How do you determine the humidification distance?

1. Definition

The humidification distance consists of the mist zone and the subsequent expansion and mixing zone. The mist zone refers to the section downstream of the air humidifier – from the introduction of the steam to its complete absorption by the system air. This is followed by the expansion and mixing zone. In this section of the duct, the introduced moisture mixes evenly with the air flow. The length of the required humidification distance depends on the component located downstream in the air flow. Correct dimensioning of the humidification distance is extremely important to prevent condensation within the air ducts. Knowledge of this is also of fundamental importance for correct humidity control, as the control sensors should only be positioned where balanced humidity levels are present.

The determination of the humidification distance ‘BN’ depends on various factors. The humidification section table can be used to easily determine the humidification section ‘BN’. The guide values given in the table refer to a supply air temperature range of 15°C to 30°C.

Humidification distances for a simple steam distributor

Humidification distance BN in m as a function of inlet humidity φ1 and outlet humidity φ2:

For φ1 = 5 %r.F., the values for φ2 = 40/50/60/70/80/90 %r.F. the values 0.9/1.1/1.4/1.8/2.3/3.5 m.

For φ1 ≥ 10 %r.F.: 0.8/1.0/1.3/1.7/2.2/3.4 m.

For φ1 at 20% RH: 0.7/0.9/1.2/1.5/2.1/3.2 m.

For φ1 at 30% RH: 0.5/0.8/1.0/1.4/1.9/2.9 m.

For φ1 at 40% RH, for φ2 at 50/60/70/80/90% RH: 0.5/0.8/1.2/1.7/2.7 m.

For φ1 at 50% RH, for φ2 at 60/70/80/90% RH: 0.5/1.0/1.5/2.4 m.

For φ1 = 60% RH for φ2 = 70/80/90% RH: 0.7/1.2/2.1 m.

For φ1 = 70% RH for φ2 = 80/90% RH: 0.8/1.7 m.

φ1 = relative supply air humidity before humidification

φ2 = relative supply air humidity after the steam distribution pipe

2. Influence of humidification on control accuracy

In steam-air humidification, the humidification distance is of particular importance. The water vapour emerging from the steam distribution pipes first condenses in the air stream and is visible as mist within a certain distance (humidification section). This is followed by the expansion and mixing zone, where the system air is evenly mixed with the introduced steam. This factor must be taken into account when dimensioning the humidification sections, particularly with regard to hygienic operation. For precise control results, optimal humidity distribution at the sensor installation point is crucial. The humidification distance depends on various factors and forms the basis for determining the required minimum distances to downstream system components and sensors.

3. How can the humidification distance be shortened?

Condensation is, to a certain extent, promoted by the contact of water vapour with the cooler system air. However, the main reason for condensation is the unfavourable distribution of the steam volume across the duct cross-section along a steam distribution pipe. Particularly in retrofit projects, the required humidification runs are not available. In such cases, the use of multiple steam distribution systems often proves successful. This allows for the most homogeneous distribution of water vapour across the entire air flow with correspondingly short humidification sections. A valuable benchmark for assessing vapour distribution is the homogeneity index.

humidification distance for multiple steam distributors

humidification distance BN in m as a function of inlet humidity φ1 and outlet humidity φ2:

For φ1 ≥ 5 % RH, the values for φ2 ≥ 40/50/60/70/80/90 % RH. are 0.22/0.28/0.36/0.48/0.66/1.08 m.

For φ1 = 10% RH: 0.26/0.26/0.45/0.45/0.64/1.04 m.

For φ1 = 20% RH: 0.16/0.22/0.30/0.41/0.58/0.96 m.

For φ1 at 30% RH: 0.10/0.17/0.25/0.36/0.52/0.88 m.

For φ1 = 40% RH for φ2 = 50/60/70/80/90% RH: 0.11/0.20/0.30/0.45/0.79 m.

For φ1 = 50% RH for φ2 = 60/70/80/90% RH: 0.13/0.24/0.38/0.69 m.

For φ1 = 60% RH for φ2 = 70/80/90% RH: 0.16/0.30/0.58 m.

For φ1 = 70% RH for φ2 = 80/90% RH: 0.20/0.45 m.

φ1 = relative supply air humidity before humidification

φ2 = relative supply air humidity after the steam distribution pipe

For duct widths < 600 mm, the humidification distance for multiple steam distribution systems is extended by

4. Minimum distances to be maintained in the expansion zone

To ensure that the water vapour escaping from the steam distribution pipe does not condense on the downstream system components, downstream system components must be located at a certain minimum distance (based on the humidification distance ‘BN’) from the steam distributor.