Moisture content of the air
When drying, it is important to know whether the air is dry or humid. By just looking at the RMC, we don’t know anything yet; The RMC indicates how much moisture is present in the air, RELATIVE to or in RELATION to the maximum moisture content that can be absorbed into the air at that temperature.
In order to absorb moisture (water molecules), energy must be present to allow the molecules to move into the air. Higher temperature means more energy which enables a greater moisture absorption capacity. With decreasing energy, the water molecules clump together to droplets (fog) or condense on cold surfaces. By measuring the RMC and the temperature of the air, we can calculate how much water is in the air; Absolute Moisture Content (AMC).
Tabel met links/rechts temperatuur en boven/onder RMC. In het midden het Absoluut vochtgehalte (AMC).
Warm air has more energy than cold air. In this case there is more energy to make the water molecules move. So warm air can contain more moisture than cold air.
The amount of moisture that air of 18°C may contain is about 12.9 grams of water per kg of air.
- With an 60% RMC, the air contains 0.6 x 12.9= 7.7 grams.
The amount of moisture that air of 30°C may contain is about 27.2 grams of water per kg of air.
- With an 60% RMC, the air contains 0.6 x 27.2 = 16.3 gr/kg
- With an RMC of 30%, the air contains 0.3 x 27.2 = 8.2 gr/kg
Air with an RMC of 60% and a temperature of 18°C is drier than air with an RMC of 30%. At a temperature of 30°C. Only at 29% RMC and 27°C is the moisture of the air equal to each other; 7.7g/kg.
When the air is warmed up, it may contain more moisture;
- When air is warmed up from 18°C with 60% RMC to 27°C, the moisture content present gradually decreases compared to the maximum moisture content of the air.
- At 27°C, the 7.7 gr/kg moisture relative to 27.2% decreased to 29%.
- Both air quantities are equally humid (or dry), there is only a difference in energy content (temperature).
Conclusions applied – (The Agratechniek Drying Method)
In order to withdraw moisture from the product, the relative moisture content (RMC) must be as low as possible. Then there is energy left to absorb moisture before the air is saturated. By heating air, the energy increases and even more moisture can be absorbed; the RMC is falling. When the air flows through a moist product, the air absorbs moisture; the air becomes relatively humid at the maximum moisture content; the RMC is rising. The absorption of moisture requires energy. This energy is extracted from the heat; the air cools down.
As the air cools, the maximum moisture content decreases from the air. The moisture content present is therefore also increasing because the air cools and may contain less moisture. The RMC is therefore increasing rapidly and the maximum moisture content (98-100%) is then reached.
The warm incoming air is cooled in the product by absorption of moisture.
40 years ago, the temperature of the incoming air was compared to the air from the product the onions. In the case of a slight difference, the grower knew that the product was also at the bottom of the coffin or hope was dry.
Drying with air
Which air is best for garlic drying? The choice depends on the outside climate condition,
- the desired conditions for the garlic and
- energy costs and
- availability of gas and electricity.
There are three options:
- Unheated dry outside Air
- Warmed dry outside air
- Conditioned air with often warmed dry outside air