Trane HUV - ASHRAE Cycles

34 UV-PRC001-EN
Control Cycles-General
There are a variety of control sys-
tems available today in unit venti-
lators. The exact method of
controlling the amount of outside
air and heating element capacity
can vary. However, all systems
provide a sequence of operation
designed to provide rapid class-
room warm-up and increasing
amount of ventilation air to offset
classroom overheating.
Classroom air conditioners are
normally controlled according to
ASHRAE Cycle I, II or III and varia-
tions of these control cycles. The
Trane classroom unit ventilators
only utilize cycles I and II. See Fig-
ure 31 for cycle charts.
ASHRAE Cycle I
(Spring Return)
ASHRAE Cycle I admits 100% out-
door air at all times except during
a warm-up cycle. During warm-up,
the outside air damper is closed.
As room temperature approaches
the thermostat setting, the outside
air damper opens fully, and the
unit handles 100% outside air. Unit
capacity is then controlled by
modulating the heating element
capacity.
ASHRAE Cycle I can be used in
those areas where a large quantity
of outdoor air is required to offset
the air being exhausted relieving
the room of unpleasant odors and
particles.
ASHRAE Cycle II
(spring return)
ASHRAE Cycle II is the most wide-
ly used of the three types of series.
Similar to ASHRAE Cycle I, the
outside air damper is closed dur-
ing warm-up. But with Cycle II, the
unit handles recirculated air
through the return-air system. As
temperature approaches the com-
fort setting, the outside air damp-
er opens to admit a
predetermined minimum amount
of outside air. This minimum has
been established by local code re-
quirements and good engineering
practices per ARI 840 to provide
adequate ventilation. Unit capaci-
ty is controlled by varying the
heating element output. If room
temperature rises above the ther-
mostat setting, the heating ele-
ment is turned off and an
increasing amount of outside air is
admitted until only outside air is
being delivered.
This cycle may incorporate a min-
imum discharge air temperature
sensor that overrides the other
controls to maintain an acceptable
discharge temperature. When the
outside air temperature is very
cold, the minimum air tempera-
ture control modulates the
amount of outside air being deliv-
ered. This keeps the mixture tem-
perature delivered to the room at
55 F or 60 F.
ASHRAE Cycle II is a very econom-
ical control sequence since only
the minimum amount of outside
air is heated and free natural cool-
ing is available to offset the heat
gains in occupied classrooms.
Figure 31: Cycle charts
Controls
ASHRAE Cycles