How it Works
McQuay
Loading...
ALS125B
Installation Manual
92 pgs2.71 Mb0

McQuay ALS125B Installation Manual

Operating Manual OM ALSMICRO
Group: Chiller Part Number: 573865Y Effective: March 2000
Supersedes: IM 549-1
MicroTech Control System for Air-Cooled Screw Compressors
Models: ALS 125B through 425B
© 1999 McQuay International
Table of Contents
Introduction.....................................3
General Description........................4
Control Panel Features...................4
Software Identification....................5
Controller Layout............................5
Component Data.............................6
Sensors and Transducers................8
Standard Sensors..........................................8
Optional Sensor Packages...........................8
Thermistor Sensors.......................................8
Pressure Transducers.................................10
Liquid Presence Sensor ..............................10
Sensor Data...................................11
Sensor Locations........................................11
Digital Inputs...............................................14
Optional Analog Outputs ..........................15
Digital Outputs............................................16
Installation.....................................18
Forced EXV Position Change...................25
EXV Evaporator Pressure Control...........25
Chilled Water Reset Options........26
Condenser Fan Control.................28
Condenser Fan Staging.............................28
Head Pressure Control ...............................28
Lift Pressure Dead Band............................29
Condenser Fan Stage Up ..........................29
High Pressure Stage Up ............................30
Condenser Fan Stage Down.....................30
SpeedTrol Logic .........................................30
Pumpdown......................................31
Safety Systems..............................32
ALS Unit – MicroTech Control Alarms...32
MicroTech Controller Test
Procedures.....................................36
Keypad/Display.............................37
MicroTech Component Test
Procedures.....................................38
Unit Sequence of Operation..........20
Compressor Control......................21
Compressor Staging Control
Sequence........................................22
Four Compressor Unit................................22
Three Compressors Unit............................22
Two Compressors Unit...............................22
Project-Ahead Calculation.........................23
Interstage Timer..........................................23
Anti-Cycle Timer.........................................23
Lead-Lag of Refrigerant Circuits.24
Electronic Expansion Valve..........24
Overview......................................................24
EXV Superheat Control..............................24
McQuay" is a registered trademark of McQuay International
"Illustrations and data cover the McQuay International products at the time of publication and we reserve the right to make changes in design
1997 McQuay International
and construction at anytime without notice"
Keypad Key Functions..................49
Personal Computer Specification.51
MicroTech Menu Structure..........52
Menus for Two (2) Screw
Compressor Units..........................53
Menus for Three (3) Screw
Compressor Units..........................61
Menus for Four (4) Screw
Compressor Units..........................68
Schematics and Drawings.............77
Introduction
This manual provides installation, setup and troubleshooting information for the MicroTech controller provided on McQuay air-cooled screw compressor chillers. Please refer to the current version of installation manual IOMM ALS for unit application information as well as water and refrigerant piping details. All operating descriptions contained in this manual are based on the current MicroTech controller software version at time of publication. Contact McQuay Technical Response Center at 1­877-349-7782 for information on specific code versions. Chiller operating characteristics and menu selections may vary depending on the actual software version installed.
This equipment generates, uses and can radiate radio frequency energy and if not installed and used in accordance with the instructions manual, may cause interference to radio communications. It has been tested and found to comply with the limits for a class A digital device, pursuant to part 15 of the FCC rules. These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment.
Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at his own expense. McQuay International disclaims any liability resulting from any interference or for the correction thereof.
CAUTION
CAUTION
The McQuay MicroTech control panel contains static sensitive components. A static discharge while handling electronic circuit boards may cause damage to the components.
To prevent such damage during service involving board replacement, McQuay recommends discharging any static electrical charge by touching the bare metal inside the panel before performing any service work.
CAUTION
Excessive moisture in the control panel can cause hazardous working conditions and improper equipment operation.
When servicing equipment during rainy weather conditions, the electrical devices and MicroTech components housed in the main control panel must be protected.
The MicroTech controller is designed to operate within an ambient temperature range of minus 40 to plus 185°F and a maximum relative humidity of 95% (non-condensing).
OM ALSMICRO 3
General Description
The MicroTech Unit Control Panel, available on all McQuay ALS products, contains a Model 250 or 280 Microprocessor based controller, which provides all monitoring, and control functions required for the safe, efficient operation of the unit. The operator can monitor all operating conditions by using the panel's built in 2 line by 16 character display and keypad or by using an IBM compatible computer running McQuay Monitor software. In addition to providing all normal operating controls, the MicroTech controller monitors all safety devices on the unit and will shut the system down and close a set of alarm contacts if an alarm condition develops.
Important operating conditions at the time an alarm occurs are retained in the controller's memory to aid in troubleshooting and fault analysis. The system is protected by a password scheme, which only allows access, by authorized personnel. The operator must enter a valid password into the panel keypad before any setpoints may be altered.
Table 1, Unit Identification
ALS Air-Cooled Chiller with Screw Compressors
Control Panel Features
Ø Flexible control of leaving chilled water with convenient reset capability. Ø Enhanced head pressure control on air-cooled units resulting in increased total unit SEER. Ø Convenient, easy to read 2 line by 16-character display for plain English readout of operating
temperatures and pressures, operating modes or alarm messages.
Ø Keypad adjustment of unit safeties such as low water temperature cutout, high pressure cutout,
suction pressure cutout, and freeze protection. The operator can use the keypad to monitor various operating conditions, setpoints or alarm messages.
Ø Security password protection against unauthorized changing of setpoints and other control
parameters.
Ø Complete plain English diagnostics to inform the operator of system warnings and alarms. All
alarms are time and date stamped so there is no guessing of when the alarm condition occurred. In addition, the operating conditions that existed at the instant of shutdown can be recalled to aid in isolating the cause of the problem.
Ø Soft loading feature to reduce electrical consumption and peak demand charges during chilled
water loop pulldown.
Ø Easy integration into building automation systems via separate 4-20 milliamp signals for chilled
water reset and demand limiting. McQuay's Open Protocol feature is fully supported.
Ø Flexible internal time clock for on/off scheduling. Ø Communications capabilities for local system monitoring, changing of setpoints, trend logging,
remote reset, alarm and event detection, via IBM compatible PC. The optional modem kit supports the same features from an off-site PC running McQuay Monitor software.
Ø Special service modes may be used to override automatic unit staging during system checkout
and service.
Unit Identification
Software Identification
Version
Software
Controller software is factory installed and tested in each panel prior to shipment. The software is identified by a program code that is printed on a small label attached to the controller. The software version may also be displayed on the keypad/display by viewing the last menu item in the Misc. Setup menu.
