ASHRAE Journal - June 2011 - 28

worst-case condition—a relatively lowfrequency event. In this strategy (Figure 3), a local anemometer is connected to the building automation system (BAS) and the minimum required exhaust flow rate is varied based on current wind conditions (direction and speed). When the wind conditions are other than worst-case, the exhaust system may be turned down to more closely match the building demand. Essentially, the air quality minimum setpoint is specified for each wind direction/ speed combination. This usually results in air quality setpoints well below building demand for many wind conditions. This strategy requires physical exhaust dispersion modeling in a wind Figure 2: Passive variable air volume. tunnel. Minimum air quality setpoints as a function of wind direction (WD) and wind speed (WS) require concentration predictions at all sensitive locations (receptors) for all wind directions, wind speeds, stack heights and exhaust flow parameters. Typically, initial testing and discussion is conducted to identify an acceptable stack height. Subsequent testing is conducted for all wind directions and speeds using a fixed stack diameter to produce concentrations for each stack/receptor combination for all combinations of wind direction, wind speed and volume flow rate (Figure 4). Similar data for all receptors is then compiled into either a single lookup table or a series of wind-direction-spe- Figure 3: Active variable air volume. cific polynomial equations for the BAS. Table 1 presents a lookup table of the air quality setpoint as nel exhaust dispersion analysis. If a monitoring system is used, percent of design flow. Note that for most directions, the air the “normal” mode would be to establish a minimum air quality quality setpoint is essentially zero (exhaust flow can match exhaust setpoint that allows higher than normal plume impact. building demand exactly), although a few conditions require Plume impact would still be limited, just to a less conservative 80% of the design flow. criterion than otherwise might be allowed. If contaminants are detected in the exhaust stream, the exhaust flow would be inStrategy 3: Active VAV with Chemical Monitor creased to achieve a more stringent criterion. Figure 5 shows An alternative to monitoring the local wind conditions could 1500 μg/m3 per g/s as an example of the “normal” allowable be to monitor the contents of the exhaust stream. When the impact and 400 μg/m3 per g/s as the criterion when a contamimonitor does not detect any adverse chemicals in the exhaust nant is detected. stream, the exhaust system is allowed to operate at a reduced Data collected at operating research laboratories with air volume flow rate. While there may be an increase in the plume quality monitors in the exhaust manifold indicate that emisconcentrations at the nearby air intakes, air quality will not sion events that would trigger the higher volume flow rate typdegrade since the exhaust plume is essentially “clean.” ically occur no more than one hour per month (12 hours per The usual assumption is that a contaminant is present in the year; 0.1% of the time).12 This means that a typical system is exhaust stream, and the exhaust design is specified to achieve able to operate at the lower air quality setpoint more than 99% acceptable air quality through either mathematical or wind tun- of the time, resulting in significant energy savings.
28 ASHRAE Journal ashrae.org June 2011



