Vapor Compression Cycle

11/27/17

“How do most AC systems work?”

 

While absorption refrigeration systems are great for many applications, sometimes we just want a more traditional technology for our cooling needs. So how exactly do most AC systems actually work? Well, let’s use our engineering mindset to find out.

 

Let’s start with the basics. Our goal is to take heat from one space and transport it to another. One very common way to do this is to use a heat exchanger. But to ensure continuous cooling, we must ensure that we have a constant supply of fluid at a temperature we desire. To build this system, let’s start with four components: a compressor, a condenser, an expansion valve, and an evaporator. Then, let’s have a refrigerant enter the compressor at a low temperature in a gaseous state, and do work on it to increase its temperature and pressure. Then, let’s have it enter the condenser, where heat is transferred into a nearby medium. Afterwards, let’s run it through an expansion valve to cool it and release pressure while turning some of the gas into a liquid. Finally, let’s pull it through an evaporator, which is basically a heat exchanger that allows the cold fluid to absorb heat from the surrounding medium (and therefore cool it) and gasify it. Once the fluid leaves the evaporator, it enters the compressor and the process (known as the vapor compression cycle) will start again.

Absorption Refrigeration

11/25/17

“How do can we apply the laws of physics and chemistry to make a refrigerator?”

 

Refrigerators allow us to survive in the modern world, whether it be in keeping our buildings cool or our food fresh. However, how exactly do these systems work? Well, let’s look at one such approach.

 

First, let’s start with a solution of hydrogen and ammonia in a boiler. Then, let’s apply heat to separate out these two substances, leading the ammonia gas to a higher pipe and the hydrogen to a lower pipe to be mixed later. Once this is complete, let’s send the ammonia gas through a heat exchanger to condense it into a liquid. Afterwards, let’s shove this new liquid with the hydrogen gas in a freezer (evaporator). The highly reactive chemical reaction will absorb a large amount of heat from the surroundings. The ammonia gas will then be evaporated with a second heat exchanger and sent back to the original boiler, starting the process all over again!

Absorption Refrigeration is commonly implemented in food storage in commercial vehicles and in buildings for waste heat recovery.

Psychrometric Charts

11/20/17

“How can we create a chart of a temperature comfort range?”

 

We all know that many different external factors can affect one’s temperature comfort zone. For example, 28 degrees centigrade can feel either pleasant or horrific depending on the relative humidity. So how can we relate all of these complex variables together in a coherent way? Well, many good Engineers, Architects, and Scientists took on this challenge, and decided to create a chart that takes in Dry-bulb (or absolute) temperature, Wet-bulb, Dew Point, Relative Humidity, Humidity Ratio, Specific Enthalpy, and Specific Volume of the local atmosphere to produce a visualization of a temperature comfort range. This is known as a psychometric chart and is one of the most important foundations of modern HVAC engineering.