MILK-PLANT- EQUIPMENT. 19 
ammonia in the remaining sections. Where the regenerative system 
of pasteurization is used the cool milk is often allowed to pass 
through the inside of the first sections of the cooler. With the 
open-surface cooler the milk is exposed to the air in the room, and 
for that reason the air should be kept pure. It is important that 
the cooler be of sufficient capacity to cool the required quantity of 
milk. When the cooler is not large enough there is a tendency for 
the operator to allow the milk to go over the cooler too fast for 
proper cooling. 
The internal-tubular coolers are constructed on the same prin- 
ciple as the internal-tube heaters; that is, the milk passes through 
a pipe which is itself inclosed in a pipe or jacket containing the cool- 
ing medium. The length of tubing required in cooling coils is about 
double that of heating coils. With this type of cooler the milk does 
not come into contact with the air at any time. A claim made for 
the open cooler is that the milk is aerated, while with the internal- 
tube cooler bad odors have no chance to escape. On a commercial 
scale, however, little trouble has been found from this source. There 
is no loss of milk from evaporation with the internal-tube cooler, 
while there may be considerable loss from this source with the open 
cooler, especially if the cooler is in a draft. 
If the open cooler is covered, contamination from the air will be 
reduced ; however, it is more desirable to have pure air in the room 
where the cooler is located, as it requires considerable care to keep 
these covers clean, and they are often clumsy to handle. If covers 
are used, the sterilization of the cooler is made easier. 
It is much easier to sterilize the internal-tube cooler than the open- 
surface cooler. If steam is introduced into the internal cooler and 
the apparatus closed, it can be thoroughly sterilized. One way to 
sterilize the surface cooler is to admit steam to the inside of the 
tubes and at the same time run hot water over the outside of the 
cooler. The hot water will be vaporized and quite thorough sterili- 
zation will result. Another method which is perhaps more satisfac- 
tory is to close the covers tightly over the cooler and introduce live 
steam. 
In some forms of regenerative pasteurization the incoming milk 
will pass through the first few tubes of the cooler. This saves steam, 
but the tubes are difficult to clean. 
The open-surface coolers should be so constructed that water can 
be used in the top sections and brine or direct-expansion ammonia 
in the bottom sections. The capacity should be great enough to cool 
the milk as fast as it comes from the holder. The tubes should not 
be too long in comparison with their height. When the tubes are 
too long their whole length sometimes is not covered by the milk, and 
refrigeration is wasted. In some cases, however, it is necessary to 
