IiftTuTURBINE 7 CONDENSER | | TURBINEtin8 CONDENSER |

£ □□□ £

Fig. 2.58 CW system mimic panel

15 Heating, ventilation and air conditioning

15.1 Introduction

The objectives of a heating, ventilation and air con­ ditioning (HVAC) plant on a power station can be considered to include one or more of the following:

• To comply with Statutory Regulations such as the Health and Safety at Work Act and the Nuclear Installations Act.

•To comply with CEGB Policies and Safety Rules.

•To maintain specified environmental conditions for plant and personnel.

•To remove toxic and hazardous materials present in the atmosphere.

•To remove heat .or moisture generated by equip­ ment.

•To assist in the containment of radioactive material and to minimise discharges to the environment.

•To assist in controlling the spread of smoke and fire in order to support the escape of personnel and the protection of plant.

To ensure that the identified design objectives are met it is essential that the HVAC requirements are con­ sidered as early as possible. HVAC equipment such as fans, ducts and filters require a lot of space and plant rooms and duct routes must be identified at an early stage In the layout of the station.

During the design phase each area of the station is studied in turn. Maximum and minimum temperature limits will be determined from equipment specifications and personnel requirements. Humidity levels will be determined and the need for specialist ventilation to dissipate toxic gases or control smoke and hot gases will be also determined. From these detailed requirements for each area, and with a knowledge of the building fabric, the solar gain to the buildings and the maximum and minimum external design conditions, estimates can be made of the heating, cooling and ventilation loads for each area. With this information a decision can be made concerning the type of system to be adopted for each area and preliminary plant sizes can be estimated enabling plant rooms and duct routes to be drawn up. As the design of the main plant progresses and more information regarding plant heating and cooling loads becomes available, then the preliminary estimates will need to be checked. As soon as a reasonable degree of confidence in the cooling and heating loads is available then final sizing and ordering of equipment can take place.

15.2 Ventilation of nuclear stations

HVAC systems on nuclear stations also serve to supplement physical containment in order to prevent the escape of radioactive particles. Air patterns are arranged so that air is extracted from the areas of higher activity and supplied to the cleaner areas. The arrangement is designed to ensure that air always flows from the cleaner areas at-sufficient velocity to prevent the back-diffusion of radioactive particles. Elaborate studies are made and sufficient equipment is installed to ensure that these air flows are maintained under all conceivable operating conditions. In addition to pre­ venting the spread of contamination within the build­ ing, the ventilation system has two further important roles. By providing sufficient air changes within the ventilated space, the system also helps to minimise the build-up of activity within the space by continuously purging it with clean air; and by fitting high efficiency particulate filters to the extract system the discharge of activity to atmosphere is kept to within allowable limits. The filtration system has a collection efficiency of greater than 99.9%, and before being discharged to atmosphere the air is sampled to check for levels of

activity in case of accidents such as a burst filter. These filtration systems are regularly checked to ensure that the collection efficiency does not deteriorate.

The foregoing requirements inevitably place a high burden of reliability on the ventilation system. This leads generally to the requirement for standby fans and filters, which in turn results in complex control systems. The necessity for standby plant, high air volumes and large banks of filters, inevitably leads to a high require­ ment for space. It is therefore important that these requirements are identified at an early stage in the design. For a detailed treatment on the design of active ventilation systems, reference should be made to the Atomic Energy Code of Practice 1054 — Ventilation of Radioactive Areas.

Figure 2.59 is a simplified schematic of an AGR nuclear station central control room heating, ventila­ tion and air conditioning system.

15.3 Smoke and fire control

In order to prevent the spread of fire and smoke between lire compartments and to assist in search and rescue operations, smoke or lire venting facilities are usually incorporated into the design. For large single­ storey buildings such as a turbine hall, automaticallyinitiated fire vents are installed in the roof. These ventilators operate at a predetermined temperature, or on a signal from the fire detection system, and opcn-up allowing fire and hot gases to escape to atmosphere.

To prevent fire and hot gases spreading between compartments which share the same ventilation system, the HVAC systems are fitted with fire dampers at penetrations through the fire compartment walls. These fire dampers possess the same degree of fire resistance as the partitions they penetrate and are automatically initiated on detection of heat or from the fire detection system. In addition, it is common practice to include a facility to enable the ventilation system to be used as a vehicle for extracting smoke following a fire. This is usually accomplished by fitting a fireman’s switch to the control system, which enables the system to be switched between normal operation, smoke extract and off. These switches are usually located in an easily accessible entrance lobby.

•J

15.4 General layout of HVAC plant

15.4.1 Turbine hall and boiler house

Traditionally the main buildings of a fossil-fired station have been naturally ventilated. Advantage has been taken of the tall boiler house, the high internal heat losses and the forced draft (FD) fans to draw air in through inlet louvres located at low level around the turbine hall and boiler house. The air rises through the building and is drawn into the FD fan intake with the balance being rejected through roof vents. The advan­

tages of this system are that no energy is expended in driving the system, and heat losses from the boiler and turbines are collected and passed back into the boilers; collection efficiencies greater than 60% are not uncom­ mon. Disadvantages, however, include problems such as high boiler house temperatures, dust and condensa­ tion in the turbine hall and smoke logging in the turbine hall during a fire situation. These problems are usually associated with too little inlet area at low level in the turbine hall. Increasing the area, whilst solving these problems, leads to cold weather problems, for this reason consideration is now being given to separating the buildings with a partition wall and ventilating them separately along the lines of a nuclear station turbine hall.

15.4.2Coal bunkers

This area is usually mechanically ventilated by means of an extract fan and fresh air inlet louvres. Air patterns are arranged to suppress dust levels, and sufficient air changes are provided to disperse any carbon monoxide which may be given off by a slow burning bunker fire. Consideration is being given on the latest stations to fitting filters on the extract to reduce the dust dis­ charged to atmosphere. This puts a heavy burden on space requirements in the area.

15.4.3Electrical equipment annexes

With the exception of computer suites and control rooms, which usually have their own packaged air conditioning units, most electrical annexes housing cableways, battery rooms, switchgear rooms, etc., are mechanically ventilated for cooling with duct-mounted heater batteries. Air is usually ducted from a central fan room and the temperature of the room is regulated by a room-mounted thermostat controlling a heater battery located at the duct entry to the room. Occa­ sionally several rooms are served by one heater battery, in which case an average of room temperatures control the temperature. On occasion when equipment is sensi­ tive to high humidity, humidistats are installed, which on sensing a high humidity, e.g., greater than 70%, override the heating system, increasing the room temperature, thereby dropping the humidity. The air volume allocated to each area is determined from an estimation of the maximum heat load and the maxi­ mum outside air.temperature. During winter conditions 90% recirculation of extract air is employed to conserve heating costs.

Figure 2.60 shows a heating and ventilation system for an essential electrical supplies building.

15.4.4Auxiliary buildings

Wherever possible, auxiliary buildings such as the cooling water (CW) pumphouse and compressor houses are naturally ventilated. Such buildings are

layout^ and design Station

2 Chapter

rarely heated, reliance being placed on the fact that plant is working during cold spells. Buildings such as the water treatment plant, oil pumphouse, ash/slurry pumphouse, fire fighting pumphouses, etc., which have low heat loads and which are sensitive to cold weather, usually employ thermostatically controlled roof extract

fans and fresh air inlet louvres. On occasion heating is also provided, usually in the form of radiant panels or fan-coil units. In the case of the administration block and amenities buildings, these usually employ a mecha­ nically ventilated system with heating coils similar to that used in the electrical annexes.