McQuay AGS 225D, Operating Manual

Page: 6 / 82

6 OM AGSD-1
Definitions
Active Setpoint
The active setpoint is the setting in effect at any given moment. This variation occurs on
setpoints that can be altered during normal operation. Resetting the chilled water leaving
temperature setpoint by one of several methods, such as return water temperature, is an
example.
Active Capacity Limit
The active setpoint is the setting in effect at any given moment. Any one of several external
inputs can limit a compressor’s capacity below its maximum value.
Condenser Saturated Temperature Target
The saturated condenser temperature target is calculated by first using the following
equation:
Sat condenser temp target raw = 0.833(evaporator sat temp) + 68.34
The “raw” value is the initial calculated value. This value is then limited to a range defined
by the Condenser Saturated Temperature Target minimum and maximum setpoints. These
setpoints simply cut off the value to a working range, and this range can be limited to a
single value if the two setpoints are set to the same value.
Dead Band
The dead band is a range of values surrounding a setpoint such that a change in the variable
occurring within the dead band range causes no action from the controller. For example, if a
temperature setpoint is 44
°
F and it has a dead band of
±
2 degrees F, nothing will happen
until the measured temperature is less than 42
°
F or more than 46
°
F.
DIN
Digital input, usually followed by a number designating the number of the input.
Discharge Superheat
Discharge superheat is calculated for each circuit using the following equation:
Discharge Superheat = Discharge Temperature – Condenser Saturated Temperature
Error
In the context of this manual, “Error” is the difference between the actual value of a variable
and the target setting or setpoint.
Evaporator Approach
The evaporator approach is calculated for each circuit. The equation is as follows:
Evaporator Approach = LWT – Evaporator Saturated Temperature
See page 38 for more details
Evap Recirc Timer
A timing function, with a 30-second default, that holds off any reading of chilled water for
the duration of the timing setting. This delay allows the chilled water sensors (especially
water temperatures) to take a more accurate reading of the chilled water system conditions.
EXV
Electronic expansion valve, used to control the flow of refrigerant to the evaporator,
controlled by the circuit microprocessor.