Honeywell AUTOMATIC CONTROL Engineering Manual page 374

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
FROM
ADDITIONAL
PANELS
TO
ADDITIONAL
PANELS
IMMERSION
SENSOR
T2
PUMP
Fig. 94. Radiant Panel with Scheduled Water
Temperature Using Converter.
Table 5. Typical Hot Water Reset Schedule.
OA Temperature
70F
35F
0F
Before determining a digital converter control scheme, two
issues must be explored; the nature of the hot water load and
the steam source.
If the load is affected by the OA temperature, then an
appropriate reset schedule should be used. If the load has a
step change, such as going from a warm-up mode (no sun, no
people, no lights or internal heat load) in the early morning; to
the occupied mode (lights, people, office equipment, sun) at a
specific time, then the reset schedule should also shift from a
high temperature reset schedule hot water during the warm-up
mode, to a low temperature reset schedule as the occupied loads
come into effect.
If the heating load includes 100 percent OA preheat coils,
special care must be given to assuring high water flow rates
through the coil when the OA temperature is below freezing.
Preheat coil valve-position reset of hot water temperature could
be applied such that anytime the OA temperature is below
freezing, the hot water temperature setpoint can be lowered as
required to keep the most demanding preheat coil hot water
valve greater than 85 percent open, and raised incrementally
anytime any valve is greater than 95 percent open. This can be
accomplished via applying (incrementing) a multiplier of 0.75
to 1.25 to the OA reset schedule-derived setpoint. Setpoint
calculations should complement the objectives of the AHU
optimum start programs.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
RADIANT
PANEL
T3
OUTDOOR
AIR
SENSOR
CONTROLLER
STEAM
SUPPLY
CONVERTER
C2916
Water Temperature
80F
100F
120F
Another digital control function is steam valve demand
limiting. Water systems are usually balanced such that if all
water valves are full open, design flow is delivered to all load
coils (unless diversity is used). Steam systems are usually not
balanced. From size-to-size, control valve capacities typically
increase about 35 percent, for example 1-1/2 in. valve, Cv =
25; to 2 in. valve, Cv = 40. When steam valves are sized and
valve selections are rounded up to the next larger size, on the
average they are about 17 percent oversized (and worse case
more than 30 percent oversized). If a valve with a Cv of 40 is
furnished for a load requiring a Cv of 30 and the oversizing has
a negative impact on the steam source, software can "destroke"
the control valve. A destroked valve has the maximum stroke
reduced, such that as the load varies from 0 to 100 percent, the
oversized valve is positioned from 0 to 75 percent open.
Destroked valves may be specified to display their actual stroke
percent open or to display 0 to 100 percent open as the valve
varies from 0 to the destroked maximum position (destroking
is normally transparent to HVAC system operators).
When hot water converter loads are scheduled (shut down at
night and started in the morning), the morning start-up is usually
greater than 100 percent load every morning because the hot
water has cooled down such that the steam valve starts full
open. In mild weather, when the actual load may be only 15 to
20 percent, this momentary 100 percent plus start-up demand
causes boilers to surge up to full capacity (or in a multiple boiler
system to stage multiple boilers on) only to unload down to the
operating load of 15 to 20 percent. Digital systems can be
programmed such that as the OA temperature varies from 60F
to 15F, the maximum start-up steam valve positions vary from
20 to 100 percent (this start-up destroking limit could be
removed at 0900).
Common practice on large converters provides two parallel
piped steam control valves (usually sized for 1/3 and 2/3 capacity)
such that the small valve handles light loads (preventing a single
large valve from throttling down to the point where the valve
plug approaches the valve seat where noise and seat erosion occur)
and the large valve is sequenced in after the small valve is full
open. At the point where the 2/3 sized valve starts opening, the
same noise and erosion is possible.
With digital systems, the valves may be staged (rather than
sequenced) such that as the load varies 0 to 33 percent, the
small valve modulates from 0 to full open, and as the load varies
33 to 66 percent, the small valve closes and large valve
modulates from 50 percent to full open, and as the load varies
66 to 100 percent, both valves modulate in unison from 2/3 to
full open.
The following converter control example is for a dual valve
converter with demand limiting and a warm-up shifted
temperature reset schedule.
364