Natural Gas Engine-Driven Air Compressors - Case Studies - Army

acfm	actual cubic feet per minute
bhp	brake horsepower
Btu	British thermal unit
CERL	U.S. Army Construction Engineering Research Laboratory
cfm	cubic feet per minute
CO	carbon monoxide
ESPC	Energy Savings Performance Contract
FL	Full load
gm/bhp/hr	grams per brake horsepower per operating hour
hr	hour
HVAC	heating, ventilating and air conditioning system
IGV	inlet guide vanes
IR	Ingersoll Rand
kW	kilowatt
MBtu	Million British Thermal Units
NGEDAC	natural gas engine driven air compressor
NOx	nitrogen oxides
psig	pounds per square inch gauge
scfm	standard cubic feet per minute
TMS	Technology and Management Services, Inc.
yr	year

1.1 BACKGROUND

The Watervliet Arsenal has a very extensive compressed air system linking many separate buildings and spread over a large geographical area. The air system reaches most production sectors and runs building to building, eventually completing a full ‘loop’ system (see sketch PLANT SURVEY Section). The compressed air supply is primarily generated in Building 110 with one large 2,000 cfm (450 hp) class Joy centrifugal compressor and two 125-hp Ingersoll-Rand XLE (650 cfm per machine) reciprocating compressors.

There are six other major compressors tied in to the main air system in surrounding buildings. There are also a number of smaller air-cooled reciprocating units throughout the Arsenal either as part of the separate "controls air system" or dedicated air to a particular process.

Air drying is provided by both desiccant and refrigeration units and appears to be working well according to plant personnel and survey results. Most of the compressors are water cooled, but some have their own air-cooled, radiator-type, closed-cooling systems, which also appear to be working well.

The complete air system appears to be very well laid out, well maintained and operated consistent with the type of controls on each compressor unit. However, on the demand side of the system, there are a number of areas that should be reviewed in the future in more detail, as they appear to be significant opportunities reducing air consumption.

The overall usage in the full system today is on the order of 2,000 to 2,500 cfm. In the past, when there was a higher level of production at the site, the overall usage was larger. The results of the preliminary site survey suggests there are leaks amounting to at least 300 cfm that could be identified and repaired, which would reduce annual electric costs by over $22,000.

There seem to be some tank agitation applications that could perhaps be powered by low-pressure air compressors or blowers rather than costly high-pressure air. These and other demand-side savings opportunities will be enumerated in the Level II Assessment if Watervliet Arsenal is selected as a NGEDAC demonstration site.

1.2 CURRENT AND RECONFIGURED SYSTEM BASELINE

The key characteristics describing the performance and economics of the current and proposed compressed air system are summarized in Tables 2, 3 and 4. The table was developed based on the data collected during the site visit and in discussions with plant personnel. The proposed system estimates are technically and economically conservative and reflect the observed performance of each compressor compared to load cycle. Performance factor calculations details are provided in the PLANT SURVEY Section of this report.

Observations of Plant Personnel

At current load, the Joy centrifugal 450-hp will "carry the plant" with some assistance from the IR LLE-5 (Building 25), which provides on the order of 100 cfm for several hours a day. Usage levels for the second and third shifts do not seem to fall much, probably due to high use of aeration air, vortex cooling, leaks, etc., which occur 24-hours per day. The estimated average system flow is approximately 2,000 to 2,500 acfm.

Observations of Audit Team

The main air supply was on and running during the plant survey period from 10:30 am to 1:00 pm on October 31, 2000. The system supply pressure was observed in the operating range of 83 - 85 psig. The pressure at the compressors was observed in the operating range of 90-100 psig. In most cases, the centrifugal unit is operated at full load. However, on the day of the visit, the centrifugal unit was not operating.

Blended electric rate equals $0.09/kWh.

Note 1 - Proposed system includes 1,500 cfm natural gas engine drive and one existing IR 125 hp XLE operating at full load. Natural gas system parameters include 8,170 Btu/hp-hr, 341 BMP and 8,760 hours annually.

