McQuay ALS125A Installation Manual

Installation and Maintenance Manual IM 549

Packaged Water Chiller with Screw Compressors

Models: ALS125A thru 204A

ALS205A thru 280A ALS300A thru 380A PFS150A thru 200A

Group: Chiller Part Number: 5714335Y

Date: August 1996

Table of Contents

Introduction ....................................................................... 4

General Description ........................................................... 4

Control Panel Features ....................................................... 5

Software Identification ....................................................... 5

Controller Layout ............................................................... 6

Component Data ............................................................... 6

Microprocessor Control Board...................................... 6

Analog/Digital Input Board (ADI Board) ........................ 7

Output Board ................................................................. 7

Electronic Expansion Valve Board (EXV Board)............ 7

Analog Output Board (AOX Board) ............................... 8

Keypad/Display.............................................................. 8

Sensors And Transducers ................................................... 9

Standard Sensors .......................................................... 9

Optional Sensor Packages ............................................ 9

Thermistor Sensors ....................................................... 9

Pressure Transducers.................................................. 11

Liquid Presence Sensor .............................................. 11

Sensor Locations ............................................................... 12

Analog Inputs............................................................... 13

Digital Inputs................................................................15

Analog Outputs............................................................ 16

Digital Outputs ............................................................. 17

Installation ....................................................................... 18

Controller Calibration................................................... 18

Field Wiring .................................................................. 18

Analog Sensors and Transducers .......................... 18

Digital Input Signals ................................................ 18

Digital Outputs ........................................................ 18

Interlock Wiring ....................................................... 18

External Alarm Circuit ............................................. 18

Power Wiring........................................................... 18

Power Supplies ....................................................... 18

Demand Limit and Chilled Water Reset Signals .... 18

Communication Ports ............................................. 18

Modem Kit............................................................... 19

Telephone Line for Remote Modem Access.......... 1 9

Unit Sequence of Operation............................................. 19

Off Conditions.............................................................. 19

Start-up........................................................................ 1 9

Waiting for Load .......................................................... 19

Start Requested........................................................... 19

Prepurge ...................................................................... 1 9

Opened EXV................................................................. 20

Low Ambient Start ....................................................... 20

Cool Stage (0 through 8) ............................................. 20

Compressor Control ......................................................... 20

Normal Compressor Staging Logic............................. 20

Project-Ahead Calculation .......................................... 21

Interstage Timer........................................................... 21

Anti-Cycle Timer .......................................................... 21

Compressor Heater Control ........................................ 21

Lead-Lag Of Refrigerant Circuits..................................... 22

Automatic Lead-Lag .................................................... 22

Manual Lead-Lag......................................................... 22

Electronic Expansion Valve .............................................. 22

Overview ...................................................................... 22

Exv Superheat Control ................................................ 22

Forced EXV Position Change ...................................... 22

EXV Evaporator Pressure Control ............................... 23

Chilled Water Reset Options ........................................... 23

Chilled Water Reset (Remote 4-20mA) ....................... 23

Ice Mode ...................................................................... 23

Network Reset ............................................................. 23

Return Water Reset ..................................................... 24

Remote Demand Limit................................................. 24

Network Demand Limit................................................ 24

Soft Loading ................................................................ 24

Max Pull Down............................................................. 24

Condenser Fan Control................................................... 24

Condenser Fan Staging............................................... 24

Head Pressure Control ................................................ 24

Lift Pressure Calculation ............................................. 25

Lift Pressure Dead Band ............................................. 25

Condenser Fan Stage Up ............................................ 26

High Pressure Stage Up .............................................. 26

Condenser Fan Stage Down ....................................... 26

SpeedTrol Logic .......................................................... 26

Pumpdown ....................................................................... 26

Automatic Pumpdown ................................................. 26

Manual Pumpdown................................................. 2 7

Service Pumpdown................................................. 27

Safety Systems .................................................................. 27