The software "version" is the 6th & 7th location of the software number. In the example, the version is "19" and the revision to the software is "A".
Revisions are released in alphabetical order.
Number of Compressors
Controller Layout
All major MicroTech components are mounted inside the control section side of the unit's control cabinet. The individual components are interconnected by ribbon cables, shielded multi-conductor cables, or discrete wiring. Transformers T-2 and T-4 provide power for the system. All field wiring must enter the control cabinet through the knockouts provided and be terminated on field wiring terminal strips. The standard ALS keypad/display is located inside the control cabinet for protection from the weather. See Figure 1 for typical control cabinet layout.
Figure 1, Typical control cabinet layout
Hardware
Screw Chiller
Refrigerant
Type 2 = R22
Type 3 = R134a
SC 3 2 E 19 A
Revision
E = I-P S = SI
OM ALSMICRO 5
Component Data
Microprocessor Control Board (MCB1)
The Model 250 or 280 Microprocessor Control Board contains the electronic hardware and software required to monitor and control the unit. It receives input from the ADI Board and sends commands to the Output Board to maintain the unit's optimum operating mode for the current conditions. Status lights are mounted on the control board to indicate the operating condition of the microprocessor.
Figure 2, MCB1
Analog/Digital Input Board (ADI Board)
The ADI Board provides low voltage power for the temperature and pressure sensors. It also provides electrical isolation between the Microprocessor Control Board and all 24V switch inputs. LEDs are furnished on the board to give a visual indication of the status of all digital inputs. All analog and digital signals from sensors, transducers and switches are received by the ADI Board and then sent to the Microprocessor Control Board for interpretation.
Figure 3, ADI
Output Board
The Output Board contains up to 24 solid state relays, which are used to control all compressors, condenser fans, solenoid valves and alarm annunciation.
It receives control signals from the Microprocessor Control Board through a 50-conductor ribbon cable.
Figure 4, Output board
Electric Expansion Valve Board (EXV Board)
Each EXV Board will directly control up to two electronic expansion valves. The boards may be cascaded together for units with more than two EXV's. Control instructions for the boards are generated by the M250 controller.
Figure 5, EXV board
Analog Output Board (AOX Board) (With Optional SpeedTrol)
The AOX Board converts control instructions from the M250's expansion bus into an analog control signal suitable for driving a variable speed condenser fan. Each AOX Board is factory set via jumper to provide an output signal of 0 - 10 VDC.
Figure 6, AOX board
The Keypad/Display is the primary operator interface to the unit. All operating conditions, system alarms and setpoints can be monitored from this display and all adjustable setpoints can be modified from this keyboard if the operator has entered a valid operator password.
OM ALSMICRO 7
Figure 7, Keypad display
Sensors and Transducers
Standard Sensors
Evaporator Leaving Water Temperature Evaporator Refrigerant Pressure, Circuit #1, 2, 3 & 4 Condenser Refrigerant Pressure, Circuit #1, 2, 3 & 4 Suction Temperature, Circuit #1, 2, 3 & 4 Liquid Line Temperature, Circuit #1, 2, 3 & 4 (Provides direct display of subcooling and superheat) Entering Evaporator Water Temperature Outside Ambient Air Temperature
Optional Sensor Packages
Percent Unit Amps on 2 Compressor Units (Percent total unit amperage including compressors and condenser fans. Does not include externally powered equipment such as water pumps.)
Percent Compressor Amps on 3 Compressor Units and Percent Circuit Amps (1 & 3, 2 & 4) on 4 Compressor Units.
Thermistor Sensors
MicroTech panels use a negative temperature coefficient thermistor for temperature sensing. A normal sensor will measure 3000 ohms at 77°F.
Figure 8, Thermistor sensor
Table 2, MicroTech Thermistors
°F Ohms Volts °F Ohms Volts °F Ohms Volts
15 16,104 4.145 77 3,000 2.373 139 761 0.932 16 15,627 4.124 78 2,927 2.343 140 746 0.917 17 15,166 4.102 79 8,357 2.313 141 731 0.902 18 14,720 4.080 80 2,789 2.283 142 717 0.888 19 14,288 4.057 81 2,723 2.253 143 703 0.874 20 13,871 4.034 82 2,658 2.223 144 689 0.859 21 13,469 4.011 83 2,595 2.194 145 676 0.846 22 13,076 3.988 84 2,534 2.164 146 662 0.831 23 12,698 3.964 85 2,474 2.135 147 649 0.818 24 12,333 3.940 86 2,416 2.106 148 637 0.805 25 11,979 3.915 87 2,360 2.077 149 625 0.792 26 11,636 3.890 88 2,305 2.049 150 613 0.779 27 11,304 3.865 89 2,251 2.020 151 601 0.766 28 10,983 3.839 90 2,199 1.992 152 589 0.753 29 10,672 3.814 91 2,149 1.965 153 578 0.741 30 10,371 3.788 92 2,099 1.937 154 567 0.729 31 10,079 3.761 93 2,051 1.909 155 556 0.717 32 9,797 3.734 94 2,004 1.882 156 546 0.706 33 9,523 3.707 95 1,959 1.855 157 535 0.694 34 9,258 3.608 96 1,914 1.828 158 525 0.683 35 9,002 3.653 97 1,871 1.802 159 516 0.673 36 8,753 3.625 98 1,829 1.775 160 506 0.661 37 8,512 3.597 99 1,788 1.750 161 496 0.650 38 8,278 3.569 100 1,747 1.724 162 487 0.640 39 8,052 3.540 101 1,708 1.698 163 478 0.629 40 7,832 3.511 102 1,670 1.673 164 469 0.619 41 7,619 3.482 103 1,633 1.648 165 461 0.610 42 7,413 3.453 104 1,597 1.624 166 452 0.599 43 7,213 3.424 105 1,562 1.600 167 444 0.590 44 7,019 3.394 106 1,528 1.576 168 436 0.580 45 6,831 3.365 107 1,494 1.552 169 428 0.571 46 6,648 3.335 108 1,461 1.528 170 420 0.561 47 6,471 3.305 109 1,430 1.505 171 413 0.553 48 6,299 3.274 110 1,398 1.482 172 405 0.544 49 6,133 3.244 111 1,368 1.459 173 398 0.535 50 5,971 3.213 112 1,339 1.437 174 391 0.527 51 5,814 3.183 113 1,310 1.415 175 384 0.518 52 5,662 3.152 114 1,282 1.393 176 377 0.510 53 5,514 3.121 115 1,254 1.371 177 370 0.501 54 5,371 3.078 116 1,228 1.350 178 364 0.494 55 5,231 3.059 117 1,201 1.328 179 357 0.485 56 5,096 3.028 118 1,176 1.308 180 351 0.478 57 4,965 2.996 119 1,151 1.287 181 345 0.471 59 4,714 2.934 121 1,103 1.247 183 333 0.456 60 4,594 2.902 122 1,080 1.227 184 327 0.448 61 4,477 2.871 123 1,058 1.208 185 321 0.441 62 4,363 2.839 124 1,036 1.189 186 316 0.435 63 4,253 2.808 125 1,014 1.170 187 310 0.427 64 4,146 2.777 126 993 1.151 188 305 0.421 65 4,042 2.745 127 973 1.133 189 299 0.413 66 3,941 2.714 128 953 1.115 190 294 0.407 67 3,842 2.682 129 933 1.076 191 289 0.400 68 3,748 2.651 130 914 1.079 192 284 0.394 69 3,655 2.620 131 895 1.062 193 280 0.389 70 3,565 2.589 132 877 1.045 194 275 0.382 71 3,477 2.558 133 859 1.028 195 270 0.376
OM ALSMICRO 9
72 3,392 2.527 134 842 1.012 196 266 0.371 73 3,309 2.496 135 825 0.995 197 261 0.364 74 3,328 2.465 136 809 0.980 198 257 0.359 75 3,150 2.434 137 792 0.963 199 252 0.353 76 3,074 2.404 138 777 0.948 200 248 0.348
Pressure Transducers
These transducers are selected for a specific operating range and provide an output signal, which is proportional to the sensed pressure. The typical range for evaporator sensors is 0 to 150 psig with a resolution of 0.1 psi. Condenser pressure sensors have a range of 0 to 450 psi and a resolution of 0.5 psi. The pressure transducers require an external 5 VDC power supply to operate that is provided by the MicroTech controller. This connection should not be used to power any additional devices.
