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Calculating non-wetting distances

Sample exercise

Read through this exercise to learn more about specifying a

dispersion unit based on non-wetting distance. Assume you have

chosen to use Ultra-sorb units because you want pre-assembled

panels.

Assume the entering air is 20% RH, and the leaving air needs to

be 70% RH. Design for a non-wetting distance of 24" (610 mm)

maximum.

Solution

Refer to Figure 25-1. Find 20% entering RH. Proceed vertically until

you intersect the 70% leaving RH line. Draw a line horizontally

from that point to the right to see that for 24" (610 mm) of non-

wetting distance, 6" (152 mm) tube spacing would be the closest

match.

Verify capacity

From Table 25-1, note that for 6" (152 mm) spacing, maximum

capacity is 18 lbs/hr/ft

(88 kg/h/m

). Multiply this value by the

active face area of the Ultra-sorb to determine if the unit will

produce adequate output capacity. If the capacity is inadequate, go

to the next smaller tube spacing.

Steam absorption considerations

1. Non-wetting distance is the dimension downstream from the

leaving side of the steam dispersion assembly to the point where

wetting will not occur, although wisps of steam may be present.

Solid objects at duct air temperature, such as coils, dampers,

fans, etc., downstream of this dimension will remain dry.

CAUTION! Non-wetting distances described in this catalog

do not apply when installing a steam dispersion assembly

upstream of filter media. If you need to install a steam dispersion

assembly upstream of filter media, consult your representative or

DRI-STEEM directly for special recommendations.

3. Note that the rise (Δ) in RH (the difference between entering

and leaving RH) has a direct bearing on the non-wetting

distance. As the rise increases, more vapor needs to be dispersed

into the air, and thus the non-wetting distance increases.

4. Uneven airflow over the cross-section of a dispersion assembly

can result in nonuniform mixing of steam with air which, in

turn, will adversely affect the non-wetting distance.

mc_042710_0850

Notes:

• Final equipment selection should account for

condensate loss. See the

DRI-STEEM Design

Guide

for steam loss tables.

• Dispersion assembly should accommodate

maximum output capacity of humidifier.

Guaranteed absorption

• Cataloged and guaranteed steam absorption

distances

• Unique tubelets in dispersion tubes eliminate

condensate drips

• Published absorption tables for sizing and

selecting the correct dispersion option

• Dri-calc

software available for system

selection and absorption distance calculations

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DriSteem Vaporstream Spécifications Page 24

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