Sabroe UniSAB III Sequence Operating instructions

Version 1.00

Unisab III Control

Sequence manualen



Preface

This manual describes the Multisab sequence function in Unisab III.

Chapter 1. Multisab regulation describes how to set up the system and how to adjust the reg-

ulation parameters.

Chapter 2. Start and system numbers describes the combination of reciprocating and screw

compressors and how to use the different strategies when choosing master regulator.

Chapter 5. Rotatune Multisab regulation explains the combination between so-called “rotatune”

compressors and conventional compressors. A rotatune compressor is a compressor where the

speed of the motor drive is controlled by a frequency converter in order to reduce the power

consumption in part load situations on the compressor. A rotatune compressor may be either a

reciprocating or a screw compressor as long as the motor drive is fitted with a frequency con-

verter.

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Contents

1. Multisab regulation

4

1.1.1. Introduction 4

1.1.2. System set-up 5

1.1.3. Regulation set-up 8

2. Start and system numbers

11

2.1.1. Pref. master = compr# 11

2.1.2. Start and system numbers - pref. master = Start# 12

2.1.3. Example of regulation - screw compressors only 13

2.1.4. Loading sequence 15

2.1.5. Unloading sequence 16

2.1.6. Operating sequence 17

2.1.7. Example of regulation - reciprocating compressors only 17

2.1.8. Loading sequence 18

2.1.9. Unloading sequence 19

2.1.10. Example of regulation - combination of screw and recip-

rocating compressors 20

2.1.11. State of transfer 22

2.1.12. State of take-over 23

2.1.13. Unloading sequence - sequence B 25

3. Troubleshooting

28

3.1.1. The plant cannot start 28

3.1.2. The plant does not run in sequence 28

4. Checklist

29

5. Rotatune Multisab regulation

32

5.1. Introduction

32

5.1.1. Preface 32

5.1.2. Requirements 32

5.1.3. Purpose 32

5.1.4. Multisab set-up 33

5.1.5. Different rota compressor types 33

5.2. Rota compressor set-up and calibration

34

5.2.1. Type 1 rotatune screw compressor (with end code HR,FV

and LR/SR) 34

5.2.2. Type 2 Rota combi screw compressor (Manual Vi) 36

5.2.3. Type 3 Rota combi screw compressor (automatic Vi) 38

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5.2.4. Type 4 rota reciprocating compressor 38

5.3. Rotatune master/slave sequence control

41

5.3.1. Set-up/configuration 41

5.3.2. Seq. (Sequence) 43

5.4. Examples of combinations between rota and

conventional compressors

45

5.5. Examples of several rotatune compressors

running in parallel operation

55

5.6. Limitations to rotatune Multisab function

59

5.6.1. Examples with compressors of different sizes 59

5.6.2. Regulator setting 61

5.7. Setting list (EXAMPLE)

64

5.7.1. Vacon frequency converter setting list (EXAMPLE) 64

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1. Multisab regulation

1.1.1. Introduction

Multisab is a distributed compressor control system which can regulate capacity in reciprocating

and/or screw compressors according to detailed rules and in a sequence set up by the user.

Multisab is included in all Unisab III and Unisab II units on delivery. The Unisab III Multisab

system does not support communication with the old Prosab II and Unisab I systems. If your

systems contain such controllers, please contact Johnson Controls Denmark, Technical Support

Controls for upgrading possibilities.

Multisab connects up to 14 compressors in the same communication system.

14

3

2

1

Unisab IIIUnisab IIIUnisab III

Unisab III

The Multisab system is based on the following basic rules:

• The system is controlled and optimized according to suction pressure, process temper-

ature, discharge pressure or external 4-20 mA signal, depending on the chosen type of

regulation.

• The system always follows the programmed starting sequence.

• Screw compressors in operation run at the highest capacity possible.

• It is acceptable that reciprocating compressors operate at part load.

Multisab is a regulating system for general application. In case of special requirements regarding

optimization of the compressors, it may be necessary to regulate the compressors externally.

