Single Zone VAV

Defining Quality. Building Comfort.
Single Zone VAV
Discover how to save money, reduce energy
consumption and lower sound levels.
What is single zone VAV?
S
ingle zone VAV, or single zone variable air volume, is an HVAC application in which the HVAC unit varies
the airflow at constant temperature to provide space temperature control. A constant volume HVAC unit
supplies constant airflow with variable temperature to provide temperature control. In the cooling mode,
to meet ventilation requirements, the fan operates continuously and the compressor cycles on and off to
meet the space cooling load. The fan and compressor operate at full capacity until the temperature drops
to a set lower limit below the setpoint; then the compressor turns off. The compressor turns on again at full
capacity once the space temperature increases to a set upper limit above the setpoint. The on/off nature of
the constant volume unit causes the temperature to constantly fluctuate above and below the room setpoint
temperature.
In a single zone VAV unit, a variable speed fan controls the amount of airflow provided to the space by
modulating the fan motor speed based on the difference between the actual space temperature and the
temperature setpoint. The modulating compressor uses the temperature of the supply air leaving the unit to
determine how much refrigerant flow is needed to maintain the supply air temperature setpoint.
Packaged Single Zone VAV System
Outdoor
Air Intake
Filters
Return Air
Damper
Variable
Frequency Drive
Cooling
Coil
Supply
Air Sensor
Variable
Capacity
Compressor
Exhaust
Air
Zone
Return
Air
Supply
Air
Thermostat
The fan and compressor continue to modulate to precisely meet the desired space temperature. For part load
conditions, the single zone VAV unit will operate at a lower fan speed for a greater amount of time, saving
valuable energy and providing the space with more constant temperature and humidity control. HVAC systems
generally operate at part load conditions for a majority of the year, and during these part load conditions the
operation of a single zone VAV system provides many benefits. First, a single zone VAV system will operate
at a lower fan speed than a constant volume system, resulting in less fan energy consumption. Second,
with the modulation capabilities of both the fan and compressor a single zone VAV system can provide
precise temperature control and additional passive dehumidification. Third, the modulation capabilities of the
compressor reduce the amount of compressor on/off cycling, reducing wear on the compressor and providing
greater energy savings than hot gas bypass systems. Fourth, lower fan speeds reduce the amount of sound
produced by the supply fan. Finally, with the entire modulating control for part load operation provided within
the HVAC unit, a single zone VAV system is simple to install, set up and maintain.
2
Single zone VAV is used for areas where the occupancy or space cooling needs vary throughout the day
such as classrooms, conference rooms, assembly halls, auditoriums, libraries, hospitals, supermarkets,
convenience stores, restaurants, churches, health clubs, museums, office buildings, manufacturing facilities,
lodgings, retail buildings, warehouses, etc.
AAON is leading the industry in single zone VAV technology with both variable speed fans and variable
capacity compressors which have a wide range of modulation capabilities that adapt to full and part
load conditions. Benefits of single zone VAV include a more comfortable indoor environment with precise
space temperature control and improved humidity control, significant energy savings, less wear on the
compressor, a reduction in fan noise and simple installation and maintenance.
Single Zone VAV - Required by Energy Standards
S
ingle zone variable air volume is required in two of the most prominent national energy standards, ANSI/
ASHRAE/IES Standard 90.1-2010 and ANSI/ASHRAE/IES Standard 189.1-2009. Following are excerpts
from the ASHRAE standards showing the requirement for Single Zone Variable-Air-Volume applications.
Standard 90.1-2010
Purpose: The purpose of this standard is to provide minimum requirements for the energyefficient design of buildings except low-rise residential buildings.
ANSI/ASHRAE/IES Standard 90.1-2010.
Section 6.4.3.10 Single Zone Variable-Air-Volume Controls.
HVAC systems shall have variable airflow controls as follows:
a. Air handling and fan-coil units with chilled-water cooling coils and supply fans with motors greater than
or equal to 5 hp shall have their supply fans controlled by two-speed motors or variable-speed drives.
