where maintenance can.be easily carried out. The floors provided inside the windshield are usually located at levels which coincide with the aircraft warning light doors. On chimneys without internal access, i.e., single-flue chimneys, lamp replacement and maintenance must be carried out by steeplejacks, and because of this duplication of fittings is required.

A lightning protection system is essential on such high structures as chimneys. Air terminals arc provided on the top of the windshield and flues, and coronary bands are provided at various levels to intercept strikes below the top of the chimney. If a strike occurs, the down conductors are at a high potential and to avoid the risk of a flashover of other metal structures inside the chimney all reinforcement and steelwork are tied in to the down conductors to ensure that no difference of potential occurs.

BS6651 — Code of Practice for Protection of Struc­ tures Against Lightning [33] — permits the use of steel reinforcement to concrete as down conductors, pro­ vided the leinlbiccmcnl cage is adequately earthed and tests on completion show that there is adequate continuity. Cast iron cappings, handrails or other air terminations must be bonded to the reinforcement.

Although access sockets in which steeplejacks may screw their ladder fixing hooks are provided on the outer face of single-flue chimneys, sockets are not provided on multi-flue chimneys.

22.2 Cooling towers

If a piled foundation is used for a cooling tower (Fig 3.70), the column loads are transmitted direct to the pile caps and the pond wall serves only to retain water. However, if ground conditions permit, a strip

T-beam.

The design of the hyperboloidal shell is based upon BS4485 Part 4 [34], but as towers increase in size (165 m), the dynamic response of the tower shell causing an enhancement of the stress pattern in the shell has been studied in both model and full scale tests and revised design requirements form the basis of a new draft of BS4485. Within the tower from just above the open column area down to pond level is the ‘cascading’ structure. The water from the condensers is pumped through the CW culverts up to the top of this structure or ‘stack’. At this level it enters a large box culvert running diametrically across the top of the stack and into pipes connected at intervals along both sides of this culvert, which feed the sprayers arranged over the top of the stack. The stack is constructed independently of the tower shell so as not to throw any load upon it in-the event of differential settlement, and is usually constructed from precast concrete columns supporting perforated precast concrete beams. Laced through the perforations of these beams are triangular section

timber fillets at varying levels within the cascade structure Io ensure a diverse path for the water falling from (he spray no/./.lcs. The damp wunii conditions in which the timber is located make it verv susceptible t<> wet rot attack. A high degree of protection is given by subjecting the timber to a pressure impregnation pro­ cess of salt solution using mercury, copper or arsenical salts. Asbestos cement sheets are also used in cooling stacks instead of timber and in view of (heir anticipated longer life their use may be economic. Before deciding the type of stack some assessment of durability must be made after analysing the cooling water as this may affect the life and economics of a particular form of construction.

Since the early 1980s a new type of packing compris­ ing closely spaced corrugated PVC sheeting formed into modules for ease of handling has begun to replace these long established forms of packing. Its use shows a marked increase in efficiency and its light weight allows the use of a pack support structure with wider spaced columns (9 m) thus improving the air flow within the tower.

This type of packing has shown a tendency in certain areas of the UK to biofouling, i.e., the build up of algae type growths in the packing matrix due to the warm humid conditions. Massive load increases can result, exacerbated bv silt deposits and sometimes by carbontile deposits in hard water areas, lienee the predicted build-up of loading must be allowed for in the structural design of the pack support structure.

Eliminators are usually constructed in a similar manner to the cooling stack but at a higher level.

Lightning protection is not normally provided on cooling towers as the shape of the tower and the rein­ forcement contained in the concrete are considered adequate protection.

22.3 Precipitators

Precipitators (Fig 3.71) are essentially civil engineering work, although they are designed by the boiler con­ tract©/ and are included as part of the boiler contract. Occasionally steel precipitators are used but the com­

on the concrete, the consequent differential thermal movement between hot and cold inner and outer faces of concrete walls, thermal expansion of the structure and the formation of acid condensates inside the precipitators are the major problems associated with precipitator design. Concrete which contains a high percentage of pulverised fuel ash is particularly suited to precipitator casing construction owing to its good resistance to acid and high temperature. Hair cracking of the finished concrete is also reduced owing to the slower hardening of.this type of concrete. In some cases exterior insulation is employed to reduce condensation.