Enthalpy temperature change

The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1. 

The enthalpy change AH for a temperature change from to T2 can be obtained by integration of the constant pressure heat capacity... 

Alternative representations of stream temperature and energy have been proposed. Perhaps the best known is the heat-content diagram, which represents each stream as an area on a graph (3) where the vertical scale is temperature, and the horizontal is heat capacity times flow rate. Sometimes this latter quantity is called capacity rate. The stream area, ie, capacity rate times temperature change, represents the enthalpy change of the stream. 

If the amount of evaporation is small, the change in enthalpy in the Hquid phase can be taken as a result of temperature change alone. Using an average (av) hquid flow rate, the following is derived ... 

Example 4 Evaporative Cooling Air at 95 F dry-bulb temperature and 70 F wet-bulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated makeup water enters at 70 F. Determine exit dry-bulb temperature, wet-bulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air. 

The enthalpy of the adsorbed phase is evaluated along a path for which the gas-phase components undergo temperature change from Tref to T and then are adsorbed isothermaUy, giving... 

As a check on the assumptions made, a comparison can be made to a different method of checking the derivation of the head. Enthalpy difference, as a function of temperature change, for an adiabatic pro is... 

Ldmister, Wayne C. and McGarry, R, J., Gas Compressor Design, Isen-tropic Temperature and Enthalpy Changes, Chemical Engineering Progress, Vol. 45, No. 7, July, 1949, pp. 421-434. 

Stream Rowrate > specific heat Btu/hr °F Supply temperature, °F Target temperature, °F Enthalpy change lb Btu/hr... 

To calculate the heat duty it must be remembered that the pressure drop through the choke is instantaneous. That is, no heat is absorbed or lost, but there is a temperature change. This is an adiabatic expansion of the gas w ith no change in enthalpy. Flow through the coils is a constant pressure process, except for the small amount of pressure drop due to friction. Thus, the change in enthalpy of the gas is equal to the heat absorbed. 

Strictly speaking, AH° calculated from enthalpies of formation listed in Table 8.3 represents the enthalpy change at 25°C and 1 atm. Actually, AH is independent of pressure and varies relatively little with temperature, changing by perhaps 1 to 10 kj per 100°C. 

STRATEGY The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation as written. If the temperature rises, the... 

As described in Section 14-1. when AR and ZlS have the same sign, the spontaneous direction of a process depends on T. For a phase change, enthalpy dominates AG at low temperature, and the formation of the more constrained phase is spontaneous, hi contrast, entropy dominates AG at high temperature, and the formation of the less constrained phase is spontaneous. At one characteristic temperature, A G = 0, and the phase change proceeds in both directions at the same rate. The two phases coexist, and the system is in a state of d Tiamic equilibrium. 

In each stage calculation it will necessary to estimate the stage temperatures to determine the K values and liquid and vapour enthalpies. The temperature range from top to bottom of the column will be approximately 120 — 60 = 60°C. An approximate calculation (Example 11.7) has shown that around fourteen ideal stages will be needed so the temperature change from stage to stage can be expected to be around 4 to 5°C. 

To evaluate the true temperature difference (driving force) in a mixed vapour condenser a condensation curve (temperature vs. enthalpy diagram) must be calculated showing the change in vapour temperature versus heat transferred throughout the condenser, Figure 12.48. The temperature profile will depend on the liquid-flow pattern in the condenser. There are two limiting conditions of condensate-vapour flow ... 

The heat capacity cp is negligible compared to the phase change enthalpy (cp AT latent heat at the phase change temperature has to be considered. 

For non-pure materials, when the phase change temperature is not constant, the phase change enthalpy can be calculated using the following expression, which includes the temperature change (between Tmi and 7m2) during phase change ... 

Note particularly that the equation has no temperature change in it and that the enthalpy of fusion term has no unit of temperature in it. 

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