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Understanding Transport Refrigeration Units

 
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Dołączył: 10 Sie 2019
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PostWysłany: Pią Kwi 24, 2020 09:15    Temat postu: Understanding Transport Refrigeration Units Odpowiedz z cytatem

We looked at some functional issues concerning the operation of transport refrigeration units and the handling of temperature sensitive loads. The objective is to eliminate spoilage and keep fresh.


The principles of physics that allow refrigeration to work


Heat is a form of energy.

Heat energy travels from warmth to cold.


Refrigerants are gases at room temperature and atmospheric pressure.


When compressed, refrigerant gases liquefy.
When pressure is released, the gases vaporize.
The gases are contained in a closed system, going through the complete cycle.
When a liquid expands to a gas, it absorbs heat, getting warmer itself but cooling the immediate surroundings.
When a gas is compressed to liquid state, it releases heat, thus getting cooler itself, but warming its immediate surroundings.



In a truck reefer unit, the compressor is driven by a small diesel engine. The compressor draws gaseous refrigerant in and compresses it. The pressure liquefies the gas. The now-liquid refrigerant gives off heat to the body of the compressor, and ultimately, to the air. It is still relatively warm, so it is pumped to the condenser. The condenser is a heat exchanger. Warmth flows from the liquid to the walls of the tubing, to fins on the tubing. The fins present more surface area to cooling outside air drawn through the condenser by a fan. This is similar to the way a radiator cools an engine.





The refrigerant, having given up much of its heat in the condenser, is now a cool liquid under pressure. It now flows through a metering valve into the evaporator. The evaporator is located in the trailer or compartment to be cooled. The metering valve controls the amount of refrigerant released into the evaporator, acting like a throttle to control the amount of cooling. It also helps maintain backpressure in the high-pressure part of the system, which runs from the compressor to the evaporator.





In the evaporator, the refrigerant rapidly expands, once again becoming a gas. As it does, it absorbs a great amount of heat from its surroundings. Those surroundings are finned coils, which help transfer heat from air flowing over the fins to the refrigerant. Air from inside the trailer is blown over the evaporator. The refrigerant gas, now under low pressure, is drawn back to the compressor where the cycle starts again. The trailer air, now cooled by giving up some of its heat to the evaporator, circulates back into the trailer to keep the cargo cool.





By the way, when you want heat in your truck to keep a load from freezing in winter, you operate the reefer in reverse. The evaporator becomes the condenser, giving off heat instead of absorbing it, while the condenser becomes the evaporator, absorbing heat from the outside air. Remember, at anything above absolute zero (459.69 F below zero, the temperature at which all molecular motion ceases) some amount of heat exists. It is, therefore, possible to extract heat from what we would normally consider extremely cold air. That’s how a reefer warms during a cold winter night.
You can read more information here:
How to Best Understand Truck Refrigeration? Its Usage in Our Daily Life

What’s the Classification of Refrigerated Vehicles Chiller Units
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