FUNDAMENTALS OF ENERGY SYSTEM OPTIMIZATION IN INDUSTRIAL BUILDINGS
This section briefly introduces the energy aspects that have to be taken into account in system design. The building-related features are covered in Chapter
11, mainly from the energy-calculation point of view, with less attention paid
To the aspects of optimization of the energy used to run the air distribution systems.
In this section, attention is paid to the energy demand of ventilation and air-conditioning systems as a whole. On the system level, considerable efforts have been made to reduce the heating and cooling demands, and the electrical energy consumption has been regarded as a marginal part. But after the first energy crisis in the mid-1970’s, the situation started to change, so rapidly that in many modern air-handling installations, the share of electrical energy can be as high as about 90%. This figure has recently been reported in cold climates, too.
One simple example describes the problem: the efficiency of a heat recovery device can be increased from approximately 50% up to approximately 75% by using a double unit. This will double the pressure drop from, say, 100 Pa to 200 Pa, resulting in the fan energy’s increasing respectively.
Measures undertaken to improve the indoor air quality (IAQ) have the same effect by upgrading the filter class, increasing the air change rate, etc. These small improvements have grown both in number and in size, little by little.
Example: A ventilation system (Fig. 9.64) handling 20 m3/s of air needs to heat the supply air from 10 °C to 20 °C. Doubling the number of heat exchangers from one to two increases the heat-recovery efficiency from 50% to 75% and introduces an extra pressure drop of 300 Pa. As we can see from Table 9.19, this is probably a cost-efficient measure.
Therefore, it is essential also in industrial environments to pay serious attention to energy usage of components and systems. This will bring a need to develop standards, guidelines, and other tools for practitioners to achieve the optimum. Some aspects are described in Chapters 15 and 16 relating to LifeCycle issues. Some criteria to estimate the total energy consumption for the building, and for individual systems, air-handling units, and fans have been developed. These will be described in more detail in the Systems and Equipment volume.
Space requirements for air-handling systems are briefly described in the draft European Standard prEN 13779, which, however, is targeted to commercial, public, and office buildings. These requirements also take into account the need for service and maintenance. In industrial applications the
TABLE 9.19 Ventilation System Characteristics (Example)
Ij = 50%, Ap = 100 Pa
Tj = 75%,
Ap = 200 Pa
Qv = 20 m3/s
—pj 62.5 kW 8 = 20 °C
FIGURE 9.64 Ventilation system (example).
Same principles apply, but the location of air-handling units, service routes and many other factors have to be considered more from the production-process point of view than for nonindustrial applications.
In ductwork design, attention shall be paid to reduce energy losses using properly designed fittings and proper duct sizing. Balancing also becomes a critical factor in order to ensure that the system operates at its most efficient designed condition. Systems that are designed for easy balancing arc good from the energy-consumption point of view, provided that the pressure drops in dampers and terminal devices are within reasonable values. Special attention to equal pressure drops in different duct branches is necessary in design of such ductworks for contaminated air where the use of dampers is not possible. See also Section 9.7 of this chapter.
Demand-controlled ventilation (DCV) is one approach to reduce energy consumption due to ventilation, that is gaining popularity in both industrial and nonindustrial applications. It is used in cases where ventilation require ments vary with time, regularly or irregularly. The control is based on a specified level of indoor air quality by means of continuous measurement of the parameters, that are expected to primarily determine the IAQ, such as the concentration of the main contaminant liberated from the production process. The principle is thus similar to the one in some better-known nonindustrial applications, e. g., C02 levels in rooms with dense human occupancy (theaters, classrooms, etc.) or nicotine concentration in smoking rooms. See also Section 9.6.
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