=How Rated.=--The rating of refrigerating compressors as usually stated by manufacturers is expressed in tons. This refers to the tons of duty that a machine will develop in a period of twenty-four hours continuous operation under assumed conditions of about fifteen pounds gauge, back or suction pressure, and 185 pounds head pressure. To perform this duty the compressor should be of sufficient size to displace or pump a volume of gas equal to 4¹⁄₄ cubic feet per minute. This rating of the machine is proper when you are producing temperatures of about 32° F. or over, and presupposes the plant to be properly balanced as to condensers, and to be properly provided with liquid receivers, oil extractors and other complementary equipment. =Freezer-Requirement.=--The growing demand for freezer space in and about packing houses, however, is so important that special means and methods must be provided to meet the conditions. It is impossible to produce freezer temperatures and conditions under the same back pressure as described above; the back pressures must be lowered and in doing so the capacity of the compressor is reduced very rapidly. The same compressor producing one hundred tons refrigeration duty at fifteen pounds back pressure and 185 pounds head pressure will only perform half the work when operating under a back pressure of five pounds, the head pressure remaining the same. The purchaser must never lose sight of the fact that in cooling freezer spaces the compressor capacity is reduced practically by half and that this is applicable to all portions of the system working under these conditions. =System to Adopt.=--No hard and fast rule can be adopted regarding the system of refrigeration to be adopted, whether by use of brine circulation or direct expansion. The case in hand and the nature of the business to be done seems to govern. For example, if the plant in which the business is mixed, wherein hogs, cattle and sheep are killed, curing performed, and lard made, with a limited amount of freezing, brine circulation for general use would seem the better. The excellent results obtained by the use of spray coolers, later described, seem to point to its use in chilling coolers, with brine circulated through ceiling or wall coils for cooling storage rooms. =Freezers in Small Plants.=--If the plant be quite moderate in size and only sufficient to justify the installation of one compressor the amount of space required for freezing purposes is thus limited, and if commercial freezer space is available it is a question as to whether the operator can afford to deplete his small equipment capacity by installing freezers. [Illustration: FIG. 16.--SUCTION CONNECTION FOR DUO-PURPOSE COMPRESSOR.]