REFRIGERANT LEVEL CONTROL
(REFER TO FIG. 51 & 52) The chiller can be provided with an optional Con dens er
Refrigerant Level Control. A Variable Orifi ce, lo cat ed
in the refrigerant liquid line between the Evap o ra tor and Condenser, is used to control the re frig er ant level in the Condenser. It is modulated by an Actuator that is driven by open and close output sig nals from Triacs on the I/O Board. These control sig nals orig i nate at the Microboard. Automatic or Manual lev el con trol is allowed. If Auto- matic control is se lect ed, the Pro gram modulates the Variable Orifi ce to maintain the Condenser refrigerant to a programmable Setpoint lev el. If Manual control is selected, the Variable Orifi ce can be manually controlled with the Keypad keys. This Manual control can also be used to place the Orifi ce in a fi xed position.
Since the Level Control feature is optional, the Pro gram operation described here must be ENABLED on those chillers so equipped and DISABLED on all oth er chill ers. This procedure, along with the pro gram ming of Setpoints described below, is performed on the Re frig er ant Level Control/Tuning Screen using in struc tions in the “System Calibration, Service Setpoints and Reset Procedures Programming Procedures” section of this book. A Liquid Level Sensor (LLS) detects the Refrigerant
Level in the condenser and outputs an analog voltage to
the Microboard that rep re sents this level. The level is expressed as a percentage and is displayed on the Con- denser Screen and the Refrigerant Level Control Screen as the “Re frig er ant Level Position = xxx%”. The Level Sensor is calibrated so that the re frig er ant level is dis- played as 0% when the level is at minimum; 100% when the lev el is at maximum. Levels between these extremes are linearly scaled. The lev el is at min i mum when the when the chiller is shutdown with the orifi ce in the fully open position. The level is at max i mum when the level is above the site glass, with the sensor fully covered. The Level Sensor calibration procedure is in the “System Calibration, Service Setpoints and Reset Procedures” section of this manual. There are different versions of the Level Sensors as shown in fi gure 50. The operation of the sensors is the same. However, the differences affect the way the sensors are calibrated. Some sensors are equipped with adjustable potentiometers, while others
as “Re frig er ant Lev el Setpoint = xxx%” on the Condenser Screen and the Refrigerant level Con trol Screen. This Setpoint is pro grammed by a Service Technician at chiller com mis sion ing using the Refrigerant Level Con- trol Screen. It is pro gram ma ble over the range of 20% to 80%.
AUTOMATIC OPERATION
While the chiller is shut down, an Open signal is ap plied to the Actuator, driving the Orifi ce to the fully open po si tion. This causes the Condenser refrigerant level to be ap prox i - mate ly 0%. Elevated Evaporator pres sure with respect to Condenser pressure could cause the lev el to be higher. When a chiller start is initiated, there are two different courses of action taken, depending on the Flash Memory Card version as follows:
If equipped with Flash Memory Card version C.MLM.01.06.xxx and earlier or “P” compressors with C.MLM.04.02.xxx and earlier: After the chiller is started, when the Vane Motor Switch (VMS) opens after entering
SYSTEM RUN, if the ac tu al level is greater than the
Level Setpoint, the Microboard begins controlling the level to the Level Setpoint. How ev er, if the actual level is less than the Level Setpoint, a linearly increasing ramp is applied to the Level Setpoint. This ramp causes the Setpoint to in crease from the ini tial refrigerant level to the pro grammed Level Setpoint over a period of 15 minutes. While this ramp is in ef fect, the ramp value is dis played as “REFRIGERANT LEVEL TAR GET = XX%” and replaces the Level Setpoint message on the Con dens er Screen. While the ramp is in effect, “RAMP UP TIME RE MAIN ING = XX MIN” is dis played. Af ter the 15 minute ramp period has elapsed, the re frig er ant level is con trolled to the pro grammed Level Setpoint.
