the conditions under which the plant can be operated most economically, the cost of power must be checked against the cost of water.

The type of condenser used may have some effect upon the pressure which must be carried on the high or compression side of the system. The atmospheric condensers usually show a higher pressure than do the submerged condensers, even with the same initial and final temperatures, but the quantity of water used is small compared with that used in the other type of condenser. Another advantage of the atmospheric condenser is that where there is a tendency for the water to deposit mineral salts, the coils can be cleaned more easily because they are exposed. The disadvantage of this type of condenser, aside from the higher pressure which, as a rule, must be carried, is that they are not uniform in action, but depend to a greater or less extent upon weather conditions. That is, they work best when the air is dry, because of the rapid evaporation of water from the surface of the coils. Of course, this evaporation will not take place when the air is saturated with water vapor.

The double pipe condenser has displaced the atmospheric condenser in many places because it is independent of atmospheric conditions and therefore more uniform in action. The most striking advantage of the double pipe system, however, is that the ammonia gas is brought into very intimate contact with the cold surface of the condensing coils. The result is that this type of condenser is very efficient in its operation, provided the cooling water is not too warm when it reaches the condenser. The flow of water through the coils may also be adjusted to any condition of temperature and pressure.

The proper use and regulation of power and cooling water

will do much to reduce operating costs, but unless provision is made to prevent the absorption of heat at other points than those areas which it is intended to cool, the gains made in the engine room may be lost in the cooling rooms because of poor or insufficient insulation.

The insulation is one of the most costly and important items in the construction of cooling and hardening rooms. The greater the difference between the temperature inside the cooling room and the external temperature, the greater will be the need of sufficient insulation of the best sort.

To be a good insulator a material must, of course, be a first-class non-conductor of heat. It should also be fire and moisture-proof and non-odorous.

The insulating materials commonly employed are mineral wool, hair-felt, mill shavings, cork and certain special cork preparations. Dead air spaces were originally used quite extensively and are still used to a certain extent in refrigerator construction as a means of insulating. It is so difficult, however, to build in such a way as to prevent a certain amount of air circulation in what should be the dead air spaces, that the tendency now is towards solid construction. A wall so built simply consists of layers of various materials, using mineral wool, saw-dust, or cork for insulating, and pitch, asphaltum, or water-proof paint to protect the insulation from moisture. The proper protection from moisture is very important. Many substances are good insulators when dry but take up moisture readily and become wasteful because of the fact that they are not good insulators when in a moist condition.

Cork is one of the best materials to use because it comes very near having all of the qualities of a good insulator. It is a first-class non-conductor, is not affected by moisture,

and although it is not fire-proof, it is a very slow burning substance.

The insulation of hardening rooms requires special attention. In the dry air hardening rooms a temperature of about zero is maintained constantly. It requires the best of insulation, properly protected from moisture, to maintain such a low temperature economically. Eight to ten inches

Fig. 59. Interior of a Dry Air Hardening Room.

of ground cork, cork board, or some similar insulator furnish the necessary insulation for the dry air hardening rooms. In some cases one or more dead air spaces are provided also. Water-proof paint, special kinds of paper, prepared roofing, and pitch are the substances frequently used to protect the insulation from dampness.

Slush boxes and brine tanks should be insulated with equal care and special precautions must be taken to protect the insulation from dampness. In all cases insulation will be found quite expensive, but economy of operation should not be sacrificed in order to reduce the cost of construction. What has already been said regarding the operation of a compressor for cooling rooms and other ice cream refrigeration, applies also to the manufacture of ice. Since many

Fig. 60. Freezing Tank, Can Filler, Hoist, and Dump in an ice cream factory.

factories, both large and small, aim to make all or a large part of the ice which they use, the subject of ice making is one of no little importance to ice cream manufacturers.

The ice making capacity of a compressor is rated at onehalf its refrigerating capacity. Theoretically, a one ton machine should make one ton of ice in 24 hours after the water has been cooled to 32° F., but in actual practice the ice making capacity of a machine is about half its re