Industrial Utility Efficiency    

A Compressed Air Management System for Five Compressors

Introduction

This building products factory spent an estimated $240,000 annually on energy to operate the compressed air system at their Midwestern facility.  This figure will increase as electric rates rise from their current average of 7.8 cents per kWh.  The set of projects recommended, by the system assessment, reduced these energy costs by an estimated $104,336 or 43% of current use. Project costs totalled $73,000, representing a simple payback period of 8 months.

The system assessment included installing a central compressed air management system. This project delivered the majority of the energy savings, providing 1,165,077 kWh in annual savings adding up to almost $91,000. The project also included some demand-side and storage projects. Due to article length constraints, we will focus on the installation of the compressed air management system.

 

The Existing Compressed Air Installation

The main compressed air system consists of three Sullair 20-100L, single-stage, lubricated, water-cooled rotary screw compressors.  Each draws 110 bhp and produces 500 cfm at full load.  They are rated for 100‑psig discharge pressure.

The primary compressor, a Quincy QSI1000 that was acquired from a sister plant closing, is a 200-hp class (211 bhp) producing 1,014 cfm at full load.  It is also a single-stage, lubricated rotary screw.  It runs as “base load” with the Sullair units and the Kaeser compressor is used as trim units.  The three Sullair units and the Quincy compressor are located in the Main Compressor Room. 

There is a Kaeser DS140 compressor located in between the Blending and Regrind areas.  It was installed to alleviate the low pressure sometimes experienced in the Blending area.  The Kaeser is a 100-hp (122 bhp), single-stage, lubricated rotary screw producing 543 cfm at full load and is equipped with auto start/stop. The Kaeser was placed in this Blend and Regrind area to supply air to the Blend process when the pressure fell to an inadequate level to that area.

All the compressors are controlled manually with the exception of the Kaeser unit.  With that in mind, multiple units are running at part load “just in case something happens.”  This practice, which may be necessary to keep the plant running, prevents the whole system from running efficiently, costing significant energy dollars to be wasted.

Each compressor is paired with a similarly sized refrigerated dryer.  The Sullair units and the Kaeser unit have Sullair refrigerated dryers and the Quincy unit has a Zeks heat-sink dryer.

 

Figure 1.  Current Compressed Air System:  Main Compressor Room

Figure 1

 

Figure 2.  Current Compressed Air System:  Blend and Regrind Area

Figure 2

Establishing the Energy Baseline

Three units were running during the system assessment - along with seven production lines.  During the visit, production changed from six to seven lines with no noticeable change in compressor load.

The full load kW of the Quincy unit at 100‑psig discharge pressure is 169 kW, while the full load kW of the Sullair units is 89 kW at 100‑psig discharge pressure.  The individual kW lines indicate that the Quincy was running at an average of 134 kW or at 80% power.  Sullair Unit #1 was running at 72 kW or 80% power and the Sullair Unit #2 was at 84 kW or 94% power.

During the morning one day, at approximately 9:00 am, the two Sullair Units #1 and #2 were shut off to see what the effect would be on the Quincy unit and on the entire system.  During this test, the Quincy unit loaded in further and the plant did not experience any long-term pressure shortage.  This test indicates that all three compressors were running at part load.

Figure 3.  Main Compressor Room Metering Data

 

Annual plant electric costs for compressed air production, as operating today, are $228,898 per year.  If the electric costs of $11,184 associated with operating ancillary equipment such as refrigerated dryers are included the total electric costs for operating the air system are $240,082 per year.  These estimates are based upon a blended electric rate of $0.078 /kWh.

The air system operates 8,760 hours per year.  The load profile or air demand of this system is relatively stable during all shifts.  Overall system flow ranges from 740 acfm during low production to 1,440 acfm during high production. The system pressure runs from 97 to 95 psig in the headers during production.

According to plant personnel, production is seasonal with higher production during the middle of the year and lower production during the rest of the year.  Each production level averages six months.  Also, the compressor load for the 14 lines was estimated based on input from plant personnel.  Each compressor load was estimated using the same information provided by personnel.