If equipped with Flash Memory Card version C.MLM.01.07.xxx and later: Upon entering chiller Prelube, the Refrigerant Level close output is energized for the length of the programmable Valve Preset Time Setpoint (0 to 100 seconds; default 50). After pre-posi- tioning, the valve is held in this position until the fi rst 3 minutes of Chiller Run Time has elapsed (Setting the
14
been running 3 minutes, if the level is greater than or equal to the Refrigerant Level Setpoint, it begins con- trolling to the Setpoint. However, if it is less than the Refrigerant Level Setpoint, a linearly increasing ramp limit, called the Refrigerant Level Target, is applied to the Refrigerant Level Setpoint. This ramp limit allows the level to go from the present level to the Refrigrant Level Setpoint over a period of time programmed as the
Ramp-Up Time setpoint (3 to 15 minutes; default 8).
During this ramp-up period, the Refrigerant Level Target is used to control the refrigerant level in the condenser and the Refrigerant Level Setpoint message on the Con- denser Screen is replaced by “REFRIGERANT LEVEL TARGET = XX%”. While the ramp is in effect, “RAMP UP TIME REMAINING = XX MIN” is displayed. After the ramp period has elapsed, the refrigerant level is controlled by the Refrigerant Level Setpoint for the remainder of chiller run. If the Valve Preset Time is set to 0 seconds and the Ramp-Up Time Setpoint is set to 15 minutes, the control will operate the same as previous Software versions, as described above.
The Program applies an open or close signal, as re quired, from the Microboard to the actuator to main tain the level to the Level Setpoint. The duration of the signal de ter - mines the magnitude of change to the Or i fi ce po si tion. The duration of the signal and whether it is an open or close signal depends upon the Proportion Error and the Rate of Change of the actual level com pared to the Lev el Setpoint in a recurring period of time called a Lev el Control Pe-
riod. At the end of each Lev el Con trol Pe ri od, the Pro-
portion error and Rate of Change are com pared to control thresholds Pro por tion Limit Close, Proportion Lim it
Open and Rate Limit Close and Rate Lim it Open. The
result of this comparison determines the signal that will be applied to the actuator at the end of the Level Control Period as explained below. The setpoint values used by the program is determined by the Software vintage. In earlier versions, the values are programmable setpoints as shown below. In later versions, they are fi xed and programmable values applied in two different Zones, as determined by the error relationship of the actual refrigerant level and the Level Setpoint as shown below. Software version C.MLM.01.11.xxx (and earlier) or C.OPT.01.11.xxx (and earlier)
Level Control Period-3.5 to 30.0 seconds (default 3.5) Proportion Limit Open-10% to 50% (default 15) Proportion Limit Close-10% to 50% (default 45) Rate Limit Open-5% to 50% (default 10) Rate Limit Close-5% to 50% (default 10) Proportion Error Deadband - +3%
Software version C.MLM.01.14.xxx (and later) or C.OPT.01.14.306(and later)
In this version, some control thresholds are fi xed while others are programmable. This provides more stable control in certain operating conditions. The control thresholds are applied in two different zones, as de- termined by the error relationship between the actual refrigerant level and the Level Setpoint as shown below. Zone 1 parameters are used when the error is < 9%. Zone 2 parameters are used when the error is >9%. When transitioning from Zone 2 to Zone 1, the error must be < 9% for 60 seconds before the Zone 1 parameters are used. If the error is > 9%, the Zone 2 parameters are immediately implemented.
On the Refrigerant Level Control Screen, the Zone Control State status box displays which zone of control is being used: “Zone 1”, Zone 2”, “Zone Control Off” (if chiller shutdown). When transitioning from Zone 2 to Zone 1, “Zone 2 to Zone 1” is displayed and the “Zone Control Time Remaining” status box displays the amount of time remaining in the 60-second countdown timer.
