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JOURNAL ASHRAE Journal’s
The following article was published in ASHRAE Journal, April 2000. © Copyright 2000 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.
Damper Sizing Using Damper Authority By Evans Lizardo L izardos, s, P.E. P.E. Fellow ASHRAE and and
Kenneth M. Elovitz, P.E. Member ASHRAE
A
utomatic dampers damp ers control airflow airf low in HVAC HVAC systems. Applications include controlling mixed air temperature in economizer cycles and airflow to the space in variable air volume systems. Selecting and sizing dampers to provide linear control action aids proper system operation. With linear control, a given change in damper position produces a proportional change in air quantity. quantity. If control is not linear, a given change in control signa l might produce a consistent change in damper position but a different change in air quantity. The result is unstable or inaccurate control. The resistance of the wide open damper can be expressed as a fraction of the total system resistance. That fraction is called “Damper Authority” or “Characteristic Ratio.” Damper Authority (%) =
Open Damper Resistance Total System Resistance
×
100%
The resistance is a pressure drop, so Damper Authority (%) =
Open Damper Pressure Drop Total System Pressure Drop
×
100%
air ductwork. Therefore, the outside air damper authority is determined by the pressure drop across the outside air damper as a percentage of the pressure difference between outside air and Point A, the mixing box plenum. The outside air damper only controls the amount of flow. It does not control the direction of flow. For outside air to enter the system, other features of the control system must make the pressure pre ssure in the mixing box (Point A) less than atmospheric. Similarly, the total system pressure drop for selecting the exhaust/relief damper is the pressure drop between Point B and outdoors. For determining the damper authority, authority, the total system pressure drop includes the pressure drop across the relief duct and louver. It does not include the pressure drop across the exhaust/relief damper. damper. The total system pressure drop for selecting the return damper may be less obvious. It is the pressure drop from Point B (the discharge of the return fan) to Point A (the mixing box plenum). The return air damper does not control the flow through the return fan. It simply proportions the flow between the relief duct and the return air duct. The pressure at Point B must always be greater than atmospheric or air will not leave the system. For a VA VAV box, the total tota l system pressure press ure drop is the pressure pressu re from the primary duct to the space. A separate control loop (typically variable speed drive, variable inlet vanes, or fan discharge damper) controls fan capacity to maintain relatively constant pressure in the primary air duct. The VAV box damper does not appreciably affect the flow through the main duct. It only affects the portion of the main duct flow that passes through the th e VA VAV box. Therefore, the th e total system syste m pressure drop d rop for a VA VAV box damper is from fr om the primary air a ir duct to the space. s pace.
Note: Total system resistance and total system pressure dro p are for the system without the without the wide open damper. It is important to realize that “Total “Total System Resistance” or “Total System Pressure Drop” relates only to the part of the system where the damper controls the flow. It is not the entire system pressure drop or the fan total static pressure. The total Damper Types system pressure drop for selecting dampers is usually the pres Figure 2 shows the two damper arrangements used in our sure drop from a constant pressure point in the system to the industry. destination for the air. Figure 1 shows a traditional airside economizer econo mizer cycle with a About the Authors Evans J. Lizardos is founder and president of Lizardos Engineering Associates, return fan. The total system pressure drop for the outside air damper is damper is Mineola, Long Island, N.Y. He is a past Handbook chairman, and serves on the pressure difference between outside air and Point A, the TC.10.9, Refrigeration Applications for Foods and Beverages. Ken Elovitz is an mixing box plenum. The outside air damper only controls the engineer and in house counsel for Energy Economics, Foxboro, Mass. He is flow through the weather louver, the damper, and the outside ASHRAE Journal’s Practical Guide editor.
April 2000
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1. Parallel blade dampers: all blades move in the same direction and in parallel. 2. Opposed blade dampers: adjacent blades move in opposite directions. Figure 3 is a set of damper authority curves for parallel blade dampers. It shows the best selection to obtain linear proportional control with a parallel blade damper is a damper authority of 30% to 50%. Figure 4 is a set of damper authority curves for opposed blade dampers. It shows that the best selection to obtain linear proportional control with an op posed blade damper is a damper authority of 10% to 15%. Table 1 is a selection guideline to determine the damper velocity for a given damper authority. It is based on a wide open damper flow coefficient of 0.5.
