Evaporation temperature curve

The family of short curves in Fig. 29-45 shows the power efficiency of conventional refrigeration systems. The curves for the latter are taken from the Engineering Data Book, Gas Processors Suppliers Association, Tulsa, Oklahoma. The data refer to the evaporator temperature as the point at which refrigeration is removed. If the refrigeration is used to cool a stream over a temperature interval, the efficiency is obviously somewhat less. The short curves in Fig. 29-45 are for several refrigeration-temperature intervals. A comparison of these curves with the expander curve shows that the refrigeration power requirement by expansion compares favorably with mechanical refrigeration below 360° R (—100° F). The expander efficiency is favored by lower temperature at which heat is to be removed. 

Example. A water-cooled unit with an evaporator temperature of-40°F will require 3 horsepower/ton of refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. Here are equations for these curves in the fonn ... 

Example 13.1 In the rating curves for an air-cooled condensing unit shown in Figure 13.3, what is the cooling capacity at an evaporating temperature of - 25°C and with air onto the condenser at 25°C By how much does this drop with condenser air at 35°C ... 

See the cooling curves in Figure 33.1. The evaporating temperature will now fall to about 0.2°C. Compressor manufacturers tables show 10.3% loss in duty for 1.5% less power at the new condition - an overall power increase of 9%. A more accurate estimate can be obtained by calculating a new basic rating for the reduced air flow. This shows about 8% extra power. 

There will also be a gain in usage of the evaporator coil and a corresponding rise in the evaporator temperature, giving a further increase in compressor capacity. This would need to be evaluated from the compressor curves, but might be a further 1%. 

The electronic interaction of the relatively large molecules of phthalocyanine shows (Fig. 30) a considerable temperature effect (77a). In an experiment demonstrating this effect, the platinum foil (B in Fig. 2) was covered by the dye molecules until the work function was lowered to 4.32 volts at room temperature. If B was cooled by pouring liquid air into the upper tube of the photocell (a in Fig. 30), the photoelectric sensitivity increased and remained constant as long as liquid air was added. If the liquid air evaporated (6 in Fig. 30), the photoemission dropped to the original value at room temperature. This effect was arbitrarily reproducible. The calculation of the work function 4> and the constant M by the curves of Fowler [see Equation (5) in section III,la] in Fig. 31 gives = 4,32 volts, log M = —12.17 at room temperature (curve I), and = 4.15 volts, log M = —12.17 at low temperature (curve II). While... 

Figure 15 shows the results of the cost calculations. The curves show minimum operating costs of 1.28, 1.31, and 1.40 per 1000 gallons for evaporating temperatures... 

The curves reproduced correspond to relative abundance of saccharide plus fatty acid ions (see formulas) of glycolipids lacking hexosamine as a function of evaporation temperature. A total of 200 pg was evaporated by a temperature rise of 5°C/min, and spectra were recorded each 38 sec. The electron energy was 34 eV, acceleration voltage 4 kV, trap current 500 pA, and ion source temperature 280°C. 

Fig. 2.27. Cooling capacity of both compres- capacity for both compressor types at different sors operating with R404A at a liquefying tern- evaporating temperatures, perature of 30 °C. The curves show the cooling...

The curves show the cooling efficiency for both compressor types at different evaporating temperatures. 

These calibration curves also reveal the huge increase of deposition rate with temperature, as expected from the Clausius-Clapeyron equation. For small flows the deposition rate increases linearly with source flow whereas at flows > 500 seem the increase is sub-linear. With higher flows the deposition rate is dominated by the flow restriction from the process chamber to the source container. If the evaporation temperature is increased from 306 to 312 °C the deposition rates increases approximately 47%, from 19.2 to 28.2 A s-1. 

Now, each droplet temperature curve is attracted by two corresponding air temperature curves, T(z) and TE(z). At top levels of the EPR, the droplet temperature exceeds both air temperatures, t(z) > T(z) and t(z) > TE(z). Therefore, both the heat and mass flows are directed from droplets to air, and both flows jE and jH are positive on these levels z as seen from Figs. 3.14, C and D. However, starting from a certain level labelled by small arrows, the droplet temperature finds itself lower that the air temperature but still tends to the formal temperature, t(z) < T(z) but t(z) > Te(z). This means that the evaporation still exists and cools droplets, but air began to warm them. Therefore, the possible minimum droplet temperature lies always between T(z) and Te(z). The last phenomenon is well known in meteorology. 

With respect to the adiabatic temperature increase observed, it has to be checked whether the final temperature truly is a consequence of the complete consumption of all educts, or whether it is an artificial final value because evaporation has prevented the temperature to rise any further. If all runs are started from different initial temperature levels, which is recommendable anyhow, then the dependency of the adiabatic induction time on temperature can directly be seen. For this purpose the measured temperature curve are differentiated once with respect to time and the resulting gradient profile is evaluated. For the above example this is shown in Figure 4-92. 

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