Defining Quality. Building Comfort.
Re
tur
n
Air
Bypass
Indirect Dehumidification Enhancement
for Constant Volume Systems
Return Air Bypass is an airflow configuration
with three independently controlled dampers return air dampers, return air bypass dampers and
outside air dampers. During the return air bypass,
or dehumidification, mode of operation, return
and outside air are cooled and dehumidified as
the air passes through the evaporator coil and
then reheated with return air bypassed around
the evaporator coil. This return air reheat is used
to prevent overcooling of the zone during the
dehumidification mode of operation.
Bypassed return air provides reheat with air that
has already been dehumidified by the system
during the cooling mode of operation and the
heat in this air comes from the load of the zone,
not an auxiliary source. Return air should be used
as a reheat source instead of mixed air (outside
air + return air) when the humidity ratio of the
mixed air is greater than the humidity ratio of the
return air.
Return air bypass provides dehumidification
capabilities to a constant volume system at a low
initial cost. Return air bypass can often control
the humidity of zones which experience interior
loads with at least a 0.7 sensible heat ratio (SHR).
Zones with interior loads less than 0.7 SHR
often require a dehumidification system such as
modulating hot gas reheat.
Benefits of Return Air Bypass
There are many benefits to including return air
bypass to control the zone relative humidity.
Occupant comfort and well being will increase
during the cooling season, because the zone
humidity will be controlled better than with
a conventional constant volume system.
Also, because return air bypass includes fully
modulating damper control dramatic zone
temperature swings will not occur, even during
the dehumidification mode of operation.
Occupant productivity will increase because less
time will be spent by the occupants adjusting the
zone temperature or using auxiliary sources of
cooling (i.e. personal fans) to compensate for the
uncontrolled humidity.
Indoor air quality will improve because high
relative humidity conditions will be better
controlled. Out of control humidity can lead to
condensation within the building, wood rot, paper
deterioration, and mold and microbial growth.
Occurrences of sickness and allergies will
decrease with the addition of humidity control to
a zone because humid spaces are more likely to
include mold, fungi, bacteria and other allergens
all of which can cause heath issues.
Mixed Air (MA)
Evaporator Coil
Filters
Outside Air
Dampers
Outside Air
Return Air
Dampers
Compressor
Access
Return Air Bypass
Dampers
Supply Air
after Return Air Bypass
Cooling Coil
Leaving Air (CCLA)
Return Air RA
AAON Packaged Rooftop Unit with Return Air Bypass
Ventilation standards can
be met or exceeded while
still meeting energy use
standards because return
air bypass can control
additional light latent
loads, or be matched with
modulating hot gas reheat
to meet larger latent loads,
while still using return air
reheat and saving energy.
0.014
70
Data based on ISO 7730
and ASHRAE STD 55
65
Upper Recommended Humidity Limit 0.012 humidity ratio
Humidity Ratio
0.012
60
0.010
0.008
0.006
0.004
0.002
50
1.0 Clo
55
0.5 Clo
%
90%
80
%
70
%
60
%
50
%
40
%
30
20%
10% RH
55
50
45
40
35
30
25
20
10
No Recommended
Lower Humidity
Limit
60
65
70
75
80
85
90
95
Dew Point Temperature, ˚F
0.016
100
Operative Temperature, ˚F
ASHRAE Standard 55 - Acceptable Range of Zone Temperature and Relative Humidity
for Occupant Thermal Comfort
1.0 CLO = Clothing insulation worn during cool ambient conditions
0.5 CLO = Clothing insulation worn during warm ambient conditions
Ranges are based on 80% occupant acceptability
Limitations of Return Air Bypass
Return air bypass is an ideal dehumidification
option for constant volume systems which serve
zones with light latent loads. Zones with large
latent loads and zones that have peak sensible and
latent loads occurring at the same time require
an additional dehumidification system, such as
modulating hot gas reheat.
As with any system which attempts to satisfy
the latent load of a zone, additional energy use is
required with return air bypass. Because the latent
load of the zone is now being satisfied, however,
energy savings is possible if the zone temperature
setpoint is adjusted based on the humidity
control provided by return air bypass. According
ASHRAE Standard 55 - Thermal Environmental
Conditions for Human Occupancy, as the zone
humidity is reduced and controlled the zone
temperature can be increased without sacrificing
occupant thermal comfort.
Return Air Bypass with AAON
Return Air Bypass is available on AAON
packaged rooftop units, from 6-230 tons, to
control the humidity of any constant volume
application with light latent loads. Ideal
applications for return air bypass include
supermarkets, schools, libraries, office buildings,
lodgings, printing facilities, industrial facilities,
data centers and LEED or green buildings.