System Alarms............................................................. 27

Loss of Chw Flow ........................................................ 27

Bad Phase/Voltage ................................................. 27

No 5VDC @AI#5 ...................................................... 27

Chilled Water Freeze Protect.................................. 27

Bad Leaving Chilled Water Sensor......................... 27

Circuit Alarms .............................................................. 27

Mechanical High Pressure...................................... 27

High Condenser Pressure....................................... 27

High Condenser Pressure Stage Down.................. 27

High Condenser Pressure Stage Hold ................... 28

No Liquid Start........................................................ 2 8

No Liquid Run ......................................................... 28

Can’t Start - Low Evaporator Pressure .................. 28

Low Evaporator Pressure ....................................... 28

Freeze Protect Stage Down and

Freeze Stat Protect ................................................. 28

Failed Pre-Purge ..................................................... 28

Failed EXV or Low Refrigerant Charge................... 28

Failed Low Ambient Start ....................................... 2 8

Can't Pump Down................................................... 28

Bad Evaporator Pressure Sensor ........................... 28

Bad Condenser Pressure Sensor ........................... 28

Wait Flooded........................................................... 28

Wail Flooded ........................................................... 28

MicroTech Controller Test Procedures ........................... 29

Service Test (Digital Outputs)...................................... 29

Service Test (Digital Inputs)......................................... 29

2 IM 549

Keypad/Display ............................................................... 29

Overview ...................................................................... 29

Status Category........................................................... 2 9

Control Category ......................................................... 29

Alarm Category............................................................30

Display Format............................................................. 30

MicroTech Component Test Procedures & ALS Units ...... 30

Status LED Diagnostics............................................... 30

Red LED Remains On ............................................. 30

Red and Green LEDs Off ........................................ 30

Troubleshooting Power Problems............................... 30

Troubleshooting Communications Problems ............. 31

Troubleshooting the Keypad/Display Interface .......... 31

Display is Hard to Read .......................................... 31

Back Light Not Lit ................................................... 31

Display Is Blank or Garbled .................................... 31

Troubleshooting Analog Inputs ................................... 32

Analog Input Not Read by the MCB....................... 32

Troubleshooting Digital Inputs .................................... 32

Digital Input Not Read by the MCB........................ 32

Troubleshooting Analog Outputs ................................ 32

Analog Output Device Is Not Operating Correctly ... 32

Troubleshooting Output Boards.................................. 33

One LED Out ........................................................... 33

All LEDs Out............................................................ 3 3

LED Lit, Output Not Energized ............................... 33

Output Energized, LED Not Lit ............................... 33

Contact Chatter ...................................................... 33

Troubleshooting Solid-State Relays............................ 34

MCB Replacement ...................................................... 34

Connecting The Communications Trunk .................... 35

Communications Cable Check............................... 35

Level-1 Controller Connection................................ 35

Level-2 Controller Connection................................ 35

Keypad Key Functions ..................................................... 36

Keypad Password........................................................ 36

Category Group ........................................................... 36

Menu Group................................................................. 3 7

Item Group................................................................... 3 7

Action Group................................................................ 37

Example of Keypad Operation .................................... 37

Personal Computer Specification ..................................... 37

MicroTech Menu Structure ............................................. 38

Status Menus............................................................... 38

Control Menus ............................................................. 38

Alarm Menus................................................................ 38

Schematics And Drawings ................................................ 60

Control Cabinet Layout ............................................... 60

Wiring Legend.............................................................. 63

PFS Unit Control.......................................................... 64

PFS 8-Stage Output .................................................... 65

PFS MicroTech ............................................................ 66

ALS Unit Control.......................................................... 67

ALS 8-Stage Output .................................................... 68

ALS MicroTech ............................................................ 69

ALS Field Wiring .......................................................... 70

IM 549 3

Introduction

This manual provides installation, setup and troubleshooting information for the MicroTech controller provided on McQuay screw compressor chillers. Please refer to installation manual IM548 for unit application information as well as water and refrigerant piping details. All operating descriptions contained in this manual are based on MicroTech controller software version SC2-X18B, SC3XX19 and SC4XX19A. Chiller operating characteristics and menu selections may vary with other versions of controller software. Contact McQuayService for software update information.

