31
Dehumidification of Fresh Air with DCV Control
Information from ASHRAE indicates that the largest humidity
load on any zone is the fresh air introduced. For some
applications, a device such as an energy recovery unit can be
added to reduce the moisture content of the fresh air being
brought into the building when the enthalpy is high. In most
cases, the normal heating and cooling processes are more than
adequate to remove the humidity loads for most commercial
applications.
If normal rooftop heating and cooling operation is not adequate
for the outdoor humidity level, an energy recovery unit and/or a
dehumidification option should be considered.
Step 7
—
Adjust Evaporator--Fan Speed
Adjust evaporator-fan speed to meet jobsite conditions.
Tables 8 and 9 show fan rpm at motor pulley settings for the
551C 024--060 and 551B 036--072 units. Tables 10 and 13 show
maximum amp draw of belt-drive motor. Table 12 shows sound
data. Refer to Tables 14-33 for performance data. See Table 34
for accessory static pressure drop. See Fig. 39 for the Perfect
Humidity
™
system static pressure drops.
For units with electric heating, required minimum cfm is 900 for
551B/C 036; 1200 for 551B/C 048; 1500 for 551B/C 060 and
1800 for 551B 072 and 600 for 551C 024.
Belt Drive Motors
Fan motor pulleys are factory set for speed shown in Table 1.
Check pulley alignment and belt tension prior to start-up.
To change fan speed:
1. Shut off the unit power supply and tag disconnect.
2. Loosen the belt by loosening the fan motor mounting nuts.
(See Fig. 40.)
3. Loosen movable pulley flange setscrew (See Fig. 41).
4. Screw movable flange toward fixed flange to increase
speed and away from fixed flange to decrease speed.
Increasing fan speed increases load on motor. Do not
exceed maximum speed specified in Table 1.
5. Set movable flange at nearest keyway of pulley hub and
tighten setscrew. (See Table 1 for speed change for each
full turn of pulley flange.)
6. Adjust belt tension and align fan and motor pulleys per
guidance below.
NOTE
: Once the required flange position is determined for the
correct blower rpm, it is recommended (but not required) that the
variable pitch pulley be replaced with a corresponding size fixed
sheave pulley.
To align fan and motor pulleys:
1. Loosen fan pulley setscrews.
2. Slide fan pulley along fan shaft.
3. Make angular alignment by loosening motor from mount-
ing.
To adjust belt tension:
1. Loosen fan motor mounting nuts.
2. Slide motor mounting plate away from fan scroll for
proper belt tension (
1
/
2
-in. deflection with 7 to 10 lb of
force).
3. Tighten motor mounting nuts.
4. Adjust bolt and tighten nut to secure motor in fixed
position.
Table 7—CO
2
Sensor Standard Settings
SETTING
EQUIPMENT
OUTPUT
VENTILATION
RATE
(cfm/Person)
ANALOG
OUTPUT
CO
2
CONTROL RANGE
(ppm)
OPTIONAL
RELAY SETPOINT
(ppm)
RELAY
HYSTERESIS
(ppm)
1
Proportional
Any
0-10V
4-20 mA
0-2000
1000
50
2
Interface w/Standard
Building Control System
Proportional
Any
2-10V
7-20 mA
0-2000
1000
50
3
Building Control System
Exponential
Any
0-10V
4-20 mA
0-2000
1100
50
4
Proportional
15
0-10V
4-20 mA
0-1100
1100
50
5
E
i
Proportional
20
0-10V
4-20 mA
0- 900
900
50
6
Economizer
Exponential
15
0-10V
4-20 mA
0-1100
1100
50
7
Exponential
20
0-10V
4-20 mA
0- 900
900
50
8
Health & Safety
Proportional
—
0-10V
4-20 mA
0-9999
5000
500
9
Parking/Air Intakes/
Loading Docks
Proportional
—
0-10V
4-20 mA
0-2000
700
50
LEGEND
ppm —
Parts Per Million
551B
,C
