Purpose (Intent) of Test
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). This is done directly with a flow sensor.
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 Form.
Compressor Specifications
Note the following data on the Acceptance Form. 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.
System Specifications
Note the online system capacity on the Acceptance Form. 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 Form. The operating system pressure should match up with the rated operating pressure of each of the compressors, also found in the specification sheets.
Method for Determining Current Air Demand
Note the method for determining the current air demand on the Acceptance Form. 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)
Method for Determining the State of the Compressors
A compressor, at any given time, is operating in one of the following states:
•Off (0% of Rated Power)
•Unloaded (15-35% of Rated Power)
•Partially Loaded
•Fully Loaded (100% 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% of the rated power. If the compressor is unloaded, it will be approximately 15-35% of rated power. If the compressor is off, it should be 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% 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. If there is no flow, but the motor is still running, the compressor is Unloaded. If there is no flow and the motor is not running (the power reading is near 0 kW), the motor is Off.
Note the method for determining the compressors’ states on the Acceptance Form.
'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)
•Blow off (venting compressed air at the compressor itself)
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.
Blow off 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 blow off at a particular compressor should be noted during the 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 Form 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 Form 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 a duration of 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% or 80-90% 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 Form 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 Form NRCA-PRC-01-A.
•No compressor exhibits blow off
•If any compressor is venting pressurized air to the atmosphere, this portion of the test fails. Circle N in Form 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 Form NRCA-PRC-01-A.
If this is not a new system, Circle NA in Form NRCA-PRC-01-A.
Step 5: Return system to initial operating conditions.