12 BULLETIN 890, U. S. DEPARTMENT OF AGRICULTURE. 
and the capacity of the outfit will be increased. Another advantage 
of precooling is that the records will show the exact time the milk 
was held at 145° F. if a recording thermometer is used. 
The covers of this type of heater should be kept closed ; otherwise 
there may be a considerable loss from evaporation and the milk may 
be contaminated. It is important with the vat system that the coils 
revolve at the proper speed. If they are allowed to run too fast the 
milk will be unduly agitated and injury to the cream line may result. 
The ordinary 300-gallon vat should be run at 45 revolutions a minute. 
Some of the advantages of vat heaters are : 
1. Simplicity. The apparatus is very simple. 
2. Easily operated. 
3. The vats are durable and comparatively inexpensive. One vat may 
serve for heating, holding, and cooling, though the last is not usually advis- 
able, as previously mentioned. 
4. For small plants the vats are economical, on account of the many uses 
to which they may be put. Besides serving as heater, holder, and cooler, 
the vat also may serve as a dump tank and storage tank for the milk. 
The milk may be placed in the vat when received and be held in the vat at 
the desired temperature until ready for pasteurization. Where a whole- 
sale business is conducted, the milk may be held in the vats at low tem- 
perature after it is pasteurized and cooled until the cans are filled and sent 
out to the wholesale trade. 
5. The time of holding can be positive if operated properly. 
6. During the holding process the milk comes into contact with no new 
surfaces between the completion of the heating process and the time the 
milk is cooled ; thus there is less chance for contamination and less appa- 
ratus to clean. 
7. The capacity of the plant may be readily increased by adding another 
unit. (A small plant may get along with one vat for a while, and as the 
business increases additional vats may be added and a continuous system 
established. ) 
Some of the disadvantages of the vat system are : 
1. Some operators have difficulty in obtaining a good cream line with 
this system. 
2. A comparatively long time is required to heat the milk. 
3. It is not automatic. The attendant must spend a considerable por- 
tion of his time operating the valves for filling and emptying the vats. 
4. With some types of vats the bearings or stuffing boxes of the coils, 
unless properly taken care of, may come into contact with the milk, and 
leaks may cause considerable trouble. 
5. All milk is not held at the pasteurizing temperature the same length 
of time. Some of the milk may be held more than 30 minutes, and thus 
the cream line may not be so clear. If the milk is precooled a few degrees 
in the vat before going to the cooler this disadvantage may be eliminated. 
CONTI XUOUS-FLOW HEATERS. 
The most common continuous-flow heaters may in general be 
classified into three types, which have sometimes been designated as 