Table 1.  Key Air System Characteristics – Current System*

Measure

7 Lines

14 Lines

Total

Average System Flow

720 cfm

1,440 cfm

NA

Avg Compressor Discharge Pressure

102 psig

102 psig

NA

Average System Pressure

97 psig

97 psig

NA

Input Electric Power

295 kW

375 kW

NA

Operating Hours of Air System

4,380 hrs

4,380 hrs

8,760 hrs

Specific Power

2.44 cfm/kW

3.84 cfm/kW

NA

Electric Cost for Air /Unit of Flow

$139.97 /cfm yr

$88.96 /cfm yr

$228.93 /cfm yr

Electric Cost for Air /Unit of Pressure

$403.13 /psig/yr

$512.46 /psig/yr

$915.59 /psig/yr

Ann’l Elec Cost for Compressed Air

$100,783 /year

$128,115 /year

$228,898 /year

*Based upon on a blended electric rate of $0.078 per kWh and 8,760 hours/year.

 

Table 2.  Compressor Use Profile – Current System

Unit

#

Compressor:

Manufacturer/Model

Full Load

Actual Elec Demand

Actual Air Flow

Demand (kW)

Air Flow (acfm)

% of Full kW

Actual

kW

% of Full Flow

Actual

acfm

7 Lines:  Operating at 102 psig discharge pressure for 4,380 hours

1

Sullair 20-100L (Unit #1)

89

500

80%

72

35%

175

2

Sullair 20-100L (Unit #2)

89

500

94%

84

32%

160

3

Sullair 20-100L (Unit #3)

89

500

OFF

4

Quincy QSI1000 (Unit #4)

169

1,014

73%

134

38%

385

5

Kaeser (Unit #5)

99

543

OFF

TOTAL (Actual):                   295 kW                   720 acfm

14 Lines:  Operating at 102 psig discharge pressure and 4,380 hours

1

Sullair 20-100L (Unit #1)

89

500

83%

73.8

48%

243

2

Sullair 20-100L (Unit #2)

89

500

83%

73.8

48%

243

3

Sullair 20-100L (Unit #3)

89

500

83%

73.8

48%

243

4

Quincy QSI1000 (Unit #4)

169

1,014

91%

153.7

70%

709

5

Kaeser (Unit #5)

99

543

OFF

TOTAL (Actual):                  375 kW                   1,440 acfm

                 

 

The Proposed Compressed Air System

The overall strategy for improving the air system is based upon operating each compressor efficiently and shutting off unnecessary units.  This can be accomplished by installing a high-quality central control system.

The two most effective ways to run air compressors are at “Full Load” and “Off.”

Capacity controls are methods of restricting the output air flow delivered to the system while the unit is running.  This is always a compromise and is never as efficient as full load on a specific power (cfm/hp) basis. 

 

Rotary Screw Air Compressor Controls

The two most common control methods used for rotary screw compressors are modulation and on-line/off-line.  Modulation is relatively efficient at higher loads, but less efficient at lower loads.

On-line/off-line controls are very efficient for loads below 60% when properly applied with adequate time for blow down.  There are several other control types ‑‑ e.g., “variable displacement” (75% to 100% load) and “variable speed drive” (25% to 75% load) ‑‑ that have very efficient turn down from when applied correctly.  Two-stage, oil-free, rotary screws generally are not applied with modulation.  As a result they use either two-step (full-load/no-load) or VSD capacity controls.

These controls must be installed correctly to operate efficiently.  Piping and storage should be available close to the unit with no measurable pressure loss at full load to allow the signal to closely match the air requirements.  The current system has modulation controls on the Quincy unit and on the Sullair units and two-step control on the Kaeser compressor.

The current units have capacity controls capable of translating “less air used” into a comparable reduction in electric cost.  These controls will work effectively with the current piping and air receiver storage situation.

