Improving Aging Refrigeration Systems in the Fruit Industry
IntroductionThis paper addresses key issues to upgrading aging R-22 and ammonia systems for energy efficiency, improved control, and updated data logging. Issues such as hardware upgrades, optimum control strategies and common difficulties are demonstrated through two case studies.
The recent trends in the fruit storage industry have been clear and consistent. Ammonia refrigeration systems, both with and without modern screw compressors have become the standard. And in nearly all cases, a computer control system is included to manage all refrigeration system components, from evaporator coils to compressors. In addition, control systems provide the mandatory monitoring and control for room temperature and atmosphere conditions.
Unfortunately, there are a significant number of refrigeration systems greater than 10 years old with substantial life left. For these systems, standard electro-mechanical controls (e.g., pressure switches) and "seat-of-the-pants'' operations are most typical. Although it is possible to operate a refrigeration system somewhat efficiently through these means, it has been our experience that many of these systems are set up for minimal hands-on interaction and worst-case operating conditions. The result is often low suction pressure, high head pressure, and little or no trimming of evaporator fans. Although this method of operation may provide adequate control of room temperature, energy use is excessive and documentation of temperatures, pressures and other key information is infrequently, if ever, gathered.
Upgrading these older refrigeration systems can be straight forward when proper precautions and planning take place. Although not quite "bolt-on" technology, control systems and other modern energy efficiency sub-systems are well established throughout the industry.
Technologies to ConsiderWe will assume for this discussion that reciprocating compressors are the standard for older systems (e.g., Carrier's 5H series). In addition, for R-22 systems, thermal-expansion valves feeding evaporator coils are most common. Finally, we can assume that evaporative condensers are most common, with simple fan cycling control based on Penn-type or Mercoid pressure switches. In these cases, four primary technologies are anticipated:1. Computer Control 2. Compressor Unloaders 3. Liquid Pressure Amplifiers 4. Variable Speed Drives
Computer ControlComputer control can be considered the "backbone" of any system upgrade, particularly when energy efficiency is targeted. The control system provides the modern user interface necessary for pressure management and fan control, both on evaporator coils and condensers. And in some cases, the control system can actually optimize operations, looking to push system pressures to allowable extremes while maintaining target room temperatures.
Compressor UnloadersMost of the aging reciprocating compressors have manually-adjusted cylinder unloaders. These spring-tensioned systems have a range of approximately 10 to 15 psig in suction pressure over which the compressor moves from fully unloaded to fully loaded. With two or more compressors, these loading ranges are commonly staggered. The result may be a 20 to 40-psi range of suction pressures between the first compressor starting to load and the last compressor becoming fully loaded. Since the last compressor is commonly set to come on within the acceptable target suction pressure, the result is an average suction pressure well below that required, particularly during light system loads. This low suction pressure means reduced compressor capacity and increased compressor energy consumption. Most reciprocating compressors can be retrofit with electrically activated unloaders. Coupled with a computer control system, the unloaders can be used to maintain tight control over suction pressure.
Liquid Pressure AmplifiersLiquid pressure amplifiers (LPAs) are designed to circumvent the limited capabilities of R-22 systems. With minimal latent heat (relative to ammonia), R-22 is highly susceptible to pressure drop and flash gas. This flash gas nearly shuts off the refrigerant flow through thermal expansion valves (TXVs). Furthermore, typical TXVs are sensitive to pressure differential. When suction pressure rises or discharge pressure falls, TXV performance may lag due to insufficient pressure differential. In this situation, the LPA shines. The LPA pump is a small, seal-less, magnetically driven pump. This pump is installed on the liquid line between the condenser outlet and TXV. The pump boosts liquid line pressure, eliminating any flash gas. In addition, the TXV is served high-pressure liquid refrigerant to ensure adequate flow rate. With LPAs installed, a system can be operated at higher suction pressures and lower discharge pressures to reduce compressor power and increase system capacity.
Variable Speed Drives
Variable speed drives (VSDs) are now quite common, with over 100 manufacturers worldwide. Simply put, a VSD converts the 60 cycle power from the utility into a user selectable frequency. In this way, induction motors that would normally operate at a single speed can now operate at any speed between 0% and 100%. This operation provides substantial energy reductions in fan energy, following the "cubic" law.