Average annual electric rates at the plant are $0.09/kWh. The actual plant electric cost for air production, as currently operated, is in excess of $300,000 per year. The load profile or demand of this system is relatively stable during all shifts. The full load operating range is 24 hours a day, 365 days a year, for 8,760 hours a year. The system pressure appears to operate in the range of 83 to 85 psig at the headers during production periods. There are no flow meters in the system.

The standard performance measure used for this analysis is "electric cost per hour per loaded cfm" of air. Annual electric cost was selected as the key project evaluation factor, since it is a good overall indication of system costs and savings associated with potential measures. It is an quantitative number and not a subjective opinion, i.e., if the compressed air is used, these dollars are spent.

All paybacks are estimated utilizing "Full Load Operating Efficiencies," which are very conservative. If the compressed air is not used, the compressor either shuts off or unloads. If it shuts off, there is a 100 percent saving of the electric cost. If it unloads, there is a 25 to 90 percent savings of the electric cost.

It is important to note that other recoverable compressed air costs should also be considered, e.g., maintenance, cooling water costs, and depreciation. Usually, the electricity cost is between 75 and 90 percent of the total "variable compressed air costs." Associated maintenance and other costs will be, in all probability, at least 20 percent or more of the identified electric cost. Existing plant records may already have these identified.

1.3 ENERGY COST BASELINE

Shown below is a recent history of energy expenditures at Watervliet Arsenal.

Gas costs averaged $4.21 per million Btu in Fiscal Year 1999. This average was up about 10 percent over Fiscal Year 1998 and by about 20 percent over Fiscal Year 1997. These gas prices include $.60 per million Btu transportation costs. An estimate of $5 per million Btu was used as the baseline for this assessment with $4 and $6 per million Btu used as a sensitivity analysis. A $1 increase in gas price increases operating costs by about $25,000 for the NGEDAC.

Electric costs averaged $0.83 per kWh during Fiscal Year 1999. At the end of December 2000, a special contract that Watervliet had with NIMO expired. The net impact of this change will be an increase to $.09 per kWh as the average rate for Watervliet in moving forward. This level of impact was provided by Watervliet staff and confirmed by project staff. The value of $.09 per kWh was used in the project assessment.

2.1 PRIMARY AIR COMPRESSOR SUPPLY

The following is an overview of the compressed air supply system as observed on October 31, 2000.

Building 110

Units 110N and 110S are each 125-hp class Ingersoll Rand, two-stage, double-acting reciprocating XLE compressors and are water-cooled. They are also of a continuous duty design. These are the most power-efficient air compressors at full load and when at part load to meet varying demand. They appear to be in good operating condition, although the inspection team did not perform any tear down inspection. There is no reason from a power efficiency standpoint to replace these units.

Unit 110 Center is a 450-hp Joy three-stage centrifugal (oil-free) TA18 compressor delivering 1,850 to 2,000 acfm at 100 psig at 450 bhp. This is a dynamic compressor, and actual air delivered and performance will vary with operating conditions. From a full load power efficiency standpoint, the TA18 is about the same as the XLE. However, the TA18 does not unload or meet part load demands as efficiently in "turndown" much below 25% when operating correctly. This unit is very power efficient from about 2,000 to 1,500 acfm flow. This TA18 is equipped with inlet guide vanes (IGVs), which allow almost "perfect turndown" from 2,000 to 1,500 acfm load. At flows below 1,750, it will be less efficient, and at lower loads it will be very inefficient with the current installed control system. Other than a more efficient unloading central controller, there is no reason from a power standpoint to replace or modify this unit.

Because of its central location on the system, proximity to other compressor units, available physical space, and easy access to gas, Building 110 is the leading candidate as the site for the proposed NGEDAC system. Preferred location is probably along the south wall of building.

