Calculating the frigories or the power that an air conditioner needs is a difficult task, for this reason we show and explain all the factors that intervene in said calculation.
What is a fridge?
The frigoria (fg) is a unit of energy that measures heat absorption and, although it is not accepted by the International System of Units, its use is widely extended in refrigeration systems such as air conditioners.
Any type of air conditioning has a number of frigories that indicate the amount of heat that it is capable of extracting from an environment. Although on some occasions, this measurement can be expressed in watts (W) or in British thermal units (BTU) . Therefore, if you want to convert those measurements to frigories, you must use the following formula.
Frigories conversion factor : 1 fg = 1.163 kW = 3.96 BTU
The range of frigories most used in homes is between 2,000 and 6,000 frigories (fg) depending on the rooms to be heated and the size of the home. If you have a large home and you want to air-condition the entire home, the most normal thing is for the user to resort to ducted air conditioning with a large number of frigories, such as 6,000 to 12,000 fg.
But since it is difficult to calculate the number of frigories needed by eye, we show you the following calculation that closely approximates the frigories needed for a home. Although, if what the user wants is an exact calculation, we recommend contacting a professional.
What power of air conditioning do I need?
When we are going to choose an air conditioner, the first thing we think about, apart from the price, is the number of frigories we need to air-condition our home or room. As is logical, the greater the power of an air conditioning unit, the colder it produces but also the more it consumes. That is why it is not necessary to oversize the power of the equipment, but to calculate the frigories that it will require depending on the home.
With the following calculation we can approximate the frigories that our home needs and, therefore, know how much the air conditioning equipment will cost us:
Formula to calculate the frigories of the air conditioner
Formula for calculating frigories of air conditioning Power = Walls coefficient + Windows coefficient + Ceiling and floor coefficient + Ventilation coefficient.
In the following example we are going to calculate the frigories required for a split air conditioner for a rectangular room of 60 m 2 with two of its walls measuring 6 m long and the other 10 m, with the height of all being 2.5 m. In the room there is a 5 m 2 window and the temperature difference we want with the outside is 20 ºC.
Wall coefficient :
Wall coefficient = dimensions (m 2 ) * insulation factor * thermal difference.
The first thing is to know the dimensions of the 4 walls of the room. To do this we multiply the width and height of each of them and add it. From this surface you have to subtract that of the windows because it is calculated separately.
Once the surface of the walls has been calculated, a correction factor must be applied depending on whether they have good thermal insulation (0.692) or not (1.09). In addition to applying the thermal difference between the outside and the temperature we want.
Practical example: Let’s imagine that the room does not have good insulation, the difference in temperature between the inside and the outside is 20 °C (40 °C vs 20 °C with the air on) and it has the shape of a rectangle, with two walls of 6 meters, two walls of 10 meters and that the height is 2.5 meters.
- Dimensions : [2 walls of (6m * 2.5 m = 15 m 2 ) = 30 m 2 ] + [2 walls of (10 m * 2.5 m = 25 m 2 ) = 50 m 2 ] = 80 m 2 – window (5 m 2 ) = 75 m 2
- Insulation factor : 1.09
- Thermal difference : 20 °C
- Total : 75 m 2 * 1.09 * 20 = 1,635 W
Windows coefficient :
To calculate its coefficient, as with walls, its surface must be multiplied by the thermal difference and the insulation factor: if it is a simple window (5.8), if it is a double glazed window (1, 62).
The window in the example is 5 m 2 and is made of simple glass, so its coefficient would be:
Window coefficient : area (5 m 2 ) * factor (5.8) * thermal difference (20 °C) = 580 W
Roof and floor coefficient :
The same thing happens with the ceiling and the floor, you have to multiply the surface by the insulation factor and the thermal difference. If it is a flat, the difference in temperature is less because the floor and the ceiling are between other houses, not facing the outside.
Floor and ceiling coefficient: surface (60 m 2 floor + 60 m 2 ceiling) * factor (0.692) * thermal difference (10 °C) = 830 W
The ventilation coefficient is directly affected by the air flow in the device. This measures the amount of air that is renewed per hour and is measured in m 3 /h. The higher this coefficient, the more cooling capacity the device has.
Ventilation coefficient = specific weight of the air * specific heat of the air / 0.86 * Air flow rate of the appliance * Temperature difference
Ventilation coefficient = [(1.2 kg/m3 * 0.24 kcal/kg ºC) / 0.86] * 500 m3/h * 20 °C = 3,348 W
Total power calculation
To calculate the power of the air conditioning, add the 4 coefficients:
- Wall coefficient: 1,635 W
- Windows coefficient: 580 W
- Floor and ceiling coefficient: 830 W
- Ventilation coefficient: 3,348 W
- Total: 6,393W
- Total in frigories: 6,393 W * 0.86 = 5,498 fg
Can I calculate the frigories of a ducted air conditioner?
Ducted air conditioning is an air conditioning system that is used to condition different rooms. This consists of an exterior device and an interior one that, through ducts hidden in a false ceiling, manages to renew the air in the different rooms.
Calculating the frigories that a ducted air conditioner needs is quite complicated, since each installation is a tailored suit that depends on the home in question. Therefore, it is not feasible for a user to calculate the frigories of this type of air conditioning.
Many factors influence this calculation, such as the materials of the ducts, the distribution of the rooms, the diameter of the ducts, whether there are return ducts or if it is done through the false ceiling. Due to this, the calculation must be carried out by a professional who has the appropriate computer program for its execution.