Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
358
Where:
∆
T = Low flow water temperature rise across the
pump
The minimum flow for a 7.46 kW pump with a 5 kelvins
∆
T
(attributed to the pump heating the water) is only 0.37 L/s. The
bypass may be fixed or if there is a remote AHU in the 0.75 to
1.52 L/s size, using a three-way valve on that AHU with the
bypass in the bypass leg of the three-way valve prevents bypass
water from flowing during full-load periods. Another option is
an automatic bypass valve programmed to open anytime all
AHU control valves are less than 10% open.
Decoupled Variable Speed Pump Control,
Reverse Return
In these examples, the design differential pressure for all
AHUs is 48 kPa with reverse return, since the balancing-valve
drop becomes negligible. If approximately 54 kPa is maintained
at AHU 1, design flow is available to all AHUs during any
non-symmetrical flow condition. With a reverse return system
locate the differential pressure sensor across AHU 1 with a
setpoint of 54 kPa. Advantages of valve position load reset are
much less with reverse return systems, however, resetting the
setpoint from 54 kPa to 24 kPa or 30 kPa based upon load is
worthwhile on large systems.
HOT WATER DISTRIBUTION SYSTEMS
GENERAL
Advantages of hot water compared to steam for heat
distribution within commercial buildings are:
— Water temperature can be varied to suit variable loads.
— Hot water boilers typically do not require a licensed
operating engineer.
— Heat loss from piping is less than steam systems because
fluid temperatures are usually lower.
— Temperature control is easier and more stable because
water temperature is matched to the load.
— Fewer piping accessories to maintain than steam systems.
— Reduced air stratification at light loads with reset hot water.
The control objectives for a hot water distribution system
providing space heating are:
1. Provide adequate hot water flow to all heating units.
2. Maintain a stable pressure difference between supply and
return mains at all load conditions.
3. Match hot water temperature to the heating load based
on outdoor air temperature and/or occupancy.
a. Avoid heat exchanger flow velocities dropping into
the laminar range.
b. Keep water velocities up to prevent freezing.
c. Reduce mixed air temperature stratification.
– No stratified cold air to trip low temperature
controllers or cause freeze-up.
– Minimizes stratified air from having adverse
effects on coil discharge temperature sensing.
– Minimizes possibilities of hot air flowing out
some duct outlets and cold air flowing out others.
Refer to METHODS OF CONTROLLING DISTRIBUTION
SYSTEMS for additional information on control objectives.
HOT WATER CONVERTERS
A hot water converter uses either hot water or steam to raise
the temperature of heating system water to a temperature which
satisfies the heating requirements of the system. The most
widely used hot water converters use steam.
Steam is often supplied to remote buildings or mechanical
rooms from a central plant. Within the buildings or zones, hot
water is often preferred because of its ease of control. Steam to
water converters are used for these applications.
A converter consists of a shell and tubes. The water to be
heated passes through the tubes. Steam or hot water circulates
in the shell around the tubes. Heat transfers from the steam or
hot water in the shell to the heating system water in the tubes.
If the pressure on the water in a converter drops below the
vaporization pressure for the temperature of the water, the water
can flash to steam. Pumps are usually located on the return
side of the converter to provide a positive pressure and
minimize flashing.
Figures 83 and 84 show commonly used controls for hot water
converters. See HOT WATER RESET for converter control.
Minimum flow (L/s) =
kW
4.2 x
∆
T