This rule states that as fan speed is reduced, airflow will change proportionally while power will change cubically. For example, an evaporator fan operating at 50% speed will provide 50% airflow across a coil. However, the fan will only require 50%, or 12.5% of full speed power. In real-life however, the motor and VSD efficiency falls off some, with actual input power in the range of 15% to 17% at half speed.
VSDs can be installed on evaporator fans and condenser fans. On evaporator fans, the computer can slow fan speed when room temperatures are satisfied, down to some minimum user setpoint (commonly 40% to 75%). Temperature probes scattered throughout the space ensure proper room conditions. In systems that utilize backpressure regulators (BPRs) for control, the fan and BPR can operate in tandem to obtain energy savings while minimizing moisture loss.
On condenser fans, speed can be used in place of fan cycling for minimum head pressure control. In addition to energy savings, wear on belts and sheaves is reduced due to the gentle starting and stopping of VSD-driven fans.
Application CautionsAlthough each of these technologies is proven, improper installation or inadequate design can result in problems. Specifically, LPAs should be installed following factory recommendations for proper net-positive suction head. This will ensure no cavitation at the pump inlet. In addition, VSDs should be carefully and thoughtfully applied. Issues such as motor "robustness", lead lengths, bypassing, line reactors and motor grouping should be considered by qualified personnel prior to installation. With all applications carefully researched and implemented, outstanding results can be anticipated.
Case Study No. 1: Bear CreekOur first case study documents a system upgrade researched and eventually implemented at Bear Creek Operations, also known as Harry and David, located in Medford, Oregon. Bear Creek is widely known for mail order specialties ranging from chocolates and fruit to flowers. To cool and store these products, Bear Creek employs five separate R-22 engine rooms serving a number of cold storages and freezers.
As part of Pacific Power's Industrial FinAnswer program, an energy review of all five engine rooms was performed by Cascade Energy Engineering. The result of the survey was a group of six recommendations applied to each of the engine rooms. Overall, computer control, evaporator fan VSDs and condenser fan VSDs were recommended. In addition, the Carrier 5H series compressors would require retrofit with factory option unloaders.
As a first go around, Engine Room A (ER-A) was chosen for retrofit. An outside view is shown in Figure 1, with the evaporative condenser on the roof.
Four reciprocating compressors totaling 400 hp serve a single suction, as shown in Figure 2.
A total of 123 hp in _ hp evaporator fans serve rooms 20, 22, 23 and 24. These rooms act as 32 °F coolers, with a catwalk down the center of the ceiling between two rows of opposing evaporator coils, as seen in Figure 3.
Prior to improvement, the system was controlled by pressure switches and simple thermostats. From daily engine room logs, a reasonably accurate history of operating pressures was available, as shown in Figure 4 and Figure 5.
Note that the suction pressure ranges from zero to 20 psi, or temperatures of 40 °F to -4 °F. These temperatures are much lower than necessary for a cooler application. To improve system control and reduce energy use, several major steps were taken.
First, a computer control system was installed. The computer was placed inside a protected, conditioned cabinet within the engine room, as shown in Figure 6.
Next, factory-option electric unloaders were installed on all compressors. The smaller 4-cylinder compressor required only one unloader, while the larger 8-cylinder units required three unloaders. A sample unloader application is shown in Figure 7. The unloaders provide not only the flexibility to maintain target suction pressure, but also allow flexibility in sequencing compressors to avoid excessive starts per hour.
A total of 11 VSDs were installed. VSD control was implemented for rooms 22, 23 and 24, and for both condenser fans. Each room was divided into three control zones, with independent fan speed control for each. The drives are shown in Figure 8.
To monitor improvements in system operations, three 4-channel data loggers were installed. VSD speed was directly monitored from each drive, as was suction pressure, discharge pressure and compressor current. The data loggers were programmed and downloaded using a simple laptop computer. The data logging system is shown in Figure 9.
The improvements in suction pressure and discharge pressure are shown in Figures 10 and 11, respectively.