Applications The following examples illustrate damper selections using the concept of damper authority. The technique may be used to select either parallel or opposed blade dampers for linear control. Figure 5 shows a typical economizer cycle damper arrangement with recommended spacing between the damper and nearby fittings. Table 2 shows the pressure drop through two types of fittings at various duct velocities. Fitting FT-1 is a clinch type fitting with 90° side entry airflow. Fitting FT-2 is a clinch type fitting with straight through airflow. Figure 5 and Table 2 show how ducts and fittings can be sized to select dampers by damper authority.
Example 1: Outdoor Air and Exhaust/Relief Damper Design A typical pressure drop to bring outdoor air through the intake louver and subsequent ductwork to the mixing box is 0.3 in. w.g. (75 Pa). Therefore, the pressure in the mixing box must be –0.3 in. w.g. (75 Pa) relative to outdoors. A pressure drop of 0.3 in. w.g. (75 Pa) also is typical for the exhaust air path from the return fan discharge to the exhaust air louver outlet. Therefore, the pressure at the discharge of the return fan must be +0.3 in. w.g. (75 Pa). Using the legend in Figure 5 with data from Table 1, the following recommended damper authorities are computed for the outdoor and exhaust/relief air dampers: 38
ASHRAE Journal
W i d e Op e n Da m p e r Ch a r a ct e r i s t i cs
A p p r o a ch Ve l o ci t y f p m
Ve l o ci t y P r e s s u r e i n . w.g .
P r e s s u r e Dr o p i n . w.g .
To t a l Sy s t e m P r e s s u r e Dr o p V e r s u s Da m p e r A u t h or i t y
Op p o s e d B l a d e
P a r a l l e l B l a d e
1 0%
1 5%
3 0%
5 0%
300
0.01
0.005
0.05
0.03
0.02
0.01
600
0.02
0.010
0.10
0.07
0.03
0.02
700
0.03
0.015
0.15
0.10
0.05
0.03
800
0.04
0.020
0.20
0.13
0.07
0.04
900
0.05
0.025
0.25
0.17
0.08
0.05
1000
0.06
0.030
0.30
0.20
0.10
0.06
1100
0.08
0.040
0.40
0.27
0.13
0.08
1200
0.09
0.045
0.45
0.30
0.15
0.09
1300
0.11
0.055
0.55
0.37
0.18
0.11
1400
0.12
0.060
0.60
0.40
0.20
0.12
1500
0.14
0.070
0.70
0.47
0.23
0.14
1600
0.16
0.080
0.80
0.53
0.27
0.16
1700
0.18
0.090
0.90
0.60
0.30
0.18
1800
0.20
0.100
1.00
0.67
0.33
0.20
1900
0.22
0.110
1.10
0.73
0.37
0.22
2000
0.25
0.125
1.25
0.83
0.42
0.25
2200
0.30
0.150
1.50
1.00
0.50
0.30
2400
0.36
0.180
1.80
1.20
0.60
0.36
2600
0.42
0.210
2.10
1.40
0.70
0.42
2800
0.49
0.245
2.45
1.63
0.82
0.49
3000
0.56
0.280
2.80
1.87
0.93
0.56
3200
0.64
0.320
3.20
2.13
1.07
0.64
3400
0.72
0.360
3.60
2.40
1.20
0.72
3600
0.81
0.405
4.05
2.70
1.35
0.81
3800
0.90
0.450
4.50
3.00
1.50
0.90
4000
1.00
0.500
5.00
3.33
1.67
1.00
* T h e p r e s s u r e d r o p a c r o s s a w i d e o p e n o p p o s e d o r p a r a l l e l b l a d e d a m p e r i s t h e v e l o c i t y p r e s s u r e t i m e s a f l o w c o e f f i c i e n t f a c t o r o f “ 0 .5 .”
Table 1: Opposed blade and parallel blade damper selection. April 2000
Dampers V
Figure 1: Airside economizer cycle with return fan.