Figure 9, Pressure Transducer
Red Dot - Condenser
Blue Dot - Evaporator
Liquid Presence Sensor
A liquid level sensor mounted at the liquid injection port in the compressor casting determines the presence of liquid refrigerant. Whenever the glass prism sensor tip is in contact with liquid, the sensor output signal will be high (>7 VDC). If no liquid is detected, the output will be low (O VDC).
Figure 10, Liquid Presence Sensor
Sensor Data
Sensor Locations
Analog Inputs
Analog inputs are used to read the various temperature and pressure sensors installed on the chiller as well as any customer supplied 4-20mA reset signals. The controller's internal regulated 5 VDC and 12 VDC supplies provide the correct operating voltage for the sensors.
Figure 11, Sensor Locations, Two Compressor Units
Table 3, Analog Inputs - 2 Compressor Units
Sensor
Number
S00 Evaporator Leaving Water Temperature Leaving Chilled Water Nozzle S01 Evaporator Pressure Transducer Circuit #1 Common Circuit #1 Suction Line S02 Evaporator Pressure Transducer Circuit #2 Common Circuit #2 Suction Line S03 Condenser Pressure Transducer Circuit #1 Compressor #1 Discharge Cover S04 Condenser Pressure Transducer Circuit #2 Compressor #2 Discharge Cover
Input05 Transducer Power Voltage Ratio (Internal) Input06 Reset-Evaporator Water Temperature External 4-20 mA Signal Input07 Demand Limit External 4-20 mA Signal
S08 Entering Evaporator Water Temperature Entering Chilled Water Nozzle S09 O.A.T Back of the Control Box S11 Percent Unit Amps CT1 and Signal Converter Board S12 Suction Temperature Circuit #1 Well Brazed to the Circuit #1 Suction Line S13 Suction Temperature Circuit #2 Well Brazed to the Circuit #2 Suction Line S14 Liquid Line Temperature Circuit #1 Well Brazed to the Circuit #1 Liquid Line S15 Liquid Line Temperature Circuit #2 Well Brazed to the Circuit #2 Liquid Line
OM ALSMICRO 11
Description Sensor Location
Figure 12, Sensor Locations - 3 Compressor Unit
Table 4, Analog Inputs - 3 Compressor Units
Sensor Number Description
S00 Evaporator Leaving Water Temperature S01 Low Pressure Transducer Circuit #1 S02 Low Pressure Transducer Circuit #2 S03 High Pressure Transducer Circuit #1 S04 High Pressure Transducer Circuit #2 S06 Evaporator Water Temperature Reset (Field Supplied) S07 Demand Limit (Field Supplied) S08 Evaporator Entering Water Temperature S09 Outside Air Temperature S10 Percent Circuit Amps Circuit #1 (CT1) S11 Percent Circuit Amps Circuit #2 (CT2) S12 Suction Temperature Circuit #1 S13 Suction Temperature Circuit #2 S14 Liquid Line Temperature Circuit #1 S15 Liquid Line Temperature Circuit #2 S16 Low Pressure Transducer Circuit #3 S17 High Pressure Transducer Circuit #3 S18 Suction Temperature Circuit #3 S19 Liquid Line Temperature Circuit #3 S20 Discharge Temperature Circuit #1 S21 Discharge Temperature Circuit #2 S22 Discharge Temperature Circuit #3 S23 Percent Circuit Amps Circuit #3 (CT3)
Figure 13, Sensor Locations - 4 Compressor Unit
Table 5, Analog Inputs - 4 Compressor Units
Sensor Number Description
S00 Evaporator Leaving Water Temperature S01 Low Pressure Transducer Circuit #1 S02 Low Pressure Transducer Circuit #2 S03 High Pressure Transducer Circuit #1 S04 High Pressure Transducer Circuit #2 S06 Evaporator Water Temperature Reset (Field Supplied) S07 Demand Limit (Field Supplied) S08 Evaporator Entering Water Temperature S09 Outside Air Temperature S10 Percent Circuit Amps Circuit #1 & 3 (CT1) S11 Percent Circuit Amps Circuit #2 & 4 (CT2) S12 Suction Temperature Circuit #1 S13 Suction Temperature Circuit #2 S14 Liquid Line Temperature Circuit #1 S15 Liquid Line Temperature Circuit #2 S16 Low Pressure Transducer Circuit #3 S17 High Pressure Transducer Circuit #3 S18 Suction Temperature Circuit #3 S19 Liquid Line Temperature Circuit #3 S20 Low Pressure Transducer Circuit #4 S21 High Pressure Transducer Circuit #4 S22 Suction Temperature Circuit #4 S23 Liquid Line Temperature Circuit #4
OM ALSMICRO 13
Digital Inputs
Note: All digital inputs are 24 VAC.
