- 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 100 bar
Pump type: piston pump (water-lubricated)
Nozzle type: swirl nozzle with locking function
- Injection volume control as a combination of pulse-width and pressure modulation
- Water meter to determine water loss percentage
Application areas for continuous-flow water spray humidifiers, implemented as high-pressure humidifiers
- Especially economical in the case of low to medium increase in humidity (0.0 to 4.0 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 from the mains and sprays it directly via nozzles. In the “high-pressure humidifier” model, the required injection pressure is raised up to 100 bar either via a piston pump or a piston diaphragm pump. Specially developed anti-drop high-pressure nozzles are used as nozzles, with the water exiting from the nozzle opening at high pressure. The injection spray is dissolved, in line with the principle of pressure atomisation, by means of turbulent speed fluctuations that are fanned by the frictional forces between the fluid and the ambient air. In contrast to the circulating water spray humidifier, the high injection pressure requires a much higher injection speed and hence much higher inner turbulence of the spray. The resulting inertia forces increase significantly in relation to the existing surface tension forces, causing the injection spray to break down into aerosol-like droplets.
An integrated check valve with a pre-tensioned spring immediately locks the nozzle opening at a pressure of around 5 bar. This prevents the nozzles from dripping when a nozzle rack is switched off.
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 high-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. In partial load operation, the water volume to be atomised may be so small that the predefined injection pressure is exceeded even at minimum speed. In this case, the pressure is adjusted to the desired value by means of a bypass channel containing a proportional valve. This type of adjustment is required, in particular, if the pressure outlet is completely closed due to the switch position of the valves, in which case pressure is fully discharged via the bypass. To prevent pressure overload in the event of the proportional valve becoming defective, an additional bypass channel with an overflow valve is provided.
The pump’s suction pipe has a further pressure switch 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, water outlet and tank drain. When determining the water loss percentage, this allows a distinction to be drawn between flushing water eliminated via the pressure relief valves and spray water that has not evaporated. The sensors are evaluated and water balanced via the system control unit.
To be able to determine the water loss percentage, a flow sensor is placed in both the water supply and water outlet. There is also a flow sensor at the tank drain to enable a distinction to be drawn, when determining the water loss percentage, between flushing water eliminated via the pressure relief valves and spray water that has not evaporated.
In addition to synchronisation (pulse-width modulation) of the nozzle racks, pressure modulation can also be activated. This can significantly lengthen the opening time of the nozzle racks in partial load operation. Pressure modulation is superimposed on pulse-width modulation in this case.
In respect of the valve configuration, two 2/2-way directional control valves (pressure valve, pressure relief valve) are used for each nozzle rack. There are also 2 further 2/2-way directional control valves as drain valve and bypass valve, respectively. Depending on the switch position of the two valves, the return water coming from the nozzle racks is conveyed either to the pump’s suction side or to the sewer. To drain the nozzle racks, all valves are opened. If no draining or sterilisation of the system has been provided, the water discharged during flushing operations and during pressure relief is channelled back to the water supply. This valve configuration not only provides pressure relief, but also makes it possible to overlap the times between pressure relief and pressure application, enabling the pressure flanks of the switch process to be modelled within certain limits.
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 and pressure relief valve for a short time. In this hygienic flushing mode, the flushing valve is also opened, which discharges the potentially hygienically compromised water contained in the nozzle rack into the sewer.
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 relevant 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 pre-saturation 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.