Natural Gas Engine-Driven Air Compressors

The Sierra Army Depot is located in Herlong, California approximately 50 miles from Reno, Nevada. The base mission is to provide Operation Project Stock services to customers including storage, repair and issue of equipment and various equipment components. In addition, the base stores, maintains and demilitarizes conventional ammunition.

The Base purchases electricity from Lassen Municipal Utility District on rate schedule #70: Industrial Service. A summary of the rate is as follows:

Demand:	$7.00/kW
Energy:	$0.1050/kWh ( Basic Charge + Rate Adjustment)
	Basic Charge: $0.0650/kWh
	Rate Adjustment (2/6/01): $0.0400/kWh

The demand charge is based on the maximum average power taken during any 15-minute interval in a month.

Demand:	1,981.7 kW	$ 13,867.70
Energy:	809,907 kWh	$ 85,040.28

Total Bill:		$99,137.98
Average Electric Cost:	$0.1220/kWh	($99,137.98 / 809,907 kWh)

The Base purchases natural gas from Texas Ohio Energy. Gas bills for the last two calendar years are presented in the following tables. The data in the Tables show that the cost of natural gas purchased by the Base nearly doubled from 1999 to 2000.

Table 1. Gas Costs of 1999

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Table 2. Gas Costs of 2000

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Table 3. Year to Date 2001 Gas Usage and Costs

During the month of March 2001, the base negotiated a natural gas rate of $0.795/therm with their gas supplier so that the natural as cost would be competitive with the cost of diesel.

On April 19, 2001 a compressed air survey was conducted by Science Applications International Corporation (SAIC) and the United States Army Construction Engineering Research Laboratory (CERL) personnel. The purpose of the survey was primarily to evaluate the site as a candidate for a CERL-funded project to demonstrate the operation of a natural gas engine driven air compressor (NGEDAC). It was also intended to identify opportunities for reducing energy operating costs associated with the existing compressed air system. Mr. Dan Moore was the SIAD point-of-contact for the survey and provided information about the compressed air system and energy usage and costs.

2.1 Compressed Air System Overview

The survey focused on the compressed air system for buildings 208 (maintenance shop), 209 (metal shop), and 210 (paint shop). The compressed air system consists of three compressors, of which only one is normally operated. Operation of the backup compressors is undesirable, since their use appears to be related to a problem with moisture in the system that has an adverse affect on the breathing air supply. The compressor that is typically operated is located in building 210, the paint shop. There is a 3.5" compressed air line that serves as the compressed air distribution for the three buildings.

The compressed air loads consist of paint booths, shot blast booths, paint sprayers, and various hand-held air tools. The paint booths utilize air for both the paint sprayers and breathing air. Breathing air is provided by the central air compressor and is then processed through a purifier that consists of filters for moisture, CO, and odor removal. The system has two purifying systems for breathing air. Only one system is operated at any given time with the other utilized for backup in the case of a failure or the need to replace filters. The focus of the survey was on the electric compressor in located in building 210 and the potential of installing an engine-driven unit to operate in its place. Specific nameplate information on this compressor is as follows:

Manufacturer: Gardner Denver
Model: EDMQNA
Nominal Power: 125 HP
Rated Capacity: 600 scfm
Rated Supply Pressure: 100 psig
Voltage: 460 VAC/ 3phase/ 60 Hz
Full Load Current: 154 amps
Date of Manufacture: December 1994

The specifications for the refrigerated dryer is as follows:

Manufacturer: Gardner Denver
Model #: 7000100
Rate Capacity: 800 scfm
Compressor Size: 5 HP
Compressor FLA: 18 amps
Fan Size: 1/3 HP
Fan FLA: 3.4 amps
Electric: 230 VAC/ 3 phase/ 60 Hz

The facility is currently operating at one shift. This results in the compressor operating 10 hours per day, four days per week. This represents 2100 hours of operation per year.
The compressed air load varies significantly based on the work being processed through the three buildings. The largest impact occurs when the shot blast booths are being utilized. They utilize both process and breathing air. The shot blast booth usage is very sporadic and infrequent.

Measurements taken at the site are as follows:

The higher reading at 12:00 take place at the end of the lunch break.

Compressor output: 100 psig @ 175°F
Compressor Hours of Operation: 11,890 Hours

Dryer Inlet: 104 psig @ 75°F
Dryer Outlet: 102 psig @ 65°F

Pressure of air supply in Building 208 (furthest point from compressor): 100 psig

Based on the above information, the gauge on the compressor for output pressure is reading low.

A detailed compressed air system survey conducted by CERL in 2000 identified 7 opportunities to reduce energy operating costs, including the installation of a natural gas engine driven air compressor (see Lin, et al, Compressed Air System Survey at Sierra Army Depot, ERED/CERL TR –00-37, November 2000. The six opportunities other than the NGEDAC included:

Collectively, these six opportunities represented annual cost savings of $15,541 in electricity costs, energy savings of 181,409 kWh, and a demand reduction of 49.9 kW. Based on our survey, it does not appear that these opportunities have all been implemented. Given the recent price increases in electricity, these savings opportunities are now worth more than $25,000, and should be increasingly attractive to SIAD.

