humidification distance

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Humidification Made Simple:

How is the humidification distance determined?




The humidification distance is understood as the distance required for optimum take up of steam by the air. Essentially, it comprises the mist zone and the expansion and mixing zone.

1. Definition
The humidification distance is made up of the mist zone and the subsequent expansion and mixing zone. The mist zone is the path downstream of the humidification system—from the injection to the complete admission of the quantity of steam by the system air. The expansion and mixing zone then follows it. The introduced humidity mixes evenly with the air current in this stretch. The length of the required humidification distance depends on the component following in the air direction. The correct dimensioning of the humidification distance is extraordinarily important to prevent condensation phenomena inside the air lines. Knowledge of this distance is also of fundamental significance for correct humidity control since control sensors should only be installed where there are balanced humidity values.

Determination of the humidification distance «BN» depends on a number of different factors. The humidification distance table may be used for simple determination of the humidification distance «BN». The reference values in this table relate to a supply air temperature of 15°C to 30°C.

Humidification Table for a Simple Steam Distributor

Inlet moisture
φ1 in % rel. hum.
Length of the humidification distance BN in m
Outlet moisture φ2 in % rel. hum.
40 50 60 70 80 90
5 0,9 1,1 1,4 1,8 2,3 3,5
10 0,8 1,0 1,3 1,7 2,2 3,4
20 0,7 0,9 1,2 1,5 2,1 3,2
30 0,5 0,8 1,0 1,4 1,9 2,9
40 - 0,5 0,8 1,2 1,7 2,7
50 - - 0,5 1,0 1,5 2,4
60 - - - 0,7 1,2 2,1
70 - - - - 0,8 1,7
φ1 in % rel. hum.: Relative supply air humidity before humidification at the lowest supply air temperature
φ2 in % rel. hum.: Relative supply air humidity after the steam distribution pipe at maximum output


2. Influence of humidification on the control accuracy
The humidification distance has special significance in steam humidification. The steam coming from the steam distribution pipes first of all condenses in the air current and is visible as mist over a certain distance (humidification distance). This is followed by the expansion and mixing zone where uniform mixing of the system air takes place with the injected volume of steam. This circumstance alone must be taken into account when sizing humidification distances for a hygienic operating mode. Exact control results depend on optimal humidity distribution at the location of the sensing elements. The humidification distance depends on different factors and forms the basis for establishing the required minimum distances to downstream system parts and sensing elements.

3. How can the humidification distance be shortened?
To some extent, condensation is aided by the contact of the steam with the cooler system air. However, the main reason for condensation is the unfavorable distribution of the steam through the duct cross-section along a steam distribution pipe. The required humidification distances are not available in retrofits in particular. In these cases, the use of multiple steam distribution systems often leads to success. Homogeneous distribution of the steam can occur as much as possible on the entire air current at appropriately short humidification distances. The homogeneity index is one valuable reference parameter for steam distribution.

Humidification distance tables with multiple steam distribution systems

Inlet moisture
φ1 in % rel. hum.
Length of the humidification distance BN in m
Outlet moisture φ2 in % rel. hum.
40 50 60 70 80 90
5 0,22 0,28 0,36 0,48 0,66 1,08
10 0,26 0,26 0,45 0,45 0,64 1,04
20 0,16 0,22 0,30 0,41 0,58 0,96
30 0,10 0,17 0,25 0,36 0,52 0,88
40 - 0,11 0,20 0,30 0,45 0,79
50 - - 0,13 0,24 0,38 0,69
60 - - - 0,16 0,30 0,58
70 - - - - 0,20 0,45
φ1 in % rel. hum.: Relative supply air humidity before humidification at the lowest supply air temperature
φ2 in % rel. hum.: Relative supply air humidity after the steam distribution pipe at maximum output

For channel widths <600 mm, the humidification distance for multiple steam distribution systems increases by approx. 50%.


4. Minimum expansion zone distances to be maintained
To prevent steam delivered by the steam distribution pipe from condensing on installation components downstream, such components must be located a specific minimum distance (based on the humidification distance «BN») from the steam distributor.

before/after a constriction after a width increase
Expansionszone vor/nach Verengung Expansionszone nach Erweiterung
before an elbow before a branch
Expansionszone vor Krümmer Expansionszone vor Verzweigung
before an air grille before a humidity controller/humidity sensor
Expansionszone vor Luftgitter Expansionszone vor Feuchteregler
before/after a heater pack/filter
Expansionszone vor/nach Heizregister
before/after a ventilator/zone exit
Expansionszone vor/nach Ventilator


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