IRC Staff. Director Doug Reindl 608/ or 608/

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1 The Cold Front IN THIIS ISSUE Liquid Injection vs. 1-6 Thermosiphon Oil Cooling of Screw Compressors Upcoming Ammonia Classes 2 Noteworthy 2 Join the IRC in R&T Forum Advanced 7 Notice IRC Staff Director Doug Reindl 608/ or 608/ dreindl@wisc.edu Jim Elleson 608/ jselleson@wisc.edu Todd Jekel 608/ tbjekel@wisc.edu Dan Dettmers 608/ djdettme@wisc.edu Toll-free Phone 608/ FAX 608/ Mail 1415 Engineering Drive Room 2342 Madison, WI Vol. 3 No. 3, 2003 LIIQUIID INJECTIION VS. THERMOSIIPHON OIIL COOLIING OF SCREW COMPRESSORS In this article we focus on the energy benefits associated with thermosiphon oil cooling. We begin with a brief introduction of the role of oil and oil cooling in today s screw compressor technologies and then proceed to a case study on a recent conversion retrofit project. INTRODUCTION Oil is an essential ingredient to make today s screw compressor technologies functional. Oil is primarily used as a lubricant and sealant. Bearings and rotors ride on a thin film of oil to minimize wear. Oil also serves as a medium to seal the mating surfaces of rotors and the enclosing housing to minimize internal leakage of the vapor being compressed; thereby, maintaining high compression efficiency. Because industrial refrigeration compressors operate in an oil-flooded configuration, oil influences other aspects of compressor operation. As oil circulates through compressors at a relatively high flow rate (of gpm per 100 hp ), it absorbs some of the heat of compression resulting in lower compressor discharge temperatures. One only has to consider the difference in discharge temperatures between reciprocating ( F) and screws ( F) to understand the heat sink action of oil. Finally, the movement of oil offers a mechanism to flush contaminants from the mating surfaces within the compressor and remove them by filtration. Pillis, J.,W., 1998, Basics of Operation, Application & Troubleshooting of Screw Compressors, Frick. Website info@irc.wisc.edu heat of compression is the rise in refrigerant vapor temperature experienced as it undergoes the compression process 1

2 In order to maintain acceptable oil temperatures, the portion of the compressor s heat of compression absorbed by the oil needs to be removed in some way. This function is provided by a screw compressor oil cooling system. DESCRIPTION Oil cooling methods can be categorized as either internal or external to the compressor. Internal oil cooling is often called liquid injection since it utilizes liquid refrigerant for direct oil cooling within the compressor. External oil cooling relies on the use of a secondary medium to cool oil outside of the compressor. Let s look at how each alternative works. Liquid injection oil cooling (LIOC) and its variants rely on high pressure liquid refrigerant fed into the compressor through a port drilled into the housing in order to cool the oil. As the liquid refrigerant enters the liquid injection port within the compressor, it expands and cools. The cool liquid refrigerant that remains after expansion absorbs heat from the hot oil and partially-compressed refrigerant vapor. The liquid refrigerant feed rate is modulated to maintain the compressor discharge temperature at the required oil supply temperature, generally 130 F. Valve types for metering in liquid injection arrangements include thermal expansion, motor controlled expansion, and temperature regulating valves. The following are consequences associated with liquid injection oil cooling arrangements: Upcoming Ammonia Courrses Introduction to Ammonia Refrigeration October 8-10, 2003 Madison, WI Ammonia Refrigeration Piping October 27-29, 2003 Madison, WI Intermediate Ammonia Refrigeration December 3-5, 2003 Madison, WI Process Safety Management Audits January 6-8, 2004 Madison, WI Ammonia Refrigeration: Uncovering Opportunities for Energy Efficiency Improvements February 11-13, 2004 Madison, WI Introduction to Ammonia Refrigeration March 10-12, 2004 Madison, WI Ammonia Refrigeration System Safety April 14-16, 2004 Madison, WI Design of Ammonia Refrigeration Systems for Peak Performance and Efficiency September 13-16, 2004 Madison, WI See for more information. Noteworthy Chris Meyer, Sargento Foods, and Todd Jekel, IRC, successfully completed the requirements and obtained a Professional Engineer s license in the State of Wisconsin. Congratulations! Our new website ( is up. Please browse and let us know what you think. Send items of note for next newsletter to Todd Jekel, tbjekel@wisc.edu. 2

