SECONDARY COOLANTS

Where a secondary refrigerant fluid is to be circulated, and the working tempera­tures are at or below 0°C, then some form of non-freeze mixture must be used.

Aqueous solutions of sodium chloride and calcium chloride were the first types of sub-zero secondary fluids, and this is why the collective term brine is some­times used. Propylene glycol-water mixtures are the most common of the gly­cols used as secondary coolants in refrigeration installations and are permissible where contact with food is possible. Ethylene glycol is another type and there are many other products on the market, some of which do not contain water. The thermal properties of Propylene glycol-water mixtures are shown in Figure 12.1 .

With any solution, there will be one concentration that remains liquid until it reaches a freezing point, and then it will freeze solid. This is the eutectic mix­ture, and its freezing point is the eutectic point of the solute (see Figure 12.2). At all other concentrations, as the solution is cooled it will reach a temperature where the excess water or solute will crystallize out, to form a slushy suspen­sion of the solid in the liquid, until the eutectic point is reached, when it will all freeze solid. For economy of cost, and to reduce the viscosity (and so improve heat transfer), solutions weaker than eutectic are normally used, provided there is no risk of freezing at the evaporator.

The concentration of a solute has a considerable effect on the viscosity of the fluid and so on the surface convective resistance to heat flow. There is little published data on these effects, so applications need to be checked from basic principles.

Brine may be pumped to each cooling device, and the flow controlled by means of shut-off or bypass valves to maintain the correct temperature (see Figure 12.3). The brine pump is usually in the return line to the chiller as shown, so the pumping rate is based on the return temperature density.

Where a brine system services a multiple-temperature installation such as a range of food stores, the coolant may be too cold for some conditions, causing excessive dehydration of the product. In such cases, to cool these rooms the brine must be blended. A separate three-way blending valve and pump will be required for each room (see Figure 12.4).

Air and moisture must be kept out of the system to avoid corrosion, and use of a closed system rather than an open one is preferable where possible. Pressure controlled dry nitrogen can be applied above the expansion tank or storage tank. The preferred brine circuit is that shown in Figure 12.3, having the feed and expansion tank out of the circuit, which is otherwise closed. This avoids entrain — ment of air and too much surface exposure. The same arrangement can be used with the divided storage tank as shown in Figure 12.4. except that the tank will be enclosed, with a separate feed and expansion tank. To reduce the effects of corrosion inhibitors are added, typically sodium chromate in the salt brines, and sodium phosphate in the glycols. These are alkaline salts and help to counteract the effects of oxidation, but periodic checks should be taken, and borax or similar alkali is added if the pH value falls below 7.0 or 7.5.

Brines are hygroscopic and will weaken by absorbing atmospheric moisture. Checks should be made on the strength of the solution and more salt or glycol is added as necessary to keep the freezing point down to the required value.

Posted in Refrigeration and Air Conditioning