A couple of reminders:1. Heat may be used on the various components of the system to help in the evacuation.2. When setting up for this type of evacuation, all material that enters the system MUST PASS THROUGH A DRYER of sufficient capacity to thoroughly dehydrate the material entering the system. A 5 cu. in. dryer would be large enough. Use it for one job and dispose of it.ADDING REFRIGERANTWhen charging a unit, follow the manufacturer's recommended charges and procedures. Normally the static charge method works well and is dependable, requiring no additional tools or equipment. The only caution here is to make certain that the unit and the ambient temperature are in the manufacturer's specified range. After unit is charged, start it and allow it to come to specified temperature. In some cases you may have to fine tune the charge by adding or purging small amounts of refrigerant. Refrain from this if at all possible. When you start this procedure one is "flying by the seat of his pants" and it is very easy to over- or under-charge the system and create more problems.The frostback on a system can be used as an indicator of the charge in most cases, normally about 4 to 6 inches from the compressor. But do not depend upon this indication wholly, as the temperature of the returning refrigerant gas is extremely cold.REFRIGERANT OILSome manufacturers supply their compressors and oil separators with oil of the correct amount and type for the unit. Always use the manufacturers' recommendations as to the type of oil to use in their equipment. The synthetic refrigeration oils normally have a lower floc point which is ideal for low temperatures.On most small systems it is impossible to check the oil after the system has been assembled and started. One must be sure that the oil is correct before installation. On larger units that have visual oil level indicators, a check should be made after a few hours of operation.CASCADE UNIT ASSEMBLYFigure 8 shows a two-stage unit assembly on the base for a 10 cu. ft. cabinet which might be used for metal treating (light duty) or biological storage at -120°F. It is air cooled and, except for this, the flow chart in Figure 2 could apply. A small water cooled condenser is incorporated in case of high ambient temperatures, and an extra fan is used to give more air circulation over both compressors. In this case the low stage hot gas desuperheater is a coil of tubing in front of the auxiliary fan. This is the base assembly ready for mounting in the cabinet.