There may also be special requirements for the plant regarding regulating speed and accuracy,

which makes a central external regulation necessary. Such an alternative regulation can be car-

ried out in several ways:

• controlling the digital input External start permission, normal stop

• using the digital output Auxiliary output

• using the 4-20 mA input Auxiliary input

• changing capacity set points from PC/PLC via Comsab II/ RS2LAN

14

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1

Unisab IIIUnisab IIIUnisab III

Unisab III

PC/PLC

possible modem

COMSAB II/RS2LAN

Multisab regulation

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If compressor capacity is controlled by Comsab II or RS2LAN from a central computer, note the

time delay in the communication system. It is possible to make an approximate calculation of

this time delay. See Comsab II instruction no. 0171-400.

1.1.2. System set-up

To be able to use Multisab, you must complete the points on the checklist in chapter 4. The

points on the checklist can be explained as follows:

A

All units to be included in the system must be connected physically. See the Engineering manual

to find information on the physical linking of Unisab III units.

B

All units must be configured for intercommunication. See the Engineering manual.

In Set-up/Config./Plant you must enter a compressor number (Compr. no.) for each unit starting

with no. 1 and upwards. The numbers correspond to the number of compressors in the system.

Important!

It is recommended to mark the compressors with this number. If two compressors have the

same compressor number entered in Unisab III, there will be no communication between the

units.

In Set-up/Config./Communication the communication speed, Baud rate, must be entered. Usu-

ally choose the highest baud rate (19200 baud). Note that it must be checked that all units on

the network are able to run at the selected rate.

C

All units, especially screw compressors, must have a swept volume value entered.

In Set-up/Configuration/Compressor block enter Swept volume, which can be read on the com-

pressor name plate. See the Engineering manual.

Multisab regulation

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D

In all units it must be entered whether the system contains a common evaporator and a common

condenser.

In Set-up/Configuration/Plant enter the current combination of common evaporator and com-

mon condenser in Common evap./cond. See the Engineering manual.

E

All units must be have a system no. and a start no.

In Set-up/Configuration/Plant enter Start no. and System no. See the Engineering manual.

F

All units in the same system must agree on the way a regulating master (sys. regulator) is

chosen.

In Set-up/Compr. control select the same Pref. master in all Unisab III units, Compr# or Start#.

See the Engineering manual.

If selecting Compr#, the Unisab III unit with the lowest Compr. no. (point B above) will always

be System regulator exept if that particular Unisab III is off power or disconnected from the

network. In this case, the Unisab III with the next Compr. no. will be System regulator. Note

Multisab regulation

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that a defective controlled input sensor does not automatically change the System regulator in

this case.

If selecting Start#, the Unisab III unit with the lowest Start no. (point F above) will be System

regulator for as long as the compressor belonging to this Unisab III unit is running and ready.

If the compressor is not available, Unisab III is not in Remote mode, the controlled input sensor

(point H below) is defective or Unisab III is off power or disconnected from the network, the

Unisab III unit with the next Start no. will be System regulator.

The System regulator is master of the coordinated capacity control, which also means that the

controlled input sensor (point H below) on this particular Unisab III unit is in use. If only one

compressor is required, it will usually be the compressor of the System regulator which is in

operation. However, if Compr# is selected, it may very well be a different compressor.

Guidelines for selecting Compr# or Start#:

• Compr# is easier and safer if the controlled input is not available to all Unisab III units.

• Start# is easier and safer if equalizing running hours is important, especially if all (or all

except one) compressors must always be available.

• Make sure that Compr. no. is a fixed value, ie never change it to equalize running hours.

Start no., on the other hand, should be changed from time to time.

Note: If there is more than one rotatune configured Unisab III in the system, make sure you

choose Start# as the Preferred master. See Set-up/Configuration/Drive.

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G

All units must be adjusted to remote control.

In Set-up/Compressor control, select Remote mode. See the Engineering manual.

1.1.3. Regulation set-up

H

All units must be configured to the desired regulating method.

In Set-up/Compr. control, adjust Control on to the desired regulating method. See the Engi-

neering manual.

The following four regulators are available: (The last two have both a cooling and a warming

function)

• Suction side (regulation of suction pressure)

• Discharge side (regulation of discharge pressure)

• Process out/hot water (regulation of process out/water temperatures)

• Ext. cooling/Ext. heating (regulation of a user defined measuring value: temperature/

pressure/level)

Note: All units which may become master must measure the same value to be used for capacity

control. If eg process out/Hotwater temperature control is selected, a separate temperature

sensor must be available to all the relevant Unisab III units. Point F above explains how to select

the master.

