Refrigerants

s Based on Heat Transmission Tests (B.T.U. per Hour) (From Bulletin 102, Eng. Experiment Station, University of Illinois, by Willard and Lichty) It has been found that the surface film offers a high resistance to the transmission of heat and in the formula the values and refer to this surface resistance. For outside weather condi tions it is usual to consider the value of F2 as equal to 3F1.

The coefficient of heat transfer (k) for refrigeration piping is extremely important, but is known to a slight degree only. The reason for this is that a large number of values are obtainable unless exactly the same conditions exist in each case. The value of k is influenced to a very large degree by the effect of stagnant films of fluids adjacent to both surfaces, to the kind of material, the kind of fluid on either side of the metal, the amount of scale or dirt on the surfaces and particularly on the velocity of flow of the fluids receiving or giving up heat to the metal. In calcu lating piping the best that can be done, then, is to give average values for k or a range of values which can be obtained by alter ing the conditions either by the velocity of the water, brine or other fluid past the heat transfer surface, the mean temperature difference on the two sides of the surface or by cleaning the sur face. Table IV. gives values usually accepted in good practice. The surface of the piping is usually taken as the outside surface, except for ammonia condensers where the surface is taken as that adjacent to the refrigerant.

Methods of Cooling.

Mechanical refrigeration as under stood at the present time means the absorption of heat by a volatile liquid in consequence of which the liquid boils. The usual method of procedure is to use direct expansion piping whereby the liquid boils in the pipes situated in the rooms to be cooled, or by first cooling brine and then by circulating the brine through the piping. The arrangement of the piping may be along the walls or under the ceiling one example of which is shown in fig. 2. In

cold storage refrigeration natural air circulation is resorted to almost entirely, in which case the velocity of the air in contact with the pipes is very low thereby resulting in a low value of the coefficient of heat transfer.

In some exceptional cases a bunker room is installed. Such an arrangement of piping, which may be in an ante-room, a corridor or in a separate room, is the suitable nesting of the piping in an enclosure whereby the air may be forced past the piping and into the room. The bunker system requires, then, a forced circulation of the air. The advantages are in being able to use less pipe and in being able to keep the refrigerant out of the room if such a matter is an asset. The disadvantages are in having to provide power for circulation and in the difficulty in cleaning the piping should frostation accumulate.

At times there is an advantage in having a hold-over arrange ment of tanks, as shown in the figure as 8 in. tubes. By this scheme the brine in the tubes is cooled during the day time opera tion of the refrigerating machine and this cold brine will assist in holding the temperature by absorbing the heat generated by the effect of heat leakage, chemical action of the various commodities, etc.

When brine is used the method of cooling is by the use of the multi-pass shell and tube brine cooler. Such an apparatus has the brine inside the tubes, which are usually 2 in. in diameter and it is designed for 2, 4, 6 or 8 passes of the brine. The ammonia is inside the shell and its boiling action is similar to that of the horizontal return tubular boiler in the generation of steam. The gas-releasing surface and the gas volume are sufficient to prevent the liquid returning through the suction pipe line to the com pressor. Besides the shell and tube brine cooler the shell and coil and the double pipe designs are used for carbon dioxide installa tions and the double pipe for ammonia. In the case of carbon dioxide the unit pressure is so great that the refrigerant is kept inside the pipes.