To specify Return Air Bypass on your next system
contact your local AAON sales representative.
Sensible Design Application
Latent Design Application
A zone is served by a constant volume system,
at the sensible design ambient conditions of
95˚F DB/75˚F MCWB. It requires 2,000 cfm of
supply air and 300 cfm of ventilation outside air
(15%) and includes 41 MBtu/h of sensible zone
load and 10 MBtu/h of latent zone load (0.80
SHR). The desired zone conditions are 74˚F DB
and 50-55% RH.
A zone is served by a constant volume system,
at the latent design ambient conditions of 85˚F
MCDB/75˚F WB. It requires 2,000 cfm of supply
air and 300 cfm of ventilation outside air (15%)
and includes 27 MBtu/h of sensible zone load
and 10 MBtu/h of latent zone load (0.73 SHR).
The desired zone conditions are 74˚F DB and
50-55% RH.
At the sensible design load the constant volume
system can directly control the zone temperature
to the desired 74˚F DB, with no return air bypass,
and indirectly control to the desired maximum
zone relative humidity at 55% RH. However,
with 350 cfm of return air bypassed around the
evaporator coil, the system can control to 50%
zone relative humidity.
At the latent design load the constant volume
system can directly control the zone temperature
to the desired 74˚F DB, however, the system
cannot indirectly control the relative humidity
and the resulting zone relative humidity is 68%
RH. With 850 cfm of return air bypassed around
the evaporator coil the system can control to
the desired maximum zone relative humidity at
55% RH.
40
40
0.45
0.45
.019
130
.018
0.40
75
35
.017
WE
TB
UL
BT
EM
PE
TU
RE
- °F
OA
70
.015
75
80
85
90
20
CCLA
.007
% 50
0.15
50%
15
.003
95
100
DRY BULB TEMPERATURE - °F
15%
40%
30%
0.05
8%
10% RELATIVE
45
50
55
VE
RELATI
ITY
HUMID
6%
20%
0.25
70
0.20
60
50
0.15
40
30
0.10
20
0.05
10
4%
HUMIDITY
2%
60
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Senible Design Application with Return Air Bypass
Outside Air - 95˚F DB/75˚F WB
Zone Conditions/Return Air - 74˚F DB/50% RH (W=0.0089 lbw/lba)
Mixed Air - 78˚F DB/64˚F WB (W=0.0095 lbw/lba)
Cooling Coil Leaving Air - 51˚F DB/50˚F WB
Supply Air - 55˚F DB/52˚F WB
%
25
WET BULB, DEW POINT, SATURATION TEMP - °F
60%
45
.005
0.10
RA
SA
70
.006
.002
55
%
80
VAPOR PRESSURE - PSIA
%
90
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
VAPOR PRESSURE - PSIA
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
0.20
IR
70
60
.008
.004
80
YA
65
25
.001
2%
60
.009
0.25
0.30
90
MA
DR
6%
4%
HUMIDITY
.010
65
LB.
55
0.35
100
ER
50
- °F
T. P
8% REL
20%
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
0.30
13.0
13.0
ITY
HUMID
ATIVE
30%
45
RE
U.F
IR
YA
DR
15%
40%
10% RELATIVE
TU
0.40
110
-C
LB.
WET BULB, DEW POINT, SATURATION TEMP - °F
50%
15
RA
ME
ER
T. P
U.F
% 50
%
25
60%
45
120
PE
LU
-C
ME
RA
55
70
.011
LU
CCLA
EM
VO
VO
60
SA
BT
14.0
14.0
MA
25
%
OA
UL
70
.012
65
%
80
TB
30
.013
90
WE
0.35
.014
30
20
75
35
.016
RA
65
70
75
80
85
90
95
100
DRY BULB TEMPERATURE - °F
Latent Design Application with Return Air Bypass
Outside Air - 85˚F DB/75˚F WB
Zone Conditions/Return Air - 74˚F DB/55% RH (W=0.0098 lbw/lba)
Mixed Air - 77˚F DB/67˚F WB (W= 0.011 lbw/lba)
Cooling Coil Leaving Air - 53˚F DB/52˚F WB
Supply Air = 62˚F DB/57˚F WB
For more information on AAON products, contact your local AAON sales representative.
AAON is a Registered Trademark of AAON, Inc.
Defining Quality. Building Comfort.
AAON • 2425 South Yukon Avenue • Tulsa, Oklahoma 74107 • (918) 583-2266 • Fax (918) 583-6094 • www.aaon.com
ReturnAir • R97970 • 100830