B4.1 AUTOMATIC CONTROL SYSTEM (ACS)

PART B - GENERAL TECHNICAL REQUIREMENTS (INSTALLTION METHODOLOGY)

The Automatic Control System (ACS) is the Master Control of a building that controls and monitors the proper, effective and efficient operation of the Air-conditioning and other Building Services System(s) in building(s). An ACS can be a complicated Central Control & Monitoring System (CCMS) or as simple as an electronic controller with built-in real time clock for scheduling controls.

The appropriate ACS shall be so designed to meet the requirements and operation needs of the building operation & management team and the operation requirements of the end users in building(s). As different equipment and systems may have different operation schedules, the ACS shall have adequate capacity for scheduling all the operation needs.

The ACS described below shall be applied to buildings which do not require sophisticated CCMS (if so specified, the designed ACS shall have provisions for the proper interfacing and direct connection to the network of the future CCMS) for the normal operation and control of the air conditioning, refrigeration and ventilation systems, and other building services and electrical & mechanical (E&M) systems installed in the buildings if so specified. The main function of an ACS is for on/off controls, monitoring of operation status and alarms generation of the connected systems in building(s). Comprehensive system controls and monitoring functions of each individual system shall be carried out by the system controller of each individual air conditioning, building services or E&M system. The ACS shall be so designed to achieve a reliable control in performing the control and monitoring functions set in the design criteria and shall be of electronic or direct digital control type as specified. Unless otherwise specified, the ACS controller shall be located inside the A/C Supervisory Control Panel.

B4.1.1 Electronic Controller with Micro-processor and REAL Time Clock Control

With the application of micro-processor and real time clock, all building services systems specified shall be turned on and off in accordance with the time schedules pre-set in the built-in program including any holiday and special schedules so required by the users.

A monitoring and alarm system shall be incorporated in the controller in giving audio and visual status and alarm of faults for each building services system in the building.

For easy programming, user friendly input/output (I/O) devices shall be provided for the setting and re-setting of the operation schedules.

To prevent malfunctioning of the real time clock and losing of memory, back-up battery with power healthy indicator and alarm shall be provided.

B4.1.2 Direct Digital Controller (DDC)

(a) Each controller shall operate as a standalone unit capable of performing its specified control responsibilities independently of other controllers in the network.

(b) Local keypad and display shall be provided for local viewing and operation.

(c) Power Fail Protection – All system setpoint, control algorithm and other programmable parameter should be stored such that power failure of any duration does not necessitate reprogramming the DDC controller.

B4.1.3 Control of Chiller Plant (a) General

(i) Controllers

Built-in micro-processor based electronic controller or DDC for each chiller plus a micro-processor based electronic controller or DDC for chiller plant Sequential/Step/Interfacing controls. (for chillers and chilled water pumps controls and interfacing with ACS and other system controllers)

(ii) Start – Stop Control

Make provision for the following control functions unless otherwise specified in the Particular Specification or Drawings:-

- Manual on/off (maintenance/manual mode);

- On/off by the Automatic Control System as scheduled; and

- On/off by special demand overriding the pre-set schedule for dehumidification and/or cooling during night purging or other special duty cycles as specified.

(iii) Faults and Abnormal Operation Controls and Monitoring by the Chiller Controller

- High condenser pressure;

- Low refrigerant pressure/temperature;

- High motor temperature;

- Motor overload;

- Low oil pressure;

- Low oil sump temperature;

- High oil sump temperature;

- Chilled water flow interlocked;

- Condenser water flow interlock (water cooled only);

- Back-up for the low chilled water temperature controller (if required by the chiller manufacturer);

and

- Plus other items as specified or recommended by the chiller manufacturer.

(iv) Fault Surveillance

Fault surveillance and safety controls of chiller shall be carried out by the built-in electronic controller or DDC of the chiller. Interfacing shall be provided for communication between the built-in chiller controller and the chiller plant controller. Interfacing system shall also be provided for the chiller plant controller in receiving operation instructions from the ACS and sending out chiller status and alarm signals to the ACS and the A/C Supervisory Control Panel.

