End users constantly complain about air conditioning problems in different spaces (hotels, cinemas, nightclubs, offices). The solution is at our fingertips.
by Camilo Botero*
Analyzing topics that could be of interest to the appreciable readers of ACR Latin America, the Editor suggested this one about the problems that arise in the air conditioning installations of human comfort, which I found very interesting because in so many years of talking about the subject, placing it as design parameters, discussing its limits for example in the elaboration of the RITE; I had not written an article about the frequent problems that arise in this subject, as is very noticeable for example in cinemas, airplanes, offices, nightclubs and in some bioclimatic solutions.
The feeling of absence of comfort can be given by high or low temperatures, humidity ditto, noise in the equipment, in the ducts or in the diffusers, dust or odors, gusts of cold or hot air, feeling of stagnation, etc., or simply by the state of health or mood of the people.
Solving these problems requires excellent design, compliance with standards, suitable assembly, strict and impeccable operation and maintenance, adequate control and precise information to users of what are the comfort conditions that your installation must achieve.
Definition of thermal comfort
I will rely on Chapter 9 of the ASHRAE Fundamentals and obviously on the ASHRAE Standard 55; he defines it in a very subtle way saying that it is "that condition of the mind that expresses satisfaction with the environment". Obviously this definition is a bit ethereal, since the terms "condition of the mind" and "satisfaction" are ambiguous, but clearly emphasize that the perception of thermal well-being is a cognitive process, which involves a wide variety of parameters, influenced by physical, physiological, psychological processes, among others.
The important thing is that both the engineers and technicians who design, install and maintain the air conditioning systems, understand the fundamentals of human thermoregulation, the feeling of comfort and sanitary conditions so that the end users feel satisfied.
Obviously, owners, project managers, auditors and users of air conditioning must be given training related to these aspects, so that they do not have erroneous notions or false expectations, because finally the issue of comfort is subjective and circumstantial what each person expects from the air conditioning system is different. For this reason there is the ASHRAE standard 55 and the RITE, so that these comfort conditions are convenient so that a high percentage of occupants feel comfortable.
Another way to put it is that a thermally comfortable HVAC system is one in which there is a balanced balance of mass and energy between the human body and the environment around it and there the body temperature is kept within low ranges, the humidity of the skin is low and the physiological efforts of regulation are minimized. Sometimes I add to my clients to make the concept very understanding: "It's like good health... you don't feel like you have it."
Comfort also depends on behaviors that are initiated consciously or unconsciously, guided by thermal and humidity sensations that reduce discomfort. Examples include weather-appropriate clothing, altering activity, relocating outside the discharge effect of an air conditioning diffuser or direct solar radiation, changing the thermostat reference value, opening a window, complaining, or leaving space.
Surprisingly, even though in the world climates, living conditions and cultures differ very considerably, the temperature that people choose for comfort under similar conditions of clothing and activity, humidity and air movement, have been found very similar.
Human thermoregulation
The metabolic activities of the body result almost entirely in heat generation that must be dissipated and regulated, to maintain the normal body temperature (38 °C), insufficient dissipation leads to hyperthermia and too much heat loss to hypothermia.
Skin temperature above 45 ° C and below 18 ° C, causes discomfort and even pain. When you have comfort this skin temperature will be between 33 and 34 ° C. The regulatory center of the brain tries to keep it between 36.8 and 37.4 °C; The hypothalamus is the central organ of control and receives signals from the skin and blood and regulates temperature, for example with vasodilation, vasoconstriction or sweat which is a powerful means of cooling the core of the body.
At rest an adult produces on the order of 100 W (341 Btu / hr) of heat and as this dissipates mainly through the skin it is convenient to typify it per unit area (1.8 m2 selected) or approximately 58 W / m2 and this unit is called a met. Obviously there are gender and ethnic differences. Other activities other than rest are defined in terms of the met unit, for example it can be said that a rough job or a sports activity would have a metabolic activity, say 5 met.
Energy balance
Equation (1) (See Page 9.2 Fundamentals 2013), which represents the mathematical model of the interaction of the human body with its environment; it's complex (that's why I don't put it here).
It describes that the total metabolic rat M, can be dissipated through total work W, and different dissipation mechanisms such as the following:
- Total heat losses through the skin and breathing.
- Loss of sensitive heat and evaporation through the skin.
- Rat of convective and evaporative heat loss through respiration.
- Heat rat stored in the compartments of the skin.
- Heat rat stored in the core of the body.
Sensitive and latent heat losses from the skin are typically expressed in terms of environmental factors, their temperature and their humidity. These factors also take into account thermal insulation and moisture permeability in clothing.
The speed of the air on the skin of people is a decisive factor of comfort or discomfort, as well as the pressure of water vapor in the environment. The heat storage rat in the body equals the rat increasing its internal energy. The body can be thought of as two thermal compartments: the skin and the core.
Heat exchange with the environment
If chapter 9 of the Fundamentals is studied in detail, all the phenomena, quantitative information, and calculations of heat and mass transfer between people and their environment appear. Mathematical descriptions of energy and mass balances combine rational and empirical approaches to such estimates. The fundamental principles of heat and mass transfer are used in these calculations as well as empirical expressions to determine the values of heat transfer rats, as well as thermo-physiological control mechanisms, such as skin and core functions of the human body.
Conditions for thermal comfort
ASHRAE Standard 55 addresses this issue and its committee periodically reviews the parameters, which specify comfort zones, where 80% of sedentary or slightly active people find the environment thermally acceptable.
Because people wear different levels of clothing, depending on the situation and climate, standard 55 defines comfort zones for different levels of clothing: 0.5 and 1.0 clo (0.078 to 0.155 m2-K/W) (it could be said as a reference that 1.0 clo is winter clothes and 0.5 clo is summer clothes). Here the reader can refer to Fig. 5 of page 9.12 where the comfort zones for summer and winter appear.
For a humid tropical climate like the one prevailing in my region, I specify as comfort conditions:
- Temperature 24 °C +/- 1 °C
- Relative Humidity 55% +/- 5%
- Air velocities of the order of 0.5 m/s (100 fpm).
Indoor air quality
Finally, great importance must also be given for the issue of indoor air quality to filtration (ASHRAE 52 standard) and outdoor air rats (ASHRAE 62 standard) and of course to the rational use of energy and care of the environment (ASHRAE 90 standard), these aspects can be extended in later columns. * Camilo Botero is the current Secretary of the Federation of Ibero-American Associations of Air Conditioning and Refrigeration - FAIAR; he was president of ACAIRE and is president of Camilo Botero Ingenieros Consultores Ltda. He has worked as a teacher in several Colombian universities, guilds and currently in ACAIRE in diploma courses of air conditioning projects, energy efficiency in air conditioning and refrigeration, cogeneration and trigeneration, applied psychometrics, thermodynamics, fluid mechanics, heat transfer and turbomachinery. ([email protected]).
