Ice formation mechanism
Ice and icicle formation on a warm roof (DE-VI):
1 - snow;
2 - water;
3 - ice;
4 - heat flow
Precipitation in the form of snow, being on the roof, does not pose any danger. However, if conditions are created for the snow to melt under the influence of any heat source, it turns into water. If the formed melt water has no way to quickly leave the roof, when the corresponding negative temperature approaches, it freezes, turning into ice. Since the conditions for melting (and the rate of melting) for ice and snow are different, with the next short-term action of the heat source, it is possible not melting, but, on the contrary, an increase in the ice plug. This mechanism of ice formation can lead to the formation of icicles tens of meters long and weighing hundreds of kilograms.
Heat sources are:
Atmospheric warmth. If the daily air temperatures fluctuate with an amplitude reaching 15 ° C, then with fluctuations in the range of +3 0: +5 ° C during the day and -6 0: -10 ° C at night, the most favorable conditions for the formation of ice are created. In the spring, you can add solar radiation to them. Although the surfaces of snow and ice reflect most of the radiation incident on them, even a small coating of dirt dramatically increases the absorption coefficient. In addition, the exposed areas of the roof quickly heat up, and thawing occurs from the inner side of the layer. Therefore, ice formation in spring is always more intense than in autumn.
Inherent heat dissipation of the roof. Heat dissipation takes place on any roof. This happens to a minimum on roofs with a ventilated attic. However, the recently widespread use of the attic space for living (attic), or as a technical floor (where a large number of powerful equipment for heating, ventilation and air conditioning is installed) dramatically changes the requirements for the roof structure. Insufficiently effective thermal insulation leads to the fact that under the surface of the snow lying on the roof (which is a good heat insulator) there is a constant drip melting of snow, and this process occurs on the entire surface of the roof. Such roofs can be called warm. They are characterized by the formation of ice in a wider range of air temperatures,which in fact could mean the danger of icicles forming during most of the cold season.
Today, the most common way to combat the formation of ice is the use of anti-icing systems based on heating cables.
Anti-icing systems based on heating cables
Application of a heating cable de-icing system
The introduction of anti-icing systems based on heating cables, provided that the design is correct, taking into account the peculiarities of the roof structure, allows to completely eliminate the formation of ice at relatively low prices and insignificant energy consumption and also to ensure the operability of the organized drainage system in the spring and autumn periods.
Installation of heating cables
The operation of anti-icing systems at temperatures below -18 ° … -20 ° C is usually unnecessary. First, at such temperatures the formation of ice does not take place by the first mechanism and the amount of moisture by the second one sharply decreases. Secondly, under these conditions, the amount of precipitation in the form of snow also decreases.
Third, melting snow and removing moisture along a sufficiently long path requires large electrical power.
When installing the system, it should be borne in mind that the designer must ensure that the water resulting from the 'operation' of the system has a free path of complete drainage from the roof.
An example of heating a valley.
1 - Clamp
2- Heating section
3 - Bracket
4 - Copper strip
There are also limits for the capacity of the heating part of the systems, established on the basis of practice, non-observance of which leads to ineffective operation of the equipment in the specified temperature range, and a significant excess of the latter leads only to an overrun of electrical power without any improvement in the operation of the system.
- specific power of heating cables installed on the horizontal parts of the roof. The total specific power per unit of surface area of the heated part (tray, chute, etc.) must be at least 180-250 W / m2; the specific power of the heating cable in the gutters must correspond to at least 25-30 W / per meter of the gutter length and increases as the gutter lengthens to 60-70 W / m.
All of the above allows us to draw several general conclusions:
Anti-icing systems generally 'work' only during the spring and autumn seasons and during thaws. The 'operation' of the system in the cold period (-15 ° … -20 ° C) is not only unnecessary, but can be harmful.
The system must be equipped with a temperature sensor and a corresponding specialized thermostat, which can rather be called a mini weather station. He must control the operation of the system and allow the possibility of adjusting the temperature parameters, taking into account the specific features of the climatic zone, location and number of storeys of the building.
