The American Society of Mechanical Engineers (ASME) released their Wire-to-Air Performance Test Code for Blower Systems in 2018. Designated PTC 13-2018, it has rapidly become a standard for inclusion in blower specifications.
Several areas should receive special consideration when writing a specification that includes performance testing. These are areas where the code permits flexibility or where the performance parameters are open to interpretation. Unless they are specified clearly they can result in disagreements between the parties and failure of the test to provide the expected assurance that actual performance coincides with expected performance.
Intent of PTC 13
The intent of PTC 13 is identifying apparatus, procedures, data reporting requirements, and calculation methods needed to establish the total energy consumption of blower systems. The code addresses all configurations, types of blowers, and methods of capacity modulation. Bare blower and complete package systems are compatible with code tests. PTC 13 also provides the methodology to conduct a test and predict performance if inlet conditions at the test differ from specified conditions.
The code does not identify specification requirements. It does not restrict the system configuration to be tested. These items must be established by agreement between the parties to the test: the engineer, the owner, and the supplier. The surest path to success is to include the essential elements of the test in the documents used by the supplier for bidding.
The specification should identify the action to take if the test results are unsatisfactory. This can include financial penalties, rejection of the supplier’s equipment or allowing the supplier to modify equipment and retest at their own expense.
Number of Blowers to Test and Extent of Testing
Most new blower system purchases include more than one blower. The amount of testing is a question of economics and the engineer’s judgement. Ideally the full spectrum of performance testing would be conducted on all blowers in all systems. However, testing is expensive and time consuming, and therefore it influences the supplier’s pricing. It is hard to justify the same level of testing on a system with 20 hp blowers as one with 1,000 hp blowers.
Systems consisting of large blowers typically require testing every blower at all specified operating conditions. If the system is composed of small blowers it may be sufficient to test a single representative blower at worst-case conditions. In between these two extremes is testing a representative blower at multiple specified operating conditions and other units at a single worst-case design point.
Commonly specified testing includes:
- Testing to provide a full performance curve of power and discharge pressure based on maintaining the control method at a specific setting. The curve is usually based on the control setting needed to meet worst-case inlet conditions at 100% design capacity.
- Testing at a spectrum of flows, discharge pressures, and inlet conditions simulating the expected system duty cycle.
- Testing at worst-case inlet conditions and maximum design flow and pressure.
Special consideration must be given to packaged systems with multiple motor/blower modules in a single enclosure – commonly referred to as “dual core” packages. The test arrangement is influenced by the site piping design and operating strategy. It is common practice to treat the package as a single blower. The discharge pipes of each module are connected upstream of the customer piping. In that case, the specification should require the test configuration to join the discharge of all modules prior to any measurement locations. Check valves and other items required for parallel operation of modules should also be included inside the test boundary.
Components to Include in Test Envelope
In the past blower systems were often assembled in the field from components and accessories that were shipped loose. Testing a bare blower was normal. The most common blower system today, however, is the factory package - either skid-mounted or installed in a sound reduction enclosure. Packages often incorporate instrumentation, controls, and accessories.
Some package components, such as Variable Frequency Drives (VFDs), directly affect power consumption because of inherent inefficiencies. Others, such as silencers or check valves, indirectly increase blower power consumption by creating a pressure drop. Still other accessories, such as cooling fans, are direct power demands and increase total package power consumption.
The specifications should identify all the components that are to be included within the test boundary. Ideally, anything upstream of the point of connection between the blower system and the customer piping should be in the test boundary. The end of the test boundary should correspond to the point that the designer used to establish the specified discharge pressure. Components not included in the designer’s pressure drop calculations should be included in the test boundary.
It may be inconvenient to have some components shipped to the testing facility. In these cases PTC 13 allows the inclusion of a calculated value in the test results. This modification can accommodate mechanical or electrical components. Either a specific value or the method of establishing the calculated value should be identified in the specification.
The code provides guidance on components to consider in defining the test boundary. In PTC 13 Table 3-5.2-1 covers mechanical components and Table 3-5.2-2 covers electrical components. The project specifications should provide a similar listing of components for inclusion in the test.
A broad statement that “all components necessary for operation are to be included in the test” is insufficient. It can lead to multiple interpretations and disagreement between the parties. It is best if the specification explicitly identifies each component.
It is not feasible to exactly match test stand piping to the site configuration. Differences in pipe diameters are accommodated in the code calculation procedures. Other piping differences should have minimal impact on power demand.
Static vs. Total Pressure
Distinguishing between absolute and gauge pressure is standard practice in most specification formats. Typically absolute pressure is used to identify inlet and barometric pressure as psia, and gauge pressure is used to specify discharge pressure as psig.
Making a distinction between static pressure and total pressure is less common.
Static pressure is the measurement from a pressure gauge or transmitter screwed into a pipe wall with the measurement port at right angles to the direction of air flow. This is the pressure most operators and many designers consider in evaluating pressure. Total pressure is the measurement from a pitot tube with the measurement port pointed into the flow. It is the sum of the static pressure and the dynamic or velocity pressure. The dynamic pressure is a function of air velocity and density. (See Figure 1)
Because the total pressure represents the energy transfer from the blower to the air stream, PTC 13 uses it in the test procedure. Total pressure is composed of two parts: static pressure and dynamic pressure. Although total pressure may be measured directly, it is more common to directly measure only static pressure. The dynamic pressure is then calculated and added to the measured static pressure to obtain total pressure.
The dynamic pressure may be small compared to static pressure and is often ignored during design. Some designers include dynamic pressure in calculating the discharge pressure. Others consider only static pressure. Both methods are acceptable so long as the specification matches the designer’s intent. The specification should clearly identify whether specified pressures are static or total.