V
V P VP
P
FT- 1C
FT- 1
FT- 2C
FT- 2
1 00 0
0.06
1.00
0.06
1.0
0.06
1 10 0
0.08
1.10
0.09
1.0
0.08
1 20 0
0.09
1.20
0.11
1.0
0.09
1 30 0
0.11
1.30
0.14
1.0
0.11
1 40 0
0.12
1.40
0.17
1.0
0.12
1 50 0
0.14
1.50
0.21
1.0
0.14
1 60 0
0.16
1.60
0.26
1.0
0.16
1 70 0
0.18
1.70
0.31
1.0
0.18
1 80 0
0.20
1.80
0.36
1.0
0.20
1 90 0
0.22
1.90
0.42
1.0
0.22
2 00 0
0.25
2.00
0.50
1.0
0.25
2 20 0
0.30
2.00
0.60
1.0
0.30
2 40 0
0.36
2.00
0.72
1.0
0.36
2 60 0
0.42
2.00
0.84
1.0
0.42
2 80 0
0.49
2.00
0.98
1.0
0.49
2 90 0
0.52
2.00
1.04
1.0
0.52
3000
0.56
2.00
1.12
1.0
0.56
Table 2: Duct fitting pressure drop table.
Figure 2: Typical multiblade dampers. DP R - V V
DP R
T SD
DA D A DA
DP R Ty p e
P r e s s u r e i n M i x i n g B o x
1 ,0 0 0
0.03
0.30
10%
Opposed
–0.33
1 ,2 5 0
0.05
0.30
15%
Opposed
–0.35
1 ,7 0 0
0.09
0.30
30%
Parallel
–0.39
2 ,2 0 0
0.15
0.30
50%
Parallel
–0.45
From the table, the outdoor air and exhaust air dampers should be sized at a face velocity of 1,000 to 1,200 fpm (5 to 6 m/s) if they are opposed blade dampers. They should be sized for 1,800 to 2,200 fpm (9 to 11 m/s) if they are parallel blade dampers. Based on this sizing criterion, outdoor air and exhaust air dampers should not be mounted directly to their respective louvers, which usually have face velocities of 300 to 500 fpm (1.5 to 2.5 m/s) to avoid sucking in rain and snow.
than 0.5 in. w.g. (125 Pa), as follows: Return fan discharge static pressure available to overcome exhaust louver, bird screen and duct losses: +0.3 in. w.g./75 Pa. Air-handling system suction pressure available to overcome outdoor air louver, bird screen and duct loss to the air-handling system mixing box: –0.3 in. w.g./75 Pa. Available pressure from return air fan discharge to the air-handling system mixing box is 0.6 in. w.g./150 Pa (absolute). In most applications, the return air duct pressure drop from the return fan discharge to the mixing box is in the order of 0.075 to 0.20 in. w.g. (MPD in the legend to Figure 5). The “Duct Fitting Pressure Drop Guideline” (Table 2) is used to size ducts and fittings to absorb the excess available pressure to achieve damper authority of 10 to 15% for opposed blade dampers or 30 to 50% for parallel blade dampers. DP R - V V
DP R
FT- 1
FT- 2
MPD
TSD
DA D A DA
DP R Ty p e
1 ,4 0 0
0.06
0.34
0.06
0.14
0.54
10%
Opposed
1 ,6 0 0
0.08
0.32
0.06
0.18
0.52
15%
Opposed
Example 2: Return Air Damper Design
2 ,1 0 0
0.14
0.22
0.06
0.18
0.46
30%
Parallel
In most cases, the return air damper and associated ductwork pressure drop from the return fan discharge to the air-handling unit mixing box must dissipate a differential pressure of more
2 ,5 0 0
0.20
0.10
0.06
0.10
0.40
50%
Parallel
April 2000
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Figure 3: Damper authority curves of installed parallel blade dampers. Advertisement in the print edition formerly in this space. Example: Damper Pressure Drop (DPR) + TSD must equal 0.60 in. w.g. (150 Pa) for 30% authority, DPR = 0.30 × TSD DPR + TSD = 0.3 × TSD = TSD =0.60 1.3 TSD = 0.60; TSD = 0.46 From the table, the return air damper should be sized at face velocity of 1,400 to 1,600 fpm (7 to 8 m/s) if it is an opposed blade damper or 2,200 to 2,400 fpm (11 to 12 m/s) if it is a parallel blade damper. Whichever damper is selected, duct and fitting pressure drop of 0.40 to 0.55 in. w.g. (100 to 135 Pa) must be included to achieve damper authorities of 10% to 15% for opposed blade dampers and 30% to 50% for parallel blade dampers.
