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During boiler operation, a three way blending valve shall
position to place the boiler flow in a recirculating mode until
the water entering the boiler exceeds a low limit value of 63 C,
at which time the blending valve shall modulate to maintain
the secondary water temperature between 43 C and 82 C as
the OA temperature varies from 13 C to –15 C.
The lag boiler shall be locked out from operation for 60 minutes
after the lead boiler starts. Thereafter, anytime one boiler control
valve is commanded full open by the secondary temperature
control loop for greater than 5 minutes and the secondary water
temperature is a temperature less than 2.5 kelvins below the
secondary water temperature setpoint, the "off" (lag ) boiler pump
shall start, and upon proving flow, the "off" boiler shall be enabled
to fire under its factory controls to maintain 82 C. The just-started
boiler blending valve shall be controlled by an entering water
63 C temperature low limit sensor and setpoint similar to the lead
boiler, and subsequently, in unison with the other boiler blending
valve to maintain the reset secondary hot water temperature.
Anytime both boilers are operating and their control valves
are less than 40% open to the secondary return line, the boiler
and pump that has run longest shall shut down.
MODULAR BOILERS
Modular boilers provide heat over a large range of loads and
avoid standby and other losses associated with operating large
boilers at small loads. Figure 50 shows a primary-secondary
piping arrangement where each modular boiler has its own
pump. The boiler pump is on when the boiler is on.
HOT AND CHILLED WATER DISTRIBUTION SYSTEMS CONTROL
INTRODUCTION
Hot and chilled water pumping, distribution, and control
systems have similar characteristics. A hot and/or chilled water
system distributes heating or cooling energy through a building.
The water is pumped from a boiler or chiller to coils or terminal
units. Effective control of this energy requires understanding
the control loops and related control valves and also an
understanding of the pressure/flow relationships between the
piping and pumping components of the system.
CLASSIFICATION OF WATER
DISTRIBUTION SYSTEMS
Water distribution systems used in buildings include:
— LTW. Low temperature water systems supply water at
temperatures up to 120 C and working pressures up to
1200 kPa. Although, most LTW boilers have a maximum
working pressure of 300 kPa.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
335
Boilers that are off have no flow and are allowed to cool.
Each boiler that is on operates at or near full capacity. Avoiding
intermittent operation prevents losses up the stack or to the
surrounding area when the boiler is off.
Normal control of modular boilers cycles one of the on-line
boilers to maintain water temperature in the supply main to
meet load requirements. The supply main control sensor cycles
the boilers in sequence. If the load increases beyond the capacity
of the boilers on-line, an additional boiler is started. The lead
(cycling) boiler can be rotated on a daily or weekly basis to
equalize wear among all boilers or when using digital control,
the program can start the boiler that has been off the longest.
HWS
HOT WATER
SUPPLY SENSOR
PRIMARY
PUMP
5
3
4
Fig. 50. Typical Primary-Secondary
Piping for Modular Boilers.
— MTW. Medium temperature water systems supply water
at temperatures between 120 C to 180 C with pressures
up to 1200 kPa. Maximum medium temperature boiler
temperature is 180 C.
— HTW. High temperature hot water systems supply water
at temperatures over 180 C, usually in the 200 C to
230 C range, with working pressures up to 2100 kPa.
— CHW. Chilled water systems supply water at temperatures
from 4.5 C to 13 C with pressures up to 860 kPa.
— DTW. Dual temperature water systems supply LTW
during the heating season and CHW during the cooling
season to the same terminal units.
TYPICAL WATER DISTRIBUTION SYSTEM
A typical system (Fig. 51) illustrates the principles of water
distribution in a system. The system consists of a heating or
cooling source, a pump, distribution piping, and valve controlled
coils. The pump provides force to push the water through the
system and valves control the flow through the individual coils.
The air separator removes entrapped air from the system.
HWR
HOT WATER
RETURN SENSOR
SECONDARY
PUMPS
MODULAR
1
2
BOILERS
C2906
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