In the expendable refrigerant s ray system, liquid nitrogen or carbon dioxide is sprayed directly into the vehicle space that is to be cooled. This system. uses liquid containers, a control box, a fill box, spray headers. emergency switches. and safety vents.
The fill box is normally located on the front of the vehicle. It contains the valves, gauges, and connections that allow the liquid containers to be filled. The liquid containers are insulated cylinders similar to thermos bottles. The control box contains the valves, gauges, and thermostats that are necessary for safe release of the liquid to the spray headers. Once the liquid is received at the spray headers, the nozzles spray it into the vehicle. The remaining two components are primarily safety devices. These emergency interlock switches are attached to each door. Thus, whenever a door is opened, the system shuts down. The safety vent is a small trapdoor that vents air directly to the atmosphere whenever the air inside the truck box exceeds atmospheric pressure.
A benefit of this system is that liquid nitrogen or carbon dioxide replaces the oxygen inside the space being refrigerated. Therefore when fruits, vegetables, meats, and fish are being refrigerated, they are also preserved by the inert atmosphere.
A vehicle equipped with this type of system must display the following safety sign:
CAUTION: THE TEMPERATURE OF LIQUID NITROGEN, AS IT COMES FROM THE SPRAY NOZZLES, IS BELOW (0F.
Liquid nitrogen will instantly freeze any part of the human body that it touches. Since liquid nitrogen can be dangerous, you should always inspect the refrigerated space before closing the doors. An expendable spray system for a refrigeration truck is shown in Figure 14-6. In this system, the liquid nitrogen is in an insulated container that is installed vertically inside the truck body. Another similar type of spray system with the refrigerant container mounted horizontal under the truck body is shown in figure 14-7.
This type of system is used to move heat from one area to another by use of electrical energy. The electrical energy, rather than the refrigerant, serves as a "earner." The primary use of thermoelectric systems has been in portable refrigerators, water coolers, cooling of scientific apparatus used in space exploration, and in aircraft. The main advantage of this system is there are no moving parts. The system is compact, quiet, and requires little service.
Multistage refrigeration systems are used where ultralow temperatures are required but cannot be obtained economical] through the use of a single-stage system. The reason for this is the compression ratios are too high to attain the temperatures required to evaporate and condense the vapor. There are two general types of -systems presently in use-cascade and multistage (compound.)
The cascade system has two separate refrigerant systems interconnected in such a way that the evaporator from the first unit cools the condenser of the second unit. This allows one of the units to be operated at a lower temperature and pessure than would otherwise be possible with the same type and size of single-stage system. It also allows two different refrigerants to be used, and it can produce temperatures as low as -250F. Refer to figure 14-8. In this typical cascade system,
Figure 14-6. - An expandable spray SystemContinue Reading