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DESIGN &

ENGINEERING

SERVICES

Indoor humidity plays a significant role in the thermal performance and

energy usage of supermarket display cases.

IMPACTS OF INDOOR

HUMIDITY ON REFR IGERATION

ENERGY USE

Increasing indoor relative humidity (RH) has

an immediate impact on the frost formation

on the coils and the latent cooling load of

the refrigeration system. Increased frost

formation on the coils and the latent load

directly impact the compressor electric

demand and, ultimately, thetotal system

energy usage.

To help supermarket operators realize the

importance of maintaining proper humidity

levels in their stores, Southern California

Edisons engineers at the utilitys

Refrigeration and Thermal Test Center

tested the effects of RH variations on the

performance and energy use of four

different types of display cases commonly

found in supermarkets. Their study focused

on identifying the effects of indoor RH on

frost mass, fog clearance time on glass

doors, refrigeration system performance,

and total power use.

RESULTS POINT TO ELECTRIC

DEMAND AND ENERGY SAVINGS

The results of this study indicate that there

is a direct relationship between the indoor

RH and the weight of moisture removed

from the air during the refrigeration

process. Other findings include:

Open vertical meat and dairy fixtures

demonstrated more vulnerability to

humidity variations and removed

more moisture from the air than

coffin and reach-in display cases.

Southern California

Edison engineers

conducted the testsat Edisons state-of-the-

art Refrigeration

and Thermal Test

Center in a controlled

environment chamber.

Results of testing

these cases under

various indoor

humidity conditions

showed that

refrigeration energy

use is dependent

on humidity

75F & 35% RH

75F & 40% RH

75F & 45% RH

75F & 50% RH

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Decreasing indoor RH had an

immediate impact on the mass of

condensate removed from the cases

and the latent load of all fixtures

Since infiltration of warm and moist

air accounts for approximately 80% ofthe cooling load of open vertical cases,

decreasing RH caused a significant latent

load reduction for these fixtures.

The effect of latent load reduction,as a result of lowering indoor humidity,was reflected directly on the evaporatorcoil load and, consequently, the powerconsumption.

Open vertical meat and dairy casesshowed the largest reduction incooling load and, subsequently, incompressor power demand asindoor RH fell.

STUDY MEASURED IMPACT

ON FOG RECOVERY TIME

The study also examined the effect of indoor

RH on the fog recovery time of a glass door

reach-in display case. Fog recovery time

affects the merchandising facet of the fixture

Findings include:

The fog recovery time variednoticeably as a function of indoor RH.

Lowering the RH from 50% to 35% resultein a 69.2% reduction in fog recovery time

Operating the case at 35% RH withanti-sweat heaters (ASH) off yielded thesame results in fog recovery time asrunning the display case with ASH on atan equivalent RH of 47%.

ENERGY-SAVING RECOMMENDATION

Supermarket designers and operators should

consider cost-effective ways to maintain

reasonably low RH in their stores, while

complying with occupant comfort conditions

In doing so, they should evaluate the trade-

off between refrigeration savings and air

conditioning penalty. Furthermore, they can

save additional energy under low indoor

humidity conditions by using smart defrost

and ASH controls.

FOR MOREINFORMATION

about howenergy-efficienttechnology canimprove your

operational costsand add to yourbottom line, callyour Southern

California Edisonaccount executive

or call SCEsRefrigeration and

Thermal Test Center at

(626) 812-7660 orvisit the Web site atwww.sce.com/rttc

457-0703

The increase in

refrigeration electric

demand under

more humid conditions

resulted from a

combination of

increased cooling

load and deterioration

in the effectiveness

of the evaporators

heat transfer

characteristics

and the blockage

of air circulation

across the coil.

,

,

,

,

,,

Comparison of Latent Cooling Load vs. Relative Humidity

Comparison of Condensate Mass Removed vs. Relative Humidity

Test ScenarioFog

Recovery Time(seconds)

75F / 35% RH 24

75F / 40% RH 55

75F / 45% RH 60

75F / 50% RH 78

75F / 35% RHwithout ASH

69