186 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
(d) The internal or double-tube cooler (Fig. 21) 
is a system of double pipes, one within the other. The 
milk flows through the inner pipes and the cooling fluid 
through the outer. In this type of cooler the milk is 
protected from possible contamination from the air. To 
facilitate cleaning, the connections between the individual 
pipes are removable (detachable return bends). 
With a cooler, the temperature of milk can be low- 
ered to within a few degrees of the cooling fluid in a 
few minutes, provided the apparatus is not pushed be- 
yond its capacity. If the cooler is not large enough, 
the milk is likely to be permitted to flow over the cooling 
surface too rapidly for much of the heat to be absorbed 
by the cooling fluid. The size of the cooler required 
will depend on the quantity of milk to be cooled and 
the number of milkers. The capacity of coolers as stated 
by manufacturers is usually based on 20 square feet of 
cooling surface per 1000 pounds of milk. 
Milk should be cooled to as low a temperature as 
possible, and should be kept cool. The lower the tem- 
perature, the slower the bacterial growth and the longer 
the milk will keep in good condition (see page 43). 
Above 60° F. the bacteria multiply rapidly, and at 70° 
F. or above growth is not only very rapid, but the de- 
velopment of the more objectionable bacteria is favored. 
When well water or spring water is used for the cooling 
fluid, the temperature of the milk cannot be reduced 
much below 60° F. and often not that low. The tem- 
perature of well and spring water in the section around 
Philadelphia ranges from 52 to 55° F. in the spring 
and summer months, but in the late summer and early 
fall it is usually higher, especially in sandy regions, rising 
to 69 and 70° F. in some sections. With ice water, the 
