Moisture from duct humidifiers can constantly accumulate in downstream duct systems and in some cases actually cause large pools of water to form within the duct system.
Humidification in many HVAC systems is of prime importance.It is well recognized that most facilities should be at a relative humidity of between 35 to 60% RH and never above 60%. Fungi, black mold and bacteria develop very well at a humidity above 60%.
One of the more common methods of providing humidification in duct HVAC systems is through the use of clean steam humidifiers, usually classified as the dry type humidifier.
The duct type of humidifier is usually comprised of horizontal delivery manifolds. The humidifier's manifolds are usually placed in a horizontal position in the duct. It is important that when the manifolds are placed in the horizontal position, they
are in a perfectly level position and the discharge holes are facing into the direction of the air flow. In this way, the steam is injected into the air stream against the air flow.
It is also important that when utilizing duct humidification, the system is designed to eliminate the chance of water (condensate) from collecting in the duct and causing breeding grounds for bacteria and other microorganisms. For this reason, the vapor dissipation stage is important to define and evaluate.
When steam is discharged from the humidifier against the air flow in the air duct, it will change from the invisible gas which it is when it first discharges, into a moist vapor with large droplets.... sometimes 8 microns or greater in size. After
being carried along with the air, it will re-evaporate into an invisible steam gas once again.
When the steam first condenses out, it gives up its latent heat of 1000 btu/pound of vapor to the duct air. This in turn causes the air to warm up slightly. As the air vapor mixes, the heat previously given off, re-vaporizes the condensate particles
back into an invisible vapor. It is important for this to occur within a distance that has no obstructions or other devices that will cause the condensate particles to drop out of the air and cause duct wetness and possible areas of microorganism
It is important to have the humidifier controllers out of the area where the visible vapor zone occurs. This is because the combination of the locally warmer air in this zone coupled with the moist vapor particles will create a false indication for the
As a rule of thumb, the controller should be at least 15 feet downstream of the humidifier manifold. Other system characteristics will also have to be considered.
The ratio of the duct height to its width is an important factor and is known as the duct's aspect ratio. If all other parameters are equal, if we compare two ducts with the same cross sectional area, the duct with the higher height (larger aspect
ratio) will have a shorter manifold and therefore its vapor output comes in contact with a much smaller percentage of duct air, causing a longer visible vapor slip stream.
This must be considered in selecting the manifolds and in selecting the distance you need for the visible vapor stream to re-evaporate.
Duct temperature is another important consideration in selecting a humidifier. A duct with an air temperature of 75 deg.F can have a visible vapor zone of approximately 12 inches. If the duct air temperature is 55 deg.F the visible moisture zone can increase to as much as 15 feet.
Duct air velocity also affects the length of the visible moisture zone. The higher the air velocity the longer the length of the visible vapor zone.
Other then improper capacity, one of the major causes of actual operating problems is the improper calculation of the position of the visible vapor zone.
Placing controls, insulation and other important system items within the zone causes these items to become saturated with water and to fail.
In some cases I have worked on, final filters became so saturated that they facilitated the growth of aspergillus and other fungi causing serious air quality problems and many illnesses in several different types of buildings.
In many installations the practical method for reducing the length of the visible moisture zone is to utilize multiple manifolds. With multiple manifolds you can provide the full steam capacity you require but at a reduced visible moisture zone
length. This is very important when you have space constraints, airflow temperature is below 70 deg. F, duct air velocities are greater then 500 fpm, filters are utilized downstream of the humidifier, height of the duct exceeds 3 feet and the visible vapor length may impinge upon coils, fans,dampers, filters, insulation (any internal duct insulation that is within the visible vapor zone will have to be removed), duct work, turning vanes, etc. located downstream of the humidifier.
Sample Load Calculation:
Total Air Flow = 23,300 cfm @ 55 deg. F
Min. Outdoor Air = 3,725 cfm, Design = 10 deg. F @ 60% RH.
Return Air = 19,575 cfm, Design = 70 deg. F @ 45% RH.
Max. Outdoor air = 23,300 cfm at 10 deg.F
From standard humidity tables:
10 deg. F @ 60% RH. 0.40 lbs H2O per 100 cfm per
Net difference to be supplied 2.70 lbs H2O per 100 cfm
Under Min. 3,725 Outdoor cfm/100 x 2.70 = 100.6 lbs. H2O per
For 55 deg. F supply air @ 90% RH. (maximum upper limit) the air
will hold 3.76 lbs. H2O per 100 cfm per Hr. We therefore
cannot attempt to have the air hold more then this amount. To
do so will cause moisture to fall out of the air stream.
19,575 cfm (return air) x 3.1 lbs. H2O/Hr/100 cfm = 606.8 lbs.
3,725 cfm (outside air) x 0.40 lbs. H2O/Hr/100 cfm = 0.40 lbs.
Moisture added by the Humidifier (load) = 100.6 lbs
Total moisture which will be contained by the air = 722.4 lbs.
H2O/Hr which for the 23,300 cfm (total supply air) at 55 deg. F =
722.4/23,300/100 = 3.1 lbs H20/100 cfm; a value below the
maximum which the air could hold which as indicated above is
3.76 lbs. H2O per 100 cfm.
Keep in mind that for human health it is very important in all air conditioned buildings to keep the R.H. below 60% and to prevent condensate from collecting in A/C ducts. If you would like more details on this topic please see the many books I have written on Air Contamination control and A/C system operations.