Example Problem with Complete Solution

10B-2 : Refrigerant Selection for a Home Refrigerator 4 pts
Why do modern home refrigerators use R-134a instead of ammonia or Freon-12 (R-12)?
Read : Compare the performance of these three refrigerants in a vapor-compression refrigeration cycle using the 7 criteria presented in Lesson 10B. Some calculations are necessary to assess the P*-T Relationship and the Heat of Vaporization.
Given: Consider  three refrigerants : R134a Ammonia R12
Find: Why is R-134a the most common choice for home refrigeration systems?
Assumptions: None.
Equations / Data / Solve:
Here is a list of criteria used to choose the best refrigerant for a given application.
1 - Cost
2 - The P*-T Relationship 5 - Corrosiveness
3 - ΔHvap 6 - Toxicity
4 - Chemical Stability 7 - Flammability
Cost : R-134a is much more expensive than ammonia, while Freon is not easy to buy since it was banned in 1990 because it damages the ozone layer in the upper atmosphere. Ammonia can cost as little as $0.40/kg while R-134a can cost several US dollars per kg.
The P*-T Relationship
A home rerigerator must keep food in the freezer colder than -10oC. Cool air from the freezer compartment is used to keep the refrigerator compartment colder than 3oC. As a result, the freezer serves as the Cold Reservoir.
The air surrounding the refrigerator serves as the Hot Reservoir. A high value of TH results in the lowest COP for the refrigerator and it also requires the highest pressure in the condenser. So, let's use a rather high temperature for the surroundings.
TC -10 oC TH 30 oC
The temperature of the refrigerant in the evaporator must be greater than TC and in the condenser it must be less than TH.
A rule of thumb in heat exchanger design is that you would like to have a DT of about 10oC between two fluids entering the heat exchanger. In this case, since TC is -10oC, assume that T1 is -20oC. Since TH is 30oC, assume that T4 is 40oC. This gives our refrigerator the desired DT of 10oC. in both the condenser and the evaporator.
T1 -20 oC T4 40 oC
Now, we need to use thermodynamic tables or the NIST Webbook to determine the operating pressures of the condenser and the evaporator. The operating pressures are the saturation pressures that correspond to T1 and T4, determined in the previous paragraph.
PEvap (kPa) PCond (kPa)
Ammonia 190.1 1555.4 Ammonia generally requires a fairly high PCond.
R134a 132.7 1016.6 R-134a and R-12 allow the use of lower PCond and PEvap than Ammonia does.
R12 150.7 958.8
The R-12 is the winner in this test, but the R-134a does almost as well.
A higher latent heat of vaporization results in a smaller mass flow rate for the refrigerant. This may allow the use of smaller, less expensive equipment.
Consider the values of ΔHvap at the values of PCond and PEvap that we determined above.
ΔH @ PEvap
ΔH @ PCond
Ammonia 1329.1 1099.3 Ammonia has the largest ΔHvap, by far.
R134a 212.9 163.0 R-134a takes 2nd place this criterion.
R12 162.0 129.7 R-12 is the biggest loser by this criterion.
Chemical Stability
All three of these refrigerants are relatively stable. Ammonia is perhaps more reactive and slightly less stable than R-134a and R-12.
R-134a and R-12 are non-corrosive to ferrous metals unless exposed to flame or very high temperatures. So, they score well by this criterion.
Ammonia is non-corrosive to ferrous metals unless water is present. In the presence of water, ammonia becomes highly corrosive. So, Ammonia does not score quite as well by this criterion, because water has a way of penetrating devices that are not rigorously maintained.
R-134a and R-12 have very low acute inhalation toxicity, but are irritants to eyes and skin
Ammonia is a strong irritant to eyes, skin, mouth, nose, throat and respiratory tract, but has low inhalation toxicity.
Ammonia may be the loser by this criterion, but all three refrigerants have fairly low toxicity overall.
There is some debate about the flammability of R-134a and R-12, but they are definitely less flammable than Ammonia. Ammonia is quite flammable and can present an explosion hazard.
Verify: None.
Answers : R-134a is the best of the three choices for home refrigeration because it... 
… Allows the condenser and evaporator to operate at pressures that are greater than atmospheric pressure, but not dangerously high.
… It is stabile, non-corrosive, has low toxicity and low flammability.
… It does not damage the ozone layer.
Ammonia is a commonly used industrial refrigerant because…
… Allows the condenser and evaporator to operate at pressures that are greater than atmospheric pressure, but not dangerously high., by industrial safety standards.
… The price of Ammonia is very low.
… With proper industrial safety practices, the problems with corrosion, toxicity and flammability can be overcome.