Building 125

Building 125 houses a Joy WN112 75-hp two-stage, double-acting, water-cooled compressor delivering 405 acfm at 100 psig at 77.3 bhp. This unit also appears to be in excellent shape and, according to plant personnel, runs very well. Even though it is an older unit (circa 1956), it is of the best designs for its type. There is no reason from a power efficiency or application standpoint that it should have to be replaced.

Building 35

Building 35 has three Joy WN112 compressors, the same as described above. One unit is a 75-hp (405 acfm @ 100 psig) and the other two are 100-hp (564 acfm @ 100psig). They all appear to be in good working order and well maintained.

Building 25

Building 25 houses a 125-hp Ingersoll Rand LLE-5 two-stage, double-acting, water-cooled compressor delivering 653 acfm @ 100 psig @ 125 bhp. This is the newest of the double-acting, water-cooled units and is of "leading edge technology." Key characteristics include:

As in the case of the rest of the double-acting units, the unit runs well and appears to be in good shape and very well maintained. There is no power efficiency reason to replace this unit. As in the case of the other compressors, it is continuous duty rated.

Building 20

Building 20 has a new Ingersoll Rand EP100 single-stage, lubricant-cooled, rotary screw air compressor. This unit is air cooled, but it is also continuous duty. The EP100 is obviously state-of-the-art and very conservatively applied. Its 100-hp motor is designed to run with a 1.15 service factor and the basic unit delivers 446 acfm at 125-135 psig at full load. It has been applied in the system very professionally with an operating band of 90-100 psig. This puts a load of 96.25 bhp or less on the 115-hp rated motor. It should do very well in the long run and, of course, save energy.

General Comments on the Air System

1. The above listed units are the main or primary air compressors used to support manufacturing and test operations at Watervliet. All but one (a rotary screw) are water-cooled units and each unit has its own polyglycol closed-cooling system. This utilization of available equipment is an excellent operational strategy and appears to be working well. This type of operation eliminates many of the problems associated with water-cooled units.

The 450-hp Joy centrifugal has a closed-radiator-type system also, and according to plant personnel, it works well except for several hours a day during extremely hot weather (>90°F). To alleviate this problem, there is a manually operated spray line set up to super cool when necessary. Centrifugal and rotary screws are more sensitive to cooling conditions in both life and performance than industrial reciprocating units. The sprayer is currently working. In the future, some consideration could be given to an automatically controlled high-performance secondary inline cooler between the radiator discharge and the compressor water inlet.

2. In Buildings 133 and 40, there are some Worthington M-Line, single-acting, air-cooled reciprocating units which are not operating under continuous duty. These type units are not well suited to industrial production applications. They are rated very low in power efficiency. One of these is inoperable now; these units should be kept only for emergency back-up air, if at all.

3. In addition to these 50- and 100-hp air-cooled units, there are at least nine 25-hp air-cooled Ingersoll Rand compressors in Building 15; one 15-hp air-cooled Wayne compressor in Building 120; and one 25-hp Champion (Speedair) compressor in Building 120. These types of units are well applied at or near the point of end-use production, particularly where higher than the 85 psig systems pressure is needed, to feed an intermittent demand. They are not continuous duty and should be applied on about a 50% duty cycle for normal life, operating, and maintenance costs. They are not particularly power efficient and should not be run in place of general system units unless higher pressure is required.

4. Well over twenty 5-hp and smaller air-cooled reciprocating compressors are set up on appropriately sized horizontal air receivers and refrigerated air dryers throughout the Arsenal. Most of these are not part of the control system and separate from the main system air. Where a 5-hp or fractional-hp unit is run instead of the general air system, utilization of these units should be questioned unless it is for higher air pressure than the main system. These units are not even close in power efficiency performance to the main air system units.

5. There is also a Breathing Aid compressor and system in Building 110 South and 123 for painting processes. These are well applied and only used when painting is in progress.

6. All units have their own local capacity control system and all, except the 450-HP Joy centrifugal, are set up to automatically start when air is needed and automatically shut off when not needed. This control strategy appears to work very well and is a very positive step in air conservation already taken.