CAUTION

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

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).

General Description

The MicroTech Unit Control Panel, available on all McQuay ALS and PFS products, contains a Model 250 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. A valid password must be entered into the panel keypad by the operator before any set points may be altered.

Table 1.

Unit Identification

ALS Air Cooled Chiller with Screw Compressors PFS Water Cooled Chiller with Screw Compressors

4 IM 549

Control P anel Featur es

●

Flexible control of leaving chilled water with convenient reset capability.

●

Enhanced head pressure control on air cooled units resulting in increased total unit SEER.

●

Convenient, easy to read 2 line by 16 character display for plain English readout of operating temperatures and pressures, operating modes or alarm messages.

●

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, set points or alarm messages.

●

Security password protection against unauthorized changing of set points and other control parameters.

●

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

Software Identification

Controller software is factory installed and tested in each panel prior to shipment. The software is identified by a program code which 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.

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.

●

Soft loading feature to reduce electrical consumption and peak demand charges during chilled water loop pulldown.

●

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.

●

Flexible internal time clock for on/off scheduling.

●

Communications capabilities for local system monitoring, changing of set points, 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.

●

Special service modes may be used to override automatic unit staging during system checkout and service.

The software “version” is the 5th & 6th digit of the software number. In the example, the version is “17” and the revision to the software is “G”.

Revisions are released in alphabetical order.

Hardware Software

Screw Chiller Number of Compressors Refrigerant

Type 2 = R22 Type 3 = R134a

SC 3 2 E 19 A

Revision

Version English

IM 549 5

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. Power for the system is provided by transformers T-2 and T-4. All field wiring must enter the control cabinet through the

Figure 1. Typical control cabinet layout — 2 compressor unit

Keypad

F1 CB F2

High voltage wireway

wireway

High voltage

TB3 TB2

ADX EXV

Low voltage wireway

Modem

MDBI

Low voltage wireway

TB4 TB5

Low voltage wireway

Fax alarm

option

ADI

Mech. relays

High voltage wireway

Output

board

Low voltage wireway

GD1

GD2

RES1

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 while the PFS Keypad/Display is accessible through the exterior of the control cabinet. See Figure 1 for typical control cabinet layout.

FB6

M11 M12 M13 M15 M23 M25

T10

M21 M22 M14 M24

OL5

CB5 CB1

M5 M1

FB8FB7

OL1

FB9

PVM

FB10

FB11

CT1

OL2 OL6

CB2

M2 M6

FB5

GRD

PB1

CB6

GFP

Component Data

Microprocessor Control Board (MCB1)

The Model 250 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.

6 IM 549

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

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.

Figure 4. Output board

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.

It receives control signals from the Microprocessor Con-

trol Board through a 50 conductor ribbon cable.

Electronic Expansion Valve Board (EXV Board)

Each EXV Board will directly control up to two electronic expansion valves. The boards may be cascaded together for

Figure 5. EXV board

units with more than two EXV’s. Control instructions for the board are generated by the M250 controller.

IM 549 7

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

Figure 6. AOX board

driving a variable speed condenser fan. Each AOX Board is factory set via jumper to provide an output signal of 0 - 10 VDC.

Keypad/Display

The Keypad/Display is the primary operator interface to the unit. All operating conditions, system alarms and set points can be monitored from this display and all adjustable set

points can be modified from this keyboard if the operator has entered a valid operator password.

Figure 7. Keypad display

8 IM 549

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 Saturated Suction Temperature, Circuit #1, 2, 3 & 4

Optional Sensor Packages

Water cooled units only:

Entering Condenser Water Temperature Leaving Condenser Water Temperature

Thermistor Sensors

MicroTech panels use a negative temperature coefficient thermistor for temperature sensing. A normal sensor will measure 3000 ohms at 77°F.

Liquid Line Temperature, Circuit #1, 2, 3 & 4 (Provides direct display of subcooling and superheat) Entering Evaporator Water Temperature Ambient O.A. Temperature

Air and water cooled units:

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.