 

Central Monitoring and Air Management Control System

We recommend installing a compressed air system central monitoring and control for multiple units.  These systems of PC hardware and Windows®-based software will allow your personnel to effectively monitor, operate, and sequence your compressed air system from any PC on your computer network.  Your monitoring system should be able to monitor and record system flow and pressure and interface with the local on-board unit control system as it stands or is modified.  These systems provide information and trending data to maximize system efficiency, reduce maintenance costs, and minimize unscheduled downtime.  They can be set to alert personnel via a cell phone or email of a compressor warning, alarm, or shut down condition to ensure prompt attention to an emergency situation.

This type of monitoring software typically analyzes the sensed operating condition and brings the compressor on- or off-line, as required, to best handle the demand.  They should not operate on a fixed sequence cycle.  This ensures that one unit runs at part load and all others are at full load or off.

The monitoring function is not a direct energy issue but does help you retain the efficiencies your air system program has obtained.  A well-applied monitoring system can become an operating part of a full central compressed air management control system.

 

Table 3.  Compressor Use Profile – Proposed System

Unit

#

Compressor:

Manufacturer/Model

Full Load

Actual Elec Demand

Actual Air Flow

Demand (kW)

Air Flow (acfm)

% of Full kW

Actual

kW

% of Full Flow

Actual

acfm

7 Lines:  Operating at 102 psig discharge pressure for 4,380 hours

1

Sullair 20-100L (Unit #1)

89 x .96

500

OFF

2

Sullair 20-100L (Unit #2)

89 x .96

500

3

Sullair 20-100L (Unit #3)

89 x .96

500

4

Quincy QSI1000 (Unit #4)

169 x .96

1,014

88%

136

60%

609

5

Kaeser (Unit #5)

99 x .96

543

OFF

TOTAL (Actual):                   136 kW                   609 acfm

14 Lines:  Operating at 102 psig discharge pressure and 4,380 hours

1

Sullair 20-100L (Unit #1)

89 x .96

500

OFF

2

Sullair 20-100L (Unit #2)

89 x .96

500

3

Sullair 20-100L (Unit #3)

89 x .96

500

89%

72.6

63%

315

4

Quincy QSI1000 (Unit #4)

169 x .96

1,014

100%

156

100%

1,014

5

Kaeser (Unit #5)

99 x .96

543

OFF

TOTAL (Actual):                  228.6 kW                   1,329 acfm

                 

 

Table 4.  Summary of Key Compressed Air System Parameters and Projected Savings

 

Current System

Proposed System (Reduced Demand)

SYSTEM COMPARISON

7 Lines

14 Lines

7 Lines

14 Lines

Average Flow (cfm)

720

1,440

609

1,329

Compressor Discharge Pressure (psig)

102

102

92

92

Average System Pressure (psig)

97

97

90

90

Electric Cost per cfm

$139.97 /cfm/yr

$88,96 /cfm/yr

$76.79 /cfm/yr

$58.26 /cfm/yr

Electric Cost per psig

$403.13 /psig/yr

$512.46 /psig/yr

$194.04 /psig/yr

$324.69 /psig/yr

 

PROJECTED ENERGY COST SAVINGS FOR PROPOSED SYSTEM

Air System Component

Annual Electric Cost of Current System

Annual Electric Cost Proposed System

Anticipated Annual Savings

Estimated Project Cost

Compressor System Operations

$228,898

$124,562

$104,336

$73,100

Ancillary Air Equipment (dryers, etc.)

$11,184

$11,184

$0

Total Compressed Air System

$240,082

$135,746

$104,336

 

Conclusion

This building products factory spent an estimated $240,000 annually on energy to operate the compressed air system at their Midwestern facility.  The set of projects recommended, by the system assessment, reduced these energy costs by an estimated $104,336 or 43% of current use. Project costs totaled $73,000, representing a simple payback period of 8 months.

 

For more information contact Don van Ormer, Air Power USA, tel: 740-862-4112, email: don@airpowerusainc.com, www.airpowerusainc.com.

To read more System Assessment articles, visit www.airbestpractices.com/system-assessments/compressor-controls.