I

All units must (normally) be configured to both automatic start and automatic stop. If not, the

compressor will not start up automatically when cooling is required or stop automatically when

cooling is no longer required and the temperature becomes too low.

In Set-up/Compr. control, choose:

Auto start = Yes

Auto stop = Yes

See the Engineering manual.

J

The chosen regulator must be set in all units.

In the parameter picture to be regulated, set the following:

• Regulator set point, Sp1 and possibly Sp2

• Regulator neutral zone, Nz

• Regulator proportional band, Pb

See the Engineering manual.

Regulation of the entire system will usually take place on the basis of the regulator in the current

regulating master (system regulator), ie the compressor with the lowest compressor or starting

number.

For this reason it is particularly important that the parameters of the configured regulator has

been set correctly. Always set the same set point on all other units in the same system (ie with

the same System no.)

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K

Timers must be adjusted in all units.

In Set-up/Timers/Timer set-up the following timers are set:

•Start delay, time delay before start.

•Stop delay, time delay before stop

See the Engineering manual.

Both timers can be set to work proportionally (ie depending on the size of Pb and only when

configured as a reciprocating compressor).

In Set-up/Timers/P band factor a proportional factor can be set for each timer.

This means that if there is only a little difference between the desired (Sp) and the current

temperature, the time delay timers will count down very slowly (up to 10 times slower than the

set time depending on the set factor).

If the difference between the desired and the current temperature is considerable (outside the

P Band), the time delay timers will count in seconds.

See the Engineering manual.

For reciprocating compressors, another two timers must be set:

•Delay up, time delay during loading of stages

•Delay down, time delay during unloading of stages

These two timers can also be set to work proportionally (ie depending on the size of Pb).

In Set-up/Timers/P band factor a proportional factor can be set for each of the timers.

L

In plants with a combination of screw and reciprocating compressors, a number of factors can

be set to optimize compressor operation and get the most out of the two compressor types.

These factors can only be set for reciprocating compressors and will be effective only if all re-

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ciprocating compressors have lower starting numbers than the screw compressors in the same

system and only if Take-over is set to Yes in Set-up/Config. See the Engineering manual.

The optimization is carried out by the Transfer function and the Take-over function. It is not

economical to run a screw compressor at low capacity. The Transfer function attempts to avoid

this by reducing capacity of the reciprocating compressor(s), ie by transferring load to the screw

compressor. The Take-over function increases the capacity of the reciprocating compressor(s)

to make the screw compressor stop, ie by taking over load from the screw compressor.

See section 2.1.10. Example of regulation - combination of screw and reciprocating compres-

sors . As described below, several parameters are available to adjust the Transfer and Take-

over functions.

In Set-up/Timers/Transfer a proportional factor Factor down (1-10, default 1) can be set for

timer Delay down. It is used when Multisab wants to let a reciprocating compressor decrease its

capacity in favour of a screw compressor at low capacity. A larger Factor down makes Multisab

wait longer before reducing the reciprocating compressor capacity another stage.

See illustrative examples of the Transfer and Take-over situations in section 2.1.10. Example of

regulation - combination of screw and reciprocating compressors . As described below, several

parameters are available to adjust the Transfer and Take-over functions.

In Set-up/Timers/Transfer the size of a transfer Zone can be set, ie the part of the proportional

band (0-100%, default 15%) where transfer may take place.

In Set-up/Timers/Take-over a proportional factor Factor up (1-10, default 1) can be set for the

Delay up timer. It is used when Multisab wants to increase capacity of the reciprocating com-

pressor to force the screw compressor down below 5% capacity which makes it stop. A larger

Factor up makes Multisab wait longer before increasing the reciprocating compressor capacity

another stage.

In Set-up/Timers/Take-over a proportional factor Factor start (1-10, default 1) can be set for

the Start delay timer. It is used when Multisab wants to make a reciprocating compressor take

over operation from a screw compressor which is running at low capacity.

In Set-up/Timers/Take-over the size of a take-over Zone can be set, ie the part of the propor-

tional band (0-100%, default 15%) where take-over may take place.

In Set-up/Timers/Timer set-up the following timers are found:

•Take-over delay, time delay at take-over, fixed at 300 seconds. When a reciprocating

compressor has calculated that it is possible to take over a subsequent screw compres-

sor, it will switch to take-over mode and start Take-over delay. If the reciprocating

compressor during the entire count down of take-over delay can continue to take over,

thus staying in take-over mode, Start delay will be activated once Take-over delay ex-

pires.

