Frost formation

Replacement of orchard heaters by wind machines is the most desirable control measure. These large propellers force the warmer air aloft to the ground, where it mixes with the cold air, minimizing frost formation. 

Advanced Genetic Science Field-tested Frostban —a GE bacterium that inhibits frost formation on crops the first outdoor tests of a GE organism... 

This was verified in a subsequent study by NARCO [25] in which a more detailed explanation of the mechanism of frost formation was given. It is well known that silica bricks exposed to soda vapor develop a liquid phase at around 1450°F. The amount of liquid in the brick will increase as the amount of flux is increased. The liquid thus formed is gradually absorbed into the interior of the brick until the voids are filled. On the... 

We have developed an in situ cooling system to perform IR measurements at very low temperatures under atmospheric pressure by combining a commercial thermostatic stage (Linkam FT-IR 600) and nitrogen gas flow system to avoid frost formation (Figure 1). 

Next consider the courts. Early this year, the U. S. Court of Appeals for the District of Columbia affirmed a lower court decision that kept NIH from approving a field test of recombined bacteria to be sprayed on plants to inhibit frost formation. The bacteria to be produced were not novel and were already isolated in the environment in small quantities. From an environmental protection standpoint, most scientists regarded the scaled-down experiment approved by NIH s Recombinant Advisory Committee as innocuous and looked forward to evaluating obtained field data. 

AUTOMATIC FROST CONTROL - Control which automatically cycles refrigerating system to remove frost formation on evaporator. 

FREEZE-UP - (1) Formation of ice in the refrigerant control device which may stop the flow of refrigerant into the evaporator. (2) Frost formation on an evaporator which may stop the airflow through the evaporator. 

Operating and safety personnel can assist in the proper maintenance of equipment by conducting periodic inspections in the course of their work. An operator is often the first person to detect a malfunction (frost formation, decreased production rate, evidence of contamination, variation in operating temperatures and pressures, appearance of leaks, etc.). Accordingly, operating personnel must work closely with the maintenance and safety personnel to prevent serious breakdowns and accidents. 

This paper provides expressions for heat and mass transfer to the frost surface and information regarding the frost formation which can be used by designers of cryogenic systems with uninsulated surfaces, and furthers the general understanding of the frost phenomena. 

Since a quasi-steady-state analysis was used, it was necessary to choose a point in time to establish an empirical method for the evaluation of (5). The point in time chosen was at the time when the frost formation acquired a maximum resistance to heat transfer, or when the experimental heat-leak data (from test section boil-off) showed a minimum value. At this point, analysis of heat transfer data indicated that Ts approaches a maximum value (see Fig. 4) and one may observe glazed patches on the frost surface and, on some tests, liquid droplets. Since Ts remains relatively constant from this time on, q will vary primarily with frost surface area and, at this point, will show a minimum value because all subsequent areas will be larger. This point in time is called the < min point and is used as a reference point in subsequent discussions. 

After the min point there was melting, which increased the density and conductivity (and in some cases substantially reduced the thickness) of the frost formation, thus lowering Tg. The conduction equation through the frost may be written as... 

This lower Ts will cause a rise in qfA and m and produce a less dense, layer of frost with lower conductivity on the previously higher-density frost surface. This low-density frost formation will continue until the surface melting condition is reached again and the cycle repeats. Thus, beyond gmin, the frost surface behavior is somewhat like a self-stabilized automatic control system fluctuating near the qfA)q xn value and Ts-gn n value shown in Fig. 1. 

It should be possible to use (5) with the indicated modifications for frost formed on vrarmer surfaces, after the gmin point. However, it is expected that the frost formation itself would be quite different on the warmer surface. 

Points of low relative humidity with higher wind velocities fell above and to the right of the curve, that is, showed a greater for the same value of the correlating parameter. The apparent reason for this longer tg iin is the lower mechanical strength of this frost formation, and the loss of frost by mechanical failure increases the time required to reach the min point. The increased loss of apparent condensate from the frost formation at low specific humidities is discussed later and is shown in Fig. 6. 

