TESTING OF AIR HUMIDITY
in gastronomic display-cases

A. Dvornitsyn
RD Division Manager of AIsberg Ltd

A problem of importance of ambient/air humidity in gastronomic cabinets has been raised many times during different trainings and seminars carried out by Aisberg for technicians and commercial staff both of Aisberg and other companies. It’s quite important question because humidity affects refrigerated food quality. That’s why we have measured air humidity level inside refrigeration gastronomy cabinets, manufactured serially by Aisberg Ltd.

Testing objectives

Influence of below mentioned factors to a relative humidity inside gastronomy cabinets:

• refrigerant boiling-point;
• night covers availability;
• cross refrigeration system application.

Testing method and subject

Experimental research. The work was done in 2007 in the Aisberg RD Division laboratory. Ellipse CS and Ellipse S cabinets have been taken for tests. Measurements were made using electronic hygrometers (LAE Electronic) with periodical data registration by Assman psychrometer.

1. Refrigerant boiling-point

Refrigerant boiling point temperature influence has been studied in Ellipse S cabinet at constant temperature inside a cabinet itself.

1 phase of measurements

It has been determined that air humidity is the same in any point of the cabinet (fig. 1). This fact let us use for measurement only one humidity sensor, placed under air inlet grid of the cabinet.

 

2 phase of measurements

Refrigerant boiling point has been modified by its boiling pressure variation. For this purpose a corresponding adjusting valve (fig. 2) has been installed in the refrigerant return line and used to vary refrigerant pressure in the evaporator. Laboratory ambient temperature has been kept at 22±1°С, relative humidity of 34±4%. The findings have been got at boiling point range of -24…0°C. At that for the rage of -15…0°С thermostating temperature has been set to +9°С in order to provide thermostat operation at increased boiling points. For other boiling points thermostat has been set to +3°С. Air humidity in the cabinet varies periodically from minimal to maximal values and depends upon position of solenoid valve, that supplies refrigerant to evaporator. Thus, when refrigerant is being supplied to evaporator, humidity decreases. When refrigerant is not supplied, humidity starts increasing. During defrost cycle humidity values reach its maximum (fig. 3). The data obtained (fig. 4 and 5) shows that relative air humidity inside a gastronomic cabinet hardly depends on refrigerant boiling point in its evaporator. It happens because while holding preset parameters thermostat changes air cooling time in accordance with boiling point changes. Bigger air desiccation, caused by refrigerant boiling point reduction, is compensated with cooling time reduction.

fig.1

fig.2

fig.3

fig.4

fig.5

When refrigerant is not supplied to evaporator any more, moisture is back to the air by ice sublimation that is facilitated by air temperature increase during evaporator down-time period.

 

Night Covers Influence

Night covers that prevent outside air admission into a cabinet from a seller’s side (fig. 6), also reduce relative humidity. In the fig. 7 there is a diagram of air relative humidity time variation. Arrows indicate time of night covers installation (on the left) and taking off (on the right) So, night covers availability reduces air relative humidity for 3-5%. Moreover, maximum humidity is reached during evaporator defrost cycle and does not depend on night covers availability.

fig.6

fig.7

Cross refrigeration system application

At the beginning of test Ellipse CS cabinet has worked in standard refrigeration mode (both evaporators have worked simultaneously). After a while the cabinet has been switched into a cross refrigeration mode, then back to a standard one. Humidity changes occurred during this experiment are given in fig. 8 and 9.

fig.8

fig.9

Thus, cross refrigeration system application results in 10% increase of relative air humidity level inside a cabinet.

 

SUMMARY

1. Refrigerant boiling point influence to relative air humidity in a gastronomic refrigeration cabinet is hardly noticeable.
2. Night covers reduce relative air humidity in a cabinet. However, this reduction is quite low, about 3-5%. Leaky closure of covers, residual moisture in the bottom of a cabinet (left after evaporator defrost and water drainage) significantly compensate humidity losses occurred during evaporator defrost.
3. Application of cross refrigeration system makes considerable influence upon air humidity in a gastronomic cabinet. It results in 10% increase of relative air humidity level inside a cabinet while switching to a cross refrigeration mode.