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Defrost
Defrost is a temporary and infrequent period during normal
heating operations on Rebel heat pumps. The purpose of
defrost is to remove frost that has built up on the outdoor coil
during mechanical heating. In heating mode the outdoor coil
acts as an evaporator to “pull” heat out of the ambient air. As
a result the surface temperature of the outdoor coil is below
the ambient temperature and depending on conditions maybe
below freezing. During prolonged mechanical heating while
the surface temperature of the outdoor coil is below 32ºF, dew
point frost will form.
The defrost operation is similar to mechanical cooling. In defrost
the four way valve will de-energize and the hot gas from the
compressor will be forced into the outdoor coil, rejecting heating
to the ambient, and melting any frost formed on the coil. To
speed up the melting process during a defrost cycle the OA
damper will close and the outdoor fan will de-energize. During
this period the supplemental (gas, electric, hot water) heat will
ramp/stage up to maintain the unit’s DAT Setpoint.
Rebel heat pump unit’s have demand based defrost control
and will operate in defrost only as long as necessary to remove
frost from the outdoor coil.
Charging
Rebel units have advanced charge management systems
that obsolete many common techniques for determining
over or under charged conditions. The charge management
system means that super heat and subcooling values will
float to achieve the peak real time energy efficiency possible
at current operating conditions (building load and ambient
temperature). Rebel units also use electronic expansion valves
that can not be adjusted manually. Refrigerant should never
be added or removed from the system based on the desire to
achieve an arbitrary subcooling value. It will always be Daikin's
recommendation that unit’s suspected of being over/under
charged have all of their refrigerant removed, leak tested with
nitrogen, and then re-charged based on the unit name plate.
Table 11:
Refrigerant Charge
Unit size
Refrig . charge - cooling model
Refrig . charge - heat pump model
Standard unit
Standard unit
w/MHGRH
Standard unit
Standard unit
w/MHGRH
3
8.6
11 .3
12.0
14 .4
4
8.5
11 .3
12 .6
15.0
5
15 .3
18.2
16.8
19 .7
6
15 .3
18.2
16.8
19 .7
7 .5
11 .1
17.8
26.0
31 .2
10
20.0
25.8
40.0
45.8
12
20.0
25.8
40.0
45.8
15
24 .4
30.2
46.0
51.8
16
30.3
30.7
53.0
53 .4
18
30.3
30.7
53.0
53 .4
20
32 .5
32 .9
56.0
56 .4
25
35 .5
35 .9
Not Available
28
35 .5
35 .9
DPS 016–028 Ton Component Description
Variable Speed Compressor
A variable speed compressor (COMP1) is used on all DPS
016-028. On DPS 16–20 ton units, the variable speed
compressor will be the only one present, and be on the right.
The discharge of the variable speed compressor is located on
the top and the suction is located on the side.
These pipes can also be identified by recalling that suction
lines will always be larger than discharge lines. The side
suction design is used to cool the motor with cold refrigerant.
Figure 34: Compressor Suction and Discharge on DPS
025–028 units
Fixed Speed Compressor (DPS 025–028 only)
Like the variable speed compressor, it has the suction line
on the side of the dome entering the scrolls and a discharge
exiting from the top of the shell.
High Pressure Switch
All Rebel Units will have a high pressure switch on each
compressor. HP1 switch is on the variable speed compressor
(COMP1) and HP3 is on the fixed speed compressor
(COMP3). These switches are normally closed devices
that are brazed directly to the refrigerant piping. When the
pressure at the switch exceeds 580 PSIG the switch will open.
This opening will interrupt the control signal to the variable
compressor drive or de-energize the contactor coil on the fixed
speed compressor, Both acts will shut down the compressors
and generate an alarm at the MicroTech III keypad.
Refrigeration System
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IM 1125-13 • REBEL ROOFTOPS