Screw Compressors: Selection Considerations for Efficient Operation - II

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1 THE COLD FRONT Vol. 2 No. 4, 2002 IN THIIS ISSUE Screw Compressors 1-4 Selection Considerations for Efficient Operation: Part II Upcoming ammonia classes 2 Noteworthy 2 Ammonia Refrigeration: 5-6 Uncovering Opportunities for Energy Efficiency Improvements R&T Forum Reminder 6 IRC Staff Director Doug Reindl 608/ or 608/ Jim Elleson 608/ Todd Jekel 608/ Dan Dettmers 608/ Phone 608/ Toll-free FAX 608/ Mail 949 East Washington Ave Suite # Website info@irc.wisc.edu Screw Compressors: Selection Considerations for Efficient Operation - II (This is the second in a two-part series on screw compressor selection for efficient operation. In this article, we present a summary of our findings in conducting a more detailed analysis of screw compressor efficiency characteristics. In the analysis, we considered the performance characteristics of both fixed volume ratio and variable volume ratio machines the two most common screw compressor configurations used in industrial refrigeration today. For brevity, we have omitted details of the analysis in this article. Those interested in learning more about the analysis summarized here are encouraged to download our TechNote Selection of Screw Compressors for Energy Efficient Operation from the IRC website at: Screw Compressor Selection Considerations There are several factors that one should consider when evaluating potential selections for a new screw compressor or changing the operating conditions of an existing screw compressor. Items warranting your consideration include: 1. What is the expected range of operating suction and discharge pressures in either a. single stage or two stage operation (booster or high-stage) b. swing duty (e.g. boosters operating as a single stage) c. load variability over time (large pull-down loads vs. relatively constant loads) 2. What is the climate type and system minimum head pressure constraints? 3. What is the basis for oil separator selection/sizing? 4. What is the method for oil cooling? 5. Have system and package losses (that include effects of check valves, service valves, strainers, and oil separators installed around the compressor) been properly accounted for? 6. What is the expected maintenance costs over machine s life? One of the key selection criteria is the performance, in terms of both capacity and efficiency, of the compressor over the range of expected operating suction and discharge 1

2 pressures. Many compressors operate with a fixed or narrow range of compressor suction pressures; however, some compressors are designed for swing duty. Swing compressors are configured to serve loads at different suction levels. Regardless of suction conditions, all compressors will operate over a range of discharge pressures. Variation in the spread of operating discharge pressures will depend on seasonal fluctuations in weather and constraints in operation for the system. Screw compressors that operate over a wide range of both suction and discharge pressures pose the greatest challenge for selection when considering fixed volume ratio machines. Table 1 below provides suggested volume ratio selection ranges for fixed volume index (Vi) compressors configured for single-stage duty that are expected to operate over high, medium, and low ranges in head pressure. In all cases, the data in Table 1 assumes that the design or maximum saturated condensing pressure is 180 psig (95 F saturation temperature). The head or condensing pressure of a system is dictated, in part, by weather. Specifically, the condensing pressure will be influenced by the outside air wet bulb temperature. For a fixed load, as the outside air wet bulb temperature decreases, the system s saturated condensing pressure (and temperature) decreases until its minimum is reached. The minimum saturated condensing pressure for a given system depends on a number of specific constraints such as: presence of thermostatic expansion valves, hot gas defrost (main and run-out sizing, defrost relief regulator setpoints, etc.), liquid injection oil cooling, sizing of high pressure liquid lines and others. Figure 1 illustrates the theoretical frequency of saturated condensing temperatures for a system with a lower limit on condensing pressure constrained at 100 psig (63 F) operating in. This particular system will spend 3,925 hours or 45% of the year operating at its minimum condensing pressure. From an energy efficiency perspective, it would be desirable to select a compressor that offers good performance at these low head pressure conditions since they represent the greatest frequency of operation over the year. This means that a lower volume index on a fixed volume ratio compressor would be preferred over a higher volume index compressor that would better match design conditions more closely than Upcoming Ammonia Courses Process Safety Management Audits January 22-24, 2003 Energy Efficiency Improvement Strategies February 10-12, 2003 Introduction to Ammonia Refrigeration March 5-7, 2003 Ammonia Refrigeration System Safety April 23-25, 2003 Design of Ammonia Refrigeration Systems September 15-19, 2003 Introduction to Ammonia Refrigeration October 8-10, 2003 Ammonia Refrigeration Piping October 27-29, 2003 Intermediate Ammonia Refrigeration December 3-5, 2003 IRC members receive a discounted fee for enrollment in UW ammonia refrigeration courses. See for more information on the courses. Noteworthy The final version of the Ammonia Sensor Overview is now available on the website for IRC member download. Send items of note for next newsletter to Todd Jekel, tbjekel@facstaff.wisc.edu. 2

