Evaluation and analysis of the air conditioning systems of the "Centro Técnico, Palmas y Almendros" of the Pontificia Universidad Javeriana de Cali, Colombia.
by Camilo Botero*
This article will be divided into two editions: in this first part the theoretical definitions that were taken into account for the project will be broken down. The second part will talk about the project as such, from the systems that were found to the renovations and the consolidated cooling district.
Diagnosis
This is the report of the general performance of the Air Conditioning systems (Technical Center, Palms and Almond Trees) with emphasis on the repowering of the Technical Center as a kind of main Cold District, as proposed in the design for this purpose of 2011.
It is proposed according to what was discussed with the group of engineers and technicians of the PUJ CALI, to leave the cold water systems of Palmas and Almendros with their original loads and include the recommendations and suggestions to substantially improve the current installation, taking into account possible future loads in the short and medium term. Also the conversion of direct expansion equipment (DX) to equipment that uses cold water (CW).
It will seek to achieve the ideal air conditioning project. This concept is in perfect coherence with the improvement of the energy efficiency of the PUJ CALI, its new and modern improvements of the university campus.
Introduction
The Pontificia Universidad Javeriana (PUJ), asked the Consulting Engineer Camilo Botero G, to carry out the study to evaluate the performance of the current Air Conditioning systems (Technical Center, Palmas and Almendros) installed on the university campus and that supply the air conditioning needs of the different facilities of the university, emphasizing each of the cold water plants and evaluating the feasibility of implementing new generation chiller alternatives.
Through a respective analysis, detailing a higher thermal efficiency and lower noise level, which better fits the partial loads that it currently presents and adjust to future loads to two years, leaving the possibility of scaling the system for future growth in air conditioning demand.
It will seek to achieve the ideal air conditioning project, which must meet the following parameters:
- Maximum Energy Efficiency.
- Minimal Impact on the Environment
- Compliance with Specifications
- Ease of Operation
- O&M maintainability
- Stability during its Life Cycle
Some considerations about: thermal comfort and air quality inside the enclosures.
Definitions
Understanding the above concept is essential for this project, which seeks thermal well-being in the different environments of the PUJ CALI Campus. Based on Chapter 9 of the ASHRAE Fundamentals and the ASHRAE Standard 55, it 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, involving a wide variety of parameters, influenced by physical, physiological, psychological and other processes.
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 end users feel satisfied.
Of course, owners, project managers, auditors and users of air conditioning should be given training related to these aspects, so that they do not have erroneous notions or false expectations, because since 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 make the concept very sympathetic: "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 climates, living conditions and cultures differ considerably in the world, the temperature that people choose for comfort under similar conditions of clothing and activity, humidity and air movement have been found to be 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 the 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 of 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, of 5 met.
Energy Balance
Sensitive and latent heat losses from the skin are typically expressed in terms of environmental factors, their temperature and 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 of the environment.
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 empirical expressions to determine the values of heat transfer rats, as well as thermo-physiological control mechanisms, which are functions of the skin and core of the human body.
Conditions for thermal comfort
ASHRAE Standard 55 deals with 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: for example 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 of the Fundamentals, where the comfort zones for summer and winter appear. For a humid tropical climate such as that prevailing in this region, specifically as comfort conditions:
- Temperature 24 °C +/- 1 °C
- Relative Humidity 55% +/- 5%
- Air velocities, on the order of 0.5 m/s (100 fpm)
- Finally, great importance must also be given to 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 environmental care (ASHRAE 90 standard).
Note: In the second part of this article we will talk about the current system of the PUJ, the conclusions after the analysis of the system until reaching the Consolidated Cooling District.
* 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]).
