Refrigeration systems are simply machines that move heat from somewhere cold to somewhere warmer. This would not be possible (it would break the second law of thermodynamics!) if no energy was added to the system.
In a vapour compression refrigeration cycle, this energy almost always comes in the form of an electric motor driving a compressor; in an absorption system, this energy comes in the form of a heat input that drives a process of absorption and desorption. We spend a lot of time talking about compressors and the refrigeration cycle, but in all cases, the system cannot absorb the heat in the cold space without rejecting it somewhere else. This is the job of our condensers.
Condensers come in a few different configurations depending on how they reject heat. There are three main types of heat rejection and we are going to discuss the first two:
- Air – sensible heat only
- Air – latent and sensible heat
- Fluid – water, glycol, etc.
The most common type, of course, is air-cooled condensers (shown in Figure 1). They are used in the majority of cars, homes and small commercial air conditioning and refrigeration systems in the country. They reject heat into the ambient air using sensible heat only; in order to reject heat, they obviously have to be warmer than the air around them. Fig. 1 shows a typical air-cooled fin-tube condenser.
This leads to some important considerations regarding where they are placed. A condenser in a hot kitchen, for example, will have a more difficult time rejecting heat than one outside in cold ambient conditions.
In order to look a little closer at condensers, we can consider a generic convective heat transfer formula to think about how condensers accomplish their heat rejection. It is important to note that this is a simplification, the entire heat transfer analysis would include several other factors. We are also assuming for this discussion that the condenser is using a material that is good at conduction of heat, like copper or aluminum, as we are going to ignore the conduction of heat through the pipe wall.
If our goal was to maximize heat transfer, there are three components that we could use in this equation. We could increase the difference in temperature between our refrigerant and the air, we could increase the surface area of our heat exchanger, or we could increase the heat transfer coefficient. The heat transfer coefficient is based on several factors including the velocity, fluid and geometry of the heat exchanger.
Achieving higher efficiencies
Unfortunately, increasing the temperature of the refrigerant results in increased pressure and condensing temperature in the refrigeration system. Which creates inefficiency. In fact, we are usually trying to decrease the refrigerant temperature in order to achieve higher refrigeration system efficiencies.
Older condensers were often designed to have a 20F-25F temperature difference. This means that if the condenser was in a 90F ambient space, the refrigerant would have to be 110F-115F for the system to reject the right amount of heat at full capacity. In more recent years, the drive for energy efficiency has reduced this temperature difference to 10F-15F.
This leads to the second thing we could change, surface area. If you were in the air conditioning industry when the efficiency rules changed from 10 SEER to 13 SEER, you would have noticed something interesting: the condensing units got way bigger. This is because, all other things being equal, a decrease in the condensing temperature (needed to get our efficiency higher on the refrigeration system) had to result in an increase in heat exchanger surface area.
In theory, we could have an incredibly large heat exchanger and reject heat with a very small temperature difference. This has very obvious diminishing returns when the condenser for your house AC becomes bigger and more expensive than your house.
There are, however, several clever ways to increase the surface area of the condenser. For the exterior heat transfer, we usually use fins to do this. Again, there are diminishing returns because as you increase the surface area it becomes more costly to move air around it; this leads to the last variable we can change, the heat transfer coefficient.
This variable encompasses a lot of things like the velocity of the air and the geometry of the heat transfer surface. We typically want as much surface contact as possible between the air and the heat exchanger to increase the heat transfer coefficient, so we do things like offset fins/tubes in the condenser, or make the fins wavy. We can also use larger fans to move more air.
How condensers fail
Imagining and simplifying condenser operation this way allows a person to think about how condensers fail and what the resulting consequences or symptoms would be. Or, conversely, it allows measurements of condenser parameters to help troubleshoot the refrigeration cycle. Consider finding a condenser with only a 2°F temperature difference. It is unlikely that it was ever designed that way and it presumably didn’t grow more tubes and fins… so something is up with your refrigeration system.
One thing we can do to decrease the temperature of the refrigerant and still reject the same amount of heat is to use the latent heat of water to absorb some energy. This is how evaporative condensers operate (Fig. 2). In addition to having air drawn or blown over the tubes, they are sprayed with water that evaporates. The result of this is that the condenser “sees” the wet-bulb temperature.
In many places in Canada, the wet-bulb temperature is 20F lower or more than the dry-bulb temperature. This has the effect of allowing smaller heat exchangers and decreasing the refrigerant temperature at the same time. There are several downsides to evaporative condensers, however, such as water treatment requirements and additional pumping energy.
A newer version of a condenser is called an adiabatic condenser (Fig. 3). They are almost as large and expensive as air-cooled condensers and operate air-cooled most of the time. However, they use media pads that have water running through them in high ambient conditions to drop the temperature of the air before it hits the condenser coil using a similar effect to evaporative condensers.
If you are familiar with “swamp coolers”, the premise is similar. Water is used to cool the air before it hits the condenser fins and tubing. These units don’t require water pumps or water treatment, so they remove some of the disadvantages of water-cooled units but keep the benefit of being able to cool based on the ambient wet-bulb temperature.
As is the case with most refrigeration components, there are a lot of things to consider when choosing a condenser or troubleshooting a refrigeration system. Understanding how the condenser accomplishes heat transfer can help you come up with solutions in both cases.