Indirect Heat Transfer Substations - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Control Strategies:
1. Control valve Y 11 maintains the SSF temperature as
dictated by TIC 13 which is reset by the outdoor air
temperature. Room temperature sensor TI 31 shifts the
reset schedule up or down. Controller TC provides night
setback and other unoccupied programs.
2. Pump Y 12 provides constant SSF flow through the check
valve.
3. Summer operation shuts down the entire station.
However, it is recommended that the summer function
include a function to exercise the pump and therefore all
the devices once a week.

Indirect Heat Transfer Substations

Indirect substations use heat exchangers and physically
decoupled or independent heating circuits. Applications range
from small substations for a one family house to large
substations for industrial types of networks. Three applications
for heat transfer substations with different heat exchanger
configurations, primary flow and/or the differential pressure is
controlled by control valves, heat flow controlled by modulating
control valve to vary the primary flow, and circulating pumps
to provide the secondary supply and return flow, follow:
Indirect heat transfer substations:
– Transfer the required heat from the supply (primary) side
to the consumer (secondary) side.
– Meter heat.
– Provide safety functions to protect consumer and
equipment against overheating, frost, and harmful agents
in hot tap water.
– Provide optimization functions to reduce energy
consumption to the lowest possible level.
– Provide hydraulic separation between the high-
temperature high-pressure system and the low-
temperature low-pressure system.
The main parts of an indirect substation are:
– Primary side.
– Heat exchanger.
– Circulating pumps.
– Expansion and storage tanks.
– Feed water facilities.
– Secondary side.
Functional principle:
The primary side of the substation (Fig. 132) contains the
differential pressure control and a normally closed temperature
control valve with safety spring return, if the secondary medium
temperature exceeds 212F. The differential pressure control is
used in large networks where over time significant pressure
differences exist.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
The primary supply flow (from the district heating network)
enters the heat exchanger, transfers the heat to the secondary supply
flow, and returns to the heat source through the primary return.
In large networks with distributed pressure and temperature
parameters the primary flow and/or the differential pressure is
controlled by electric or self regulating control valves. This
ensures a constant inlet pressure differential regardless of the
actual pressure differential pressure in the network. Separate
control valves or valves with combined functionality in the
return or supply flow ensure this.
PRIMARY
SUPPLY
PRIMARY
RETURN
M
Fig. 132. Primary Side of Indirect
Heat Transfer Substation.
The heat exchanger (HEX) separates both circuits and
maintains the heat flow between primary and secondary side
(Fig. 133). Both plate or tube type heat exchangers are used in
either parallel or series configurations. The heat flow is
controlled by modulating the control valve position varying
the primary flow.
The secondary supply flow temperature setpoint is usually
set from the outdoor air temperature during the day and set
back during the night. A high limit safety device reduces the
supply flow temperature, if the return flow temperature exceeds
high limit setpoint.
In addition HEXs are used for domestic hot water either
directly or via a charge pump and hot water storage tanks.
HEAT
EXCHANGER
PRIMARY SUPPLY
PRIMARY RETURN
Fig. 133. Heat Exchanger Portion of
Indirect Heat Transfer Substation.
388
SECONDARY
SUPPLY
HEAT
EXCHANGER
SECONDARY
RETURN
∆P=CONSTANT
DIFFERENTIAL
M11440
PRESSURE
CONTROL
SECONDARY SUPPLY
SECONDARY RETURN
M11438