•Take-over max., maximum duration of a take-over attempt. When Take-over delay

expires, the time delay Take-over max. will also be activated to ensure that a recipro-

cating compressor will not use more than the Take-over max. time during a take-over

attempt. If the screw compressor has not stopped before the Take-over max. expires,

the reciprocating compressor will give up and stop. Cancel this delay function by setting

the delay for 0 (default).

Multisab regulation

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2. Start and system numbers

2.1.1. Pref. master = compr#

Example A

Plants with one temperature system and 5 compressors where pref.master =

Compr#.

System no. 11111

Starting no. 12345

Compressor no. K1 K2 K3 K4 K5

Example B

Plants with one temperature system and 5 compressors where pref.master =

Compr#.

System no. 11111

Starting no. 54321

Compressor no. K1 K2 K3 K4 K5

In examples A and B, K1 will be the regulator (system regulator) of the entire system.

Example C

Plants with two temperature systems and 5 compressors (eg two-stage plants)

where pref.master = Compr#.

System no. 11222

Starting no. 21312

Compressor no. K1 K2 K3 K4 K5

In example C, K1 will be system regulator of system no. 1 and K3 will be system regulator of

system no. 2.

Set-up/Sequence/Reg. shows which unit has been chosen as System regulator.

If there are two or more regulating systems, the compressor with the lowest number in the

individual system will be system regulator.

The regulating compressor (Sys. regulator) will regulate the other compressors in the system

even if the compressor is in Manual, Auto, Stopped or Shutdown.

Only if the voltage supply or the communication is disconnected, the system automatically se-

lects a new system regulator. This will be the unit with the second lowest number.

A compressor will not be part of the Multisab system if it is in either Manual, Auto, Stopped or

Shutdown or if the digital inputs External start-normal stop and External start-immediate stop

have not been connected.

Moreover, the compressor will not be part of the Multisab system if it is in Remote/capacity

remote control with either a 4-20 mA external signal or a communication signal.

The way the system operates can be varied depending on whether a reciprocating or a screw

compressor comes first in the sequence. Likewise, the way the system operates can be varied

with screw compressors of different sizes depending on whether a small or large compressor

comes first.

If mixing compressors of different sizes, unfortunate part load conditions may occur depending

on the sequence.

Be aware that if a screw compressor has the lowest starting number, it may go down to low

capacity even at small loads.

In case of mixed systems, it is recommended to place all reciprocating compressors in a system

one after another followed by the screw compressors or vice versa. The take-over and transfer

Start and system numbers

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functions will only be effective if all the reciprocating compressors have lower starting numbers

than the screw compressors in the same system.

It is important, however, to consider the actual capacity requirement. If the plant includes eg 4

reciprocating compressors and 2 screw compressors, 2 of the reciprocating compressors may

be placed on the other side of the screw compressors in periods where a lower total cooling

capacity is required. Consequently, one of the screw compressors does not run at a too low load

close to the take-over and transfer function limits. When the total load starts to increase, the

reciprocating compressors may be placed in front of the screw compressors again according to

requirement.

The purpose of this is to respect the fact that there is always a need for the capacity of one or

more screw compressors plus x number of reciprocating compressors.

2.1.2. Start and system numbers - pref. master = Start#

Example A

Plant with one temperature system and 5 compressors where pref.master = Start#.

System no. 1 1 1 1 1

Starting no. 1 2 3 4 5

Compressor no. K1 K2 K3 K4 K5

In example A, K1 will be the regulator (system regulator) of the entire system but only if it is

operating or able to start.

If K1 is stopped because of an alarm or if one of the digital inputs External start - normal stop

or External start - immediate stop is disconnected and a cooling requirement still exists, the

next compressor in the starting sequence will be the regulator of the entire system provided

that it is operating or able to start - and that it does in fact start. In the above example, K2 will

become system regulator.

Example B

Plant with one temperature system and 5 compressors where pref.master = Start#.

System no. 1 1 1 1 1

Starting no. 5 4 3 2 1

Compressor no. K1 K2 K3 K4 K5

In example B, K5 will be the regulator (system regulator) of the entire system, but only if it is

operating or able to start.

