There are a number of factors used when calculating moisture loads. These factors are important when determining the requirements to control the humidity tolerances within a given space. Once the specific project factors are understood the designer can then apply the best methods for moisture removal and control of the room. Usually dehumidification is integrated as part of the dedicated air conditioning system. One method is the use of desiccant technology to sufficiently depress the supply air to the room to offset the potential internal moisture loads discussed in this article.
Using Desiccant Technology to Remove Moisture
A desiccant dehumidification system employs both a process air and reactivation air path in a counterflow configuration. Moisture laden process air is passed though a rotating desiccant impregnated media such as silica gel. The large pore surface area coupled with an extremely low vapor pressure attracts and absorbs water from the airstream that will be delivered to the controlled space. The counterflow reactivation air is a smaller and separate airstream that is first heated and then passed through a separate section of the rotating desiccant media. The absorbed water is released to the reactivation airstream and is rejected to the outside. The desiccant media is slowly rotated to achieve a continuous moisture removal and rejection process.
What methods can be used to successfully calculate moisture loads?
Space moisture load, this is a combination of permeation and infiltration and both are necessary for determining the load. So, what is permeation? This is a straight line function of the difference in interior and exterior vapor pressures that are determined by gr/lb.
Therefore, each pound of the air that is entering the provided space will impose a kind of moisture load that will be determined by the difference in the interior and exterior moisture content. Since this particular moisture pressure differs significantly with the moisture content, the vapor moves at a higher velocity than the air. A combination of these two factors, therefore, results in a space moisture increase at an increasing rate, as the difference between the interior and exterior moisture content increases. Incorporating appropriate vapor barriers into the room construction will help to reduce the impact of permeation.
Note: In regard to the above explanation, the F1 factor is used to adjust the increased vapor velocity. Therefore, the combination of the F1 and F2 factors represent the space moisture load that is anticipated from both permeation and infiltration.
How is Permeation Calculated?
To be able to determine correctly the amount of moisture that penetrates the construction into a controlled space, you can use the following calculation:
C x ∆G x F1 x F2 x F3 x F4 = Grs/hr. (To determine grains/minute divide answer by 60).
= Amount of vapor able to permeate the closed space through construction and vapor barriers.
The above equation can be used to solve a typical example as follows:
Problem- Find the amount of moisture that will permeate the room defined below.
Sample Calculation – Space to be controlled:
Room with 12″ masonry walls.
Two coats of aluminum paint as a vapor barrier.
Volume of room – 22,000 ft.3
Outside Design: 95°F db 77°F wb (Table I Shows 130 gr/lb)
Required – To hold in room – 40 gr/lb
C x ∆G x F1 x F2 x F3 x F4 = Grains per hour.
22,000/14 x 90 x 2.29 x 0.58 x 1.0 x 0.75 = 140,884 grs/hr
140,884/60 = 2348 grs/min (Moisture through Intermittent Openings)
Similar equations are used to determine Infiltration though doors, fixed openings and other points of infiltration. Even if a room is positively pressurized, the delta vapor pressure will still allow infiltration through the smallest of openings.
Permeation and infiltration are used to determine moisture loads from the construction of the space however the Internal room moisture loads must also be accounted for. This includes the number of people and the type of work they are doing, open water baths, sinks and potential moisture release from the product.
Again, proven moisture factors and equations and are used to determine internal moisture loads. Available selection software from manufactures has incorporated these equations and manufactures performance knowledge to determine moisture removal loads and equipment selections. Understanding the sources of moisture will ultimately help you to select the most cost effective approach to achieve the desired humidity control requirements.
Q Applied Systems Corp is a manufacturers’ representative of the HVAC and moisture control equipment and systems as well as fume and dust collection products. In case, you would like to learn more about these products you can contact Q Applied Systems or visit our website.