It was found that the mass of water vapor diffused and condensed as indicated by (5) was not equal to the experimentally determined mass in the frost formation (see section on... 

Some of the apparent water vapor condensate does not become a permanent part of the frost formation. 

The m/A for the cylinder used in this experiment has been determined by placing a removable test section in the wind tunnel used in [ ] and weighing the condensate collected on the test section. A frost cage was slipped onto the test section after the wind tunnel was stopped, excess frost was removed from around the cage, and then the test section with cage was placed in a plastic bag. Hot water was introduced inside the test section to melt the frost and dry the test section before the bag was removed and weighed repeatability was within 1 %. The mass transferred, which becomes part of the frost formation for the special case of a 2j-in.-diameter cylindrical test section, can be calculated from the following expression ... 

Three things must be pointed out at this time At first there is an additional component of heat transfer due to diffusion from the air stream through the porous frost which must be added to the derived conductivity expression this component appears to be proportional to the rate mjAy but disappears when the frost becomes sufficiently dense (indicated by the increased conductivity shown in Fig. 8). At about the same time, the expression for density (10) also breaks dowm and the net mass transfer becomes much smaller due to melting, dripping, etc. Some of the runs checked seemed to go through this transition at a density of about 35 Ib/ft. Exact expressions for frost properties will be difficult to obtain because we have observed two kinds of frost on the test section where the boundary line was at the separation point on the cylinder. In fact, Pingry, et al. p] reports three kinds of frost formation and the data tend to indicate that the properties differ somewhat for the different kinds of frost. 

P. M. Chung and A. B, Algren, Frost Formation and Heat Transfer on a Cylinder Surface in Humid Air Cross-Flow, Heating Piping Air Conditioning, (Sept.-Oct. 1958). 

With respect to the use of our expressions for surfaces at a higher temperature than LN2 temperatures, we would expect the frost formations to be different for other surface temperatures. Therefore, during the transient portion of the frost formation (to time min) we would not expect agreement with our work. However, after the jmin point is reached we would expect general agreement for all frost formations. 

Answer by Author We believe the frost formation follows a growth pattern which consists of an initial dendritic, low-density structure with the voids subsequently filled by diffusion processes and, after gmin, by some flooding with liquid. 

Figure 4.7. PVC assembly for crankcase ventilation system preventing frost formation. [Adapted from Spix, T. A., US Patent 6,782,878, Aug. 31, 2004.1...

Consider that the outer film heat transfer coefficient, /a-i, is a function of the Grashof and Prandtl numbers and that the emissivity factor F is equivalent to the emissivity of snow. If it is further assumed that the enthalpies of water vapor, and hi, are functions of temperature alone, then measurements of the unit area heat transfer rate q /A, the ambient temperature, and the time rate of frost formation w, can now be made in order to yield the remaining unknown in (la), the temperature of the frost surface Ti. Additional measurements of the container skin temperature, T2, the frost thickness, and... 

To minimize end effects, spherically shaped containers were utilized in the experimental apparatus. These containers were of soft aluminum (2S), having an outer diameter of 5.375 in. and a wall thickness of 0.029 in. Each sphere was fitted with a standpipe welded into its top. Two types of experiments were performed. These were (1) the determination of frost formation rates and thicknesses as functions of time, and (2) the determination of liquid oxygen vaporization rates and subsequent heat transfer rates under varying ambient conditions of temperature, pressure and humidity. 

Frost formation rates were obtained by graphical differentiation of experimentally determined frost mass—time functions. These latter functions were obtained by measurement of the difference in empty container weights before the start of and immediately following the completion of a timed test period. During such periods, the container was maintained essentially full of liquid oxygen. Measurements of frost thickness were accomplished with the use of a cath-etometer and were taken at the center line of the container at incremented times during each test. 

Fig. 2. Rate of frost formation on an aluminum container as a function of time and specific humidity.

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