Approach temperature has a price
A smaller approach reduces the refrigerant lift and can improve COP, but it needs more heat-transfer area and may be more sensitive to fouling. A larger approach reduces exchanger size but makes the compressor work harder.
Count both sides
If the cold-side approach increases by 3 K and the hot-side approach also increases by 3 K, the refrigerant lift rises by roughly 6 K at the same process temperatures. The actual performance effect must come from the selected machine map.
Design for real streams
Wastewater, exhaust condensate and food-process streams can foul. Specify cleaning access, filtration, pressure drop, materials and degraded-condition performance. A clean-design COP is not an annual forecast when exchanger temperatures drift.
Sources
Primary and authoritative sources used for the material claims on this page.