AEC NEC Central Station Air and Water Cooled Chiller - Manual (Page 36)

NEC Series Water- and Air-Cooled Central Chillers Page 35
4-10 Control Nipple
All NEC chiller evaporators have two control nipples. The flow switch and freezestat are located
in the To Process control nipple. Optional pressure gages and thermometers are mounted in the
control nipples to aid in achieving proper flow through the evaporator and balancing flows on
NEC-2 and NEC-3 chillers.
Coupling mounts are provided for customer-installed pressure gauges and thermometers on
standard NEC chillers. See Figure 8 — Evaporator GPM Pressure Drop Curves to convert
pressure drop between evaporator entering water pressure and evaporator leaving water pressure
to gallons per minute.
4-11 Control Probe
Figures 3 through 5 — Location
The control probe senses the temperature of the chilled water returning from the process. The
temperature reading is used by the compressor sequencing controller to cycle the compressors
according to the load.
On NEC-1 chillers, the control probe is factory-installed in the return control nipple. On NEC-2
and NEC-3 chillers equipped with optional manifolds, the control probe is factory-installed in
the return manifold.
On standard NEC-2 and NEC-3 chillers (without manifold), the control probe thermocouple and
well must be customer-installed in the common return line ahead of the evaporators. This permits
proper control function if a circuit is isolated for service.
4-12 Hot Gas Bypass
Optional
NEC chillers use multiple compressor unloading stages to achieve capacity stages. Under light
cooling loads, the electrically-controlled hot gas bypass valve is the last stage off and the first
stage on to give close temperature control. It prolongs compressor life by reducing compressor
cycling. No customer adjustments are necessary.
The hot gas bypass system is an electric bypass and shutoff service valve run between the
compressor discharge, downstream of the thermal expansion valve near the entry to the
evaporator. The controller cycles the electric valve to achieve capacity reduction by false loading
the evaporator with hot discharge gas.
On NEC-2 and NEC-3 chillers with the hot gas bypass and Lead/Lag options, two hot gas
circuits and additional controls are used.