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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
HIGH TEMPERATURE WATER HEATING SYSTEM CONTROL
INTRODUCTION
HIGH TEMPERATURE WATER (HTW) HEATING
High temperature water systems operate with supply water
temperatures of 120 to 230 C and pressures from 500 to
3000 kPa.
HTW is typically generated by boilers; however, experimental
systems utilizing geothermal or solar energy have been built.
First costs are similar for steam and high temperature water
systems, however, maintenance and operating costs are
generally lower for HTW systems. The use of the same boiler
for both HTW and steam generation is not recommended
because feed water requirements for steam eliminate some of
the advantages of HTW.
When relatively small amounts of steam are required, steam
can be produced from HTW at the required location. A steam
generator using 175 C HTW (900 kPa) will produce 200 kPa
steam and using 190 to 210 C HTW (1300 to 1900 kPa) will
produce 800 kPa steam allowing a HTW temperature drop of
30 to 35 kelvins.
A HTW system offers several advantages over a steam system:
— Boiler sizes can be smaller than low pressure boilers
because of the high heat capacity in a HTW system.
— Diameter of distribution piping can be reduced.
— The piping system requires no grading for return of
water to boiler.
PRESSURE CONTROL
EXPANSION
TANK
BOILER
NITROGEN BOTTLES
SYSTEM
CIRCULATING
PUMP
Fig. 117 Typical Nitrogen Pressurized High Temperature Water System.
MANUAL AIR VENTS
AT HIGH POINTS OF
SUPPLY AND RETURN
EXPANSION
PIPING FOLLOWS
CONTOURS OF
LAND.
ALL DISTRIBUTION
PIPING IS WELDED.
CHECK
MAKE-UP
VALVE
FEED PUMP
— Feedwater requirements are minimal, eliminating
treatment costs and introduction of air which is a source
of corrosion. HTW systems tend to remain clean.
— Steam traps and condensate receivers, sources of heat
loss and maintenance costs are eliminated.
— Heat not used in terminal heat transfer units is returned
to the HTW generator.
Several major design characteristics are typical of HTW
systems:
1. The HTW boiler is controlled by pressure rather than
temperature to eliminate flashing if heating load fluctuates.
2. Multiple boiler systems must be designed so that loads
are divided between the boilers. Generally it is less costly
to operate two boilers at part load than one at full load.
3. HTW systems can be pressurized by steam or air in the
expansion tank but typically an inert gas such as nitrogen
is used because it absorbs no heat energy and excludes
oxygen.
4. All piping is welded except at mechanical equipment
which must be maintained. Connections at equipment are
flanged, including provision for removal of small threaded
control valves.
5. Terminal units are rated for the high temperature and
pressure.
Figure 117 illustrates the elements of a typical HTW system.
UNIT
HEATERS
LOOPS
INSTANTANEOUS
CONVERTER
STORAGE
CONVERTER
STEAM GENERATOR
ENGINEERING MANUAL OF AUTOMATIC CONTROL
374
OTHER HEATING COILS
LOAD
DOMESTIC HOT WATER
FOR BUILDING
STEAM FOR PROCESS
OR AIR CONDITIONING
CONDENSATE
REVERSE
RETURN
C2581
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