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What is a desiccant?
Desiccants or desiccant granules are hygroscopic substances, which means that they attract and store water from the ambient air. The envelopes in which they are placed are used to isolate the desiccant granules from their environments and prevent dirt or contamination; they remain permeable for water vapour to permit the absorption of ambient moisture.
Desiccants can be used directly to influence the climate, as the desiccant's absorption of moisture lowers the absolute and relative humidity.
Desiccant is the term used for this kind of product in industrial applications, while dehumidifier is more commonly used in domestic settings. Learn more about dehumdifiers here.
The principles presented here refer to the shipping of products and only have limited usefulness for domestic settings.
What is the "desiccant method“?
The "desiccant method" is used to protect products from moisture damage. This involves adding desiccant to a sealed envelope, the so-called barrier layer (i.e. an enclosed space), to protect the goods. The desiccant absorbs moisture from the air and in doing so lowers the relative humidity to a harmless level.
What do the terms "relative humidity" and "absolute humidity" mean?
Absolute air humidity describes the volume of water present in the air, expressed as grams per cubic metre. Absolute humidity does not depend on the temperature and provides information on the actual quantity of water vapour in the air.
In contrast, relative humidity is far more important for the shipment of products. Air can only accommodate a certain volume of water before becoming "saturated". This property is dependent on the temperature of the air: Hot air can hold more water vapour than cold air. This is where the relative humidity steps in. It represents the ratio between absolute humidity relative to the potential saturation volume and hence expresses how full the air is at a particular temperature. The temperature-dependence of relative humidity means that its value rises sharply when the temperature falls at constant absolute humidity, as the capacity of the air also drops at lower temperatures. The Climate table here illustrates this phenomenon.
The table shows the water vapour contained in one cubic metre of air, depending on the temperature and relative humidity.
- · The water vapour content present in the air is expressed in g/m³ in the upper line.
- · The lower line shows the temperature of dew point in ºC.
1 m³ of air contains 39.50 g of water at 45 ºC and 60 % relative humidity. Cooling of this air to 35 ºC (dew point) produces relative humidity of 100 %. So the same volume of water in 1 m³ of air is equivalent to 60 % relative humidity at 45 ºC and to 100 % relative humidity at 35 ºC. The air will no longer be able to hold the water if the temperature drops further to 15 ºC. It precipitates on the products and the packaging, or is absorbed by them. Relative humidity remains constant at no more than 100 % at 15 ºC. Only 12.82 g of water can be held. The rest is absorbed by the goods or the packaging: 39.5 g–12.82 g = 26.68 g water.
How does the moisture enter the products?
The air stores considerably more water at high temperatures than at low temperatures. The maximum adsorption capacity of 100 % is 30.4 g/m³ at 30 °C, but only 6.8 g/m³ at 5 °C. It follows, therefore, that if the temperature falls by 25 °C at constant absolute humidity of 26 g/m³, 20 g/m³ will be lost. Given that the moisture cannot escape through the outer shell when the temperatures fall, it gathers on the products or the packaging. Condensation will even form on the surface of the goods if they do not warm up as quickly as the air.
Vice versa, the moisture escapes from the products or packaging into the air when the temperatures rise. This means, for instance, that while packing the goods in the warehouse, a damp pallet can cause the originally low relative humidity to rise from 30 % to 80 %. Transported goods are exposed to severe temperature fluctuations, and not just when moving through different climate zones. Merely the difference between a hot sunny day and the following night can produce temperature variances of more than 50 °C in transport containers.
What are the sources of moisture during transport?
The following diagram visualises the various sources of moisture:
All four moisture sources must be taken into consideration to ensure correct dimensioning. In a closed system, moisture levels can be calculated precisely using formulas or estimated based on empirical values.
What types of desiccant are available?
Desiccants are classified according to their granule composition. Here, the various granule types are used for different purposes.
The most common types of desiccant are:
- · Silica gel
Often packaged in the form of transparent beads. Can absorb 25–39 % of its own weight in water, depending on the quality. Silica gel is regenerable, but the low cost of the material makes regeneration inefficient in most cases. Specially impregnated silica gels for laboratories or test facilities are the exception here.
- · Clay granule-based desiccants
The most common desiccant material in Germany for the shipping of relatively small volumes is clay. Clay granules can store 30 to 35 % of their own weight in water. For instance, clay granules like bentonite are frequently used in desiccants according to DIN 55473.
- · Salt-based desiccants
Salt-based desiccants can be divided into drip systems made entirely of salt and hybrid systems with swelling agent. Aside from in domestic settings , drip systems are used for storage in particular. Salts dissolve, so a drip system produces a brine that can present a risk for goods and surfaces. This led to the development of swelling agents that bind the brine and hence prevent leakage immediately after its production.
- · Molecular sieves
Molecular sieve is the functional term for synthetic zeolites (or other substances) that possess a rapid absorption capacity for certain, possibly modifiable, molecule sizes. Zeolites can be modified for all kinds of gases or liquids and are used in a wide variety of systems in many industries.
When used as a desiccant, the sieve is adjusted to the size of the water molecule.