The installation of an engine-driven air compressor to operate in place of the existing electric air compressor is straightforward for this site. The NGEDAC that is appropriate for this site is a 125 HP (137 bhp) unit that is capable of producing 630 scfm at 100 psig. The main compressor room has a second room located in the structure that has the appropriate space for installing the engine-driven unit. The room dimensions are 19’ long, 20, wide and 9’ tall. The room contains a unit heater that can be utilized for freeze protection during the winter months (if needed). The condition of the structure is below average and will require some refurbishing. Areas of focus are the ceiling and an access door.

Compressor Room Enclosure

Natural gas is accessible at the building. The gas line and regulator is located outside, at a distance of approximately 120 feet from the main compressor room. The current gas regulator is set at 14" w.c. The NGEDAC will require gas pressure of 2 - 5 psig. Increasing the pressure at the main pressure regulator will require that the regulators for the existing space heating units be replaced to accommodate the increased pressure. Base personnel indicted that this can be easily accommodated. The gas line would need to be extended into the building 210 approximately 20 feet and then run along the ceiling down to the compressor room and back to the location of the unit. The estimated total natural gas piping run is 200 feet.

Stub-up for Natural Gas Line

Compressed Air Piping Interface

Since the NGEDAC capacity is the same as the electric unit’s capacity, the compressed air output from the engine-driven unit can be interfaced to the compressed air output of the existing electric unit. The point of interface would be the piping between the output of the electric unit and the input into the receiver. Thus, the existing receiver, filter, dryer and breathing air purifiers can be utilized. The total compressed air piping run is estimated to be 25 feet.

Heat Recovery

There are no process hot water loads within the three buildings. The proposed heat recovery option is to install a hydronic space heater in building 210. There are currently gas-fired space heaters suspended from the ceiling throughout the building. The proposed heating system utilizing heat recovered from the engine-driven air compressor would directly offset the heating requirement of the existing suspended gas heaters. The heat recovery equipment would consist of the heat recovery heat exchanger option on the engine-driven air compressor, interface copper piping, a circulation pump, a forced air hydronic heating unit, and controls. The hydronic heating unit would be suspended form the ceiling near the entrance of the main compressor room. The heat recovery heat exchanger would be capable of recovering approximately 600,000 Btuh of heat from the new compressor.

The building is approximately 18,000 ft2 and does not appear to be insulated. The ceiling in the center of the building is in excess of 30 feet high. It is projected that the new space heater would be capable of providing the 600,000 Btuh from the new compressor into the conditioned space 100% of the occupied hours of the building during the months of October through April.

Ducting Modifications

The installation of the engine-driven air compressor will require some ducting to vent the engine exhaust from entering the compressor inlet air. The engine exhaust may trip the alarm for the breathing air purification system. To accommodate this requirement, it is proposed that ducting for the inlet air to both the new compressor and the existing electric unit be installed. The inlet ducting would be run towards the building and extended upward. The resulting distance between the inlet air and the engine exhaust would be approximately 50 feet.

The principal benefits of the NGEDAC unit for SIAD include:

ß Net savings in operating costs
ß Hedge against power disruptions – operates on natural gas, not electricity.
ß Added capacity/redundancy for the compressed air system

The following estimates the operating costs and benefits.

Operating Cost Comparison

A 125 hp NGEDAC unit capable of providing 600 scfm at 100 psig was evaluated to serve 100% of the load currently met by the existing 125 hp Gardner-Denver electric motor drive air compressor. Table 3.2-2 below summarizes the energy performance and costs associated with the proposed unit. The load is based on a 10 hr/day, 4 day/week schedule (2080 hours/yr.), assuming an average loading of 90% of design capacity (540 scfm).

Table 4.2-1 Compressor Performance Characteristics at Design Load

Table 4.2-2 Annual Energy Use and Operating Costs

1 Electricity Costs: $.145/kWh average – includes demand @$7/kW and energy charges @.105/kWh (2/6/01 rate)
Natural Gas Costs: $7.28/MBtu (Average for April 2000 – March 2001)
2 Based on (.22/.8) *heat value of natural gas into the engine, where .22 is the fraction of recoverable heat and .8 is assumed efficiency of heating unit.

The results shown are based on the most recent electric and gas prices as indicated. The table below shows changes in the annual operating costs of the NGEDAC system based on possible changes in future electric rates or gas prices. Note also that the maintenance costs for the NGEDAC are a function of the hours of operation for a given size unit.

Table 4.2-3 Annual Operating Costs ($) – Sensitivity to Changes in Energy Prices

Table 4.2-4 NGEDAC Capital Cost