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The amount of desiccant is the most critical point in the application of desiccant. If the desiccant is used less, it will not play an effective role in moisture-proof, and the product will be affected by moisture; if the desiccant is used more, it will be wasteful and uneconomical.
Before we start calculating the amount of desiccant, we must set a target humidity, which is the highest humidity allowed in the package. Generally speaking, ordinary products can be stored safely in a humidity environment below 50%. There are also many products that require lower humidity, such as a 20% environment. The lower the target humidity setting, the more desiccant needs to be used to keep the package dry.
The water vapor transmittance (g/m2·day) refers to the gram weight of water vapor that penetrates through the test material after 24 hours (one day) of test time in a static environment under stable temperature and humidity conditions. The water vapor transmittance of barrier packaging has a great impact on the amount of desiccant. The same volume of packaging requires the same time to be stored. If a material with good barrier properties such as aluminum foil is used, the desiccant can be used less. If a material with poor barrier properties such as thin PE bags is used, the desiccant can be used less. A lot of desiccant needs to be put in to have the same effect. Therefore, when conditions permit, we recommend that barrier materials with good barrier properties (that is, low water vapor transmittance) be used to package products as much as possible. In addition, the sealing of the barrier packaging must be strict, and there must be no pores. Any tiny air leakage holes will greatly reduce the effectiveness of the desiccant. Under different temperature and humidity conditions, the water vapor transmittance of the same barrier material is also different. The higher the temperature and humidity, the higher the water vapor transmittance. But when we calculate, we can only use an average value. DIN55474 is the value recommended for us to use the water vapor transmittance at 23℃ and 85%Rh. If the value is taken in combination with the actual situation, the calculation will be more accurate.
In a particular package, there are three sources of moisture. The mission of the desiccant is to absorb these three parts of water vapor and control the humidity in the package below the target humidity.
(1) When the product is packaged, the initial moisture contained in the air in the package. For example, the volume of product packaging is 1 cubic meter, and when the initial temperature and humidity of the packaging are 23℃ and 85% relative humidity, the air contains 17.47 grams of water.
(2) The moisture contained in the packaging accessories in the product packaging will gradually evaporate during storage and transportation. For example, wood usually has a moisture content of 15-30%.
(3) During storage and transportation, water vapor penetrates into the product packaging by blocking the packaging. For example, a barrier PE bag with a water vapor transmittance of 2 g/m2·day has an area of 6 m2. After 90 days, it will penetrate 1080 g of water into the package.
n = 1/a * (V*b + m*c + A*e*WDD*t)
n: The number of desiccant units (units) required to be used
a: Under the target humidity conditions, the amount of water vapor that a unit of desiccant can absorb (grams g)
V: The content volume of the package (cubic meters m3).
b: The water content of the air under temperature and humidity conditions during the initial packaging (g/m3 g/m3)
m: The weight of the water-containing packaging accessories in the packaging (kg kg)
c: The moisture content of the water-containing packaging accessories (G/kg g/kg).
A: The surface area of the barrier package (m2 in square meters)
e: The correction coefficient under the target humidity conditions
WDD: The water vapor transmittance of the barrier package (g/m2·day g/m2·d)
t: The number of days of storage and transportation (days d)