A substantial, year-round improvement in pressures has occurred. Evaporator fan speed varied by season and load, as expected. Various minimum speeds were tried, with a minimum of 45 Hz (75%) finally chosen. A sample of fan speed data from the heavy-load month of October is shown in Figure 12.
From the monitoring results, annual savings were calculated. Relative to implementation cost, the total project economics could then be summarized. This summary is provided in Table 1. Following the success of this project, Bear Creek plans on implementing similar upgrades in four other engine rooms, with a target of 2,500,000 kWh/yr additional energy savings.
Table 1. Case 1, Summary of Project Economics
Case Study No. 2: Stadelman FruitOur second case study documents an upgrade of three engine rooms at Stadelman Fruit's Milton-Freewater, Oregon facility. This project was initiated through OMECA, a consortium of small utilities participating in energy efficiency projects.
This particular facility provides "bread and butter" receiving, sorting, boxing and storage of apples from the neighboring area. Three engine rooms (one ammonia and two R-22) were surveyed. Engine Room #1 houses three reciprocating compressors and an ammonia system for two common storage rooms. A view of the engine room is shown in Figure 13, with the two evaporative condensers on the roof. An interior view of Room #1 is shown in Figure 14, with the evaporator coils near the ceiling.
Engine Room #2 is serving cold storage Rooms #3 and #4 with two isolated R-22 reciprocating compressors sharing a dual circuit evaporative condenser on the roof. The engine room is shown in Figure 15.
Finally, CA rooms #5 and #6, as well as cold storage #7 are served by Engine Room #3. Two reciprocating compressors again operate within an R-22 system to provide cooling. Similar to Engine Room #2, a single evaporative condenser is mounted on the roof, as seen in Figure 16.
Following a detailed system review, similar recommendations were made for these engine rooms. A single computer now manages all three engine rooms, as shown in Figure 17.
In Engine Room #2 and #3, one compressor was retrofit with electric unloaders to allow desired sequencing and suction control. The unloaders for Engine Room #3 are shown in Figure 18.
The computer system is used to cycle evaporator fans in Rooms #3 - #7. However, the evaporator fans in Rooms #1 and #2 were retrofit for VSD duty. These rooms operate year-round, and provided acceptable economics. For this application, all motors were retrofit with new inverter-duty motors. A VSD was installed on each coil, as shown in Figure 19. The computer manages all fan speed control, with a minimum of 40% speed.
To improve condenser operations, VSDs were installed on each condenser. The VSD for Engine Room #2 condenser fan is shown in Figure 20.
Finally, to allow floating head pressure operation, LPA pumps were installed on both R-22 systems. In Engine Room #2, two pumps were installed (one per suction system) to obtain sufficient capacity. In Engine Room #3, a small pit was dug in order to mount the pump low for sufficient suction head. The LPA pumps for Engine Room #2 are shown in Figure 21.
With the computer control system, external data logging equipment is not necessary. Periodically, system logs are downloaded from the computer for analysis and commissioning. This data can be compared with pre-retrofit engine room logs for quantification of energy savings.
As shown in Figure 22, suction pressure in Engine Room #1 has improved dramatically. In Figure 23, the improvement in discharge for Engine Room #3 is evident. In fact, operating pressures in all engine rooms have improved.
Evaporator fan speed has provided excellent savings. As shown in Figure 24, the fans have been able to operate at reduced speed, even during the summer months. Although average speed was 62%, average power was only 36%! In addition, energy savings results from reduced fan motor heat to be removed by the compressors.
A summary of the project implementation cost, savings and final customer economics is shown in Table 2. Note that on this project, significant contributions by OMECA and Oregon Business Energy Tax Credits created an acceptable payback for the customer.
Table 2. Case 2, Summary of Project Economics
SummaryFrom these projects, it can be seen that energy efficiency can be implemented in aging refrigeration systems with acceptable economics and outstanding efficiency improvements. For these two example projects, refrigeration energy use was reduced by over 40%! With a careful, well-thought out approach, many other aging refrigeration systems could benefit from similar upgrades.
Marcus H. Wilcox
Cascade Energy Engineering, Inc.
6 1/2 N. 2 Ave; Suite 310, Walla Walla, WA 99362
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