Published Date 26/10/17 08:00
Paper is a hygroscopic material that tends to reach a condition of equilibrium with the temperature-humidity conditions of the surrounding environment. Any swings in temperature and humidity can cause dimensional variations and loss of paper flatness.
These phenomena are well-known to those who work in the printing industry as, if not appropriately controlled, they can lead to printing defects or unwanted machine stoppages due to incorrect paper feed.
How are the correct ambient working conditions determined?
Firstly, it is important to know the paper’s absolute and relative humidity values, in order to ensure the right conditions during all stages of storage and subsequent printing and finishing of the product.
So what are the absolute and relative humidity of paper?
The absolute humidity of paper is the amount of moisture as a percentage of total mass, and varies based on grammage, typically from 5% to 10%.
Relative humidity, on the other hand, refers to the value at which the paper does not absorb or release moisture from/to the surrounding environment. Also for the latter, the value varies according to the grammage and type of paper, and is typically between 45 and 60%.
Absolute and relative humidity of paper are measured using specific instruments, both at the paper mill (before packaging), and once unpacked, before printing.
So how can these values be kept constant throughout the storage and processing stages?
In the paper mill, paper and board are sealed in waterproof packaging, which prevents exchanges of moisture with the outside environment.
The product thus remains in its packaging until it is used. In addition, if paper is left unopened for several hours, it is recommended to wrap and seal the product again.
Where there is a significant difference in temperature between the paper storage area and the area where the printing machinery is located, the paper needs to be kept close to the machine for a few days before unpacking it, so it can adapt to the ambient temperature where it will be used. The time needed to reach equilibrium depends on the size of the ream and the difference in paper-ambient temperature. In general, a time of at least 24/72 h is recommended.
Even during the printing process, temperature and humidity conditions should be kept constant and controlled within a defined range, to ensure efficiency, quality and reproducibility of the printing process.
Generally, ambient temperature is kept at around 23°C, while the best relative humidity level is between 55-60% for offset printing, and between 50-55% for other types of printing.
Once the set point has been defined, these levels must be maintained with a maximum tolerance of +/- 5%.
What are the factors that alter ambient conditions?
The ideal storage and process conditions are continually altered by factors such as heating systems, air flow taken in by printing machinery, the opening of doors and windows and the introduction of latent loads. All of these factors require the use of an air handling unit, which also includes a humidification system.
Air relative humidity is the ratio between the amount of water vapour contained in the air and the maximum value the same air can hold. As the maximum amount of water vapour that can be contained in the air varies with temperature, relative humidity, even with the same absolute vapour content, varies quite considerably with temperature. Consequently, in winter, even if the outside air is very humid, once this is heated to comfort level, its relative humidity drops drastically. For example, saturated air at 0°C, when heated to 23°C will have a relative humidity below 22%.
Opening doors and passageways
Another factor that can affect the set point being reached and maintained is the opening of doors and windows for circulation of air from the outside or other environments with different temperature and humidity levels.
Aspiration by printing machinery and emission of harmful substances
Printing machinery produces harmful emissions that need to be extracted before mixing with the ambient air.
The “2015 ASHRAE Handbook-HVAC Applications” specifies that the ventilation system should provide at least 2.5 l/(s * m2) of fresh outside air in order to keep the concentration of these substances below a threshold limit.
Printing machines take in thousands of m3/h of air, needed to carry away heat and restore the correct process temperature. Sizing of the ventilation system, including the humidifier, thus needs to take into account both the air intake of the machinery and the need to ensure an adequate air change volume.
Latent loads due to hygroscopic materials
Hygroscopic substances that have a moisture content other than the amount held at equilibrium either absorb or give up water vapour from/to the environment and thus represent an additional load for the air-conditioning system.
Water vapour is exchanged with the environment over times that depend on the difference between the current and the tendential content, on the air velocity and on the layout of the material; the phenomenon in fact occurs more actively on the outside surface than on the inside of the material or the pile of stacked or packaged items.
To ensure the correct ambient conditions, an air handling unit is needed, which also includes a suitable humidification system.
What are the most suitable humidification systems?
For the printing industry, adiabatic systems are often chosen, as compared to traditional isothermal systems they have the following benefits:
- Reduced energy consumption (generally less than 10% of the energy consumption of a traditional steam system);
- Evaporative cooling: the possibility to use these systems not only in the winter months to control humidity, but also to cool the air in summer and help absorb the heat generated by the printing machinery.
Why does the printing industry require humidity control?