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In sterilization operations, the lower temperature vapor is widely applied. Most commonly, the higher temperature is obtained in parabolic trough collectors, which is more effective method also among other sources of solar collectors. There are three working theories established [21] in a PTCs to produce steam as follows: steam flash, direct, and unfired boiler.

Figure 1.8 Flash type [1].

Figure 1.9 Direct method [1].

In the first method, pressurized heated fluid is rushed in a separate vessel to obtain steam. The second method is an in situ; in this type, steam production is reached through the way of two phase flow type, and in the latter method, heat exchanger device helps to produce steam. In all these above working procedures, the heated water flows into the collectors.

Figure 1.8 shows the steam flash type where fluid is increased in the range of pressure to eliminate boiling, and then, it enters into collector and further moved into flash vessel. Fluid range is steadily maintained throughout the makeup water inlet.

Figure 1.9 denotes the direct type, and the change in this method compared with the former method is flash valve detached from the system. The feed water directly flows to the receiver end as heated water. Both these methods are quite lesser cost to installation [22].

Figure 1.10 describes the unfired boiler systems; this type of system is simpler than the previous described systems. However, pressure is quite less in this design.