Air Conditioner Fans and Blowers
|Let us make a quick trip into knowing air conditioner fans and blowers. These are used in improving heat transfer at the condenser and evaporator.|
Air conditioner fans are used for low pressure loss systems.
The function of air conditioner fans and blowers:
We have learned in Air Conditioner Parts that fans function as air movers. This equipment will enable forced convection to occur, thus improving the heat transfer rate.
In addition to that, it makes compact design of evaporator and condenser, possible. Depending on natural convection would otherwise mean very large evaporator and condenser design. Not only that, you’ll have to wait almost forever before cool air reaches you.
Therefore, function of air conditioner fans and blowers is not negligible thing. Not now, not forever.
Blowers have the exact same function as a fan. Only the construction is different. Fans are normally used at condensing units, while blowers are used for evaporation units.
How air conditioner fans and blowers operate:
Air conditioner blowers are used for high pressure loss systems.
- fans – operate by displacing air, perpendicular to its rotation, while
- blowers – operate by displacing air, parallel to its rotation
These units operate under low pressure difference, and high mass flow rate principle. Therefore, we wouldn’t need electric motors as powerful as compressor motors.
What are the characteristics of air conditioner fans and blowers?
The characteristics of fans and blowers are quite similar to that of compressors.
These equipments have,
- maximum rotational speed – limited by the driver’s speed and power, and
- maximum pressure difference between inlet, and outlet of the fluid motion – limited by the driver’s power
|The efficiency is determined by the ratio of work done on moving the fluid per second, to the amount of power input by the driver.|
Work done on moving the fluid per second, is achieved by multiplying the pressure difference, Δp, and the volume flow rate, Q.
Therefore, the efficiency of the fan will be 0, if
- a duct system is fully restricted, i.e. no flow is possible, or
- the pressure difference is 0, i.e. when the fluid flow is not at all, influenced by the fan
What do the statements mean? It means, we have to ensure that the duct system, condenser, or evaporator, has as low as possible restriction.
It also means that, we have to have fans or blowers, sized and placed at a correct distance from each other in a ducted system, so that the efficiency of each fan or blower will not be affected. If and only if, we need more than one fan or blower in a duct system.
Basic principles of fan design:
Very large fans are designed similar to an aerofoil design. Smaller fans however, have very little resemblance to an aerofoil. Nevertheless, these equipment use the principle of angle of “attack” or angle of incidence, to determine the flow rate capability.
We have to have fan blades, tilted at a certain angle to force the fluid perpendicularly to the blade motion. However, that also means that we will have drag opposing the motion of the fan blade.
We will have, lift and drag. Lift provides the perpendicular motion of the fluid, and drag opposes the fan’s motion.
Ideally, we would like to have the fan to provide the highest lift, with lowest drag. That is, to have the highest lift to drag ratio. This is achieved by increasing the angle of incidence.
However, we wouldn’t want to increase the angle too much, as this will cause stalling of the fan, and hence reducing the efficiency.
Maximum angle of incidence (before stalling), for a specific fluid and fan design, depends on the fan speed. The lower the speed, the higher the stalling angle.
Therefore, the blade angle for air conditioner fans and blowers are designed based on required flow, and available space. The angle of incidence and speed will be adjusted during design stage, until the optimum performance is achieved.
Manufacturers will normally have a set of catalogue to select standard air conditioner fans and blowers, for a given air conditioner size.
Other fan characteristics that has to be considered during design and manufacturing are,
- the weight material
- rigid construction
- difficult to oxidise
Air conditioner fans and blowers, concluded:
Though it may seem like simple equipment, air conditioner fans and blowers have to be designed properly, or you’ll have unsatisfactory air conditioner performance, and a very noisy system.
Even a very good system has some noise. And the major contributor to that, are air conditioner fans! Surprised? Don’t be.
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