IM 738-2
103
Check, Test, and Start Procedures:
Oil Pressure (sizes 115 to 135C only)
115 to 135C only)” on page 103. If the compressor cycles
on its low pressure switch, do the following:
a
Verify that the circuit is not short of refrigerant.
b
Check for low airflow.
c
Check for clogged filters.
d
Check for restricted ductwork.
e
Check for very low temperature return air entering the
unit.
f
Verify that the liquid line components, expansion valve,
and distributor tubes are feeding the evaporator coil.
g
Verify that all air handling section panels are closed.
h
Verify that the suction service valve (size 115 to 135
only) and the liquid line service valves are completely
open.
8
Verify that the compressors stage properly. As the circuit
loads up the second compressor (if available) will be
energized. Any unloaders (reciprocating compressors only)
are energized when unloaded. For more information on
staging sequences, see the
Binary Outputs-Auxiliary
Control Boards
section of bulletin IM 696, “MicroTech II
Applied Rooftop Unit Controller.”
9
Verify that the condenser fans are cycling and rotating
properly (blowing air upward). When the compressor
starts, at least one condenser fan should also start. The
CCB1 should control the remaining condenser fans based
on outdoor air conditions. Look at keypad
menu Setup/
Service \ Compressor Setup \ Cond Fan1
= (also look at
Cond Fan2
=,
Cond Fan3
=,
Cond Fan4
=). Tables 4 and 5
on page 20 shows recommended setpoints based on the unit
size. Cond Fan1 controls BO5, Cond Fan2 controls BO6,
Cond Fan3 controls BO7, Cond Fan4 controls BO8. Refer
to the unit wiring diagrams and “Condenser Fan
Arrangement” on page 4.
10
Check the oil level in the compressor sightglass. If low oil
is observed, it is possible that liquid refrigerant is returning
to the compressor. Check the suction superheat, see
“Expansion Valve Superheat Adjustment” on page 103. It
should be between 10°F (5.5°C) and 13°F (7.2°C). See
“Expansion Valve Superheat Adjustment” below.
11
Close solenoid valve SV1 by turning switch PS1 to OFF
The circuit should pump down and then the compressor(s)
should stop. Place the unit into the “Fan Only” mode
through the keypad menu
System Summary \ System \ Ctrl
Mode= Fan Only
.
12
Check refrigerant circuit #2 by repeating steps 2 through 9,
substituting circuit #2 nomenclature for circuit #1
nomenclature (CS2, PS2, TD2, SV2, CCB2, and
compressor #2 (and #4).
Note:
The unit is wired for one time pumpdown. The
compressor will not restart on the off cycle even if the
low pressure switch closes due to small leaks in the
solenoid valve or compressor check valve.
13
Verify that the condenser refrigerant subcooling at full
capacity is between 13 and 20°F.
Expansion Valve Superheat Adjustment
It is very important that the expansion valve superheat setting
be adjusted to be between 10° F (–12° C) and 13° F (–11° C).
Insufficient superheat will cause liquid floodback to the
compressor which may result in slugging. Excessive superheat
will reduce system capacity and shorten compressor life.
Turn the adjustment stem clockwise to increase superheat. Not
exceeding one turn, adjust the stem and then observe the
superheat. Allow up to 30 minutes for the system to rebalance
at the final superheat setting.
On refrigeration circuits with multiple expansion valves, the
superheat adjustment should be approximately the same for all
valves in the circuit.
R-407C Superheat
Due to refrigerant glide, when measuring and/or adjusting
TEV superheat, it is important to use SATURATED VAPOR
(Dew Point) TABLES. Example: The Pressure/Temperature
(P/T) chart shows that the saturated vapor temperature, at the
dew point, of R-407C for 79 psig is approximately 51°F. If the
actual refrigerant temperature is 60°F, the superheat is 9°F.
Checking Superheat
Following are recommendations for checking superheat:
1
Close the unit section doors. Running the unit with its
doors open will affect expansion valve and system
operation considerably.
2
For units with one expansion valve per circuit, check the
pressure and temperature at the compressor suction valve.
3
For units with multiple expansion valves per circuit, check
the pressure at the compressor, and check the temperature
at the suction header that is fed by the valve.
Oil Pressure (sizes 115 to 135C only)
When the compressor has operated long enough to stabilize
conditions, proper oil pressure should be maintained. The
actual oil pressure value varies from compressor to compressor
and depends upon the temperature, oil viscosity, compressor
size, and the amount of clearance in the compressor bearings.
Oil pressure values from 20 psi to 60 psi (138kPa to 414 kPa)
(over suction pressure) are not uncommon.
The oil level in the compressor sightglass can vary widely and
depends upon the same factors listed above. In fact, it is not
unusual for two compressors that serve the same circuit to
have very different oil levels. Therefore, it is recommended
that oil pressure, not sightglass level, be used to judge whether
there is enough oil in a refrigerant circuit. If the oil pressure is
low, add additional. Refer to page 43 to find acceptable
refrigerant oils.