The primary compressed air supply is produced by relatively efficient air compressors that are capable of delivering the 100 psig full load pressure in a continuous manner. The units are well applied. They appear to be in good operating order and well maintained. Key characteristics of the units are summarized in Table 5.

* For more precise performance measures, see OEM curve or measure actual flow and input kW – compare in scfm, unit was down for repairs during the site visit. Data were obtained from plant personnel. Blended electric rate equals $0.09/kWh.

2.2 COMPRESSOR CAPACITY CONTROLS

The two most effective ways to run air compressors are at "Full Load" and "Off." Capacity controls are a means of restricting the output cfm delivered to the system while the unit is still running. This is always a compromise and it is never as efficient as full load on a specific power (cfm/bhp) basis. For details on unloading, see the MISCELLANEOUS SECTION.

Controls for Reciprocating Compressors

Reciprocating compressors are double-acting, water-cooled units with multi-step unloading. This is an efficient compressed air unloading system. Reciprocating multi-step unloading will efficiently translate percentage of "less air used" into almost the same proportional reduction in energy cost. (See article and production curves in the MISCELLANEOUS SECTION.)

The current system has two-step, free air unloading on the Ingersoll Rands and two-step total closure on the Joys. They are very responsive and power efficient. There are also newer electronic Intelysis controls on the IRs.

Controls for Rotary Screw Compressors

The two most common controls used rotary screw compressors are modulation and online/offline. Modulation is relatively efficient at very high loads—and inefficient at lower loads. Online/offline 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., "rotor length adjustment" or "variable displacement" and "variable speed drive") that have very efficient turndown from 100% load to about 60% load.

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 online/offline controls with an automatic electronic upper range modulator on the new IR rotary screw. It is very well applied and installed and appears to be working well.

Controls for Centrifugal Compressors

The two most common controls used for centrifugal compressors are modulation and blow off. Modulation is relatively efficient at very high loads, but will not work much below 75 percent load. After modulation or turndown, the compressor then just blows off excess air. The basic power draw at the blow off point then stays the same regardless of the load. The Watervliet unit has this, plus IGVs to allow efficient turndown.

There are modern electronic control systems that can be applied today that will effectively close off the inlet and will blow the unit down to idle and significantly reduce the kW draw. The Quad II control system installed now is somewhat limited, but the new Quad 2000 by Cooper (Joy) would do this with some system storage and piping modification. There is no reason to pursue this as long as the unit stays in base load and does go into continuing blow off.

The centrifugal units involved have capacity controls capable of translating "less air used" into a comparable reduction in electric cost. These controls will work effectively with current piping and the air receiver storage situation at today’s conditions.

Long-term Recommendation

With the system stabilized and balanced and with the primary air supply centrally located, consider a microprocessor-driven, centralized, full networking electronic control system. This will automatically place the most efficient machine online and assure use of no more than one partial loaded unit at a time. It will operate at a fixed system target pressure.

2.3 AIR TREATMENT AND AIR QUALITY

2.3.1 DRYERS

Aftercoolers

Aftercoolers are mostly water cooled and appear capable of delivering 100°F or lower temperature compressed air to the dryer during all seasons. The new rotary screw unit has a high performance air-cooled aftercooler.

Refrigerated dryers require a refrigeration system to mechanically cool the air. The lowest possible consistent pressure dew point with a non-cycling dryer is +40ºF. Cycling and variable speed-driven dryers not only save power (60-75%), but also can deliver a lower pressure dew point (down to 35-38ºF) when:

Desiccant dryer regeneration equipment removes moisture vapor by "adsorbing" it to desiccant beads (see MISCELLANEOUS SECTION). These dryers can consistently deliver a pressure dew point to -40ºF or lower, which removes much more water 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. Desiccant dryers are usually rated at the same 100ºF inlet, 100 psig conditions. They also require:

The current system has a refrigerated dryer on most of the air compressors, and they all appeared to be well applied and maintained. Those that were in use were running well. There are also two heatless, twin-tower regenerative dryers (670, 730 scfm each), which deliver dryer air to specific areas. These are also relatively well applied and, even though they use 15% purge air, they are equipped with new point removal purge controllers, which will usually reduce this by about 50%.

