Features of the water-chemical mode of hot water boilers

Ivan Tikhonov

This article discusses what changes occur in the make-up water when it enters the boiler circuits of heat supply systems.

Heat supply systems can be with a single- and double-circuit. In the first case, mains water circulates in the heat supply system directly through the hot water boiler. In the second case, mains water circulates through the mains heater, in which it is heated by boiler water. Boiler water circulates directly through the hot water boiler, getting heated up in the boiler and giving heat to the mains water in the heater. In the first case, the quality of the make-up water must meet the stringent requirements for hot water boilers. In the second case, it is customary to impose somewhat less stringent requirements on the quality of make-up water, since only the network circuit is fed. The boiler circuit is closed and, with proper operation, does not require recharge.

To begin with, let’s look at a single-circuit heating system. That is, all the water of the heat supply system passes through a hot water boiler.

In the heat supply circuit, the water is heated. At the same time, gases dissolved in it begin to be actively released from the water. So, at a temperature of 100 ⁰C at atmospheric pressure, the solubility of gases in water is zero. Thus, when the water is heated in the boiler circuit, oxygen and carbon dioxide are released. The reduction of carbon dioxide in the water shifts the carbon dioxide equilibrium of the boiler circuit water towards the release of solid calcium carbonate. Carbon dioxide equilibrium was observed in the make-up water, i.e. calcium carbonate was not released from the water because the carbonates were in the form of bicarbonates. The removal of carbon dioxide due to an increase of water temperature caused the transition of bicarbonates into carbonates and, accordingly, precipitation of calcium carbonate.

The less dissolved carbon dioxide in the water is, the higher the pH value of such water will be. This is due to the fact that carbon dioxide is removed from the water when water is heated and it is no longer involved in the hydrolysis process to form carbonic acid. As a result, calcium bicarbonate, which was obtained in water during its dissolution by carbonic acid from a solid sedimentary rock of calcium carbonate, again passes into calcium carbonate and it is released from the water.

At pH = 8.37, carbon dioxide is almost completely absent in the water, so carbonates begin to form in the water, which tend to pass into the solid phase in the form of calcium carbonate. At low concentrations of the carbonate ion, this process is probably hindered by the dipole properties of water and the ionic strength of water. The higher the ionic strength of the water is, the more carbonate ion can be in the water. But as the water temperature increases, the concentration of dissolved carbonate (carbonate anion) decreases significantly. The product of solubility for calcium carbonate at a temperature of 25 ⁰C is 4,4*10^-9, at 100 ⁰C is 0,47*10^-9.

This means that with a calcium concentration equaling 0.1 mmol/l at a temperature of 25 ⁰C, the carbonate concentration will be 0.06 mmol/l. At the same concentration of calcium, but at a temperature of 100 ⁰C, the carbonate concentration will be 0.0065 mmol/l. Above these concentrations, the release of the solid phase of calcium carbonate will begin.

As can be seen, at such high temperature in the boiler circuit, even with small concentrations of carbonate and calcium, a solid precipitate of calcium carbonate can begin to form in the water.

A simple conclusion can be made – it is necessary to ensure a deep softening of the make-up water of the boiler circuit in order to exclude the formation of calcium carbonate deposits in the boiler circuit.

It is possible to talk about any sufficient amount of calcium in the boiler circuit only if the pH value of the boiler water is significantly lower than 8.37. In this case, there are practically no carbonate ions in the water (only bicarbonates exist) and calcium is in the form of a cation.

But can we say that the pH of the boiler circuit water will be below 8.37?

In the circuit, water is heated and carbon dioxide is intensively released through the air vents. In the absence of recharge or low recharge, there will be practically no carbon dioxide in the boiler circuit, and the pH value of such water will be kept in the range from 8.0 to 8.5 or more (depending on the temperature and the recharge rate). Therefore, in such water, the achievement of thermodynamic equilibrium will definitely lead to the release of calcium carbonate even in the case of their small concentrations.

Therefore, for the boiler circuit, under any conditions, it is necessary to produce a deep softening of the make-up water.

It should be noted that at pH values of about 8.5, calcium carbonate precipitates as a thin dense layer on the heating surfaces. Unlike steam boilers with a water space, where a high pH value of boiler water (about 12) transforms dissolved calcium bicarbonate of feed water (if there is a slip of hardness salts) into sludge in boiler water. In a hot water boiler (at relatively low pH values), calcium carbonate will fall out as a solid precipitate.

In a two-circuit heat supply system the boiler circuit does not require recharge, and this will allow the boiler to work even when it is filled with water with increased hardness. After a certain time of operation, the water in the boiler circuit will reach thermodynamic equilibrium for conditions of elevated temperature, and part of the hardness will precipitate. But the amount of this sediment will be small. For example, if the volume of the boiler circuit is 20,000 liters. The concentration of calcium in water is 1.5 mmol/l (1.5 *40 = 60 mg/l). The concentration of carbonate in water is 1.5 mmol/l (1.5*60 = 90 mg/l). Total CaCO₃ = 60+90 = 150 mg/l. If all the calcium precipitates with carbonate, then the precipitate mass will be equal to 20,000 *150 = 3,000,000 mg = 3 kg.

Nevertheless, even with such a small amount of calcium carbonate, it will be evenly deposited on the heating surfaces of the boiler. In the event that the boiler circuit is filled with water with a low pH value, active hydrogen corrosion will be observed. Naturally, the processes of corrosion and sedimentation will take place only at the stage when water reaches a new thermodynamic equilibrium in the boiler circuit. Then, when equilibrium is reached, the corrosive and sedimentation processes will stop, but the resulting products will settle on the heating surfaces, additionally binding to each other. In this case, the efficiency of heat exchange will decrease, but the boiler will be able to work quite well. This will happen only if there is no recharge of the boiler circuit.

The network circuit of the heat supply system does not have heating surfaces with high temperature voltage, as in a hot water boiler. Therefore, it is considered that the quality of the make-up water of the network circuit can be significantly lower than that of the boiler. This circumstance reflects the criterion for assessing the quality of network water, called the carbonate index. But it must be remembered that this criterion does not mean that there is no release of calcium carbonate in the network circuit. The criterion only assumes that the precipitation will have a certain low rate. This circumstance requires periodic inspections of network heaters and regular chemical flushes.

It can be concluded that when feeding the contour of a hot water boiler, deep-softened water is always required as well as in the case of steam boilers. When feeding the mains circuit, the hardness of the make-up water may be slightly higher, but due to the release of carbon dioxide, calcium carbonate will be released at a higher or lower rate. With a large flow of make-up water of the network circuit, it may be economically unprofitable to soften and alkalize it. Then it is necessary to organize periodic inspections and flushing of heat exchange equipment. Also, in the case of a large recharge of the network circuit, you should be prepared for constant leaks of pipelines caused by corrosion processes. The large flow rate of the supply circuit does not allow to achieve thermodynamic equilibrium of water depending on temperature. Therefore, corrosion and sedimentation processes will constantly occur. The speed of these processes will directly depend primarily on the make-up water consumption, the temperature of mains water and the concentration of calcium bicarbonate in the make-up water.

In conclusion, I want to say that the best water treatment for a heat supply system is the organization of a two—circuit system and the maximum possible reduction in the supply of the network circuit, and the absence of supply of the boiler circuit. In this case, the system can operate without water treatment at all, under condition there will be an annual inspection of the network heaters and, if necessary, chemical flushing of the heat exchange surface of the heaters. These arguments are presented for heat supply systems with mains water temperature of no more than 100 ⁰C.