At 7.5 VAC to 24 VAC nominal the digital input contacts are considered closed, and the signal level is high. Below 7.5 VAC nominal, the contacts are considered open, and the signal level is low.
Table 6, Digital Inputs - 2 Compressor Unit
Number Description Low Signal High Signal
0 Mechanical High Pressure Switch, Circuit #1 Alarm Normal 1 Liquid Presence Switch, Compressor #1 Alarm Normal 2 Motor Protect, Compressor #1 Alarm Normal 3 High Liquid Pressure Drop, #1 Alarm Normal 4 (Reserved) - - 5 System Switch (S1) Stop Run 6 Phase Voltage Monitor Alarm Normal 7 Pump Down Switch, Circuit #1 Normal Pumpdown 8 Mechanical High Pressure Switch, Circuit #2 Alarm Normal
9 Liquid Presence Switch, Compressor #2 Alarm Normal 10 Motor Protect, Compressor #2 Alarm Normal 11 High Liquid Pressure Drop, # 2 Alarm Normal 12 (Reserved) - - 13 Unit Remote Stop Switch Stop Run 14 Evap Water Flow Switch Alarm Normal 15 Pump Down Switch, Circuit #2 Normal Pumpdown
Table 7, Digital Inputs - 3 Compressor Unit
Number Description Led On Led Off
0 Mechanical High Pressure Switch, Circuit #1 Alarm Normal
1 Liquid Presence Sensor Compressor #1 No Liquid Liquid
2 Motor Protect, Compressor #1 Alarm Normal
3 High Liquid Pressure Drop, #1 Alarm Normal
4 Not Used - -
5 System On/Off Switch Off On
6 Phase Voltage Monitor Compressor #1 Alarm Normal
7 PumpDown Switch Compressor #1 Normal Pumpdown
8 Mechanical High Pressure Switch, Circuit #2 Alarm Normal
9 Liquid Presence Sensor Compressor #2 No Liquid Liquid 10 Motor Protect, Compressor #2 Alarm Normal 11 High Liquid Pressure Drop, #2 Alarm Normal 12 Not Used - - 13 Remote Start Stop Switch Stop Start 14 Evap Water Flow Switch No Flow Flow 15 PumpDown Switch, Circuit #2 Normal Pumpdown 16 Mechanical High Pressure Switch Circuit #3 Alarm Normal 17 Liquid Presence Sensor Compressor #3 No Liquid Liquid 18 Motor Prot Compressor #3 Alarm Normal 19 High Liquid Pressure Drop, #3 Alarm Normal 20 Not Used - - 21 Phase Volt Monitor Compressor #2 Alarm Normal 22 Phase Volt Monitor Compressor #3 Alarm Normal 23 PumpDown Switch Compressor #3 Alarm Normal
Table 8, Digital Inputs - 4 Compressor Unit
Number Description Led On Led Off
0 Mechanical High Pressure Switch, Circuit #1 Alarm Normal 1 Liquid Presence Sensor Compressor #1 No Liquid Liquid 2 Motor Protect, Compressor #1 Alarm Normal 3 High Liquid Pressure Drop, #1 Alarm Normal 4 Not Used - - 5 System On/Off Switch Off On 6 Phase Voltage Monitor Compressor #1 Alarm Normal 7 PumpDown Switch Compressor #1 Normal Pumpdown 8 Mechanical High Pressure Switch, Circuit #2 Alarm Normal
9 Liquid Presence Sensor Compressor #2 No Liquid Liquid 10 Motor Protect, Compressor #2 Alarm Normal 11 High Liquid Pressure Drop, #2 Alarm Normal 12 Not Used - - 13 Remote Start Stop Switch Stop Start 14 Evap Water Flow Switch No Flow Flow 15 PumpDown Switch, Circuit #2 Normal Pumpdown 16 Mechanical High Pressure Switch Circuit #3 Alarm Normal 17 Liquid Presence Sensor Compressor #3 No Liquid Liquid 18 Motor Prot Compressor #3 Alarm Normal 19 High Liquid Pressure Drop, #3 Alarm Normal 20 Not Used - - 21 Phase Volt Monitor Multi Point Alarm Normal 22 Not Used - - 23 Pumpdown Switch compressor #3 Normal Pumpdown
0 Mechanical High Pressure Switch Circuit #4 Alarm Normal
1 Liquid Presence Sensor Compressor #4 No Liquid Liquid
2 Motor Prot Compressor #4 Alarm Normal
3 High Liquid Pressure Drop, #4 Alarm Normal
4 Not Used - -
5 Not Used - -
6 Not Used - -
7 Pumpdown Switch Compressor #4 Normal Pumpdown
Optional Analog Outputs
Table 9, Analog Outputs
Number Description Signal Range
0 SpeedTrol, Circuit #1 0-10 VDC 1 SpeedTrol, Circuit #2 0-10 VDC 2 SpeedTrol, Circuit #3 0-10 VDC 3 SpeedTrol, Circuit #4 0-10 VDC
OM ALSMICRO 15
Digital Outputs
Table 10, Digital Outputs - 2 Compressor Unit
Relay Description Off On
0 Alarm LED and Contact (Programmable) (Programmable) 1 Chilled Water Pump Stop Run 2 EXV Serial Data 1 3 EXV Serial Data 2 4 MCR Relay. Compressor#1 Stop Run 5 Top Solenoid, Compressor #1 Hold Load 6 Bottom Right Solenoid, Compressor #1 Hold Load 7 Bottom Left Solenoid, Compressor #1 Hold Load 8 MCR Relay, Compressor #2 Stop Run
9 Top Solenoid, Compressor #2 Hold Load 10 Bottom Right Solenoid, Compressor #2 Hold Unload 11 Bottom Left Solenoid, Compressor #2 Hold Load 12 Condenser Fan #1, Circuit #1 (M12) Off On 13 Condenser Fan #2, Circuit #1 (M13) Off On 14 Condenser Fan #3, Circuit #1 (M14) Off On 15 Condenser Fan #4, Circuit #1 (M15) Off On 16 Condenser Fan #1, Circuit #2 (M22) Off On 17 Condenser Fan #2, Circuit #2 (M23) Off On 18 Condenser Fan #3, Circuit #2 (M24) Off On 19 Condenser Fan #4, Circuit #2 (M25) Off On 20 Liquid Solenoid Valve, Circuit #1 Close Open 21 Liquid Solenoid Valve, Circuit #2 Close Open 22 (Spare) 23 (Spare)
Table 11, Digital Outputs - 3 Compressor Unit
Relay Description
0 Alarm Circuit 1 Chilled Water Pump Relay 2 EXV Control 3 EXV Control 4 Compressor #1 Contactor 5 Compressor #1 Top Solenoid Valve 6 Compressor #1 Bottom Right Solenoid Valve (feed) 7 Compressor #1 Bottom Left Solenoid Valve (vent) 8 Compressor #2 Contactor
9 Compressor #2 Top Solenoid Valve (feed) 10 Compressor #2 Bottom Right Solenoid Valve (feed) 11 Compressor #2 Bottom Left Solenoid Valve (vent) 12 Condenser Fan Contactor M-12 13 Condenser Fan Contactor M-13 14 Condenser Fan Contactor M-14 15 Condenser Fan Contactor M-15 16 Condenser Fan Contactor M-22 17 Condenser Fan Contactor M-23 18 Condenser Fan Contactor M-24 19 Condenser Fan Contactor M-25 20 Compressor #3 Contactor 21 Compressor #3 Top Solenoid Valve (feed) 22 Compressor #3 Bottom Right Solenoid Valve (feed) 23 Compressor #3 Bottom Left Solenoid Valve (vent) 24 Condenser Fan Contactor M-32 25 Condenser Fan Contactor M-33
