Industrial Utility Efficiency

Compressed Air System Optimized at Cheese Packaging Plant


A cheese processing and packaging plant is located in the upper Midwest of the U.S. The primary activities are Grade B cheese processing as well as cutting, wrapping, grinding, shredding and packaging of a variety of cheese products. The company expects manufacturing to expand and is looking at how the compressed air system must evolve to accommodate this.

Our firm was asked to conduct a supply-side and demand-side compressed air system assessment, with a focus on getting a few more years life out of the existing air compressors.  The demand-side assessment provides recommendations on how to reduce the flow and pressure requirements of the system in order to generate energy savings.

This assessment identifies a path to reduce the energy consumption from \$85,000 to \$51,000 per year. This can be done with little capital by fixing poppet-valve control problems on the air compressors and reducing flow and pressure requirements. Due to article space limitations, this article does not provide detail on the flow reduction projects. It focuses only on the impact these projects have on the air compressors and provides readers with a template on the information they should have on their units, by shift.

 

Characteristics of the Existing Compressed Air System

The compressed air system is supplied by four (4) 100 horsepower, single-stage, lubricant-cooled, rotary screw air compressors and one (1) 150 horsepower, single-stage, lubricant-cooled, rotary screw air compressor. All of the units are water-cooled.  

The compressed air is dried by a 2000 scfm rated blower purge regenerative desiccant dryer providing a -40°F (-40°C) pressure dew point.  

All energy calculations in this study use the current blended electric rate at the plant of an average \$0.05/kWh.  

According to plant personnel, most of the plant can run on 80-85 psig (5.4-5.8 bar) plant pressure. The exception is one special production area requiring a minimum of 95 psig (6.5 bar). Currently the average compressor discharge pressure is 108 psig.

        Table 1. Existing Compressed Air System Characteristics

 

Measure

 

1st Shift

 

2nd Shift

 

3rd Shift

 

Holidays

 

Total

Average System Flow (cfm)

1500 cfm

1500 cfm

750 cfm

335 cfm

NA

Average Compressor Discharge Pressure (psig)

108 psig

108 psig

108 psig

105 psig

psig

Average System Pressure (psig)

95 psig

95 psig

95 psig

95 psig

NA

Input Electric Demand (kW)

276 kW

276 kW

123 kW

71.4 kW

NA

Operating Hours of Air System (hrs)

2344 hrs

2344 hrs

2344 hrs

1728 hrs

hrs

Specific Power

5.43 cfm/kW

5.43 cfm/kW

6.09 cfm/kW

4.69 cfm/kW

NA

Electric Cost for Air – per unit of flow (\$/cfm/year)

\$21.56

\$21.56

\$19.22

\$18.41

\$80.75

Electric Cost for Air – per unit of pressure (\$/psig/yr)

\$161.74

\$161.56

\$72.08

\$30.84

\$426.40

Annual Electric Cost for Air (\$/yr)

\$32,347 / yr.

\$32,347 / yr.

\$14,416 / yr.

\$6169 / yr.

\$85,279 / yr.

*Current electric rates at the plant average \$0.05/kWh.

To establish the observed and perceived base load we interviewed plant personnel and measured the input kW to each unit with a Fluke Model 41B motor analyzer. Specific pressure readings were taken with a Helcoid DPG 200 Digital test gauge. 

The plant runs three shifts, 24 hours a day, six days a week. The first and second shift are about the same and are usually served by the 150 HP air compressor (750 cfm) and by two or three of the 100 hp air compressors at various loads. 

The third shift includes a sanitation phase, with reduced compressed air demand. This uses the 150 HP air compressor (750 cfm) and two or three of the 100 hp air compressors at various loads. On Sundays and Holidays all production is off, but they run a 100 horsepower air compressor to handle the HVAC and Fire system controls. This unit feeds the complete system and runs about 50-60% loaded, using 42-43 kW.

