NON-SLIPPERY DRIVING AREAS DURING WINTER TIME
Having applied heating cables underneath the surface the garage driveways, ramps in multi-storey car parks and other exposed driving areas can be kept free from snow and ice during the winter time.
The cables can be applied in many a different ways:
Bed of sand with a surface covering of for example concrete slabs or asphalt.
On concrete which is then provided of for example covering of asphalt.
In concrete, when the construction is out of concrete and is cast in one step, the heating cables are applied in the concrete by lashing the cable in the upper part of the armouring.
Power required: Depending on the geographical position of the application and the construction of the building the power required could vary between 200 – 400 Watt/m². There are some things you have to think of when deciding the power required, for example how fast the melting off is going to start, to what extent the surface is exposed to falling snow and cold and if vast quantities of snow are coming down from the adjacent areas.
In the previous chapters we have described the design and the installation of heating cables in beds of sand and in asphalt. What now remains is to describe how to apply heating cables on concrete.
Dimensioning heating cables for ramps in multi-storey car parks
Conditions: A multi-storey car park in a city is planned. To solve the problems with snow-clearance and slippery ways heating cables are going to be installed in the ramps.
Calculated – ramp in storey one 350 W/m²
Power required- ramp in storey two 250 W/m² .
The car park is built out of prefabricated concrete panels. The ramps are constructed of concrete panels 1.500 x 4.500mm (6.75m2). Six panels per ramp.
Construction: the heating cable is applied in the panels in the factory. The heating cables are connected to junction boxes which are cast inn the short sides of the panels. The power required of each panel is calculated.
Heating cable calculations
As the surface power of the concrete panels is not the same in ramp one and ramp two, two different calculations must be made. The method is the same and that is why we here only will explain the calculation of ramp one.
When calculating the cable resistance and length this is the best way:
- The required power is determined.
- Distribution on # of cable lengths.
- Suitable cable and voltage feed is calculated.
Then calculate for each length:
- Total resistance (Rt).
- Minimum allowed cable length (l).
- Cable resistance (Ω/m) are chosen.
- The real length of the cable (l).
- Check calculations.
- Distance between the cable runs (c/c).
- Required power: Surface x required power:
Example: 6.75 x 350 = 2.365 W
- There is no reason or distributing this power into several cable lengths. It is sufficient with one length per concrete panel.
- The best cable type is: The TCPR is applied directly in the concrete (max 25 W/m cable).Voltage feed: If you calculate the max. power at a voltage of 415V it will be possible to supply also with 230V via an Y/D-change-over switch and thus a power of 1/3. This can be of advantage if you for example want a basic heating which is always switched on when the temperature drops down below the adjusted value.
This possibility is used in ramp one where it can be of advantage to have a certain maintenance heating when it gets very cold in order to get the melting of snow and ice to start quickly when there is a change in the weather and precipitaions. On the other hand this is not needed for ramp two, which is not subject to snowfalls in a direct way but only to what is brought in.
The heating cables for the panels in ramp one re calculated for 415V/230V. The heating cables that are to be installed in the panels in ramp two are calculated for one voltage only
- the total resistance of the cable lenth (Rt):Ohms law for power R = U² / P
Example: 380² / 2.365 = 61 Ohm
- The shortest possible length of the cable length (l). l = Pt / P/mExample: 2.365 W / 25 W/m = 95m.
- Required resistance per meter cable (R/m): R/m = Rt / l
Example: 61 Ohm / 95 m = 0,64 Ohm/m.
In the TCP cable data sheet you will find that the nearest higher resistance value is 0.65 Ohm/m and the closest lower value is 0.45 Ohm/m.We will here abandon the rule always to choose the lower value, as it, in this case, differs too much from the required value and the closest higher value is almost perfect.
- The real length of the cable length: l = Rt / R/m
Example: 61 Ohm / 0.65 = 94 meter
- Check calculations: 94m TCPR 0.65; 6.75m²
P = 380² / (94 x 0.65) = 2.363 W
P/m = 2.363 / 94 = 25.1 W/m
P/m² = 2.363 / 6.75 = 350 W/m²
In this example it is shown that the power per meter is somewhat high. If you then increase the length by a little and make new calculations until you find the correct length.
New check calculation: 95m TCPR 0.65; 6.75m²
P = 380² / (95 x 0.65) = 2.340 W
P/m = 2.363 / 95 = 24.6 W/m
P/m² = 2.363 / 6.75 = 346 W/m²
c/c = 6.75 x 100 / 95 = 7cm
- Calculating the c/c-distance between the cable runs: c/c = surface x 100 / l
The answer is obtained in cm if you put the surface into m² and l into meter.
Example: 6.75 x 100 / 95 ≈ 7 cm
- The solution: 1 length á 95m TCPR 0.65 Ohm/m in each panel is installed with a c/c of 7 cm.
Ramp 2: Use the same method as above and the solution will be: A heating length á 67m TCPR 1.30/1.660W/380V in each panel is installed with a c/c-distance of 10cm.