CompAir_ReclaimHeat

7/28/2019 CompAir_ReclaimHeat

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Compressed Air Systems: Reclaim Heat



AR X: Use Exhaust Heat from Air Compressors for Space Heating in

PlantAnnual Savings Project Cost Simple

Payback Resource CO2 (lb) Dollars Capital Other Total

Natural Gas 1,350 mmBtu 152,550 $8,050 $1,000 $800 $1,800 3 months

Analysis

Four air-cooled 30-hp rotary-screw air compressors provide compressed air for the plant.The compressors are located in an equipment room also containing the compressed air dryers, and about 8 vacuum pumps. We measured the temperature of the exhaust air from each of the compressor cooling units to be about 150 F. Currently, the hot air fromeach compressor cooling unit is ducted through the exterior wall year round. In addition,outdoor air is brought in to cool the room through thermostatically controlled damperslocated at each end of the room and the room air is exhausted through a ceiling fan.

Recommendation

We recommend reconfiguring the ventilation in the compressor room to route the wasteheat from the compressor cooling units into the plant to offset space heating loads duringthe winter months. To do this, we suggest disabling the outdoor air dampers and theceiling fan and installing a wall fan above the compressor room door and a wall vent atthe other end of the room. The outdoor air dampers and ceiling fan should be openedagain in the summer to cool the room. The diagrams below illustrate the recommendedconfiguration.

Estimated Savings

In the Air Compressor Analysis section of the report, we calculated that the total average power draw of all four compressors is about 105 kW. We conservatively estimate that75% of the compressor’s energy consumption is rejected as heat by the compressor cooling system and that all of this is recoverable for space heating. According to

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maintenance personnel, the compressors operate around the clock for 50 weeks per year.Assuming that heat is routed into the plant for 24 weeks per year, the recoverable heatfrom the air compressors would be about:

105 kW x 75% x 24 hours/day x 7 days/week x 24 weeks/year = 317,520 kWh/yr

317,520 kWh/yr x 3,413 Btu/kWh = 1,080 mmBtu/yr

Assuming that the natural gas-fired air makeup units in the plant are 80% efficient, thenatural gas savings from capturing rejected heat from the compressors would be about:

1,080 mmBtu/year / 80% = 1,350 mmBtu1,350 mmBtu x $5.96 /mmBtu = $8,050 /year

This would reduce CO2 emissions by the electric utility by about:

1,350 mmBtu/year x 113 lb CO2/mmBtu = 152,550 lb CO2 /yr

Estimated Implementation Cost

Maintenance estimates that it would cost about $1,000 for materials and about $800 for labor to install ductwork, dampers and a fan in order to properly direct warm air from theair compressors into the plant.

Estimated Simple Payback

($1,800 / $8,050 /year) x 12 months/year = 3 months



AR 3754: Reclaim Heat from Air Compressors

Annual Savings Project Simple

Resource CO2 (lb) Dollars Cost Payback

Electric Demand -17.3 kW -$1,196

Electric Use -41,520 kWh -95,496 -$955Gas Use 47,120 ccf 527,477 $16,492

Net 432,248 $14,341 $15,000 13 months

Analysis

Compressed air is supplied by two 500-hp water-cooled reciprocating compressors. Theycurrently run 5 days per week. One unit runs continuously at 75% loading and the other unit cycles to supply the system peaks.

Recommendations

Maintenance personnel report that the cooling water leaving the compressor is nearly

100°F. This is warm enough to use for space heating. We recommend that you install aheat exchanger and fan, and reclaim this energy for plant heating in winter.

Estimated Savings

Compressor electricity useEach 500-hp air compressor, with motor efficiency of 95% and at full load, would drawabout:

500 hp x 0.746 kW/hp / 95% = 392.6 kW

We measured the loading times for your compressors. One unit operates continuously at

75% loading. Hence, power draw for this unit is about:

75% x 392.6 kW = 294.5 kW

The second unit's control panel showed that it fluctuates between 100%, 75%, and 50%loading. For a 40-second cycle, we measured the cycle time as 10 seconds at full load, 5seconds at 75% load and 25 seconds at 50% load. Assuming that this is normal operation,the unit's average power draw is about:

[392.6 kW x (10 sec / 40 sec) x 100% load] + [392.6 kW x (5 sec / 40 sec) x 75% load] +[392.6 kW x (25 sec/40 sec) x 50% load] = 257.7 kW

