1. Thermodynamics, Lesson 5-5: The Vapor Compression Refrigeration Cycle
MECE-251
Thermodynamics, Lesson 5-5:
The Vapor Compression Refrigeration Cycle
VCR is used extensively in the transportation sector to moves perishable goods. National and International regulations are changing the design space rapidly.
Refrigerant selection is based on several factors:
Performance: provides adequate cooling capacity cost-effectively.
Safety: avoids hazards (i.e., toxicity).
Environmental impact: minimizes harm to stratospheric ozone layer and reduces negative impact to global climate change.
The Vapor Compression Refrigeration Cycle is the dominant refrigeration cycle used in mobile applications all over the world.
The most important design parameter lies in the selection of the working fluid to be used as a refrigerant. A key aspect of the working fluid is that we need to be able to have it exist in both the liquid and vapor phase at both the hot temperature and cold temperature of the cycle by manipulating the pressure of the fluid. We need the liquid refrigerant to boil at low temperatures and pressure, and we need it to condense to a liquid at high temperature and pressure.
Finding fluids that achieve these phase changes over the desired range of cold temperatures (for the ice box of the refrigerator) and warm temperatures (the ambient environment) without requiring EXTREME pressures is a big challenge.
Many candidate refrigerant fluids are quite nasty and toxic to humans or other mammals. Others are non-toxic, but have very low specific heat, so you need a LOT of the fluid. The vast majority of refrigerants are very damaging to the ozone layer AND they make carbon dioxide look good by comparison from a global warming perspective.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

2. The Vapor-Compression Refrigeration Cycle Consists of 4 processes
liquid
EVAPORATOR Process 4-1: two-phase liquid-vapor mixture of refrigerant is evaporated through heat transfer from the refrigerated space.
COMPRESSOR Process 1-2: vapor refrigerant is compressed to a relatively high temperature and pressure requiring work input.
CONDENSER Process 2-3: vapor refrigerant condenses to liquid through heat transfer to the cooler surroundings.
EXPANSION VALVE Process 3-4: liquid refrigerant expands to the evaporator pressure.
vapor
Two-phase
liquid-vapor mixture 3 2
Let’s study the VCRC cycle by looking at the evaporator first. As we study this system, please look at the refrigerator in your home or apartment.
Process 4-1: The evaporator is located inside the icebox of your freezer. It’s probably where the ice builds up when it’s time for you to defrost your fridge. The evaporator is a heat exchanger. Inside of the coils of the evaporator, there is a liquid/vapor mixture of refrigerant, with a quality of around 20%. The saturation temperature of the mixture MUST be lower than the temperature in the freezer. The second law (and intuition) tells us that heat will flow from the cold freezer into the even colder refrigerant. The heat flowing into the refrigerant causes its quality to rise and it becomes a superheated vapor
Process 1-2: The gaseous refrigerant flows out of the freezer to the inlet of the compressor. Locate the compressor on your refrigerator. It is usually near the floor, in the back. The compressor accepts work in, most commonly in the form of electricity for residential applications, and as rotating power from an engine in mobile applications. The gaseous refrigerant enters the compressor at low pressure, and leaves the compressor at high pressure. As we know from our previous work, the compression work upon the gas raises its temperature. We have a high temperature, high pressure gaseous fluid at the exit of the compressor.
Process 2-3: The fluid leaves the compressor at high temperature, and enters the condenser. The temperature of the fluid inside the condenser must be higher than the ambient temperature of the room where the refrigerator sits. Heat flows from the high temperature refrigerant through the condenser coils into the room air of the kitchen. The condenser is usually a set of serpentine coils on the back of your refrigerator, against the wall. When the coils get coated with dust bunnies, the refrigerator does not work well. If the fridge is located outside or in a warm garage in the summer, its COP will reduce dramatically, or it may even cease to operate as a refrigerator at all. As the heat leaves the refrigerant its quality decreases, and it exits the condenser as a compressed liquid.
Process 3-4: The next process is where the “magic” really happens! Have you ever picked up a warm garden hose, full of warm water from laying in the sun, and held the spray nozzle in our hand? Notice how your hand gets cold quite rapidly? If you use compressed air in a spray can to blow the dust out of your computer the can gets quite cold. This throttling effect is called the Joule-Thompson effect. It’s a natural phenomena that occurs when high pressure compressed liquids undergo a rapid expansion as the result of a significant decrease in pressure. This throttling effect in the expansion valve dramatically lowers the temperature of the refrigerant and produces a liquid vapor mixture with a quality of about 20%.
We’re back where we started, and the cycle repeats in a continuous fashion. This is how your refrigerator, and the vast majority of mobile refrigeration systems work. There are other refrigeration cycles, but we don’t have time to look at them here. 1 4
vapor
mixture
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

