We’ve spent some time on the second law of thermodynamics - energy flows naturally from high potential to low, from hot to cold. Most of us have refrigerators or air conditioners in our dwellings - devices which seem to violate this law. They somehow move heat out of a colder space and into a hotter space. So what is going on here?
The short answer is that the refrigerant in these devices acts as a temporary energy storage medium - by some clever manipulation, we can move the refrigerant through a cycle such that at the point it travels through the space we want to cool it absorbs energy, which it needs to dump when it gets to the warmer exhaust space.
Here’s a typical refrigerator schematic.
Refrigerant enters the compressor as vapor. The compressor does work to increase its pressure. Increasing its pressure also increases its temperature, so by the time it hits the condenser it’s relatively hot. In fact, it’s hot enough relative to the surrounding environment that it sheds heat as it flows through the condenser. By the time it leaves, it’s condensed and much cooler, although it’s still at a very high pressure. It goes through an expansion valve from here, which drastically drops both its temperature and pressure. So it enters the evaporator as a low temperature, low pressure liquid. In this state, it is low enough energy that it absorbs energy from the surrounding environment going through the evaporator. When it leaves, it’s a vapor again, ready reenter the compressor and start another cycle.
This works because we are injecting energy into the cycle in the form of work done by the compressor in order to get the refrigerant to a state where the heat it absorbed from the refrigerated space can be dumped to the outside environment.
We have a heat engine taking in heat at a rate of 100 MW from a high temperature reservoir. 75 MW of this energy is rejected to a low temperature reservoir. The rest is converted to useful work. How much net work output does this heat engine produce and how efficient is it?
The only source of energy in the system is Q_H, so we know that the energy leaving in the form of W_net out and Q_L must be equal to the energy input, Q_H. Or, put another way, W_net out must be equal to the difference between Q_H and Q_L.
In this case, we get a net work output of 25 MW from this engine.
Efficiency is the ratio of your desired output to your input. In this case, the output we care about is useful work, and the input to the system is Q_H. So we get a thermal efficiency for the engine of 25%.
So I am an avid reader of Hackadayfor a long time now and they have been putting out a lot of great introductions to tools and processes to get makers up to speed on the resources that are available. This is just a splattering of links that I have found lately that you guys might be interested in.
Hackaday has done a terrific write up of the engineering behind the Othermill cnc machine. In the article, which is way too long to post, they compare it to another generic desktop CNC kit, but at the same time they point out all the key areas that the designers had to take into account in order to build the machine. One detail is the hdpe frame vs aluminum extrusion that results in greater machinabililty (yep, it is a word), lighter weight, and cheaper cost of goods. It’s a great demonstration of how engineering doesn’t happen in a vacuum. There are many factors that have to be taken into account like materials, vibrations, loose material, wiring, fasteners, cost of goods, and ease of use.
While Hackaday is mainly an electronics and software blog, they occasionally branch out into the mechanical area. Definitely check it out.
Note: Just like Hackaday, we have not received any form of compensation for posting this from Othermill, I just think its a well designed machine.
If we’re going to talk about heat transfer, we’ve got to talk a little bit about thermodynamics. We’ll take it one law at a time.
The first law of thermodynamics just boils down to conservation of energy.
In a closed system, the total energy present remains constant. The only way the amount of energy present can change is if energy is put into the system or taken out. There are two ways to make energy cross system boundaries like this: either by heat transfer or by work done. So for a closed system, the total change in energy of the system is the net amount of heat put in minus the net amount of work out.
In addition to a closed system, this principle can also be applied to a control volume - that is, a defined region of space that mass can enter and leave. Mass entering will carry energy in with it, and mass leaving will carry energy out with it.
In the situation in which you are considering a control volume in the midst of a constant flow of incompressible fluid, you can consider the heat transfer occurring to be a function of the temperature difference between the fluid entering and the fluid exiting, the mass flow rate of the fluid (mass transferred per unit time), and the specific heat of the fluid, c, which is a physical property of the fluid - basically, how much energy you have to put into it to raise its temperature.
This is a simplified equation, and many situations involving fluid flow require consideration of additional factors, but for now it’ll work for us. We’ll get into the more complicated stuff later.