Figure 1. Existing Compressed Air System Schematic

 
The Existing Compressed Air Dryer

The blower purge desiccant dryer is rated to handle 2,000 scfm at standard CAGI rated inlet conditions. These dryers can consistently provide a -40°F (-40°C) pressure dew point, which removes more water vapor than conventional refrigeration units. To regenerate the wet tower while the other tower is drying requires the use of heat in some form and some dry air to “sweep” or “purge” the exchanged moisture out. The unit has a 10 kW blower and 41 kW heater whose use can be minimized by operating the dryer in “demand control mode”. Currently the dryer is operating on fixed timer-driven cycles, regardless of the demand on the dryer. Switching to demand control mode will reduce the energy cost from \$21,550 to \$6,500 per year at current load profile levels.

 
Figure 2. Existing Compressed Air System Schematic (continued from Figure 1)

 

The Existing Air Compressors

Current electric rates at the plant average 0.05/kWh.  The actual plant electric cost for air production, as running today, is probably in excess of \$85,279 per year.

The load profile or demand of this system is relatively stable during all shifts. The full load operating range is 293 days a year, 24 hours a day, 7032 hours a year. There is one flow meter in the system.

The system pressure appears to run from 95 to 105 psig in the headers during production. System cost on a per unit of pressure basis run \$426.40/psig.

The 100 horsepower air compressors are aging but well maintained. They are, however, experiencing down-time with increasing frequency and should be replaced in the future. They use poppet valve control systems. The low input kW combined with the lower operating temperatures indicate that units (3) and (4) probably have have poppet valves stuck open not allowing them to get to full load but keeping them at part load. Maintenance needs to keep a close eye on this.

Typical Operating Band of Poppet Valve Control

100%   100% power 
87.5% 90% power
75%   84% power
62.5%  75% power
50%  68% power

 

 

 

 

Table 2. Annual Load Profiles of Existing Air Compressors

Unit #

Air Compressor – Horsepower

Percent of Load

Percent of Power

Full Load kW X Percent of Power

Net Demand (kW)

Actual Flow(cfm)

First & Second Shift:  Operating at 1500 cfm and 108 psig

1

150 HP

100

100

123 x 1

123 kW

750 acfm

2

100 HP

100

100

85 x 1

85 kW

450 acfm

3

100 HP

67

80

85 x .80

68 kW

300 acfm

Total 276 kW   1500 acfm

Third  Shift:  Operating at 750  cfm and 108 psig

1

150 HP

100

100

123 x 1

123 kW

750 acfm

Total 123 kW    750 acfm

Holiday  Shift:  Operating at 335 cfm and 108 psig

1

100 HP

74

84

85 x .84

71.4 kW

335 acfm

Total                                                                                                                  71.4 kW   335 acfm

     

Compressed Air System Assessment Recommendations

Summarized below are the key characteristics describing the performance and economics of the proposed compressed air system.  Tables 3 and 4 are modifications of similar tables displayed previously that described the current system.  The tables were modified to reflect the system performance and operating cost changes resulting from implementing the set of projects recommended in this report.

All the compressors in the current system have poppet valve capacity controls, which should work very well from 50% load to 100%.  The piping system, however, is creating a great deal of back pressure fluctuations in the header causing some significant control instability.  
    
Often multiple units are running at part loads rather than having a unit shut off. This is the way the air compressors were operating when we arrived.  When we left, we had momentarily eliminated the problem through machine selection and operating band adjustment. With the inherent instability with the 4” header we would expect the condition to continue to appear. Currently, the system is wasting about 45 to 50 kW depending on the conditions.   
    
This has an annualized energy cost of \$15,000 or more if undetected and not readjusted. These potential savings are not in our proposed system modifications and payback since it can be adjusted out. 
    
We recommend the piping header be corrected to eliminate this back pressure (5 psig) and instability. The project is to correct capacity control operation by replacing the 4” header with an 8” header and make all connections a 30º to 45º directional angle entry. This will stabilize the central sensing pressure, increase storage, and eliminate 5 psig of lost pressure at high loads. 
    
We also recommend installing a small 5 HP tank-mounted (120 gallon) industrial air compressor with a refrigerated air dryer to run the HVAC controls and Fire Systems controls on Holidays and Sundays.  Set it up dedicated to run HVAC controls only and not through the main compressed air system.  

