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The evaporator approach temperature is the leaving brine temperature minus the refrigerant saturation temperature at the evaporator pressure.
The approach temperature is a measure of how well the heat exchange is happening vs. the design approach temperature.
Approach comes from Q = 1/k * area * (TD), Q is the overall heat transfer, 1/k, the transfer coefficient of the material, and TD, is the temperature difference across the tube, essentially the approach temperature.
For "off design" operation approach must be compensated for change in flow since, Q = Mass flow * Cp * TD, changes in flow change Q, since the transfer coefficient and area can't change in the above equation, TD must change.
Convert the evaporator pressure to a temperature for refrigerant type using a TP chart.
Subtract the found value above from the leaving brine temperature (Should not be negative.)
The remainder is the evaporator approach temperature.
Below is an example of how approach can change with flow:
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.