If K5 is stopped and a cooling requirement still exists, the next compressor in the starting se-

quence will become the regulator of the entire system provided that it is operating or able to

start - and that it does in fact start. In the above example K4 will become system regulator.

Example C

Plant with two temperature systems and 5 compressors (eg two-stage plants)

System no. 1 1 2 2 2

Starting no. 2 1 3 1 2

Compressor no. K1 K2 K3 K4 K5

In example C, K2 will be the system regulator of system no. 1 and K4 will be system regulator

of system no. 2 provided that K2 and K4 are operating or able to start.

Set-up/Sequencing shows which unit has been chosen as Sys. regulator.

In case of two or more regulating systems, the compressor with the lowest number in the indi-

vidual system will be system regulator.

The regulating compressor (Sys. regulator) will regulate the other compressors in the system,

but only if the compressor is in Remote/Multisab and at the same time operating or ready to

start.

Start and system numbers

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A compressor will not be part of the Multisab system if it is in Manual, Auto, Stopped or Shut-

down mode or if the digital inputs External start-normal stop and External start-immediate

stop are not connected.

Further, the compressor will not be part of the Multisab system if it is in Remote/capacity remote

control with either a 4-20 mA external signal or communication signal.

The way the system operates can be varied depending on whether a reciprocating or a screw

compressor comes first in the sequence. Likewise it can be varied with screw compressors of

different sizes depending on whether a small or a large compressor comes first.

If compressors of different sizes are mixed, unfortunate part load conditions may occur de-

pending on the sequence.

Be aware that if a screw compressor has the lowest starting number, it may go down on low

capacity even at small loads.

In case of mixed systems it is recommended to place all reciprocating compressors in a system

one after another followed by the screw compressors or vice versa. The take-over and trans-

fer functions only work if all the reciprocating compressors have lower starting numbers than

the screw compressors in the same system.

It is however important to consider the actual capacity requirement. If the plant includes eg 4

reciprocating compressors and 2 screw compressors, 2 of the reciprocating compressors may

be placed on the other side of the screw compressors during periods with low total cooling ca-

pacity requirements. Thereby, one of the screw compressors does not run at a too low load close

to the take-over and transfer function limits. When the total load starts to increase, the recip-

rocating compressors may be placed in front of the screw compressors again according to

requirement.

The purpose of this is to respect the fact that there is always a need for the capacity of one or

more screw compressors plus x number of reciprocating compressors.

2.1.3. Example of regulation - screw compressors only

The working of Multisab will be described on the basis of the below example and with the indi-

cated compressors.

Compressor K1

SAB 163 Mk2

K2

SAB 163 Mk2

K3

SAB 202 S

K4

SAB 202 S

System no. 1111

Starting no. 1234

The following settings have been made in all units: The digital inputs External start - normal

stop and External start - immediate stop are connected.

Start and system numbers

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Configuration

Control on = Process out

Auto start = Yes

Auto stop = Yes

Compr.no. = XX

(Compressors are numbered 1-4)

Pref. master = compr#

(ie K1 is system regulator)

Common evap./

cond.

=Y/Y

(Common evaporator and common condenser)

Swept

volume

= XXXX

(Corresponding to compressor name plate)

Baud rate = 9600/19200/38400

(All compressors are fitted with Unisab III)

Regulating parameters (for process out regulator)

SP = -2°C

(Chosen according to operating conditions of the plant)

NZ = 1°C

PB = 5°C

Timers

Start delay = 60 sec.

Stop delay = 45 sec.

During the running-in time, adjustment of the values is required. If Nz is reduced, the plant will

react to minor temperature variations in relation to the set point. This will result in frequent

regulation of the compressors. By increasing Nz the temperature can vary within a larger area

before a temperature regulation becomes necessary.

If Pb is reduced, the plant will react faster to temperature changes and return to its set point.

This usually leads to temperature variations close to the set point, but with frequent loading/

unloading of compressor capacity.

If Pb is increased plant reaction will be slower which can prevent frequent oscillations around

the set point.

The purpose of Start delay is to prevent a subsequent compressor from starting up inadver-

tently. If the compressor runs above 95% capacity and the temperature is above the set point,

Start delay will be started on the next compressor in the sequence. Should the temperature

reach the set point, ie be within the neutral zone before Start delay expires, the following com-

pressor will not start.

Should the temperature rise excessively before the next compressor starts, Start delay must be

reduced.

