units now used in place of the conventional radiators. Most heaters consist of a bank of finned tubes located behind a pressed metal cover with inlet and outlet grilles. Circulation of air through this type of heater may be controlled by a simple damper type of operating gear or, for locations where a greater flexibility of control is required, fans can be incorporated to speed the air flow over the heater tubes thus increasing the rate of heat transfer.

Ventilation to individual rooms is normally by means of opening windows except in special locations in the laboratory. In the canteen kitchen extract fans are used to create an induced plenum system to prevent the ' spread of air contaminated by odours. Special inlets may be provided in the kitchen area to provide the comparatively large quantity of air necessary to give the required air changes. If these inlets cannot be located in positions to ensure reasonably clean air, then some kind of filter should be incorporated in the inlet duct.

The standard of finishings in these buildings is usually fairly high, with plastered walls and ceilings, and thermoplastic or wood block floors. In offices it is usual to complete floor finishings between outer walls before partitions arc erected. As these are normally supplied in sections, they can be easily taken down and rc-erected to a different layout if required.

Welfare amenities are provided as necessary throughout the station, separate facilities being pro­ vided for coal gangs and other personnel working in isolated locations. Toilet and washing facilities are also provided at central points in the station buildings but the main locker and ablution facilities are normally provided in a central block adjacent to the gate house and clocking arrangements. Quarry tiles are the most commonly used floor finish in the welfare blocks, walls being plastered or tiled as necessary.

A typical layout of offices, welfare, laboratory and canteen facilities is shown in Fig 3.67.

There are many other buildings located in different parts of the station which require special treatment. Amongst these are the water treatment plant house where special consideration must be given to the floor and wall finishes to resist acids and alkalis.

The main switching station consists of large lattice frames cladded with lightweight wall cladding and roof­ ing. A brick plinth wall is normally provided and the floor finish is usually bitumen macadam. The steelwork is usually designed to support electrical equipment in addition to the wind, snow and dead load of the structure.

Heating and ventilation is not normally provided in these buildings, and a small structure is usually con­ structed from load bearing brickwork with a concrete roof to house the office, stores and welfare facilities for the switching station. A typical arrangement of a 132 kV air blast switch house is shown in Fig 3.68.

Chimneys, cooling towers and precipitators

22 Chimneys, cooling towers and precipitators

22.1 Chimneys

Although single flue chimneys have been used on 2000 MW power stations the present trends are to use multi­ flue stacks for all future stations of this type. For power stations with four boilers, four flues, one to each boiler, are contained in a single concrete cylinder which acts as a windshield. Figure 3.69 shows a typical arrangement

the flue gases from two boilers into one .flue may be necessary for larger stations. The outer cylinder of concrete is constructed to withstand the greater part of the wind forces and its own dead load. The flues are built off the lowest intermediate floor or foundation slab and an acid resisting brickwork or other suitable lining is provided. There is no structural connection between the flues and the windshield, the flues being permitted to expand or contract freely according to temperature variations. The space between the wind­ shield and the flues is accessible at all times, including the period when the boilers are operating. Access to the various levels is provided by a concrete or steel staircase and in some chimneys of this type, a lift is also provided.

Other types of multi-flue chimneys have windshields supporting floors at 9 m to 12 m intervals, the flues consisting of brick cylinders with external insulation built off each of these floors to form continuous gas ducts.

On the first type of chimney the windshield and flues are of reinforced concrete not normally exceeding 305 mm thick, apart from the lower section below the floor supporting the concrete flues. Brick linings to the flues arc normally only one half brick thick and are selfsupporting for heights up to 12 m. Concrete corbels, therefore, are cast on the inner face of the flue to carry each successive lift of brickwork. The use of corbels at these intervals has the advantage that in addition to restricting the height of brick lining sections it also provides joints for accommodating differential expan­ sion between the lining and chimney, the joint being packed with glass wool and covered with a lead flash­ ing. Brickwork is built from best quality engineering bricks jointed in acid resisting compound with a cavity between the concrete and brickwork. On single-flue chimneys the cavity is filled with vermiculite or glass wool insulation or is ventilated by holes in the concrete, and on multi-flue chimneys the cavities are either sealed or filled with insulation as for single flue chimneys.

In the second type of chimney the construction of the intermediate floors governs the construction pro­ gramme and therefore it is advantageous to reduce the number of floors. With the development of tongued

and grooved blockwork in lieu of plain no-frog bricks it is feasible to space these floors at intervals up to 30 m.

Protection against acid attack can be given to the concrete at the top of chimneys by the use of engineer­ ing brickwork or protective coatings applied to the outer surface of the concrete windshield and flues. Quarry, or similar tiles are used on the flat surface on the top of the windshield and cast iron segments are used on the top of the flues.