At cooling demands less than or equal to 50%, the supply fan controls shall be able to reduce the airflow
to no greater than the larger of the following:
1. One half of the full fans speed, or
2. The volume of outdoor air required to meet the ventilation requirements of Standard 62.1.
b. Effective January 1, 2012, all air-conditioning equipment and air-handling units with direct expansion
cooling and a cooling capacity at AHRI conditions greater than or equal to 110,000 Btu/h that serve
single zones shall have their supply fans controlled by two-speed motors or variable-speed drives. At
cooling demands less than or equal to 50%, the supply fan controls shall be able to reduce the airflow
to no greater than the larger of the following:
1. Two-thirds of the full fan speed, or
2. The volume of outdoor air required to meet the ventilation requirements of Standard 62.1.
Standard 189.1-2009
Purpose: The purpose of this standard is to provide minimum requirements for the siting, design,
construction, and plan for operation of high performance, green buildings to:
a. Balance environmental responsibility, resource efficiency, occupant comfort and well being, and
community sensitivity, and
b. Support the goal of development that meets the needs of the present without compromising the ability
of future generations to meet their own needs.
3
ANSI/ASHRAE/IES Standard 189.1-2009.
Section 7.4.3.7 Controls.
The following requirements shall apply:
a. DX systems with a capacity greater than 65,000 Btu/h (19 kW) shall have a minimum of two stages of
cooling capacity.
b. Air handling and fan-coil units with chilled-water cooling coils and supply fans with motors greater than
or equal to 5 hp shall have their supply fans controlled by two-speed motors or variable-speed drives.
At cooling demands less than or equal to 50%, the supply fan controls shall be able to reduce the airflow
to no greater than the larger of the following:
1. One half of the full fans speed, or
2. The volume of outdoor air required to meet the ventilation requirements of ANSI/AHSRAE Standard
62.1.
c. All air-conditioning equipment and air-handling units with direct expansion cooling and a cooling capacity
at AHRI conditions greater than or equal to 110,000 Btu/h (32.2 kW) that serve single zones shall have
their supply fans controlled by two-speed motors or variable speed drives. At cooling demands less
than or equal to 50%, the supply fan controls shall be able to reduce the airflow to no greater than the
larger of the following:
1. Two-thirds of the full fan speed, or
2. The volume of outdoor air required to meet the ventilation requirements of ANSI/ASHRAE Standard
62.1.
d. All DX and chilled-water VAV units shall be equipped with variable-speed fans that result in less than
30% power at 50% flow.
Energy Savings
O
ne of the main advantages of varying the fan speed is energy savings. It takes less energy to run a
fan at lower rotational speeds. The fan law that relates fan input horsepower to fan rotational speed is:
5
rpm 2 3 t2
m :
HP2 = HP1 : c D 2 m : c
D1
rpm 1
t1
where D is the fan diameter, 𝜌 is the air density, HP is the input horsepower, and rpm is the fan rotational
speed. This law says that, assuming the diameter and air density do not change, the fan power input is
proportional to the cube of the fan rotational speed:
1
1
5
rpm 2
rpm 2 3
t
m : 2 & HP2 = HP1 : c
m
HP2 = HP1 : c D 2 m : c
t1
D1
rpm 1
rpm 1
3
4
torque
airflow
For the same fan diameter and air
conditions, cutting the fan rotational
speed in half cuts the required
input horsepower by eight! This law
is illustrated graphically in Figure
1, where brake horsepower is the
amount of input power needed for the
given fan rotational speed.
brake
horsepower
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
A single zone VAV unit also saves
energy due to reduced cycling losses
fan
speed
(rpm)
in the compressor. When power is
first applied to a motor, the current is
Figure 1: Brake Horsepower, Torque, and Airflow as a Percentage of Full
Capacity versus Fan Speed
significantly higher for a short period
of time until the motor reaches its
normal operating current. This initial
inrush current can be as much as twenty times the normal operating current. Fans and compressors
consume significantly more power when the motor is first turned on than at steady state operation. This
initial inrush does no useful work but is required to take the motor from a stopped state to a state of motion.
In addition to the inrush current, the compressor also loses energy during on/off cycling due to the work
the compressor does to initiate the flow of refrigerant through the compressor. As the compressor is turned
on, a pressure difference is created from the suction to the discharge of the compressor to start the flow
of refrigerant. It takes more work to initially create this pressure difference and start the refrigerant flow
than it does to maintain the refrigerant flow during reduced load. For constant volume systems in which the
compressor is cycled on and off more often, more energy is consumed during frequent startups.
