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LEARNING ABOUT HUMIDITY IN THERMAL COMFORT
ROSET, J. (1); MARINCIC, I.; OCHOA, J.M.; SERRA , R. (2)
(1) Dpt. Física Aplicada - ETSAB (UPC)
(2) Dpt. Construccions Arquitectòniques I - ETSAB (UPC)
Diagonal, 649. E-08028 Barcelona, Spain.
Tel. +34 93 401 6380, Fax. +34 93 401 6426
E-mail: JAUME.ROSET@FA.ETSAB.UPC.ES
ABSTRACT. The purpose of this work is to explain in a 'common language' to technicians (physicists, engineers, ...) and architects a technical topic of importance in the practice of architecture such as the influence of humidity on human thermal comfort. To evaluate the contribution of humidity, Fanger's PMV treatment (ISO7730) is recommended only for water vapour pressures under 2700 Pa.
A study of Fanger's formulation shows that an increase in relative humidity is ALWAYS perceived as a warmer sensation. It seems to be contradicted by experience at low temperatures and high relative humidities..
In this paper, we want to present the subject in an educational way. So, we begin from classical thermodynamics and, pointing out CLEARLY the approximations done, we finish at actual applications of the humidity concepts.
We have used this method to obtain, in a reasoned way, the contributions of humidity to Fanger's formulation of PMV. Approximations like stationary modelisation (that means: no consideration of transient effects) or linearisation, are discussed.
The main idea of this work is our conviction that an 'architects oriented' explanation of the principles, processes and results of physics knowledge is essential to improve the understanding of technical applications in buildings.
1. INTRODUCTION
Presence of water vapour in the atmosphere causes different phenomena that vary at the different climates we are in. In humid climates we can see, in winter, dew on the fields, condensation on walls , ... and, in summer, people sweating. In dry climates at the hot station, humidity is a great help for extracting heat of the ambience.
The amount of water vapour is due to natural causes (big masses of water, abundance of raining , ...) or to the presence of man (people, fountains, industrial processes generating it, ...).
Propagation occurs by contact (rain filtration's, capillarity, ...) or by the entrance of humid air in the considered sites.
In the field of architecture the presence of water vapour implies only obligations preventing direct infiltration, condensation (superficial and interior). When condensation becomes unavoidable, like in kitchens or bathrooms, we must dispose of water resistant materials .
For people, there are positive and negative effects. Positive effects can be achieved in dry hot climates when liquid water takes heat of the ambience for its vaporisation. Negative effects appear when:
- High humidity make the hot and cool sensations worse
- Low humidity dissect the nasal mucouses, ...
We could hypothesize that exist a narrow band of humidities where people can feel comfortable (o near to comfort), outside this band people can feel very uncomfortable.
2. BASIC PHYSICS
How can we help to understand, in a scientifical basis, the important subject of the water vapour contained in the air? Lets begin defining the variables that show its behaviour. In this way, absolute humidity is defined by the mass of water vapour (usually in grams) contained in a given quantity of dry air (usually kilograms). The same concept could be evaluated by the vapour pressure. With respect of the behaviour is important the quantity of water that saturates the air, because when saturation is achieved, the air can't contain more water vapour, and it must be transformed into liquid (sweat drops, condensation on glass surfaces, ...). It is observed that this saturation quantity increases with temperature. When we talk about pressures, this quantity is reflected by the saturation pressure.
The quotient, in per cent, between vapour pressure and saturation pressure is called relative humidity. Representation of the state of a given mass of air is usually made by a point in the psychrometric chart , as can be seen on the figure.
As an analytic tool, the psychrometric chart is usually used to compare two different states of the air, having generally different temperatures and absolute humidities.
Psychrometric chart can also be used as a calculation tool. This way, if we want to know the energetic exchange between two different states of the air, we can decompose it in two components: a change in the air temperature (sensible heat exchange) and a change in the humidity contents (latent heat exchange). As these exchanges take usually place at approximately constant global pressures, they are called enthalpy variations.
Other specialists' studies (see Serra 1995) could give us information about comfort zones or how to achieve them (by ventilation, inertia, ...).
Another important option is vegetation. In outdoor spaces, vegetation is one of the landscape elements that can affect microclimate, and so human comfort , by several ways. It can obstruct solar radiation, change wind's speed and direction, affects the radiant exchange between the soil and the space, modifies the ambience temperature and, obviously, modifies the humidity of the air.
