Honeywell AUTOMATIC CONTROL Engineering Manual page 242

AIR HANDLING SYSTEM CONTROL APPLICATIONS
FEATURES
1. The multiple inline coil system heats below-freezing air
with little danger of freezing the coils or overheating the
space.
2. The supply temperature is constant (or may be dictated
by other HVAC controls).
3. In this sequence, individual Coil 1 and 2 Valves are
opened fully just before freezing air enters to prevent
freezing.
4. The low temperature control (Item 2) is intended to
protect the coils from freezing due to the loss of heat.
Low temperature controls cannot be installed in the
leaving air of Coils 1 and 2 because leaving air
temperatures below freezing may be normal. If water flow
is lost through Coils 1 or 2, they could freeze without
detection by the low temperature control (Item 2).
Differential temperature (software) monitor points across
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
32°F and the differential temperature is less than 22
Fahrenheit degrees, and shut the system down if the
differential temperature is less than 18 Fahrenheit degrees.
If the hot water temperature is reset, or if the design OA
temperature is less than 0°F, 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 35°F 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.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
The heating Coil 1 valve shall open upon loss of actuator
motive force, shall close upon fan shutdown if the OA
temperature is above 35°F, and shall open upon fan shutdown
if the OA temperature is below 35°F. 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 40°F
to 35°F.
The valve to Coil No. 2 shall position from 0% to 100%
open as the air temperature leaving Coil 1 varies from 40°F to
35°F.
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
30°F to -10°F. 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 70°F.
3. Design OA temperature is –20°F.
4. Temperature rises through coils: Coil 1 = 30 Fahrenheit
degrees; Coil 2 = 30 Fahrenheit degrees; Coil 3 = as
required for the balance of the heating load.
5. OA temperature is 35°F DB.
OA 35°F DB
1
2
232
DISCHARGE OF COIL NO. 1
DISCHARGE OF COIL NO. 3
3
C2556-1