The pressure that is reached is 1.2-1.3 inH2O. At such low pressures the gas compressibility actually does not influence the working process, related to the transportation of energy, so it is generally accepted that the fluid is incompressible.
Dimensioning of fans and their operation in combination with air distribution networks is the same as that of centrifugal pumps. The pressure of fans is measured in Pa or in inH20. Fig 1.1 shows exemplary characteristics of a centrifugal fan (blue line). The characteristics of such turbo machines can be theoretical and real. Theoretical characteristics show clearly how the blade angles influence the machine parameters provided there are no internal losses e.g. it works with a perfect fluid.
In order to obtain the actual characteristic using the theoretical one it is necessary to measure the fluid that flows through the machine and which is related with hydraulic losses. Another very important thing which has to be taken into account when choosing a fan for a particular system is the exact points of intersection of the two curves. And that point has to be between A and B. The machine efficiency at point A is almost 100% and about 85% at point B. The lower is the position of intersection of the two curves the more energy will be used by the turbo machine to operate. This characteristic is given by the manufacturer after completing the necessary laboratory tests on the turbo machine.
The hydraulic efficiency of the fan depends on:
Gas density. Lb/Ft3 (Air density has different parameters in winter and in the summer period)
Peripheral speed at the outlet of the working blade.
Projection of the absolute speed onto the direction of peripheral speed.
Dimensionless pressure coefficients depend also, to a great extent, on that how blades, that deliver air to the impeller, are coupled. For blades with an angle smaller than 90% (these are backwards curved blades) the pressure is between 0.35 and 0.60 and for forward-curved blades where the angle is more than 90% the pressure produced by the fan is between 0.60 and 0.80.
This is the right time to mention how the production of forced air equipment in the country has developed over the years. Only 10-15-20 years ago all manufacturers assembled machines that were quite big and took a lot of space in the existing premises. Efficiency was between 50 and 70%.
The fans installed in the machines at that time were very big and the speed of rotation was comparatively slower than the speed of nowadays fans. In result those machines were much more noiseless (a big fan with slow rotation speed, while modern ones have small fans with high rotation speed). On the other hand this increases the level of aerodynamic noise, coming from the fan and which in return is distributed over the entire air duct going much further compared to the previous models. Some manufacturers are trying to stick to this concept only because of this.
In most of the cases, when replacing a low efficient machine with equipment, which is better and more efficient, homeowners don’t see a great reduction in their heating bills. The reason is that the new equipment can not work efficiently with the old air duct system. A bigger flow rate is required to pass through the heat exchanger. The geothermal heating system or air to air heat pump would not be able to work with a system like this. It is strongly recommended to redesign or replace it.
