Hybrid Heat Transfer Substations - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
PRIMARY
SUPPLY
P=CONSTANT
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.
Control loops:
Circulating pumps provide the differential pressure between
secondary supply and return flow. In facilities where the load
varies over a wide range, variable speed pumps are used to
meet the pressure requirements and save electrical energy.
The absolute pressure in the secondary circuit must be kept
within the safety limits (upper limit to avoid pipe and pump
damage, lower limit to avoid water flashing to steam and
cavitation).
SECONDARY
SUPPLY
HEAT
EXCHANGER
SECONDARY
RETURN
DIFFERENTIAL
M11440
PRESSURE
CONTROL
SECONDARY SUPPLY
SECONDARY RETURN
M11438
In general three different configurations are used:
– Open systems using water column static pressure and
expansion tanks.
– Closed systems (Fig. 134) using pressurized air and
expansion tanks (static pressure holding system).
– Dynamic systems using a combination of pressure
holding pumps and vent-valves.
VENT VALVE
EXPANSION TANK
PRIMARY
SUPPLY
PRIMARY HEX
RETURN
Fig. 134. Feed Water System in Secondary of
Indirect Heat Transfer Substation.
Feed water facilities maintain the water level in a heating
system. Two ways of feeding water exist depending on the
requirements placed on the water quality.
The first method takes water from the primary circuit. This
water is of sufficient quality for use in the secondary circuit.
Pressure reducing, cooling, and safety (close off) equipment
may be necessary depending upon the pressure and temperature
level in each circuit.
The second method uses treated tap water. Use of untreated
water is not recommended as it can coat system with minerals
precipitated from the water and dissolved oxygen can corrode
the piping. This method can also be used as a back up to the
first method.

Hybrid Heat Transfer Substations

This combines direct heating with indirect domestic hot water
supply (DHW) as shown in Figure 135. Sizes range from small
substations to large heat exchanger substations supplying
several blocks of buildings.
As described before, there are usually three main parts to
the substation:
– Primary side including pressure/flow regulating
equipment.
– Heat exchanger or hot water storage tank (chargeable) for
DHW and heat flow regulating equipment for heating.
– Circulating pumps for secondary side (heating) and for
DHW.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
386
COMPRESSOR
FEED WATER
PUMP
WATER STORAGE
TANK
SECONDARY SUPPLY
SECONDARY RETURN
CIRCULATING PUMP
M11437