Example 3: Outdoor Air Damper Design in an Exhaust/Relief Fan Economizer Cycle In a relief/exhaust air fan economizer cycle, the pressure in the mixing box reflects the pressure drop for return air to flow from the space, through the return duct, and into the mixing box. The pressure in the mixing box is typically on the order of –0.6 in. w.g. (150 Pa). If the pressure drop through the return duct is much higher, the system requires a return fan to overcome that resistance and avoid excessive negative pressures in the return duct and mixing box. The pressure in the mixing box must be the same regardless of which path air takes. Accordingly, if the pressure in the mixing box must be –0.6 in. w.g. (150 Pa) on account of the return air path, the pressure drop through the outside air path must likewise be –0.6 in. w.g. (150 Pa). The outdoor air damper introduces the resistance required to bring the outside air path pressure drop to 0.6 in. w.g. (150 Pa). The outside air louver, bird screen, and intake plenum duct box typically have a pressure drop of about 0.3 in. w.g. (75 Pa). The duct and fitting that connect the intake plenum to the outside air duct (a straightthrough, “FT-2” style fitting) must be sized to achieve the desired damper authority. 40
ASHRAE Journal
April 2000
Dampers
Figure 4: Damper authority curves of installed opposed blade dampers.
DP R - V V
DP R
FT- 1
FT- 2
MP D
1 , 40 0
0.06
0
0.24
0.34
1 , 60 0
0.08
0
0.22
0.22
2 , 10 0
0.14
0
0.16
0.16
2 , 50 0
0.20
0
0.10
0.10
T SD 0.54 0.52 0.46 0.40
DA D A DA
DP R Ty p e
10%
Opposed
15%
Opposed
30%
Parallel
50%
Parallel
From the table above, the outdoor air damper sho uld be sized at face velocity of 1,400 to 1,600 fpm (7 to 8 m/s) when using opposed blade dampers. It should be sized for 2,200 to 2,400 fpm (11 to 12 m/s) when using parallel blade dampers. Whichever damper is selected, a duct and fitting pressure drop of 0.40 to 0.55 in. w.g. (100 to 135 Pa) must be added to achieve damper authorities of 10% to 15% for opposed blade dampers and 30% to 50% for parallel blade dampers.
Example 4: Return Air Damper Design in an Exhaust/Relief Fan Economizer Cycle The total system pressure drop for the return air damper is the pressure drop from the space to the mixing box. It is in the order of 0.6 in. w.g. (150 Pa). DP R - V V
DP R
T SD
DA D A DA
DP R Ty p e
1, 4 0 0
0.06
0.54
10%
Opposed
1, 6 0 0
0.08
0.52
15%
Opposed
2, 1 0 0
0.14
0.46
30%
Parallel
2, 5 0 0
0.20
0.40
50%
Parallel
April 2000
Figure 5: Economizer cycle damper arrangement. From the previous table, the return air damper should be sized at a face velocity of 1,400 to 1,600 fpm (7 to 8 m/s) when using opposed blade dampers and 2,100 to 2,500 fpm (11 to 12 m/s) when using parallel blade dampers.
Example 5: Exhaust/Relief Air Damper Design in an Exhaust/Relief Fan Economizer Cycle The exhaust air damper controls a total system pressure drop of about 0.80 in. w.g. (200 Pa). This pressure drop is the difference between the pressure required to push air out the exhaust duct and louver (0.30 in. w.g./75 Pa) and the pressure in the return duct at the inlet to the exhaust/relief fan (–0.50 in. w.g./ 125 Pa). DP R - V V
DP R
T SD
DA D A DA
DP R Ty p e
1 ,5 0 0
0.07
0.73
10%
Opposed
1 ,8 0 0
0.10
0.70
15%
Opposed
2 ,4 0 0
0.18
0.62
30%
Parallel
2 ,9 0 0
0.27
0.53
50%
Parallel
From the above, the exhaust air damper should be sized at a face velocity of 1,500 to 1,800 fpm (8 to 9 m/s) when using ASHRAE Journal
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opposed blade dampers and 2,400 to 2,900 fpm (12 to 14 m/s) when using parallel blade dampers.
