- 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.
Pressure range: up to 16 bar
Pump type: multi-stage centrifugal pump
Nozzle type: swirl nozzle with/without lock function
Water meter for determining the water loss percentage
Application areas for continuous-flow water spray humidifiers, implemented as medium-pressure humidifiers
- Especially economical in the case of low humidification (0.0 to 2.0 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
- Can be supplied either with stainless steel housing or as installation kit for empty housing
- Pump station with 2 pumps (redundant design)
- Several humidification systems can be operated via one pump station (master-slave)
The continuous-flow water spray humidifier takes water directly from the mains and sprays it via nozzles. In the “medium-pressure humidifier” model, the required injection pressure is raised up to 18 bar via a multi-stage centrifugal pump.
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. A check valve (anti-drop valve) upstream of the nozzle locks the nozzle supply at a pressure of approx. 5 bar. Due to the position of the swirl creator directly in front of the nozzle bore, the check valve is not placed directly on the nozzle bore, as is the case with high-pressure nozzles. The water volume contained between check valve and nozzle bore therefore causes the nozzles to drip slightly after they are switched off. However, this volume is minimised insofar as it does not cause any significant water loss in relation to the injected water quantity.
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 with the airflow 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 pre-saturated 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 injected 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.
The injection pressure is measured via a pressure sensor and kept at a predefined value via the pump speed. Due to the pumping characteristics of a multi-stage centrifugal pump, the pressure range required for atomisation is usually only reached at higher speeds. This means that, particularly in partial load operation, the flow rate attainable is many times higher than the humidification requirement.
Unlike with a high-pressure humidifier’s piston pump, the pump’s excess flow rate does not need to be discharged, but is buffered due to the low pressure-resistant pumping characteristics of the pump impeller. This property of the piston pump obviates the need for active pressure relief via a bypass channel containing a proportional valve towards the suction side. The fact that there is no risk of a pressure overload means that the safety valve standard on centrifugal pumps can be dispensed with.
However, the conversion of mechanical energy into heat causes the temperature within the pump to rise, which means that it makes sense, not least for energy-related reasons, to switch off the pump during extended interruptions of the injection process.
The pump’s suction pipe has a further pressure sensor for monitoring the minimum water pressure in the supply. Similarly, water heating within the unit is recorded via a temperature sensor. Both process parameters have defined limit values which, if exceeded, cause 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 humidity requests 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.
To make more effective use of a pump station, particularly in the case of smaller systems or systems with low humidity requirements, it is possible to operate up to 5 humidifiers with one pump station (master-slave). For reliable operation of the system or for high system availability even in the event of maintenance, we recommend equipping the pump station with 2 identical pumps in a redundant arrangement.