The International Steam Pages

An introduction to Java's sugar mills and their stationary steam engines

Although much original equipment has been replaced, there is an astonishing variety of steam power in the sugar mills. This introduction attempts to show you some of the highlights and you can then click on the links for more examples. It will also try to explain the basic process in which sugar is produced, but it is not a definitive guide to what is a complex process! More details are available on other pages linked below.

Special thanks go to veteran Java sugar mill engineer Rob van Dort and retired sugar technologist Kees Morks who have provided a lot of back-up, but responsibility for remaining errors resides entirely with me.

A guide to the crusher/mill engines in Java's sugar mills 
A guide to the large steam pumps/compressors in Java's sugar mills 
A guide to the belt drive engines in Java's sugar mills 
A guide to the smaller pumps in Java's sugar mills 

In addition to this general survey, there are two special features on the two mills with the best array of surviving machinery Olean and Gondang Baru.

For details about the equipment in individual mills you should read Steam Equipment in Javan Sugar Mills

After the cane has first entered the mill, the juice has to be removed. Often up to six large engines in series will be used, traditionally the first was a 'crusher' followed by up to five 'mill' engines although to people like me they all look very similar! This is the most spectacular part of the operation as here at Jombang. Below is the mill train at Purwodadi. Preceding the 'crusher' there may have been one or two electrically driven knives or shredders - these are a relatively new addition. These days most of the crushers have been replaced by electrically powered 'unigrators' which effectively homogenise the cane. Click here for more about crusher/mill engines.

The juice will now be need to be transferred to the boiling house, the small pumps like this one at Krembung are the unsung heroes of the operation. Click here for more about pumps.

Juice clarification is usually carried out by sulphitation which involves controlled burning of sulphur to produce sulphur dioxide. Some of the sulphur comes from Kawah Ijen. (Previously many mills used a carbonatation process with carbon dioxide, but just Gondang Baru and Tasik Madu still do.) In sulphitation, first excess milk of lime (alkali) is added to the juice and later neutralized with sulphur dioxide gas (acid). The purpose of the lime and sulphur is to manipulate the acidity (pH) to condition the juice for maximum settling of impurities in the subsequent clarification step. (Much of the impurities are present in colloidal suspension and will not settle easily.)

However, before the juice is concentrated, it has to be filtered to remove solid residues like mud, which is then traditionally removed in skips, as here at Kanigoro (above is a vacuum filter which leaves the mud caked on the outside:

Water boils at lower temperature if the pressure over it is reduced and for this reason, sugar mills use condensers and vacuum pumps.  These are immediately recognisable because they have an extra cylinder ahead of the engine. This is a typical example at Gending. Click here for more about larger pumps.

The evaporators traditionally work in series (there may be more than one set in a large mill), with the concentrated juices being drawn when required into the next evaporator by its relatively higher vacuum. The evaporated vapour from the first few evaporators in a set is hot enough for heating the incoming juices. Below are evaporators at Sumberharjo.

After initial evaporation, the 'syrup' is bleached in a second sulphitation and passed to a 'pan station' where it is concentrated for crystalisation (also under vacuum as more water has to be evaporated) - the result is massecuite, a mixture of crystallising sugar and molasses. This is the pan floor at Wonolangan with the crystalisers below:

wonolangan044.jpg (52726 bytes)

Condensers are needed to condense the vapors produced from the evaporators and sugar pans by the vacuum pumps to remove entrained air and incondensable gas and typically these are high speed engines such as this Bellis and Morcom pump at Purwodadi:

Again pumps are needed.

When low grade massecuite is cooled (and churned) further crystalisation occurs. For high grade massecuite, producing the commercial sugar, the crystallisers act as holding vessels prior to the centrifugals only.

Separation is achieved using centrifugation, a task mainly achieved these days by electrically powered machinery. These water powered centrifuges and the engines which drive them at Krembung are rare survivors. 

This engine is driving the centrifugals at Rejosari. Click here for more about belt drive engines.

The sugar that emerges needs to be dried and is then transferred to the packaging department by a series of shaking trays - again mainly achieved these days by electrically powered machinery. This Tangye (Birmingham, England) engine still does the job at Karangsuwung. Click here for more about belt drive engines.

The sugar is then bagged and weighed and placed in store for distribution. This is Panji.

and this is Purwodadi:

There are other important ancillary processes.

The cane residue (bagasse) is recovered and used as boiler fuel, this usually requires a complicated conveyor belt system, which in a few cases is powered by small engines like this one at Jatibarang. Behind is another machine to turn loose bagasse into bales (lower picture, right hand side).

Working with the furnaces which fire the boilers to produce steam is best compared to Dante's inferno. This is Sumberharjo:

This is the modern furnace at Tersana Baru

While much of the steam is used directly to power machinery, all mills have their own generator systems, diesel for start up and mostly (but not all) modern steam turbines. This is an old generator at Sudhono.

Rob and Yuehong  Dickinson