10.8 Compressed Air Systems (§120.6(e))

§120.6(e) applies to all new compressed air systems with a total installed compressor capacity of ≥25 hp. It also applies to existing compressed air systems that are being altered on the supply side (equipment upstream of the distribution system). For alternations there is an exception for systems that include one or more centrifugal compressors.

As described in the following paragraphs, there are 3 main requirements in this section:

10.8.2 Mandatory Measures §120.6(e)

This requirement targets the performance of a compressed air system across its full range. This requirement excludes alterations that are not making a large change to the system. A large change is defined as adding or replacing more than 50% of the online capacity.

• Using a compressor or set of compressors as the Trim Compressor (§120.6(e)1B)

Both of these paths aim to reduce the amount of cycling of fixed speed compressors by utilizing a better-suited compressor that operates well in part-load.

In order to avoid control gaps - portions of the compressed air system range with poor performance - it’s important to have a trim compressor sized to handle the gaps between base compressors. This minimum size is determined with the Largest Net Capacity Increment - the biggest step increase between combinations of base compressors.

With equally sized compressors this is fairly intuitive: in a system with two-100 hp (434 acfm) rotary screw compressor system, the largest step increase would be the size of one of the compressors (434 acfm). For systems with uneven compressor sizes, it requires going through the following steps:

a) Determine all combinations of base compressors (including all compressors off).

This largest difference is what must be covered by the trim compressor(s) in order to avoid a control gap.

Example 10-55

Question

Given a system with three base compressors with capacities of 200 acfm (Compressor A), 400 acfm (Compressor B) and 1,000 acfm (Compressor C), what is the Largest Net Capacity Increment?

Answer

As shown in the image below there are 8 possible stages of capacity ranging from 0 acfm with no compressors to 1,600 acfm with all three compressors operating. The largest net increment is between stage 4 with compressors A and B operating (200+400=600acfm) to stage 5 with compressor C operating (1,000 acfm)

For this system the Largest Net Capacity Increment is 1,000 acfm-600 acfm = 400 acfm

Once the Largest Net Capacity Increment is calculated, this value can be used to satisfy the first compliance option. Option one mandates that the rated capacity of the VSD compressor(s) be at least 1.25 times the largest net increment.

Example 10-56

Question

Using the system from the previous example, what is the minimum rated capacity of VSD compressor(s) that are needed to comply with Option 1?

Answer

As previously shown, the Largest Net Capacity Increment is 1,000 acfm-600 acfm = 400 acfm. The minimum rated capacity for VSD compressor(s) is 400 acfm X 1.25 = 500 acfm.

For compliance option 1, the system must include primary storage that has a minimum capacity of 1 gallon for every acfm of capacity of the largest trim compressor.

Example 10-57

Question

What is the required minimum primary storage capacity for the trim compressor from the previous example to comply with Option 1?

Answer

Assuming there is a VSD compressor with a rated capacity of 500 acfm, per §120.6(e)1A it must have 1 gallon of storage per acfm of rated capacity or 500*1 = 500 gallons of storage.

The second compliance option offers more flexibility but requires looking at both the Largest Net Capacity Increment of the system, as well as the Effective Trim Capacity of the trim compressor(s).

The Effective Trim Capacity is the range across which a trim compressor has adequate part-load performance. Performance is measured in power input over air volume output or specific power (kw/100acfm). Many VSD compressors come with a compressor performance graph in a CAGI data sheet that looks similar to the graph in Figure 10-40.Figure 10-40–Example Compressor Power vs. Capacity Curve

The capacity of the compressor is along the x-axis, while the power is on the y-axis. The curve in Figure 10-41 is a typical shape of a performance curve for a VSD compressor. The lower the specific power, the more energy efficient the compressor is at that condition.

The Effective Trim Capacity uses the minimum of the compressor power vs. capacity curve to determine the range of adequate part-load performance. This can be done in the following steps and is illustrated in the graph below.

c) Determine the endpoints of the capacity where the specific power is less than or equal to the upper bound.

Figure 10-41 –Determination of Effective Trim Capacity from a Compressor Curve

This definition of Effective Trim Capacity, along with the Largest Net Capacity Increment of the system, will be used to assist in sizing the trim compressor appropriately in the next section.

