Find the total heat energy
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4 kilograms of steam at 180°C is cooled to 50°C. The saturation temperature of steam is 143°C, and the heat of vaporization is 2114. Determine the formula for the total heat transferred.
(180-143)*cp+2114+(143-50)*cp
Using this and multiplying by the mass flow rate gives the desired Q, right?
This kind of calculation is acceptable when dealing with problems in textbooks, but it leads to significant errors in engineering calculations... The key issue lies in the enthalpy value of saturated steam; it’s very difficult to achieve saturated steam in actual engineering applications – either the steam is superheated or it contains a certain amount of moisture... 100% saturation is practically impossible, even under vacuum conditions. If the temperature of the steam drops to 50 degrees after heating, then the level of moisture will be quite high... In such cases, the error in the calculations is around 5–20%……
Hello, if it’s a engineering calculation, how can we minimize the error?
The enthalpy values for 143 degrees and 50 degrees need to be calculated separately……
It means looking up the corresponding enthalpy values in a table directly and then adding them together?
The corresponding enthalpy value is determined first, and then it is multiplied by a dryness coefficient – this coefficient relies on empirical values; it could be 97%, 90%, or even lower... It is called an empirical coefficient because it also depends on the operating conditions!