Why the APR Control Improves Dehumidification

As cooling demand falls, fixed-capacity compressors cycle on and off more frequently. Moisture collected on the evaporator coil during the cooling cycle can then evaporate back into the supply air after the compressor shuts off, particularly when the supply fan continues running.

Published laboratory and field research cited in this paper has shown that this cycling behavior can reduce delivered latent capacity to less than 50% of steady-state performance under some part-load conditions.

The APR Control® addresses the problem by reducing effective compressor capacity as the cooling load falls. This allows the compressor to operate longer to satisfy the same space load.

The result is longer compressor runtime, fewer off cycles, and more time with the evaporator coil actively removing moisture.

Why Do DX Systems Struggle With Humidity at Part Load?

Constant-volume DX equipment typically matches cooling capacity to changing building loads by cycling fixed-speed compressors on and off.

The equipment is selected to satisfy peak design conditions, but those conditions represent only a small portion of annual operating hours. For much of the cooling season, the equipment has more available capacity than the building actually requires.

That excess capacity can create a familiar sequence:

Oversizing can make the problem worse, but the condition does not require an incorrectly sized system. Even properly selected equipment operates at part load during most off-design conditions.

The thermostat may be satisfied quickly while the building's moisture load remains.

How Does the APR Control Improve Dehumidification?

The APR Control is a mechanical, self-regulating, modulating external compressor unloader.

As cooling load falls and suction pressure approaches the APR Control's setpoint, the device reduces effective compressor capacity while allowing the compressor to continue operating.

For the same building load, reduced instantaneous capacity requires additional runtime.

That additional runtime is important because the evaporator coil removes moisture while it remains cold and below the dew point of the entering air.

Instead of repeatedly cycling between cooling and off periods, the system can operate longer at a capacity that more closely matches the actual load.

What Did Rawal Devices' Testing Show?

Rawal Devices evaluated an APR-410-3 installed on one compressor of a nominal 10-ton, two-compressor R-410A packaged rooftop unit in a psychrometric test chamber.

The testing provides evidence for three important parts of the dehumidification mechanism.

1. Latent Capacity Was Preserved When Full Capacity Was Needed

Testing showed no resolvable latent-capacity penalty when latent load was present and full capacity was demanded.

Across eight matched APR-enabled and APR-disabled test conditions, the differences were within or only marginally outside the established repeatability range of the dataset.

At face value, the largest full-stage difference was 3.3%. The paper notes this result rather than suppressing it, while also showing that the magnitude is much smaller than the greater-than-50% latent degradation documented in published research for cycling constant-volume equipment.

The measured conclusion is that when full capacity and moisture removal are required, the APR Control leaves latent capacity essentially unchanged.

2. Moisture Removal Continued During Modulation

The system continued removing moisture throughout the tested APR Control modulation conditions.

Across applicable valve-enabled test runs, measured latent capacity remained approximately 20,000 to 42,000 Btu/hr, depending on operating conditions.

Leaving-air dew point remained below entering-air dew point in every applicable modulation test.

At one representative condition, the system removed approximately 41,539 Btu/hr of latent heat while the APR Control maintained suction pressure near its setpoint.

Even at the deepest modulation conditions observed in the test program, the system continued removing approximately 20,000 to 21,000 Btu/hr of moisture.

3. Measured Capacity Reduction Corresponded to Longer Runtime

At deep part-load conditions, bench testing measured effective capacity reductions of up to:

• 18.7% when considering both compressor stages together
• 37.5% on the compressor stage controlled by the APR Control

For a fixed space load, those measured capacity reductions correspond mathematically to approximately:

• 1.23× the runtime for the combined system condition
• 1.60× the runtime on the controlled compressor stage

These runtime multipliers are calculated from measured steady-state capacity reductions. They were not measured through a thermostat-cycling test.

The valve modulates proportionally, so the amount of runtime extension depends on how far the cooling load has fallen and how much capacity reduction is required: up to approximately 1.6× the calculated runtime at the deepest controlled-stage modulation condition tested.

By reducing effective compressor capacity as load falls, the APR Control can allow the compressor to operate longer to satisfy the same space load.

