We present a special on the Thermal Districts analyzed under the perspective of an independent consultant.
by Eng. Camilo Botero G.*
The Thermal Districts seek to achieve optimal air conditioning projects, which must mainly comply with the following characteristics, in which I have been reiterative in several of my articles and under different contexts: first of all the Thermal Districts are projected to achieve Maximum Energy Efficiency with a Minimum Impact on the Environment. Of course this implies a design with the highest degree of detailed engineering possible and in its execution it is essential to comply with the specifications of said design. In addition, the Thermal District must have ease of operation and have adequate maintainability during its occupation phase and maintain stability during its life cycle.
For reasons specific to the activities of the users of the cold water of a Thermal Cooling District of those that exist in the Latin American tropical zone, not all the facilities are permanently occupied, but they are occupied sporadically, due to the characteristics of the particular activity of each building, and there are appreciable variations in demand.
In case of this being occupied at 100% load and taking into account the new climatic conditions, the system at full capacity could only really cover a small of the demand in the best of cases, that is why there are many inconveniences for the air conditioning of the areas, if the Thermal District has not been well conceived. At this time with very appreciable global warming, extreme high temperature weather is recurrent, which has an impact on a greater demand for central cold water systems.
When evaluating cold water systems, the alternative is presented regarding the type of condensation, that is, whether to use the water condensation system or use air condensation. For a long time it has been a paradigm to use water condensation because this would give, in theory, more efficiency, but with an increase in the consumption of replacement water that is already very high today, and water is a resource that is becoming scarce.
With the technology available today, a Cooling District can be made with air condensation equipment, with variable speed compressors, with efficiencies similar to those of water condensation, in capacities up to 300 TR. The counterpart is that they are very bulky and require more space for their location. It should also be defined whether the new system would be fully centralised and/or repower some of the current systems. I usually recommend a central system, with the maximum efficiency available at that moment, and improve the current systems by optimizing the interconnection between them with a properly designed pipe ring. This involves a thorough understanding of the principles of fluid mechanics and heat transfer, which are often underlying and overshadowed by design programs.
It is also necessary to carry out the recognition of the system and analyze the information provided by the engineering personnel, the surveys of the interconnection of existing pipes and the previous technical information of equipment catalogs and the analysis of said data, to make a conceptual engineering as adjusted to reality; with reference to this, perform the basic engineering and of course the detailed engineering with great rigor, leaving perfectly documented all the required technical information.
Because there are centralized cold water systems of constant speed, already installed in buildings, condensation by water and / or air; the improvement using more efficient chillers with variable speed compressors and condensation by air, and / or water, has a favorable return on investment, to pay the project for energy savings; this requires information on current energy and water consumption, its values and thermal demand profiles. All interconnection pipe networks of current and future cold water systems should be redesigned; generally the existing ones are not adequate.
The process of selecting the buried pipe for the ring is a task of great responsibility. The insulated and protected pipe with some flexibility and suitable for burial is privileged; the manufacturer must give a minimum warranty of 25 years. It is essential to study the topography of the terrain, as it is necessary to know the levels of the cold water systems, since being interconnected these will interact their static pressures, by the different heights; the process of assembling the pipe is cumbersome and must be rigorously programmed.
The appropriate water expansion systems must be selected to absorb volume variations, due to temperature change and systems for the deaeration of the pipes, since the air in them produces harmful effects on pumping.
It is essential to make an inventory of present, future immediate and future loads of medium term, in relation to the update of the central systems from cold water and change of direct expansion (DX) to cold water in the Thermal District and define a factor of simultaneity (capacity in the centralized system vs demand in the buildings), that for this type of project to have a favorable return on investment, it must be 0.5 or less, so that there is considerable energy savings and the initial investment is the most appropriate.
It is essential to invest considerable design time, taking into account mathematical models that reasonably reflect the performance of thermofluids. As they are systems of considerable electricity consumption, the state-of-the-art electrical power substation must be designed, preferably with dry transformer and with its energy backup. It is very important to consider the use of alternative energies such as ice banks and photovoltaic, wind or geothermal energies, etc. The management of the project and the auditing must be carried out by engineers, who understand this technology, because otherwise the probability of failures in the development of the project is very high.
Thermal Comfort and Indoor Air Quality
I will expand on this concept even when it seems to be repeating myself, because it is essential for the projects of Thermal Districts, which seek thermal well-being in the thousands of people who inhabit the different environments of the cities and its fulfillment ultimately depends on the success of the Thermal District in cities.
Based on Chapter 9 of the ASHRAE Fundamentals and 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 and psychological processes.
The important thing is that both the engineers who design, install and maintain the Thermal Districts, understand the fundamentals of human thermoregulation, the feeling of comfort and sanitary conditions so that the 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 it very understanding, the concept: "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.
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.
Sensitive and latent heat loss from the skin is typically expressed in terms of environmental factors, its 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.
If you study in detail Chapter 9 of the ASHRAE Fundamentals, 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.
ASHRAE Standard 55 addresses this issue and its committee periodically reviews the parameters that 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 should 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 care for the environment (ASHRAE 90 standard). The thousands of users of a Thermal District in its general conception will be its judges.
Control
This is a primary issue for the optimal performance of the Thermal Districts, in what has to do especially with the achievement of maximum energy efficiency, accommodating the chiller plant to partial loads. Therefore, it is necessary to hold specific technical meetings, with the participation of control technicians who must understand the correlation between the behavior of thermal inertia of thermofluid systems and the microsecond responses of electronic controls, to address this issue of control in all its aspects.
The strategy and design of the control for the Thermal Districts, it is necessary that it appears in all phases of the quality assurance of the project from the RDP (Requirements of the Owner for the Project), the BdD (Bases of the Design), the Design itself, the specifications for the Construction, Assembly and Start-up of the equipment, and training with assimilation verification in the O&M (Operation and Maintenance) phase, as rigorously described by the ASHRAE 202 Commissioning standard (Project Quality Assurance).
* 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]).
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