AReCoS

Embedded controller · ammonia

Interstage pressure optimization for two-stage ammonia plants.

On two-stage ammonia, discharge temperatures are won or lost on a pressure set by the machine's actuators: the split between two stages. The AReCoS controller is designed to choose it from a physics model of the machine, on the equipment.

In the AReCoS cycle model, a split that takes 11.6 °C (20.9 °F) off the hottest point in the machine costs a sixth of a percent of efficiency.

  • U.S. Patent 12,768,557
  • A hardware test on a vapor-compression system (US 2024/0295337 A1, FIGS. 4 to 7)
  • Under test in a research agreement (CRADA) with UT-Battelle, LLC, which manages Oak Ridge National Laboratory for the U.S. Department of Energy
  • Patent and CRADA record
Request a technical evaluationDownload the ammonia brief PDF

Every circuit hides a pressure someone has to choose

In an ordinary system the weather, load and fans set the condenser pressure, and the optimum is wide and flat. Run CO2 above its critical point, or compress in two stages, and a pressure appears that someone has to choose.

Ammonia

Two-stage ammonia plants

One controller designed to run the whole ammonia plant, in 90 seconds: a design explainer, not footage of an installation.

Two-stage ammonia: where to split the compression, air and water in the plant, and the grid.

Ammonia

When one stage is not enough

Ammonia is already the efficient choice on industrial duty. From about −26 °C (−15 °F) down, plants usually compress it in two stages, which leaves one choice: where to split the compression.

A line chart with evaporating temperature along the bottom, from about 2 degrees Celsius down to minus 40, and compressor discharge temperature up the side from 55 to 315 degrees. Both scales are repeated in Fahrenheit, evaporating along the top and discharge down the right. Both curves are drawn for gas that loses no heat while it is compressed; on the two-stage curve the intercooler cools the gas to saturated vapour between the stages, and that cooling is what keeps it low. The one-stage curve starts at 125 degrees and climbs to 287 at minus 40. The two-stage curve rises only from 70 to 128. A dashed horizontal line at 88 degrees Celsius, 190 Fahrenheit, runs the full width, labelled as the temperature a one-stage oil-injected screw typically stays below. A shaded vertical band between minus 16 and minus 22 degrees marks where the pressure ratio reaches 7 to 9, the usual limit for a piston compressor. A dashed vertical line at minus 26 degrees Celsius, minus 15 Fahrenheit, marks the handbook's guidance for where plants commonly go to two or more stages; against this 40 degree condenser a piston machine reaches its ratio limit sooner, inside the shaded band.
Compressor discharge temperature against evaporating temperature, condensing at 40 °C (104 °F): one stage against two at the coolest split, both assuming no heat leaves the gas while it is compressed.

Ammonia

Nearly twelve degrees off the compressor, for a sixth of a percent, in the model

In the AReCoS model, moving the interstage pressure from the most efficient split to the coolest takes 11.6 °C (20.9 °F) off the hottest point in the machine. Efficiency barely notices: at the duty in the figure below, a sixth of one percent. A lower modeled discharge temperature may reduce thermal stress and the risk of a high-temperature trip; any effect on oil life and maintenance needs plant-level validation.

AReCoS is designed to take as many of those degrees as the plant's loads allow, and to find the coolest split without hunting for it.

Two stacked line charts sharing one horizontal axis, the pressure between the two compression stages in bar absolute, from 2.2 to 10. The upper chart is cooling delivered per unit of power. Its curve is very flat, peaking at 4.3 bar and barely lower at 5.1 bar. Its heading reads: efficiency barely notices where the split goes, the coolest one costs 0.17 percent. The lower chart is the hottest point in the machine in degrees Celsius, repeated in Fahrenheit down its right-hand side. Its curve is a sharp V, falling from 177 at 2.2 bar to a minimum of 111 at 5.1 bar and climbing again to 186 at 10 bar. Its heading reads: the compressor notices a great deal. A filled dot on each chart marks the most efficient split and an open ring marks the coolest; a vertical line through both charts marks the coolest split. A label notes that the textbook rule of thumb lands at the efficient split too, 12 degrees Celsius, 22 Fahrenheit, hotter than it needed to be, and an arrow at the minimum reads 11.6 degrees Celsius, 20.9 Fahrenheit, cooler. Both of those are temperature differences, so they scale without the freezing-point offset.
One two-stage ammonia machine with an open flash intercooler, fed liquid that leaves the condenser 5 °C (9 °F) subcooled, evaporating at −30 °C (−22 °F) against a 40 °C (104 °F) condenser, with no heat leaving the gas while it is compressed. Above, what the split costs in cooling per unit of power; below, what it does to the hottest point in the machine.

Ammonia

Air in, water in, charge out

Below about −33 °C (−28 °F) ammonia boils below atmospheric pressure, so a cold plant can draw in air, and moisture with it. Air holds the condensing pressure up; water makes the coldest vessel's liquid boil warmer. Where a plant reports each vessel's pressure and liquid temperature, the controller is designed to read both faults from them, to tell either from a drifting sensor, and to count the charge.

Ammonia

When the grid asks

On two-stage ammonia, the frozen product is the store, and the controller is designed to follow the coolest split while load is cut.

One controller: efficiency on CO2, cooler compressors on ammonia

On carbon dioxide the decision buys efficiency, and it moves with the weather. On ammonia it buys cooler compressors; what that is worth in reliability is for plant tests to show. Same controller, same kind of decision, different payoff.

The same controller on transcritical CO2

Ready for evaluation on your machine

The controller is available for evaluation today, fitted to your ammonia plant, and licensed per unit.

The controller is designed to make this decision on two-stage ammonia, as it does on HFC and HFO systems.

In the evaluation, AReCoS builds your machine's model from its components, fits it to your plant and checks it against your own test data.

The cycle work behind this page is the AReCoS team's own.

Let's talk.

Tell us what you run: a refrigeration system or a machine you build.

Email info@arecos.net Download the ammonia brief PDF
625 Kenmoor Ave SE, Suite 350
Grand Rapids, Michigan 49546