(v) Energy Conservation

If specified in the Particular Specification or Drawings, sufficient control provision shall be allowed in the chiller controllers for the proper operation of the following energy systems:-

- Heat recovery condenser for the supply of heat source as specified;

- Cycling of condenser fans by head pressure control for energy saving;

- Variable Speed Drive for chiller. The application of electronic speed controller in maintaining higher chiller energy efficiency at partial load operation; and - Reset of supply chilled water temperature to optimize

the chiller energy efficiency.

(b) Single Chiller

(i) Capacity Control

Make provision for the following control functions plus other special requirements specified in the Particular Specification or Drawings:-

- Chiller output capacity modulated to maintain the pre-set supply or return chilled water temperature (after mixing with the by-pass); and

- The chiller controller shall modulate its refrigeration capacity to meet the cooling load demand in the chilled water circulation system.

Make provision for the following control functions plus other special requirements specified in the Particular Specification or Drawings:-

(1) Built-in controller of each chiller modulates output capacity to maintain a constant supply chilled water temperature pre-set at the outlet of each operating chiller; and

(2) The Chiller Plant Controller shall control the sequencing of chillers and cut-in or cut-out a chiller in accordance with the return chilled water (with by-passed chilled water well mixed) temperatures pre-set in the controller.

The sequencing logic shall be as follows :- - Chiller that has run longest since last

start is first to stop;

- Chiller that is idle for the longest time is first to start; and

- Chiller(s) in manual or maintenance mode shall be by-passed from the automatic sequencing control operation.

(d) Make provision of monitoring and Alarm Signals to ACS and the A/C Supervisory Control Panel

Other special requirement as specified in the Particular Specification or Drawings.

B4.1.4 Control of Chilled Water Pumping System (by chiller plant controller) Unless otherwise specified in the Particular Specification or the Drawings, chilled water control system shall perform the following:- (a) Circulation Pumps with Differential Pressure By-Pass

(i) For a single chiller system, the duty chilled water pump shall be operated as long as the system is in operation mode. During partial load condition, excessive chilled water supply shall be by-passed by the differential pressure by-pass valve and hence lower the system return chilled water temperature. Dropping of the return chilled water temperature shall then call for refrigeration capacity control in the chiller. There is possibility that the cooling load demand is so low that the chiller will be cut off and at idle condition for a period of time. However it is important to keep the

chilled water pump to run continuously for sensing the correct chilled water temperature. The pump shall only be cut-off when the chiller plant is turned off by the ACS;

(ii) For a multiple chiller system, the number of chillers put into operation and chiller refrigeration capacity controls shall be in accordance with the system return chilled water temperature, which is affected by the amount of chilled water by-passed by the differential pressure by-pass valve. Each chilled water pump shall be operated in accordance with its associated or delegated chiller. The delegated chilled water pump shall be cut-in earlier than the chiller in order to maintain the chilled water flow required for the chiller operation. The same logic of leaving at least one chilled water pump to operate for monitoring the chilled water temperature shall be applied; and

(iii) Manual on/off for pumps (maintenance/manual mode).

(b) Primary and Secondary Pumps (Decoupled) System

In a typical decoupled pumping system, the primary pumps are of constant flow and the secondary system can be of all variable flow pumps or a combination of constant and variable flow pumps. Exact configuration shall refer to the Particular Specification or the Drawings.

(i) Primary chilled water pump

Primary chilled water pump shall be of constant flow type, which shall be operated with time leading its associated or delegated chiller and cut-out when the associated chiller is turned off. When a reverse flow is detected in the by-pass, an additional chiller shall be cut in and when the by-pass flow is greater than the designed full capacity flow rate of a chiller, an operating chiller shall be cut off. The associated primary chilled water pumps shall then be operated in the same arrangement. The logic of keeping at least one pump operated in the primary circuit (when the system is in operation mode) is still valid.

(ii) Secondary chilled water pumps

Secondary chilled water pumps shall be operated to meet the flow demand at the load side. Discharge head pressure or differential pressure control (across supply and return mains of the secondary loop) shall be used to monitor the flow demand. The secondary pumps (no matter constant or variable flow) shall be operated

in such a configuration that a constant chilled water supply pressure (i.e. in proportion to the amount of chilled water supplied) is maintained and hence meets the chilled water flow demand.