3. The Vapor-Compression Refrigeration Cycle
Engineering model:
Each component is analyzed as a control volume at steady state.
Dry compression is presumed: the refrigerant is a vapor.
The compressor operates adiabatically.
The refrigerant expanding through the valve undergoes a throttling process.
Kinetic and potential energy changes are ignored.
Based on the work we have done so far, you have all the tools needed to rigorously analyze a refrigerator.
The compressor can be modeled adiabatically, just like we did on the previous case study.
The condenser and the evaporator are both heat exchangers. We’ll take a look at them in more detail when we get to the heat transfer module of the course.
The expansion valve is really a novel device. As a very good first approximation, we can estimate that the enthalpy (or entropy) at the inlet to the valve is almost the same as the enthalpy (or entropy) at the exit of the valve.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

4. The Vapor-Compression Refrigeration Cycle
Applying mass and energy rate balances
Evaporator
The term is referred to as the refrigeration capacity, expressed in kW in the SI unit system or Btu/h in the English unit system.
A common alternate unit is the ton of refrigeration which equals 200 Btu/min or about 211 kJ/min.
Refrigeration systems are often sized in terms of their cooling capacity, or ability to remove energy from the cold box.
All over the world, even where SI units are extensively employed, it is very common to refer to the refrigeration capacity of industrial and commercial refrigeration systems using the term “ton” of refrigeration.
A ton is 12,000 BTU (British thermal units) per hour. The term tons comes from the amount of energy absorbed by one ton of ice (the latent heat of fusion) as it changes phase from solid to liquid at 32F over a 24 hour period. This terminology was developed when the means of cooling food was to harvest natural ice from lakes and rivers during the winter months, and store it in ice houses for use in the summer months. A ton of ice was a useful measure for commercial food processors (such as cheese makers) to estimate their cooling needs for a season.
This practice was used throughout most of modern human civilization, and really only became unpopular in the late 1960’s, when a lot of companies took their ice houses out and replaced them with VCRCs. For example, I grew up near the New York State Thruway, and very well knew the Kutter Family, who operated a cheese factory. They welcomed the thruway authority to dig a large pond on their land to use the soil for making embankments on the thruway. The Kutter family then used the pond to harvest ice in the winter, which they used all year long in their cheese factory.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

5. The Vapor-Compression Refrigeration Cycle
Applying mass and energy rate balances
Compressor
Assuming adiabatic compression
Condenser
Expansion valve
Assuming a throttling process
We’ve studied compressors at some length already. Enough said.
The condenser is another heat exchanger, just like the evaporator. The only difference is the direction of heat flow.
The expansion valve can be a simple as a tiny pipe discharging into a large diameter pipe through an abrupt area change. See if you can locate the expansion valve on your home refrigerator.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

6. The Vapor-Compression Refrigeration Cycle
Performance parameters
Coefficient of Performance (COP)
The actual COP of the refrigerator is given by the ratio of desired cooling effect over the compressor work needed as input to the cycle.
The actual COP must always be less than the Carnot cycle COP. If you propose a thermal cycle that violates the Carnot efficiency limits, you’ll never make it past the first stage of patent review for your invention!
Carnot Coefficient of Performance
This equation represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between cold and hot regions at TC and TH, respectively.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

7. Refrigerant Types and Characteristics
The table here shows some of the most common refrigerants. R-11 and R-12 were the work horse of refrigeration systems for decades. ChloroFlouroCarbons, or CFC’s were eliminated from release in Prior to that, it was common for maintenance personnel to simply purge the refrigerant to the atmosphere when performing maintenance.
CFCs were identified as the reason for the “hole in the ozone layer” that appeared over Antartica. The peak hole size occurred in 2006.
Internationally, with the increased emphasis on global warming and greenhouse gases, many common refrigerants are getting a lot of attention. The table shown here provides a comparison of common refrigerants against the baseline of Carbon Dioxide. A larger value means that the refrigerant has that much greater atmospheric energy capture of sunlight than carbon dioxide. A value less than 1 indicates that the refrigerant has less energy absorption than CO2.
Obviously, from this table, Ammonia looks pretty appealing. It is commonly used today in large scale industrial applications, and used to be used in residential refrigerators in the USA in the 1950’s. Ammonia used a completely different cycle, called “absorption cycle” to achieve cooling. One of the biggest negative aspects of ammonia is that it is toxic to humans.
Global Warming Potential (GWP) is a simplified index that estimates the potential future influence on global warming associated with different gases when released to the atmosphere.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.