26 Condenser Fan Contactor M-34 27 Condenser Fan Contactor M-34 28 Optional Hot Gas Bypass - SV5 29 Optional Hot Gas Bypass - SV6
Table 12, Digital Outputs - 4 Compressor Unit
Relay Description
0 Alarm Circuit 1 Chilled Water Pump Relay 2 EXV Control 3 EXV Control 4 Compressor #1 Contactor 5 Compressor #1 Top Solenoid Valve 6 Compressor #1 Bottom Right Solenoid Valve (feed) 7 Compressor #1 Bottom Left Solenoid Valve (vent) 8 Compressor #2 Contactor
9 Compressor #2 Top Solenoid Valve (feed) 10 Compressor #2 Bottom Right Solenoid Valve (feed) 11 Compressor #2 Bottom Left Solenoid Valve (vent) 12 Condenser Fan Contactor M-12 13 Condenser Fan Contactor M-13 14 Condenser Fan Contactor M-14 15 Condenser Fan Contactor M-15 16 Condenser Fan Contactor M-22 17 Condenser Fan Contactor M-23 18 Condenser Fan Contactor M-24 19 Condenser Fan Contactor M-25 20 Compressor #3 Contactor 21 Compressor #3 Top Solenoid Valve (feed) 22 Compressor #3 Bottom Right Solenoid Valve (feed) 23 Compressor #3 Bottom Left Solenoid Valve (vent) 24 Condenser Fan Contactor M-32 25 Condenser Fan Contactor M-33 26 Condenser Fan Contactor M-34 27 Condenser Fan Contactor M-35 28 Optional Hot Gas Bypass - SV5 29 Optional Hot Gas Bypass - SV6 30 Not Used 31 Compressor #4 Contactor 32 Compressor #4 Top Solenoid Valve (feed) 33 Compressor #4 Bottom Right Solenoid Valve (feed) 34 Compressor #4 Bottom Left Solenoid Valve (vent) 35 Condenser Fan Contactor M-42 36 Condenser Fan Contactor M-43 37 Condenser Fan Contactor M-44 38 Condenser Fan Contactor M-45
OM ALSMICRO 17
Installation
Controller Calibration
The control software is installed and tested by the factory prior to shipping therefore no periodic calibration of the controller is required. All control and safety setpoints will be checked and adjusted if necessary by the McQuayService start-up technician prior to starting the unit. The MicroTech controller contains default setpoints that will be appropriate for most common installations.
Field Wiring
Analog sensors and transducers
All sensors and transducers required for normal chiller operation are installed and wired by the factory. Any optional analog signals provided by the installing contractor require twisted, shielded pair wire (Belden #8760 or equal).
Digital input signals
Remote contacts for all digital inputs such as the chilled water flow switch and the remote start/stop switch must be dry contacts suitable for the 24 VAC control signals produced by the screw chiller panel.
Digital outputs
Devices wired to the digital outputs typically are an optional Chilled Water Pump control relay or an Alarm Annunciator. The MicroTech output device is a normally open solid state relay with an on board, replaceable 5 amp fuse. The model 250 controller activates a solid state relay by sending a "trigger" signal to the output board via the attached ribbon cable. The relay responds to the trigger by lowering its resistance that allows current to flow through its "contacts". When the controller removes the trigger signal, the relay's resistance becomes very high, causing the current flow to stop. The status of all outputs is shown by individual red LEDs for ease of determining output status.
Interlock wiring
The installing contractor provides all interlock wiring to field devices such as flow switches and pump starters. Refer to the Field Wiring Drawing as well as the unit wiring schematics and typical application drawings at the end of this manual for details.
External alarm circuit
The MicroTech panel can activate an external alarm circuit when an alarm or pre-alarm condition is detected. A 24 VAC voltage source is available at field wiring terminals #102 through #107 to power an external alarm device such as a bell, light or relay. An alarm annunciator rated for a maximum load of 1.8 Amps at 24 VAC is to be provided and wired by the installing contractor. The normal and alarm states for the 24 VAC alarm signal are programmable by the operator. Available settings are:
Ø Pre-alarm annunciation: Close-or-Open-or-Blink Ø Alarm annunciation: Close-or-Open
Power wiring
115 VAC power for the control transformer is derived from the 3-phase power connection provided by the electrical contractor.
A separate disconnect for the cooler heating tape and control circuit transformer may be supplied as options on some installations. Wiring for these circuits is to be provided by the installing contractor and should conform to the National Electrical Code and all applicable local building codes.
Power supplies
There are several internal power supplies used by the controller and its associated circuitry. The regulated 5 VDC power on terminal #42 is used to support the analog inputs on the ADI Board and
should not be used to operate any external devices. An unregulated 12 VDC power supply is available on field wiring terminal #56 and an unregulated 24 VAC supply is provided at terminal #81. Both of these may be used for powering external devices such as low current relays and lights.
Demand limit and chilled water reset signals
Separate 4-20 milliamp signals for remote chilled water reset and demand limit can be provided by the customer and should be connected to the appropriate terminals on the field wiring strip inside the control cabinet. The optional demand limit and chilled water reset signals are 4 to 20 milliamp DC signals. The resistive load used to condition the milliamp input signals is a 249 ohm resistor factory mounted on the ADI Board.
