In large capacity refrigeration plants with evaporators located at widely spread out areas or locations of application of cooling, it become necessary to run long lengths of refrigerant lines to the evaporators, which will raise the refrigerant charge, increase the pressure drop in the refrigerant side and also increase the chances of refrigerant leakage.
For such applications, a liquid is chilled in a centrally located chiller package and it is circulated by pumps between the chiller and the various cooling coils (in AHUs) or heat exchanger jackets in industrial processing, located at the cooling application points, for effecting transfer of heat from the substance to be cooled. Thus the liquid becomes a carrier of refrigeration and so is known as ‘Secondary Refrigerant.’ As is evident the secondary refrigerant does not undergo any change of state.
Water is used as the secondary refrigerant for temperature applications above its freezing point of 0ºC and for sub-zero application brines or glycols are employed.
Brine is a solution formed by dissolving a soluble substance in water. The soluble substance could be a salt, like, sodium chloride or calcium chloride or a glycol. On mixing a soluble substance in water, its freezing point is lowered, or in other words, the solution so formed has a lower freezing point than water.
The weight of the salt or glycol in the solution, expressed as a percent of the total weight of the solution, is known as the ‘strength’ or ‘concentration’ of the brines. As the concentration of the solution increases, its freezing point goes down. Thus, for a particular concentration of a solution, there is a definite freezing point. When a solution at a particular concentration is cooled below its freezing point, some portion of water in the solution will freeze out as pure ice. With the reduced water content, the remaining solution attains higher concentration and consequently a lower freezing point. Ultimately, on further cooling the solution itself freezes and the temperature at which this happens is known as its eutectic point.
With the addition of a soluble substance in water, the density or specific gravity of the resulting solution will be higher; but its specific heat becomes lower than that of water. Since both specific gravity and specific heat of water are one, the specific gravity of a brine / solution will always be higher than one, while its specific heat will be less than one.Further, the specific gravity of a brine at a particular concentration decrease as its temperature is increased, while its specific heat increases. Therefore, while calculating the heat removal from brine, its specific gravity and specific heat values for the particular concentration should be used.
Following are the important factors to be considered in selecting the brine.
Freezing Point: The brines should have a concentration for which the freezing point has necessarily to be lower than the brine temperature to be maintained for the application – generally by about 5 to 8ºC. This difference is to prevent sudden crystallization of the brine, if the temperature of the brine falls down accidentally.
Safety: The brine should be non-inflammable and non-toxic.
Suitability: Should be compatible with the materials of the equipment.
pH Value: Ideally should be neutral, to minimize corrosion. But neutral or near neutral brine can become corrosive with contamination during the operation.
Specific Heat: Determines the rate of flow of brine required – higher the specific heat, lower will be the rate of flow required.
Density: has no bearing on heat transfer aspects, but is helpful in finding the strength of a brine with the help of a hydrometer and thermometer.
Viscosity: again does not influence the heat transfer aspect; but where the viscosity of a brine rises fast as its temperature falls, the pumping head and so the pumping horsepower will go uneconomically high. A typical example is propylene glycol – its viscosity rises very high below a temperature of 7ºC (20ºF).
The brines in common use are of sodium chloride, calcium chloride and glycols, such as ethylene glycol, propylene glycol, etc.
Salt Brines: Sodium Chloride (common salt) is cheaper than calcium chloride. But since the freezing point of sodium chloride brine is comparatively high, it can be used only in applications requiring brine temperature not lower than about – 12ºC (10ºF). Calcium chloride brine is favoured for most applications, because of its lower freezing point. Where contact with calcium chloride brine is not permitted, sodium chloride brine is used, such as in fast freezing / glazing of fresh catch of fish in fishing trawlers and other foods.
Ideal pH value for sodium and calcium chloride brine is 7.5 to 8.5; a brine slightly alkaline is considered safer than being slightly acidic. To correct acidic condition of these brines, caustic soda (an alkaly) dissolved in warm water is added, while for correcting an alkaline condition, acetic or chromic or hydro-chloric acid is used.