0%
300
600
900
1200
1500
1800
2100
Inrush Current
Amps
0
Ta
Normal Operating Current
Time
Switch
Closes
Figure 2: Example of Motor Inrush Current
5
Sample Savings by ASHRAE Zone
How much energy does a single zone VAV unit save? The monthly fan and direct expansion cooling energy
usage for a constant volume unit and a single zone VAV unit are shown in Figures 4 through 10 for various
ASHRAE climate zones. Both units are 25 tons in capacity and are evaluated for a single zone of 10,000
square feet that is occupied Monday thru Friday from 7 am to 7 pm and Saturdays from 8 am to noon with
100 people doing light work. The occupied heating and cooling setpoints are 68°F and 74°F, respectively.
The unoccupied heating and cooling setpoints are 55°F and 90°F, respectively. The single zone VAV unit is
controlled with a fixed 54 degree coil temperature setpoint. The constant volume unit is controlled with zone
control reset with a range of 54 to 58 degrees.
All of Alaska is in Zone 7 except for
the following boroughs which are in
Zone 8:
Bethel
Dellingham
Fairbanks N. Star
Nome
North Slope
Zone 1 includes
Hawaii, Guam,
Puerto Rico,
and the Virgin Islands
Northwest Artic
Southeast Fairbanks
Wade Hampton
Yukon-Koyukuk
Figure 3: ASHRAE Zone Map
Figure 4: Sample Fan and DX Cooling Energy Usage for ASHRAE Zone 1
6
Figure 5: Sample Fan and DX Cooling Energy Usage for ASHRAE Zone 2
Figure 6: Sample Fan and DX Cooling Energy Usage for ASHRAE Zone 3
Figure 7: Sample Fan and DX Cooling Energy Usage for ASHRAE Zone 4
7
Figure 8: Sample fan and DX cooling energy usage for ASHRAE Zone 5
Figure 9: Sample Fan and DX Cooling Energy Usage for ASHRAE Zone 6
Figure 10: Sample fan and DX cooling energy usage for ASHRAE Zone 7
8
The constant volume unit contains a standard, single speed fan and two standard fixed capacity scroll
compressors. The single zone VAV unit contains a variable speed fan and two variable capacity scroll
compressors.
The energy usage of the constant volume unit is significantly more than the energy usage of a single zone
VAV unit. The energy costs are greater for the constant volume system due to both increased usage charges
and increased demand charges. Utility companies bill commercial energy on the basis of both usage and
demand. The usage charge is simply the total energy used multiplied by the usage rate. The demand charge
takes the greatest peak load during a given time period and multiplies the peak load by the demand usage
rate. The single zone VAV unit only runs at full energy load when the space conditions require it. The single
zone VAV unit saves a greater amount in energy costs than a comparable constant volume system because
of the ability to adapt to the space needs.
Noise Reduction
O
ne of the main sources of sound in an HVAC unit is the fan operation. The faster the fan rotates, the
more sound the system produces. The relationship between fan speed and A-weighted sound power
level for an example AAON unit with a backward curved plenum fan is shown in Figure 11.
Because a single zone VAV unit varies the fan speed as needed by space conditions, the fan will be running
at lower speeds than a constant volume unit unless the space conditions require the fan to operate at full
capacity. The difference in sound power level as the fan speed increases from 1,000 rpm to 2,100 rpm, a
little over twice the fan speed, increases the discharge sound power level by 18.5 dBA and the return sound
power level by 17 dBA. The sound increase caused by the increase in fan speed is perceived by the human
ear as an increase in loudness of about four times the sound of the original, lower speed fan. This means
that by simply reducing the fan speed by half, the unit is almost four times quieter!
Fan
Speed
vs
Sound
Power
Level
Sound
Power
Level
(dB
A)
90
85
80
75
discharge
70
return
65
60
1000
1200
1400
1600
1800
2000
Fan
Speed
(rpm)
Figure 11: A-weighted Sound Power Level versus Fan Speed
9
Passive Dehumidification
A
Airflow
s warm air passes over a cold cooling coil, the warm
air transfers heat to the coil resulting in colder air as
shown in Figure 12.
79°F db
56°F db
The amount of energy that is removed from the air as it
passes over the cooling coil is given by the following
equation:
66°F wb
55°F wb
.