For a system with constant and variable flow pumps, the constant flow pump(s) shall only be operated as long as the demand flow is greater than the full capacity of that constant flow pump. The variable speed pump shall always be operated first to handle any partial load demand for energy saving.

(iii) Manual on/off for all pumps (maintenance/manual mode)

Other special requirement as specified in the Particular Specification or Drawings.

B4.1.5 Control of Cooling Towers and Condenser Water System (by condenser plant controller)

Micro-processor based electronic controller or DDC for Condenser Water Plant Sequential/Step/Condenser Water Temperature By-Pass/

Cooling Tower Fans Cycling/Interfacing controls. (for condenser water plant controls and interfacing with the chiller plant controller and ACS)

(a) Start-stop Control

Condenser water pumps shall be interlocked with their associated water-cooled chillers and shall be started in advance and turned off after their associated chillers.

(i) Manual on/off (maintenance and manual mode) Condenser water pumps on/off are interlocked with the chiller plant, which is started /stopped by the ACS as scheduled.

(ii) Cooling towers and their associated control valves shall be opened for the reticulation of condenser water.

(iii) Once the condenser water plant is turned on, its ancillary equipment such as water treatment plant, condenser water filtration and cleansing devices shall be energized accordingly.

(iv) For cooling tower system installed with multiple cooling towers and/or standby cooling tower(s), sequence control

shall be provided to allow each cooling tower to be operated at least one hour per week.

(b) Capacity Control

(i) For energy saving, cycling of cooling tower fans shall be used to maintain the supply condenser water temperature entering the chillers at the designed value.

(ii) If the supply condenser water temperature drops below the pre-set value even with all the cooling tower fans cut off, the temperature by-pass valve shall be modulated to by-pass condenser water leaving the chillers from entering the cooling tower but to mix with the condenser water supply.

(i) High condenser water temperature.

(ii) Extra high condenser water temperature. (chillers to be cut off one by one if so specified)

(iii) Low water level of cooling towers.

(iv) Faults of cooling tower fans.

(v) Faults of condenser water pump. (associated chiller to cut off)

(vi) Faults of water treatment plant(s).

(vii) Faults of condenser water filtration and cleansing equipment.

(viii) Plus other items as specified or recommended by the condenser water plant manufacturer.

(d) Make Provision of Monitoring and Alarm Signals to ACS and the A/C Supervisory Control Panel

Other special requirement as specified in the Particular Specification or Drawings.

B4.1.6 Control of Primary Air Handling Unit (PAU)

Micro-processor based electronic controller or DDC as specified shall be used for the control of the PAU, its associated equipment and the interfacing and communication with the ACS and its associated zone AHUs, FCUs, etc. The following functions shall be provided unless otherwise specified:-

(a) Start-stop Control

(i) Automatic fire tripping as specified or required by the FSD;

(ii) Manual on/off control (maintenance/manual mode);

(iii) On/off control by the ACS as scheduled;

(iv) On/off controls by special demand overriding the pre-set schedule for dehumidification and/or cooling during night purging or other special duty cycles as specified;

If so specified in the Particular Specification or the Drawings, the demand control function in sub-clause (v) below shall be adopted; and

(v) Demand on/off control. The operation of any AHU/FCU served by the unit shall energize the PAU.

Unit will only be cut-out when all associated AHUs/FCUs are turned off or the whole system is turned off by the ACS as scheduled.

(b) Capacity Control

(i) For constant air flow design

- Constant flow, full capacity at all the time during operation.

(ii) For variable air flow design

- Air flow modulated by variable speed drive in order to reduce the indoor CO2 level within the specified range (applicable for single zone system only); and - Air flow modulated by variable speed drive to

maintain the pre-set static pressure at the main supply air duct. Amount of fresh air supply to each zone shall be controlled by motorised damper, which is modulated to reduce the indoor CO2 level at that particular zone to be within the range specified (applicable for multi-zone system).

- Cooling (outdoor air temperature higher than the pre-set value)

If the supply air temperature rises above the pre-set upper temperature, chilled water flow through the cooling coil of the PAU shall be increased by a modulating valve and vice versa when the supply air temperature drops below the pre-set lower temperature. The controller shall respond to modulate the control valve in maintaining the pre-set supply air temperature.

