6 May 2024

From wood steaming to drying

When freshly cut wood is available, in whatever format it may be, it must be subjected to specific drying processes, so that the massive water content within it is brought to a level where it can be used. Incomac never misses an opportunity to tell you about this fundamental procedure, which must, however, be preceded by other preparatory processes: either spraying with water or, if it is necessary to act more quickly, steaming the wood. We would therefore like to take this opportunity to discuss this latter type of treatment in more detail, also analysing Incomac’s solutions in this regard.

Why wood steaming is useful in the drying process 

Let’s start by explaining why it is of prime importance to put careful and effective steaming before drying. When wood is cut in nature, the water in it is not evenly distributed throughout the trunk, but has moisture peaks scattered here and there. Because of this aqueous imbalance, if one were to proceed directly with drying, one would obtain a wood compromised by widespread harmful tensions in its structure, unable to ensure the desired performance. This is essential for more complex woods to be dried.

To dry wood without defects such as cracks or deformations, an intermediate process such as steaming must be used. In fact, this is a process that precedes drying , in principle, consisting of subjecting the wood, placed in specific chambers, to powerful jets of steam, which uniform the water content inside, creating the conditions for optimised drying.

However, we must also point out that steaming is indeed effective, but it is used when we need a quicker drying process. Pre-treatment of wood is usually done by water spraying (low or high pressure). This process reduces conditioning and drying times by at least 10%. It is mainly used for treating thicker, particularly hard woods, or for more ‘nervous’ woods such as eucalyptus. In all these cases, treatment with pressurised water makes the subsequent drying of the wood more efficient due to the greater opening of the wood pores.

Spraying is the commonly used solution in wood pre-treatment, but steaming is a better solution, although used to a lesser extent. Returning to the latter process, in fact, we can state that its benefits do not only turn into time reduction, but also into two other beneficial actions of absolute importance:

  1. It modifies the natural colouring of the wood, evenly colouring the whole surface without any intervention by chemical agents;
  2. It softens the wood fibres, allowing them to be more easily processed in subsequent handling (including at the veneer machine or veneer stage);

In light of all this, it is clear that steaming is a useful preamble to drying, which is also able to produce top-quality end results thanks to the process that precedes it.

Direct and indirect steaming of wood 

Steaming takes place in special chambers, in which, however, the process is not always carried out in the same way. In fact, there are two types of steaming:

  1. Direct steaming. In this case, the steam is fed directly into the steaming chamber through a series of perforated pipes placed underneath the woodpile, which comes into direct contact with the steam as it is enveloped in a cloud of it. (If the steam were blown directly onto the woodpile, this would lead to problems of partial drying or overheating. That is why the pipe holes face the floor of the chamber). There is, however, one condition that must be met for this process to run smoothly: the steam must be saturated and expand in the chamber until it reaches atmospheric pressure level. If, on the other hand, it does not reach the necessary saturation levels, it must be moistened: in this case, the steam is first blown to a channel filled with water located under the pipes;
  2. Indirect steaming. Here, there is no direct injection of saturated steam into the chamber, but it is obtained by evaporation. In indirect steaming chambers, there are water tanks on three sides (excluding the door side), below the floor and stack level, which must be brought to the boil in order to obtain the steam required for the process. Evaporation is traditionally stimulated by heating coils immersed in water containers, in which superheated water, steam or diathermic oil can flow as a thermal agent. In some cases, however, the coil in the tanks can be activated by the flame of a gas burner (indirect steaming with direct burner). But, regardless of how the boiling is caused, it is essential in this type of steaming that the water in the tanks is kept at a constant level.

Each of the two processes provides specific advantages. On the direct steaming side, the real plus is the structural simplicity of the system, where the pipe system can be implemented quickly. Indirect steaming, on the other hand, is preferable both because it treats the wood in a mild manner, minimising the risk of damage, and because it allows the recovery of condensation, guaranteeing significant energy savings.

Our steaming solutions

Incomac is also technologically at the forefront of steaming. This is evidenced by its high-temperature wood steaming system Vap, which is capable of transferring to each wood species all those characteristics that must be present in the wood before drying, from the reduction of internal tensions to colour uniformity.

Our steaming chamber can be fed by steam, superheated water or diathermic oil, or even by direct gas or oil heating systems, and is characterised by a special feature that makes all the difference in this respect: high thermal insulation. The steaming cycle is controlled at all times by two temperature probes, which allow it to be managed according to the requirements of the wood to be treated, and consists of recreating an environment filled with saturated steam in which the wood species is exposed to high temperature and humidity. The duration of the process can be adjusted according to the final purpose of the treatment: Vap systems are not only limited to colour uniformity, but can also impart particular colour gradations to the wood. Contact us to get more details about our steaming machinery!




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