Danfoss AK-PC 420 User guide

User Guide

Capacity controller for dry cooler

AK-PC 420

ADAP-KOOL® Refrigeration control systems

Introduction

Application

The controller is used for capacity regulation of a dry cooler including fans, three-way valve and pumps.

Advantages

•Complete dry cooler control

•Several combination options for sensor selection

•Heat recovery acc. to external voltage signal.

Regulation

Capacity regulation can include fans combined with a three-way valve or just fans. The following sensor signals can be used in regulation:

•Condensing pressure Pc

•Brine temperature S7, located immediately after the three-way valve outlet

•Brine temperature S8, located by the outlet from the dry cooler

•Outdoor temperature sensor Sc3, located by the air inlet to the dry cooler.

•Either S7, Pc or S8 can be used as a regulation sensor.

Functions

•Three-way valve control

•Speed regulation or step-by-step coupling of up to six fans

•Safety monitoring of fans

•Overriding of reference acc. to external voltage signal or outdoor temperature

•Separate reference for heat recovery

•Control and monitoring of twin pumps

•Monitoring of flow switch

•Contact inputs to alarms

•External start/stop of regulation

•Data communication via extra module

Operation

All operation takes place either via data communication or via connection of a display type EKA 164 .

Contents
Introduction........................................................................................................	2
Function...............................................................................................................	3
Survey of functions..........................................................................................	8
Operation...........................................................................................................	13
Menu survey.....................................................................................................	14
Connections......................................................................................................	16
Data......................................................................................................................	17
Ordering.............................................................................................................	17
Montage.............................................................................................................	17
Appendix - Reference and regulation.....................................................	18

Manual RS8EL402 © Danfoss 03-2010

AK-PC 420

Function

Application

Regulation sensor and system type are selected via one single setting. The setting will define both the regulation sensor, if a threeway valve is being used, and how the fans are to be controlled. The fans are controlled either in steps or together with a speed control. If steps are used, up to six fans can be controlled.

Application 1-4

The capacity here is controlled via the fans combined with a threeway valve.

Applications 1 and 2 use brine return temperature S7 at the three-way valve outlet as regulation sensor and the fan capacity is controlled either in steps or via speed control.

The S8 sensor, which is fitted to the dry cooler’s outlet should be used in the control if there is a large distance between the dry cooler and the three-way valve. Use of the S8 sensor will provide more robust regulation, which takes long pipe lengths into account. The S8 sensor must be fitted close to the dry cooler’s outlet.

Applications 3 and 4 use brine condensation pressure Pc as regulation sensor and the fan capacity is controlled either in steps or via speed control.

For PI control the use of the S7 and S8 sensors is optional but they should be fitted if there is a large distance between the dry cooler and the three-way valve. Use of the sensors will provide more robust regulation, which takes long pipe lengths into account. The S8 sensor must be fitted close to the dry cooler’s outlet.

For P control the S7 and S8 sensors are not used in the regulation, but they can be fitted to ensure that the emergency control functions in the even of the Pc signal failing.

Applications 5-6

Here the capacity is controlled via step-by-step coupling or speed control on the basis of the dry cooler’s discharge temperature S8. This application is used in particular when another controller takes care of the regulation of the three-way valve, e.g. to cool several parallel-coupled condensers.

NB.

The sensors Pc and S7 must not be located in positions other than those specified, as they are used for emergency regulation if the primary regulation sensor develops a fault.

The S8 sensor can be located in a different position than the one indicated if it is not used in the control.

If the S8 sensor is required in the control, the setting "o96, S8 optional" must be selected as ON.

If the S8 sensor is not used in the regulation, it can be located anywhere.

Regulation

Optional

3-way

Fan

(o61)

sensor

valve

control

(P + PI)

Step

(P + PI)

Speed

/ S7+S8 (PI)

Step

/ S7+S8 (PI)

Speed

Step

Speed

AK-PC 420

Manual RS8EL402 © Danfoss 03-2010

Capacity regulation

Three-way valve + step-by-step coupling of fans (applications 1 and 3)

Capacity regulation takes place via a P or PI regulation, which controls the three-way valve and subsequent step-by-step coupling of the fans.

First of all regulation takes place on the three-way valve via the analogue output. The output can be set at 0-10 V or 10-0 V, depending on which valve function is desired. Only when the three-way valve is fully open do the fans start up.

The capacity of the three-way valve is 50% of the total capacity. The P/PI controller has separate regulation settings (proportional band and integration time) for the three-way valve and the fans respectively.

Capacity overlap – start of fans

As mentioned above, the three-way valve comprises a fixed 50% of the total capacity. In some systems it may be necessary to start the fans slightly before (or slightly after) the three-way valve has been fully opened. An overlap is created here between the three-way valve and the start of the fans. This is done by changing the setting “FanCap OFF%” from 50% to, for example, 40% (the setting defines the capacity at which the last fan stops). In this case the fans are started before the three-way valve is fully open.

Three-way valve + speed control of fans (applications 2 and 4)

Capacity regulation takes place via a P or PI regulator, which controls the three-way valve and subsequent speed control of the fans. Here the analogue output signal is divided between the three-way valve and the frequency converter (50% of the analogue output signal is always used for the three-way valve), i.e.

if the analogue output signal is set at 0-10V, 0-5 V is used for the three-way valve.

The frequency converter/fans are controlled via the relay output for fan 1 on the basis of the setting “FanCapOFF%”.

In the event of falling capacity, the relay output for fan 1 will be disabled when the actual capacity has fallen to “FanCap OFF%”. To achieve hysteresis when starting/stopping the frequency converter, the relay output for fan 1 will, when capacity is increasing, be enabled when the desired capacity is “FanCap OFF%” + “10% fan capacity”.

