How the APR‑E Provides Externally Commanded Continuous Capacity Modulation

The APR‑E® provides another option: externally commanded continuous capacity modulation.

The APR‑E is an electronically controlled external compressor unloading device. It uses the same gas-diversion hardware and piping arrangement as the mechanical APR Control, but there is an important difference:

The APR‑E does not regulate itself based on suction pressure. Its valve position is controlled entirely by an external controller.

That allows the system to continuously adjust refrigeration capacity based on the objective selected by the controls strategy.

Field data and published research examined in this paper document APR‑E operation under two different controller architectures, ranging from basic cycling reduction on a large rooftop unit to BAS-controlled precision applications.

How Is the APR‑E Different From the Mechanical APR Control?

The mechanical APR Control is self-regulating. It senses suction pressure and mechanically modulates in response to changing load.

The APR‑E uses the same general diversion arrangement, including a desuperheating path that returns cool gas to compressor suction, but the APR‑E does not respond directly to suction pressure.

Instead, the valve opens and modulates according to an external command.

This gives the APR‑E two primary control approaches.

Standalone Controller

The standalone controller modulates the APR‑E against a fixed setpoint, typically discharge air temperature.

It is intended for applications where the primary objective is load matching and cycling reduction without requiring BAS integration or precision control.

The paper notes from field experience that the effective control point can drift approximately 3°F to 4°F from the nominal setting, so this architecture is not presented as a precision-control solution.

0-10 VDC / 4-20 mA BAS Control

The second architecture accepts an analog command from an external building automation or control system.

In this configuration, the BAS determines the modulation objective and commands valve position accordingly.

Depending on how the control sequence is programmed, the objective can include:

• Discharge air temperature
• Humidity
• Vapor pressure deficit
• Coil temperature limits
• Other variables the BAS can monitor and calculate

This architecture allows the APR‑E to become part of a more sophisticated control strategy rather than operating from a single fixed internal setpoint.

What Does the Field Evidence Show?

The paper evaluates three independent sources of evidence. Their results are reported separately and are not averaged or combined.

1. Compressor Stage Cycling Fell on a 150-Ton Rooftop Unit

A multi-tenant medical office building provided a field comparison between two identical nominal 150-ton packaged rooftop units.

One unit was retrofitted with a 20-ton APR‑E on its lead refrigeration circuit using the standalone controller. The second unit remained untreated and served as a comparison unit operating in the same building.

Because weather became warmer between the pre-installation and post-installation periods, the paper also compared performance within the same 65°F to 75°F outdoor-temperature range.

Within that matched temperature range:

• The APR‑E unit's stage cycling decreased 76%
• The untreated unit's stage cycling decreased 40%

These are measured results from 15-minute building-automation data.

The comparison does not establish that every APR‑E application will produce the same cycling reduction. It documents what occurred on this particular system and installation.

How Can BAS Control Change What the APR‑E Modulates For?

The second field application demonstrates a very different use of the APR‑E.

A controlled-environment agriculture facility uses APR‑E valves with the 0-10 VDC control board, commanded by the facility automation system.

The control objective is vapor pressure deficit, or VPD, rather than simply maintaining one fixed air temperature.

VPD depends on both temperature and relative humidity. As the facility's lighting changes, its control system changes temperature and humidity setpoints together to maintain a relatively consistent VPD target.

During the documented transition, setpoints changed from 76°F / 70% RH during the dark period to 82°F / 75% RH during the light period.

The VPD calculated from those two setpoint combinations was approximately 0.92 kPa and 0.93 kPa respectively.

This indicates that the automation strategy was changing temperature and humidity together while maintaining essentially the same VPD objective.

How Closely Did the Space Track the BAS-Controlled Target?

During the settled light period, the measured zone temperature tracked its active setpoint from approximately 0.2°F below to 1.6°F above, with an average deviation of approximately +0.5°F.

Relative humidity averaged approximately 0.6 percentage points below its active setpoint.

The resulting VPD calculated from the logged temperature and humidity averaged approximately 0.97 kPa, compared with an approximately 0.93 kPa setpoint-derived target.

These results demonstrate the programmable nature of the APR‑E control architecture in this particular application.

The VPD values are derived from measured temperature and humidity data. VPD itself was not directly logged in the data export.

What Did the ASHRAE Research Application Show?

The third source comes from ASHRAE Research Project RP-1604, which involved an automated cleanroom test laboratory.

