Economizer Cycle; Chilled Water Temperature; Controller (6U11) - Trane RAUJ-C20 Installation, Operation And Maintenance Manual

O O p p e e r r a a t t i i n n g g P P r r i i n n c c i i p p l l e e s s

Economizer Cycle

The economizer is only allowed to function if the
ambient conditions are below the setpoint of the
enthalpy switch.
If the ambient air conditions are above the enthalpy
setpoint, the W7100A will open the Fresh Air dampers
to the minimum setpoint position.
To take full advantage of the "free cooling" provided
by the economizer, the W7100A "resets" the discharge
air setpoint. The amount of "reset" between the actual
discharge air setpoint and the economizer control point
is equal to 1/2 of the W7100's control band setpoint.
E E x x a a m m p p l l e e : : With a typical control band setting of 6°F, the
amount of discharge air "reset" is 3ºF (1/2 of the
control band setpoint). Therefore, if the discharge air
setpoint is 55°F, the economizer control point is 52°F (i.
e., 55°F - 3°F).
A second economizer "algorithm" within the W7100A
is the response time of the controller. The greater the
amount of deviation between the discharge air
temperature and the economizer control point, i.e., as
the temperature strays further from the control point,
the response time becomes faster; and, as the
discharge air temperature approaches the control
point, the response time becomes slower.
When the discharge air temperature is within the
"Deadband" (± 1.5°F of the economizer control point);
the W7100A maintains the economizer's present
position.
Figure 65. W7100A staging sequence
Adjustable Control
Band set at 6°F
Chilled Water Temperature Controller
(6U11)
The chilled water temperature controller used with EVP
chiller applications is a Honeywell W7100G. This
microprocessor controller is designed to maintain an
average leaving water temperature using an
integrating control band concept that matches the
required operating capacity to the chiller load. The
integral action, unlike "proportional only" type
90
58°F
D/A 55°F
Setpoint
52°F
When the economizer cannot handle the cooling
requirement or when the outdoor ambient conditions
are unsuitable for "economizing", the W7100A
activates the unit's mechanical cooling section.
N N o o t t e e : : As long as ambient conditions are suitable for
economizing, the economizer works in
conjunction with the mechanical cooling
operation.
The control algorithm used by the W7100A to add
stages of cooling is illustrated in the figure below.
When the discharge air temperature drifts above the
setpoint, "Region 1", a stage of mechanical cooling is
added based on time and the amount of deviation from
setpoint. If the discharge air temperature remains
above the setpoint, the W7100A energizes additional
stages of mechanical cooling.
If the operating cooling stage is capable of satisfying
the cooling requirement, as the discharge air
temperature falls below the setpoint for a sufficient
period of time, the W7100A turns the stages of
mechanical cooling "Off", "Region 3".
The W7100A determines the length of the time before
stages of mechanical cooling are turned "On" and
"Off". When the system is operating within the control
band, the delay is longest at setpoint, and decreases to
a minimum of 4 minutes when the discharge air
temperature exceeds the upper or lower limit of the
control band. See figure below.
Region 2
Region 1
Region 3
controllers, minimizes the amount of offset from the
control setpoint.
The control band setting is centered on the leaving
water setpoint. It is adjustable from 0°F to 10°F [0ºC to
6°C] and is used to stabilize system operation. The
control algorithm used by the W7100G to add stages of
cooling is illustrated in the figure below. As the water
temperature rises above the upper control band limit, a
stage of mechanical cooling is added, provided the
Minimum On/Off
Delay between
Stages = 4 minutes
Minimum On/Off
Delay between
Stages > 4 minutes
Minimum On/Off
Delay between
Stages > 4 minutes
Minimum On/Off
Delay between
Stages = 4 minutes
SS-SVX11K-EN