MILK-PLANT EQUIPMENT. 
15 
Some of the disadvantages are : 
1. Some of the heaters contain several parts and are difficult to clean and 
to keep in good running condition. 
2. They are not especially adapted to increase in capacity by adding 
additional units. 
Internal tubular heaters. — With the internal tubular type of heater 
(fig. 8) the milk passes through a tube inclosed within another tube or 
in a jacket containing the heating medium. In some of them the outer 
pipe or jacket is insulated to prevent loss of heat by radiation. The 
milk flows in an opposite direction to that of the heating medium, 
and in this way the heat units of the heating medium are economi- 
cally utilized and the milk is heated without any extreme fluctuations 
of temperature. The heating medium is hot water, which is heated 
by the introduction of steam before it enters the coil. As this heating 
water can be forced through rather rapidly and the area of the 
heating surface is comparatively large, the temperature of the heat- 
ing medium need not be much higher at any time than that to which 
the milk is to be heated. 
Some of the advantages of this type of heater are : 
1. Simplicity. 
2. Some types are comparatively inexpensive. 
3. Some types are adaptable to increase in capacity by adding additional 
units. 
4. Durability. There are few parts to get out of repair. 
5. Regenerative principle of heating can be utilized, permitting economy 
of heat and refrigeration. 
Fig. 7. — Continuous pasteurizer of the film or drum type. Clarifier on right. 