In case of too frequent compressor starts/stops, Start delay must be increased.

Start and system numbers

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Stop delay determines the time the compressor runs at minimum capacity before stop. If quick

stop is required, reduce Stop delay. If the compressor should run for a long period, increase

Stop delay.

The above is a general description of the various setting possibilities. The final settings depend

entirely on the actual plant.

2.1.4. Loading sequence

Compressor K1 is assumed to be in operation at eg 70% {Runs by itself}, K2 {Not my turn},

K3 and K4 {Not my turn} are stopped.

The Multisab status, shown in the picture Set-up/Multisab/Multisab state is marked with {--}.

At increasing cooling requirement and consequent capacity loading, compressor K1 will increase

capacity until reaching 100%.

The loading speed depends on how far the immediate temperature is above the set point which

has been entered into the regulator.

If a cooling requirement still exists, ie the immediate temperature is above the set point and the

capacity of K1 is above 95%, the time delay (Start delay) will start up in Unisab III on com-

pressor K2. The timer will count down from its set point to zero and compressor K2 will start.

(Should the temperature reach the set point, ie be within Nz before Start delay expires, K2 will

not start).

When the above has taken place, screw compressor K1 {Lead compr.} will run slowly down from

100% while K2 will increase its capacity {Lag compr.}. When the compressors meet, ie when

they have the same slide position, they will continue to regulate in parallel (see Fig. 1).

How much and how fast screw compressor K1 decreases in capacity in order to meet K2 depends

entirely on the strength of the regulating UP signal.

With screw compressors K1 and K2 at 100% and with a persistent cooling requirement, Start

delay will be started on screw compressor K3. After a countdown to zero, K3 will start up. K1

will be kept at 100% {Runs at max. capacity} and K2 and K3 will regulate in parallel.

With a persistent cooling requirement and with K1, K2 and K3 at 100%, Start delay will be

started on screw compressor K4. The timer will count down to zero and K4 will start up. K1 and

K2 will be kept at 100% {Runs at max. capacity} and K3 and K4 will regulate in parallel.

However, the actual capacity requirement must be considered. If there are four reciprocating

and two screw compressors in the plant, two of the reciprocating compressors can be moved to

the other side of the screw compressors during low load operation. This prevents a screw com-

pressor from running at too low load in the border area of the take over/transfer function. When

load increases again the reciprocating compressors are moved back in front of the screw com-

pressors as needed. The purpose of this is to keep a constant need for the capacity of one or

more screw compressors and x number of reciprocating compressors.

Start and system numbers

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25

100

50

75

25

100

50

75

5

%

1050 152025

1050152025

Lead

Lead = lag

Lag

Start

Stop

Lag

Lead

min.

Increasing capacity

Decreasing capacity

Fig. 1: Lead/lag control for screw compressors

When two screw compressors regulate in parallel, it means that they follow each other up and

down in capacity and that both units will try to keep the same capacity +/- approximately 2%.

If the two compressors regulating in parallel have equal capacities, the capacity change at any

given relocation of the capacity slide will be doubled compared to the regulation of a single

compressor.

Multisab compensates for this by reducing (by half) the UP and DOWN regulating speed.

2.1.5. Unloading sequence

Three of the four plant compressors are assumed to be in operation, K1 at 100% {Runs at max.

capacity}, K2 {Lead compr.} and K3 {Lag compr.} both in parallel operation at eg 90%.

At decreasing cooling requirement and a consequent capacity unloading, compressors K2 and

K3 will decrease their capacity until both are below the value called parallel capacity. The parallel

capacity is automatically calculated by Multisab. It depends on the size and types of the involved

compressors and the operating conditions. The parallel capacity is the limit at which K2 is able

to take over for certain. If the two compressors are of equal size, the parallel capacity will usually

be about 55%.

The Parallel capacity percentage can be read in Set-up/Multisab.

K3 will now ramp down towards 0% {Stop Ramp Down} with a speed of approximately 12%/

min. When K3 is below 5% {May stop} the Stop delay timer will start up. When Stop delay

expires, K3 will stop {Blocked}. Until K3 stops, K2 will ensure regulation all alone {Stop Ramp

Up} as K1 remains at 100% {Runs at max capacity}.

Screw compressor K1 {Lead compr.} and K2 {Lag compr.} will join each other in parallel op-

eration.

