- Requirements for an ideal coolant
- Heat carrier – water
- Heat carrier – antifreeze
- How to choose the optimal coolant
In terms of their prevalence, heating systems with circulation of a liquid coolant beat all records – their continued popularity is largely due to the harsh winter climate of Russia. Liquid heating systems include a whole range of equipment, including boilers, heat exchangers, pumping stations, often many kilometers of pipelines. The correct operation of the heating complex directly depends on the characteristics of the coolant, so what kind of liquid is best used in this capacity and why?
Requirements for an ideal coolant
It should be noted right away – there is no such coolant. Any of the existing ones regularly performs its functions only in a certain temperature range, going beyond which leads to dramatic changes in its quality characteristics.
The heat carrier is obliged to transfer the maximum amount of heat per unit of time with minimum heat loss. The viscosity of the coolant has a serious effect on its pumping within the heating system, so the less viscous it is, the better.
The coolant should not have a corrosive effect on a variety of structural materials of pipelines and heating devices, otherwise the choice of these materials will be strictly limited. In addition, the lubricating ability of certain coolants imposes restrictions on the structural material of circulation pumps and other mechanisms in contact with them..
From the standpoint of household safety, the coolant must have certain (safe) characteristics in terms of toxicity, the temperature of ignition of the liquid and the outbreak of its vapors.
And the last – the liquid used as a heat carrier must be affordable or, in the case of a high cost, maintain its characteristics and volume for a long time during operation in the heating system.
Heat carrier – water
Of all the liquids that exist on Earth in a natural state, water has the highest heat capacity – on average 1 kcal / (kg deg), that is, if one kilogram of water is heated to 90 ° C and cooled in a heating radiator to 70 ° C , then 20 kcal of heat will enter the room heated by this radiator.
This liquid has a high density (917 kg / m3), decreasing with heating or cooling. By the way, water is the only natural liquid that expands both when heated and cooled..
The ecological and toxicological characteristics of water surpass those of any synthetic heat transfer fluids – an accidental leak from the heating system will not create problems for the health of households, unless it gets directly onto the human body. And in the event of such a leak, it is very easy to restore the original volume of water – you just need to add the required amount to the open expansion tank of the natural circulation heating system.
In terms of cost, water is also out of competition, since there is no cheaper and more affordable heat carrier..
However, this coolant has a number of disadvantages – ordinary water, that is, in its natural state, contains oxygen and salts, which causes internal corrosion of the elements of the heating system, as well as overgrowing of their walls with scale, which reduces heat transfer and the internal volume of heating devices.
The simplest way to soften water is well known to everyone – thermal (boiling), using a metal container without a lid. In the course of heat treatment, part of the salts will be deposited at the bottom of the tank, and carbon dioxide will be removed from the volume of water. By the way, the larger the bottom area of the boiling tank, the more salts can be removed from the water – the salts will be deposited at the bottom in the form of scale. The disadvantage of the thermal method is that only unstable magnesium and calcium bicarbonates can be removed from the water in this way, and their stable compounds will remain..
The chemical or reagent method is more effective, it allows you to transfer the salts contained in water to an insoluble state. For its implementation, slaked lime, soda ash or sodium orthophosphate are used – the introduction of the first two reagents into the water volume will cause the formation of a carbonate precipitate, the latter – a precipitate of magnesium and calcium orthophosphates. At the end of the chemical reaction, the precipitate formed is removed by water filtration. The last reagent – sodium orthophosphate – provides the best water softening, but its use requires precise dosage.
Distilled water is most suitable for heating systems, since it is completely free of any impurities. Its only drawback is that you have to spend money on the purchase, the cost of a liter of distilled water will be about 14 rubles. Before pouring distilled water into the heating system, it is necessary to thoroughly rinse the heating devices, pipes and the boiler with ordinary water, and wash both the previously used system and the newly installed one – there will be contamination inside it in any case.
You can use pure melt or rainwater, since it contains much less salt than tap, well or artesian.