Example 10-58

Question

Continuing with the system from the previous examples, what is the required minimum Effective Trim Capacity of the trim compressor(s) to comply with Option 2?

Answer

As previously shown, the Largest Net Capacity Increment is 1,000 acfm-600 acfm = 400 acfm. Per §120.6(e)1 the minimum Effective Trim Capacity is equal to the Largest Net Capacity Increment or 400 acfm.

Example 10-59

Question

A manufacturer provided the following data for their compressor; would this provide the minimum Effective Trim Capacity for this system to comply with Option 2?

Answer

From the manufacturer’s data the minimum specific power is 18.2 kW/100 acfm. The upper limit would be 18.2 * 1.15 = 20.9 kW/100 acfm. Interpolating from the manufacturer’s data this appears to go from 155 acfm to 605 acfm for an Effective Trim Capacity of 605-155= 450 acfm. This is larger than the Largest Net Capacity Increment of 400 acfm so this compressor would comply as a trim compressor for this system.

For compliance option 2, the system must include primary storage that has a minimum capacity of 2 gallons for every acfm of capacity of the largest trim compressor.

Example 10-60

Question

What is the required minimum primary storage capacity for the trim compressor from the previous example to comply with Option 2?

Answer

This compressor has a rated capacity of 603 acfm, per §120.6(e)1B it must have 2 gallons of storage per acfm of rated capacity or 603*2 = 1,206 gallons of storage.

The last example used a VSD compressor, but other technologies can be used for compliance option 2. The next example examines a 250-hp load-unload, single stage, rotary screw compressor coupled with 10 gallons/cfm of storage. Generally, higher levels of storage improve part-load performance and this combination was chosen to meet the part-load performance mandated by code. This data was generated from theoretical curves used in AirMaster+, a tool created by the U.S. Department of Energy.

Example 10-61

Question

Part-load data was approximated below for a 250-hp load-unload, single stage, rotary screw compressor coupled with 10 gallons/cfm of storage; would this provide the minimum Effective Trim Capacity for this system to comply with Option 2?

Answer

Using the previous examples, a compressor with an effective trim capacity of at least 400 acfm is necessary.

Looking at the graph, the minimum specific power (labeled as A below) occurs at full load - a capacity of 1261 acfm, with a specific power of 17.4 kW/100acfm. Using this minimum specific power, the upper bound is 17.4 * 1.15 = 20.01 kW/100acfm or 15% higher than the minimum specific power. This puts the ends of the effective trim capacity at 1261 acfm (labeled as B) and 845 acfm (labeled as C), resulting in an effective trim capacity of 1261 – 845 = 416 acfm. This is larger than the Largest Net Capacity Increment of 400 acfm so this compressor would comply as a trim compressor for this system.

For compliance option 2, the system must include primary storage that has a minimum capacity of 2 gallons for every acfm of capacity of the largest trim compressor.

Example 10-62

Question

What is the required minimum primary storage capacity for the trim compressor from the previous example to comply with Option 2?

Answer

This compressor has a rated capacity of 1261 acfm, and per §120.6(e)1B it must have 2 gallons of storage per acfm of rated capacity or 1261 * 2 = 2,522 gallons of storage.

However, in order to meet the performance necessary for a large enough Effective Trim Capacity, there must be 10 gallons of storage per acfm of the rated capacity, or 1261 * 10 = 12,610 gallons.

The next example also utilizes option 2, but with a 250-hp Variable Capacity compressor, with part-load performance approximated by theoretical curves used in AirMaster+, similar to the last example.

Example 10-63

Question

Part-load data was approximated below for a 250-hp variable capacity compressor; would this provide the minimum Effective Trim Capacity for this system to comply with Option 2?

Answer

Using the previous examples, a compressor with an effective trim capacity of at least 400 acfm is necessary.

Looking at the graph, the minimum specific power (labeled as A below) occurs at full load - a capacity of 1218 acfm, with a specific power of 15.3 kW/100acfm. Using this minimum specific power, the upper bound is 15.3 * 1.15 = 17.56 kW/100acfm or 15% higher than the minimum specific power. This puts the ends of the effective trim capacity at 1218 acfm (labeled as B) and 804 acfm (labeled as C), resulting in an effective trim capacity of 1218 – 804 = 414 acfm. This is larger than the Largest Net Capacity Increment of 400 acfm so this compressor would comply as a trim compressor for this system.