3 Table 1: Fixed volume ratio screw compressor selection ranges for single-stage duty. Saturated Suction Temperature [ F] High psig (95-65 F SCT 2 ) Head Pressure Range 1 Medium psig (95-70 F SCT) Low psig (95-80 F SCT) or higher 5.0 or higher 5.0 or higher The term Head Pressure Range is defined as the difference between the maximum (always assumed to be 180 psig or 95 F in this analysis) and minimum saturated condensing pressure (temperature). 2 SCT is saturated condensing temperature. off-design conditions. Keep in mind that any compressor selection will need to meet the peak load requirements for the application. In the course of selecting a screw compressor for peak performance during offdesign conditions, oil separator sizing becomes important because the full-load volume flow rate of gas at the discharge of the compressor increases as the head pressure decreases. The increase in gas volume flow rate is attributable to the combination of gas specific volume increasing (primary) and compressor capacity increasing (secondary) as the discharge pressure decreases. The discharge volume flow rate will also increase with an increase in suction pressure because the mass flow rate of refrigerant through the compressor increases. Individually or combined, lowering condensing pressures and raising suction pressures are two widely pursued strategies for effectively improving the energy efficiency of refrigeration systems. Both have the net effect of increasing the volume flow rate of gas through the compressor and through the oil separator. If the volume flow rate of gas through the oil separator exceeds the rate assumed in the sizing of the separator, the efficiency of oil separation from the compressor discharge gas stream will decrease. As the oil separation efficiency decreases, the concentration of oil leaving the separator and migrating out into the system increases. This results in the need for greater frequency of oil draining from points out in the system with corresponding increases in labor costs and risk. For efficient and flexible compressor operation, work with your compressor manufacturer to select an effective oil separator for operation at full-load with the maximum expected suction pressure coincident with the lowest expected discharge pressure. The choice of oil cooling methods also influences the compressor efficiency. Liquid injection oil cooling is the least first cost option; however, it results in a loss of compressor capacity and necessitates a higher minimum head pressure. Higher head pressures are needed to maintain the required pressure differential across the oil cooling thermostatic expansion valve (TXV) to maintain control authority. (Note that other valves can be used in place of the TXV to relax the minimum head pressure requirement for liquid injection.) In addition, maintenance costs for liquid injected oil cooled compressors will be higher than alternative oil cooling methods such as thermosiphon oil cooling. Thermosiphon oil cooling is the most efficient and lowest compressor maintenance cost option but has the largest capital cost. The payback thermosiphon oil cooling is often less than 3 years when including energy and maintenance cost savings. It is worthwhile to recognize that the selection of components around the compressor itself will influence its efficiency when integrated into the system. All compressor manufacturers have provisions for selecting alternative trim components including service valves, check valves, and strainers. Those options include low pressure drop (also called oversized ) components for minimum parasitic losses. Finally, maintenance costs for compressor selections should be included with energy costs 3

4 Hours per Year Dry Operation Wet Operation Saturated Condensing Temperature [F] Figure 1: Frequency analysis of expected condensing temperatures for an evaporatively condensed industrial refrigeration system in. in the economic analysis of alternatives being considered. In general, maintenance costs for liquid injection oil cooled compressors are greater than thermosiphon (or water-cooled) oil cooled counterparts. Maintenance costs for variable volume ratio screw compressors are higher than fixed volume ratio machines. The increased maintenance is attributed to the additional components needed for volume ratio control. In addition to capital cost, be sure you take into account the following considerations when selecting your next screw compressor. The compressor should be able to properly operate at the highest expected suction pressure coincident with the lowest expected discharge pressure. This means that the oil separator, oil cooling method and valve train should not limit the envelope of its operation. The compressor should be able to meet the capacity required while operating at the lowest expected suction pressure and its design condensing pressure. In comparing compressor alternatives, consider the efficiency of each while operating at the suction and discharge conditions expected during the majority of the year (as illustrated in Figure 1). For fixed volume ratio machines, select the volume ratio to match the suction and discharge conditions expected during the majority of yearly operating hours but check to be sure it will meet the peak load requirements at design conditions (see Table 1). Variable volume ratio compressors will always deliver better energy performance but at a higher capital and maintenance cost compared to fixed volume ratio machies. Consider specifying and installing thermosiphon oil cooling due to its desirable efficiency and maintenance characteristics. If you have an opportunity, perform a lifecycle analysis for alternative compressor selections. The life-cycle cost should include capital, operating, maintenance, and replacement costs over a specified time horizon. Keep in mind that, based on evidence from the field, some ancillary equipment alternatives (such as liquid injection oil cooling) will lead to shortened compressor lifetimes when compared to others (such as thermosiphon oil cooling). For additional details on compressor selection for efficient operation, download our TechNote: If you have questions or comments on this article, please contact Doug Reindl at (608) or dreindl@wisc.edu. 4

