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THE AUTOMATIC SASH POSITIONING SYSTEM
PROVIDES ENERGY AND CAPITAL SAVINGS
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What is the benefit of laboratory fume hood Variable Air Volume (VAV) systems when the
fume hood sashes are not closed? On average, a fume hood only requires the sash
to be open 20 minutes per day. Normally, once a sash has been opened they rarely
ever are shut. As in turning off a light, the inconvenience of closing a sash lends
itself to leaving the fume hood sash open knowing you will be back soon. This habit
then leads to the sash being left open as the norm, not the exception. Other factors
leading to open sashes are the worry over cross contamination or exposures caused by
touching the sashes or the need for using both hands to move chemicals into and out of the
hood.
Just like any store entrance door that automatically opens and closes for you, a fume hood
sash can be easily automated. When a fume hood sash is automated, fume hood sashes are
closed whenever possible providing maximum safety and compliance with NFPA-45*
6.8.3 "Laboratory Hood Sash Closure".
The Cost of Air Airflow exhausted
by fume hoods first needs to be pulled from the outside, filtered, cooled and heated, and finally
supplied to the laboratory. Once all of this energy is used to condition the outside air,
the air is drawn into the fume hood and the contaminated air is exhausted out of the building.
In 2008 the commercial rate for conditioning and exhausting one CFM of outside air is $5.00
per year. This cost varies by climate, utility costs and HVAC equipment efficiency.
To provide year round containment within the laboratory the boilers and chillers are sized
to handle the peak heating and cooling loads. Because of the cooling and dehumidification
of the laboratory air, the chillers are a substantial cost of the laboratory HVAC system.
Usually the peak cooling needs of outside air are about 200 CFM/ton. For a typical
six foot fume hood with a 28 inch sash travel and maintaining 100 FPM capture velocity
(NFPA-45* A.6.4.6), the exhaust and supply requirements are 1230 CFM with the sash
open. From an HVAC system standpoint, this outside air would require 6.2 tons
of cooling when at peak outside air cooling loads.
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With VAV fume hoods the exhaust/supply airflow can be proportionately
reduced as the sash is lowered until a minimum of 25 CFM/sq.ft. of working surface is reached
(NFPA-45* A.6.4.6), a six foot fume hood requires 265 CFM. This means if the
fume hood were shut at peak cooling loads the HVAC system cooling load would be
reduced from 6.2 tons to 1.4 tons.
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FIGURE 1.
Research laboratory with 48-benchtop
fume hoods, surveyed for open fume hood sashes. On average, at least one technician was
assigned to work in one fume hood. No fume hoods were used as storage.
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Shutting the Sash Understandably, not all fume hood
sashes can be shut at all times or at peak cooling loads. With the addition of the Automatic
Sash Positioning System (ASPS™), the fume hood sashes are closed when the fume hood is unattended.
This automatic closure ensures that a minimum number of fume hoods will be open at any time.
To determine this usage, on a random basis over several days, visually survey technicians using the
fume hood to provide a statistical baseline. Figure 1 shows that the average fume
hood usage at this research facility is 6 of 48 hoods open at one time. From an energy usage
standpoint, the average for an eight-hour day is 6 fume hood sashes open and the other 42 fume hood
sashes closed for one-third of a day. For the other two-thirds of a day all 48 fume hoods will
be closed. Using six foot fume hoods, the average airflow per hood is:
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Knowing that the average fume hood airflow is 305 CFM
using the ASPS™ and VAV the average energy savings compared
to a 1230 CFM constant volume fume hood is:
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This energy savings assumes that the minimum
airflow of the fume hood is greater than the minimum air change rate of
the laboratory and no extra cooling airflow is needed for equipment.
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HVAC Equipment Sizing What can be even more important
than yearly energy savings is an HVAC system capacity reduction. Fume hood usage can be calculated
using mathematical probability equations similar to the "Hunter's Curves" used for plumbing
fixtures. HVAC system sizing can be calculated using probability and safety factors to account
for the importance of the HVAC equipment relative to the containment of the fume hood.
When the average of 6 of 48 fume hoods open is applied to the "Probability of Use" a 5%
probability is found, see Table 1, Column 3. Using a 3 times safety factor,
or 15% probability, can be used to find the load for Chillers and Boilers.
Table 1, Column 2 shows
that 13 of 48 fume hoods would be open. As in Figure 1, a peak of 11 of 48 fume hoods was seen
open. Sizing the chiller for 13 fume hoods open produces:
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In comparison if the chiller were sized for all 48 fume hoods at maximum airflow the required
chiller size would be:
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At a capital cost of $ 2,000/ton installed, the 169 ton reduction would
equate to a $338,000 capital cost savings on the chiller system alone. This savings
could pay for the ASPS™ and VAV fume hood systems. The $3,237/hood/year energy savings is additional
the capital cost reduction. This equates to an average 3.5 ton/hood reduction in peak load to the
chiller system would be achieved.
Similarly, applying a safety factor of 5 times to a single ganged exhaust and single supply fan
system will find that these systems can be be sized for 19 of 48 hoods open.
(see Table 1, Column 1)
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In comparison if the exhaust and supply were sized for all 48 fume hoods at maximum
airflow the required fan sizes would be:
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1 Exhaust & Supply |
2 Chiller & Boiler |
3 Average Use |
Using a 5X Safety Factor
These system can be sized
for a 25% Probability |
Using a 3X Safety Factor
These system can be sized
for a 15% Probability |
5% Probability of
Used for Energy Analysis |
| Number Of Hoods |
Hoods Open |
Number Of Hoods |
Hoods Open |
Number Of Hoods |
Hoods Open |
1 |
1 |
1 |
1 |
1
– 3 |
1 |
2 |
2 |
2
– 3 |
2 |
4
– 9 |
2 |
3
– 4 |
3 |
4
– 6 |
3 |
10
– 17 |
3 |
5
– 6 |
4 |
7
– 10 |
4 |
18
– 26 |
4 |
7
– 9 |
5 |
11
– 13 |
5 |
27
– 38 |
5 |
10
– 11 |
6 |
14
– 17 |
6 |
38
– 48 |
6 |
12
– 13 |
7 |
18
– 21 |
7 |
49
– 66 |
7 |
14
– 16 |
8 |
22
– 25 |
8 |
67
– 81 |
8 |
17
– 19 |
9 |
26
– 30 |
9 |
82
– 97 |
9 |
20
– 22 |
10 |
31
– 34 |
10 |
98
– 112 |
10 |
23
– 24 |
11 |
35
– 39 |
11 |
113
– 128 |
11 |
25
– 27 |
12 |
40
– 43 |
12 |
129
– 143 |
12 |
28
– 30 |
13 |
44
– 48 |
13 |
144
– 159 |
13 |
31
– 33 |
14 |
49
– 55 |
14 |
160
– 175 |
14 |
34
– 36 |
15 |
56
– 60 |
15 |
176
– 191 |
15 |
37
– 39 |
16 |
61
– 65 |
16 |
192
– 207 |
16 |
40
– 42 |
17 |
66
– 70 |
17 |
208
– 223 |
17 |
43
– 45 |
18 |
71
– 75 |
18 |
224
– 239 |
18 |
46
– 48 |
19 |
76
– 80 |
19 |
240
– 255 |
19 |
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TABLE 1.
Probability of Use for Fume Hood Systems using the ASPS™
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Improving Safety while Saving Energy
A laboratory is not
built to save energy, it is built to provide maximum safety for the people using the facility. The
ASPS™ not only provides maximum safety but when combined with any VAV system provides maximum energy
savings and minimum HVAC system costs.
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