How much desiccant do I need?
What kind of transport climate does my product need?
For this question, and all others you would like to ask, our experienced team of consultants in customer service would gladly be of assistance. You will benefit from years of practical experience in an array of industries or applications.
Desiccant for container shipment
Which desiccants are suitable for container shipping?
Container shipping presents fresh challenges for the desiccant method. Extremely high-performance desiccants are needed to cope with the higher rate of air exchange compared to in-box shipping, extreme climactic conditions and protracted journeys.
Familiar dry granule desiccants according to DIN 55473 can only satisfy the requirements to a limited extent. The quantity of desiccant needed to create humidity of 40 % (for example) would be uneconomically high. Moreover, the desiccant becomes saturated after just a few days, a condition that may have been reached before the container even departs the port. Afterwards your products are left unprotected.
Salt-based desiccants were developed to satisfy the requirements of maritime container shipping. Salts possess high hygroscopic properties and hence have the capacity to absorb vast quantities of water. Two categories can be distinguished:
- · Salt/swelling agent blend
- · Salt drip systems
Clay and starches are the most common swelling agents, whereby starch is many times more effective. Our SeaDry products, high-performance desiccants for container shipping made of virtually pure calcium chloride and modified starch, are found here.
Drip systems consist of a salt reservoir with access to the air and a drip basin. ThoMar OHG does not use drip systems for container shipping and urgently advises against them due to the significant risk potential for brine leakage, especially during the vessel's pitch and yaw movements.
How much desiccant do I need for my container transport project?
What must I consider when buying desiccants for container shipping?
Pay attention to the quality of the products on offer. In this regard, tests of water absorption capacity or rate by independent institutions are strong indicators of product quality. The best procedure is to enquire with your suppliers.
Moreover, the quality of the envelope material is important in order to reduce the risk of product contamination. You should select a tear-resistant envelope material to prevent desiccant leakage on all accounts.
ThoMar OHG recommends: Do not compare the price per product, rather the price per absorbed gram of water. A better product quality will automatically mean less handling workload, additional weight and risk for your products.
Click here for our product, the SeaDry container desiccant.
Where should desiccants be placed in the container?
1. Desiccants should be distributed evenly in the container.
2. Attach desiccants to the highest possible point. This is because warm air carrying more moisture will rise in the container. Hence, the greatest humidity is found below the container ceiling.
3. Check for damage to the container shell, for instance rust holes, cracks and defective door seals. Make them tight before container transport if possible.
What kind of damages are possible through relative humidity?
Damages vary from good to good. Subsequent, you will find a selection of possible damages:
- · Corrosion damage: ferrous products (machines, technical instruments or canned food) tend to corrosion at higher humidities. Corrosion limit from 40 % relative humidity.
- · Mould: cargo or cargo securing organic origin, including unprocessed food, textiles, leather or wood and wood products tend to mould formation. Non-organic substances, like pottery may tend to superficial mould formation as well. Mould growth limit is at 75 % relative humidity.
- · From 100 % relative humidity condensation occurs from steam to water.
Consequences thereof may be: floor and colour stain, collapse and failure of cardboard packing, labels become detached or clumping and swelling of bagged goods.
What is cargo space meteorolgy and what does it have to do with desiccants?
Cargo space meteorology is a sub-area of applied maritime meteorology. It deals with the meteorological influences and risk factors for goods transported by water. Using scientific methods, the influence of solar radiation and heat distribution within the goods can be determined.
The humidity level inside the load carriers also represents a broad area of cargo space meteorology.In this way, knowledge about air currents, heat conduction, as well as the consequentoal damages of condensate such as container sweat and cargo sweat, can be traced back to the scientific principles of cargo space meteorology.
What is cargo sweat?
Cargo sweat is the condensation of liquid on the surface of the goods. This happens under the following conditions:
In direct contact between air and goods, the air temperature must cool below the dew point to form condensate. This happens when the product is cooler than the air and has at a maximum dew point temperature.
This effect is created when goods are shipped from cold to warm regions or, sometimes, when a sea container which becomes cold at night is exposed to strong solar radiation.
What is container sweat?
Container sweat is the accumulation of condensate on the inner container surface, which usually occurs under the ceiling. As the number of deposits increases, the drops formed start to rain down. It's called "container rain".
This effect is caused by direct contact between a cold container wall and warmer air. When the air is cooled below the dew point temperature by contact, condensation forms. The temperature discrepancies required for this occur when the container is transported from warm to cold regions. Cooling down of the container walls at sea in the evening can also have this effect.
How long can desiccant be stored?
Generally, durability of container desiccants depends on package and closure.
ThoMar OHG uses HDPE pouches for storage which commands a high water vapour barrier. In addition, it is resealable, so individual products can be removed. Thus, desiccants can usually be stored for more than two years.
For how long does the desiccant act in the container?
The effective duration is essentially dependent on the absorption rate and capacity of the desiccant. In turn, this is defined largely by the ambient climate.
Each substance – so desiccants as well – is compelled to seek moisture balance. The absorption of water by objects such as products, packaging or desiccants will rise in a direct proportionate relationship with the humidity present in the system.