The centrifugal goes through a 2,500 scfm rated Van Air internally heated twin-tower regenerative dryer which is the most energy efficient type of dryer available except heat of compressors. It takes less intensive energy because of induction compared to the condition heating of the bead with other types and uses much less purge air. It is also equipped with a dew point demand purge controller.

Water or Oil Carryover in System

Water (condensate) and oil carryover problems in the current air system are not significant. The correct way to eliminate water and oil in the air system is to clean and dry the air immediately after it is produced in the compressor room. Then, clean dry air can be stored in a separate air receiver and delivered to the system, as required. Some guidelines for controlling oil and water carryover include the following:

2.3.2 PRE-FILTERS AND AFTER-FILTERS

Pre- and after-filters are generally either particulate or coalescing type, and their use depends on the type of dryer being used and various installation considerations.

Desiccant dryers always require a high-quality coalescing pre-filter to keep liquid oil and water out of the drying tower. They also always require an effective particulate filter after the dryer to keep "desiccant dust" from migrating into the system.

Refrigerated dryers may or may not need pre- and after-filters depending on the piping, type of compressor, and desired degree of cleanliness. If the inlet air is apt to be dirty and fouled with carbon scale, etc., a particulate pre-filter is required. If the inlet air is liable to have significant liquid or heavy oil mist, a coalescing (or combination coalescing particulate) pre-filter may be needed. If oil or water mist is leaving the dryer, a coalescing after-filter may be in order.

Care in selection must be taken in all cases because:

The pre- and after-filter(s) in this system are well applied and apparently well maintained.

2.3.3 AUTOMATIC CONDENSATE DRAINS

The configuration and performance of condensate drains in the plant’s system do not need to be modified. However, there still are some dual-timer drains that should ultimately be replaced with level-actuated ones.

3.1 BASIC SYSTEM HEADER AND PIPING

It is the job of the main header system to deliver compressed air for production use from the compressor area to all sectors of the plant with little or no pressure loss—with 1-2 psig being a reasonable target. It is also desirable that the compressed air velocity in the main headers be kept below 20 fps to allow effective dropout of contaminants and to minimize pressure losses caused by excessive turbulence. The magnitude of the turbulence effect also depends on piping design and layout.

Headers were checked at appropriate points with a single test gauge and there is a pressure loss of approximately 1 psig or less in the header systems. Consequently, we believe that the header system today can deliver the required air to any area without any significant pressure loss. Low-pressure problems encountered may be in the feeds from the header to the area.

3.2 MINIMUM EFFECTIVE SYSTEM PRESSURE

The system is currently running at 83 to 85 psig. However, there are additional direct power cost savings that will accrue from continuing to lower the overall system operating pressure. A steady delivered pressure to the system will allow follow-up programs at each process to establish the lowest effective pressure. This will enhance productivity, quality, and continue to reduce air usage.

The cornerstone of any effective demand-side air conservation program is to identify and operate at the lowest acceptable operating pressure at various sectors and operating units in the plant. This conservation program should be a part of any training, operating, and maintenance procedures.

Check Regulator

Some regulators are probably set at higher feed pressures than necessary for the process, with some regulators set for wide open to full header pressure. Arsenal personnel should always keep certain questions in mind. Is there a minimum effective pressure at operation established at the unit for each product run? If so, is it being adhered to?

In this type of operation, it is very important that the actual inlet pressure to the process be known and that the lowest effective pressure be held steady for the proper product quality. Installation of storage bottles downstream of the regulator may be needed to "close up" the pressure readings at rest and at operation.

>> RECOMMENDED INVESTIGATION – Determine whether regulators and regulated flow at process can be modified to reduce overall system pressure.