Pressure Reduction

  • Reduce pressure to 90 % of plant to 85 psig (95 psig to Packaging Process)
  • Replace 4” header with 8” header – 5 psig

Flow Reduction – 456 cfm total flow reduction 

  • Leak Repairs – 300 cfm
  • Replace 10 timer drains – 31 cfm
  • 90% of flow has 15 psig reduction in pressure – 75 cfm
  • Install 5 air amplifiers on blow–offs – 50 cfm
  • Run 5 HP tank mounted air compressor on holidays to handle the HVAC and Fire system controls.  

Other projects or savings not reflected in the tables include:

  • Eliminate cooling water cost
  • Repairs to water coolers due to untreated water
  • Run the blower purge in demand control mode

Table 3. Proposed Compressed Air System Characteristics

Measure

1st Shift

2nd Shift

3rd Shift

Holidays

Total

Average System Flow (cfm)

1044 cfm

1044 cfm

294 cfm

20 cfm

NA

Average Compressor Discharge Pressure (psig)

100 psig

100 psig

100 psig

150 psig

NA

Average System Pressure (psig)

95 psig

95 psig

95 psig

60 psig

NA

Input Electric Demand (kW)

184.29kW

184.29kW

65.75 kW

5 kW

NA

Operating Hours of Air System (hrs)

2344 hrs

2344 hrs

2344 hrs

100 hrs

hrs

Specific Power

5.66 cfm/kW

5.66 cfm/kW

4.47 cfm/kW

4.00 cfm/kW

NA

Electric Cost for Air – per unit of flow (\$/cfm/year)

\$20.69

\$20.69

\$26.21

\$1.35

\$68.84

Electric Cost for Air – per unit of pressure (\$/psig/yr)

\$107.99

\$107.99

\$38.53

\$.13

\$254.64

Annual Electric Cost for Air (\$/yr)

\$21,598 /yr

\$21,598 /yr

\$7706 /yr

\$25 /yr

\$50,927 /yr

 

A Note on the Current Air Compressor Control System

All the air compressors have poppet valve operated variable displacement controls. These have an electronic indicator when the valve solenoid or activator is told to close the valve. These do not necessarily mean the valve is actually closed and seated. If the valves leak, they will reduce capacity (12.8% per valve) and power (5 to 8% per valve).
    
When we arrived at the plant, the system was running (and had been running for some time) with the 150 hp unit and 2 of the 3 of the 100 HP units for a total energy consumption of approximately 290 kW. When we reviewed the units, we found that No. 3 & 4 100 hp air compressors apparently have leaking poppet valves. By running the 150 hp unit as the base load unit and the No.1 100 hp unit as the first follow-unit, we ran the plant with one 100 hp unit turned off. This represents a significant savings. 
    
The potential for leaking poppet valves should be constantly monitored as part of your daily maintenance.  We recommend installing kW monitors on each control box to compare flow vs kW.

Table 4. Proposed Annual Air Compressor Operation/Use Profile

 

Air Compressor – Horsepower

Percent of Load

Percent  of Power

Full Load kW X Percent of Power

Net Demand (kW)

Actual Flow(cfm)

First Shift:  Operating at 1044 cfm and 100 psig

1

150 hp

100

100

123 x .98

120.54 kW

750 acfm

2

100 hp

65

77

85 x 77

63.75 kW

294 acfm

Total                                                                                                          184.29 kW     1044 acfm

Second Shift:  Operating at 1044 cfm and 100 psig

1

150 hp

100

100

123 x .98

120.54 kW

750 acfm

2

100 hp

65

77

85 x 77

63.75 kW

294 acfm

Total                                                                                                          184.29 kW     1044 acfm

Third Shift: Operating at 294 cfm and 100 psig

1

100 hp

65

77

85 x 77

63.75 kW

294 acfm

Down the road, new air compressors will need to be purchased to replace the 100 hp units. At this time, a centralized controller, able to manage all the air compressors, should be purchased.

This assessment recommends reducing compressed air demand and system pressure. The existing air compressors still have some years of life left in them if maintenance keeps an eye on the poppet valve control system and the dryer control system. This will provide the firm with energy cost savings of \$34,000, with little capital deployed, and an optimized demand-side system ready for new air compressors when that day comes.

For more information on APenergy visit apenergy.com or call 740.862.4112

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