Hybrid Heat Transfer Substations - Honeywell AUTOMATIC CONTROL Engineering Manual

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).
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
CIRCULATING PUMP
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.
PRIMARY SUPPLY
PRESSURE
CONTROL
PRIMARY RETURN
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
COMPRESSOR
FEED WATER
PUMP
WATER STORAGE
TANK
SECONDARY SUPPLY
SECONDARY RETURN
M11437
∆P=CONSTANT
HEAT
EXCHANGER
(DHW)
Fig. 135. Hybrid Heat Transfer Substation.
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.
The primary side includes shut off valves, differential
pressure, and flow control equipment provide safety functions
and a required differential pressure or flow. Heat metering is
also included. The water flow here is divided into two parts.
The HEX transfers the heat from the primary system to the
DHW loop. A valve placed in series with the HEX controls the
flow. The other part of the primary flow enters the secondary
circuit and transfers the heat to the consumer directly. The
supply flow temperature is controlled by one three-way valve,
two two-way valves, or one jet pump making an additional
circulation pump redundant.
In cases without a jet pump, circulation pumps provide the
needed differential pressure.
389 ENGINEERING MANUAL OF AUTOMATIC CONTROL
DHW SUPPLY
DHW RECIRCULATED/
COLD WATER
HEATING SUPPLY
HEATING RETURN
M11433