7 g/kg)
  • From small to large volumes of air (1,000 to 2,000,000 m³/h)
  • Low cleaning overhead required for adherence to hygienic operating conditions
  • Fitting to existing housing of an air handling system

Product details

The continuous-flow water spray humidifier takes water directly from the mains and sprays it via nozzles. In the “low-pressure humidifier” model, there is no pressure pump. The injection pressure of 4 to 6 bar required for spraying is made available by the demineralised water system. The salient feature of low-pressure humidifiers is therefore their simple, cost-efficient and robust design. The disadvantage they have compared with high-pressure and medium-pressure systems is the poorer spraying quality, which is reflected in a slightly higher water loss percentage. It therefore makes economic sense to use low-pressure humidifiers only for low humidifications (<0.7 g/kg).

Swirl nozzles especially designed for this pressure range are used to spray the water. The water flows into the nozzle’s swirl chamber via tangential inlet channels. The swirl flow’s high centrifugal forces give rise to an air-filled core within the nozzle bore, with the water forming a rotating film around it. The thickness of the water film reaches only a fraction of the nozzle diameter. Compared with unswirled injection for high-pressure humidifiers, this results in relatively small droplets even at low injection pressures. Due to the air core that forms, the nozzle bore of a swirl nozzle is many times larger, at the same flow rate, than is the case with conventional nozzles. Sensitivity to the deposit of particles therefore decreases. 

Due to the low injection pressure, it is not possible to use a check valve (anti-drop valve) upstream of the nozzle. Compared with high-pressure and low-pressure humidifiers, nozzle dripping is therefore unavoidable. The water loss caused thereby is, however, tolerated in the light of the systems’ very low water requirement.

Only part of the generated water mist evaporates directly within the airflow. Whereas, in the case of the circulating water spray humidifier, the unevaporated droplet fragments, owing to their size, either fall back directly into the water tank or are eliminated via the eliminator, the aerosol-like droplet mist in the medium-pressure humidifier is carried to the agglomerator and eliminated there, with the droplets forming a liquid film on the surface of the agglomerator. The evaporation process continues in contact with the air. Ideally, particularly in the case of low degrees of humidification, any unevaporated droplet fragments will undergo complete subsequent evaporation in this area. With increasing humidification and strongly presaturated air at the humidifier inlet, droplet evaporation is made more difficult. Droplet elimination within the agglomerator therefore increases. As the elimination efficiency increases, the subsequent evaporation effect no longer suffices to evaporate the water completely. Part of the water therefore flows via the agglomerator into the humidifier tank and from there into the drain. Particularly in the case of high flow rates and intensive droplet elimination in the agglomerator, the water film may give rise to further droplets. In such cases, this droplet entrainment is eliminated via a downstream eliminator.

The humidifier’s performance is adjusted to the request for humidity by synchronising the individual nozzle racks.

To prevent the injection pressure from falling below a value that is critical for spray quality, the supply has a pressure sensor for monitoring the pressure in the demineralised water system. If the pressure falls below a predefined limit value, it causes the humidifier to go into fault lockout mode.

To be able to determine the water requirement and the water loss percentage during system operation, a flow sensor is available as an optional accessory for the water supply and tank drain. The sensors are evaluated and water balanced via the system control unit.

In respect of the valve configuration, a 2/2-way directional control valve is used for each nozzle rack.  To drain the nozzle racks, all valves are opened and the water they contain discharged to the drain via a mechanical ball valve.

With regard to water hygiene, nozzle racks that have not been used for an extended period are switched on for the humidification operation as an alternative to the most used nozzle block, in line with a predefined time program.

If requests for humidity are very low, this flushing program might not be effective due to the nozzle racks’ extremely low turn-on time. To prevent germ formation, the individual racks are flushed in this case by opening the injection valve for a short time.

Apart from such automatic activation of hygienic flushing, there is also an option for manual activation, with venting of the nozzle racks during initial startup or draining during shutdown also being possible.

In the event of low humidifications, the water loss percentage is mainly caused by hygiene-related flushing operations. As humidification increases, the effectiveness of droplet evaporation becomes increasingly important to the water loss percentage. Under favourable conditions, the water loss percentage is less than 10% and can rise to up to 30% or more under unfavourable conditions. High humidity presaturation of the air, a high flow rate, high humidification requirements and small installation length are particularly detrimental in relation to the water loss percentage.

Your contacts for low-pressure humidifiers


Chris Koppenhöfer

Head of sales & marketing

Phone +49 7774 93 11-25
Fax +49 7774 93 11-29




Mattias Ziegler

Head of engineering & design

Phone +49 7774 93 11-20
Fax +49 7774 93 11-29