Application Guide

63-706237
CC1
CC2
Evaporator 1
Evaporator 2
M
V2
M
V1
Inverter
RST
PG
MON PARREFBTNS
RUN READY FAULT
Honeywel
l
Fig. 44 Application with VFD.
Application Description
Chillers with evaporators piped in parallel (see Fig. 43) and controlling on either flow or return from the machine. Often, under
light load conditions, chiller two will close down (to prevent short-cycling). This results in a rise in CHW supply temperature to
the building, as 50% of water circulates through the
off
chiller (supply temperature is average of flow from the two machines).
A more energy efficient means of controlling the plant would be to install valves V1 and V2 and a VFD on the circulation pump
(as in Fig. 44). Under light load conditions, one machine running and the other isolated, by closing valve V2 the VFD speed can
be reduced to give the design flow rate through the “on” machine. As the demand increases the VFD speed can be increased
to give full design flow for the two machines, the valve for number two chiller opened and the second machine started.
Benefits
— Energy savings: Chilled water pump and second chiller off for longer periods.
— Reduced maintenance: Fewer starts and stops and machine not having to run under light loads.
— Supply temperature to the building will be more stable.
— Reduced noise.
VFD Features Used
— Digital speed control.
— Auto-restart.
IMPORTANT
The VFD speed must be increased and full flow achieved, before the second valve is opened. Otherwise a flow failure
trip may occur on the number 1 machine. It is also advisable to pulse this valve open as slowly as possible.
Primary Chilled Water Pump with DDC Control of Plant Option 2 (See Fig. 45 and 46)
CC1
CC2
Evaporator 1
Evaporator 2
Fig. 45. Application without VFD.