Why Does Longer Compressor Runtime Matter for Dehumidification?

Published laboratory and field research identifies runtime fraction as a major driver of delivered latent capacity in cycling constant-volume systems.

When the compressor shuts off, moisture remaining on the evaporator coil can re-evaporate into the supply air.

Longer compressor cycles provide:

• More time with the evaporator coil cold and condensing moisture
• Fewer compressor shutdowns
• Fewer opportunities for retained condensate to evaporate back into the air
• Better utilization of the coil's available latent capacity

The APR Control's role is not to make the coil remove more moisture during every instantaneous moment of operation.

Instead, capacity modulation allows the system to remain in a moisture-removing operating condition for a greater portion of its operating time.

What About Evaporator Coil Freezing at Low Load?

Deep part-load conditions can also drive evaporator temperatures below freezing in an unmodulated system.

In the documented bench dataset, 10 of 154 APR-disabled runs fell below 32°F saturated suction temperature, with a minimum of 24.2°F.

By comparison, none of the 26 documented APR-enabled runs fell below freezing, with the lowest observed saturated suction temperature at 32.3°F.

An evaporator operating below freezing may initially show greater instantaneous latent capacity because the coil is colder, but that operating condition cannot necessarily be sustained. Frost accumulation can progressively restrict airflow and end moisture removal, or the equipment may cycle off on a low-pressure safety.

The APR Control instead maintained an above-freezing operating condition throughout the documented valve-enabled test runs.

Key Technical Takeaway

The APR Control improves dehumidification by helping constant-volume DX equipment operate longer at part load while continuing to remove moisture.

The technical argument is supported by four distinct evidence classes:

• Measured
  No resolvable latent-capacity penalty was observed when full capacity was demanded under a latent load.
• Measured
  Moisture removal continued throughout the tested modulation conditions while saturated suction temperature remained above freezing.
• Measured + Calculated
  Measured capacity reductions of up to 37.5% on the controlled compressor correspond to approximately 1.60× the calculated runtime for the same load.
• Published Research
  Longer runtime and fewer off cycles improve delivered latent performance by reducing opportunities for condensed moisture to re-evaporate into the supply air.

Together, these findings explain why matching compressor capacity more closely to the actual cooling load can improve humidity control in constant-volume DX systems.

Scope and Limitations

The findings summarized here apply specifically to the mechanical APR Control evaluated in this test program. They should not be generalized to the electronically controlled APR‑E.

• The measurements are steady-state bench results from a single R-410A rooftop unit with the APR Control installed on one of two compressors.
• The study did not conduct thermostat-cycling tests, annualized testing, or field-performance measurements.
• Latent capacity was derived from measured airflow, temperature, and total-capacity channels rather than logged directly.
• The approximately 1.23× and 1.60× runtime figures are mathematical consequences of measured capacity reductions, not directly measured cycle-duration results.

These limitations are important to interpreting the results correctly and are discussed in greater detail in the complete technical paper.

About the Research

This paper combines psychrometric chamber bench testing of an R-410A packaged rooftop unit with published laboratory and field research on part-load latent-capacity degradation.

The bench testing evaluates latent capacity, moisture removal during modulation, evaporator operating temperature, measured capacity reduction, and the corresponding calculated effect on compressor runtime.

Sources and Technical References

1. Suction Gas Modulation ResearchWang et al. (2012), Applied Thermal EngineeringView the research
2. Part-Load Latent Capacity ModelHenderson & Rengarajan (1996), ASHRAE TransactionsView the research
3. Part-Load Dehumidification Model ValidationHenderson (1998), Proceedings, ASHRAE IAQ & Energy 98
4. Dehumidification Performance at Part LoadShirey, Henderson & Raustad (2006), Florida Solar Energy Center / U.S. Department of EnergyView the research
5. Resolving Customer Complaints for High HumiditySantos (2020), RSES JournalView the article

Read the Full Whitepaper

The complete technical whitepaper provides the full methodology, test conditions, measured data, tables, calculations, limitations, technical discussion, and supporting references behind these findings.