Section 2 - Enthalpy Theory And Controllers
27 Honeywell Economizers 63-8594-02
Psychrometric Chart of
Enthalpy Economizer Control
A standard dry bulb economizer discussed
earlier causes the air handler to switch over
from outside air to return air at the setpoint of
the outside air high limit. This will vary based
on the climate. According to ASHRAE 90.1,
the changeover temperatures for the US are
65F, 70F and 75F based on the region of the
country. Dry bulb economizers only control the
outside air dampers based on temperature. If
it is a cool but rainy day, the outside air will be
brought in and extra cooling capacity will be
required to dehumidify it or the humidity will be
released into the ducts or occupied space.
Enthalpy economizers take temperature and
humidity into account. With enthalpy control,
humid air below a conventional dry bulb
temperature setpoint is locked out. Cooling
costs are lowered in most climates when using
enthalpy instead of dry bulb temperature with
the economizer.
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