Q = m : Dh
Figure 12: Heat Transfer Through
where Q is the capacity of the coil, m is the airflow across
Cooling Coil
the coil, and Δh is the change in enthalpy from before
the cooling coil to after the cooling coil. The change in the
enthalpy of the air as it passes over the cooling coil is equal to the total change in internal energy (or the
total amount of heat gained or lost). As sensible cooling occurs and without moisture removal from the air,
heat is removed from the air resulting in a reduction in the supply air temperature. As latent cooling occurs
and moisture in the air is removed, condensation appears on the cooling coil and energy is removed from
the air without a reduction in temperature. The total energy lost by the air as it cools is equal to the sensible
heat removal plus the latent heat removal.
Sensible Cooling
Latent Cooling
6. 44 7 44 8 6. 44 7 44 8
Q = m : c p : D T + m : h fg : D W
where Q is the capacity of the coil, m is the airflow across the coil, cp is the specific heat capacity of the air,
ΔT is the temperature change of the air as it passes over the coil, hfg is the enthalpy of vaporization, and
ΔW is the change in the humidity ratio as the air passes over the coil.
If the airflow is reduced, and in order to maintain the same amount of cooling, Δh must increase. For two
different units, one constant volume and one single zone VAV, with the same return and outside air conditions,
the reduction in airflow of the single zone VAV unit compared to the constant volume unit means that the
air is exposed to the cooling coil for a longer amount of time, resulting in the single zone VAV unit having a
lower supply air temperature than a constant volume system under the same operating conditions. Cooler
air holds less moisture than warmer air. When the cooling coils are colder than the dew point temperature of
the entering air, dehumidification occurs as can be seen in the psychrometric chart in Figure 13.
0.20
55
0.25
0.30
SENSIBLE HEAT RATIO = Qs / Qt
60
95
90
0.35
200
50
15.0
180
45
160
PE
RA
TU
RE
150
- °F
140
75
130
25
%
P
TE
M
N
UR
AT
IO
AT
IN
T,
S
PO
EW
%
ET
90
BU
LB
,D
AL
PY
80
%
W
55
%
60
O
%
%
40
AT
8%
13.0
4%
20%
IDITY
TIVE HUM
10% RELA
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
110
.020
.019
120
125
.017
.016
.015
100
.014
.013
90
.012
80
.011
70
.010
.009
60
0.50
0.45
0.60
0.65
0.70
0.80
0.90
0.30
50
0.25
0.95
1.00
.005
.004
.003
20
0.35
55
0.85
.006
30
0.40
0.75
.007
40
0.830
60
0.55
.008
50
0.20
45
0.10
0.810
0.790
0.770
0.750
0.730
0.710
0.15
40
0.690
2600
0.05
.002
10
2%
10
15
20
25
Figure 13: Cooling Coil Psychrometric Example
10
.021
2800
3000
3200
3400
3600
3800
4000
.001
130
.000
35
Airflow
(cfm)
DRY BULB TEMPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
5
REL
6%
30%
25
H
IVE
Y
IDIT
UM
R
Y AI
. DR
50
45
40
35
%
15
R LB
50
15
20
MA
RA
60
T. PE
SA
40
35
30
65
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
- °F
ten
t
La
AIR
Δh
DR
Y
F
O
le
UN
D
ns
ib
PO
Se
R
Δh
PE
-B
TU
65
CU.F
50
25
OA
70
MEVOLU
EN
TH
60
CC
20
70
%
70
45
20
120
75
14.0
20
15
32
.022
.018
55
30
.023
Sensible
Heat
Ratio
EM
0.850
0.55
VAPOR PRESSURE - PSIA
BT
0.50
.024
ENTHALPY - BTU PER POUND OF DRY AIR
UL
80
25
Airflow
vs
Cooling
Coil
Sensible
Heat
Ratio
0.60
.026
HUMIDITY RATIO - POUNDS OF MOISTURE PER POUND OF DRY AIR
TB
SENSIBLE HEAT RATIO = Qs / Qt
170
WE
80
40
30
0.65
0.45
.027
.025
85
10
.029
.028
190
85
35
0.40
.030
90
30
Figure 14: System Sensible Heat Ratio versus Airflow
4200
4400
As the air is passed over the cooling coil in a single
zone VAV unit, more of the moisture condenses on the
coil, dehumidifying the air, than in a constant volume
unit. This means that although a constant volume
and a single zone VAV unit maintain the same room
temperature for a given space, the single zone VAV
unit provides more space dehumidification, providing
more comfortable space conditions. The relationship
between airflow and system sensible heat ratio is
illustrated in Figure 14, and the required air volume
for given latent loads and desired absolute humidity
difference is shown in Figure 15.