It follows, therefore, that low desiccant doses will usually attempt to absorb water at a faster rate – and hence reach saturation more quickly – as there is less material available to accommodate the moisture. The duration of effective dehumidification can be adjusted using several desiccant pouches. The various climate zones encountered along the route, as well as the climate during loading and unloading, are relevant factors as well.
Use our desiccant calculator to define the necessary duration of effectiveness and hence calculate the quantity of desiccant you require.
How do I dispose of my container desiccant?
Strictly adhere to the manufacturer's instructions for disposal.
SeaDry – the container desiccant by ThoMar OHG – is disposed of conveniently with industrial waste.
In-box desiccant for storage and shipment
What kind of desiccants are there?
The most common types of desiccant are:
- · Silica gel
- · Clay granules-based desiccants
- · Calcium chloride-based desiccants
- · Molecular sieves
You will find a more detailed description of the desiccant types under Basics.
When do I use each desiccant?
What do "desiccant according to DIN 55473-B" and "MIL D 3464E" mean?
These are standards that define the properties of certain in-box desiccants. DIN, standing for “Deutsches Institut für Normung”, is the German Industrial Standard, while MIL is the American equivalent. The technical pouch properties are almost identical.
All desiccants according to DIN 55473 are directly comparable with each other, as the hygroscopic and absorption speed of the desiccant pouches are defined by the standards.
What does the letter behind the DIN code mean, for instance DIN 55473-B?
The letter after the DIN number refers to the dust permeability of the wrapping material:
- · A: Low-dust
- · B: Dust-proof
In this regard, DIN 55473 defines as follows:"A: low-dust"
"No more than 10 mg of dust may pass through the envelope per desiccant unit."
[DIN 55473:2015-12, 4.3.2 a) para 2]
"Irrespective of the volume of desiccant units in the pouch, no more than 1 mg of dust must be measurable following completion of the test."
[DIN 55473:2015-12, 4.3.2 b) para 2]
What does DU or Unit mean and how do the terms differ?
DU stands for desiccant unit and describes the amount of desiccant granulate that absorbs a given amount of water within a defined climate in a specified time. This is not an indication of weight.
Depending on the quality and origin of the clay granulate, the weights of a DU may vary.
Units are constant designations in English. Depending on the origin of the desiccant, this designation may also be printed on the product.
Important: This information always refers to desiccants according to DIN 55473
How do I determine the required amount of desiccant?
We have developed the desiccant calculator for this purpose. Here you can calculate the required quantities. Calculation of the desiccant volume is based on DIN 55474: "Desiccant pouch – Use and calculation of the required number of desiccant units".
How do I use the desiccants in packaging for shipping?
Like dimensioning, DIN 55474 also specifies how to use the desiccant pouches.
According to the desiccant method, a barrier envelope must be added to the packaging for shipping. It is defined by its extremely low permeability for water vapour. The barrier envelope must be sealed tight and must not have any holes, tears or defective seals.
The size of the desiccant pouch is determined according to DIN 55474, and it is introduced immediately before sealing the barrier envelope.
The desiccants must be removed after opening the barrier envelope and replaced before the envelope is sealed again.
Can desiccant pouches be regenerated?
Industrial desiccants are not intended for regeneration. The cost of the required energy usually far exceeds the cost of replacement. The exception here is loose silica gel, which is used in laboratories and can be regenerated at approx. 130 °C.
Dehumidifier for private use
What is the difference between a dehumidifier and a desiccant?
These terms describe the same systems and substance combinations. Over time, humidifier has become the standard term in a domestic setting, while the word desiccant is used for industrial applications.
What types of humidifiers are available?
Essentially there are two systems:
- · Drip systems
- · Ab- and adsorbent systems
Drip systems consist of a salt reservoir with access to the air and a drip basin. The salt reservoir attracts moisture from the air, causing it to dissolve. The brine this produces drips into the basin. There is a risk of brine spillage if the system is moved, so ThoMar OHG recommends using drip systems in a stationary setting only. This does not necessarily apply to particularly large drip system with lateral guards like the Air Dry Cup.
Drip sysemts are especially suitable for:
- · Cupboards, larders, safes
- · Mothballed caravans
- · Basements and storerooms
- · Rooms (especially bathrooms)
Ab- and adsorbent systems usually consist of silica gel, clay or salt mixtures. They bind the water by physical or chemical processes. Physical adsorption is reversible, and so the desiccant can be regenerated.
In contrast, absorbent dehumidifiers are disposable products.
Can dehumidifiers be regenerated?
The regenerable qualities of dehumidifiers are usually described by the manufacturer on the packaging.
If you look for information on the Internet, only use the information provided directly by the manufacturer.
Incorrect regeneration of dehumidifiers may cause the granules to decompose or can damage the envelope.
Can I dry dehumidifiers in the microwave?
Regeneration in the microwave is not advisable, with very few exceptions. The packaging or the product itself will state clearly if it is suitable for microwave regeneration.
In case of doubt, contact the retailer or manufacturer of the product.