Análisis de Riesgos

The HVAC system installed in this building is designed to regulate indoor climate: both air temperature and moisture content. Moisture is extracted from the outdoor air supply by a cooling coil which is either located in an air handling unit (AHU) or fan coil unit (FCU). The cooling coil dehumidification rate depends on the coil surface area as well as the surface temperature. The mechanical engineer, who was involved during the design process, was responsible for the determination of the correct condensation rate by sizing the coil relative to conditioned space requirements. On the other hand, air temperature is controlled by the volume of cooled air supplied to the conditioned spaces. This air supply which combines outside air and air returned from the conditioned spaces, is cooled by cooling coils supplied with chilled water. Waterside primarily consists of plate heat exchanger, chilled water variable speed pump and cooling coils.

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3.3.1

Plate heat exchanger

Two numbers of plate heat exchanger are installed in this building with capacity of 505 RT each. Building cooling load contained in the secondary loop is transmitted to the primary loop via these plate heat exchangers. The primarily loop is the chilled water piping that connects the central gas district cooling (GDC) plant to all plate heat plate exchangers located in various buildings, and the secondary loop is the chilled water piping loop that connects the plate heat exchangers to all the cooling coils in one specific building. The chilled water temperature supplied from the GDC plant to the plate heat exchanger is ideally maintained constant at 7oC and returned back at 14oC while for the secondary loop the chilled water temperature is maintained at 8oC supply and 15oC return. With building management system (BMS) installed, these temperatures can be observed from the centralised monitor located in the control room. Figure 3-1 shows the layout of the pipe-work.

3.3.2

Chilled water variable speed pump

There are three variable speed drive (VSD) chilled water pumps installed here. Only two pumps are operated when the HVAC system is switched on, while the other one functions as a stand-by unit. Each of them has 60 l/s pumping capacity. This pumping system is also installed with one motorised valve to bypass the chilled water when the load falls below the minimum capacity of the VSD pumps. The pumps generate the flow and head required at anytime during operation. This is performed by the pump controller which regulates the pump speed via the adjustable frequency drive and modulates the bypass motorised valve. The controller regulates the pump speed and valve position in response to building cooling load conditions. The pump controller receives a signal from pressure differential sensor located across the chilled water supply and return pipes. Pipes connections of the pumping system are shown in Figure 3-1.

74 Figure 3-1: Diagrammatic diagram of chilled water piping connection from GDC Plant to Heat Exchangers and Pumps

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3.3.3

Cooling coil

In total, there are 62 chilled-water cooling coils connected to this HVAC system. The cooling coil is either located in an AHU or FCU depending on the size of the conditioned space served. Similar to other cooling coils, the chilled-water cooling coils installed here are finned tube heat exchangers. They consist of rows of copper tubes that pass through numbers of aluminium formed fin sheets. Heat is transferred from the air to the water flowing through the tubes as air carrying the load from the airside passes through the coil and contacts the cold fin surfaces. The temperature of the air that exits the coil, which is normally referred as ‘off-coil temperature’, is controlled by a Proportional + Integral (PI) controller. The PI controller which is usually located adjacent to the AHU or FCU uses either a two or three way modulating valve to modulate the chilled water flow into the cooling coil. Figure 3-2 shows a typical chilled water piping connection for an AHU with a two way modulating valve installed. For this building, the set-point value for the off-coil temperature is specified by the building facilities engineer via the BMS System and the value varies within 15oC to 18oC in each floor. Figure 3-3 shows a screen view of the level 6 off-coil actual temperature and the set-point from the BMS system.

The waterside HVAC control system of this building is designed to maintain the desired off-coil temperature to each floor. The control system of the airside on the other hand is responsible to sustain the indoor air temperature relatively close to the indoor set-point temperature. To implement this task, BHEUU has variable air volume system (VAV) installed. Similar with the off-coil set-point temperature, the value of indoor set-point of each conditioned space is also entered to the system using the BMS. The VAV System installed has 2 primary components: a central supply fan and VAV room terminal units.

76 Figure 3-2: Typical chilled water piping connection of AHU installed with 2-way modulating valve

Figure 3-3: A screen view of level 6 off-coil actual temperature and the set-point from BMS system. It also shows the supply fan static pressure set-point and the relevant supply duct static pressure.

3.3.4

Central supply fan

The supply fan has a variable speed drive (VSD) and is usually located in the AHU room as part of the air handling unit. This drive responds to fluctuations in the air flow demand from each VAV terminals unit. A constant speed supply fan would not be suitable in this application because it would increase the static pressure in the ducting system when the terminal units modulated towards closed. Therefore it creates an increase in energy consumption as well as a risk of damage [67]. There are several control strategies available as mentioned in CIBSE Guide H, and one of them is the use of ‘multiple pressure sensors’ which are used in this building. Here, the fan speed

77 is controlled using a PI controller connected to static pressure sensors positioned at each branch of the air duct, to maintain duct static pressure at the set-point. When the cooling load in the condition space drops, the room terminal units modulate towards closed, causing the duct pressure to increase. The PI controller then decreases the fan speed to maintain the pressure at the set-point. Figure 3-4 shows an example of multiple brunches duct layout with multiple pressure sensors control strategies applied. The value of the supply fan static pressure set-point as well as the relevant supply duct static pressure can be controlled and monitored by BMS system as shown in Figure 3-3.

Figure 3-4: An example of multiple brunches duct layout with multiple pressure sensors control strategies applied. Supply fan is controlled to ensure that all pressure sensors meet or slightly exceed the set-point pressure. This ensures all branches receive an adequate air supply [67].

3.3.5

VAV room terminal unit

Every AHU in this building supplies several VAV Room Terminal Units to maintain the desired indoor climate of the condition spaces. In level 6 for instance, the only AHU supplies 19 room terminal units. There are several types of room terminal unit as described in CIBSE guide H: the one used here is the ‘pressure independent VAV terminal’[67]. All terminal units installed in this building are designed only for cooling purposes, i.e. they lack of reheat coil. Each terminal unit is fitted with their own local controller functions to control cool air supply by throttling the damper built in the terminal with the indoor air temperature as its control variable. The indoor set-point temperature of the terminal units are entered using the BMS as shown in Figure 3-6. The term pressure independent means that at a given damper position, the cool air supply to the space is a function of indoor air temperature only and not dependent to static air pressure in the supply duct. The system uses cascade controller to modulate

78 the cool air supply to the conditioned spaces. The first stage of this controller reset the air flow set-point with indoor air temperature as a control variable using a PI controller. The second stage also uses PI controller to operate the damper position as a function of the measured supply air flow. Figure 3-5 shows an example of pressure independent VAV terminal unit with a cascade controller is used to reset the air flow set-point.

Figure 3-5: Pressure independent VAV terminal unit with a cascade controller is used to reset the air flow set-point [67]

Figure 3-6: VAV Room terminal units table that shows the space actual and set-point temperature as well as actual and set-point supply air. In between of the temperature and the box blow are the terminal’s damper position and static pressure. The floor plan shows the position of the VAV Room Terminals on level 6. This figure is a screen shot of BMS computer when resetting the indoor set-point temperature of VAV room terminal units in level 6.

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