MILK-PLANT EQUIPMENT. 
13 
the " kettle " or " Danish " heater, the " drum " or " film " heater, 
and the " internal tubular " heater. 
Kettle or Danish heater. — The kettle or Danish heater (fig. 6) 
consists of a kettlelike receptacle in -which the milk is heated by a 
water jacket surrounding it. Into this jacket steam is introduced 
in order to bring the contents to the desired temperature. The milk 
enters the inside of the apparatus at a point near the bottom and is 
forced out at the top by means of revolving paddles. These paddles 
may force the milk upward for 5 to 10 feet, and thus the machine 
acts as a pump. The heating surface is comparatively small, and it 
is therefore necessary to use a high temperature in the heating 
medium. For this reason it is very difficult to get a uniform tempera- 
ture for all portions of milk going through this heater. 
This tj^pe of heater is often used where milk is heated and not held 
for any length of time ; but it may also be used with nearly any type 
of holder. The system of heating milk to high temperatures for short 
periods is known as the " flash " process of pasteurization. This 
system is not desirable, but the heating units are satisfactory if used 
in connection with a holding device. 
The principal advantages of this type of heater are: 
1. Inexpensiveness. 
2. Small floor space required. 
3. It is economical and easy to operate, 
4. It is quite easily cleaned. 
5. It may act as a pump to elevate the milk to the holder. 
The principal disadvantage of this type of heater is the difficulty 
of maintaining a uniform temperature. The area of the heating 
Fig. 6. — Pasteurizer or the Danish or "kettle" type, with retarder. 
