ASHRAE Journal - February 2010 - 34

data on TVOCs, particles of a size range of 0.3 to 2.5 microns, carbon dioxide, and dew point (absolute humidity). This article will focus only on the data collected on TVOCs and particulates since this data is most directly related to the demand-based control of air change rates. For particles and TVOCs, most measurements were taken as differential measurements of the room conditions compared to the environmental conditions of the supply air feeding the lab or vivarium. This was done to significantly reduce potential effects of any sensor drift, as well as to subtract out any impact of the outdoor conditions on the measured room conditions. Since all measurements were taken using a multiplexed sensing system, the measurements of the room conditions and the supply air feeding these rooms were taken with the same sensor, thereby creating very accurate differential measurements. To simplify the analysis, all sensor data was placed into bins representing the number of counts or times that a parameter exceeded a specific threshold level corresponding to that bin. The data was then normalized based on the total number of data points or counts to generate the percent of time the data exceeded the bin value thresholds. A cumulative graph was created showing the percent of time that each bin value was exceeded. Regarding the average air change rates of the labs involved, generally the typical “clean” or minimum rate was between 2 and 6 air changes based on the specific site involved. When significant levels of TVOCs or particles were sensed, the commanded purge rates of airflow typically were between 12 and 16 ach based on the site involved. Regarding the vivariums, the reduced flow rates varied between 6 and 10 ach when the rooms were sensed to be clean and the commanded purge rates, when significant contaminants were sensed, varied between 15 and 20 ach. Note that airflow rates higher than the minimum flow rates mentioned previously may have also occurred occasionally, even if the lab rooms were clean due to VAV fume hoods that may have been opened or cooling loads that demanded additional airflow. Review of the Lab Room Data 1.0% Percentage Time Over Threshold 0.9% 0.8% 0.7% 0.6% 0.5% 0.4% 0.3% 0.2% 0.1% 0.0% 0.1 0.25 0.5 1 2 4 6 TVOC Level Threshold (ppm) 8 16 Figure 2: Average TVOC level percentages over threshold (1.5 million hours of lab operation). 16 Minimum ach Command Value 14 12 10 8 6 4 2 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TVOC Value (ppm) 1.4 1.6 1.8 2.0 Figure 3: Typical control relationship between TVOC values and commanded airflow. Figure 2 shows a graph of the average TVOC levels over all of the lab locations representing about 1.5 million hours of operating data. As mentioned previously, this is a cumulative graph so that the value of 0.84% at 0.10 ppm means that, on average, this is the amount of time that a lab location has a TVOC value greater than or equal to 0.1 ppm. Since this represents the average, some locations can be much higher than this and others potentially near zero. However, the average gives a good idea of the potential energy savings across all these different locations. The highlighted blue area on the graph in Figure 2 represents a typical set of values for where the dynamic control of air changes operates in a proportional control fashion. For the indicated example, below about 0.2 ppm, there is no override of the room airflow. At 0.2 ppm and above, the override of the minimum air change rate starts to occur with a maximum command value of 15 ach reached when the sensed TVOC value reaches 1.5 ppm. This typical control relationship between the 34 ASHRAE Journal sensed TVOC values in a room versus the generated command for the minimum ventilation rate is shown in Figure 3. As can be seen in Figure 2, labs are, on average, typically clean of most chemical contaminants more than 99% of the time at least to a background level of less than about 0.2 ppm. This means that energy can be saved by using a reduced minimum air change rate of down to 2 ach for more than 99% of the time in labs with respect to at least the TVOC sensor. Looking at this same data in another light, Figure 4 takes the data of Figure 2 and displays it in terms of approximately how many TVOC events occur in an average week in one of the lab room locations. Most TVOC events where chemical vapors are sensed in a lab room typically vary in duration from about 15 minutes to 90 minutes, although the more common small events are often 15 to 30 minutes long. For the purposes of the graph, the total amount of time that the average TVOC event lasts was assumed to be 15 minutes. For events over about 0.2 ppm, there are about four of these events or about one hour of total time per week, on average, that the TVOC sensor senses over 0.2 ppm. Based on this data and a lab building with 50 lab rooms, on average, there may be events occurring almost every hour of at least the occupied and potentially some part of the unoccupied schedule somewhere in the building. This provides further evidence for the relatively common need to use higher airflow ashrae.org February 2010

ASHRAE Journal - February 2010

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