The only drawback of water used as a heat carrier is that at temperatures below 0 ° C it will freeze, expand and cause serious damage to the heating system. And therefore, homeowners who irregularly operate a heating system during the cold season, as well as living in areas where power outages are especially frequent, another group of heat transfer fluids is more suitable – antifreezes..
Heat carrier – antifreeze
Non-freezing, poured into the heating circuit, allows you to completely solve the threat of freezing of the system in the cold season – the low temperatures for which this antifreeze is designed do not change its physical state. Antifreezes are able to ensure the transportation of thermal energy inside the heating system, do not cause corrosive processes and scale deposits.
The main quality of antifreezes is that they do not harden up to certain extremely low temperatures, in the case of curing, they do not expand like water and do not destroy the elements of the heating system, but turn into a gel-like mass, the volume of which does not change. In other words, if the temperature of the frozen antifreeze is increased, then it will return from a gel-like to a liquid state without any consequences for the heating circuit..
Manufacturers introduce additional additives into the composition of antifreezes in order to increase the service life of the heating system – corrosion and mineral deposits inhibitors that eliminate corrosion foci and scale in systems that have been in operation for many years. When choosing antifreeze, it should be borne in mind that its composition is not universal – the additives contained in it are designed for certain structural materials and alloys, the wrong choice will cause electrochemical corrosion or, for example, the destruction of polymer materials used in the construction of the heating system.
As a rule, antifreezes are produced, designed for two extremely low temperatures – up to -65 and up to -30 ° C. If necessary, you can change the concentration of the saturated composition to the desired one, from the proportion of one part of distilled water to two parts of antifreeze (for example, if a liter of antifreeze of the first type, designed for a lower temperature, is diluted with 0.5 liters of water, then such a composition will work up to – 30 ° C).
The chemical composition of antifreeze is designed for 10 heating seasons or 5 years of operation, after which the entire volume of anti-freeze must be replaced.
Compared to water, antifreezes have not only advantages, but also disadvantages:
- the heat capacity of non-freezing devices is 15% lower, that is, they give off heat worse;
- their viscosity is at least twice as high, which requires the introduction of powerful circulation pumps into the heating system;
- a higher volumetric expansion during heating, an expansomat (closed-type expansion tank) and heating radiators are required, the capacity of which is 50-60% larger than their counterparts used in systems with a water heat carrier;
- fluidity is 50% higher than that of water, that is, detachable connections in a system with antifreeze must be sealed with great care;
- ethylene glycol-based antifreezes are toxic to humans, therefore such antifreeze can only be used in single-circuit boilers.
For domestic needs, i.e. for heating systems of private houses, antifreezes are produced based on two types of polyols – ethylene glycol (monoethylene glycol) and propylene glycol. Compositions based on the first type of polyols are more common and cheaper than those based on expensive propylene glycol, but they are very toxic – when ingested, 350 mg of ethylene glycol is enough to cause serious harm to health and even cause death. Working with antifreezes containing ethylene glycol requires mandatory protection of the skin, respiratory system and eyes.
During operation, ethylene glycol-based antifreezes are especially sensitive to overheating – with any, even a short-term rise in temperature above the limit set by the manufacturer for a given brand of anti-freeze, thermal decomposition of the polyol and additives in the antifreeze occurs, insoluble sediment and acids are formed. Sediment, if it gets on the surfaces of heating elements, forms carbon deposits, impairing heat exchange at the local level and causing overheating with repeated sludge formation, etc. Acids formed as a result of the decomposition of ethylene glycol react chemically with the structural metals of the heating system, causing multiple foci corrosion. As a result of the decomposition of the additives, the protective properties of the coolant, previously provided by it for the material of the seals of the detachable joints, are sharply reduced, and with high fluidity this will immediately cause a leak. In addition, overheating increases the foam formation of the antifreeze, which, in turn, entrains air into the heating system. For the reasons described, it is necessary to carefully monitor the heating temperature of boilers and the heating system, however, not all boiler models allow this..
