13.5.3 Limited Capacity at Low Pressure Ratios
Figure 13-2 Centrifugal Performance Dynamics illustrates the capacity reduction imposed by compressor logic
when a compressor is operated at low lift conditions without a low lift pump and without the low lift function
activated. This capacity limitation is common during a warm building pull down cycle during low ambient
conditions but can also be encountered under other operating conditions with extended operation with low
pressure ratios. If more capacity is desired during operation during low lift conditions the condensing temperature
can be increased, or alternatively the low lift pump and low lift function can be activated if the refrigeration system
is so equipped with these features. Reference Figure 14-1 Typical Refrigeration Piping Schematic. and the
OEMProgramming Manual for more details on operation of the low lift function.)
13.5.4 Low Lift Application
The standard TTS/TGS control limits compressor speed and capacity above a 1.5 pressure ratio to ensure adequate
motor/inverter cooling. When enabled, the low lift option is meant to enable increased compressor speed and
capacity at pressure ratios below 1.5. To ensure adequate cooling for extended operation with pressure ratios below
1.5, the chiller system must provide the subcooled refrigerant flow specified in Table 13-2 Low Lift Pump Sizing. An
OEM-supplied liquid refrigerant pump will be required in most circumstances/designs. If an adequate supply of
subcooled liquid is not provided, the compressor will limit speed and capacity to maintain safe operating
temperatures. If safe temperatures cannot be maintained, the compressor will fault. Repeated operation without
adequate motor cooling could result in damage to the compressor and evidence of such operation could limit
warranty coverage.
Refer to Figure 14-1 Typical Refrigeration Piping Schematic for a possible pump design and Table 13-2 Low Lift
Pump Sizing. Though each OEM may choose to use more sophisticated logic, a simplified control logic would turn
the pump on at pressure ratios below 1.5 and off when the pressure ratio rises to 1.7. By default, the low lift option is
not enabled. More details on enabling the low lift option and the alarm and faults associated with it are included in
the OEM Programming Manual.
Table 13-2 Low Lift Pump Sizing
Model Cooling Mass Flow (kg/s) Head (kPa)
ALL 0.06 310
Page 78 of 136 - M-AP-001-EN Rev. S 9/8/2021
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