charcoal filter and a sponge filter are always useful adjuncts, particularly as it is one of the peculiar properties of the former to absorb any bad smell or odour that the water may have. When, however, ice is made from distilled water produced by condensing the exhaust steam of the engine, the way is paved for numerous defects in the ice. The most prominent of these are as follows:

Red Core. This is a reddish tinge which shows itself in the centre of the blocks of ice, and can generally be traced to rust in the distilling plant. It usually reveals itself during the second season after the first winter's rest. Το avoid this rust the distilling plant should be of galvanized iron throughout. Sometimes the inside of the filters and tanks are painted at the beginning and end of each season. It is advisable to keep the distilling plant full of distilled water during the winter. Red core can frequently be got rid of if the forecooler chills the water to from 36 to 33° F. before it is passed through the sponge filter. Sometimes the water itself contains iron oxides-this can be ascertained by analysis which are much the most troublesome to deal with. When hard town water, or water derived from a well in the chalk, is used for ice-making, it will be found to contain, not only carbonate of lime, but carbonic acid, which, being a gas, passes over with the steam. As a matter of fact, distilled water nearly always contains a small proportion of carbonic acid, which, in contact with the iron pipes of the plant, forms two kinds of iron oxide-one the ordinary ferric oxide or iron rust, which is caught by the filter; and the other a solution of ferrous oxide, which is not caught, and which passes to the ice-cans.

When the water is being frozen, what little air it may still contain is brought into contact with the oxide, which

is turned into rust and causes the red core. No amount of filtering will get rid of this oxide. The only satisfactory way is to cure the evil at the start and soften the water before it enters the boilers by means of suitable and cheap chemical reagents. These get rid of the carbonate of lime, iron salts, and magnesia, which are the chief scale-forming impurities, and not only keep the ice clear, but considerably prolong the life and efficiency of the boiler. Watersoftening has been dealt with in Chapter VII.

There is another reason why it is important to eliminate the carbonic acid, for this substance in contact with blackbone will produce carbonate of lime, giving a white core to the ice.

Red ice is not always due to rust, but to a peculiar deposit from the lubricating oil used in the cylinder. This should always be caught by the skimmer, and seldom reveals itself unless the plant is being forced. If the red deposit is due to oil, it will float in warm water and sink in cold, and can be readily tested in this way. When oil is appearing in the distilled water, the oilseparator is not doing its duty, and the chances are that the steam condenser will be found coated with oil. If so, it must be blown out with live steam1 until it is clean. A flannel bag placed over the delivery into the forecooler tank has been recommended, but this is hardly necessary where a good sponge filter is in use.

White Core. The usual cause of white core or cloudy ice is the presence of air in the water, but it can be caused in other ways, and is sometimes due to carbonate of lime and magnesia, which has been carried over with the steam, or which has been formed by carbonic acid coming in contact with the blackbone in the filters. It

1 See method of cleaning surface condensers, page 182.

is also due to the priming of the boiler and to impurities derived from overworked filters. As before stated, most of these troubles can be got rid of if the water is forecooled to 33 or 34° F.

Charcoal for Filters.-Probably the best insurance against bad ice is the use of good charcoal. It is well known that this substance will chemically absorb odours and purify the water, as well as mechanically rid the water of any impurities. Almost any hard wood charcoal will do for filters. In the United States sugar maple is preferred, but beach charcoal is equally good. Charcoal should be renewed from time to, time, as it loses its properties of absorption, and these cannot be restored to it.

Starting up a Distilling Plant.-When a distilling plant is first put down, and at the beginning of every season, it should be thoroughly cleaned. In the first case oil, red lead, and dirt will have been left in the pipes and coils by the workmen, and, if not got rid of, will spoil the ice. These impurities can generally be displaced by blowing out the apparatus with hot water and live steam; but there is another source of trouble due to the zinc oxides from the galvanized pipes becoming dissolved in the distilled water. As a precaution against this, it is recommended that a plant before being started should be filled with a strong solution of soda ash. After this has stood for several hours, it should be blown out with live steam or live steam and hot water. Of course, before this is done the charcoal must be carefully removed from the filter, for, if the soda once gets into that, it means new charcoal.

Blackbone and Blood Charcoal.-If, after taking all the above precautions, the ice is still unsatisfactory from

either bad taste, smell, or colour, a cure can generally be effected by mixing finely powdered blackbone or bone charcoal with the distilled water. This is a powerful absorbent, and a pound or two of blackbone will be sufficient to withdraw the taste or smell from a ton of water. Blood charcoal, which is a bye-product in the manufacture of prussiate of potash, will do equally well, but is rather more expensive.

CHAPTER XI.

ICE-MAKING.

ONE of the most important uses to which mechanical refrigeration has been put is the artificial freezing of water to produce ice.

Water freezes at a temperature of 32° F., and in freezing changes its chemical constitution. In order to effect the change, the latent heat must be withdrawn from the water. The latent heat of ice is 142-4 British Thermal Units-that is to say, in order to convert 1 lb. of water at 32° F. into 1 lb. of ice at 32° F., 142-4 B.T.U. must be withdrawn by the refrigerating machine. To this latent heat must, of course, be added the sensible heat of the water, and, assuming that when freezing operations were started the water was at a temperature of 60° F., 60−32+142·4, or 170·4 heat units per lb., would have to be abstracted before it would be completely frozen. After freezing is complete, the block of ice is reduced to a temperature of about 20° F. As the specific heat of ice is 0.5, 32-2012, 12 x 5 = 6 B.T.U. have still to be extracted. This makes a total of 176.4 B.T.U. per lb.

Water for Ice-making-Practically any water that is fit to drink will make ice fit to use, but if the ice is to be clear the air must be eliminated. The same remark applies to the carbonates of lime or magnesia or to other