The laboratory used four cleanrooms ranging from ISO Class 8 through Class 1. The research strategy continuously varied supply airflow based on particle counts, requiring cooling capacity to respond as airflow changed.

The condensing unit used in the application was a standard single-stage, non-modulating 3.5-ton unit.

The project team added an APR‑E controlled by the building management system using a tuned control algorithm.

According to the published project source, discharge air temperature remained within ±1°F of setpoint despite continuously changing airflow.

This application demonstrates the more tightly engineered end of the APR‑E's external-control capability: a fixed-capacity compressor used with externally commanded modulation to maintain precise discharge-air conditions.

Why Do These Findings Matter?

The three applications demonstrate that the APR‑E is not limited to one control objective.

With the standalone controller, the field data show a large reduction in compressor staging without requiring a highly tuned BAS sequence.

With BAS command, the modulation objective can instead be defined by the application.

That can allow a controls engineer to use continuous refrigeration capacity modulation as part of strategies involving variables such as discharge air temperature, humidity, VPD, or other calculated control objectives.

The important distinction is that the APR‑E itself does not decide what condition to maintain. The external controller does.

The APR‑E provides the refrigeration-capacity modulation that allows the control system to respond continuously rather than relying solely on fixed compressor stages.

Key Technical Takeaway

The APR‑E allows DX refrigeration capacity to be continuously modulated according to an external control command.

The paper documents both ends of its controller architecture:

• Measured field data
  On one 150-ton rooftop unit using the standalone APR‑E controller, average stage transitions decreased from 11.0 per day to 1.0, with a median of zero stage transitions after installation.
• Measured and derived field data
  In a controlled-environment agriculture application, BAS-commanded APR‑E operation was part of a control strategy that maintained an approximately constant VPD objective as temperature and humidity setpoints changed across a lighting transition.
• Published research
  In ASHRAE RP-1604, a BAS-controlled APR‑E installation maintained discharge air within ±1°F of setpoint on a single-stage compressor in a cleanroom research laboratory.

Together, these sources document the APR‑E's ability to support externally commanded continuous capacity modulation across both basic standalone and fully programmed control architectures.

Scope and Limitations

The evidence in this paper comes from three different sources, and the results should not be combined into one universal performance claim.

• The medical-office field study used two identical rooftop-unit models, but the two units experienced different baseline zone loads and cycling rates. Each unit's change was therefore evaluated primarily against its own baseline.
• The standalone APR‑E controller's actual control setpoint was not logged, so the medical-office study makes no discharge-air precision claim.
• The agriculture data represents a single-day export from one room and does not include an untreated comparison space or equipment-side refrigeration channels. It documents that the control objective was achieved, but does not determine how much of that performance should be attributed independently to the APR‑E versus the facility's overall control system.
• The paper also makes no energy or power-savings claim. The available field datasets did not include the instrumentation necessary to support one.

Finally, findings established in other papers for the mechanical APR Control are not automatically generalized to the APR‑E. Although the products share diversion hardware, their control methods are different and each requires its own supporting evidence.

About the Research

This paper evaluates the APR‑E using three independent evidence sources:

• Building-automation data from two nominal 150-ton rooftop units at a medical office building
• Control-system and facility trend data from a controlled-environment agriculture application
• Published information from ASHRAE Research Project RP-1604

The paper distinguishes measured results from derived calculations, engineering inference, field experience, reported information, and published sources.

Sources and Technical References

1. Suction Gas Modulation ResearchWang, Han, Shi & Li (2012), Modulation Method of Scroll Compressor Based on Suction Gas Bypass, Applied Thermal Engineering. Provides the published technical background for suction-gas modulation of DX compressor capacity.View the research
2. ASHRAE RP-1604 APR‑E Case StudyRawal Devices, Inc. (2020), ASHRAE Research Project (RP-1604) Succeeds with APR‑E Valve. Documents the APR‑E application in the cleanroom research project.View the case study
3. ASHRAE Research Project RP-1604ASHRAE (2018), Active Research Project Listing, RP-1604 Project Abstract. Provides the published research-project context for the cleanroom laboratory application.View the ASHRAE project listing

Read the Full Whitepaper

The complete technical whitepaper provides the full field-study methodology, data windows, exclusions, tables, control architectures, limitations, VPD analysis, published research discussion, and supporting references behind these findings.