Explain with neat schematic Low temperature control in VCRcycle.

The Vapor Compression Refrigeration (VCR) cycle is a common method used in refrigeration systems to control temperature, especially at low temperatures. Here's a step-by-step explanation of how low temperature control works in a VCR cycle, along with a simple schematic:

1. Compression: The refrigerant, at low pressure and temperature, enters the compressor. Here, it is compressed, which increases its temperature and pressure.

2. Condensation: The high-pressure, high-temperature refrigerant then moves to the condenser. Here, it loses heat to the surroundings and condenses into a high-pressure liquid.

3. Expansion: The high-pressure liquid refrigerant then passes through an expansion valve or device. This rapidly drops its pressure, causing it to partially evaporate and significantly decrease in temperature.

4. Evaporation: The low-pressure, low-temperature refrigerant then enters the evaporator. Here, it absorbs heat from the space that needs to be cooled, causing the refrigerant to evaporate and return to the compressor, completing the cycle.

Schematic:

[Space to be cooled] <---- (Evaporator) <---- | ---- (Expansion device) ---- | ---- (Condenser) ---- | ---- (Compressor) ---- |
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In this cycle, the temperature is controlled by manipulating the pressure of the refrigerant. Lowering the pressure lowers the temperature, and vice versa. This is why the expansion device is crucial for achieving low temperatures.

In a typical refrigeration system, the desired temperature is set using a thermostat, which controls the compressor. When the temperature in the space to be cooled rises above the set point, the compressor is turned on, starting the VCR cycle. When the desired temperature is reached, the compressor is turned off, stopping the cycle. This maintains the temperature within a certain range.

For very low temperatures, a cascade refrigeration system may be used. This involves two or more VCR cycles in series, with each cycle using a different refrigerant suitable for a particular temperature range. This allows the system to reach much lower temperatures than a single VCR cycle.