The effects of retrofitting on an older display case

September 12, 2000
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Medium-temperature display cases consume roughly 18% of the total electricity use of a supermarket. Open, multi-deck meat/dairy/deli cases can contribute up to 25% of refrigeration energy consumption.

In addition to this significant contribution to energy use, meat/dairy/deli cases are required to maintain a desirable meat temperature. Fresh meat, deli, and dairy products, according to Food and Drug Administration (FDA) classification, are required to be stored at less than 41°F at all times, including post-defrost periods, at any location within the case.

It is a continuing challenge of food chain operators to manage the power use of their fixtures and maintain acceptable product temperatures, while constantly attempting to improve their profit margins.

In the past decade, many improvements have been made to the design of refrigerated, open, multi-deck display cases which have enhanced their energy efficiency and overall performance. In order to take advantage of these improvements and reduce operating costs, supermarket operators may replace their older and less efficient fixtures with new energy-efficient units, which can be an expensive investment.

A possible alternative to the replacement of older-model fixtures is to retrofit them with commercially available energy-efficient technologies.

Display case study

As part of a multiphase study, Southern California Edison (SCE) conducted a series of tests to determine the effects of commonly available retrofit technologies on the performance of an older-model refrigerated meat display case.

The effects of these retrofit strategies on the energy use and performance of the case can provide effective economic insight for supermarket operators. Association of cost with the obtained performance results can assist supermarket operators in evaluating the cost-effectiveness of fixture replacement as opposed to retrofit alternatives.

The issue that SCE ultimately intended to address in this study was to quantify how much the energy-efficient retrofit technologies can improve the performance of an older-model meat display case, and why.

Two retrofits

This test specifically focused on two common energy-efficient technologies: T8 lamps equipped with electronic ballasts (T8EB) and electronically commutated fan motors (ECMs).

The incremental effects of these two retrofit technologies were evaluated by testing the display cases under the same environmental condition three times.

The three test runs were conducted in a 75° drybulb (db), 55% relative humidity (rh) ambient condition, in compliance with ASHRAE Standard 72-83.

In the first test run, the standard case (with no retrofit improvements) was tested to establish the base case. The standard lighting system in the case was then replaced by T8 fluorescent lamps and electronic ballasts, and the unit was retested.

Lastly, with the lighting improvements still in place, the case’ standard fan motors were replaced with ECMs and the unit was tested under the same indoor conditions.

The specific display case tested is shown in Figure 1. The fixture has a dual-band air curtain with one ambient and one refrigerated discharge air grille.

Display case lighting replacement is one of the most convenient case retrofits. The more-efficient T8 lamps draw 0.49 amps at 120 V, compared to the standard T12 lamps that draw 0.73 amps at 120 V.

In addition, replacing the evaporator and ambient standard shaded-pole fan motors with ECMs provides higher-efficiency motors.

Test procedure

SCE conducted this test at its Refrigeration Technology and Test Center (RTTC), located in Irwindale, Calif. The center’s sophisticated instrumentation and data-acquisition system provided detailed tracking and acquisition of the refrigeration system’s mass flow rate and critical temperature and pressure points, as well as essential psychrometric data, during the test period.

These readings were then imported into an engineering analysis model developed by SCE to quantify various heat transfer and power-related parameters within the refrigeration cycle.

The ambient condition, saturated condensing temperature, and target product temperature remained constant for all tests. The display case was served by a refrigeration system using an HFC refrigerant, R-404A, operating at a constant discharge pressure of 212 psig, or saturated condensing temperature of 93° (±1°).

The maximum allowable product temperature (including post-defrost periods), which was the critical control point for all tests, was 38.5° (±0.5°). If the product temperature exceeded this level at any time, the control settings, specifically target suction pressure, were modified and the test was repeated.

Using the product temperature as a control point in conjunction with the fixed indoor condition and condensing temperature, helped ensure a compatible evaluation of retrofit technologies on the tested display case.

A Kaye Instruments “Digi-4” data scanner was used to log the test data. The Digi-4 has a special emphasis on temperature measurement, with excellent thermocouple signal processing.

The data scanner processed 127 data channels throughout the test. The scanner was calibrated at the factory, and is traceable to the National Institute of Standards and Technology’s (NIST) standards.

To ensure that the data collection was not compromised by the control sequence’s priority over data acquisition, the data-acquisition system for the project was designed to be completely independent of the supervisory control computer.

Every 10 sec the data-acquisition system sampled the scanned data and created time-stamped, 2-min averages. The 2-min data was then saved to a file which was closed at the end of each 24-hr period. Edison engineers reviewed the real-time data initially on-site at the RTTC, to ensure that the product temperature and other key control parameters were within acceptable ranges.

In the event that any of the control parameters fell outside of acceptable limits, the problem was flagged. In such cases, test runs were repeated until the problem was corrected. Once the data passed the initial screening process, it was downloaded remotely to SCE’s San Dimas office for further screening and processing.

Results

Figure 2 compares the various cooling load components of the tested meat display case under the base-case condition. Conduction seems to have the least influence on the case load, while infiltration and radiation play significant roles.

The improved lighting system resulted in an increase of 2° and 1° in evaporator temperature and discharge air temperature, respectively, while the maximum product temperature stayed relatively constant at 38°.

Less heat was introduced into the refrigerated space by the energy-efficient lamps and ballast. As a result, the product temperature rise after defrost was reduced by 25%. Obviously, exposure of the product to less temperature fluctuations could potentially result in improved product quality. The lighting power consumption decreased by 31% as a result of the replacement.

The efficient lights reduced the case cooling load (Btuh-ft) by 3%, resulting in a 4% increase in suction pressure, which translated into a reduction of 3% and 5% in refrigerant mass flow rate and total system power (W/ft), respectively. The total system power in this test included the consumption of evaporator fan(s), lights and ballast, and the compressor.

The test was continued to evaluate the impact of evaporator fan motor replacement. The ECM fan motors replaced the standard, shaded-pole fan motors.

The more-efficient ECMs dissipated less heat into the case while consuming less power to circulate the air within the fixture. Consequently, the case cooling load dropped by 13%, and the system operated at 50-psig suction pressure (15% higher than the base case) to maintain the same product temperature.

The increase in suction pressure as a result of the cooling load decrease caused a reduction of 15% and 27% in refrigerant mass flow rate and total system power, respectively.

Clearly, the case retrofitted with T8EB and ECM was exposed to a noticeably lower cooling load and consumed less power than the base case throughout the entire test period.

The power requirement of the fan motors for the base case and the T8EB retrofit is almost equal. The reasons for the slight increase in fan power use can possibly be attributed to an increase in air density as a result of the slightly lower return air temperatures.

The reduced cooling load of the display case, despite the increased suction pressure and discharge air temperature, caused a slight reduction in return air temperature. The power use of the fan motors decreased by more than 50% with the ECMs installed.

As the display case was retrofitted with both energy-efficient technologies, the refrigerant mass flow rate, required to absorb the refrigeration load of the case, decreased by 15%. Throughout the test, a variable-speed drive controller modulated the compressor’s speed with respect to the fluctuation in suction pressure and discharge air temperature.

Table 1 summarizes the test results. Both the lighting and motor retrofits cause the display case to operate at slightly lower temperatures (as shown by the decrease in entering air temperature), and at slightly higher discharge air temperatures, while the maximum product temperature remains nearly unchanged.

The installation of energy-efficient lamps, ballasts, and fan motors reduced the cooling load and system power consumption of this particular older-model meat display case by 13% and 27%, respectively.

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