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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
With both valves operating, as the total demand drops below
the capacity of the large valve for five minutes, the small valve
shall close. With the large valve operating, as the demand drops
below the capacity of the small valve for five minutes, the large
valve shall close and the small valve shall assume the load.
Demand Limiting:
A large valve Kv limiting parameter shall be provided and
set such that upon full demand, the valve Kv shall not exceed
the design Kv of 51.
All valve timings and parameters shall be field adjustable by
the owner.
THREE-WAY VALVE
The supply water temperature to the radiators (Fig. 96) is
reset by the controller modulating the three-way valve. The
controller modulates the three-way valve to maintain the reset
schedule in response to temperature changes at the outdoor air
sensor and the supply water sensor. Pump runs continuously
during the heating season. As water temperature changes, the
heat output changes linearly. This allows accurate changes in
heating plant capacity as a function of outdoor air temperature
or some other signal to match the load.
OUTDOOR
AIR
CONTROLLER
SENSOR
SUPPLY WATER
SENSOR
3-WAY
VALVE
BOILER
MINIMUM
BOILER
FLOW (BYPASS)
Fig. 96. Radiators with Scheduled Water
Temperature Using Three-Way Valve.
Coordinating Valve Selection and System Design
A prerequisite to good modulating control of water systems
is a coordinated design of the entire water system. All control
valves must be sized so that the system will deliver design flow
at full load and not generate uncontrollable conditions at
minimum load.
Control valve selection is based on pressure differentials at
the valve location, full load flow conditions, valve close-off,
and valve controllability at minimum load conditions.
Generally, for smooth modulating control, the no-flow
pressure differential should not exceed the full flow differential
by more than 50 percent.
Hot Water Control Method Selection
Supply water temperature control is suitable for controlling the
heat delivery from a heat exchanger or a secondary water pump.
Flow control is acceptable for controlling individual terminal
units such as convectors, fan coils, or induction units. An equal
percentage characteristic valve complements the water to air
heat exchanger characteristic of heat output versus flow. The
result is a relatively linear heat output as a function of stem
position (Fig. 97). The heat output is relatively constant,
providing optimized controllability of heat exchangers such as
converters, fan coils, and induction units.
VALVE POSITION
CL
Fig. 97. Control of a Water Heat Exchanger Using an
Equal Percentage Characteristic Valve.
CHILLED WATER DISTRIBUTION SYSTEMS
GENERAL
Chilled water systems for cooling of commercial buildings
usually provide water between 5 C and 10 C to finned coils in
room units or air handlers.
TO OTHER
The amount of water delivered to the cooling coils and/or
RADIATORS
produced by chillers is related to temperature difference ( t)
across the coil or chiller and energy (kilowatts of refrigeration)
C2917
by the equation:
Q =
Where:
Q = Volume flow rate (L/s)
h = Cooling rate
t = water temperature difference in kelvins
4.2 = a constant
EXAMPLE:
Given: A 3.52 kW chiller with a t of 6 kelvins.
Q =
ENGINEERING MANUAL OF AUTOMATIC CONTROL
364
VALVE POSITION
OP
CL
h
4.2 x
t W
3.52 kW
= 0.14 L/s
4.2 x 6
OP
C2918
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