In conventional control the condenser fans hold the discharge pressure at the same value summer and winter, for example at 40 °C condensing. Yet when the outside air is at 10 °C the condenser can just as well condense at 20–25 °C. In floating condensing the set point is not fixed; it is calculated by adding a set difference to the outside air temperature, typically outside air + 8–12 K. As the air cools the target falls, the compressor discharges to a lower pressure and draws less power.
The gain can be calculated directly: every 1 K reduction in condensing temperature cuts compressor consumption by 2–3% and increases capacity a little. Bringing condensing down from 40 °C to 25 °C reduces consumption in those hours by about a third. Because nights and the winter months stay cool for long periods in the inland regions of Turkey, the annual saving reaches 10–20%. The fans running more brings some additional consumption; with EC fans this extra load stays small.
There is a lower limit to which condensing can fall. A thermostatic expansion valve passes refrigerant by virtue of the pressure difference across it; if the difference becomes too small the evaporator is starved. This is why, in systems with a mechanical valve, condensing is held at a minimum of 25–30 °C. Because an electronic expansion valve can work at a much lower pressure difference, the limit comes down to 15–20 °C. The operating envelope of the compressor, the efficiency of the oil separator and the pressure needed for hot gas defrost also determine the lower limit.
What is needed to apply it: an outside air sensor, a discharge pressure transmitter, speed-controlled condenser fans and a controller that runs this logic. In plants with heat recovery the two conflict; high condensing is wanted for hot water and low condensing for efficiency, and the priority is chosen according to the season. In existing systems it can mostly be applied simply by changing the setting and adding fan control. The capacities of the condensers in our catalog are given at different temperature differences.