ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
363
Fig. 90. Effect of Flow Control on Finned
Tube Radiator Heat Output.
Supply Hot Water Temperature Control
With Flow Control
Combining flow control with supply water temperature reset
from outdoor air temperature or any other measurement of load
results in effective control.
Figure 93 shows output of a typical air heating coil with flow
and supply water temperature control. When 110F water is
supplied during light load conditions, the maximum air
temperature rise through the coil is only 10F. With the proper
water temperature reset schedule, the control valve stays in the
30 to 100 percent open range where flow control output is linear.
Thus, radical changes from desired output do not occur. This is
true for radiators, finned tubes, and reheat coils.
75% FLOW REDUCTION
FLOW, GPM
HEAT OUTPUT %
100
75
50
25
0
10 20 30 40 50
RATED OUTPUT
C2404-1
130
120
110
100
90
80
70
60
50
40
30
20
10
0
50
45
40
35
30
25
20
15
10
0
0102030405060708090100
PERFORMANCE CURVE FOR HOT WATER COIL
200°F ENTERING WATER
80°F ENTERING AIR
HEAT
OUTPUT
TEMP
DROP OF
WATER
TEMP DROP OF WATER (DEGREES F)
1000 BTU/HOUR (MBH)
PERCENT FLOW
C2408-1
Fig. 91. Effect of Flow Control on Hot
Water Coil Heat Output.
Supply Hot Water Temperature Control
If supply water temperature is varied (reset) in response to a
change in heating load, heat output varies almost linearly (Fig.
92). This mode of control appears ideal, except that it is
impractical to provide a different hot water temperature to each
heating coil or piece of radiation in a building. Varying supply
water temperature as a function of outdoor temperature provides
a good compromise.
Fig. 92. Supply Water Temperature vs
Heat Output at Constant Flow.
HOT WATER
UNIT HEATER
RADIANT
PANEL
CONVECTORS
AND
RADIATORS
100
90
80
70
60
50
40
30
20
10
0
80 100 150 200
SUPPLY WATER TEMPERATURE °F
HEAT OUTPUT, PERCENT
C2405
Fig. 93. Heat Output of Hot Water Coil
at Two Supply Temperatures.
Hot Water Reset
As previously discussed, reset of hot water supply
temperature is the most effective method of controlling Btu
output of a water supplied heating coil, panel, radiator, or
convector. Btu output of most water supplied heating is
relatively linear with respect to supply water temperature. Water
temperature reset can be provided by a steam to hot water
converter, a three-way valve, or boiler water temperature reset.
Reset of hot water using a steam to hot water converter is
discussed in this section.
HOT WATER CONVERTER
The supply water temperature to the radiant panel (Fig. 94)
is reset by the controller modulating a valve in the converter
steam supply. The controller uses the temperatures at the outdoor
air sensor and the supply water sensor to control the steam
supply valve. The valve is positioned to maintain the converter
discharge water temperature as determined by the reset schedule
and outdoor air temperature. A typical reset schedule is shown
in Table 5.
100
90
80
70
60
50
40
30
20
10
0
012 3456789
30
25
20
15
10
5
0
LINEAR RANGE
OUTPUT
110°F SUPPLY
WATER
OUTPUT
200°F WATER
FLOW-GPM
% OF RATED HEAT OUTPUT
AIR TEMP RISE THRU COIL °F
C2406
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