Evaluation of the frost properties on a semipermeable membrane

https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.116Get rights and content

Highlights

  • Using semipermeable membranes could delay the formation of the frost for 30–60 min, depending on the operating conditions.

  • Increasing the moisture transfer through the membrane results in less coverage of surface with frost.

  • Under extreme weather conditions, the membrane may become completely blocked and act as an impermeable plate.

Abstract

The onset of the formation of frost on a semipermeable membrane, which allows both heat and moisture transfer through it, can be delayed, due to the moisture transfer through the membrane however, at severe conditions, frost formation cannot be avoided completely. In this paper, the formation of frost on a semipermeable membrane is evaluated both visually and quantitatively. The frost thickness and mass, and the area of the surface covered with frost are measured for a semipermeable membrane and compared with those on an impermeable plastic surface. Moreover, the effect of operating conditions (air humidity ratio, air mass flow rate, liquid desiccant temperature and salt concentration) on the properties of the accumulated frost, on the semipermeable membrane, are investigated. The results show a 30–60 min delay in frost deposition and also the formation of a thinner, lighter frost layer on the semipermeable membrane compared to the impermeable surface. Also, results show that increasing the moisture transfer through the semipermeable membrane, by decreasing the humidity ratio of the cold liquid desiccant, can lead to frost that is less thick; however, increasing the humidity ratio of the humid air leads to a higher rate of frost formation. Increasing the air mass flow rate or decreasing the surface temperature increases the thickness of the frost layer on the semipermeable membrane.

Introduction

Condensation/frosting is the physical phenomenon of the deposition of water vapour on a surface, in the form of water droplets, or a porous structure of ice, when the temperature of the surface is below the dew point/freezing point of the surrounding humid air. Frosting occurs in many industries, such as HVAC (Heat, Ventilation, and Air-Conditioning) systems and refrigerators, aviation, electrical transmission, transportation, and wind power [1], [2]. Heat/energy exchangers, where heat/energy (heat + moisture) are transferred between two fluids, are commonly used in different industries. In cold climates, the exchangers are likely to be used under conditions where frost may develop [1], [3], [4], [5], [6], [7].

Semipermeable membrane-based energy exchangers have gained attention in the HVAC industry over the past decades. A semipermeable membrane is a porous material, which is permeable to water vapour, but impermeable to liquid water, thus allowing both heat and moisture transfer through the membrane. A few studies have investigated the performance of energy exchangers under frosting conditions [1], [3], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. From the literature, it can be concluded that energy exchangers are more frost resistant compared to heat exchangers operating at the same conditions. A previous study by the authors [18] shows that the initiation of frost formation is delayed on the membrane compared to the plate, because the moisture transfer through the membrane reduces the dew point temperature of the air. However, to completely understand the process of frost formation on a semipermeable membrane, further studies on the properties of the frost layer formed on a membrane must be performed.

The measurement of frost properties on an impermeable surface has been presented in the literature [19], [20], [21], [22], [23], [24], [25], [26]. Recently a review paper [27], summarized the frost thickness measurement techniques that have been used in the literature. They concluded that frost layer thickness is the most important parameter that can describe frost characteristics. Furthermore, the effect of operating conditions (air temperature, air relative humidity, and surface temperature) and surface properties (hydrophobicity and hydrophilicity, direct contact angle) on the properties of a frost layer on an impermeable surface have been investigated [28], [29], [30], [31], [32], [33], [34], [35], [36]. There are no existing studies that show these results for a frost layer on a semipermeable membrane.

The main goal of this paper, is to quantify the properties of frost on a membrane, including thickness and mass of frost, and the surface area covered by the frost, and to compare the properties of the frost layer on the membrane with those on the impermeable plastic. The second aim is to find the effect of operating conditions and moisture transfer rate on the frost layer shape, thickness and mass of frost layer on the membrane.

Section snippets

Test facility

A schematic of the test facility used to measure the properties of frost (mass and thickness) on a membrane and a plastic surface is shown in Fig. 1. The test facility was designed to create a cold surface, with warm humid air passing over the top of the surface. A cold liquid desiccant (LD) was passed underneath the surface to maintain a constant surface temperature and create a driving potential for moisture transfer through the membrane. More details of the test facility and the uncertainty

Comparison between frost properties on a membrane and plastic

To find the differences in frost growth on the membrane and the plastic, two experiments are conducted, at the same operating conditions. Top-view photographs of the frost layer on the membrane and plate are shown in Fig. 3. The frost layer on the plastic, Fig. 3(a), consists of a uniform layer of needle-shaped crystals at X1 (the air inlet), while plate-shaped crystals are seen at X2 and X3. The frost layer on the membrane, Fig. 3(b) consists mostly of irregular hexagonal-shaped ice clusters

Conclusions

The experimental study in this paper compares the measured thickness and mass of frost accumulated on a plastic sheet and a semipermeable membrane. The operating conditions that effect the thickness and mass of frost on the membrane are then investigated. The tests are accompanied by visual observations to help explain the influence of the operating conditions on the frost formation process. It is found that frost growth on a membrane can be reduced due to the moisture transfer through the

Conflict of interest

The authors declared that there is no conflict of interest.

References (40)

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