3.3 COMPRESSED AIR CONDENSATE HANDLING

Reviewing the condensate handling system, we understand that the condensate goes to water treatment. If this is true, and discharge condensate meets the requirements of the water treatment facility plant, there is no problem. Refer to the Article Reprint – "Do You Know Where Your Condensate Is?" in the MISCELLANEOUS SECTION. However, if condensate is discharging to a storm sewer or in some other manner to ground water (Federal EPA minimum is 10 ppm), or is required to be separated it by the local water treatment facility, this practice should be investigated in detail.

>> RECOMMENDED INVESTIGATION – Review compressed air condensate handling system to ensure compliance with environmental regulations.

3.4 LEAK IDENTIFICATION AND REPAIR

With a plant of this type, an effective leak control program could save 300 cfm or the equivalent of repairing 100 leaks averaging 3 cfm each. On a percentage basis, this leak level is then about the same as leak levels in other plants. A leak level of 300 cfm translates into an annual loss of $30,000 in electric cost @ $100/cfm. A comprehensive leak management program could reduce such levels by 75%, saving up to $22,000 annually.

>> RECOMMENDED INVESTIGATION – Consider implementing a continuing leak identification and repair program with ultrasonic locators.

There should be a continuing cost minimization program in place. Generally speaking, the most effective programs are those that involve the production supervisors and operators in a positive manner working in concert with the maintenance personnel. Accordingly, it is suggested that the program consist of the following:

Short Term – Set up a continuing leak inspection by maintenance personnel so that for a while, each primary sector (see drawings in PLANT SURVEY SECTION) of the plant is inspected once a quarter, or at a minimum, once every six months, to identify and repair leaks. A record should be kept of the findings and overall results. The PROJECT COST SECTION includes a very effective ultrasonic leak locator quotation for the Arsenal’s information.

Long Term -- Consider setting up programs where the production people (particularly the operators and their supervisors) are positively motivated to identify and repair these leaks. One method that has worked well with other operations is to monitor the airflow to each responsible section (perhaps with the use of recording the non-recording flow meters) and to identify the air usage as a measurable part of the operating expense of that area. This usually works best when combined with an effective in-house training and awareness program.

Savings associated with implementing a leak management program include:
Estimated number of leaks	100 leaks
Estimated average leak size	3 cfm per leak
Estimated leak level	300 cfm
Potential value of leak reduction (from system baseline chart)	$100 per cfm
Estimated unit electric savings	$30,000 per year
Recoverable leak losses	75%
Calculated electric savings from leak program	$22,000 per year
Costs associated with implementing a leak management program include:
Leak detection equipment	$2,800
Leak program development and detection equipment training	$1,000
Leak repair (100 leaks @ $30 materials per leak and $50 labor per leak)	$3,000
Total Program Cost	$5,000 plus $1,000 annually for ongoing repairs

3.5 AUTOMATIC BALL VALVES

Some of the most significant areas for leaks in any high-production plant involve shutting off the air supply to machinery when not in use. When these opportunities are found, there are usually some very economical and easy methods to automatically shut off machinery air supply when not in use. The PROJECT COST Section lists some electric-operated automatic ball valves that can be installed in the main feed line to a piece of equipment and be wired in so as to open and close when the machine is powered up or shut off.

3.6 CABINET COOLERS

Cabinet cooling is often required to obtain reasonable life and performance of the electronic equipment in control cabinets. There are various means of accomplishing this cooling: blowing compressed air into the cabinet, vortex coolers, refrigeration units, and heat tube cabinet coolers. Blowing straight compressed air into the cabinet is generally very inefficient.

Vortex coolers can use chilled air with no moving parts and use less air. Vortex coolers should always:

Refrigeration units should be carefully selected and equipped with automatic regulation control. Heat tubes are the most energy efficient when applied and can cool a "sealed cabinet."

There are some cabinet coolers in use in the plant. These may all be replaced with "heat tube" cabinet coolers with a potential savings of 3.5 to 4 kW each.