Latent Load and Required Air Volume
Required Air Volume (cfm)
300.00
250.0
50
200.0
60
150.0
80
100
100.0
150
200
50.0
300
0.0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Latent Load (Btu/hr)
Figure 15: Required Air Volume versus Latent Load
(Absolute Humidity Difference Between Cooling Coil Leaving Air and Room Air (grains/lb))
Traditional constant volume HVAC units are often
only able to provide adequate humidity control under
very limited space loads and outdoor air conditions. For example, consider a classroom with 30 students
doing light, seated work. Allowing 15 cfm per person of outside air, 450 cfm of outside air is needed for this
classroom. The latent load per person can be approximated at 155 Btu/hr, providing a metabolic latent load
of 4,650 Btu/hr for the fully occupied classroom. Assuming outdoor air conditions of 95°F db/ 75°F wb and
a desired room temperature of 74°F with 50% relative humidity with 450 cfm of outside air, the outside air
room ventilation provides an additional latent load of 11,500 Btu/hr for a total latent load of 16,200 Btu/hr
at full occupancy. At full load the room has a sensible load of 64,700 Btu/hr, yielding a sensible heat ratio of
0.8. These given conditions require about 3,000 cfm of airflow to the room to maintain 50% relative humidity.
What happens to this same room under reduced sensible conditions?
Constant Volume
Room
Sensible
Heat Ratio
Relative
Humidity
Single Zone VAV
Latent Cooling
Airflow
(Btu/hr)
(cfm)
Relative
Humidity
Latent Cooling
Airflow
(Btu/hr)
(cfm)
0.80
51.0
%
27,000
3,000
51.0
%
27,000
3,000
0.75
59.7
%
23,541
3,000
50.0
%
27,347
2,000
0.70
66.3
%
20,887
3,000
51.9
%
26,604
1,500
0.65
71.5
%
18,817
3,000
52.0
%
26,565
1,000
0.60
75.7
%
17,162
3,000
56.4
%
24,852
800
0.55
79.1
%
15,812
3,000
61.2
%
22,923
600
Figure 16: Relative Humidity and Airflow for Varying System Sensible Loads
60
90%
Constant
Volume
latent
cooling
80%
50
40
60%
50%
30
40%
20
30%
20%
Relative
Humidity
Latent
Cooling
(MBH)
70%
Single
Zone
VAV
latent
cooling
Single
Zone
VAV
Room
Relative
Humidity
10
10%
0
0%
0.8
0.75
0.7
0.65
0.6
0.55
Constant
Volume
Room
Relative
Humidity
Sensible
Heat
Ratio
Figure 17: Latent Cooling and Relative Humidity (Decreased Sensible Load)
versus Sensible Heat Ratio
11
Let’s assume that the room sensible load decreases with the same room metabolic latent load, as if the
computers or lights are turned off, the blinds are closed, or it’s a cloudy day outside. Figures 16 and 17
show the relative humidity and airflow for various sensible heat ratios for the same classroom with the full
latent load but reduced sensible load. The figure shows what happens when the classroom is fully occupied
but the lights are turned off or the blinds are down, thus reducing the sensible load while the latent load
stays the same.
Now let’s consider the case when the sensible load stays the same but the latent load increases. This might
happen if, for example, the classroom next door joins the class for a lesson or if the students' parents stop
by for career day. Figures 18 and 19 show the relative humidity and airflow for various sensible heat ratios
for the same classroom sensible load with increasing latent loads.
The psychrometric charts for a constant volume system and a single zone VAV system are shown in Figures
20 and 21 for the same classroom described above with a room sensible load of 16,200 Btu/hr and a
room sensible heat ratio of 0.6. The cooling coil leaving air temperature for the single zone VAV system is
much lower than the cooling coil leaving air temperature of the constant volume system. As a result of this
lower cooling coil leaving air temperature, the single zone VAV system is able to provide a greater amount
of dehumidification.
