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1、9 翻译Standard62.1-2004: System Operation: Dynamic Reset OptionSeventeen years ago, the ventilation rates established by ANSI/ASHRAE Standard 62-1989, ventilation for Acceptable indoor air quality, increased substantially over those previously required by the 1981 version of the standard. Fifteen year
2、s later, standard 62-2004 prescribed new minimum breathing zone ventilation rate and a new calculation procedure to find the minimum intake airflow needed for different ventilation systems. Described in previous articles, these new rates and procedures must be used to find the design or “worst-case”
3、 outdoor air intake flow, which establishes the required capacity of mechanical system equipment. In this article, we turn from ventilation system design to operation. Variation in occupancy or ventilation airflow The number of occupants in many ventilation zones changes during normal. Actual zone p
4、opulation often falls short of park design population used to establish the highest zone outdoor airflow and outdoor air intake flow needed. Consequently, standard 62.1 allows breathing zone and/or intake airflow to be reset in response to changes in zone population or the resulting changes in the z
5、one ventilation airflow per person. To reset ventilation based on the current population within a zone requires a reasonably accurate estimate of population or breathing zone outdoor air rate. Standard 62.1 lists four example measures used to estimate variations in occupancy and one measure to estim
6、ate variation in ventilation airflow. Direct Count of People. In some zones, occupant entry and exit may be orderly. Using appropriate sensors and counting controls, the difference between entry and exit events can be used to estimate population based on a direct “count”. Ticket sales might also be
7、used as an estimate of population. Some retail stores or auditoriums may be configured to use these approaches. Sensors that actually sense individual occupants also might be used, although such are not common. Presence of People. A variation to counting occupants directly involves simply sensing th
8、e presence of occupants. Motion detectors can sense human activity in offices, conference rooms and so on. Upon detecting motion, the control system assumes that the zone is occupied at park population and calls for ventilation accordingly. Time-of-day schedule. In some zones, population can be pred
9、icted based on time-of-day. For instance, the population in a given classroom in an elementary school may be estimated quite accurately during any hour of the day. The daily schedule for Mr. Browns fifth graders places them in Mr. Browns classroom from 8a.m. until 10a.m., in the art room from 10a.m.
10、until 11 a.m., then back in the classroom from 11 a.m. until noon, and so on. If the building control system includes an occupancy schedule for the classroom, current population can be estimate and outdoor airflow can be reset to match prescribed ventilation airflow to current population. Estimate o
11、f Occupancy Based on CO2 and Airflow. As Mumma has shown, in a single-zone system, zone and outdoor CO2 level along with intake airflow could be sensed and assuming an occupant activity level, used to estimate current population. At steady-state CO2 concentration zone population could be found ,but
12、in practice, zones seldom reach steady-state condition. So, in addition to accurate CO2 and airflow sensors, this approach requires a controller capable of solving a non-steady state equation. And, since occupant activity level and CO2 generation rate can vary widely, calculated population may not b
13、e accurate. Due to its cost, its complexity and its potential for poor accuracy, not many designers use this approach. However, do not confuse this CO2 -based “people counting” measure with our next topic: tradition CO2 based “demand- controlled ventilation.”CO2 Based Estimate of Current Outdoor Air
14、flow per Person. Since changes in population change ventilation “demand”, any dynamic reset approach that responds to zone population could be referred to as demand-controlled ventilation(DCV). However, this acronym has been used for years to refer specifically to control measures that sense CO2 con
15、centration as a surrogate for the concentration of human bioeffluents. The most popular dynamic-reset approach to part-load ventilation, DCV controls adjust outdoor airflow rate to maintain zone CO2 and thereby bioeffluents-at levels that would result from ventilating at, or above, prescribed minimu
16、m outdoor airflow rates. As explained by Taylor, CO2 based CO2 DVC assumes that people produce both odors and CO2 in proportion to their activity level, that occupant activity level in a zone can be estimated with reasonable accuracy. And that steady-state equations can be used to estimate ventilation load. Much the same as steady-state equations can be used to estimate heating/cooling load. For single- zone systems. The minimum required outdoor a
