The Processes Are Reversed, But Air Conditioner Condenser and Evaporator Share The Same Feature...
|Exactly. Condensation happens at air conditioner condenser, and boiling happens at air conditioner evaporator. The processes of condensation and boiling, are reversed in manner.|
Nonetheless, the process of heat transfer between two physically separated media, requires a similarly constructed intermediate “partition”. We need this intermediate “partition” as,
- we want the air to be clean from pollution by refrigerant, and
- we want to keep the refrigerant within air conditioning unit, to avoid continuous recharging
Definition of air conditioner condenser and air conditioner evaporator:
|Topic of Air Conditioner Parts have explained that, a condenser, or evaporator is a heat exchanger, allowing condensation or evaporation to happen, by means of giving off, or taking in heat respectively.|
The construction principle:
Refrigerant and air will be physically separated, at air conditioner condenser, and evaporator. Therefore, heat transfer occurs by means of conduction.
We would like the heat exchanger that enables these processes, to have,
- high conductivity – this property will ensure that the low temperature difference between the outside wall, and inside wall
- high contact factor – this property ensures the passing air mass, will come in contact with the tubes, as much as possible
How to achieve high conductivity in air conditioner condenser and evaporator?
We have to understand the factors that effect conduction of a material. Let’s assume a condition where the refrigerant is within the tube of an air conditioner condenser. The tube will have a circular wall.
Fourier’s law has stated that the rate of conduction heat transfer is proportional to,
- the thermal conductivity of the wall k W/m2,
- the mean surface area, A m2
- the inverse of the wall’s thickness L in metres
- and the temperature difference between the inside wall, and the outside wall
Selection of the tube for the condenser and evaporator has to meet few other criteria as well. It has to be durable, difficult to oxidise, easy to join with other lengths of similar tube, good strength and cheap.
Copper and aluminium has proven time and time again, to meet all the criteria mentioned, with excellent thermal conductivity. Of course it is not the best material with high thermal conductivity, but it is very cheap for the performance it can provide.
Have I missed something over here? Oh yes, the conduction heat transfer rate is also proportional to the mean surface area. What will this imply?
It will imply that the rate of heat transfer will increase if the total surface area of the condenser’s or the evaporator’s, is increased.
You might have a fixed tube material, with fixed wall thickness, but you want to minimise the temperature difference between the outside wall and the inside wall. You achieve this by increasing the total effective surface area of air conditioner condenser, or evaporator.
Why we would want small temperature difference between the walls? Because most of the work done by the compressor, will be converted into giving off, and taking in heat.
We will otherwise have an insulator, if the temperature difference between the walls is high. In other words, we want the tube to be as “non-existent” as possible.
But how to achieve the increased effective surface area?
|We run the tube into a set of aluminium sheets. The tube will contact the sheets, and hence the surface area for conduction heat transfer is increased!|
This way, we can have a much compact air conditioner condenser and evaporator, per desired heat transfer rate.
These fins will also serve as guide vanes for air to pass through the tube set, and improve the
It is the amount of media that needs to be heated up or cooled down, that comes directly in contact with the tube walls.
|Contact factor will be very low, if the air inside a duct is passed through a straight tube with refrigerant. This happens as the amount of air that contacts the tube will be very low.|
Therefore, we will increase the contact factor, by constructing the condenser and evaporator to have many passes within a given duct area. Thus, the passing air will “see” a lot of tubes on its passage. Hence the contact factor will be improved.
The maximum theoretical contact factor is 100%. We will have contact factors around 80% for commercially produced air conditioner evaporators and air conditioner condensers. The real figures really depend on each manufacturer. The reciprocal of the contact factor, is the bypass factor, where it is equal to 1 – contact factor.
Air conditioner condenser and evaporator effectiveness, ε:
|Is similar to the definition of heat exchanger effectiveness, and it is defined as the ratio of actual heat transfer, to maximum possible heat transfer. In detail,|
- the actual heat transfer is either the hot fluid’s heat loss or the cold fluid's heat gain
- the maximum possible heat transfer is the product between maximum temperature difference at the heat exchanger, and the minimum kJ/K (or Btu/F) value of either the hot fluid, or the cold fluid. The maximum temperature difference is the temperature difference between the hot fluid entry, and the cold fluid entry, at the heat exchanger
Value for effectiveness is measured in percentage. We have to bear in mind that 100% effectiveness is improbable to happen. However, manufacturers can improve it by clever design of the units.
For example, larger air conditioner condensers and evaporators will have a header distribution tubes, for evenly distributed refrigerant inlet.
Finally, but not the least important points:
The length and size of air conditioner condensers and evaporators have to be sized such that,
- the refrigerant is completely condensed before the condenser’s exit, and
- the refrigerant is completely boiled before the evaporator’s exit
Those two, depends mainly on the size of the compressor and refrigerant used.
Air conditioner manufacturers has to understand how conduction, as well as convection works, to design an effective, yet compact air conditioner condenser and evaporator, per unit heat transferred.
Normally, the condenser and evaporator will be designed to 110% of the intended heat transfer requirement, to cater for any performance drop during the service life.
It’s good that we know the basics now.
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