Constant Volume
Room
Sensible
Heat Ratio
Relative
Humidity
Single Zone VAV
Latent Cooling
Airflow
(Btu/hr)
Relative
Humidity
(cfm)
Latent Cooling
Airflow
(Btu/hr)
(cfm)
0.80
51.0
%
27,000
3,000
51.0
%
27,000
3,000
0.75
52.9
%
31,612
3,000
48.5
%
33,368
2,500
0.70
55.3
%
36,847
3,000
51.3
%
38,415
2,500
0.65
58.0
%
42,887
3,000
50.9
%
45,714
2,000
0.60
61.2
%
49,934
3,000
55.2
%
52,320
1,900
0.55
64.9
%
58,262
3,000
61.1
%
59,784
1,900
Figure 18: Relative humidity and Airflow for Varying System Latent Loads
80
70%
70
60%
50%
50
40%
40
30%
30
Relative
Humidity
Latent
Cooling
(MBH)
60
Constant
Volume
latent
cooling
20%
20
10%
10
0%
0
0.8
0.75
0.7
0.65
0.6
0.55
Sensible
Heat
Ratio
Figure 19: Latent Cooling and Relative Humidity (Increased Latent Load)
versus Sensible Heat Ratio
12
Single
Zone
VAV
latent
cooling
Single
Zone
VAV
Room
Relative
Humidity
Constant
Volume
Room
Relative
humidity
25
30
35
40
45
50
55
60
65
70
75
90
95
100
105
110
115
120
125
.004
.003
10
15
10
25
%
90
%
60
40
30
35
20
O
5
%
40
25
20
%
60
0%
SA 45
35
32
30
25
20
25
Figure 20: Psychrometric Chart for Constant Volume System
30
%
15
8%
6%
20%
4%
IDITY
TIVE HUM
30
35
40
45
50
55
60
5
E
ATIV
30%
10% RELA
25
.022
.021
.020
.019
65
70
75
REL
85
90
95
100
105
110
115
Y
IDIT
M
HU
120
125
.016
.014
.013
90
.012
80
.011
70
.010
.009
60
0.50
0.45
0.60
0.65
0.70
0.80
0.90
0.30
55
50
0.25
0.95
1.00
.005
.004
.003
20
0.35
0.85
.006
30
0.40
0.75
.007
40
60
0.55
.008
50
0.20
45
0.15
0.10
40
0.05
.002
10
2%
80
.017
.015
100
0.55
.023
VAPOR PRESSURE - PSIA
-°
F
P
TE
M
AT
IO
N
AT
UR
T,
S
PO
IN
EW
,D
LB
50
40
35
RA
%
55
CC45
15
.000
OA
MA
110
0.50
.024
ENTHALPY - BTU PER POUND OF DRY AIR
t
60
50
15
40
120
70
65
70
45
.001
130
La
ten
Δh
AIR
Y
DR
O
F
D
PO
UN
le
20
0.15
0.10
140
75
.001
130
.000
35
DRY BULB TEMPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
20
150
- °F
70
65
55
0.20
0.05
.002
130
25
1.00
.005
20
0.25
0.95
.006
30
50
80
0.90
.007
40
0.30
0.85
BU
0.80
0.60
.026
.018
30
R
0.75
.008
50
0.35
55
ib
0.70
0.40
ns
0.65
DRY BULB TEMPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
5
85
.009
60
10
2%
80
.010
RE
R
IDITY
TIVE HUM
.011
70
TU
AI
DRY
4%
.012
80
RA
LB.
6%
20%
.014
.013
90
160
PE
75
13.0
30%
10% RELA
15
20
R
20
8%
REL
Y
IDIT
M
HU
100
EM
PER
%
40
25
E
ATIV
AI
DRY
35
LB.
5
13.0
40
30
60
0%
45
35
32
30
25
%
PER
40
%
25
%
80
60
%
70
55
50
.015
0.60
35
W
ET
%
90
LB
55
50
%
15
.016
BT
.FT.
E- CU
LUM
.FT.
