58 
BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
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supply curve is based on the monthly percentage of the average 
supply. The temperature curve is the average of the mean 24-hour 
temperatures. Referring to the curve showing the variation in the 
supply of milk from month to month, it will be noted that there is 
practically a fixed relation between the temperature of the air and the 
supply of milk. The average of the milk supply, which is taken as 
100 per cent, is available during April and September, while the 
maximum occurs during June. The highest summer temperature 
occurs the latter part of July and the first of August, and the maxi- 
mum amount of work to be done by the refrigerating plant is during 
July. Therefore the condi- 
tions existing at this time 
should be taken as a basis for 
determining the size of ihe re- 
frigerating plant required. If 
the capacity of the refrigerat- 
ing plant is sufficient to han- 
dle the maximum load run- 
ning eight hours a day, it will 
handle the average load run- 
ning four hours a day. The 
time of running the machine 
will decrease from a maximum 
of eight hours during July until 
it can be shut down entirely 
in the Northern States during 
December, January, and Feb- 
ruary. In the South it will 
be necessary to operate the 
refrigerating plant to some 
extent during the entire year. 
The curve marked "compres- 
sor curve" shows the approx- 
imate daily hours the compres- 
sor will have to be operated to produce sufficient refrigeration to 
take care of the milk during the different months of the year. This 
curve, however, is based on the milk supply and weather conditions 
existing in the Northern States, where the dairying industry is prin- 
cipally located at the present time. In the Southern States the daily 
hours of operation will have to be increased. However, the flush 
period is not so marked as in the North, as the seasons are longer; 
consequently the refrigerating load is more uniform and the peak 
load is not so great. 
A simple and fairly accurate estimate on the size of refrigerating 
machine required to do the work of a given amount of ice may be 
made as follows: 
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Fig. 25.— B. T. U. transmitted per square foot per 24 
hours per degree difference in temperature. 
