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 and VI 3. Utilize Economizing 4. Compressor Sequencing Motor Suction Discharge Compressor Microprocessor control panel Oil separator Courtesy: Frick 3 1. INCREASE SUCTION PRESSURE 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) Every compressor has tables or curves such as detailed below. Suction Temperature Condensing -40°F 0°F +40°F 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 4 2
6/28/2016 INCREASING SUCTION PRESSURE 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) 5 COMMISSIONING SUCTION PRESSURE Low-Temperature Suction Commissioning History Low-Temperature Suction Commissioning History 20 20 Commissioning Commissioning Improvements Improvements 15 15 10 10 psig (+) or inHg (-) psig (+) or inHg (-) 5 5 - - (5) (5) (10) (10) 03/01 03/01 03/21 03/21 04/10 04/10 04/30 04/30 05/20 05/20 06/09 06/09 06/29 06/29 07/19 07/19 08/08 08/08 08/28 08/28 Date Date Sometimes this process takes months to work through 6 3
6/28/2016 2. PART LOAD CONTROL AND VI ‣ 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 ordered sequencing Mix‐and‐match with dedicated trim � 7 SCREW PART‐LOAD POWER Typically, part‐load worsens as pressure ratios widen. 8 4
6/28/2016 TYPES OF SCREW COMPRESSOR VI ‣ 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) 9 UNDER OR OVER‐COMPRESSION Examples of Over or Under-Compression 160 Beginning of Compression 140 Discharge Pressure 120 Over-Compression Pressure (psi) 100 End of Compression 80 60 Under-Compression 40 20 Suction Pressure 0 Compression Process 10 5
6/28/2016 IMPACT OF VI ON SCREW EFFICIENCY ‣ Most efficient volume ratio varies with pressure ratio across compressor ‣ Consider following screw compressor example at two different suction temperatures: Suction Cond. Tons Brake Horsepower BHP/TR Volume Ratio Volume Ratio Temp. Temp. Press. of (°F) (°F) Ratio Refrig. 2.2 2.6 3.7 4.8 2.2 2.6 3.7 4.8 0 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 Capacity is not In this case, the In this case, efficiency influenced by volume difference in power is would increase by 50% ratio, just power 153.7 hp! 11 3. UTILIZE ECONOMIZING ‣ 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! � Sideport Economizer From Condenser LTS Suction COMP To Condenser HTR HPR LTR 12 6
6/28/2016 4. COMPRESSOR SEQUENCING ‣ Need at least one efficient trimming compressor per suction ‣ Utilize mix‐and‐match strategy with compressors of different size. Stage Number Stage Number Unit Unit 1 1 2 2 3 3 4 4 5 5 6 6 7 7 100 hp 100 hp X X X X X X X X 200 hp 200 hp X X X X X X X X 400 hp 400 hp X X X X X X X X ‣ Strategy should utilized dedicated trim compressors, such as VFD‐driven screw or reciprocating compressors. 13 CONCLUSION ‣ 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 14 7
Recommend
More recommend