Communication ports
Communication port "A" is provided on the MicroTech controller for connection to an IBM compatible computer for local or remote system monitoring (Belden 8762 or equivalent). The network uses the RS232 communication standard with a maximum cable length of 50 feet. All communication network wiring utilizes low voltage shielded twisted pair cable. See the Personal Computer Specification section of this manual for specific hardware requirements.
Communication port "B" is used to link the unit controller into a MicroTech network using the RS485 communication standard. Refer to the field wiring in this manual for details.
Modem Kit
An optional modem kit may be installed for remote monitoring of the chiller from an off-site PC running McQuay's Monitor software. The kit comes complete with modem, wiring harness and installation instructions.
Remote monitoring of the MicroTech controller requires a dedicated telephone line supplied by the equipment owner. The McQuay Monitor software package used to establish a remote connection to the modem kit must be purchased separately.
Telephone line for remote modem access
A voice quality, direct dial telephone line is required if remote access and monitoring of the unit controller is desired. The phone line should be terminated with a standard RJ-11 modular phone plug.
OM ALSMICRO 19
Unit Sequence of Operation
The following sequence of operation is typical for McQuay ALS air-cooled chillers. The sequence may vary depending on various options that may be installed on the chiller.
Off Conditions
With power supplied to the unit, 115 VAC power is applied through the control fuse F1 to the compressor casing heaters, the compressor motor protector circuits, the primary of the 24V control circuit transformer and optionally, the evaporator heater (HTR5). The 24V transformer provides power to the MicroTech controller and related components. With 24V power applied, the controller will check the position of the front panel System Switch (S 1). If the switch is in the "stop, position the chiller will remain off and the display will indicate the operating mode to be OFF:SystemSw. The controller will then check the PumpDown Switches. If any switch is in the "stop" position, that circuit's operating mode will be displayed as OFF:RemoteComm if this operating mode is in effect. If an alarm condition exists which prevents normal operation of both refrigerant circuits, the chiller will be disabled and the display will indicate OFF:AllCompAlarm.
The MicroTech controller allows the operator to manually set the chiller to an off mode via the keypad. The display indicates this operating mode with the message OFF:ManualMode.
Assuming none of the above "Off" conditions are true, the controller will examine the internal time schedule to determine if the chiller should start. The operating mode will be OFF:TimeClock if the time schedule indicates an "off"' time period.
Start-up
If none of the above "Off" conditions are true, the MicroTech controller will initiate a start sequence and energize the chilled water pump output relay. The display will indicate Starting as the operating mode. The chiller will remain in the Waiting For Flow mode until the field installed flow switch indicates the presence of chilled water flow. If flow is not proven within 30 seconds, the alarm output will be activated and the chiller will continue to wait for proof of chilled water flow. When chilled water flow is re-established, the alarm will be automatically cleared.
Waiting for Load
Once flow is established the controller will sample the chilled water temperature and compare it against the Leaving Chilled Water Setpoint, the Control Band and the Load Delay which have been programmed into the controller's memory. If the leaving chilled water temperature is above the Leaving Chilled Water Setpoint plus ½ the adjustable Control Band plus the Start-up Delta Temperature Setpoint, the controller will start the lead compressor.
Start Requested
In the Start Requested Mode, the electronic expansion valve is fully closed. The MicroTech controller will read the evaporator pressure to ensure that at least 4 psi of refrigerant pressure is present. If the evaporator pressure is less than 4 psi the compressor will not be enabled and the display will read "NoStart-LoEvap".
Prepurge
In order to purge the compressor of any liquid refrigerant that may be present, the starting compressor is operated at 50% capacity while the electronic expansion valve is held fully closed. The refrigerant circuit will continue to run in this mode until either the evaporator refrigerant pressure drops to less than 40 psi or 60 seconds has elapsed. If the evaporator pressure does not drop to 40 psi within the 60 seconds, the compressor will continue to run and the display will read "Failed Prepurge". The alarm is logged in the alarm buffer.
Opened EXV
With the evaporator pressure less than 40 psi and the compressor still running, the electronic expansion valve will be driven open to 300 steps. If the evaporator pressure rises above the freeze­stat setpoint, the chiller will advance to Cool Staging Mode. If the circuit is in Cool Staging Mode and after 20 seconds, the evaporator pressure remains below the freeze state setpoint but is greater than 2 psi, the controller will transition to Low Ambient Start Mode.
Low Ambient Start
If the difference between the freeze stat setpoint and the evaporator refrigerant pressure is greater than 12 psi, the low ambient start timer will be set to l80 seconds. The compressor will continue to run for 180 seconds from the moment the expansion valve is opened in an attempt to build up the evaporator pressure. If the difference between the freeze stat setpoint and the evaporator refrigerant pressure is greater than 12 psi, the following calculation will be used to set the low ambient start timer:
Low Ambient timer = 360 - (Pressure Difference X 15)
If the calculated low ambient timer value is greater than 360, the compressor will be stopped, the alarm output will be activated and the display will indicate "FailLowAmbStart".
Cool Stage
Circuit capacity at initial start will be 50%. Once the chiller has started, the MicroTech controller will add or subtract cooling capacity to maintain the chilled water setpoint. The current cooling stage will be displayed on the keypad/display. Automatic chiller staging may be overridden by selecting "Manual Cooling" as the operating mode and then choosing the desired cooling stage.
"Manual Cooling" will by-pass all interstage timers. This will result in rapid
compressor stage up and possible chilled water temperature overshoot. The unit will
not unload as the chillers water temperature reaches the setpoint.
Compressor Control
Normal Compressor Staging Logic
The Compressor Staging Logic uses an adjustable control band and interstage timers to determine the correct number of cooling stages to activate. A project-ahead temperature calculation provides stable operation. The total number of cooling stages for each circuit is dependent upon the "number of cooling stages" setpoint.
Operation at 25% is not allowed on compressors #3 and #4. For compressors #1 and 2, 25% is selectable by setting MinStage = 1. Then 30 minutes is the
maximum timer setting allowed at 25%. If the evaporator Delta-T is less than 1 degree F. then 5 minutes at 25% load is allowed.
Operation at 25% load is not allowed if the outside ambient air temperature is below the minimum setpoint of 60°F.