This section provides a preliminary assessment of the opportunity for gas engine driven compressors at Watervliet Arsenal. The assessment is based on three key design factors:

4.1 GAS ENGINE DRIVEN SYSTEM DESIGN FACTORS

The conceptual design for the gas engine driven system is based on providing about two-thirds of the requirements of the main system at Watervliet. The system will be configured as a hybrid system in conjunction with the existing electric system. In this way, the existing electric system can serve as a backup to the gas engine system, if the gas system has a planned or unplanned shutdown or if the air requirements of the base are suddenly increased.

Using this approach, the Department of Defense can gain experience with not only operating a gas engine driven system, but also integrating it with electric systems to improve overall compressed air system reliability and reduce operating costs. This flexibility is especially important given the increasing uncertainty associated with the price and supply reliability of most energy sources.

Environmental issues are expected to be minimal in this application given the key areas to be addressed in any major project of this nature, particularly on the East Coast. The assessment is based on emitting 2.60 gm/bhp/hr for NOx and 1.75 gm/bhp/hr for CO.

4.2 OPERATING COST COMPARISON

Table 6 summarizes the electrical energy costs for the Main Power House System for both the current system (with the centrifugal compressor operating) and the proposed NGEDAC system. Annual electrical costs are $308,000 for the current system and $210,000 for the modified system, a savings of $98,000 annually, based on the cost of gas at $5 per million Btu. Adding or reducing the gas cost by $1 per million Btu would change the savings level by about $25,000 annually.

A two-year comprehensive maintenance contract is approximately $15,000 higher for the proposed system when compared with the existing system. Quoted NGEDAC system maintenance contract levels range from $3.15 to $3.85 per operating hour. The estimate is based on $75.00 per labor hour and 17,000 operating hours over two years. The price includes all parts, fluids, scheduled and unscheduled maintenance.

The net annual savings for the proposed NGEDAC system incorporating both the lower energy costs and higher maintenance costs is $61,000 to $91,000, depending on whether the centrifugal unit is used as the baseline electric system.

4.3 CAPITAL COST ASSESSMENT

Table 7 summarizes the capital cost estimates for two configurations of the natural gas engine-driven systems: a Caterpillar G3408SITA engine and a Waukesha F18GLD engine. The capital costs include the catalytic converter for the Caterpillar engine but not the Waukesha since it is a specially built lean machine. The costs include a placeholder estimate for all installation and freight costs. The capital costs also include a budget to erect a special soundproof enclosure to reduce noise levels beyond that attained with the hospital mufflers.

Without consideration to potential cost reductions resulting from negotiating or utility rebates, the capital costs for the natural gas systems are on the order of $350,000 to $400,000.

4.4 "AIR COMPRESSOR ADVISOR" ASSESSMENT

As a check on the project assessment, "Air Compressor Advisor – Version 1.82" was used to generate an economic evaluation of the proposed system. "Air Compressor Advisor" was developed by Energy International, Inc. in concert with the Gas Research Institute and the Energy Solutions Center Inc.

The results are provided for three scenarios.

The "Baseline Scenario" (reported in Table 8) reflects the parameters used in the original analysis (Table 3) for the "Current System – Typical Day" – i.e., with the centrifugal system operating. The bottom-line numbers, which show a net savings of $91,749 annually, are consistent with those of the original analysis. The payback is 4.09 years based on a capital cost of $375,000 for a hypothetical or "average" NGEDAC system.

The "Caterpillar" Scenario (reported in Table 9) adjusts the operating parameters to represent the Caterpillar system with the input parameters shown in Table 7. Compared with the "average" system used for the baseline, the Caterpillar system has a slightly better energy efficiency, but a higher capital and maintenance cost. The net annual savings estimate is $93,492 and the payback is 4.46 years.

The "Waukesha" Scenario (reported in Table 10) adjust the parameters to represent the "Waukesha" system with the input parameters shown in Table 7. Compared with the "average" system used for the baseline, the Waukesha system has a much better energy efficiency, lower capital and maintenance costs, but a higher horsepower requirement due to its being air-cooled. The net annual savings estimate is $98,478 and the payback is 3.50 years.