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Thursday, May 9, 2019

How do we determine the refrigeration or cooling capacity experimentally ?

Any refrigeration device's performance is measured in terms of its cooling capacity or refrigeration capacity in kW or Ton. There are many terms available to call capacity of refrigeration system such as cooling capacity, refrigeration capacity, refrigeration power or cooling power or refrigeration effect.


What is cooling capacity or refrigeration capacity or refrigeration power? 


Any refrigeration device removes the heat from low temperature body or space to high temperature body or space by using external power or work (or compressor electric input) as per the Clausius statement of second law of thermodynamics. The amount of heat (J/s) absorbed or removed from the low temperature body or space or object to be cooled (for example, freezer, inside part of domestic refrigerator) is called the refrigeration capacity or refrigeration power or refrigeration effect.

The cooling capacity is termed in J/s or W or in Ton capacity. 1 Ton = 3.5 kW that means a device of 1 Ton removes the 3.5 kW of energy or heat from the space to be cooled. 

If the refrigerator works on vapor compression refrigeration cycle, the heat Qa is absorbed at evaporator (low temperature space) by taking work input (Win) and rejects the heat of Qa + Win amount to the condenser via heat exchange with the air. 


How to obtain refrigeration effect experimentally? 


COP of a refrigeration device is the ratio of Refrigeration effect to work input. Thus in order to determine the COP experimentally, one needs to obtain refrigeration power experimentally. 
When a domestic refrigerator is turned ON, initially all the parts of the refrigerator are at room temperature i.e. 30 ℃. As the cycle continues or compressor runs, the heat is getting removed at the evaporator till the low temperature around -5 ℃ is reached. This temperature can be anything around 0 ℃ depending on the design and capacity of the refrigerator. It takes around 5 minutes to reach low temperature. Once the low temperature is reached, no further temperature reduction is possible because the system reaches at steady state condition. 

Now further heat load in the evaporator of refrigerator starts pumping the heat and temperature of evaporator rises depending on the load. Again the system reaches at steady state temperature if the load is steady. This steady load is called the refrigeration capacity at given steady state temperature. 

For example, a domestic refrigerator is started from room temperature at 09:00 am and the temperature reduction starts. At 09:05 am, the low temperature of -5 ℃ is reached. This condition is called steady state condition because, even if the compressor/refrigerator keep on running, the temperature will not drop. If a heating coil of 5 W capacity is started in evaporator, it will add 5 J of heating load every second. This extra heat load need to be removed and thus, compressor again starts pumping heat. Because of 5 W load, the new steady state temperature is somewhat higher than -5 ℃. Suppose it is 3 ℃. This 3 ℃ will not change if the load remains 5 W. If the load changes, the steady state temperature changes. Thus, 5 W of refrigeration capacity is obtained at 3℃. This is how cooling capacity or refrigeration power is obtained. 


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