Standard steel pipes can be used for brines piping – copper pipe cannot be used.
Glycols: Ethylene and propylene glycols are widely used in cooling as well as heating applications. Ethylene glycol solution is usually preferred, as it has more desirable properties at lower temperatures, but operation below – 50ºC (-60ºF) is not advisable. However, for food and beverage cooling processing applications, where there are chances of the glycol solution coming in contact with the food or beverages, only propylene glycol is employed, as propylene is not toxic as ethylene glycol.
Glycols are circulated by centrifugal pumps, with rubber impregnated asbestos or equivalent for the gland packing. However, to prevent / minimize drip losses of glycol solution through the pump gland, mechanical seal is preferable.
Glycols attack galvanized surfaces, forming sludge, so should be avoided. Standard steel and copper piping can be used for glycol lines.
Corrosive effect of brines and glycol solution
The salt brines can become corrosive, due to contamination, in handling and operation, like, when too much air gets mixed with the brines. So excessive aeration of brines is to be avoided – as far as possible, should be kept in closed systems, the brine tanks should be kept covered etc. These brines attack copper and steel parts, resulting in pitting of surfaces. If brine enters the refrigeration system, it will naturally affect the system, calling for elaborate flushing, cleaning, dehydrating, etc. So, in addition to correcting the pH of the brines, inhibiters have to be added to combat corrosion. Inhibiters, like sodium chromate or sodium – dichromate is found to be effective with these brine in overcoming corrosion. The recommended inhibitor concentration is :
for calcium chloride brine ……… 2 kg per 1000 litres (1.67 lb per 100 US gals) of brine
for sodium chloride brines …….. 3.2 kg per 1000 litres ( 2.67 lb / 100 US gal) of brine
Dichromate comes in granular form and it dissolves very slowly in cold brine. So it should not be added directly to low temperature brine ; dissolve it in warm water and add the solution away from the brine pump suction take off point in the brine tank, so that only dilute solution reaches the pump. A word of caution – if the chromate or its solution comes in contact with skin, rashes can develop – wash skin immediately with water.
Both ethylene and propylene glycols, when pure, are less corrosive than even water, but due to the quality of water used for preparing the solution they can turn to be corrosive, particularly when air gets mixed. Soft water or if possible distilled water or condensate water should be used for preparing the solution to avoid the effects of bad water quality. In any case, inhibited glycols, which are available, should be used. If inhibited glycol is not available, the glycol manufacturer should be approached for recommending the suitable inhibitor.
Sodium dichromate or chromate should not be used as inhibitors with glycols, as oxidation of glycol can occur, making the solution more corrosive.
Maintenance: To ensure fairly non-corrosive solutions for a long period of time, it is very important to monitor the inhibitor concentration and replenish the inhibitor as necessary. For this, the pH reading should be regularly recorded followed with periodical analysis of solution sample. This systematic approach also will prevent indiscriminate addition of inhibitor, which is also very harmful.
The rate of depletion of the inhibitor in a solution depends upon the usage of the plant- this may vary from job to job. Hence, in the initial stages on commissioning the plant, it may be necessary to do the analysis of the solution frequently to establish a pattern for the maintenance schedule.
Since these solutions are generally used in industrial applications as secondary refrigerants, it should not be difficult to adhere to these simple yet very import maintenance steps. Plant failures due to corrosion can be attributed to ignorance or callous negligence of these maintenance steps – the necessity of these steps has to be overemphasized.
Other secondary refrigerants: Many of the halocarbon refrigerants also have been in use as secondary refrigerants, because of their favourable properties such as, low freezing points, good heat transfer co-efficient, non-flammability, stability, low viscosities, etc. But because of environmental considerations now these cannot be used.
Chilled special grade oils for heat treatment purposes, chilled kerosene oil as coolant for machining cast iron, etc are others in this category.
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