ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
232
Coils 1 and 2 could be added to detect the loss of heating.
If a Building Management System is included, the
monitor could be two-stage to send an alarm warning
message if the coil entering air temperature is less than
0°C and the differential temperature is less than 12
kelvins, and shut the system down if the differential
temperature is less than 10 kelvins. If the hot water
temperature is reset, or if the design OA temperature is
less than –18°C, the two differential temperature alarm
values should increase as the coil entering air temperature
decreases. Temperature sensors in the coils leaving water
may also detect freezing danger. The numbers “22” and
“18” are arbitrary and must be carefully determined if
protection is to be achieved.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Heating Coils 1 and 2 should provide approximately the
same temperature rise and keep the air entering Coil 3
above freezing. Do not oversized any of the coils . The
rise for Coils 1 & 2 (which are full open at 1.5°C entering
air temperature) should be selected to produce no more
than the maximum leaving air temperature that can be
tolerated.
2. Coil 3 should be sized for the balance of the heating load.
3. Follow good piping practices to avoid freezing steam
coils.
4. On large systems, fan start signal should open the OA
damper, and a damper end switch should start the fan (to
prevent the ductwork from collapsing before the damper
can open).
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, heating control shall be enabled
and the OA dampers shall open for ventilation requirements.
The heating Coil 1 valve shall open upon loss of actuator
motive force, shall close upon fan shutdown if the OA
temperature is above 1.5°C, and shall open upon fan shutdown
if the OA temperature is below 1.5°C. Heating Coils 2 and 3
valves shall open upon loss of actuator motive force and shall
close upon fan shutdown.
The Heating Coil Valve 3 shall be modulated by a PID control
loop to maintain the leaving air temperature setpoint.
The valve to Coil No. 1 (nearest entering air) shall position
from 0% to 100% open as the OA temperature varies from
4°C to 1.5°C.
The valve to Coil No. 2 shall position from 0% to 100% open
as the air temperature leaving Coil 1 varies from 4°C to 1.5°C.
OPTIONAL: Anytime the Coil 1 valve is full open and the
Coil 1 air differential temperature is less than a
Value V1, an alarm message shall be issued.
Anytime the Coil 1 valve is full open and the
Coil 1 air differential temperature is less than a
Value V2, the fan system shall shut down until
manually reset. The Value V1 shall vary from
20 to 24 as the OA temperature varies from –
1°C to -23°C. Value V2 shall equal V1 minus
4. A similar monitor and alarm function shall
be provided for Coil 2.
PSYCHROMETRIC ASPECTS
This application is of greatest use in a 100 percent OA system.
The SA conditions can lie anywhere along the line of the desired
DA dry-bulb temperature.
In the following chart it is assumed that:
1. The system handles 100 percent OA.
2. The supply air PID control loop is set at 21°C.
3. Design OA temperature is –29°C.
4. Temperature rises through coils: Coil 1 = 16 kelvins; Coil
2 = 16 kelvins; Coil 3 = as required for the balance of the
heating load.
5. OA temperature is 1.5°C DB.
The following results are obtained:
Item
No. Explanation
1 Coil No. 1 is providing 100 percent capacity
raising the entering air from 1.5°C to 18°C.
2 Coil No. 2 valve is closed.
3 Coil No. 3 is modulating and provides a 2.5
kelvins temperature rise to the desired supply
temperature.
C3247
DISCHARGE OF COIL NO. 1
DISCHARGE OF COIL NO. 3
1
3
2
OA 1.5°C DB
To Next Page
Loading...
Need help?
General
