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The optimal temperature/pressure ratio depends on the length
and structure of the network, actual load, outdoor air
temperature, and pipe insulation. Because of the many
variables, a combination of both Outdoor Air Temperature
Control and Variable Flow Control or Variable Temperature
Control is often used.
STEAM SYSTEM VS HOT WATER SYSTEM
Steam networks differ mainly in the following points from
hot water systems:
–
No pumps are required, the pressure difference between
boiler and consumer drives the movement of the steam.
–
Condensate traps are required approximately every
500 meters.
–
The return line diameter is much smaller because
condensate takes up less space than steam.
–
Heat losses are significantly higher than hot water systems.
–
The heat storage capacity of steam is lower than hot water.
–
Maintenance costs are higher than hot water.
HOT WATER PIPELINE SYSTEM
Hot water systems must be protected against high pressure
peaks which could damage the pumps and pressure drops below
the evaporation point which results in the water changing to
steam. Common types of preinsulated pipelines must be
protected against temperatures exceeding 130 C.
Factors affecting efficiency include optimal temperature/
pressure ratio with respect to the length and capacity of the
network, temperature differences between supply and return
flow, heat and water losses, as well as friction between the
water and pipe wall. Higher temperatures cause greater heat
loss by radiation and conduction while greater differential
pressure in the network produces heavier pump loads. Every
network has a different optimum value for the supply and return
pipelines. Use of friction reducing chemicals to decrease the
friction losses in the pipeline can reduce the pump power
required. Extracting as much heat at the substations as possible
also reduces pumping costs.
Additionally the water in the entire district heating system
pipelines also serves as a large heat accumulator and helps
compensate for peak loads or short periods of low heat generation.
BOOSTER PUMP STATION
In large pipeline systems, using a single main pump requires a
large differential pressure to overcome the friction in the network.
Figure 127 shows a profile of a single pump system. Decentralized
pumps (booster pump stations) avoid this and keep the pressure
in every pipeline section within the required levels.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
383
PRESSURE
MAXIMUM
SAFETY
LIMIT
SUPPLY
FLOW
RETURN
FLOW
MINIMUM
SAFETY
LIMIT
HEAT SOURCE
Fig. 127. Network Pressure Profile.
A booster pump station overcomes the pressure drop in areas
with considerable differences in altitude (to overcome 50m
difference in altitude a pressure of 5 bar (500 kPa) is required).
They are often applied where going under or over obstacles is
necessary. Equip the pipeline with an emergency shut down system
(ESD) in areas with considerable differences in altitude to protect
the system from high pressures in case of power failure.
PRESSURE REDUCING STATIONS
A pressure reducing station is the counterpart to the booster
pump station. A pressure reducing station is used in lines
located in mountainous areas to protect the pipeline from over
pressure and to keep the pressure in the return line lower than
the supply line. For this application pressure reducing valves
are control valves.
MIXING STATION
A mixing station (Fig. 128) is used in hot water networks. It is
a variable speed (mixing) pump which mixes cooled return flow
directly into the supply flow to reduce the supply flow line
temperature to the required level. These facilities are used to
provide different maximum temperatures in the network pipeline.
SUPPLY FLOW (HIGH TEMP)
RETURN FLOW
Fig. 128. Principle Of A Mixing Station.
MINIMUM
PERMISSIBLE
DIFFERENTIAL
PRESSURE
DISTANCE
CONSUMER
M11443
SUPPLY FLOW (LOW TEMP)
MIXING PUMP
M11444
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