AReCoS

Embedded controller

Pressure optimization for CO2 and ammonia refrigeration.

On transcritical CO2 and two-stage ammonia, efficiency and discharge temperatures are won or lost on pressures that interact with the machine's actuators: the gas cooler's, the receiver's, the split between two stages. The AReCoS controller is designed to choose them together, from a physics model of the machine, on the equipment.

In the AReCoS cycle model, on a 35 °C (95 °F) day, running ten bar below the best CO2 pressure costs about 26% of the cooling per unit of power. That is the cost of the wrong pressure, not a saving against a well-tuned controller. On two-stage ammonia, the same model finds a split that takes 11.6 °C (20.9 °F) off the hottest point in the machine, for a sixth of a percent. Neither has yet run on a machine.

Email info@arecos.netDownload the CO2 brief PDFDownload the ammonia brief PDFHow an evaluation works

Jump to: CO2 racks · ammonia plants

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.

Carbon dioxide

Supermarket CO2 racks

One controller designed to run the whole CO2 plant, in under two minutes: a design explainer, not footage of an installation.
Download the CO2 brief PDF

Transcritical CO2 racks: the gas cooler pressure, the three valves, heat reclaim, a leak, and the grid.

Carbon dioxide

The high side stops being automatic

Carbon dioxide above about 31 °C (88 °F) does not condense at all. Heat leaves through a gas cooler, the pressure there comes loose from the temperature, and it becomes a decision the machine has to make again every time the weather moves.

A pressure-enthalpy chart of a transcritical carbon dioxide refrigeration circuit. A shaded dome marks where the refrigerant is part liquid and part gas, ending at a marked point labeled: above 31 degrees Celsius, 88 Fahrenheit, carbon dioxide no longer condenses. The circuit is drawn as a loop around and above that dome. The compressor raises pressure to about 94 bar, well above the dome. The gas cooler runs across the top at constant pressure. That pressure is above the top of the dome, so the refrigerant never condenses on this leg. A high-pressure valve then drops the refrigerant to a receiver at 45 bar, where about two fifths of it flashes to gas. A gas bypass valve returns that flash gas to the compressor inlet. A separate evaporator valve feeds the remaining liquid to the evaporator at about 37 bar, which runs across the bottom. Three labels with arrows name the high-pressure valve, the gas bypass valve and the evaporator valve.
Where the pressure goes in a transcritical CO2 circuit, and which valve sets what, for a machine evaporating at 2 °C (36 °F) with 35 °C (95 °F) outdoor air.

Drawn with the AReCoS refrigerant cycle tools, with the gas leaving the cooler 2 °C (3.6 °F) above the outdoor air, a flash tank at 45 bar and no suction-line heat exchanger. The next figure is the same machine.

Carbon dioxide

And the decision is lopsided

For this machine the best pressure sits near 94 bar on a 35 °C (95 °F) day and about 125 bar on a 45 °C (113 °F) one. Ten bar above it costs under four percent of the cooling per unit of power; ten bar below, about twenty-six. Running low feels safer for the hardware, but it is the expensive side.

A line chart with gas cooler pressure in bar absolute along the bottom and cooling delivered per unit of power up the side. Two curves. The 35 degree Celsius, 95 Fahrenheit, outdoor air curve climbs steeply from 74 bar, peaks near 94 bar, and then falls away slowly. The 45 degree Celsius, 113 Fahrenheit, outdoor air curve starts from zero at about 83 bar, below which the receiver holds no liquid, is lower throughout, and peaks much later and far more broadly, at 125 bar. A filled dot marks each peak. Two open rings on the warm-day curve sit ten bar either side of its peak and show what missing it costs. Ten bar below loses about twenty-six percent of the cooling per unit of power. Ten bar above loses under four percent. A shaded band covers the ten bar below the peak. On the warm day, the fall below the peak is far steeper than the fall above it.
Cooling delivered per unit of power against gas cooler pressure, on a warm day and a hot one, for the machine above.

On a CO2 machine that is also making hot water there is no single best value: the same pressure trades cooling against heat delivered, and the right answer depends on what the building wants at that moment.

Carbon dioxide

Three valves, three different jobs

A transcritical circuit with a flash tank has three interacting valve functions. The high-pressure valve after the gas cooler sets the gas cooler pressure. The flash-gas bypass valve returns the receiver's flash gas to the compressors. Only the expansion valve at each evaporator feeds it.

Each moves the other two. In many installations each holds its own target, correcting after the fact what the others just did; this controller is designed to set all three together from one model of the machine.

Carbon dioxide

A hot afternoon, or a leak?

On a CO2 booster the receiver level moves with weather, load, defrost and heat reclaim, and a fixed level alarm cannot tell that from a leak. The AReCoS controller is designed to predict the level from a model of the gas cooler, valves and cases, and to read what the prediction misses as charge lost, well before the receiver runs low.

The gas is cheap. A receiver that runs dry is not: flash gas reaches the case valves, and the cases go warm.

Carbon dioxide

When the grid asks

On CO2, cutting load for the grid is designed to come from compressor capacity and a floated suction, never from pushing the high side below its best pressure, where it loses cooling far faster than it saves power.

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.
Download the ammonia brief PDF

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 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.

Ready for evaluation on your machine

The controller has been tested on hardware only on a vapor-compression system in cooling, not yet on CO2 or ammonia. So the first step is an evaluation on your equipment, and the controller is planned to be licensed per unit.

Being developed first for HFC and HFO systems, the controller is designed to extend to this decision on transcritical CO2 and two-stage ammonia.

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, and a script kept with the site redraws each of its four figures exactly.

Let's talk.

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

Email info@arecos.net Download the CO2 brief PDFDownload the ammonia brief PDF
625 Kenmoor Ave SE, Suite 350
Grand Rapids, Michigan 49546