E- CU
LUM
W
ET
BU
O
65
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
-°
F
P
TE
M
AT
IO
N
AT
UR
T,
S
PO
IN
EW
,D
SA
.017
UL
80
0.45
PE
Se
Δh
AIR
Y
DR
O
F
D
PO
UN
R
PE
TU
-B
Y
LP
TH
A
EN
OA
RA 70MA
65
CC
110
0.55
TB
VO
14.0
60
120
75
70
VO
14.0
20
45
10
.019
.018
15
20
.020
Se
130
.021
170
WE
80
40
60
TU
nt
La
te
le
Δh
ns
ib
140
0.50
-B
150
- °F
.022
Δh
RE
80
75
.023
Y
TU
LP
RA
TH
A
160
PE
EN
EM
0.65
0.45
.027
.025
85
0.55
VAPOR PRESSURE - PSIA
BT
180
45
0.50
.024
.029
.028
190
85
ENTHALPY - BTU PER POUND OF DRY AIR
UL
HUMIDITY RATIO - POUNDS OF MOISTURE PER POUND OF DRY AIR
TB
80
40
25
50
0.60
.026
SENSIBLE HEAT RATIO = Qs / Qt
170
WE
0.40
.030
200
0.45
.027
.025
85
0.35
90
15.0
15.0
180
45
0.25
0.30
SENSIBLE HEAT RATIO = Qs / Qt
60
95
90
0.65
.028
190
85
30
55
.029
HUMIDITY RATIO - POUNDS OF MOISTURE PER POUND OF DRY AIR
50
35
0.20
0.40
.030
200
SENSIBLE HEAT RATIO = Qs / Qt
0.35
90
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
0.25
0.30
SENSIBLE HEAT RATIO = Qs / Qt
60
95
90
%
0.20
55
10
15
20
25
30
Figure 21: Psychrometric Chart for Single Zone VAV System
ASHRAE Standard 62.1 states that in order to maintain comfortable indoor air conditions, the room relative
humidity ratio must be below 0.012 lbw/lba which corresponds to a relative humidity of 65% at 74°F. Single
zone VAV units provide greater dehumidification at all part load conditions in which the sensible heat ratio
decreases from design specifications. During part-load conditions in which the latent load is at design
specifications and the sensible load decreases significantly, the constant volume unit is not able to maintain
a relative humidity according to ASHRAE Standard 62.1. For conditions in which humidity control is needed
while not much sensible cooling is necessary, the constant volume unit is not able to maintain acceptable
room comfort standards. A constant volume system is unable to adequately control the room humidity as the
room conditions vary from full load design conditions. A single zone VAV unit does not directly measure and
control the room humidity but still provides much more latent cooling at part load conditions than a constant
volume system due to passive dehumidification.
In passive dehumidification, the humidity is not directly measured or controlled. In the absence of a reheat
source, the cooling coil, and corresponding variable capacity compressor, can control temperature or
humidity but one must be primary. If direct humidity control is needed in addition to direct temperature
control, a reheat option can be added to a standard single zone VAV unit for use when space humidity
conditions are not being met. In dehumidification mode, a single zone VAV unit with a reheat coil will use
the cooling coil and compressor to control the space humidity by subcooling the air until the required amount
of moisture is removed from the air. The reheat option is used to reheat the air, providing only sensible
energy to the supply air. This allows the air to directly meet the space humidity and temperature needs. For
more information on reheat options and direct humidity control, refer to the “Modulating Temperature and
Humidity Control” brochure by AAON.
13
Off-Cycle Condensate Re-evaporation
I
n the previous section, all latent cooling benefits were calculated at steady-state operation (no on/off
cycling). Latent cooling benefits are even greater for a single zone VAV unit if we consider the on/off cycling
of the compressor. When air is dehumidified, moisture that is removed from the air condenses on the cooling
coil, leaving water droplets on the cooling coil. When the compressor is turned off and the fan remains on, this
condensate re-evaporates into the air, undoing the latent cooling benefits. This re-humidified air then enters
the room and is cycled back into the HVAC unit and conditioned again through return air. The HVAC unit then
has to dehumidify this air again performing dehumidification work twice and consuming large amounts of
energy in the condensation-evaporation process. The re-humidification due to on/off cycling is even greater
in constant volume units that turn the compressor off while the fan is still running. This allows the unit to
maximize the amount of space sensible cooling while the evaporator coils are still cold but the compressor
is turned off. However the air that is blowing over the cooling coils simply picks up the moisture and
transfers it to the space because the compressor is not operating and dehumidification has decreased and
evaporation has begun. The sensible and latent capacity with continuous supply fan operation was examined
by Henderson, Shirey, and Raustad and presented at the CIBSE/ASHRAE Conference on September 2003.
