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Calculation and example of the amount of desiccant
A.1 Setting of target humidity in product packaging
Before the amount of desiccant is calculated, a storage target humidity, that is, the maximum relative humidity allowed in the package, should be set.
Generally speaking, ordinary products can be safely stored under 50% humidity.
A.2 The amount of water vapor through the barrier packing material
Water vapor permeation is the weight of water vapor that permeates the barrier packing material after 24 hours of test time in a static and constant temperature and constant humidity environment. (unit: gram / square meter. Day)
Under the conditions of different temperature and humidity, the water vapor transmission rate of the same barrier material is also different. The higher the temperature and humidity, the higher the water vapor permeation rate. So, the calculation usually takes an average.
DIN55474 recommends the use of the value of water vapor transmission at 23 ℃ and 85% relative humidity as a reference value for the calculation of the amount of desiccant. The calculation can make the calculation more accurate if the value is taken in combination with the actual situation.
A.3 Three sources of water vapor in packaging
In a particular package, the moisture in the packaging generally comes from the following three aspects:
a) When the product is packed, the original water vapor in the air is contained in the packing.
b) The water vapour released by the moisture volatilization of the packing materials in the inner packing of the product.
c) During storage and transportation, the water vapor in the packaging of the product is permeated through the barrier packing material.
A.4 Formula for calculating the amount of desiccant
The amount of desiccant is calculated by the formula below： n = 1/a × (V×b + m×c + A×e×WDD×t)……………………（1）
In the formula：
n:The number of desiccant units (unit) required for use；
a:Under the target humidity, the amount of water vapor absorbed by a unit desiccant, g；
V:Internal volume of packaging，m3；
b:Air water content under temperature and humidity in initial packing，g/m3 ；
m:Weight of packing materials in packaging kg；
c:Water content of water containing packaging excipients，g；
A:The surface area of the barrier packing material，m2；
e:Correction coefficient under the condition of target humidity；
WDD:The amount of water vapor through the barrier packing material，g/m2·d；
t:The number of days for storage and transportation，d。
A.5 The meaning and reference value of the parameters
A.The value of the formula (1) in the number 4 is shown as shown in the following table：
The maximum amount of water vapor (%) absorbed by
a unit desiccant under the condition of "balanced humidity"
Correction coefficient under the
|condition of "balanced humidity"|
Air water content under the
For example, 20 C, 85% relative humidity,
condition of temperature and humidity in initial packing(g/m3)
Air water content b=15g/m3
A.6 A calculation case of the amount of desiccant
a) The description of the application case
An auto parts company production of auto parts are placed in 1 cubic packing, the packing surface area of 6 square meters; the water vapor transmission rate is 0.9 g / m2 / day (23 C, 85% 0.1mm thick) LDPE barrier sealing packaging bag; the product needs to be kept safe in the relative humidity below 50% in the environment, the storage time of 25 days, the transit time is 15 days, so after 40 days can be used; product packaging when the environment temperature is 25 DEG C, relative humidity is 80% relative humidity; in the sealed packaging and 3 kg of water rate was 30 per thousand wood as a buffer material. b) values of the relevant parameters
According to the case description, the values of the parameters required in the formula are shown as follows:
Among them, the parameters a, b, and e are calculated according to the DIN55474.c）Calculation and conclusion
n = 1/a × (V×b + m×c + A×e×WDD×t)
= 1/7.1 × (1×18.42 + 3×30 + 6×0.65×0.9×40)
= 35 (Take integer)
So in order to ensure that the moisture in the package is kept below 50% within 40 days, it is necessary to use the desiccant of 35 units.