Miscellaneous factor. This includes such factors as exposure to direct sunlight and ambient temperature surrounding the system.
There are several charts and graphs available that depict the relationship between the factors listed above. To do the job right, you must take into consideration the total effect of the factors before selecting a particular refrigeration system.
There are certain applications where an electrically powered refrigeration system cannot be used. This requires knowledge of special applications and selection of a refrigeration system that will work effective . The absorption and the expendable refrigeration systems are discussed in this section.
An absorption system uses either water, ammonia, orlithiumbrom ideas the refrigerant. The system can range from very simple (small refrigerator) to complex (commercial freezer). This type of system is used in domestic and industrial refrigeration and air-conditioning applications. The absorption system is also used in recreational vehicles. Normally, these systems are identified by the type of heat source being used to power them, such as kerosene, natural gas, steam, or electricity. Because of the high pressure (400 psi), you should remember that welded steel tube construction must be used throughout the system. Also, because of the reaction between ammonia and copper or brass, you need a set of steel manifold gauges. Figure 14-4 shows an absorption.refrigeration cycle using ammonia as the refrigerant.
An expendable refrigeration system. is for used in trucks, railroad cars, and shipping containers that transport perishable items. The three types of refrigerants presently being used in an expendable system are liquid nitrogen, carbon dioxide, and liquid helium. The evaporator system and the spray system are two types of expendable systems commonly used in the Navy, and they are discussed in this section
In the expendable evaporator system, liquid refrigerant is stored in large metal insulated cylinders. These cylinders are normally located in the front of the cargo vehicle. Each cylinder is equipped with a temperature control to provide a temperature range of -20F to 60F.
The temperature control is connected to a temperature sensor. As the temperature rises, the switch operating the control valve opens and liquid refrigerant flows into the evaporator. The evaporator can be blower coils, plates, or eutectic plates. As it passes through the evaporator, the refrigerant vaporizes. This vapor is pushed through the evaporator by the pressure difference between the cylinder and the vent. When the selected temperature is attained, the refrigerant valve closes. The vapor that has been used is then discharged from the evaporator at about the same temperature as the air in the cargo vehicle. With this system, the refrigerant does not mix with air in the vehicle space. An example of the expendable evaporator system is shown in figure 14-5. This example shows two nitrogen cylinders located inside a truck body that are connected by a manifold to regulators and to temperature control solenoid valves. The vaporizing liquid nitrogen flows into the vaporizers or cold plates to refrigerate the true box.
Figure 14-5.-Expendable evaporator refrigeration system.Continue Reading