Recently, a produce distributor wanted to improve profit margins and throughput at one of its distribution and warehouse operations. The company knew that the existing refrigeration system was consuming a significant portion of the facility's overall energy and maintenance costs. However, it was concerned about how expensive a retrofit might be. Before making a decision, the firm requested an evaluation of the existing system's capacity, energy consumption and operational maintenance, and compared this data to the potential benefits that could be achieved over the operating life of the system.
A Thorough EvaluationCapacity. The evaluation determined that the facility's refrigeration load requirement was 350 tons of refrigeration (TR). However, the capacity of the installed equipment was 254 TR, which equated to a refrigeration shortage of 96 TR. Several discrepancies were discovered in the equipment ratings, the application of the original design by the engineer and the installation by the contractor.
Originally, 10 direct expansion (DX) R-22 evaporators were installed in the facility. The initial design rated each evaporator at 11.5 TR when operating at a 10°F (5.6°C) air-to-evaporator temperature difference, for a total of 115 TR. When the refrigeration capacity proved to be inadequate, three more identical evaporators were added for a combined total refrigeration capacity of 149.5 TR. Again, the capacity was tested and did not meet the owner's required load. At the recommendation of the installing contractor, the owner agreed to adjust the air-to-evaporator temperature difference to 17°F (9.4°C). This adjustment increased the refrigeration capacity of evaporators to a total 254.2 TR. Each evaporator used 16.4 A of fan power, for a total of 148.5 kW, which equated to 0.58 kW/TR.
The original installation also included 10 condensing units, each designed to match the load from its respective evaporator. Each unit contained one compressor rated at 71 A and four fans rated at 14 A, and was rated for 11.5 TR operating 12 to 14 hours per day. To match the new requirements of the evaporators, three more condensing units were added, and all of the existing units were reset to operate at a lower suction pressure for 24 hours per day. The condensing units produced 254.2 TR to match the new evaporator load. The three additional condensing units each contained one compressor rated at 71 A and three fans rated at 11 A. This gave the 13 condensing units a total power consumption of 742.3 kW, which equated to 2.92 kW/TR.
Thirteen individual wall-mounted thermostats controlled each respective combination of evaporator and condensing unit and were accurate to ±2°F. The thermostats worked independently of each other and controlled the associated liquid solenoid valve on the condensing unit.
Energy Consumption. The owner's maintenance contractor and power utility representative were present during the energy consumption evaluation to provide information on the base energy consumption and refrigeration load. It was determined that the refrigeration capacity of 254.2 TR required 890.8 kW, or 3.5 kW/TR, which was far in excess of the original design specifications.
At an average electricity cost of $0.060/kW, the installed equipment cost $53.45/hr to operate. At the time the evaluation was performed, the equipment had been operating 24 hours per day for eight months, which equated to an operating cost of $307,872/yr. This cost was verified by examining the plant's electric bills and was confirmed by the electric utility representative. The calculated demand charge for eight months of operation was $44,896, and the total refrigeration energy cost was calculated at $352,768.
Operations and Maintenance. On average, two condensing unit compressors were changed out each month, which equated to a maintenance cost of more than $10,000/mo. Additionally, the evaporators were designed for hot-gas defrost; however, the hot-gas control circuits had been removed and the controls de-energized, which caused the units to be air defrosted instead of hot-gas defrosted. This situation extended the defrost cycle while reducing the refrigeration cycle, decreasing the overall refrigeration capacity of the plant.
A Plan of ActionBased on the evaluation, a plan of action was created and presented to the owner. During all phases of the redesign, the goals were to reduce energy, increase capacity and lower maintenance and operating costs. The new system also would be installed with a minimum impact on plant operations. The company decided to move forward with the retrofit.
The new refrigeration system used the existing evaporators to reduce downtime and costs. The direct expansion/thermostatic expansion valve (DX/TXV) option was removed from the evaporator liquid supply, and the liquid feed distribution system was redesigned. The actual evaporator surface area was not disturbed. A new liquid solenoid valve was installed in the liquid line to feed the modified distribution system, and a liquid sight glass was installed in the evaporator outlet suction line. Finally, isolation valves were installed in front of the liquid solenoid valve and after the liquid sight glass. These modifications increased the capacity of each air evaporator by 27.6 TR, for a total capacity of 358.8 TR. Each air evaporator maintained the 16.4 A of fan power for a total of 148.5 kW, which now equated to 0.41 kW/TR.
The existing condensing units were pumped down and removed from the work area. The refrigerant charge in each unit was later used to recharge the newly designed refrigeration system.
The new condensing unit consisted of three new compressors, one new condenser, one new receiver/accumulator and a new distributed control system (DCS) to operate the entire plant. This condensing system consumed a total of 297.8 kW per hour to produce 358.8 TR, which equated to 0.83 kW/TR.
Another EvaluationThe retrofit system was tested with the owner's maintenance contractor and power utility representative again present to provide the base energy consumption and refrigeration load. Using this information, it was determined that the new system required 446.3 kW to achieve 358.8 TR. At the average cost of $0.060/kW, this calculated out to be $26.77/hr. With the new refrigeration equipment operating 24 hrs/day for eight months, the new calculated operating cost was $154,195/yr. This cost was verified by examining the post-retrofit electric bills and was confirmed by the electric utility representative. The calculated demand charge for eight months of operation was $22,493.52. The total refrigeration energy cost was calculated at $176,688, which equated to an annual savings of $176,079.
The new DCS was designed to operate the refrigeration system retrofit at optimum design conditions and to achieve the desired product temperatures with the least amount of energy consumed. The DCS monitored all aspects of the refrigeration system, including compressor operating conditions, outside weather conditions, room temperatures, product temperatures, evaporator liquid temperature and evaporative condenser operating conditions. Due to the DCS monitoring, overall maintenance costs were reduced to approximately $1,100/mo. This equated to an annual savings of $71,200 for related maintenance costs.
The overall energy and TR comparisons are shown in tables 1 through 3. Using this information, it was determined that the newly installed equipment, along with the redesigned evaporators, equated to an annual savings of $247,280.
The payback of the installed cost of the new system was estimated to be approximately 18 months. Existing energy incentives also allowed the electric company to provide a rebate for the new system, which was paid directly to the owner.
Energy Costs CountWhile most equipment upgrades are a one-time expenditure, energy and maintenance costs are ongoing. For this reason, facility owners should carefully consider any options that will increase energy efficiency and reduce maintenance requirements. When evaluating equipment and designs, owners should keep in mind that utilities sell energy by kilowatts (demand) and kilowatt-hours (energy cost) of usage. Proposals submitted by equipment suppliers and design engineers should therefore include the kilowatts required to operate the equipment so that the overall equipment performance and operating costs can be accurately determined.
With today's ever-increasing energy costs, understanding the long-term “cost of doing business” is more important than ever to optimize profitability. PCE