Section 2 - Enthalpy Theory And Controllers
63-8594-02 Honeywell Economizers 28
Single Sensor Enthalpy Control
There are two enthalpy control strategies
available: single and differential enthalpy
control.
The single enthalpy control uses one enthalpy
sensor located in the outdoor air in any
orientation that exposes it to freely circulating
air and protects it from rain, snow and direct
sunlight. The enthalpy sensor replaces the dry
bulb high limit used in a standard economizer.
Instead of switching the mixed air control loop
from outdoor to return air at a preset outdoor
air dry bulb temperature, on a call for cooling
from the controller or commercial thermostat
the economizer logic module compares the
outdoor enthalpy to a preselected setpoint.
The value of the setpoint is illustrated on the
psychrometric chart on page 57 with curves
labeled as A, B, C or D. The setpoint selected
will vary based on climate, activities in the
controlled area and the type of mechanical
equipment used to provide cooling. An
installer can choose a more aggressive
setpoint A for more free cooling or a
conservative setpoint D for less free cooling.
Care needs to taken to select the correct
curve for comfort and to control the humidity to
prevent indoor air quality and other issues
caused by high humidity in a building. The
mixed air sensor, located in the area where
the return and outdoor air mix, maintains the
mixed air temperature between 50 and 56°F
(10.0 and 12.8°C).
When using the JADE
TM
the analog curves are changed to control to
dry bulb temperature, enthalpy and dew point
based on ES boundaries as shown on
modified psychrometric chart and table on
page 57.
JADE
TM
controls to a +/-1F differential versus
the 50-55F range of an analog economizer.
M23906A
SUPPLY
AIR TO
BUILDING
MIXED OR
SUPPLY AIR
SENSOR
FAN
COILS
W7220 ECONOMIZER
CONTROLLER
WITH DEMAND
CONTROL
VENTILATION
TB8220
COMMERCIAL
PRO
SINGLE SENSOR OR HIGH LIMIT ENTHALPY
COOLING AND HEATING CONTROLLED FROM COMMERCIAL
THERMOSTAT. ENTHALPY ECONOMIZER CONTROLLER
FUNCTIONS AS FIRST STAGE OF COOLING.
C7400S
ENTHALPY
SENSOR
C7400S ENTHALPY SENSORS ARE WIDELY USED WITH HONEYWELL
CONTROLLERS. IT COMBINES BOTH TEMPERATURE AND HUMIDITY
MEASUREMENTS INTO ONE DEVICE. TWO C7400A’S ARE NEEDED TO
PROVIDE DIFFERENTIAL ENTHALPY FOR ANALOG ECONOMIZERS AND
TWO C7400S SYLK-BUS SENSORS PROVIDE TEMPERATURE AND HUMIDITY
TO THE JADE FOR DIFFERENTIAL ENTHALPY.
OUTSIDE
AIR SENSOR
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