10.1 Control set point
Inputs used:
• Analogue input for remote set point variation
• Supervisor serial network Parameters used:
• Control set point
• Enable remote set point from analogue input
• Limits for the calculation of remote set point from analogue input
• Display set point used by the control
Description of operation
Temperature control, irrespective of the type, is based on the setting of two fundamental parameters: control set point and band. The control set point can be changed according to special operating requirements of the unit. There are three different methods for changing the control set point:
1. Setting on the screen: by accessing the special screen, the user can directly set the value of the parameter.
2. Setting from the supervisor: if connected to a supervisory system, by accessing the special addresses, the cooling or heating set point can be set.
3. Setting from analogue input: enabling the remote set point control from analogue input (0 to 1 V / 0 to 10 V / 4 to 20mA selectable), allows compensation of the control set point by a proportional value between the two limits for the conversion of the input signal set.
All the above conditions may be active at the same time, while condition “1” is always present; the compensation of the set point from analogue input can be enabled by a special parameter, while setting from the supervisor is only possible using a board that is configured and connected for communication to a serial supervisor system.
In units that feature chiller + heat pump operation, the changeover from cooling to heating operation and vice-versa can be selected as automatic or manual. This setting defines how the temperature control set point is managed:
• Automatic changeover – one set point only for cooling and heating operation, based on which the unit changes operating mode;
• Manual changeover – two distinct set points, one for cooling operation, the other for heating operation, activated alternatively depending on the unit operating mode selected unit.
10.2 Temperature control
Two distinct modes are available for the operation of the temperature controller:
• Control depending on the temperature of the water measured by the probe located at the evaporator inlet
• Control depending on the temperature of the water measured by the probe located at the evaporator outlet
The first case involves proportional control based on the absolute value of the temperature measured by the probe; the second case involves dead zone control based on the time the temperature measured by the probe remains over certain thresholds.
10.3 Inlet temperature control
Inputs used
• Inlet temperature Parameters used:
• Control set point
• Proportional band for inlet control.
• Type of control (proportional or proportional + integral)
• Integral time (if proportional + integral control is enabled)
• Type of unit
• Total number of compressors
• Number of load steps Outputs used
• All the compressors and the corresponding load steps
Description of operation
STPM Control set point
RBM
Example of temperature control in units with 4 compressors and 3 load steps each, in chiller operation:
STPM RBM EIWT [ºC]
C1
P1,1 P2,1 P3,1 C2
P1,2
C4
P1,4 P2,4 P2,2 P3,2
C3
P1,3 P2,3 P3,3 P3,4
STPM Control set point
RBM Control band
EIWT Evaporator water inlet temperature
C 1…4 Compressors
P 1…4,1…4 Compressor load steps
Fig. 10.2 Semi-hermetic compressors with proportional control
10.4 Outlet temperature control
Inputs used
• Outlet temperature Parameters used
• Control set point
• Dead zone for outlet control
• Step activation delay
• Step deactivation delay
• Cooling outlet temperature limit
• Heating outlet temperature limit
• Minimum compressor on time
• Differential comprising the variation in the on time.
• Minimum compressor off time
• Differential comprising the variation in the off time.
Outputs used
• All the compressors and the corresponding load steps
Description of operation
STPM
RBM
NZ
DOffZ DOnZ
EOWT [ºC]
STPM Control set point RBM Control band
NZ Dead zone
EOWT Evaporator water outlet temperature DOnZ Device start zone
DOffZ Device stop zone
Fig. 10.3 Temperature control with dead zone based on the reading of the outlet probe A temperature dead zone is identified based on the set point and band.
Temperature values between the set point and set point + band (STPM < Temperature < STPM+RBM) will not switch any compressors On/Off.
Temperature values above set point + band (Temperature > STPM+RBM) will activate the compressors Temperature values below the set point (Temperature < STPM) will deactivate the compressors
A temperature threshold is envisaged, for both cooling operation and heating operation, below/above which the devices installed will in any case be stopped, in order to avoid excessive cooling/heating output produced by the unit.
Example of temperature control in units with 4 compressors and 3 load steps each, in chiller operation:
NZ
When the temperature is greater than STP_M + NZ, the devices are activated with a delay between the activations equal to the value set for the parameter “delay between starts in dead zone”.
EOWT [ºC]
C4 P1,4 P2,4
P3,4 C1
P1,1 P2,1 P3,1 C2 P1,2 P2,2 P3,2 C3 P1,3 P2,3 P3,3
t [s]
OffDT [s]
STPM NZ
FTHR
STPM Control set point
NZ Dead zone
EOWT Evaporator water outlet temperature FTHR Forced shutdown threshold C 1…4 Compressors
P 1…4,1…4 Compressor load steps OffDT Differential shutdown compressors T Time
Fig. 10.5 Semi-hermetic compressors with dead zone control [stop]
When the temperature is less than STP_M, the devices are deactivated with a delay between deactivations equal to the value set for the parameter “delay between stops in dead zone”. When the temperature falls below the minimum limit FTHR, the devices are switched off even if the delay time set has not elapsed; this helps avoid the activation of the antifreeze protection.
The user may also set a variable time between calls depending on how far the temperature is out of the dead zone. Specifically, the step request / deactivation time decreases (within certain limits) depending on the deviation of the temperature.
To do this, the following parameters need to be configured:
• Maximum compressor on time
• Minimum compressor on time
• Differential comprising the variation in the type of call.
• Maximum compressor off time
• Minimum compressor off time
• Differential comprising the variation in the off time.
STPM
RBM
NZ
DOffZ DOnZ
EOWT [ºC]
TOnMin TOffMax
TOnMax TOffMax
DTNZ
B
DTNZ
STPM Control set point RBM Control band
NZ Dead zone
EOWT Evaporator water outlet temperature DOnZ Device start zone
DOffZ Device stop zone
DTNZ Differential comprising the variation in the time The following cases are therefore possible in the start phase:
1. Inlet temperature equal to point b
No documento
Standard Modular Chiller HP 1/8 compressors with CAREL driver
(páginas 50-53)