Temperature is also affected by air velocity. The total, or stagnation temperature, reflects the increase in temperature that occurs if the air stream is brought to rest. The code includes velocity effects on temperature to maintain theoretical correctness, but the influence on test results is often minimal. Specified temperatures normally reflect anticipated ambient conditions. Because ambient velocity is zero, the static and total temperatures are the same and it is unnecessary to distinguish total temperature from static.
Energy optimization is a consideration in purchasing blower systems. It is common to identify a financial penalty if test results exceed specified or guaranteed power consumption. The penalty is usually calculated as dollars/kW and deducted from the contractor’s or supplier’s payment.
The specification should clearly identify how the penalty is to be assessed:
- against each blower based on its individual test results
- against all blowers based on the average of all test results
- against all blowers based on the test of a single representative blower
Worst-case inlet and discharge conditions are infrequently encountered in normal operation. Although guaranteed operating points are often specified at worst-case conditions, specifying airflow as scfm at average site inlet conditions will provide a more accurate reflection of operating cost.
The discharge pressure for the power guarantee should be reflective of normal operation. The discharge pressure specified for individual duty cycle points may differ. It is common to use the worst-case design pressure for the entire load spectrum. However, if a lower pressure is anticipated for normal operation that should be used to better reflect actual power consumption. If a Most-Open-Valve or other mechanism will cause pressure to vary with flow then each duty cycle data point may have a different discharge pressure specified.
Nonmandatory Appendix F of the code offers suggestions for establishing test points.
If the specification includes power guarantees at one or more operating points, it is recommended that a financial power penalty be specified. Otherwise the owner derives little benefit from the guarantee. Without a penalty, if guaranteed performance is not verified by testing the only available recourse would be rejecting the equipment, resulting in controversy and project delays.
It is inevitable that the test performance deviates from the supplier’s standard data or projected performance. Most standard data sheets state that actual performance may vary ± some percentage from nominal performance. However, PTC 13 recommends that the test results, projected to specification conditions using the methods in the code, be used directly for comparison to specification values and power guarantees. Any allowance or tolerance would be applied by the supplier prior to providing performance values.
Pressure and flow should be specified at zero minus tolerance, and the values the test predicted at site conditions must equal or exceed the specified values. Power should be specified at zero plus tolerance, and the values the test predicted at site conditions must be less than or equal to the specified values.
There are several reasons for doing this. First, it simplifies comparing results to the specification, providing clear pass/fail criteria. Second, it negates differences in tolerances between suppliers. Third, it puts the risk of deviations on the supplier, who is in the best position to know and understand the potential variations.
Uncertainty is an indication of the confidence level for the test. It is not a tolerance. The code provides procedures for determining test uncertainty, but in practice this is usually waived. The code establishes minimum accuracy levels and calibration requirements for the test stand. These are generally sufficient to ensure the accuracy of results. The specification should explicitly identify whether pre-test or post-test uncertainty calculations are required. If uncertainty is to be calculated the acceptable level of uncertainty and the steps to be taken if it is not achieved should be identified.
Data Reported and Format
The code does not identify a specific reporting format. It does identify the minimum data to include in the report. Many manufacturers have standard reporting formats, and organizations such as CAGI have useful data sheets that can be used to supplement other formats.
Performance curves are often required in reports. They can provide useful information for projecting operating characteristics at different process demands. In some cases the manufacturer’s standard curve format is simple to understand, but in others extracting useful information is problematic.
Nonmandatory Appendix F of the code provides examples of curve formats that have proven useful.
Surge is a pulsating flow at the minimum capacity of a dynamic blower. Rise to surge is an indication of the control stability, particularly for variable speed applications. A higher rise to surge typically accompanies a greater speed range and better stability at reduced airflow. Rise to surge is identified as the difference between discharge pressure at design flow and the maximum pressure achievable at reduced flow without modulating the blower control mechanism.
Rise to surge is sometimes specified as applying under all operating conditions. This is subject to misinterpretation, and instead the minimum rise to surge should be specified as applying under all specified inlet conditions.
Efficiency is not used in the methodology PTC 13 used to establish wire-to-air performance. Efficiency may be a parameter of interest.
Nonmandatory appendix J identifies methods for determining efficiency.
Another parameter of interest is specific power, usually expressed as kW/100 cfm. It may be part of the reported data but is not typically an acceptance parameter.
Discharge air temperature is a concern in many systems. Discharge temperature increases at higher discharge pressure, potentially damaging piping, gaskets, and diffusers. Appendix J provides a calculation method for estimating discharge air temperature. Maximum discharge air temperature at worst-case operating conditions may be reported for information or as an acceptance criterion in critical applications.
The complexity of blower systems, the increasing use of packaged systems, and greater importance of energy use has generated the need for wire-to-air test codes. ASME’s PTC 13 answers this need, although other organizations have established or are establishing alternate codes. Regardless of which test code is specified, the designer should be aware of the common problems and necessary clarifications.
The designer’s goal in the test specification is to protect the interest of the owner while accommodating reasonable needs of suppliers. The more clearly items of concern are specified the less likely it is that contention will arise during equipment testing and acceptance.
About the Author
Tom Jenkins has over forty years’ experience in blowers and blower applications. As an inventor and entrepreneur, he has pioneered many innovations in aeration and blower control. He is an Adjunct Professor at the University of Wisconsin, Madison. Tom is the current Chair of the ASME PTC 13 Committee. For more information, visit www.jentechinc.com.
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