1. Refrigerants & Coolants Bartosz Zajaczkowski, PhD Lecture 2

2. Today topics Refrigerant evaluation factors Most important properties of refrigerants

3. Desired properties of primary refrigerants No single refrigerant satisfies all desired attributes for all operating conditions. Choice is always a compromise. The best refrigerant for low temperatures is not necessarily as good in moderate temperature range. The refrigerants used in vapor-compression systems are different from the ones used in absorption or adsorption cycles. The evaluation of refrigerant is based upon several factors, including its thermodynamic, chemical and physical properties as well as technical aspects of its use.

4. Thermodynamic evaluation factors Evaporator and condenser pressures Temperature of boiling/liquefaction* Temperature of melting/solidification* Critical temperature and pressure* Latent heat of vaporization* Density or specific volume* Thermal conductivity Viscosity Compression ratio

5. Other evaluation factors Chemical stability Dielectric strength Corrosive properties Miscibility with oil Toxicity and flammability (also explosivity) Methods of leak detection Cost etc.

6. Temperatures of boiling/evaporation A refrigerant should always boil at the lowest possible temperature. In other case it becomes necessary to operate at high vacuums, and thus at low efficiency and capacity. Examples: Water to be evaporated at 10°C requires pressure reduction to approx. 1 kPa. R134a at pressure approx. 2 bar evaporates at temperature ~-10°C.

7. Temperature of melting/solidification The freezing point of refrigerant should be well below any temperature which might be encountered in entire system. Otherwise, there is a risk of blocking passages during the flow of liquid through evaporator. Operating range of most of commonly used and popular refrigerants is far above their freezing points.

8. Freezing and boiling point Melting and boiling temperature of various refrigerants under atmospheric pressure.

9. Critical temperature and pressure In the case of CO 2 the pressure at the end of compression would be too large, necessitating heavy construction to withstand high pressures. A high critical temperature is desirable, as it is impossible to condense the refrigerant at a temperature about the critical, no matter how much the pressure is increased. Above critical point condensation is not possible - latent heat of phase transitions are unusable. With the exception of CO 2, critical temperatures of all popular refrigerants are sufficiently high.

10. Critical point Critical temperatures and pressures of various refrigerants.

11. Sensible heat Sensible heat is the amount of energy released or absorbed by substance during a change of temperature (in J/kg). It is „sensible” because it can be „sensed”, i.e. temperature change can be directly measured with thermometer. The amount of heat necessary to change the temperature of 1 kg of this substance by 1 K (in J/kgK) is the thermodynamic property of every substance and it is called specific heat.

12. Q: How much heat is required to raise the temperature of one liter of water by 20°C? Do you have all requried information to solve this problem? Q: How much heat is required to raise the temperature of one liter of water from 20°C to 100°C? How long will it take 2 kW heater to finish this task?

13. Latent heat Latent heat of phase transition is the amount of energy released or absorbed by a chemical substance during phase change that occurs at constant temperature (in J/kg). It is „latent” because it is hidden from direct measurement of temperature change. Although, energy is transferred temperature reading remains constant.

14. Various types of latent heats of phase transitions.

15. Latent heat Latent heat of evaporation of various refrigerants under atmospheric pressure.

16. Sensible and latent heats Sensible and latent heats and corresponding phase changes. Latent heats of water (approx.): evaporation/condensation ~2250 [kJ/kgK] fusion/melting ~334 [kJ/kgK]

17. Phase diagrams Phase diagram for carbon dioxide (left) and water (right).

18. Density / Specific volume (in gaseous phase) Refrigerant should have low volume per kg when in a gaseous state. This reduces the size of equipment and means high efficiency. In vapor-compression refrigeration systems, the refrigerant can be of... low density (high specific volume) refrigerants are preferred for roto- dynamic compressors. high density (low specific volume) refrigerants are preferred for positive displacement compressors. Water is a popular (yet, troublesome) refrigerant in absorption (LiBr-water) and adsorption refrigeration systems that are easlily recognizable for their equipment size.

19. Specific volumes of selected refrigerants Specific volumes of several refrigrants - areas are in scale, green color indicates natural refrigerant, blue color indicate synthetic. Q: Which refrigerants (either left or right section) are preferred for a piston cylinder compressor and why?