impact_rh_display_report.pdf

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The Impact of Indoor Relative Humidity on Performance of Four Different

Types of Refrigerated Display Cases

This project investigates the effects of decreasing indoor ambient relative humidity on the performance

and energy use of four different types of refrigerated display cases. The four tested cases represent

fixtures commonly found in supermarkets for storage and merchandising of medium and low temperature

products. The test results established a correlation between decreasing indoor relative humidity and the

following key parameters:

Mass of frost formed on the evaporator coil during refrigeration period

Defrost length and power Anti-sweat heaters power

Discharge air velocity

Product temperature

Test compressor power

Display case cooling load and its components

Refrigerant mass flow rate

Discharge air temperature

Frost formation pattern and uniformity

Reach-in case door fog refresh rate

The performance of these fixtures was evaluated under equal indoor conditions. Southern California

Edisons (SCEs) Refrigeration and Thermal Test Center (RTTC) conducted the tests. The controlled

environment chamber of the RTTC was maintained at fixed dry bulb temperature of 75oF, while its

relative humidity was changed from 55% to 35% in 5% segments, for all tests.

The results of this study indicate a direct correlation between the indoor relative humidity and:

1. The weight of moisture removed from the air during the process of refrigeration

2. The fog removal time on freezer glass case doors with equal anti-sweat heater power (refresh rate)

Figure 1 shows the weight of moisture in the form of condensate collected at the end of each defrost

cycle. Clearly, open vertical meat and dairy fixtures demonstrated more vulnerability to humidity

variations, and removed more moisture from the air than the coffin and reach-in units. The lowering of

indoor humidity from 55% to 35% reduced the weight of condensate collected from the meat and dairy

cases by 61.7% and 73.2%, respectively.

THE IMPACT OF INDOOR RELATIVE HUMIDITY ON 1 REFRIGERATION AND THERMAL TEST CENTER

PERFORMANCE OF FOUR DIFFERENT TYPES OF

REFRIGERATED DISPLAY CASES


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79.4

67.1

52.8

39.3

30.4

80.4

67.1

52.9

36.6

21.614.3

11.911.49. 7 6. 34. 94. 54. 03. 0

0

10

20

30

4050

60

70

80

90

35% 40% 45% 50% 55%

Test Scenarios

CondensateM

ass(lbs)

Meat Cas e Dairy/Deli Case

Frozen Food Case (Reach- In ) Frozen Food Case (Co f fin)

Figure 1 - Comparison of Total Collected Condensate vs. Relative Humidity (All Four Tested Display Cases)

The decrease in indoor relative humidity had an immediate impact on the latent load of all fixtures.Infiltration accounted for approximately 80% of the cooling load of open vertical cases. This factor

brought about a considerable latent load penalty for open vertical display cases (Figure 2). Lowering the

indoor humidity from 55% to 35% resulted in 62% and 74% reduction in latent loads of the meat and

dairy cases, respectively.

4,198

3,587

2,786

2,057

1,594

4,041

3,376

2,636

1,807

1,050

74 862159951 0 32 8

25 723 421115 9

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

35% 40% 45% 50% 55%

Test Scenarios

TotalLatent

Load(Btu/hr)

Meat Case Dairy/Deli Case

Frozen Food Case (Reach-In) Frozen Food Case (Cof f in)

Figure 2 - Comparison of Total Latent Load vs. Relative Humidity (All Four Tested Display Cases)

The effect of decreasing latent load, as a result of lowering indoor humidity, was reflected directly on thepower consumption of the test compressor1 (Figure 3). As expected, open vertical meat and

1The test compressor and its rack system were not designed specifically for the subject casework and therefore will have certain

inefficiencies that would not appear in actual system design. Therefore, refrigerant cooling load is a preferred measure of

system performance.





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dairy cases showed the highest increase in their compressor power demand as indoor relative humidity

increased. Lowering of relative humidity from 55% to 35% for these two fixtures resulted in 17.7% and

19.6% reduction in compressor power, respectively.

1.13 1.191.26

1.31

1.37

1.23 1.30

1.381.45 1.53

1.30 1.30

1.18

1.29

0.790.77 0.85 0.87

0.93

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

35% 40% 45% 50% 55%

Test Scenarios

NormalizedCompressorPower(kW)

M eat Cas e Dai ry /De li Cas e Fr o ze n Fo od Cas e (Re ac h-In ) Fr o ze n Fo od Cas e (Co ff in )

Figure 3 - Comparison of Normalized Compressor Power Consumption vs. Relative Humidity (All Four Tested

Display Cases)

Figure 4 shows that lowering of relative humidity from 55% to 35% for the open vertical meat and dairy

cases resulted in 20.7% and 20.8% reduction in cooling loads, respectively.

1,368

1,594

1,779

1,293 1,453

1,527 1,631

1,409

1,493

1,675

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

35% 40% 45% 50% 55%

Test Scenarios

TotalCoolingLoad(Btu/hr-ft)

Meat Case Dairy/Deli Case

Figure 4 - Comparison of Total Cooling Load vs. Relative Humidity (Meat and Dairy/Deli Cases)





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Fogging of the reach-in glass doors has an adverse impact on the merchandising capability of the display

case, and its quick recovery may require additional power consumption for anti-sweat heater. Figures 5

and 6 clearly illustrate the visual effect of fogging before and after the middle glass door was opened for

16 seconds in the test environment of 75oF/50%RH.

Before Opening After Opening andClosing

Figures 5 and 6 - Comparison of Visibility Due to Fogging Effect on the Middle Glass Door of Reach-in Freezer

Case at 75oF/50%RH

Quantification of the impact of reduced indoor relative humidity on the length of time required for the

fogging to be cleared was determined using a time calibrated analog video camera. Figure 7 compares the

fog recovery time requirements under various indoor relative humidity conditions. Clearly the fog

recovery time increased as a function of indoor relative humidity. Lowering the relative humidity from

50% to 35% resulted in 69.2% reduction in fog recovery time. Additionally, operating the fixture at

35%RH with anti-sweat heaters off yielded equivalent results to running the display case with anti-sweat

heaters on, under indoor relative humidity conditions between 45% and 50%RH.

60

24

78

55

69

0

10

20

30

40

50

60

70

80

90

35% 40% 45% 50% 35% w ithout ASH

Test Scenarios

FogRecoveryTime(seconds)

Figure 7 - Fog Recovery Time and Actual Fog Image on Middle Glass Door for Each Test Scenario




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