Low Temperature Protection For Steam Coils; High Temperature Water Heating System Control; Introduction; High Temperature Water (Htw) Heating - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
EQUALIZER
AUTOMATIC
CONTROL
VALVE
STEAM
MAIN
STEAM
COIL
F & T
TRAP
RETURN
MAIN
Fig. 116. Steam Supplied Air Heating Coil.
High vacuum systems are an exception to these considerations
since they lower the steam temperature and pressure as the
heating load decreases. Vacuum systems are adaptable to
automatic control valves, since usual practice is to maintain a
controlled difference between supply and return main pressures
while varying supply main pressure with heating load.

HIGH TEMPERATURE WATER HEATING SYSTEM CONTROL

INTRODUCTION

HIGH TEMPERATURE WATER (HTW) HEATING

High temperature water systems operate with supply water
temperatures of 250 to 450F and pressures from 55 to 450 psig.
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 350F HTW (120 psig) will produce 15 psig
steam and using 380 to 410F HTW (200 to 275 psig) will
produce 100 psig steam allowing a HTW temperature drop of
50 to 60F.
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.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
LINE
CHECK
VALVE
C2927
LOW TEMPERATURE PROTECTION FOR
STEAM COILS
Any steam coil exposed to outdoor air is in danger of freeze
up in cold climates. A coil begins to freeze in the 30 to 32F
temperature range. Steam coils designed for heating cold air
contain internal distributing tubes to ensure that steam reaches
all parts of the coil as the control valve modulates.
Another approach to freeze-up control is to design coils with
dampers so that the control valve does not modulate but remains
open when air entering the coil is below freezing. If too much
heat is being delivered, face and bypass dampers control the airflow
across the coil. Above freezing, the valve can be modulated.
In all cases, a low limit temperature controller, which
responds to the coldest portion of the capillary sensing element,
should be part of the design. For addition examples of control
with freezing air conditions entering a coil see Air Handling
Systems Control Applications section.
— 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.
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