The following are the control thresholds:
Zone 1 Zone 2
Proportion Limit Open (fi xed) 50% 52% Proportion Limit Close (fi xed) 45% 45% Rate Limit* (setpoint) 3%-15% 3%-15%
default 7% 5%
Period (setpoint) (seconds) 8-22 2.5-10
default 15 2.5
Zone 1 & Zone 2 Level Setpoint 20% to 80%; default 30% Valve Preset Time (setpoint) (seconds)
0 to 100; default 50 Ramp Up Time (setpoint) (minutes)
3 to 15; default 8 Proportion Error Deadband - +0%
Rate Error Deadband – +0%
*This RATE LIMIT setpoint sets the Rate Limit thresh- old for refrigerant levels both above (open) and below (close) the Refrigerant Level Setpoint.
The entire chiller run time is divided into Level Con trol Periods. They occur consecutively and con tin u ous ly. The fi rst one begins upon entering SYSTEM RUN and when it ends the next one begins, etc. This repeats until the chill er is shutdown. The duration of these periods are
At the completion of each Level Con trol Pe ri od, the ac- tu al level is com pared to the Lev el Setpoint. The re sult is the Pro por tion Error. The Pro por tion Error is com- pared to Setpoints Pro por tion Limit Open (if level is above setpoint) and Pro por tion Limit Close (if level is below setpoint). If the Pro por tion Error ex ceeds the Limit, the Pro por tion Error infl uence in the response will be large. If the Pro por tion Error is less than the Limit, the Proportion er ror in fl u ence in the re sponse is de ter mined by how close the Proportion Error is to the Limit; close yields larger infl uence, further yields smaller in fl u ence. To es tab lish the re sponse to the rate of change, the amount of change in the Level within the Lev el Control Period is com pared to the Rate Limit Close (if level less than setpoint) and Rate Limit Open (if level greater than setpoint). If the result ex ceeds the Setpoint, the rate in fl u ence in the response will be large; if less than the Setpoint, the rate infl uence is de ter mined by how close the result is to the Setpoint; close yields larg er infl uence, further yields small er in fl u ence.
Therefore, per the above, the values programmed for Proportion Limit Open/Close and Rate Limit Open/Close determine the sensitivity of the level control. Smaller val ues generally yield greater response for the same level change in the Level control Period. Also, the small er the value programmed for the Level Con trol Period, the more
often an output signal is applied to the Variable Orifi ce Actuator. The orifi ce valve movement is animated on the Re frig er ant Level Control Screen as follows: when 0-20% open, shown fully closed; 20 - 40% open, shown as 20% open; 40 - 60% open, shown as 40% open; 60 - 80% open, shown as 80% open; 80 - 100% open, shown as 100% open.
MANUAL OPERATION
The Orifi ce Actuator can be manually controlled from the Keypad using the Refrigerant Level Control Screen after logging in at SERVICE access level. Open, Close, Hold and Auto keys are used to control the Variable Or i fi ce. Using the Open, Close and Hold keys, the Vari able Ori- fi ce can be placed in a fi xed position. Pressing the Auto key returns Level Control to Automatic op er a tion. ACTUATORS
New production units use a Belimo actuator that op er ates from 24VAC. If the OptiView Control Center is retrofi t to an existing chiller, the chiller could be equipped with a Bar ber-Coleman actuator that op er ates from 115VAC. The interface for both actuators is shown in Fig. 52. The description of the operation of both ac tu a tors is in the “I/O Board” section of this book.
F/UP E/DN
MAX MIN
14
MICR O BO ARD I/O BO ARD J19 28 29 J8 13 3 14 J1 28 29 TB1 163 161 162 (REFER T O FIG. 16) CLOSE OPEN I/O BO ARD J1 28 29 TB1 163 161 162 (REFER T O FIG. 16) CLOSE OPEN LEVEL OUTPUT GND +12VDC 3 2 1 LIQUID LEVEL SENSOR
ORIFICE V A LV E A CTU A T OR BARBER-COLEMAN A CTU A T OR 115V A C FIELD L1 L2 2 3 X 5 115V A C IT 12 24 VAC ORIFICE V A LV E A CTU A T OR BELIMO A CTU A T OR CLOSE OPEN OPEN CLOSE 3 2 1 LD04092 REFRIGERANT LIQUID LEVEL CONTROL - INTERF ACE