Figure 8. Thermistor sensor

Stainless steel

tubing

Potting

Thermistor

Shielded cable

IM 549 9

Table 2. MicroTech thermistors

°F Ohms Volts °F Ohms Volts °F Ohms Volts

15 16,104 4.145 77 3000 2.373 139 761 0.932 16 15,627 4.124 78 2927 2,343 140 746 0.917 17 15,166 4.102 79 8357 2.313 141 731 0.902 18 14,720 4.080 80 2789 2.283 142 717 0.888 19 14,288 4.057 81 2723 2.253 143 703 0.874 20 13,871 4.034 82 2658 2.223 144 689 0.859 21 13,467 4.011 83 2595 2.194 145 676 0.846 22 13,076 3.988 84 2534 2.164 146 662 0.831 23 12,698 3.964 85 2474 2.135 147 649 0.818 24 12,333 3.940 86 2416 2.106 148 637 0.805 25 11,979 3.915 87 2360 2.077 1490 625 0.792 26 11,636 3.890 88 2305 2.049 150 613 0.779 27 11,304 3.865 89 2251 2.020 151 601 0.766 28 10,983 3.839 90 2199 1.992 152 589 0.753 29 10,672 3.814 91 2149 1.965 153 578 0.741 30 10,371 3.788 92 2099 1.937 154 567 0.729 31 10,079 3.761 93 2051 1.909 155 556 0.717 32 9797 3,734 94 2004 1.882 156 546 0.706 33 9523 3.707 95 1959 1.855 157 535 0.694 34 9258 3.608 96 1914 1.828 153 525 0.683 35 9002 3.653 97 1871 1.802 159 516 0.673 36 8753 3.625 98 1829 1.775 160 506 0.661 37 8512 3.597 99 1788 1.750 161 496 0.650 38 8278 3.569 100 1747 1.724 162 487 0.640 39 8052 3.540 101 1708 1.698 163 478 0.629 40 7832 3.511 102 1670 1.673 164 469 0.619 41 7619 3.482 103 1633 1.648 165 461 0.610 42 7413 3.453 104 1597 1.624 166 452 0.599 43 7213 3.424 105 1562 1.600 167 444 0.590 44 7019 3.394 106 1528 1.576 168 436 0.580 45 6831 3.365 107 1494 1.552 169 428 0.571 46 6648 3.335 108 1461 1.528 170 420 0.561 47 6471 3.305 109 1430 1.505 171 413 0.553 48 6299 3.274 110 1398 1.482 172 405 0.544 49 6133 3.244 111 1368 1.459 173 398 0.535 50 5971 3.213 112 1339 1.437 174 391 0.527 51 5814 3.183 113 1310 1.415 175 384 0.518 52 5662 3.152 114 1282 1.393 176 377 0.510 53 5514 3.121 115 1254 1.371 177 370 0.501 54 5371 3.078 116 1228 1.350 178 364 0.494 55 5231 3.059 117 1201 1.328 179 357 0.485 56 5096 3.028 118 1176 1.308 180 351 0.478 57 4965 2.996 119 1151 1.287 181 345 0.471 59 4714 2.934 121 1103 1.247 183 333 0.456 60 4594 2.902 122 1080 1.227 184 327 0.448 61 4477 2.871 123 1058 1.208 185 321 0.441 62 4363 2.839 124 1036 1.189 186 316 0.435 63 4253 2.808 125 1014 1.170 187 310 0.427 64 4146 2.777 126 993 1.151 188 305 0.421 65 4042 2.745 127 973 1.133 189 299 0.413 66 3941 2.714 128 953 1.115 190 294 0.407 67 3842 2.682 129 933 1.076 191 289 0.400 68 3748 2.651 130 914 1.079 192 284 0.394 69 3655 2.620 131 895 1.062 193 280 0.389 70 3565 2.589 132 877 1.045 194 275 0.382 71 3477 2.558 133 859 1.028 195 270 0.376 72 3392 2.527 134 842 1.012 196 266 0.371 73 3309 2.496 135 825 0.995 197 261 0.364 74 3328 2.465 136 809 0.980 198 257 0.359 75 3150 2.434 137 792 0.963 199 252 0.353 76 3074 2.404 138 777 0.948 200 248 0.348

10 IM 549

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 psig. Condenser pressure sensors have a range of 0 to 450 psi and a resolution

Figure 9.