ASHRAE Journal - June 2011

Table of Contents for the Digital Edition of ASHRAE Journal - June 2011

Contents
Commentary
Industry News
Meetings and Shows
Letters
Feature Articles
Saving Energy in Lab Exhaust Systems
System Effect: To Believe It Is to See It
What’s New in ASHRAE’s Standard on Comfort
On the Path to Net Zero: How Do We Get There From Here?
Technology Award Case Studies:
Walls to Heat, Cool
Building Sciences
Solar NZEB Project
HVAC Mythbuster
Emerging Technologies
IAQ Applications
Washington Report
Products
People
Classified Advertising
Advertisers Index
ASHRAE Journal - June 2011 - Intro
ASHRAE Journal - June 2011 - Cover1
ASHRAE Journal - June 2011 - Cover2
ASHRAE Journal - June 2011 - 1
ASHRAE Journal - June 2011 - 2
ASHRAE Journal - June 2011 - Contents
ASHRAE Journal - June 2011 - Commentary
ASHRAE Journal - June 2011 - 5
ASHRAE Journal - June 2011 - Industry News
ASHRAE Journal - June 2011 - 7
ASHRAE Journal - June 2011 - 8
ASHRAE Journal - June 2011 - 9
ASHRAE Journal - June 2011 - 10
ASHRAE Journal - June 2011 - 11
ASHRAE Journal - June 2011 - 12
ASHRAE Journal - June 2011 - 13
ASHRAE Journal - June 2011 - Meetings and Shows
ASHRAE Journal - June 2011 - 15
ASHRAE Journal - June 2011 - Letters
ASHRAE Journal - June 2011 - 16a
ASHRAE Journal - June 2011 - 16b
ASHRAE Journal - June 2011 - 17
ASHRAE Journal - June 2011 - 18
ASHRAE Journal - June 2011 - 19
ASHRAE Journal - June 2011 - 20
ASHRAE Journal - June 2011 - 21
ASHRAE Journal - June 2011 - 22
ASHRAE Journal - June 2011 - 23
ASHRAE Journal - June 2011 - 24
ASHRAE Journal - June 2011 - 25
ASHRAE Journal - June 2011 - Saving Energy in Lab Exhaust Systems
ASHRAE Journal - June 2011 - 27
ASHRAE Journal - June 2011 - 28
ASHRAE Journal - June 2011 - 29
ASHRAE Journal - June 2011 - 30
ASHRAE Journal - June 2011 - 31
ASHRAE Journal - June 2011 - 32
ASHRAE Journal - June 2011 - 33
ASHRAE Journal - June 2011 - 34
ASHRAE Journal - June 2011 - 35
ASHRAE Journal - June 2011 - 36
ASHRAE Journal - June 2011 - 37
ASHRAE Journal - June 2011 - System Effect: To Believe It Is to See It
ASHRAE Journal - June 2011 - 39
ASHRAE Journal - June 2011 - 40
ASHRAE Journal - June 2011 - 41
ASHRAE Journal - June 2011 - What’s New in ASHRAE’s Standard on Comfort
ASHRAE Journal - June 2011 - 43
ASHRAE Journal - June 2011 - 44
ASHRAE Journal - June 2011 - 45
ASHRAE Journal - June 2011 - 46
ASHRAE Journal - June 2011 - 47
ASHRAE Journal - June 2011 - 48
ASHRAE Journal - June 2011 - 49
ASHRAE Journal - June 2011 - On the Path to Net Zero: How Do We Get There From Here?
ASHRAE Journal - June 2011 - 51
ASHRAE Journal - June 2011 - 52
ASHRAE Journal - June 2011 - 53
ASHRAE Journal - June 2011 - 54
ASHRAE Journal - June 2011 - 55
ASHRAE Journal - June 2011 - 56
ASHRAE Journal - June 2011 - 57
ASHRAE Journal - June 2011 - 58
ASHRAE Journal - June 2011 - 59
ASHRAE Journal - June 2011 - 60
ASHRAE Journal - June 2011 - 61
ASHRAE Journal - June 2011 - Walls to Heat, Cool
ASHRAE Journal - June 2011 - 63
ASHRAE Journal - June 2011 - 64
ASHRAE Journal - June 2011 - 65
ASHRAE Journal - June 2011 - 66
ASHRAE Journal - June 2011 - 67
ASHRAE Journal - June 2011 - Building Sciences
ASHRAE Journal - June 2011 - 69
ASHRAE Journal - June 2011 - 70
ASHRAE Journal - June 2011 - 71
ASHRAE Journal - June 2011 - Solar NZEB Project
ASHRAE Journal - June 2011 - 73
ASHRAE Journal - June 2011 - 74
ASHRAE Journal - June 2011 - 75
ASHRAE Journal - June 2011 - 76
ASHRAE Journal - June 2011 - 77
ASHRAE Journal - June 2011 - HVAC Mythbuster
ASHRAE Journal - June 2011 - 79
ASHRAE Journal - June 2011 - 80
ASHRAE Journal - June 2011 - 81
ASHRAE Journal - June 2011 - Emerging Technologies
ASHRAE Journal - June 2011 - 83
ASHRAE Journal - June 2011 - 84
ASHRAE Journal - June 2011 - 85
ASHRAE Journal - June 2011 - IAQ Applications
ASHRAE Journal - June 2011 - 87
ASHRAE Journal - June 2011 - 88
ASHRAE Journal - June 2011 - 89
ASHRAE Journal - June 2011 - Washington Report
ASHRAE Journal - June 2011 - Products
ASHRAE Journal - June 2011 - 92
ASHRAE Journal - June 2011 - People
ASHRAE Journal - June 2011 - Classified Advertising
ASHRAE Journal - June 2011 - 95
ASHRAE Journal - June 2011 - Advertisers Index
ASHRAE Journal - June 2011 - Cover3
ASHRAE Journal - June 2011 - Cover4
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