If “FanCapOFF%” is set at above or below 50%, the frequency converter can be started after or before the three-way valve is fully opened.

The P/PI controller has separate regulation settings (proportional band and integration time) for the three-way valve and the fans respectively.

Fan control only (applications 5 and 6)

Capacity regulation takes place via a P or PI regulator, which controls the fans either with step-by-step coupling or speed control. The analogue output and a frequency converter are used for speed control. The relay output for fan 1 is used to start/stop the frequency converter.

In the event of falling capacity, the relay output for fan 1 will be disabled when the actual capacity has fallen to “FanCap OFF%”. To achieve hysteresis when starting/stopping the frequency converter, the relay will become enabled when the desired capacity is the fan capacity above “FanCap OFF%” (The 10% fan capacity will correspond to hysteresis of 1 V in the analogue output signal, when the signal is 0-10 V.)

The P/PI controller only uses regulation settings (proportional band and integration time) for the fans.

Manual RS8EL402 © Danfoss 03-2010

AK-PC 420

Linear/non-linear capacity curve for fans

The first fan step provides relatively more capacity than the subsequent capacity steps. The increase in capacity that an extra step/ speed will generate falls gradually, as more and more steps are connected/the speed is increased.

The fan regulation therefore has a crooked capacity curve, which provides optimal reinforcement at both high and low capacities. However, for some systems a straight curve is required for capacity regulation, e.g. if the analogue signal is used for more than controlling fans. In this instance the capacity curve can be set at linear. (The capacity curve for the three-way valve will always be straight.)

Step-by-step coupling of fans

The fans can either be connected in the sequence defined for them (sequentially) or they can be connected in rotation (sequentially with alternating start).

In sequential operation all fans are enabled at least once every 24 hours, so no fan rusts up due to being inactive for a longer period of time.

In rotation the various fans take turns in being first.

Monitoring fans

The controller must receive a signal of the status of each defined condenser step’s safety circuit. The signal is taken directly from the safety circuit and connected to a “DI” input.

If the safety circuit is broken, the controller will lose the signal and emit an alarm.

The associated relay output will not be disconnected. The reason is that fans are often connected in pairs, but with one single safety circuit. If there is a fault in one fan, the other will continue to operate.

If no monitoring is desired, the input must be permanently wired at 24 V.

Reference

Regulation reference

The regulation reference can be defined in one of the following two ways:

• Fixed setting

The set point for the regulation sensor is set in °C.

If displacement is required, the reference can be displaced with a 0-10 V signal. During setup you define how great the displacement is to be at the signal’s max. and min. value.

• Floating reference according to outdoor temperature

This function allows the reference to vary according to the outdoor temperature within a defined range.

The outdoor temperature is measured with the Sc3 sensor, and the reference will always fall within a fixed value (min. tm) above the measured outdoor temperature.

Heat recovery

When heat recovery is enabled via the digital input, the reference will switch to another set point “Heat SP”, although this too can be overridden via the external 0-10 V signal. At the same time the relay for heat recovery is enabled, which then transmits a signal to either a pump or a valve. The reference's max. value (r30) is overriden from the set value to 99.9° C.

If heat recovery is in progress and the temperature by the regulation sensor is lower than the reference’s set minimum value, the following happens (see also the following section on limiting the reference):

AK-PC 420

The heat recovery relay is disconnected and can only be re-ena- bled once the temperature by the regulation sensor has reached 2 K above the reference’s minimum setting.

Ramp function

To avoid overswing and underswing of the reference, a ramp function has been included to guarantee that the reference cannot change more quickly than the preset ramp in Kelvin/minute.

Limitation of reference

To protect against regulation reference that is too high or too low, a limit must be set for the reference. The limit is valid under normal regulation, but is increased during heat recovery to 99.9° C.

The upper limit is always an absolute value. The lower limit can either be an absolute value or it can vary in accordance with the outside temperature Sc3 whereby the limit has a fixed value ("r56, Min tm") above the outside temperature. This means that a reference below the minimum value, making all fans run continuously, can be avoided.

P or PI regulation with floating or fixed reference

See appendix.

Safety functions

Ref. Max.

Ref. Min.

Ref. Max.

Ref. Min.

Monitoring of condensation pressure

The controller has a safety function that provides protection against the condensation pressure being too high.

The function can be enabled in two ways.

• Digital input - HP safety

When the digital input is connected, full capacity is cut in to both fans and three-way valve. An alarm is emitted at the same time.

The capacity remains cut in until the digital input is interrupted, at which point the alarm is also cancelled. The digital input can possibly be connected to an external safety pressure control.

• Measuring the Pc/S7 temperature

This function always uses condensation pressure Pc if the pressure signal is connected. If Pc is not fitted, brine return temperature S7 is used instead.

The function cuts in all condenser steps and emits an alarm if the measured temperature is higher than 3 K below the set limit “S7/Pc max”.

Normal capacity regulation is restored when the temperature (pressure) has once more fallen to 3 K below the limit, and a delay time of 60 seconds has passed.

Pump controL

The controller can control and monitor one or two pumps that circulates the brine.

If two pumps are used and operating time equalisation is selected, the controller can also execute a switch between the two pumps if operating alarms occur.

Pump selection is performed using the following settings: 0: Both pumps are stopped

1:Pump 1 is started

2:Pump 2 is started

3:Both pumps are started

4:Automatic switch between the pumps is permitted. Start before stop

5:Automatic switch between the pumps is permitted. Stop before start

AK-PC 420

+ 14 hidden pages