Reclaiming Heat From CompressorsMaintenance personnel report that the cooling water leaving the compressor is about

100°F. This is warm enough to be used for space heating in winter. Industry data and our

measurements at other plants indicate that the air compression process is less than 25%efficient; over 75% of the energy put into a compressor is given off as "waste heat". Thus,the total heat output of the compressors is about:



(294.5 + 257.7) kW x 75% x 3,413 Btu/kWh / 1,000,000 Btu/MBtu = 1.413 MBtu/hr 1.413 MBtu/hr x 24 hours/day x 5 dy/wk x 50 wk/yr = 8,481 MBtu/yr

Assuming a five-month heating season, the heat available to displace gas space-heating isabout:

8,481 MBtu/yr x 5/12 = 3,534 MBtu/yr

Assuming that the existing space-heating equipment is 75% efficient, the natural gasenergy saved would be about:

3,534 MBtu/yr / 75% = 4,712 MBtu/yr 4,712 MBtu/yr x $3.50 /MBtu = $16,492 /yr

From our analysis of your utility bills, we estimate that you use about 12 MBtu per day,or 144 MBtu per year for process (non-space heating) applications. Thus, the quantity of

natural gas used for space heating is about:

77,838 MBtu/yr - 144 MBtu/yr = 77,694 MBtu/yr

Thus, waste heat from the compressors could displace nearly 6% of the plant's spaceheating requirement.

To utilize this heat, you would have to install heat exchangers with fans. Two 10-hp fanswould be required for appropriately sized heat-exchangers. Assuming the motor would be 80% loaded and 86% efficient, the electricity costs would be about:

2 fans x 10 hp/fan x 0.746 kW/hp / 86% = 17.3 kW17.3 kW x $13.83 /kW-mo x 5 mo/yr = $1,196 /yr 17.3 kW x 24 hours/day x 5 dy/wk x 20 wk/yr = 41,520 kWh/yr 41,520 kWh/yr x $0.023 /kWh = $955 /yr

The net savings from reclaiming heat would be about:

$16,492 /yr - $1,196 /yr - $955 /yr = $14,341 /yr


A vendor of industrial heat exchangers estimates that a "fluid cooler", operating at 35gallons per minute water flow and capable of rejecting 850,000 Btu/hr would cost $5,000.Two units would be able to handle the 1.413 MBtu/hr load that we estimated above. Weestimate that installation would add another 50% to the cost. If so, the installed costwould be about $15,000.

Estimated Simple Payback

Simple payback = $15,000 cost / $14,341 /yr savings x 12 months/year = 13 months



AR 3004: Reclaim Waste Heat from Air Compressor

Present Recommended Annual Savings

Gas Consumption 1,600 ccf; $784

Implementation Cost $1,060

Simple Payback 16 months

Analysis

At least 75% of the energy consumed by air compressors is given off as heat. This“waste heat” can be reclaimed by routing the compressor cooling air into the facility, tohelp heat the plant in winter. Recommendations

We recommend installing a circulating fan and return air grille in the wall between thecompressor room and the plant. This would entail cutting two openings in the wall. Acirculating fan would be installed in one opening. The fan would direct warm air from

the compressor room into the plant during the winter to provide free space heating. Agrille would be installed in the other opening to allow plant air to return to thecompressor room for reheating. The ventilating fan in the exterior wall would direct hotair outside in the summer.

Estimated Savings

In AR # 4, we estimate the average compressor power draw to be about 37.5 kW. Weestimate that about 50% of the electrical power to the compressor can be recovered anddirected into the plant as useful space heat. Assuming a 25-week heating season and an80% average heating system efficiency, the natural gas space-heating energy savingswould be about:

80%

1x

Btu100,000

ccf 1x

kWh

Btu3,413x50%x

year

weeks25x

week

days5x

day

shifts2x

shift

hours8kW x37.5

The cost savings would be about:

year

$784

ccf

$0.49x

year

ccf 1,600=


Product literature (see for example: Grainger, 1997) advertises industrial circulating fansfor about $300, and damper grilles for about $100. In addition to these equipment costs,

we estimate that it would take about 40 man-hours at $14 per hour labor cost, plus about$100 in supplies to install a fan and return grille. The total cost of implementation would be about $1,060.

Estimated Simple Payback months16months/yr 12/yr x$784/$1,060 =

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CompAir_ReclaimHeat