If flue gas is to be prevented from penetrating the chimney lining and entering the cavity between the lining and the concrete, a negative pressure must be maintained inside the chimney. The difference in pressure caused by the relative densities of the gases inside the chimney and the atmosphere must exceed the losses caused by the bend at the flue entry, the head loss due to friction inside the chimney and the head necessary to give the gas the required exit velocity of about 25 m/s.

A negative pressure inside the flue is more important in chimneys where flues are built in brickwork off intermediate floors, because a gas leakage in a chimney of this type means that gas would enter the occupied zone inside the windshield.

The shafts of single-flue chimneys and the wind­ shields of multi-flue chimneys must be designed to with­ stand wind and dead loads and temperature stresses. Building Code of Practice CP3: Chapter V: Part 2 [16] requires that structures whose greatest lateral or verti­ cal dimension exceeds 50 m shall be designed for a 15 second gust wind speed, but it is prudent to apply a factor to allow for dynamic effects in the preliminary design. The basic design, as a cantilever resisting over­ turning under wind forces considered as static loading, may be based on any of several well documented pro­ cedures. However, windshields have aspect ratios (i.c., height/mcan diameter) in the range 10 to 12, and it is necessary (particularly for a windshield enclosing free­ standing shafts) to investigate the ovalling stresses caused by the varying pressure distribution around the windshield, which result in positive and negative bend­ ing moments in the horizontal plane. Generally, these two aspects of design are considered separately and this is probably adequate for a ratio of mean diameter/shell thickness up to 50.

The distribution of pressure around the windshield has in the past been based on wind tunnel measure­ ments at values of Reynolds number somewhat lower than those that actually occur, but the CEGB has carried out full-scale measurements to determine a realistic pressure distribution. Less is known about the internal pressure on the windshield; the presence of ventilation louvres at top and bottom of the windshield will cause the internal pressure to vary between them.

In designing the floors inside the windshield, their effect as stiffening diaphragms should be considered, otherwise the windshield could be of uneconomic thick­ ness. The floor design must also include areas of open­ mesh flooring to allow sufficient upflow of air to cool

Chimneys, cooling towers and precipitators

the interspace, in which the temperature should not normally exceed 38°C.

Although it is usual to provide an expansion gap between the floors and free-standing concrete shafts in a windshield, the floors may be brought into contact with the shafts and load transferred laterally owing to horizontal deflection of the windshield. Hence the shafts must be designed to withstand a proportion of the total wind load based on the relative stiffness of shafts and windshield.

The design of the windshield is based on an elastic analysis for a 15 second gust wind speed. The sections should be checked using a load factor analysis for overturning moment resulting from a wind speed of 1.5 times the design wind speed.

As previously stated, it is desirable in chimney design to apply a factor to the wind forces which will ade­ quately allow for dynamic effects. This factor for a single-flue chimney is related to the natural frequency of the chimney, but for multi-flue chimneys a full investigation is required. Excessive oscillations may occur in steel chimneys owing to vortex shedding and buffeting and, whilst no significant vibration has been noticed in concrete chimneys, it is important to deter­ mine the conditions under which such vibrations could occur. In the last decade, the effect of dynamic forces has been studied in greater detail and guidelines for design purposes are now incorporated in a ‘Model Code for Concrete Chimneys’ [31] prepared by Comite International des Cheminees Industrielles (CICIND). In addition the latest edition of DIN 1056 ‘Free Standing Chimneys’ [32] incorporates requirements for the consideration of the effects of oscillation, con­ sidered in the form of a static load equivalent.

Temperature stresses have been traditionally calcu­ lated on the temperature differential which exists across the flue walls and which causes tensile strain on the cooler face. I lowcvcr, the presence of long vertical cracks in several tall chimneys built for the CEGB since 1960 suggests that temperature stresses have been underestimated and that an empirical approach based on experience would provide better answers.

In accordance with Civil Aviation Authority require­ ments, chimneys are provided with aircraft warning lights and the usual arrangement consists of lights located at the top of the chimney and at 50 m vertical intervals. Three fittings at 120° positions or four fittings at 90“ positions depending upon the arrangement of flues are provided at each level. In the past, fittings have each had four tungsten bulbs with individual red shades, but a recently-developed aircraft warning beacon has a discharge lamp in a glass fibre casing with a vertical transparent plastic front. This latter fitting has an anticipated lamp life several times greater than that for a tungsten fitting, and all future chimneys will be equipped with discharge fittings. On multi-flue chimneys, the lights are usually fixed to doors in the outer face of the windshield in such a manner that when the door opens, the light is brought into the chimney