Refrigeration Insights series. Season 2: The enemy within, part 8.
We have devoted articles to the water inside ammonia and to the air inside the condenser. Today it is the turn of the water that is exactly where it should be, but that can also degrade the system: the water in the recirculation circuit of the evaporative condenser. The source is the technical paper by Michael Sanders and Jorge Hernandez presented at the 2017 IIAR conference.
The evaporative condenser in one sentence
The paper describes it precisely: hot ammonia gas flows through the coil above and condenses as the water sprayed over the tubes evaporates. Most of the cooling comes from evaporation; the air also absorbs some of the sensible heat.
From that physics comes the consequence that governs the whole subject: if cooling depends on evaporating water over the coil surfaces, keeping those surfaces clean means keeping the capacity of the condenser. And the water that evaporates leaves behind everything it carried dissolved.
Cycles of concentration: the number you need to know
Every liter that evaporates concentrates the minerals in the water that remains. The paper quantifies this with the concept of cycles of concentration: the number of times the circuit water is concentrated relative to the make-up water. If the make-up water has 100 ppm of hardness and the condenser water has 300 ppm, that is 3 cycles. The same applies to conductivity: 300 micromhos entering against 900 in the circuit, 3 cycles.
That number is controlled by blowdown: the removal of water from the circuit, intentional through a valve, or unintentional through losses at pump seals, splashing and basin overflow. Too little blowdown and the minerals concentrate until they scale; too much and you are throwing water and chemicals down the drain.
The three enemies of the circuit
The classic triad that water treatment manages:
Scale. The concentrated minerals precipitate on the hot tubes. Each layer is an insulator that raises the condensing pressure, with the same economic effect as air in the system: more energy consumption, less capacity.
Corrosion. Aerated water with dissolved solids attacks the metal of the coil and the structure. In the article on corrosion under insulation we saw that the electrolyte is what enables the cell; here the electrolyte circulates by design.
Biological activity. Warm, aerated water is an ideal culture medium. Biofilm insulates thermally, protects the corrosion taking place underneath, and in its most serious form involves health risks that no plant can afford.
The program, not the product
The conclusion of the paper shows a good sense that is welcome in a subject full of sellers of potions: the study of the system, of the quality of the make-up water and of the inspections of the equipment is what determines the need for treatment and its type. The recommendations of a reputable consultant must be studied and reviewed together, the consultant must train the staff to implement them, and the plant must establish its own testing program with scheduled inspections.
In other words: water treatment is not a drum you buy, it is a program you manage, with your own data and periodic verification.
The practical takeaway
Three numbers to ask for this week: the hardness (or conductivity) of your make-up water, that of the circuit water, and the ratio between the two. With that you know your real cycles of concentration. Compare them against what your treatment program says they should be. If nobody knows what the target value is, you do not have a program: you have a drum.
Next week, the technical close of the season: mechanical integrity of vessels and piping, with non-destructive inspection as the protagonist.
Source: M. Sanders and J. Hernandez, "Water treatment for evaporative condensers", Technical Paper #3, IIAR Natural Refrigeration Conference, 2017. Thermomac is a corporate member of IIAR.