Their field test data, given in DOE/NETL Project #DE-FC26-01NT41253 is shown in Figure 22.
COIL1_TEST_4B_10B_16B_22B 08/30/02 07:42:04 Cycle #1 (Comp ON time: 45.0 minutes)
30
Sensible
Off-Cycle
Evaporation is
Adiabatic Process:
20
Capacity (MBtu/h)
Sensible ≈ Latent
10
0
Latent Removal
-10
Compressor
Latent Addition
-20
0
20
40
time (minutes)
60
80
100
Figure 22: Sensible and Latent Capacity with Continuous Supply Air Fan Operation
In Figure 22, the compressor and fan are both running at full capacity for the first 45 minutes of operation
then for the next 45 minutes the compressor is turned off while the fan is still running at full capacity.
Sensible cooling is represented by the red area, latent heat removal is represented by the blue area, and
latent heat addition is represented by the green area. The field test data shows that as the compressor turns
off while the fan is still running, latent heat is added to the space while some sensible heat is removed.
Figure 22 shows that as the compressor turns on, it takes a certain amount of time for the compressor to
reach its full sensible and latent capacity. This means that energy is wasted each time the compressor is
turned on while the temperature of the cooling coils reaches its steady state. In addition to this, every time
14
the compressor is turned off while the supply fan continues to run, moisture is added back to the space,
wasting a large amount of the energy that was consumed to remove the moisture. Estimating the amount
of latent removal and latent addition from Figure 22 shows that the unit provides about 5.4 MBtu of latent
cooling while the compressor is on and loses about 4.0 MBtu of latent cooling while the compressor is off,
providing only 1.4 MBtu of latent net cooling per cycle. When the compressor is off, the process is roughly
adiabatic, meaning no actual energy overall is removed from the air. The unit provides sensible cooling to the
space while the compressor is off but much of the sensible cooling results in an equal loss of latent cooling.
Not only does on/off compressor cycling waste energy and provide very little latent cooling but it also creates
large variations in the room humidity which can make the space uncomfortable for its occupants.
AAON single zone VAV systems reduce the latent cooling losses due to the cycling of the on/off compressor
by lowering the fan speed and utilizing a variable capacity compressor, which can modulate its capacity from
10% to 100%, to satisfy the cooling load instead of simply turning the compressor on and off.
Why Single Zone VAV?
S
ingle zone VAV systems modulate the supply fan speed based on the space temperature and modulate
the variable capacity compressor based on the supply air temperature to provide variable airflow at a
constant supply air temperature to control the space temperature of a single zone. With both variable speed
fans and variable capacity compressors, which have a wide range of modulation capabilities and can adapt
to full and part load conditions, AAON is leading the industry in single zone VAV technology.
There are many benefits to a single zone VAV system. With the modulation capabilities of both the fan and
compressor, a single zone VAV system can provide precise temperature control and additional passive
dehumidification. Because of the modulating capability of the variable capacity compressor, and therefore
a reduction in on/off cycling, single zone VAV systems reduce the wear on the compressor and save energy
with less cycling losses caused by inrush current. Lower fan speeds during part load operation reduce the
amount of system sound and significantly reduce the system energy consumption. Finally, because the
entire modulating control for part load operation is provided within the HVAC unit, a single zone VAV system
is simple to install, set up and maintain.
Single zone VAV units are a better alternative to all constant volume units due to the ability to adapt to
normal swings in room conditions. With a single zone VAV system, a building owner will have a more
comfortable environment, reduce HVAC system energy consumption, lower sound levels and save money.
The versatility of single zone VAV makes single zone VAV a superior choice that provides better results for
all HVAC applications.
Contact your local AAON representative to see how
an AAON single zone VAV unit will benefit your application.
15
Defining Quality. Building Comfort.
2425 S. Yukon Ave. • Tulsa, OK 74107-2728 • www.AAON.com
It is the intent of AAON to provide accurate and current product information. However, in the interest of product improvement, AAON reserves the right to change
pricing, specifications, and/or design of its product without notice, obligation, or liability. Copyright © AAON, all rights reserved throughout the world. AAON® and
AAONAIRE® are registered trademarks of AAON, Inc., Tulsa, OK.
SingleZoneVAV • R97020 • 140228