For compliance option 2, the system must include primary storage that has a minimum capacity of 2 gallons for every acfm of capacity of the largest trim compressor.

Example 10-64

Question

What is the required minimum primary storage capacity for the trim compressor from the previous example to comply with Option 2?

Answer

This compressor has a rated capacity of 1218 acfm, and per §120.6(e)1B it must have 2 gallons of storage per acfm of rated capacity or 1218 * 2 = 2,236 gallons of storage.

This requirement applies to new and existing facilities that are being altered with ≥100 hp of installed compressor capacity. The section requires an automated control system which will optimally stage the compressors to minimize energy for the given load. With new systems, this ideally means that at any given load, the only compressors running at part-load are the trim compressors. Because not all systems are required to upgrade the trim compressor, the installed controls must stage the compressors in the most efficient manner.

This requirement also mandates the measurement of air demand. The control system must be able to measure or calculate the current system demand (in terms of actual cubic feet per minute of airflow). There are a variety of ways to accomplish this, including but not limited to the following sensors:

10.8.3 Prescriptive Measures §140.9

10.8.4 Additions and Alterations

These requirements apply to existing systems which are being altered and which have a total compressor capacity of ≥25 hp. These requirements will be triggered by replacing a compressor, adding a compressor, or removing a compressor.

10.8.5 Compliance Documentation

PROJECT NAME is the title of the project, as shown on the plans and known to the enforcement agency.

DATE PREPARED is the last revision date of the plans. If the plans are revised after this date, it may be necessary to re-submit the compliance documentation to reflect the altered design. Note that it is the enforcement agency’s discretion whether or not to require new compliance documentation.

PHASE OF CONSTRUCTION indicates the status of the building project described in the compliance documents. Refer to Section 1.7 for detailed discussion of the various choices.

NEW CONSTRUCTION should be checked for all new buildings, newly conditioned space or for new construction in existing buildings (tenant improvements, see Section 1.7.11 and 1.7.12) that are submitted for envelope compliance.

ADDITION should be checked for an addition which is not treated as a stand-alone building, but which uses option 2 described in Section 1.7.14. Tenant improvements that increase conditioned floor area and volume are additions.

ALTERATION should be checked for alterations to an existing building mechanical systems (see Section 1.7.13). Tenant improvements are usually alterations.

List the total horsepower of the compressed air system capacity for the current system (if applicable) and the proposed system in the provided spaces. The person with overall responsibility must ensure that the proposed system meet the requirements of 120.6 of the standards by completing this section.

The CERTIFICATE of COMPLIANCE – Compressed Air Systems Requirements is signed by both the Documentation Author and the Principal Designer who is responsible for preparation of the plans of building. This latter person is also responsible for this compliance documentation, even if the actual work is delegated to a different person acting as Documentation Author. It is necessary that the compliance documentation be consistent with the plans.

DOCUMENTATION AUTHOR is the person who prepared the compliance forms and who signs the Declaration Statement. The person’s telephone number is given to facilitate response to any questions that arise. A Documentation Author may have additional certifications such as an Energy Analyst or a Certified Energy Plans Examiner certification number. Enter number in the EA# or CEPE# box.

The Declaration Statement is signed by the person responsible for preparation of the plans for the building. This principal designer is also responsible for this compliance form, even if the actual work is delegated to someone else (the Documentation Author as described above). It is necessary that the compliance documentation be consistent with the plans. The Business and Professions Code governs who is qualified to prepare plans and therefore to sign this statement. See Section 2.2.2 Permit Application for applicable text from the Business and Professions Code.

Page 2 of 3 includes mandatory requirements for Trim compressor and storage requirements. As stated on the page, the required information should be either listed on the form or the page from the plans or specifications section and the paragraph displaying the required information should be indicated on the form.

Page 3 of 3 includes Acceptance Testing and declaration statement. Please refer to the Nonresidential Appendices, NA 7.13 for relevant tests.