Heating cables must be installed along the entire path of the melt water, starting from horizontal gutters and trays, and ending with exits from gutters, and if there are entrances to storm sewers, up to collectors below the freezing depth.
It is necessary to comply with the standards for the installed capacity of heating cables for various parts of the system - horizontal trays and gutters, vertical gutters.
Typical, constructive solutions
The main tasks in the design of anti-icing roofing systems are to make it effective, relatively inexpensive, and to apply such fastening methods that would not damage very critical roof components and would not spoil the appearance of the building. In this case, the attachment points must be reliable, durable, and not damaging the sheath of the heating cables.
One of the basic principles of designing fasteners is to use the same materials as for the roof or compatible with them.
Heated snow pocket
In fig. 4, 5 and 6 show examples of laying heating and distribution cables on various (most common) pitched roof nodes. First of all, they relate to roofs covered with galvanized iron, copper sheets and metal tiles.
It should be noted that special methods are used for non-damaging heating cables for soft roofs. On the widespread trays of snow retention and snow removal, it is highly advisable to lay heating cables in a concrete (or cement-sand screed). This, in addition to protecting the cable from damage, significantly increases the heating efficiency due to the use of heat storage properties of concrete.
Gutter heating with heated funnel
Basic requirements are imposed from the point of view of fire and electrical safety.
To satisfy them, several conditions must be met:
the system must include only heating cables with appropriate certificates, incl. a fire safety certificate is required. Typically, these are non-combustible or non-combustible cables. For use in anti-icing systems, manufacturer's recommendations are required;
the heating part of the system must be equipped with an RCD or a differential circuit breaker with a leakage current of no more than 30mA (for electrical safety requirements - 10mA);
complex anti-icing systems must be divided into separate sections with leakage currents in each part not exceeding the values indicated above.
Heating cables from major manufacturers have all the necessary certificates and have been repeatedly tested as part of anti-icing systems.
Testing and performance evaluation
Tests of anti-icing systems can be divided into two groups: acceptance testing and periodic testing.
Routine tests usually begin with testing the insulation resistance of heating and distribution cables. RCDs (or difavtomats) are being tested. Appropriate protocols are drawn up with specific values. The most informative are performance tests, during which the efficiency of the system is checked.
It should be noted that anti-icing systems are not instantaneous systems. They are designed to work in standby mode, and turn on immediately when precipitation occurs. If the system was turned on not at the beginning of the season, and a layer of snow has accumulated on the roof, it will take time from 6 hours to a day to remove it.
There are difficulties when putting the system in the warm season. At the same time, the proper functioning of the control equipment is checked, the signals from the sensors are simulated, the transition of the system to the mode of turning on the load, turning off the trays, and then turning off the drains is checked.
Periodic tests are carried out, as a rule, at the beginning of autumn to check the technical condition of the system and prepare it for operation. First of all, the insulation resistance is checked to identify damaged areas. Then the state of the equipment is checked, its test switching is carried out. After checking the settings of the thermostats, the system is switched on, and it remains in standby mode.
Hydrophobic anti-icing compositions
Hydrophobic anti-icing compositions do not prevent the formation of ice, but provide a quick descent of newly formed water ice during repeated freeze-thaw cycles, preventing it from forming into large ice icicles and drips.
Such hydrophobic compositions are applied to metal, concrete and other substrates by hand, brush, roller or spray on clean, dry and dust-free surfaces free of rust, oil, grease, etc. Compositions harden at temperatures above +5 0С.
According to the International Academy of Cold (MAX), the adhesion force of water ice with building roofing materials is very high (steel 3 - more than 0.16 MPa, concrete - more than 0.22 MPa), during pull-off tests the internal structure of ice was destroyed, and its remnants were firmly remained on the surface of materials. At the same time, the adhesion strength of ice coated with an anti-icing composition is almost completely absent and is less than 0.22 MPa.
The anti-icing coatings are waterproof, anti-corrosion, environmentally friendly, have high strength and elasticity, retain high physical and mechanical properties in a wide temperature range, and are resistant to UV radiation and atmospheric precipitation.