3 Increased compressor power Because additional refrigerant is expanded and evaporated into the machine during the compression process, additional work is required to compress the added refrigerant mass. Reduced compressor capacity Compressor capacity is diminished in two ways. First, liquid injection oil cooling reduces the compressor s volumetric efficiency. Second, the high stage load on a two stage system increases when liquid injection is used on booster compressors The extent of these consequences depends on the location of the liquid injection port within the compression process. Injection port pressures closer to the suction pressure (i.e. earlier in the compression process) result in larger capacity reductions and a higher horsepower penalty. Figure 1 shows a booster compressor fitted with LIOC. The alternative to liquid injection oil cooling is to use a heat exchanger, external to the compressor, to cool the oil. Heat exchange media include: water, secondary fluids (such as ethylene glycol and water), and refrigerant. Water and secondary fluid coolers rely on the sensible (temperature-driven) heat exchange between the cooling medium and oil. Another approach to external oil cooling is thermosiphon oil cooling (TSOC). Thermosiphon oil cooling is accomplished by supplying high-pressure liquid refrigerant from an elevated vessel (thermosiphon pilot receiver) to one side of the heat exchanger and hot oil to the other. The hot oil causes the refrigerant to evaporate and naturally rise back to the thermosiphon pilot receiver. The vapor generated by the oil cooling heat exchanger flows through the pilot receiver and on to the condenser hot gas connection where the condenser liquefies the vapor and returns the liquid refrigerant to the pilot receiver. In contrast to liquid injection, thermosiphon oil cooling becomes a load on the condenser but not on the compressor. Regardless of the fluid that cools the oil, compressor performance is identical because the oil cooling is external to the compressor. Typical discharge temperatures from compressors with external oil cooling are in the range of F. Figure 2 shows a booster compressor fitted with Injection point High-pressure liquid piping Thermal expansion valve Figure 1: LIOC on screw compressor in booster duty. 3

4 thermosiphon oil cooling. COMPARISON Liquid injection is the lowest capital cost option for oil cooling; however, there are three primary reasons to consider thermosiphon over liquid injection: 1. reduced maintenance 2. reduced refrigeration energy use 3. increased high-stage compressor capacity With regard to maintenance, thermosiphon oil cooling is less stressful on the compressor; thereby, prolonging the interval between rebuilds. Anecdotal evidence from the field suggests that liquid injection oil cooled screw compressors have approximately half the run hours between rebuilds as compared to externally oil cooled machines. Energy benefits can be broken down into three areas: 1. reduced compressor horsepower per refrigeration load, 2. reduced load on the high-stage of a twostage system, and 3. increased opportunity to float the refrigeration system discharge pressure. The first two items were mentioned in the description of the methods above. Item 3 is an often overlooked consideration that is important in the context of achieving efficient systems and can be considered a synergistic efficiency improvement. Liquid injection oil cooled compressors often use thermal expansion valves to modulate the feed of liquid refrigerant to maintain oil temperature. Thermal expansion valves require a minimum pressure difference across the valve to properly control liquid refrigerant feed. The pressure difference across the valve is determined by the liquid injection port position (i.e. pressure) within the compressor and the high-pressure liquid feed pressure. Therefore, the minimum pressure difference across the thermal expansion valve will constrain the extent to which the system s discharge pressure can be floated during offdesign loads and ambient conditions. Liquid refrigerant supply piping Vapor refrigerant return piping Oil cooling heat exchanger Figure 2: TSOC on a compressor in booster duty. 4

5 CASE STUDY The IRC recently completed a study of refrigeration system energy efficiency improvements at a large food processing plant. One of the opportunities investigated was the conversion from liquid injection to thermosiphon oil cooling. The refrigeration system is configured with two-stages of compression, three suction pressure levels, and two-stages of liquid expansion. Sum total there are more than 10 screw compressors of various sizes and duties with nameplate power totaling nearly 5,000 hp. The plant manger requested an energy analysis to quantify the energy cost benefit of the proposed conversion. To complete such an analysis, both refrigeration loads and ambient conditions needed to be determined. Refrigeration loads on each suction pressure level were estimated and divided into several bins over each day throughout year to capture the various production schedules at the plant. Weather data was obtained from a Typical Meteorological Year (TMY2) data set for a nearby location. Gross validation of the model was accomplished in two ways, first using the calculated estimates of design loads and design weather conditions to compute the design condensing pressure of the system, and second by estimating the dry bulb temperature at which the operational staff were required to valve out one of the three condensers. The system model we developed included both the compressor efficiency effects and the operational change (i.e. floating head pressure) were included. The ability to float the head pressure below the current set points was only done in wintertime because it was determined that the liquid injection condensing pressure requirement only was controlling during wintertime. The summertime constraint was high-pressure liquid feed requirements due to the existence of direct expansion loads and the distance over which the liquid was supplied. In the energy analysis, reducing the minimum We re Moving October 15-16, 2003 The IRC offices are moving back to the engineering campus: Industrial Refrigeration Consortium 1415 Engineering Drive, Room 2342 Madison, WI All other contact information for staff will remain unchanged. We anticipate this move, although a disruption, will improve our ability to meet our members needs. condensing pressure set point was only done when the dry-bulb temperature fell below 60 F. With LIOC, the system required a wintertime minimum discharge head pressure of 140 psig (81 F saturated condensing) to maintain adequate operation of the screw compressor liquid injection oil cooling. The summer set point of 160 psig was unchanged (although the retirement of the liquid injection oil cooling feed line would allow the piping of a parallel liquid feed to plant loads to relax that set point as well, without requiring a recertification of the high-pressure receiver). Based on discussions with the operational staff, we estimated that the wintertime minimum discharge pressure could be safely reduced to 120 psig (73 F saturated condensing) immediately following the conversion to thermosiphon oil cooling. The resulting energy use and energy cost savings for the system were estimated to be 1,150,000 kwh and $53,000 per year, respectively. The demand savings are approximately 170 kw on the yearly peak. The energy savings represent an approximate 9% reduction in compressor and condenser energy use. Note that 67% of the energy cost savings is due to energy use reduction and 33% is due to demand reduction. Approximately 82% of the energy use savings 5

