Example of absorption distance nomogram for a steam humidifier Air temperature (after humidification)


8 7


6


5 4


3 2


Once the amount of moisture required for an application has been established and the humidifier selected, the absorption distance can be calculated using the nomogram. Starting at the top of the graph, plot:


• The air moisture content, before humidification: • Increase required in air moisture content: 3g/kg• Temperature of air after humidification: 20°C


5g/kg


• The humidification rate: 100kg/h • Length of steam manifold: 1.25 m • Air velocity: 5 m/s


1


4 3


Hygromatik’s Vortex Steam module, which can be placed in front of a steam manifold to spin the air to help it mix


2


Then starting at the bottom of the nomogram enter:


The lines can be plotted and the absorption distance read off as 0.8 m


1 0.8 0.5


Too often absorption distances are compromised by contractors installing a humidifier where it is easiest to fit, rather than in its optimum location


30 40 50 60 80 100 140 200 300 Steam humidity


occur. Increasing the number of humidifier injection nozzles and/or lances ensures maximum moisture dispersion into the airflow to help shorten the absorption distance. Where space in an air-handling unit


KS19: Humidification (2012)


The new CIBSE Knowledge Series publication, KS19: Humidification (2012) is available from the CIBSE Knowledge Portal at www.cibseknowledgeportal.co.uk


is limited, a simple device such as Hygromatik’s Vortex Steam module can be placed in front of a steam manifold to spin the air; this will help it mix with the steam to shorten absorption distances by up to 50%. For optimum humidifier performance, designers should ensure airflow is even across the humidifier, otherwise this can affect the air’s ability to mix the droplets or vapour. Humidifier manufacturers supply data on recommended absorption distances.


46 CIBSE Journal February 2013


Specifiers can get accurate data using an absorption distance calculator, which are generally available on manufacturers’ websites. Absorption distances can also be derived using an absorption distance nomogram. Figure 1 is an example of the nomogram used by HygroMatik (www.hygromatik.com) to calculate the absorption distance for a specific steam humidifier. The new CIBSE Knowledge Series publication, KS19: Humidification (2012) (to which the HVAC humidity group contributed) also covers this aspect on pages 31-33. Once the absorption distance is known, the location of the humidifier can be determined based on the system. Of course, all of this work will count for


www.cibsejournal.com


10


9 8 7 6 5 4 3 2


1


25 22 20 18 16 14 12 10


300 400 500 600 800 1000 1500 2000 2500 3000 4000 5000 6000 8000 2 1250


3


5 8 12


Length of steam manifold


Specific increase in humidity


Air humidity (before humidification)


Air speed


Absorption distance


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