Red dot – condenser

Blue dot – evaporator

Liquid Presence Sensor

The presence of liquid refrigerant is determined by a liquid level sensor mounted at the liquid injection port in the compressor casting. Whenever the glass prism sensor tip is

of 0.5 psig. The pressure transducers require an external 5 VDC power supply to operate which is provided by the MicroTech controller. This connection should power any additional devices.

in contact with liquid, the sensor output signal will be high (>7VAC). If no liquid is detected, the output will be low (0VAC).

not be used to

Figure 10.

IM 549 11

Sensor Locations – 2 Compressor Unit

Table 3.

Sensor Description

S00 Evaporator Leaving Water Temperature 605830-03 S01 Evaporator Pressure Transducer Circuit #1 658168B-011 S02 Evaporator Pressure Transducer Circuit #2 658168B-011 S03 Condenser Pressure Transducer Circuit #1 658168B-021 S04 Condenser Pressure Transducer Circuit #2 658168B-021

Evaporator Water Temperature Reset

S06

(Outdoor Air or Zone)

S07 Demand Limit N/A S08 Evaporator Entering Water Temperature 705830B-02

Condenser Entering Water Temperature

S09

(or Outside Air)

S11 Total Unit Amps S12 Suction Temperature Circuit #1 705830B-02 S13 Suction Temperature Circuit #2 705830B-01 S14 Liquid Line Temperature Circuit #1 705830B-01 S15 Liquid Line Temperature Circuit #2 705830B-02

Figure 11.

Part

Number

N/A

705830B-01

S00

Out

S08

S12

S13

S03

S02

S01 S14

S15S04

Inside of

control box

on power

& control

panels

S11

S09

Back of

control box

12 IM 549

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

Table 4a. Analog inputs — 2 compressor units

No. Description Sensor Location

S00 Evaporator Leaving Water Temp Leaving chilled water nozzle S01 Evap Pressure Transducer, Cir #1 Common cir #1 suction line S02 Evap Pressure Transducer, Cir #2 Common cir #2 suction line S03 Cond Pressure Transducer, Cir #1 Compressor #1 discharge cover S04 Cond Pressure Transducer, Cir #2 Compressor #2 discharge cover S05 Transducer Power Voltage Ratio (Internal) S06 Reset-Evap Water Temperature External 4-20 mA signal S07 Demand Limit External 4-20 mA signal S08 Entering Evaporator Water Temp Entering chilled water nozzle S09 O.A.T. (Ent Cond Water Temp) Back of the control box S10 Condenser Leaving Water Temp Leaving condenser water nozzle S11 Percent Unit Amps Signal converter board S12 Suction Temperature Circuit #1 Well brazed to the cir #1 suction line S13 Suction Temperature Circuit #2 Well brazed to the cir #2 suction line S14 Liquid Line Temperature Circuit #1 Well brazed to the cir #1 liquid line S15 Liquid Line Temperature Circuit #2 Well brazed to the cir #1 liquid line

internal regulated 5 VDC and 12 VDC supplies provide the correct operating voltage for the sensors.

Table 4b. Analog inputs — 3 compressor units

Sensor

Number

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 S11 Percent Circuit Amps Circuit #2 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

Description

Table 4c. Analog inputs — 4 compressor units

Sensor

Number

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

S11 Percent Circuit Amps Circuit #2 & 4

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

Description

IM 549 13

Sensor Locations – 3 Scr ew Compressor Unit

Sensor Locations – 4 Scr ew Compressor Unit

14 IM 549

Digital Inputs

Note: All digital inputs are 24 VAC.