It should be noted that ethylene glycol enters into a chemical reaction with zinc – to use it in a heating system in which antifreezes of this group act as a coolant, structural elements and devices with internal galvanizing is pointless, since all its coating will be completely destroyed during almost one heating season.
Antifreezes based on propylene glycol are much safer for households – technical propylene glycol is similar in characteristics to food propylene glycol (E1520), which is widely used in the pharmaceutical, perfumery and food industries due to its complete safety for the human body and the environment. Non-freezers with propylene glycol are allowed to be used in double-circuit boilers, since their accidental penetration into drinking water, as well as leaks in the places of detachable joints, will not harm people.
Propylene glycol heat transfer fluids, in addition to the general positive characteristics identical to those related to ethylene glycol antifreezes, have a lubricating effect inside the heating system, lower the hydrodynamic resistance and facilitate the operation of the secondary circuit pumps. The heat transfer of propylene glycol antifreeze is higher than that of ethylene glycol. There is only one minus – a higher cost, about 1000 rubles. per 10 kg (for comparison, the cost of ethylene glycol antifreeze at -30 ° C is about 550 rubles per 10 kg).
It is strictly forbidden to use antifreeze in the heating system if:
- The system uses electrolysis (ionic) boilers, in which the heating agent is heated by passing an electric current through its volume in the boiler tank. In general, before purchasing a heating boiler, make sure that the manufacturer allows it to work in the heating system with this antifreeze, otherwise the factory warranty for the boiler will not be valid;
- open type heating system. This rule mainly applies to antifreeze based on poisonous ethylene glycol;
- from savings, you expect to lower its frost resistance to more than -20 ° C, since this will seriously reduce the characteristics of the additives introduced into the antifreeze, which will lead to the formation of foci of corrosion and scale;
- sealing of detachable joints is carried out using linen winding and oil paint – antifreeze will inevitably corrode the paint and there will be no sense from winding;
- when constructing the heating circuit, galvanized pipes and fittings were used;
- the heating boiler heats the coolant to temperatures exceeding + 70 ° C (this is the limiting heating temperature of any antifreeze, it cannot be heated above due to the high temperature expansion inherent in coolants of this group).
If antifreeze is used in the heating system, the following conditions must be met:
- equip the system with a more powerful circulation pump than would be needed for hot water heating. With a long heating circuit, an external circulation pump will be needed;
- install a capacious expansomat (expansion tank), the volume of which is at least twice the volume required for the water coolant;
- use pipes of obviously larger diameter and volumetric radiators in the heating system;
- do not install automatic air vents – only manual ones (for example, Mayevsky’s taps);
- Seal detachable joints with gaskets made of chemically resistant rubber, paronite or Teflon only. You can use linen roll along with an ethylene glycol resistant sealant (if using an ethylene glycol based antifreeze). When buying cast iron radiators, it is necessary to disassemble them into sections and replace the existing rubber gaskets with paronite or Teflon;
- you can dilute the anti-freeze only with distilled water, that is, neither rain nor melt water will work here;
- before each complete pouring of antifreeze into the system, it is imperative to flush it with water (the boiler too) – manufacturers of anti-freeze devices recommend completely replacing them in the heating system every 2-3 years;
- you should not set a high heating temperature to a cold boiler at once – you need to raise the temperature gradually, giving the coolant time to warm up (non-freezing systems have a lower heat capacity than water);
- in winter, when you turn off a double-circuit boiler in a system with antifreeze for a long time, do not forget to drain the water from the hot water supply circuit, since it can freeze and damage the circuit pipes.
How to choose the optimal coolant
First of all, the issue of choosing a coolant should be decisive even at the design stage of a heating system, since if it was created for water, it will require serious reconstruction for antifreeze.
