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Thank you, sponsors Our online sponsors PLATINUM GOLD 1 - - PDF document

Jan 17, 2023 312 likes •401 views

6/28/2016 COMPRESSOR EFFICIENCY AND SEQUENCING Presented by: Rob Travis, P.E. 1 Thank you, sponsors Our online sponsors PLATINUM GOLD 1 6/28/2016 TOP 4 LOW COST COMPRESSOR ACTION ITEMS 1. Increase Suction Pressure 2. Part Load Control

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SLIDE 1

6/28/2016 1

COMPRESSOR EFFICIENCY AND SEQUENCING

Presented by: Rob Travis, P.E.

1

PLATINUM GOLD

Our online sponsors

Thank you, sponsors

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SLIDE 2

6/28/2016 2

TOP 4 LOW COST COMPRESSOR ACTION ITEMS

  • 1. Increase Suction Pressure
  • 2. Part Load Control and VI
  • 3. Utilize Economizing
  • 4. Compressor Sequencing

3

Motor Microprocessor control panel Oil separator Compressor Suction Discharge

Courtesy: Frick

  • 1. INCREASE SUCTION PRESSURE

4

System lift is the compression ratio of the compressor’s discharge pressure to its suction pressure. The greater the ratio, the more work the compressor needs to do, and the greater the input power required to do it.

  • Raise suction: More capacity (TR)
  • Lower discharge: Less power (BHP)
  • Reduce “lift”: Higher efficiency (BHP/TR)

Condensing Temperature BHP TR BHP/TR BHP TR BHP/TR BHP TR BHP/TR 75°F 269 108 2.49 347 317 1.09 350 773 0.45 85°F 301 103 2.92 389 308 1.26 419 751 0.56 95°F 336 99 3.39 436 296 1.47 496 729 0.68 105°F 374 94 3.98 486 290 1.68 582 705 0.83 Suction Temperature

  • 40°F

0°F +40°F

Every compressor has tables or curves such as detailed below.

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SLIDE 3

6/28/2016 3

INCREASING SUCTION PRESSURE

5

Rule of thumb savings:

  • 2% compressor efficiency gain per degree‐F increase in

suction temperature Common approaches:

  • Simply raise set‐point
  • Reduce pressure drop or other bottlenecks
  • Reduce evaporator approach (increased coil sizing)
  • Correctly match suctions to loads (multiple suction

systems)

COMMISSIONING SUCTION PRESSURE

6

Low-Temperature Suction Commissioning History

(10) (5)

  • 5

10 15 20 03/01 03/21 04/10 04/30 05/20 06/09 06/29 07/19 08/08 08/28 Date psig (+) or inHg (-) Commissioning Improvements

Low-Temperature Suction Commissioning History

(10) (5)

  • 5

10 15 20 03/01 03/21 04/10 04/30 05/20 06/09 06/29 07/19 08/08 08/28 Date psig (+) or inHg (-) Commissioning Improvements

Sometimes this process takes months to work through

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SLIDE 4

6/28/2016 4

  • 2. PART LOAD CONTROL AND VI

7

  • Avoid running screws unloaded
  • Need at least one efficient trimming compressor per

suction

  • Reciprocating compressor
  • VFD‐equipped screw compressor
  • Typically slowed to 20%‐50% speed
  • Once at minimum speed, the slide valve provides additional

capacity reduction

  • Needs to be large enough to handle typical system swings

in load

  • Need optimizing control system algorithms, not simple
  • rdered sequencing
  • Mix‐and‐match with dedicated trim

SCREW PART‐LOAD POWER

8

Typically, part‐load worsens as pressure ratios widen.

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SLIDE 5

6/28/2016 5

TYPES OF SCREW COMPRESSOR VI

9

  • Fixed Volume Index (VI)
  • Determined by physical configuration from the

manufacturer.

  • Manually‐adjusted VI
  • Adjustable based on typical conditions and

instructions in user manual.

  • Auto VI
  • Automatically adjusted on actual conditions by

microprocessor panel.

Typical VI Range: 2.1(low)‐5.5(high)

UNDER OR OVER‐COMPRESSION

10

Examples of Over or Under-Compression 20 40 60 80 100 120 140 160 Compression Process Pressure (psi)

Beginning of Compression End of Compression

Suction Pressure Discharge Pressure Over-Compression Under-Compression

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SLIDE 6

6/28/2016 6

IMPACT OF VI ON SCREW EFFICIENCY

11

  • Most efficient volume ratio varies with pressure

ratio across compressor

  • Consider following screw compressor example

at two different suction temperatures:

In this case, efficiency would increase by 50% Capacity is not influenced by volume ratio, just power

2.2 2.6 3.7 4.8 2.2 2.6 3.7 4.8 85 5.4 199.7 314.2 296.7 284.5 291.7 1.57 1.49 1.42 1.46 40 85 2.3 481.4 301.9 319 385.4 455.6 0.63 0.66 0.88 0.95 Suction Temp. (°F) Press. Ratio Tons

  • f

Refrig. Brake Horsepower Volume Ratio BHP/TR Volume Ratio Cond. Temp. (°F)

In this case, the difference in power is 153.7 hp!

  • 3. UTILIZE ECONOMIZING

12

  • Economizing improves efficiency by 10% to 15%
  • Cascade liquid refrigerant from one suction to the next

to maximize efficiency

  • Unloaded screw compressors have no sideport capacity
  • VFD control helps this!

HPR HTR LTR

COMP From Condenser To Condenser Sideport Economizer LTS Suction

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SLIDE 7

6/28/2016 7

  • 4. COMPRESSOR SEQUENCING

13

  • Need at least one efficient trimming compressor

per suction

  • Utilize mix‐and‐match strategy with compressors of

different size. Unit 1 2 3 4 5 6 7 100 hp X X X X 200 hp X X X X 400 hp X X X X Stage Number Unit 1 2 3 4 5 6 7 100 hp X X X X 200 hp X X X X 400 hp X X X X Stage Number

  • Strategy should utilized dedicated trim compressors,

such as VFD‐driven screw or reciprocating compressors.

CONCLUSION

14

  • Higher suction pressure increases compressor
  • efficiency. Consider seasonal adjustments or floating

suction pressure control.

  • Use most efficient part‐load compressor as the

dedicated trim compressor.

  • Ensure VI is properly set or automatically adjusting on

each compressor.

  • Utilize economizing wherever possible.
  • Sequence compressors based on capacity and load

and not simple staging