At 7.5 VAC to 24 VAC the digital input contacts are

considered closed, and the signal level is high.

Below 7.5 VAC, the contacts are considered open, and the signal level is low.

Table 5a. Digital inputs — 2 compressor unit

Number Description Lo Signal Hi 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 Oil Level Sensor, Compressor #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 Oil Level Sensor, Compressor #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 5b. Digital inputs — 3 compressor unit

No. Description Led Off Led On

0 Mechanical High Pressure Switch, Cir #1 Alarm Normal

1 Liquid Presence Sensor Compr #1 No Liquid Liquid

2 Motor Prot Compr #1 Alarm Normal

3 Not Used — —

4 Not Used — —

5 System On-Off Switch Off On

6 Phase Volt Monitor Compr #1 Alarm Normal

7 PumpDown Switch Compr #1 Normal Pump DN

8 Mechanical High Pressure Switch Cir #2 Alarm Normal

9 Liquid Presence Sensor Compr #2 No Liquid Liquid 10 Motor Prot Compr #2 Alarm Normal 11 Not Used — — 12 Not Used — — 13 Remote Start Stop Switch Stop Start 14 Evap Water Flow Switch No Flow Flow 15 PumpDown Switch Compr #2 Normal PumpDn 16 Mechanical High Pressure Switch Cir #3 Alarm Normal 17 Liquid Presence Sensor Compr #3 No Liquid Liquid 18 Motor Prot Compr #3 Alarm Normal 19 Not Used — — 20 Not Used — — 21 Phase Volt Monitor Compr #2 Alarm Normal 22 Phase Volt Monitor Compr #3 Alarm Normal 23 PumpDown Switch Compr #3 Alarm Normal

IM 549 15

Table 5c. Digital inputs — 4 compressor unit

No. Description Led Off Led On

0 Mechanical High Pressure Switch, Cir #1 Alarm Normal 1 Liquid Presence Sensor Compr #1 No Liquid Liquid 2 Motor Prot Compr #1 Alarm Normal 3 Not Used — — 4 Not Used — — 5 System On-Off Switch Off On 6 Phase Volt Monitor Compr #1 Alarm Normal 7 PumpDown Switch Compr #1 Normal Pump DN 8 Mechanical High Pressure Switch Cir #2 Alarm Normal

9 Liquid Presence Sensor Compr #2 No Liquid Liquid 10 Motor Prot Compr #2 Alarm Normal 11 Not Used — — 12 Not Used — — 13 Remote Start Stop Switch Stop Start 14 Evap Water Flow Switch No Flow Flow 15 PumpDown Switch Compr #2 Normal PumpDn 16 Mechanical High Pressure Switch Cir #3 Alarm Normal 17 Liquid Presence Sensor Compr #3 No Liquid Liquid 18 Motor Prot Compr #3 Alarm Normal 19 Not Used — — 20 Not Used — — 21 Phase Volt Monitor Multi Point Alarm Normal 22 Not Used — — 23 PumpDown Switch Compr #3 Normal PumpDn

0 Mechanical High Pressure Switch Cir #4 Alarm Normal

1 Liquid Presence Sensor Compr #4 No Liquid Liquid

2 Motor Prot Compr #4 Alarm Normal

3 Not Used — —

4 Not Used — —

5 Not Used — —

6 Not Used — —

7 PumpDown Switch Compr #4 Normal PumpDn

Analog Outputs

Table 6. Analog outputs

No. 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

16 IM 549

Digital Outputs

Table 7a. Digital outputs — 2 compressor unit

No. 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, Compr #1 Stop Run 5 Top Solenoid, Compr #1 Hold Load 6 Bottom Right Solenoid, Compr #1 Hold Load 7 Bottom Left Solenoid, Compr #1 Hold Load 8 MCR Relay, Compr #2 Stop Run

