Calculate the enthalpy difference and specific heat for heat
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There is a problem in calculating the heat consumption of heat transfer oil in computing systems: since heat transfer oil is a liquid without any phase change, the enthalpy difference values from data tables used to calculate heat changes differ significantly from those calculated using the average specific heat. Which one should I choose? How to choose among similar calculations. When calculating steam, the change in heat can be determined directly using the enthalpy difference; can the same method be used here?
This post was last edited by arpcd on 2017-8-12 20:01. Careless poster. . . Even middle school students make mistakes in unit conversions that are supposed to be easy? ℉It’s 1.8 times different from °C(K); of course, multiplying the specific heat value in K by the temperature difference in °F won’t give the correct result. Just pick any two values and use a calculator to do the calculation; for example, 0°F and 20°F. Use the average specific heat value: Cp = (1.77 + 1.81)/2 = 1.79 kJ/kg·K. Note that the unit for temperature difference here is K, which is the same as °C. So the temperature difference is ΔT = (-7) – (-18) = 11°C = 11K. The amount of heat then is Q = Cp * ΔT = 1.79 * 11 = 19.69 kJ/kg. . . If you calculate using the enthalpy difference, then Q=ΔH=19.9–0=19.9 kJ/kg. What difference can there be between these two values? ? ? Your mistake was using the specific heat of K while plugging in the Fahrenheit temperature difference (which is 20°F in this case) into the formula; of course, this resulted in a large error. Moreover, this error was exactly 1.8 times larger, which is precisely the conversion factor between°F and °C (1.8). . . Americans find this table the most convenient to use. Why? ? Because they* use imperial units, and as I’ve said many times, European and American technicians* prefer to use kcal or the imperial unit BTU when calculating calories; what the original poster posted is a typical example of this. Still with that previous example, it looks much simpler to Americans. . . No one cares about metric units. . . For specific heat, the value is simply given as Btu/lb-℉ (the temperature difference is in Fahrenheit, so it’s 20℉ here). The value in terms of enthalpy is also Btu/lb; this table uses 0℉ as a reference point, and by subtracting 20℉ from that value, we get 8.6 Btu/lb. Calculating it using specific heat yields the same result: Q = (0.423 + 0.433)/2 * 20 = 8.56 Btu/lb. Isn’t that the same? ? Our students find it troublesome just to deal with things like Fahrenheit, British thermal units, and pounds; they’re not even familiar with units like cal. . . . So be careful when looking at American materials. . . Pay special attention to the units. .