1
SYSTEM DESIGN RECOMMENDATIONS
Low pressure
MT
R-22
MTZ
R-407C
MTZ
R-134a
MTZ
R-404A / R-507A
Test pressure low side
psig
360
360
360
360
Working pressure range high side
psig
158 - 402
181 - 426
115 - 328
191 - 402
Working pressure range low side
psig
15 - 102
20 - 96
9 - 68
15 - 104
Relief valve opening pressure difference
psig
435
435
435
435
Relief valve closing pressure difference
psig
115
115
115
115
Low ambient temperature
operation
Operating voltage range
Operating voltage and
cycle rate
|V
avg
- V
1-2
|+|V
avg
- V
1-3
|+|V
avg
- V
2-3
|
2 xV
avg
Vavg = Mean voltage of phases 1, 2 and 3
V1-2 = Voltage between phases 1 and 2
V1-3 = Voltage between phases 1 and 3
V2-3 = Voltage between phases 2 and 3.
% voltage unbalance:
x 100
Cycle rate limit
A low pressure safety switch is recom-
mended to avoid compressor operation
at too lower suction pressures.
At low ambient temperatures, the
condensing temperature and conden-
sing pressure in air cooled condensers
will decrease. These lower pressures
may be insufficient to supply enough
liquid refrigerant to the evaporator. As a
result, the evaporator temperature will
sharply decrease with risk of frosting.
At compressor start-up, the compressor
can pull a deep vacuum and it can be
cut off by low pressure protection. De-
pending on the low pressure switch set-
ting and delay timer, short cycling can
occur. To avoid these problems, several
solutions are possible, all based on re-
ducing condenser capacity:
• Locating condensor indoors
• Liquid flooding of condensers (note:
this solution requires extra
refrigerant charge, which can
introduce other problems. A non-
return valve in the discharge line is
required and special care should
be taken when designing the
discharge line.)
• Reduce air flow to condensers.
Other problems can occur when the
compressor is operating at low
ambient temperature. For example,
during shut down periods, liquid
refrigerant can migrate to a cold
compressor. For such conditions a
belt-type crankcase heater is strongly
recommended.
Because Maneurop compressor motors
are 100% suction gas cooled, they can
be externally insulated.
Refer to section "Liquid refrigerant mi-
gration & charge limits" for more de-
tails.
Operating voltage limits are shown in
the table on page 4. The voltage applied
to the motor terminals must always be
within these limits. The maximum allo-
wable voltage unbalance for 3-phase
compressors is 2%. Voltage unbalance
causes high current draw on one or
more phases, which in turn leads to ove-
rheating and possible motor damage.
Voltage unbalance is given by the for-
mula:
There may be no more than 12 starts
per hour (6 when a soft start accessory
is used). A higher number reduces the
service life of the motor-compressor
unit. If necessary, use an anti-short-cy-
cle timer in the control circuit.
A time-out of six minutes is recommen-
ded. The system must be designed in
such a way to guarantee a minimum
compressor run time in order to provide
proper oil return and sufficient motor
cooling after starting.
Note that the oil return rate varies as a
function of the system design.