5 If energy efficiency is of interest to you, consider attending the upcoming course: Ammonia Refrigeration: Uncovering Opportunities for Energy Efficiency Improvements February 10-12, 2003 Why Should You Consider This Course? Many companies are seeing increases in energy costs impact their operations budgets. Focusing on improving the efficiency of industrial refrigeration systems offers the opportunity to reduce operations budgets since they are one of the single largest consumers of energy in many facilities. Four key learning objectives will guide your instruction and enhance your ability to deliver energy savings back on the job: understand factors that influence system energy efficiency an energy costs identify methods for improving system energy efficiency develop an action plan validate your efficiency improvements Establishing goals and priorities are important preliminary steps in the overall process aimed at achieving more energy-efficient refrigeration systems. However, many owners and operators of industrial refrigeration have broader concerns and interests that may include: decreasing overall plant energy consumption and energy costs increasing production capability maintaining quality of products manufactured and stored maximizing capital utilization reducing maintenance costs minimizing environmental impacts and off-site consequences minimizing lost work days By helping you improve the energy efficiency of your refrigeration systems, this course can also enhance many of the above-mentioned attributes. This course has been developed for plant engineers, energy managers, utility managers, advanced refrigeration system operators, design engineers, contractors, energy service providers and others interested in improving the energy efficiency of industrial refrigeration systems. In a course of this nature and length, we do assume attendees will have a working knowledge of industrial refrigeration. To obtain maximum benefit, you should understand the basics of industrial refrigeration systems, including refrigerant properties, property behavior, system components and configurations, and operational strategies. This is the perfect course for those who have already taken the UW s Introduction and Intermediate Ammonia Refrigeration classes. Key Topics for Energy Efficiency Single- and multiple-stage compression systems (review) 5

6 Compressor and condenser selection, operation and control strategies Evaporators and evaporator piping Heat recovery options Utility rates and rate structures Benchmarking systems Maintenance considerations Half the class time will be devoted to helping you identify specific energy-saving opportunities for improving your plant s refrigeration systems. Past Participants Say I will be able to save the cost of this course in one month. Steve Ardiana, Gerber Products The ideas I gained should show large savings. Tom Maxwell Ben & Jerry s Download a complete brochure including a course outline and registration materials by clicking here. SEASONS GREETI INGS We at the IRC hope that you and your family have a safe and happy holiday season IRC Research & Technology Forum Reminder Just a reminder to register for the 3rd Annual Research & Technology Forum to be held on January 8th, 2003 at the Pyle Center on the University of Wisconsin-Madison campus. This year s forum begins at 9:00 am CST and ends at 5:00 pm. There will presentations by IRC staff, IRC member organizations, and industry experts. There is an open invitation to the forum to anyone interested in industrial refrigeration. Registration materials can be downloaded from our website ( IRC members and prospective members should plan to stay for the IRC s annual business meeting to be held on January 9, Help us out! We are nearly at the capacity of our reserved meeting space for this event. We would be happy to accommodate a larger group but we do need advanced notice. If you plan on coming, please register soon so that we can accommodate everyone. The registration deadline is December 16, If you have any questions regarding travel arrangements or presentation topics, please contact the IRC at or info@irc.wisc.edu. Join the IRC in 2003 The IRC s mission is to improve the safety, reliability, efficiency, and productivity of industrial refrigeration systems. Our vision is to make continuous progress toward improving the safety, productivity, and efficiency of the systems and technologies that form the foundation of the industrial refrigeration industry. We believe these improvements can best be accomplished through a balanced and coordinated effort that balances research, education and technical assistance. Do these goals match your company s? Does your company have needs in the areas of education, technical assistance or strategic planning for refrigeration? If so, please contact us to see how joining the IRC can benefit you and your company. 6