Ddc Control Of A Small House Substation - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

Control Strategies
1. Primary Flow Loop Differential Pressure Control:
Provides a constant value pressure drop across the heat
exchangers. Actuator control valves (Y17, Y18) open or
close sequentially to maintain the pressure drop measured
by PDIC 14.
2. Secondary Loop Supply Flow Temperature Control: The
SSF temperature is reset from outdoor air temperature
(Fig. 137). The actual SSF temperature is measured by
sensors TIC 28 and TIC 38. The regulation of the heat
flow through the HEX is provided by the actuator control
valves Y 23 and Y 33 in the primary return flow line of
each heat exchanger.
3. Secondary Flow Loop Differential Pressure Control:
Circulating pumps Y 48 and Y 49 are switched off or on
and the speed varied to maintain the differential pressure
setpoint. A frequency converter (fc) controls pump speed.
4. SSF Loop Pressure Control: Maintains the pressure
sensed by transmitter PIC 57 within a defined range to
avoid too low pressure (vaporization) and too high
pressure (pump overload). Vent valve Y 54 maintains the
high limit and compressor Y 55 maintains the low limit.
5. Expansion Tank Water Level Control (WLC) Loop:
LIC 56 maintains the expansion tank water level. Normally

DDC CONTROL OF A SMALL HOUSE SUBSTATION

DISTRIBUTION
OUTSIDE AIR
PSF
TRANSFER
PRF
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
SUBSTATION
DDC
TI
01
HEAT
TI
STATION
12
Fig. 138. DDC Control of a Small House Heat Transfer Substation.
PRF water through Valve Y 51 provides make-up water, in
cases of a high quantity water loss, Pump Y 64 pumps make-
up water from the water supply tank. The WLC protects
the circulating pumps against running without water.
6. Water Supply Tank Water Level Control: LIC 63) controls
Valve Y 61, providing make-up water from the cold water.
7. Secondary Return Flow (SRF) and the Primary Return
Flow (PRF) temperatures are limited to provide a high
temperature drop between supply and return flow.
180
160
140
120
100
80
60
-25 -21 -17
Fig. 137. Adjustment of Temperature Curves.
TZA+ TIC
21 22
Y
11
HEX 1
M
389
LIMIT SSF SETPOINT
TEMPERATURE IS
UNLIMITED
-13 -9 -5 -1 3 7 11 15
OUTSIDE AIR TEMPERATURE (˚C)
CONSUMER
TI
31
ROOM
Y
23
SSF
TIC
24
SRF
M11436
19
M15294