If there is still no cooling requirement, K1 and K2 will decrease their capacity until both are below

parallel capacity. K2 will ramp down and stop as soon as the Stop delay timer expires.

K1 will now ensure regulation all alone {Runs by itself}.

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2.1.6. Operating sequence

Three of the four plant compressors are assumed to be in operation, K1 at 100% {Runs at max.

capacity}, K2 {Lead compr.} and K3 {Lag compr.} both in parallel operation at eg 80%.

The cooling requirement is constant.

Due to an alarm caused by high oil filter differential pressure, K2 stops {Blocked}. This makes

K1 {Lead compr.} and K3 {Lag compr.} change to parallel operation and K3 will increase its

capacity to 100%.

When K1 and K3 are above 95% the time delay (Start delay) will start in Unisab III on com-

pressor K4. The timer will count down from its set point to zero and compressor K4 will start up.

K4 will regulate upwards and the plant will have the following operating mode: K1 at 100%

{Runs at max. capacity}, K3 {Lead compr.} and K4 {Lag compr.} both in parallel operation at

approximately 65% (the parallel operation percentage is lower now than during the output state

as K4 is larger than K2).

The alarm on K2 is confirmed (after the oil filter has been replaced) and although K2 enters the

Ready status {Not my turn} it will not start until a requirement exists as Multisab does not stop

K4 to make K2 resume its position in the sequence.

2.1.7. Example of regulation - reciprocating compressors only

The working of Multisab will be described on the basis of the below example and with the indi-

cated compressors.

Compressor K1

SMC 108 L

K2

SMC 108 L

K3

SMC 106 S

K4

SMC 106 S

System no. 1111

Starting no. 1234

The following settings have been made in all units: The digital inputs External start - normal

stop and External start - immediate stop have been connected.

Configuration

Control on = Suction

Auto start = Yes

Auto stop = Yes

Compr.no. = XX

(Compressors are numbered 1-4)

Pref.master = Compr#

(ie. K1 is system regulator)

Common evap./

cond.

=Y/Y

(Common evaporator and common condenser)

Swept volume = XXXX

(Corresponding to compressor name plate)

Baud rate = 9600/19200/38400

(All compressors are fitted with Unisab II)

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Regulating parameters (for Suction pressure regulator)

SP = -10°C

(Chosen according to the operating conditions of the plant)

NZ = 3°C

PB = 5°C

Timers

Start delay = 60 sec.

Stop delay = 45 sec.

Delay up = 30 sec.

Delay down = 20 sec.

During the running-in time, adjustment of the values is required.

If Nz is reduced, the plant will react to minor suction pressure variations in relation to the set

point. This will result in frequent regulation of the compressors. By increasing Nz the suction

pressure can vary within a larger area before regulation of the compressors becomes necessary.

If Pb is reduced, the plant will react faster to suction pressure changes and return to its set point.

This usually leads to temperature variations close to the set point but frequent loading/unloading

of compressor capacity.

If Pb is increased, the plant reaction will be slower which can prevent frequent oscillations

around the set point.

The purpose of Start delay is to prevent a subsequent compressor from starting up inadver-

tently. If the compressor runs at 100% capacity and the temperature is above the set point,

Start delay will be started on the next compressor in the sequence. Should the temperature

reach the set point, ie be within the neutral zone before Start delay expires, the next compressor

will not start.

Should the temperature rise excessively before start of the next compressor, Start delay must

be reduced.

In case of too frequent compressor starts/stops, Start delay must be increased.

Stop delay determines the time the compressor runs at minimum capacity before stop. If a quick

stop is required, reduce Stop delay. If the compressor should run for a long period, increase

Stop delay.

If Delay up is increased, the reciprocating compressor will load capacity stages at a lower speed.

A lower Delay up value will result in faster loading.

If Delay down is increased, the reciprocating compressor will unload capacity stages at a lower

speed. A lower Delay down value will result in faster unloading.

Often it is an advantage if Delay up is higher than Delay down.

The above is a general description of the various setting possibilities. The final settings depend

entirely on the actual plant.

2.1.8. Loading sequence

Compressor K1 is assumed to be in operation at eg 75% {Runs by itself}, K2 {Not my turn},

K3 and K4 {Not my turn} have been stopped.

The Multisab status, as shown in the picture Set-up/Multisab/Multisab state is marked with {--}.

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