Cement Kilns

Rotary Coolers

Stokes' Cooler

A rotary cooler consists of a slightly inclined tube of similar construction to a rotary kiln, placed at a lower level than the kiln so that the clinker leaving the kiln can fall directly into the upper end of the cooler. Ambient air enters at the other end. The upper part of the cooler, because of the high temperature of the clinker, is brick lined, while the lower part is usually provided with scoops or "lifters" that pick up the clinker and cascade it through the air-stream.

The earliest rotary cooler described is in Stokes' patent of 1888, and a prototype may well have been installed on the kiln at Arlesey. He says:

In order to produce the required temperature in the furnace I prefer to use what is known as the "regenerative system" in which the air is heated previous to combustion, either by contact with the issueing (sic) hot clinker, the passage of the air being in the contrary direction to that of the hot clinker; or by the passage through regenerative chambers, or parallel flues, of well known construction, instead of the cooling drum aforesaid.

Stokes' drawings show a cooler 38 ft long and 2'3" diameter (the kiln was 28×5), lined for two-thirds of its length with 4" brickwork, while the rest has "projecting shelves" for lifters. If the kiln had worked (which it most certainly didn't) the cooler would have been very effective. In the ensuing 80 years in which they were used, rotary coolers evolved little beyond this first design.

Hurry & Seaman Coolers

Hurry & Seaman always used rotary coolers in their kiln designs, but there is a subtle change of emphasis. The early Hurry & Seaman kilns were oil fired, and getting the kiln hot enough was not a major preoccupation. On the other hand, the piles of white hot clinker that they produced needed more efficient handling, and their coolers were designed to cool the clinker. Early designs had two cooling tubes on successively lower levels. In the first, ambient air cooled the clinker to below red heat before entering the kiln as secondary air. The clinker then passed through a crusher into the second cooler, with air drawn through by a fan and wasted. Water was sprayed into the upper end, to increase the cooling effect, and also probably to hydrate free lime, which was a major problem in early rotary kiln clinker.

By the time Hurry & Seaman kilns began to be installed in Britain, single cooler tubes were the norm. Because at that time it was recommended that most or all of the combustion air should pass up the firing pipe, most cooler air was diverted into coal drying or other duties. Water sprayed into the cooler would have no detrimental effect on these.

Design features

The design of the cooler has to allow for the temperature of the clinker and air. Because clinker enters at 1200°C or more, the upper part of the cooler is lined with refractory brick. The brick lining generally prevents attachment of lifters, so that little in the way of agitation of the bed of clinker is possible in the hot zone, although "ripple" linings or cast refractory lifter blocks have been tried, with little success.

The rest of the cooler, in which the clinker is usually below 800°C, is provided with steel liner plates (mainly to protect the structural shell from abrasion), and lifters. In the hotter part, the lifters are in the form of buckets made from cast high temperature steel. In the cooler parts, mild steel "channel" is generally used. At the outlet, a grid is usually provided, allowing normal-sized clinker (<100 mm) to fall through to the clinker conveyor, while larger lumps are separated out for crushing.

The cooling effect obtainable obviously depends upon the amount of cascading of clinker that can take place. Provided that enough lifter capacity is present to lift the entire clinker bed, the best results are obtained with a low slope and high rotation speed.

Coolers on British kilns

Rotary coolers were installed on the vast majority of kilns in the first quarter of the twentieth century. They were installed either beneath the firing floor, as in the original Hurry & Seaman designs installed by Fellner & Ziegler, or beneath the kiln. The latter design had the advantage that floor space required for the kiln system could be reduced, but meant that the cooler had to pass through one or more of the kiln piers, requiring a more elaborate design of the latter.

rotary cooler
The largest British rotary cooler, by Vickers Armstrong as installed at Kent and Shoreham, 92' long and 9' in diameter. The design necessitated tall and massive kiln piers, but was easy to maintain and run.

One way of shortening the cooler so that the second kiln pier need not be pierced was the "double-back" or "concentric" cooler, produced by several manufacturers in the period 1905-1920. The later versions were frequently supplied with pressurisation air. Being very difficult to maintain, most of them were soon abandoned in favour of more conventional designs. The standard design coolers shown above were installed at Kent in 1949 to replace the original (1922) concentric coolers. On some smaller kilns, such as Wouldham 7 & 8, the concentric coolers were simply removed, and the kilns run with no cooling at all.

concentric cooler
Concentric rotary cooler layout based on those at Johnsons.

A tentative list of the concentric cooler installations is as follows:

Johnsons 1-31907FLSno
Wouldham 7-91910-12FLS?1929
Peters 1&21911-13FLSno
West Kent 41911FLSno
Rugby 1&21911-13Newell?
West Thurrock 1&21912-14FLS?
Wilmington 3&41913-20FLS1933?
Coltness 11914Pfeifferno
Magheramorne 1&21914-21FLS1949
?Kirton Lindsey 1&21920-22FLS?1949
Kent 1&21922FLS1949

From 1923, rotary coolers started to lose ground to planetary coolers, and from 1937 to grate coolers. The number of installations declined, although this to some extent reflects reduction in kiln installations, while size of coolers increased. The largest installed (90'×9'0¾") were among the last, on Kent A1 and A2 and Shoreham C1 and C2 in 1949-1950. The last installed was on the anachronistic Kirton Lindsey A5 in 1968.


 GeoScenicP538713 ©NERC
Picture: ©NERC : British Geological Survey Cat. No. P538713. This is a view (early 1960s?) into the outlet of Barrington A1 rotary cooler. Clinker cascades through the stream of cold air being drawn in by the kiln overhead. Because the cooler "lifters" are fairly worn, the hot clinker can be seen falling into the cooler at the far end. Water is being squirted in, to aid the cooling action, at the expense of cooling and diluting the secondary air.

© Dylan Moore 2011: last edit 15/01/15.