Trane RAUC-IOM-15, Installation & Maintenance Instructions Manual

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Sequence of Operation
Chilled Water Temperature Controller (6U11)
The chilled water temperature controller used with EVP
chiller applications is a Honeywell W7100G. This micropro-
cessor controller is designed to maintain an average leav-
ing 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 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
o
F to 10
o
F [0
o
C to 6
o
C] and is
used to stabilize system operation.
The control algorithm used by the W7100G to add stages of
cooling is illustrated in Figure 5-2. As the water temperature
rises above the upper control band limit, a stage of me-
chanical cooling is added, provided the minimum “Off” time
has been satisfied (Point A). The minimum “fast response”
time and the time delay between staging for the W7100G is
set for 60 seconds.
If the water temperature remains above the upper control
band limit (Point B), the next available stage of cooling will
be energized when the minimum time delay between stages
has elapsed.
As the water temperature decreases below the lower con-
trol band, the last stage that was turned “On” will be cycled
“Off” (Point C) when the minimum “On” time for that stage
has elapsed.
As the load on the water increases due to cooling stages
being cycled “Off”, the controller will maintain it’s current po-
sition, i.e., no staging of cooling “On” or “Off”, as long as the
temperature remains inside the control band.
When the temperature increases above the upper control
band limit (Point D), mechanical cooling stages will be se-
quenced “On” in the same manner as before. As a rule, any
time the water temperature is above the upper control band
limit, a stage of cooling will be “added” and anytime the wa-
ter temperature decreases below the lower control band
limit, a stage of cooling will be “Subtracted”.
Sequence of Operation
Thermostatic Expansion Valve
The reliability and performance of the refrigeration system
is heavily dependent upon proper expansion valve adjust-
ment. Therefore, the importance of maintaining the proper
superheat cannot be over emphasized. Accurate measure-
ments of superheat will provide the following information.
1. How well the expansion valve is controlling the refriger-
ant flow.
2. The efficiency of the evaporator coil.
3. The amount of protection the compressor is receiving
against flooding or overheating.
The recommended range for superheat is 10 to 16 degrees
at the evaporator. Systems operating with less than 10 de-
grees of superheat:
a. Could cause serious compressor damage due to
refrigerant floodback.
b. Removes working surface from the evaporator
normally used for heat transfer.
Systems operating with superheat in excess of 16 degrees:
a. Could cause excessive compressor cycling on
internal winding thermostat which leads to
compressor motor failure.
b. Lowers the efficiency of the evaporator by reducing
the heat transfer capability.
The outdoor ambient temperature must be between 65
o
F
and 105 o F and the relative humidity of the air entering the
evaporator must be above 40 percent. When the tempera-
tures are outside of these ranges, measuring the operating
pressures can be meaningless.
Figure 5-2
W7100G Staging Sequence
System Start-Up (Continued)