Datum temperature

Prior to the now almost universal adoption of the SI system of units, the unit of heat was defined as the quantity of heat required to raise the temperature of unit mass of water by one degree. This heat quantity is designated the calorie in the cgs system and the kilocalorie in the mks system, and in both cases temperature is expressed in degrees Celsius (Centigrade). As the specific heat capacity is a function of temperature, it has been necessary to set a datum temperature which is chosen as 298 K or 25°C. 

Assuming that the temperature of the walls remains constant at the datum temperature and that the temperature at any distance y from the walls is given by a polynomial, then ... 

The flow of fluid over a plane surface, heated at distances greater than. to from the leading edge, is now considered. As shown in Figure 11.11 the velocity boundary layer starts at the leading edge and the thermal boundary layer at a distance o from it. If the temperature of the heated portion of the plate remains constant, this may be taken as the datum temperature. It is assumed that the temperature at a distance y from the surface may be represented by a polynomial of the form ... 

X Latent heat of vaporisation per unit mass, at datum temperature J/kg i. r- ... 

If a phase transition takes place between the specified and datum temperatures, the latent heat of the phase transition is added to the sensible-heat change calculated by equation 3.11. The sensible-heat calculation is then split into two parts ... 

Estimate the specific enthalpy of ethyl alcohol at 1 bar and 200° C, taking the datum temperature as 0°C. 

For practical reactors, where the reactants and products may well be at temperatures different from the reaction temperature, it is best to carry out the heat balance over the actual reactor using the standard temperature (25°C) as the datum temperature the standard heat of reaction can then be used without correction. 

This program can be used to calculate the heat input or cooling required for a process unit, where the stream enthalpies relative to the datum temperature can be calculated from the specific heat capacities of the components (equation 3.11). 

The datum temperature in the program is 25°C (298 K), which is standard for most heat of reaction data. Specific heats are represented by a cubic equation in temperature ... 

Heat of reaction and latent heat, taken to be values at datum temperature of 298 K. There is no need to convert to kJ/kmol, providing quantities are expressed in kg. For the purposes of this example the dissociation of C02 and H20 at 1500 K is ignored. 

It is good practice to show on the flow-sheet the basis used for the flow-sheet calculations. This would include the operating hours per year the reaction and physical yields and the datum temperature used for energy balances. It is also helpful to include a list of the principal assumptions used in the calculations. This alerts the user to any limitations that may have to be placed on the flow-sheet information. 

Energy balance around mixing tee, taking as the datum temperature the inlet temperature to the oxidiser, f3. 

So that the magnitude of each source can be compared, each will be calculated separately. Take the datum temperature as 25°C. 

The steam is considered to condense at the inlet temperature and the condensate then cooled to the datum temperature. 

If the temperature of the inlet stream is taken as the datum temperature for enthalpy and internal energy calculations,... 

This example involves a heat balance over the system. 273 K will be chosen as the datum temperature, and it will be assumed that the flowrate of dry air = G kg/s. 

The problem now consists of an enthalpy balance around the unit, and for this purpose a datum temperature of 294 K will be chosen. It will be assumed that the flow of dry air into the unit is m kg/s. 

Assuming a flowrate of G kg/s dry air, then a heat balance may be made based on a datum temperature of 294 K. 

Related Calculations. The problem can be worked in similar fashion using values from enthalpy tables. In this case, the datum temperature is below the flash-zone temperature therefore, sensible heat in the two exiting streams must be taken into account. 

Here Cio, Cn, etc. are coefficients chosen to give a good fit of C° over the temperature range of interest. The standard state chosen for species i determines whether the heat capacity at constant pressure or at constant volume is required. The corresponding expansions for the standard-state energy functions around a datum temperature To are... 

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