9 Top Solenoid, Compr #2 Hold Load 10 Bottom Right Solenoid, Compr #2 Hold Unload 11 Bottom Left Solenoid, Compr #2 Hold Load 12 Condenser Fan #1, Circ #1 (M12) Off On 13 Condenser Fan #2, Circ #1 (M13) Off On 14 Condenser Fan #3, Circ #1 (M14) Off On 15 Condenser Fan #4, Circ #1 (M15) Off On 16 Condenser Fan #1, Circ #2 (M22) Off On 17 Condenser Fan #2, Circ #2 (M23) Off On 18 Condenser Fan #3, Circ #2 (M24) Off On 19 Condenser Fan #4, Circ #2 (M25) Off On 20 Liquid Solenoid Valve, Cir #1 Close Open 21 Liquid Solenoid Valve, Cir #2 Close Open 22 (Spare) 23 (Spare)

Table 7b. Digital outputs — 3 compressor unit Table 7c. Digital outputs — 4 compressor unit

Relay Description

0 Alarm Circuit 1 Chilled Water Pump Relay 2 EXV Control 3 EXV Control 4 Compr #1 Contactor 5 Compr #1 Top Solenoid Valve 6 Compr #1 Bottom Right Solenoid Valve (feed) 7 Compr #1 Bottom Left Solenoid Valve (vent) 8 Compr #2 Contactor

9 Compr #2 Top Solenoid Valve (feed) 10 Compr #2 Bottom Right Solenoid Valve (feed) 11 Compr #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 Compr #3 Contactor 21 Compr #3 Top Solenoid Valve (feed) 22 Compr #3 Bottom Right Solenoid Valve (feed) 23 Compr #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 Hot Gas Bypass - SV5 29 Hot Gas Bypass - SV6

Relay Description

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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 set points will be checked and adjusted if necessary by the McQuayService

Field Wiring

start-up technician prior to starting the unit. The MicroTech controller contains default set points which will be appropriate for most common installations.

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 control panel.

Digital outputs

Devices wired to the digital outputs typically be 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 it’s resistance which 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 are shown by individual red LEDs for ease of determining output status.

Interlock wiring

All interlock wiring to field devices such as flow switches and pump starters is provided by the installing contractor. 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 24VAC voltage source is available at field wiring terminal #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 24VAC is to be provided and wired by the installing contractor. The normal and alarm states for the 24VAC alarm signal are programmable by the operator. Available settings are:

Pre-alarm annunciation: Close-or-Open-or-Blink Alarm annunciation: Close-or-Open

Power wiring

115VAC 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 RS-485 communication standard. Refer to the field wiring drawing in this manual for details.

18 IM 549

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.

Unit Sequence of Operation

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.

The following sequence of operation is typical for McQuay ALS air cooled and PFS water cooled chillers. The sequence

Off Conditions

With power supplied to the unit, 115 VAC power is applied through the control fuse F1 to the compressor crankcase 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 (S1). 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

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

Waiting for Load

Once flow is established the controller will sample the chilled water temperature and compare it against the Leaving Chilled Water Set point, 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

may vary depending on various options which may be installed on the chiller.

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.

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.

Chilled Water Set point plus + the adjustable Control Band plus the Start-up Delta Temperature Set point, the controller will select the refrigerant circuit with the lowest number of starts as the lead circuit and initiate the compressor start sequence.

Start Requested

In the Start Requested Mode, the electronic expansion valve is assumed to be fully closed. The MicroTech controller will read the evaporator pressure to ensure 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 lead 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

IM 549 19

to less than 40 psi or 45 seconds has elapsed. If the evaporator pressure does not drop to 40 psi within the 45 seconds, the compressor will stop and the display will read “Failed Prepurge”. The alarm output will be activated.

Opened EXV

With the evaporator pressure less than 40 psi and the compressor still running, the electronic expansion valve will be driven open to 200 steps. If the evaporator pressure rises above the freeze stat set point, the chiller will advance to

Low Ambient Start

If the difference between the freeze stat set point and the evaporator refrigerant pressure is greater than 12 psi, the low ambient start timer will be set to 180 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 set point and the evaporator refrigerant pressure is greater than 12 psi, the following calculation will be used to set the low ambient start timer:

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 set point. The current cooling stage will be displayed on the keypad/display.

