13.28.      NA7.13.1 Compressed Air Systems


NA7.13 Compressed Air System Acceptance

Use Document NRCA-PRC-01-A

Purpose of the Test

The purpose of functionally testing the controls of a compressed air system is to confirm that the controls are set up in a compliant manner. A compliant system will choose the most efficient combination of compressors, given the current air demand as measured by a sensor, according to §120.6(e)2 of the Energy Standards. This test is designed for flexibility, as this covers both newer compressed air systems designed for use with controls and older compressed air systems under direction of controls for the first time.


Instrumentation to perform the test includes:

Power meter(s) for each compressor

Pressure transducer(s) for each compressor

Sensor or set of sensors to measure or infer current air demand, including but not limited to:

Flow meter

Set of pressure transducers

Pressure transducers and power meters

Test Conditions

Equipment installation is complete (including compressors, storage, controls, and piping).

Compressed air system must be ready for system operation, including completion of all start-up procedures per manufacturer’s recommendations.

For a new compressed air system, the trim compressor(s) must be identified prior to conducting the test.

Document the initial conditions before overrides or manipulation of the settings, if any. All systems must be returned to normal at the end of the test.

If using a valve to achieve a steady demand, ensure that this will not affect any equipment downstream.

Estimated Time to Complete

Construction Inspection: 1 to 1.5 hours (depending on complexity of the system)

Functional Testing: 1 to 3 hours (depending on familiarity with the controls and issues that arise during testing)

Acceptance Criteria

The states of each compressor will be observed throughout the duration of the test. By the end of the 10-minute duration, each compressor must not exhibit short-cycling or blow-off.

For new compressed air systems, the trim compressors are the only compressors that can be partially loaded. All base compressors must be either fully loaded or off by the end of the test.

Potential Issues and Cautions

For older systems, it may not be feasible to run at a steady demand for 10 minutes. In these cases, still observe the compressors to ensure that the controls are operating efficiently.

A.   Test Application

Newly Constructed and Additions/Alterations: All new compressed air systems, and all additions or alterations of compressed air systems, where the total combined online horsepower of the compressor(s) is 25 horsepower or more, must be tested.

The purpose of the installed controls is to choose the best combination of compressors for a given current demand. This test verifies that the installed controls have been set up to make these choices.

Ideally, the best combination of compressors keeps all base compressors either fully loaded or off with any given demand. The only compressors that should be partially-loaded are compressors that operate well partially-loaded, deemed as trim compressors.

This test is designed for flexibility, as this covers both older and newer compressed air systems. Older compressed air systems may be under direction of controls for the first time and may require compressors to be partially loaded.

Controls need to be able to determine real-time demand with a sensor (or calculate demand by a set of sensors). A flow sensor may do the task directly.

B.   Construction Inspection

Prior to the functional test, the system and compressor specifications must be documented. In addition, the method for determining the current air demand and the state of each of the compressors must also be documented. Having this documented will assist in determining if the controls are working properly. The following sections provide instructions on the data that must be verified during the Construction Inspection and included on the Acceptance Document.

a.    Compressor Specifications

Note the following data on the Acceptance Document. Most of this information can be identified from compressor specification sheets or the nameplate. This includes:

    Size (in rated horsepower)

    Rated Capacity (in actual cubic feet per minute)

    Control Type

o Fixed Speed

o Variable Speed

o Variable Displacement

o Inlet Modulation

o Centrifugal

o Other

    Designation as a Trim Compressor

If in doubt, contact the plant manager or controls designer, who should have this information readily available.

b. System Specifications

Note the online system capacity on the Acceptance Document. The online system capacity refers to the sum total capacity of all the compressors that will be in operation and connected to the control system. Once the compressor specifications are identified, taking the sum of every compressor’s rated capacity should yield the online system capacity.

Note the operating system pressure on the Acceptance Document. The operating system pressure should match up with the rated operating pressure of each of the compressors, also found in the specification sheets.

c.    Method for Determining Current Air Demand

Note the method for determining the current air demand on the Acceptance Document. There are a variety of ways to determine current air demand, which is the load required to safely run all downstream operating equipment. Since equipment operation is variable, the current air demand will also be variable. Tracking the real-time air demand is important to a well-functioning control system.

The controls designer should be aware of this method, as it is crucial to the operation of the controls.

It’s important to document the following in this explanation of the method:

    Sensors and tools being used to determine the current air demand.

    What each sensor is measuring.

    Calculations (if necessary) used to determine the current air demand (in acfm).

d.    Method for Determining the State of the Compressors

A compressor, at any given time, is operating in one of the following states:

    Off (0 percent of Rated Power)

    Unloaded (15-35 percent of Rated Power)

    Partially Loaded

    Fully Loaded (100 percent of Rated Power)

As with current air demand, there are a few ways you can determine the state of the compressor. All states, aside from the Partially Loaded state, can be easily determined with a power meter and the rated power of the compressor. For example, if a compressor is fully loaded, the power meter for this compressor should read near 100 percent of the rated power. If the compressor is unloaded, it will be approximately 15-35 percent of rated power. If the compressor is off, it should be at or near 0 kW of power.