CAUTION
OM ALSMICRO 21
Compressor Staging Control Sequence
Four Compressor Unit
Staging Up Staging Down
Stage
10 75% 75% 50% 50% 62.5% 75% 75% 50% 50% 62.5% 11 75% 75% 75% 50% 68.8% 75% 75% 75% 50% 68.8% 12 75% 75% 75% 75% 75.0% 75% 75% 75% 75% 75.0% 13 100% 75% 75% 75% 81.3% 100% 75% 75% 75% 81.3% 14 100% 100% 75% 75% 87.5% 100% 100% 75% 75% 87.5% 15 100% 100% 100% 75% 93.8% 100% 100% 100% 75% 93.8% 16 100% 100% 100% 100% 100.0% 100% 100% 100% 100% 100.0%
Lead
Comp.
1 - - - - 0.0% 25% 0% 0% 0% 6.3% 2 50% 0% 0% 0% 12.5% 50% 0% 0% 0% 12.5% 3 75% 0% 0% 0% 18.8% 75% 0% 0% 0% 18.8% 4 50% 50% 0% 0% 25.0% 50% 50% 0% 0% 25.0% 5 75% 50% 0% 0% 31.3% 75% 50% 0% 0% 31.3% 6 75% 75% 0% 0% 37.5% 50% 50% 50% 0% 37.5% 7 75% 50% 50% 0% 43.8% 75% 50% 50% 0% 43.8% 8 75% 75% 50% 0% 50.0% 50% 50% 50% 50% 50.0% 9 75% 75% 75% 0% 56.3% 75% 75% 50% 50% 56.3%
Lag 1 Comp.
Lag 2
Comp.
Lag 3
Comp.
Unit
Capacity
Lead
Comp
Lag 1
Comp.
.
Lag 2
Comp.
Lag 3
Comp.
Unit
Capacity
Three Compressors Unit
Staging Up Staging Down
Stage
10 100% 75% 75% 83.3 100% 75% 75% 83.3 11 100% 100% 75% 91.7 100% 100% 75% 91.7 12 100% 100% 100% 100.0 100% 100% 100% 100.0
Lead
Comp.
1 - - - 0.0 25% 0% 0% 8.3 2 50% 0% 0% 16.7 50% 0% 0% 16.7 3 75% 0% 0% 25.0 75% 0% 0% 25.0 4 50% 50% 0% 33.3 50% 50% 0% 33.3 5 75% 50% 0% 41.7 75% 50% 0% 41.7 6 75% 75% 0% 50.0 50% 50% 50% 50.0 7 75% 50% 50% 58.3 75% 50% 50% 58.3 8 75% 75% 50% 66.6 75% 75% 50% 66.6 9 75% 75% 75% 75.0 75% 75% 75% 75.0
Lag 1 Comp.
Lag 2
Comp.
Unit
Capacity
Lead
Comp.
Lag 1
Comp.
Lag 2
Comp.
Capacity
Two Compressors Unit
Staging Up Staging Down
Stage
Lead
Comp.
1 - - 0.0 25% 0% 12.5 2 50% 0% 25.0 50% 0% 25.0 3 75% 0% 37.5 75% 0% 37.5 4 50% 50% 50.0 50% 50% 50.0 5 75% 50% 62.5 75% 50% 62.5 6 75% 75% 75.0 75% 75% 75.0
Lag 1 Comp.
Unit
Capacit
y
Lead
Comp.
Lag 1
Comp.
Unit
Capacity
Unit
7 100% 75% 87.5 100% 75% 87.5 8 100% 100% 100.0 100% 100% 100.0
Project-Ahead Calculation
The Project-Ahead Calculation provides protection against an overshoot condition when the chilled water temperature is outside the control band. During cooling mode, if the Chilled Water Temperature is above the control band and the rate of temperature reduction is so great that in 120 seconds the chilled water temperature will be below the control band, the controller will stage down. The Project­Ahead Calculation also moderates the controller's response to a rapid increase in leaving water temperature.
Interstage Timer
The minimum time delay between stage up commands is set by the interstage timer setpoint (default=120 sec). The interstage timer for stage down commands is 1/5 of the stage up timer.
Anti-Cycle Timer
Anti-cycle timers are used to protect the compressors from excessive starts and high motor winding temperature. The anti-cycle timers are 5 minutes stop-to-start and 15 minutes start-to-start.
OM ALSMICRO 23
Lead-Lag of Refrigerant Circuits
The following compressor control rules are enforced in the control software.
Ø The MicroTech controller will never turn on the lag compressor until the lead compressor is at
75% capacity or greater and additional cooling capacity is required.
Ø The MicroTech controller will not turn off the lag compressor until the lead compressor is
running at 50% capacity, the lag compressor is running at 25% capacity and a reduction in cooling capacity is required. Three and four compressor units lag at 50% before pumpdown.
Automatic Lead-Lag
The controller provides automatic lead-lag of refrigeration circuits based on compressor operating hours and the number of starts. The circuit with the fewest number of starts will be started first. If circuits are operating and a stage down is required, the circuit with the most operating hours will cycle off first.
Manual Lead-Lag
The operator may override automatic circuit selection by manually selecting the lead circuit via the keypad or monitor.
When the setpoint equals "auto", the lead compressor is selected by the MicroTech controller based upon which circuit has the least operating hours. Regardless of the mode selected, if the lead circuit cannot operate due to an alarm condition or if off on cycle timers, the controller will switch to the lag circuit.
Electronic Expansion Valve
Overview
McQuay screw compressor chillers are supplied with Sporlan SE-series electronic expansion valves. The MicroTech controller generates valve positioning signals to maintain refrigerant circuit superheat to within 1.5°F of the superheat setpoint. Valve positioning signals are converted to actuator step pulses by the EXV board which in turn drives the valve's 3-phase DC stepper motor open or closed as required. A control range of 0 steps (full closed) to 760 steps (full open) is available to provide precise control of the valve position.
EXV Superheat Control
The electronic expansion valve position will be adjusted to maintain the refrigerant circuit's superheat setpoint. Superheat setpoints are based on refrigerant circuit capacity. For circuit capacity of 25% to 50%, the superheat setpoint will be 8.0°F. For circuit capacity of 75% to 100%, the superheat setpoint will be 10.0°F.
When the chiller control panel is powered up, the expansion valve will be driven closed 800 steps. This ensures that the valve is fully closed prior to a call for cooling. When all refrigerant circuit safeties are satisfied, the controller will initiate a start sequence. When the start sequence reaches "open solenoid", the expansion valve will be driven open to the First Open setpoint (default=300 steps). The current suction line temperature is compared against the Suction Line Temperature setpoint (evaporator temp plus superheat spt) to calculate superheat error (Err). The current suction line temperature is also compared with the previous reading to calculate delta superheat error (Derr). These two error values are used to determine the magnitude and direction of the expansion valve positioning signal. A new valve positioning signal is calculated every 10 seconds, however, the
interval at which these signals are issued to the EXV board is dependent on the magnitude of the required positional change. If no change is required, the internal will be 60 seconds.