6 and 84% of the energy cost savings occur during production hours. A collateral benefit is that the conversion freed up nearly 90 tons of high-stage load during peak production! A key to the successful implementation of the oil cooling conversion was coordination with a local contractor. Based on a quote the facility had in-hand for the conversion, the payback on energy alone was approximately 4 years. The conversion was made more attractive because the facility s electric utility provided a rebate that reduced the payback period by nearly one year. In addition, maintenance cost savings (although not quantified) will further reduced the payback period. If you have questions or comments on this article please feel free to contact Todd Jekel at tbjekel@wisc.edu. COMING SOON There are additional choices on liquid feed valves for liquid injection oil cooling. These valves have relaxed pressure difference requirements and can allow floating head pressure. In a future newsletter, we will analyze the energy implications of different options and estimate the cost of operation. PLAN FOR 2004! Join the Industrial Refrigeration Consortium (IRC) 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. 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. We can schedule and deliver a web-based presentation on the benefits of IRC membership to you and other decision-makers in your company. Benefits include unbiased and authoritative information on industrial refrigeration applied research results to improve refrigeration system efficiency, productivity, and safety preferred access to refrigeration education programs to expand staff capabilities In addition, IRC members have access to telephone hot-line support on technical, operational and regulatory issues, internet-based information resources (including access to IRC web courses), technical bulletins, safety updates, and specialized publications, and refrigeration education for personnel at all levels. LET US ASSIST YOU IN I ENHANCING YOUR REFRIGERATION RESOURCE.. 6

7 2004 IRC Research & Technollogy Forum January 14-15, 2004 Bradenton, FL The Industrial Refrigeration Consortium is pleased to announce that the 4th Annual Research & Technology Forum will be held on January 14-15, 2004 in Bradenton, FL. This event will feature presentations by IRC staff, IRC member organizations, and industry experts. Tropicana Products, Inc. has graciously agreed to host the 2004 R&T Forum at their Bradenton Florida plant. A highlight of the event will feature an overview and tour of their new world class North Compressor Room. Completed less then a year ago, this facility incorporates innovative safety and operation features that define world-class standards in industrial refrigeration. The 2004 IRC R&T forum will provide attendees an opportunity to receive information about current refrigeration research, regulatory updates, emerging technologies, and refrigeration education and to explore needs for future research in the area of industrial refrigeration. It also provides you an opportunity to meet and exchange ideas with other industry experts. Topics include: PSM What s the business case Standard Operating Procedures lessons learned Variable frequency drive application considerations in industrial refrigeration IRC research update o Benchmarking o Mechanical integrity and non-destructive testing o IRC Toolbox Underfloor heating problems and remediation The forum is open to anyone interested in industrial refrigeration. IRC member registration is free and there will be a nominal registration fee of $99 for non-irc members to recover meeting costs. Attendees receive a binder of all of the presentation materials. Visit our website ( to download registration materials. The registration deadline is December 24, Please note that for planning and security reasons no attendance is allowed without pre-registration. A block of rooms has been reserved at the Holiday Inn Riverfront in Bradenton at $94/night. For reservations, call the hotel directly at (941) and ask for the Industrial Refrigeration Consortium group rate. IRC members should plan to arrive early for the IRC s annual business meeting to be held the morning of January 14, Watch our website for the agenda and registration materials. If you have any questions regarding travel arrangements or presentation topics, please contact the IRC at or info@irc.wisc.edu. 7