SEQUENCE OF OPERATIONS FOR THE CENTRAL AIR CONDITIONING SYSTEM

                                                                                                                                                                                                                       

This article is about the sequence of operations for the central air conditioning system of a high rise building.

IBMS DESIGNING PROCEDURES FOR AIR CONDITIONING SYSTEM

The previous chapter of BOC headquarters building ’ energy consumption analysis shows the breakdown of energy consumption in each service and based on that it can be concluded that most dominant energy consumption factor is Air conditioning system, which is about 50%. Therefore, we planned to integrate IBMS system only for the air conditioning system. According to that, we carried out detail energy survey in central air conditioning system to identify ways and means to minimize cost of energy at the BOC headquarters. Due to that all the analyzed energy efficient improvement methods are described as below.

PRESENT STATUS OF CENTRAL AIR CONDITIONING SYSTEM

In this building various types of A/c system used to increase customer and occupants satisfaction. Four numbers of 259.8TR centrifugal Chillers, four numbers of 7.5TR packaged Chiller, split A/C units, and close control A/c system are installed at BOC headquarters building, out of which only three are in operating condition (centrifugal) and other types are running. During the monitored period only two 259.8 TR centrifugal Chiller (Chiller 2) and other types were used for catering air conditioning requirement.

Until introducing recent changes to Chiller operation, BOC building operated 259.8TR Chillers throughout the day and other types Chillers were also operated for about 8 hrs per day during the day time where heat load is high. Further, one primary chilled water pump, two secondary chilled water pumps and one condenser water pump were operated throughout the day (amounts are in one central A/C system). Five number of 450TR cooling tower was operated for 24 hrs a day and additional 450TR cooling tower was standby.

Since end of the year (December) 2010, daily operation of package (7.5T) Chiller was discontinued and total air conditioning demand is served by 259.8TR Chiller. However, it is planned to operate the 259.8TR Chiller if delivery chilled water temperature rise above 7.8 0C (460F). Requirement for operation of second Chiller in such situation will be identified by manual observation of chill water temperature.

In the present system one primary chilled water pump and secondary chilled water pump (per one) is in operation throughout the day and additional chilled water pump is used for secondary cycle during the daytime for about 2 hrs. Five cooling tower are operated (450TR) throughout the day and other one is standby. However Present operation methodology has being able to achieve considerable reduction in energy consumption for air conditioning compared to earlier operation methodology used BOC.

CURRENT PERFORMANCE OF CENTRAL AIR CONDITIONING SYSTEM

Detail study was carried out on air conditioning system covering the equipment such as Chillers, primary and secondary chilled water pumps, cooling towers, condenser water pumps and air handling units (AHUs). During the study following areas were explored for possible energy saving measures,

Ø Heat load variation in the system and performance of each Chiller under different load conditions

Ø Chilled water balance in the system

Ø Heat load variation in Chiller condenser

Ø Primary and secondary chilled water pump performance

Ø Temperature and relative humidity maintained inside the conditioned spaces

Ø Condenser water pump and cooling tower operation

Ø Performance of AHUs

Chiller Performance

Heat load of the system varies between 150TR to 260TR during 8 hrs of monitoring period. Presently 260TR centrifugal Chillers are used to cater this varying heat load in the system. However, centrifugal Chillers are less efficient when they operate under partial load conditions whereas the efficiency of other types of Chillers are not much affected by the percentage load on the Chiller compressor.

Using the data obtained during the survey, specific energy consumption of Chiller 4 is calculated for different heat load conditions. Due to that results obtained, average SEC of Chiller 4 is about 0.55 KW/TR during the day time where heat load is around 80% of Chiller rated cooling capacity and when heat load in the system reduce to a level lower than 175TR.

Chilled water pumps

Present chilled water circuit is operating with primary and secondary chilled water pumps and variable speed drives (VSD) are installed for each secondary chilled water pump. Presently They are using the VFD to control the frequency of pump.

Ten secondary chilled water pumps are operated for about 8 hrs. a day and these pumps are operated at 50Hz frequency. Only Three secondary chilled water pumps are operated during the remaining period.

Chilled water flow through Chillers

Chilled water line to each Chiller is still operating with previous settings of balancing valves set for operation of five Chillers. Present the system is mostly operating with only three.

Cooling towers, condenser water pumps and Condenser water balance

Presently 450TR cooling towers are operated from around the 7.00 am to 5.00 pm and one is operated during rest of the day. According to the result heat load and temperature difference across the Chiller condenser was monitored using temperature and flow. Due to that result concern, heat load to cooling towers varies between 175 TR to 450TR. According to this data, operating other cooling tower is economical when heat load at condenser is above 450TR.present system condenser water balance between cooling tower is adjusted to avoid split out of condenser water from cooling towers. Balancing valves installed at each Chiller condenser water line is still remaining partially closed, as they were used to balance the system while 5 Chillers were operated daily.

Air handling units (AHU)

Air Handling Units of the system have motorized modulating valves installed to the chilled water lines. There are 58 numbers AHUs in the BOC. These valves were originally designed to control using the BMS and there is no local controlling system exists in the system that can modulate these valves based on return air temperature.

Presently there is not BMS in the .BOC building. Due to this they are using the manual controlling system through motorize valve, the chilled water flow to individual AHUs. Same as there is not having automatic temperature regulating mechanism has resulted in overcooling in most of the areas served by AHUs. Optional comfort temperature in an air conditioned space is 23oC. According to the BOC staff there is no special temperature that is required by any area of the bank except for area served by AHU 43b, where required temperature is 27 C These AHUs are switched on and off by controllers installed inside operating theatres. Therefore, the maintenance staffs are not informed on the operating time of these AHUs.

BASIC STEPS OF DESIGNING A CONTROL SYSTEM

There are several steps, to design a proper building management system. Such as,

I. Creating the point schedule

II. Defining sequence of operations

III. Define I/O objects

IV. Creating PID control loops

V. Create schedules

VI. Implement intelligent controls

VII. Create Totalizers (run hours)

VIII. Create trend logs

IX. Configure Alarms

X. Scripting