Determining if a compressor is partially loaded would vary based on the compressor’s control scheme. A fixed speed compressor would cycle between loaded and unloaded (or off and on) if it were partially loaded.

Both variable speed drive and variable displacement compressors match power and air output somewhat linearly. As air output decreases, then power also decreases in direct proportion. Thus, operating between 35-99 percent rated power may qualify as partially loaded.

The best way to determine if a compressor is Partially Loaded is to install a flowmeter at the discharge of the compressor. If the acfm output is less than the rated acfm of the compressor, it is running Partially Loaded. When there is no flow, but the motor is still running, the compressor is Unloaded. When there is no flow and the motor is not running (the power reading is near 0 kW), the motor is considered OFF.

Note the method for determining the compressors’ states on the Acceptance Document.

In addition to these states, it is important that none of the compressors exhibit the following behavior:

    Short-cycling (loading and unloading more often than once per minute)

Short-cycling is easily measured with a stopwatch and a power meter or flowmeter. Simply observe if any compressors are cycling between the loaded and unloaded state. If so, measure the frequency by counting how many cycles are achieved over the 10 minute duration of the test. If it is more than 10 on-off cycles, then the compressor is short-cycling.

    Blowoff (venting compressed air at the compressor itself)

Blowoff is a state that will need to be observed rather than measured. This is sometimes used to limit flow delivered to a compressed air system, where the air is vented to the atmosphere. This is usually noisy and obvious, though compressors can be outfitted with silencers. For Centrifugal compressors, this is sometimes necessary to prevent surge (and compressor damage) when running at partial load. The reason for exhibiting blowoff at a particular compressor should be noted during the Functional Testing.

C.   Functional Testing

Step 1: Verify that the methods from the Construction Inspection have been employed by confirming the following:

    Compressor states can be observed and recorded for every compressor.

As documented in the Construction Inspection, ensure that the proper tools are installed and operational. Confirm that if external sensors are needed to determine the state of each compressor, they are calibrated. The power meter and flow meter should read levels that are at or below the rated power input and air capacity, respectively (as recorded in Document NRCA-PRC-01-A).

    The current air demand (in acfm) can be measured or inferred.

The easiest way to accomplish this is to install a flowmeter at the common header. This can be achieved by other methods, but this will need to be documented in the Notes section of Document NRCA-PRC-01-A.

Step 2: Run the compressed air supply system steadily at as close to the expected operational load range as can be practically implemented for at least 10 minutes.

Verify the following:

    System is running steadily for at least 10 minutes.

It is the intent to observe a system running normally and at steady state.

    System is running near to the expected operational load range.

Confirm that the controls are operating as expected. Running the system in the typical operational range is one way to accomplish this intent, though will require some communication with the plant manager to get an idea of this range. For example, does the system typically operate closer to 40-50 percent or 80-90 percent of the total online system capacity?

    Downstream equipment is not affected by a test valve being open, if applicable.

Running a system steadily may be difficult without a valve installed near a common header (in the distribution system upstream of the demand side of the system) that will release air to the atmosphere. If a test valve is not used, it’s recommended that the plant manager be contacted to determine a good time during the day when the system will be running steadily for a period longer than 10 minutes. For the case with a test valve, the pressure may drop below what is safe for some equipment. If there is equipment that must be running during the time of the test, take this into account when deciding how to perform the test.

If it is not possible to achieve a steady air demand for a 10 minute period of time, document the reason why and observe the state of the compressors during the 10 minute test. Observe any anomalies and document this in the Notes section.

Step 3: Observe and record the states of each compressor and the current air demand during the test.

Fill out the table for Step 3 in Document NRCA-PRC-01-A. If any state is difficult to determine, then document your specific observations and measurements in the Notes section.

Step 4: Confirm that the system exhibits the following behavior following the test:

    No compressor exhibits short-cycling.

If any compressor was cycling between loaded and unloaded during the test, and if the number of on-off cycles exceeds 10, this portion of the test fails. Circle N in Document NRCA-PRC-01-A.

    No compressor exhibits blowoff.

If any compressor is venting pressurized air to the atmosphere, this portion of the test fails. Circle N in Document NRCA-PRC-01-A

    The trim compressors are the only compressors partially loaded, while the base compressors will either be fully loaded or off by the end of the test. (only applicable for new systems)

This is a requirement for new systems because these systems are required to have properly sized trim compressors. If the new systems are designed properly, the controls should operate in a manner that has the trim compressors responsible for the trim load on top of fully loaded base compressors.

If any compressor is in the Partially Loaded state that is not a trim compressor, this portion of the test fails. Circle N in Document NRCA-PRC-01-A.

For an existing system, circle NA in NRCA-PRC-01-A.

Step 5: Return system to initial operating conditions.