Example 6: Variable Air Volume Damper Design Assuming a butterfly damper has the same damper authority characteristics as a parallel blade damper, the following table demonstrates that a manufacturer’s variable air volume terminal is designed to have a damper authority of 30%. Total system pressure drop (TSD) is assumed to be 0.70 in. w.g. consisting of 0.45 in. w.g. for the takeoff from the main duct plus 0.25 in. w.g. for the pressure drop through the low veloc ity duct and outlet(s). I n l e t Di a m e t e r Si z e ( i n i n ch e s )
CFM
FP M
V P VP
DP R
TS D
DA D A A
4
225
2,600
0.42
0.21
0.70
30%
6
500
2,600
0.42
0.21
0.70
30%
8
900
2,600
0.42
0.21
0.70
30%
10 10
1,400
2,600
0.42
0.21
0.70
30%
12 12
2,000
2,600
0.42
0.21
0.70
30%
14 14
3,000
2,600
0.42
0.21
0.70
30%
16 16
4,000
2,600
0.42
0.21
0.70
30%
Since the VAV box manufacturer has established the damper authority, the ductwork entering the VAV box must be sized so the damper has the desired authority (30%) in the system as installed. The next table shows the duct entering the VAV box in this example should be sized for 3,100 fpm (15.7 m/s) to achieve 30% authority with the parallel blade damper in the VAV box. DP R - V V
DP R
V AT- P D
M SD
T SD
DA D A DA
DP R Ty p e
1800
0.10
0.25
0.75
0.90
10%
Opposed
2200
0.13
0.25
0.75
0.87
15%
Opposed
2 70 0
0.23
0.25
0.75
0.77
30%
Parallel
4000
0.33
0.25
0.75
0.67
50%
Parallel
Example 7: Face and Bypass Damper Design The tables on the right show that the ideal selections for linear damper control are an opposed blade damper for the coil section and a parallel blade damper for the bypass section. The opposed blade damper at the coil should be sized at a velocity of 700 fpm (3.6 m/s), which matches the probable heating coil velocity. The parallel blade damper in the by-pass section should be sized for a velocity of 1,800 to 2,350 fpm (9 to 12 m/s). This sizing will reduce the height of the by-pass section.
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ASHRAE Journal
Figure 6: The return air fan economizer cycle (Examples 1 & 2).
Figure 7: The relief/exhaust air fan economizer cycle (Examples 3, 4 & 5). Fa ce Con t r ol Da m p e r Se l e ct i on DP R - V V
DP R - H T G
T SD - H TG
DA D A A
DP R Ty p e
70 0
0.03
0.33
10%
Opposed
1 ,2 5 0
0.05
0.33
15%
Opposed
1 ,8 0 0
0.10
0.33
30%
Parallel
2 ,3 5 0
0.17
0.33
50%
Parallel
B y - p a s s Con t r ol Da m p e r Se l e ct i on DP R - V V
DP R - B YP
TSD - H TG
DA D A A
DP R Ty p e
1,800
0.10
0.33
30%
Parallel
2,350
0.17
0.33
50%
Parallel
April 2000
Figure 8: Variable air volume damper control (Example 6).
Advertisement in the print edition formerly in this space.
Figure 9: Face and bypass damper control (Example 7).
Conclusion This article introduces the concept of damper authority and provides examples of how the concept can be used to design for linear damper control. The typical pressure drops used in the examples are just that—typical. The techniques presented here can be applied to actual jobs by calculating actual pressure drop for the actual system and selecting velocities accordingly. Since damper authority is defined as the ratio of the damper pressure drop to the system pressure drop without the damper, the pressure drop through the wide open damper must be added to the “system pressure drop” when sizing the fan.
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