Forced EXV Position Change
With an increase in circuit capacity, the electronic expansion valve position will be opened by a fixed percentage of its current position. This change will not occur if the superheat is less than 4°F below the superheat setpoint.
With a decrease in circuit capacity, the electronic expansion valve position will be closed by a fixed percentage of its current position.
Table 13, Staging Up
When Staging Up
From To Open
25% 50% 65% 50% 75% 50% 75% 100% 25%
Table 14, Staging Down
When Staging Down
From To Close
100% 75% 18%
75% 50% 40% 50% 25% 60%
EXV Evaporator Pressure Control
The electronic expansion valve control will maintain a constant superheat for suction line temperature up to 60°F. For suction line temperatures greater than 61°F, the expansion valve control logic will maintain a constant evaporator temperature to avoid overloading the compressor motor. The control point will be the Evap Temp setpoint (default=50°F) and the control method will be the standard MicroTech Step and Wait algorithm. When the suction line temperature drops below 57°F, the MicroTech logic will resume normal superheat control.
OM ALSMICRO 25
Chilled Water Reset Options
Chilled Water Reset (Remote 4-20mA)
The controller resets the chilled water setpoint based on an external 4 to 20mA signal. At 4mA or less, no reset will occur. At 20mA, the chilled water setpoint will be reset by an amount equal to the value stored in the Maximum Reset setpoint. The reset schedule is linear and may be calculated using Figure 14.
Figure 14, Chilled Water Reset Schedule
Ice Mode
The MicroTech controller has dual chilled water setpoints when ice mode is selected. With an external reset signal of 4mA or less, the chilled water reset will be zero. If the external reset signal is greater than 4mA, maximum reset will be in effect. The following set points should be adjusted to accommodate the reduced ice mode system temperature and pressure.
Table 15
Setpoint Monitors Default Ice Mode
FreezeStat Low Evap Pressure 54 psig
FreezeH20 Leaving Solution 36°F
StpPumpDn Final Pumpdown 34 psig
Note: Once the load is satisfied in Ice Mode, restart of chiller can not occur for 12 hours.
A pressure value equivalent to the
leaving solution temperature minus 10°F
A temperature value equal to the leaving
solution temperature minus 4°F
A pressure value equal to the
FreezeStat setpoint minus 10 psi
Network Reset
The reset mode can be set to "network" if chilled water reset via communications network is desired. The chiller controller receives a signal from the network master panel in the range of 0% to 100% of maximum reset.
Return Water Reset
When return water is selected as the reset mode, the MicroTech controller will adjust the leaving chilled water setpoint to maintain a constant return water temperature equal to the return water setpoint. The return water temperature is sampled every 5 minutes and a proportional correction is made to the leaving chilled water setpoint. The corrected leaving water setpoint is never set to a
value greater than the return water setpoint and is never set to a value less than the actual leaving chilled water setpoint.
Remote Demand Limit
The controller will limit the total number of stages based on an external 4 to 20mA signal regardless of the amount of cooling actually required. A 4mA or less signal will enable all stages while a 20mA signal will allow only 1 stage to operate. The effect of the reset signal may be calculated by using Figure 15.
Network Demand Limit
Unit demand limit via network communication may be selected if desired. The chiller controller receives a demand limit signal from the network master panel in the range of 0% to 100% with 0 equaling no limit.
Keypad Selectable Demand Limit
In the menu Demand Limit, set Manual Demand = Stage, which is below the maximum available for the unit.
Soft Loading
The soft loading feature limits the number of cooling stages which may be energized by the controller to prevent unnecessary electrical demand and possible over-shoot of the desired leaving water temperature. Soft loading is typically used during morning start-up. When the controller enters the "Cool Staging" mode of operation, the controller will start a count down timer to indicate how long the unit has been in the cool staging mode. The maximum number of cooling stages will be limited to the soft load setpoint until the soft load count down timer equals zero.
Max Pull Down
The controller can limit the rate at which the chilled water loop temperature is reduced. Whenever the rate of temperature decrease exceeds the maximum pull down setpoint, no additional cooling stages will be activated.
OM ALSMICRO 27
Condenser Fan Control
Condenser Fan Staging
The first condenser fan stage will be started in conjunction with the first compressor to provide initial head pressure control. The MicroTech controller continuously monitors the lift pressure referenced to several head pressure control setpoints and will adjust the number of operating condenser fans as required to maintain proper head pressure.
Head Pressure Control
For each circuit, the first stage of condenser fans will be wired in parallel with the compressor output so that they are energized with the compressor. For chillers with optional SpeedTrol, the first condenser fan stage will receive a control signal from the AOX board that in turn modulates the Johnson Controls S66DC-1 to provide variable speed fan operation. Each circuit has 3 additional digital outputs available for refrigerant head pressure control. Each output will energize an additional bank of condenser fans with each bank consisting of 1 or 2 fans depending on the size of the unit. Each output energizes additional heat rejection due to increased airflow across the air-cooled condenser regardless of the number of fans. If the outdoor ambient temperature is greater than 60°F when the unit is started, one additional condenser fan stage will be energized. If the outdoor ambient temperature is greater than 80°F, two additional fan stages will be energized.
ALS unit EERs are maximized by not allowing the last condenser fan stage to operate when the unit capacity is 25% and the condenser pressure is below 200 psi. The last fan stage will operate if the condenser pressure is above 220 psi at 25% unit capacity.
Lift Pressure Calculation
The expansion valve determines the minimum acceptable lift pressure. At low tonnage capacities, a minimum lift pressure of approximately 60 psid must be maintained. At high tonnage capacities, a higher lift pressure must be maintained to provide proper refrigerant flow through the expansion valve. Refer to the following table for the lift pressure values maintained at various unit capacities. Individual head pressure setpoints are provided at 25%, 50%, and 100% circuit capacity to optimize chiller operation. For operation at 75% capacity and greater with outdoor air temperatures less than 60°F, the minimum lift will automatically be reset downward. The maximum available reset at 100% capacity is 40 psid while the maximum reset at 75% capacity is 20 psid.
Table 16, Lift Pressure Values
Capacity Setpoint Adjustment Range
25% 90 psig 80 - 120 50% 100 psig 50 - 130 75% 110 psig Fixed
100% 140 psig 110 - 160
Loading...
+ 64 hidden pages
Buy Points How it Works FAQ Contact Us Questions and Suggestions Privacy Policy Cookie Policy Terms of Use