It is my objective to explain engineering topics in as simple a way as possible, so as to make it easy for the everyday person to understand. I believe learning more about these topics can greatly benefit any individual, regardless of background or career field.
Let’s play a game. We have 1 objective: Figure out a way to lower the temperature of the air inside of a building (such as a house).
Step 1: Identify a Method of Cooling
Method 1: We could open up some windows, allowing air to flow freely in and out. But what if the outside air is warmer than the inside air? This will not allow for cooling to occur.
Method 2: We could run a fan across a block of ice, but that ice will eventually melt and the cooling will stop.
Method 3: What if…we use some kind of closed system to pick up heat from the inside air and transfer it to the outside air? This may just work…but how will we engineer such a system?
Step 2: Engineering a Cooling System
First, we need to figure out what kind of medium can transfer heat from one area to another.
Well, we could use a pump to cycle water through pipes, and run fans to draw air across the pipes. This will transfer heat from the inside air to indoor pipes, and from outdoor pipes to the outside air. However, if the outside air is warmer than the water in the outdoor pipes, no heat will transfer from the outdoor pipe to the outside air, which means the water will not cool the inside air; the water will only be warmer by the time it cycles back to the indoor pipes. If there is very little heat being produced inside the building and we are trying to cool the space, it is most often going to be the case that the outside air is warmer than the inside air.
Let’s explore a different scenario. Let’s say the outside air is 65°F, the inside air is 75°F, and we are trying to cool the inside air to 72°F. The water temperature is going to be in this general range (65-75°F). Being so close in temperature to the inside air, the water will not be able to pick up as much heat. Being so close in temperature to the outside air, the water will not be able to reject as much heat.
The closer in temperature two media or objects are, the slower the rate of heat transfer. The inverse is also true: The greater the difference in temperature between two media or objects, the quicker the rate of heat transfer. For example, if you set an ice cube on the counter, it will take a while to melt. If you throw the ice cube in a pan which has been heated on the highest setting, it will melt in no time.
So, how can we utilize this idea to get past the issue at hand?
We will have to increase our delta T; the difference in temperature between the inside air and indoor water pipes, and between the outside air and outdoor water pipes. What are some ways to do this?
Well, we know that we can heat spaces by using electrical resistance. But, while this would increase the delta T between the outside air and outdoor water pipes, it would not transfer enough heat to the interior of the pipes to be drawn into the water.
We should note that increasing the pressure of a mass correspondingly increases the temperature of the mass. Okay, so what if we add a compressor, such as in the image below, to the piping system to increase the delta T between the outside air and the outdoor water pipes?
Increasing the pressure would surely increase the delta T, but 2 issues remain:
- We need another mechanism that will inversely decrease the pressure of the fluid in the system.
- Liquid does not fair well in a compressor.
First, let’s add an “expansion valve”.
This mechanism will allow us to lower the pressure of the water once enough heat has been rejected from the outdoor water pipes. Likewise, by decreasing the pressure this device will lower the temperature of the water, allowing for a greater delta T between the inside air and the indoor water pipes. Thus, more heat is rejected into the water and transferred to the outside air, cooling the building.
Second, we need to figure out how to make our water gaseous before it enters the compressor, or else the machine will start “slugging” and lose efficiency. We could evaporate the water, but that would either require more temperature or more pressure. Well, our inside air is not (in any practical sense) going to be warm enough to achieve this, and we haven’t reached a mechanism yet which increases pressure. WHAT WILL WE DO?!
Perhaps water is the wrong fluid.
Let’s try refrigerant. “A refrigerant is a substance or mixture, usually a fluid…[which has] favorable thermodynamic properties, [is] noncorrosive to mechanical components, and [is] safe, including freedom from toxicity and flammability.” https://en.wikipedia.org/wiki/Refrigerant
One of the favorable thermodynamic properties of refrigerant is it’s low boiling point, allowing us to evaporate the fluid at a relatively low temperature (slightly below the temperature of our inside air, so that the fluid is fully evaporated before it reaches the compressor), and minimizing the risk of the fluid freezing in colder temperatures.
Now that we have refrigerant working to our benefit, what else can we do to increase the efficiency of heat transfer?
We can increase the surface area of the piping, allowing for more time to draw and reject heat. We can use a design such as seen in the image below, now to be called “coils”.
So now we have a refrigerant piping system with 4 components:
- Indoor Coils
- Outdoor Coils
- Expansion Valve
The indoor and outdoor coils allow us to transfer heat, while the compressor and expansion valve increase our delta T, allowing for greater heat transfer.
Here’s a diagram of this cycle, called the Refrigeration Cycle:
Here is the sequence of the cycle:
- Refrigerant passes through the evaporator coils (inside the building) and extracts heat from the inside air, enough to evaporate the fluid from liquid to gas.
- The refrigerant is drawn into the compressor, which increases the pressure and temperature of the fluid, allowing it to reject more heat.
- The refrigerant passes through the condenser, rejecting heat to the outside air.
- The refrigerant passes through the expansion valve, which decreases the pressure and temperature of the fluid, allowing it to extract more heat.
- The refrigerant passes back through the evaporator, repeating the cycle.
A few things to note:
- The inside air contains some amount of humidity. The cool temperature of the evaporator coils causes some of this humidity to condense onto the coils as the air is passing through. This is then drained outside of the building through a condensation pipe.
- The flow of the refrigerant can be reversed using a “reversal valve”, to reject heat into the building, increasing the temperature of the inside air. This is useful in the winter time.
The images below show part of a home “split-system”. The outdoor components include the following:
- Condenser coils, the cube shape inside the exterior casing
- Compressor, located in the middle of the cube
- Expansion valve, similar location as the compressor
Note, the refrigerant is flowing in the compressor through the “lower” pipe and discharging through the “upper” pipe.