- Heating (outdoor air temperature lower than the pre-set value)

The supply air temperature is maintained within the range specified by modulating the control valve and hence regulating the amount of hot water (for space heating) from entering the heating coil of the PAU or for electric heating by staging of electric heaters to maintain the supply air temperature specified.

(ii) Humidity Control

- Dehumidification (when the outdoor air enthalpy/moisture content is higher than the pre-set cut-in value)

Despite of the PAU is in cooling mode or heating mode, the humidistat shall modulate the chilled water control valve to open for dehumidification until the humidity condition of the outdoor air drops below the pre-set cut-out value. If the supply air temperature drops below the lower limit due to the dehumidification process, the hot water coil or electric heaters shall be actuated for re-heating.

If so specified in the Particular Specification or the Drawings, the dehumidification process shall be carried out by a separate dehumidification system installed at the fresh air intake section.

- Humidification (outdoor air relative humidity lower than the pre-set value)

Humidification process is not taken place in the PAU.

If the room humidity condition at the space inside the building is lower than the value specified, humidification by means of steam or water spray humidifier shall be operated at the relevant AHUs.

For fan coil system, individual room/zone humidifier or system shall be used as specified.

(d) Filters and Air Cleaners

Controller of PAU shall also control the proper operation of its ancillary equipment such as automatic air filter, air cleaner, air sterilizer, etc. Unless otherwise specified, the following alarm signals shall be sent to the A/C Supervisory Control Panel:-

(i) Filter clog alarms;

(ii) Filter detached from filter chamber;

(iii) Time for replacement of absorption filters; and (iv) Status and fault alarms of air cleaner & sterilizer.

(e) Make provision of monitoring and Alarm Signals to ACS and the A/C Supervisory Control Panel

Other special requirement as specified in the Particular Specification or Drawings.

B4.1.7 Control of Air Handling Unit (AHU)

Micro-processor based electronic controller or DDC as specified shall be used for the control of the AHU and its associated equipment and the interfacing and communication with the ACS and its associated zone terminal, VAV units, etc. The following functions shall be provided unless otherwise specified:-

(a) Start-Stop Control

(i) Same as Clause B4.1.6(a).

If so specified in the Particular Specification or the Drawings, the demand control function in sub-clause (ii) below shall be adapted; and

(ii) Demand on/off control. The operation of any VAV unit served by the unit shall energize the AHU. Unit shall only be cut-out when all associated VAV units are turned off or the whole system is turned off by the ACS as scheduled.

(b) Capacity Control

(i) For Constant Air Volume System

- Single zone constant volume; full flow capacity to a single zone at all the time during operation (applicable for single zone system only); and

- Multi zone constant volume, the amount of air supplied to each zone is still constant. However zone reheater shall be provided at each zone for temperature control (applicable for multi zone system).

(ii) For Variable Air Volume System

Airflow modulated by variable speed drive to maintain the pre-set static pressure at the main supply air duct.

Amount of air supplied to each zone shall be controlled by motorised damper of each VAV unit. The VAV damper shall be modulated to open or close in order to maintain the room temperature/supply air flow rate to be within the range specified.

(i) Temperature Control for Constant Air Volume Systems

- Cooling

If the room air temperature rises above the pre-set upper temperature, chilled water flow through the cooling coil of the AHU shall be increased by opening the modulating valve and vice versa when the room air temperature drops below the pre-set lower temperature. By modulating the control valve, chilled water flowing through the cooling coil of the AHU shall be regulated to meet the cooling demand load.

- Heating

The room air temperature shall be maintained within the range specified by the control valve and hence the amount of heating hot water from entering the heating coil of the AHU or by staging of electric heaters to maintain the room air temperature specified.

(ii) Temperature Control for Variable Air Volume Systems

In document GENERAL SPECIFICATION FOR AIR-CONDITIONING, REFRIGERATION, VENTILATION AND CENTRAL MONITORING & CONTROL SYSTEM INSTALLATION GOVERNMENT BUILDINGS (Page 79-96)