Cool Staging Mode. If the circuit is in Cool Staging Mode and after 20 seconds, the evaporator pressure remains below the freeze state set point but is greater than 2 psi, the controller will transition to Low Ambient Start Mode.

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”.

Automatic chiller staging may be overridden by selecting “Manual Cooling” as the operating mode and then choosing the desired cooling stage.

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” set point.

Compressor Staging Sequence

Four Compressors Available

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 Lag 1 Lag 2 Lag 3 Unit Lead Lag 1 Lag 2 Lag 3 Unit

Compressor Compressor Compressor Compressor Capacity Compressor Compressor Compressor Compressor Capacity

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% 50% 50% 50% 56.3%

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Three Compressors Available

Staging Up Staging Down

Stage

10 100% 75% 75% 0% 62.5% 100% 75% 75% 0% 62.5% 11 100% 100% 75% 0% 68.8% 100% 100% 75% 0% 68.8% 12 100% 100% 100% 0% 75.0% 100% 100% 100% 0% 75.0%

Lead Lag 1 Lag 2 Lag 3 Unit Lead Lag 1 Lag 2 Lag 3 Unit

Compressor Compressor Compressor Compressor Capacity Compressor Compressor Compressor Compressor Capacity

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% 75% 75% 50% 0% 50.0% 9 75% 75% 75% 0% 56.3% 75% 75% 75% 0% 56.3%

Two Compressors Available

Staging Up Staging Down

Stage

Lead Lag 1 Lag 2 Lag 3 Unit Lead Lag 1 Lag 2 Lag 3 Unit

Compressor Compressor Compressor Compressor Capacity Compressor Compressor Compressor Compressor Capacity

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% 75% 75% 0% 0% 37.5% 7 100% 75% 0% 0% 43.8% 100% 75% 0% 0% 43.8% 8 100% 100% 0% 0% 50.0% 100% 100% 0% 0% 50.0%

One Compressor A vailable

Staging Up Staging Down

Stage

Lead Lag 1 Lag 2 Lag 3 Unit Lead Lag 1 Lag 2 Lag 3 Unit

Compressor Compressor Compressor Compressor Capacity Compressor Compressor Compressor Compressor Capacity

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% 4 100% 0% 0% 0% 25.0% 100% 0% 0% 0% 25.0%

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 Project-Ahead Calculation also moderates the controllers 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 set point (default=210 sec). The

interstage timer for stage down commands is 1⁄3 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.

Compressor Heater Control

Compressor Heater Control for PFS units is based on the suction line superheat. If the superheat reading drops below

IM 549 21

3°F, the heater will be energized. The heater will be deenergized when the superheat rises above 8°F.

Lead-Lag of Refrigerant Circuits

The following compressor control rules are enforced in the control software.

●

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.

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

Manual Lead-Lag

The operator may override automatic circuit selection by manually selecting the lead circuit via the keypad.

When the set point equals “auto”, the lead compressor is

selected by the MicroTech controller based upon which

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 set point. Valve positioning signals are converted to actuator

●

The Micro Tech controller will not turn off the lag compres-

sor 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.

be started first. If circuits are operating and a stage down is required, the circuit with the most operating hours will cycle off first.

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.

step pulses by the EXV board which in turn drive 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 set point. Superheat set points are based on refrigerant circuit capacity. For circuit capacity of 25% to 50%, the superheat set point will be 8.0°F. For circuit capacity of 75% to 100%, the superheat set point 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 set point (default=200 steps). The current

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 set point.

Table 8a.

When Staging Up

From To Open

25% 50% 65% 50% 75% 50% 75% 100% 25%

suction line temperature is compared against the Suction Line Temperature set point (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 interval will be 60 seconds.

With a decrease in circuit capacity, the electronic expansion valve position will be closed by a fixed percentage of its current position.

Table 8b.

When Staging Down

From To Close

100% 75% 18%

75% 50% 40% 50% 25% 60%

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