Figure 1. Psychrometric chart showing comfort zones (Serra 1995)
Although effects on radiation and wind are more evident (Ochoa 1998), variations in humidity and temperature can also be appreciated, mainly when talking about big landscaped areas and, even more, in zones confined by topographic or architectural barriers.
The mechanism that vegetation employs to modify temperature and humidity is called evapotranspiration, that is the evaporation of the water, leaked by plants, through their estomes of its leafs and stems. It is called evaporative cooling, which depends mainly on two factors: the quantity of leaked water and the relative humidity of the air.
3. ADVANCED PHYSICS
The word 'advanced' must always be referred to a particular subject. In this section, we want to 'advance' in the knowledge that psychrometric chart can give us, about hygrothermic comfort of people.
Fanger's formulation (Fanger 1972) for thermal comfort is advised by The European Union (ISO7730). Air temperature, mean radiant temperature, air velocity, water vapour quantity, people metabolic activity and people clothing are pointed as variables that must always be taken in consideration.
In this formulation, a 'thermal load' L is defined as the number of watts per square meter that a given person must gain or lose to achieve an idealised comfort state.
The L calculation consists on subtract the internal heat flow produced by the person from the heat flows emited to the exterior and assuming that sweating can produce enough flux to arrive to the 'proximity' of thermal comfort. The considered flows are produced by: diffusion through the skin, sweating, humid and dry respiration, radiation and convection.
The L value obtained in this way is then linearly correlated with cool and heat sensations voted by people, Predicted Mean Votes, that are finally associated with the People Percentage of Dissatisfied (this is much better explained by Fanger!).
4. HUMIDITY AND COMFORT
In the L equation, water vapour participates explicitly in two types of flows (energies per unit time and unit area) : skin diffusion (Ed) and humid respiration (Ere), which is given in I.S. units by:
Ed = 3.05 e-3 * (5733 - 6.99*MW - PA); Ere = 1.7 e-5 * M * (5867 - PA)
where PA is the vapour pressure in Pascal, M the metabolic activity in W/m2 and MW the metabolic activity minus the work done by the person.
Ed and Ere are linearly dependent on "-PA". As they take part in the L equation with other negative sign, the result is that L is always proportional to PA, which can be represented by a line with positive slope!!
This result is not applicable for big values of PA. European normative (Grosso 1997) establish that this formulae are valid for PA values under 2700 Pascal. That implies an absolute humidity value (on the psycrometric chart) of 16.5 water vapour grams per dry air kilogram.
The graph in figure 2, gives an idea about how people feel humidity effects in a wide range of temperatures:
In the graph, we can see that for high relative humidities (no matter if temperature is high or low) and for very low relative humidities, people experience very important influences. Important influences become always negative for human beings because the person must force its body mechanisms.
In the middle, around the 0 influence (comfort), exists a linear zone with positive slope, as Fanger's treatment predicts.
The zone above 2700 Pa at the psychrometric chart (Serra 1995) only admits ventilation as a solution to achieve the comfort sensation. We think that a relation among air velocity, humidity and rate of heat transfer should be added to calculations.
Figure 2. Influence of humidity on thermal comfort.
5. CONCLUSIONS
Fanger's stationary model using a constant value (in fact, L) is accurate enough in a certain range of humidities and temperatures. We think that any model with the aim of enlarging these limits, must contain Fanger's model as a limit for moderate relative humidities..
Future new models, taking better into account time variations, would require studies on people (specially about acclimatisation) and on the accuracy of the measures based on them.
ACKNOWLEDGEMENTS
J.M.Ochoa would like to acknowledge to the National Council for Science and Technology of Mexico (CONACYT) the scholarship to carry on his PhD, which this paper is based on.
REFERENCES
[1] Fanger, P.O. (1972) Thermal Comfort, McGraw-Hill, New York, USA.
[2] Grosso, M. (1997) Il raffrescamento passivo degli edifici, Maggioli Editore, Rímini, Italy.
[3] Ochoa, J.M., Serra, R. (1998) .Proceedings of PLEA'98, James & James, Lisbon, Portugal.
[4] Serra, R. (1995) Les energies a l'arquitectura, Edicions UPC, Barcelona, Spain.