If the temperature in the heating circuit during the cold season does not drop below +5 ° C, then the optimal coolant for such a system is water, from which salt compounds are removed to the maximum. If there is a possibility that the temperature in the heating system will drop to minus values, then in this case only antifreeze is needed. Of course, you can drain the water from the system, which will protect it from damage during frost, however, in this case, the circuit will fill with air, which will dramatically accelerate corrosion processes in conditions of high humidity..
It is possible to protect the water heating system from freezing by integrating electric heaters into it, controlled by temperature sensors or remotely, via GSM channels, which will allow maintaining the water temperature at a level above +5 ° C, but here there is a dependence on power supply and cellular communication – one of the these systems separately or together will lead to freezing of the coolant and multiple damage to the heating circuit.
When choosing antifreeze, you need to study in detail its characteristics, including: the permissible extremely low temperature; the composition of the additives and their purpose; how it affects the elements of the heating system (made of ferrous and non-ferrous metals, cast iron, plastic, rubber, etc.); duration of use in the system without replacement; safety for human health and ecology (after all, it will have to be merged somewhere). By the way, the color of the anti-freeze has no practical value for the heating circuit, it is only needed to emphasize belonging to a particular brand. Given the potential health risks to households, propylene glycol antifreeze is the best choice..
In view of the popularity among homeowners of the Tosol brand antifreeze, developed in the middle of the last century in the USSR, it is worth briefly describing its characteristics. So, antifreeze was originally developed as an anti-freeze coolant for motor vehicles, its composition is based on ethylene glycol, the characteristics of which are described above. It is not recommended to use antifreeze in heating systems, since this antifreeze is not intended for them – it contains specific additives for car engines, useless and even harmful in heating systems, since antifreeze is simply not designed to work at high temperatures.
In conclusion, we will name the most optimal antifreeze, which is very, very simple to purchase or prepare – a 40 ° mixture of ethyl alcohol with distilled water. The performance characteristics of this mixture when used as antifreeze coolant are as follows:
- slightly higher than that of water, but significantly lower than that of ethylene glycol and propylene glycol anti-freeze, viscosity;
- less fluidity than that of the mentioned antifreezes, which makes it possible to reduce the requirements for the tightness of detachable joints, allowing the use of conventional seals in them (alcohol is not chemically reactive to rubber);
- alcohol is an excellent corrosion inhibitor, that is, it blocks its development;
- when using water saturated with salts (hard), the alcohol in such a mixture will prevent scale deposits on the inner surfaces of the heating circuit. Salts will precipitate into an insoluble precipitate; it can be easily removed when the system is flushed;
- as a result of the heat of mixing and contraction (compression of the aqueous volume of the alcohol solution), alcohol does not evaporate separately from water (provided that its content in the aqueous solution is not less than 30%);
- the boiling point of an aqueous solution of alcohol practically corresponds to the boiling point of water, that is, when the temperature in the heating system rises to +85 ° C, which is usual for systems with water as a heat carrier, boiling with the appearance of plugs in the form of steam will not occur;
- the alcohol content in an aqueous solution sharply reduces the expansion of water during freezing, that is, even with complete freezing of a heating system with such a coolant, there will be no damage to its structural elements.
To achieve certain threshold values of resistance of an aqueous solution of ethyl alcohol to low temperatures, it is necessary to achieve the following content in a solution with water: 20.3% – freezing at -10.6 ° C; 33.8% – freezing at -23.6 ° C; 39% – freezing at -28.7 ° C; 46.3% – freezing at -33.9 ° C. It will be especially convenient to use a coolant, which is an aqueous solution of ethyl alcohol, in closed heating systems.
When preparing a water-alcohol coolant, the proportions of alcohol content in water are calculated as follows – a liter of 96% alcohol contains 960 ml of anhydrous alcohol, respectively, to get a 33% solution, you need to divide 96 by 33 and get the required volume of water equal to 2.9 liters. That is, if you introduce exactly 2.9 liters of water into a liter of 96% alcohol, then the alcohol content in the resulting solution will be exactly 33% – a coolant that does not freeze to about -22.5 ° C is ready.