HUMIDIFICATION
into a heated dwelling (or museum, in this case), its relative humidity drops drastically. Of the three diff erences listed above, the main one is that nowadays indoor environments are generally heated in winter to temperatures that in the past were only applied to living areas. Churches, for example, were never heated. Clearly then, increasing the air temperature without adding more water vapour will cause a decrease in relative humidity. In rooms that are heated for personal comfort in cold climates, this phenomenon can lead to extremely dry air. It should also be remembered that these are average minimum temperatures, and in some cities it is not rare to have days in which the temperature falls below -10°C.
It is clear that such relative humidity values are completely inadequate for the conservation of artworks, without neglecting the likewise important comfort of visitors and museum staff . The result is that above all in winter, when the ventilation system – whether forced or natural – introduces outside air into controlled indoor spaces, these also need to be humidifi ed. In a museum, trends in attendance are quite atypical, with visitors quickly entering and leaving the diff erent rooms.
In a closed environment this situation leads to
fl uctuations in temperature and humidity that may at times be signifi cant, and even harmful to sensitive objects such as paintings on canvases or manuscripts.
Indeed, occupancy by people is by far the most
signifi cant ‘passive’ cause of introduction of water vapour into such environments, via respiration, transpiration and perspiration.
As we know, these processes are very important in the expenditure of the metabolic rate, and even more so the higher the temperature and the more intense the physical activity.
An individual can in fact produce up to 2kg/h of perspiration, even though evaporation capacity does not exceed 1kg/h.
To evaluate the introduction of water vapour into rooms due to the presence of people, we need to know the average occupancy and the average level of activity.
Indicatively, we can assume that the average metabolic rate of an individual in a museum environment is around 150W (~130kcal/h). At a temperature of 21°C, typical of a heating system, the quota of the rate eliminated by latent heat exchange is around 45%, that is, around 70W (~60 kcal/h), corresponding to an hourly
production of water vapour per person of a little over 100g.
At a room temperature of 27°C, the latent quota increases and reaches 60%, with a production of water vapour per person of around 140g/h.
Consider a room measuring 50m2 (thus with a volume of 150m3
, 3m high ), occupied by 25
people. This is a reasonably frequent situation that occurs during visits by school groups or around the most prestigious works of art. These people would release around 2.5kg/h of water vapour into the environment. If the initial conditions are 18°C and 50% RH, without changing the air, saturation (100% RH) would be reached in less than half an hour.
In reality, though, we also need to consider the controlled or uncontrolled introduction of outside air.
The result of mixing outside air and inside air, combined with latent heat exchange with the people present, leads to a decrease or increase in humidity: to keep environmental conditions under control, it is essential to humidify or dehumidify the air, depending on the season and occupancy, on a case-by-case basis.
Strut Foot
www.acr-news.com
December 2016 39
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