For refrigerants to be ideal, it should possesses following properties
1. Low boiling point
2. Low freezing point
3. High critical pressure and temperature (to avoid large power requirement)
4. Low specific heat (high specific heat decreases refrigerant effect per kg or refrigerant.
5. High latent heat (increases refrigerant effect per kg of refrigerant)
6. Pressure in evaporator and condenser should be low enough to reduce material cost and must be positive to avoid leakage of air into the system.
7. Low specific volume to reduce size of compressor.
8. High thermal conductivity to reduce area of heat transfer in evaporator and condenser.
9. Non-flammable, non-explosive, non-toxic and non-corrosive.
10. High miscibility with lubricating oil and should not have reacting property with lubricating oil in the temperature range of system.
11. High coefficient of performance (COP) to reduce cost of system
12. Readily available and cheap
Comparison of refrigerants:
In the choice of refrigerant, it should be remembered that none of the substance has proved as ideal working medium under all operating conditions. The characteristics of some refrigerants make them desirable for use with reciprocating compressors. Other refrigerants are best adapted to centrifugal or rotary compressors. In some applications, toxicity is of negligible importance whereas in others, such as comfort cooling, a non-toxic and non-flammable refrigerant is essential.
The requirement of refrigerant to be used for low temperature applications are different from those for high temperature applications. Therefore in selecting the correct refrigerant, it is necessary to determine those properties which are most desirable and to choose the one most economic, approaching the ideal for particular application. Comparison of refrigerants are made on the basis of following desirable properties
1. Evaporator and condenser pressures: It is desirable that both condenser and evaporator pressure be positive, yet not too high above atmospheric pressure. Positive pressure prevent leakage of air and moisture into the system and also make it easier to detect leaks. However, if the pressure are high, heavy construction of compressor, evaporator and condenser is usually necessary resulting in an increased initial cost and less compact system. High operating pressure also tend to increase power consumption.
2. Boiling point: low boiling temperature at atmospheric pressure of refrigerant is required for an efficient refrigeration. The refrigerants usually selected for reciprocating compressors are the ones with boiling point below -1°C and pressure differentials of approximately 3.5 kg/cm2 or higher. Carbon dioxide, ammonia, F-12, SO2, F-22 and CH3Cl come within this group. Because centrifugal compressor do not offer positive displacement, they are best adopted to refrigerants operating at low pressure differentials and under low evaporator and condenser pressures.
3. Freezing Point: The refrigerant chosen must have freezing point below the operating evaporator temperature. Low freezing point is necessary because refrigerant should not freeze under required evaporator temperature.
4. Critical temperature and pressure: The critical temperature of vapor is the temperature above which vapor cannot be condensed irrespective of any high pressure. If the critical temperature of refrigerant is too near the desired condensing temperature, excessive power consumption results. It should be higher than the temperature occurring in the condenser for easy condensation of the refrigerant vapor.
5. Latent heat: High latent heat of refrigerant at evaporator temperature is desirable because the refrigerating effect per kg of refrigerant will be high. The weight of refrigerant required to be circulated in the system per ton of refrigeration will be low if the latent heat of refrigerant is high.
6. Cost: in lager plants both the initial and maintenance costs are influenced by tightness of construction and leakage tendency of refrigerant. The rate of leakage is known to be inversely proportional to the square root of molecular weight besides being dependent on factors such as pressure difference, diffusion rate, viscosity, density and capillarity. The possibility of leakage is more with discharge pressure and low density refrigerant.
7. Coefficient of performance (COP) and power requirement: The requirement of the horse power per ton of refrigeration is most important consideration from the economic point of view. Low power consumption per ton of refrigerant is always desirable. Theoretical values of COP and horse power per ton of different refrigerants are listed below, when the evaporating temperature is -15°C and condensing temperature is 30°C
8. Vapor and liquid refrigerant densities: Low refrigerant densities are generally preferable, since they permit the use of small suction and discharge lines without excessive pressure drops. The reciprocating compressors are always used with a low vapor density, whereas the centrifugal compressors require high vapor density.
9. Specific volume: Specific volume of refrigerant is the number of cubic feet of gas, which is formed when one pound of refrigerant evaporates. The vapor displacement required for a compressor is dependent on the specific volume of the refrigerant at evaporator temperature. A low vapor displacement per ton of refrigeration is necessary for satisfactory operation of reciprocating compressors because of comparatively small displacement available.
The required vapor displacement is almost directly related to the boiling point of the refrigerant at atmospheric pressure. Refrigerants having low boiling points are usually operated with reciprocating compressors, refrigerants with intermediate boiling points with rotary compressors and refrigerants with high boiling point with centrifugal compressors.
10. Stability and inertness: An ideal refrigerant should not decompose at any temperature normally encountered and should not form higher boiling point liquid or solid substance through polymerization. It is desirable that refrigerant be inert with respect to all materials with which it comes in contact. Under certain conditions, ammonia will decompose in either compressor or absorption system. Freon is stable unless exposed to temperature over 538°
11. Corrosive properties: Extreme care must be taken that the compressor chosen for the use of given refrigerant contains no metal with which that refrigerants reacts. The green refrigerants can be used with practically all metals without danger of corrosion and ammonia may be used with iron or steel but not with copper and copper compositions.
12. Dielectric strength: It is the maximum electric field that the material can withstand under ideal condition without breaking down. The electrical resistance of a refrigerant is important if it is to be used in hermetically sealed unit within the motor exposed to refrigerant. The Freons exhibit the best qualities from this standpoint.
13. Viscosity: It is desirable that both the liquid and vapor refrigerants have low viscosities. Pressure drop in passing through condenser and evaporator surface is improved at low viscosities.
14. Thermal conductivity: For efficient use of both evaporator and condenser surface, a refrigerant should possesses a high thermal conductivity. At 0°C, ammonia vapor has thermal conductivity of 0.019 and at 30°C Freon-12 has 0.0083 Kcal/m2/h/° If
15. Oil effect: Practically all refrigerant are miscible with mineral lubricating oil to a certain degree. The miscibility depends on operating temperature, pressure, viscosity and base of oil and design of lubricating system. Ammonia is non-miscible with oil while Freons are highly miscible. With non-miscible refrigerants, larger heat transfer surfaces are required because of poor heat conduction.
16. Toxicity and explosive particles: The toxicity of a refrigerant may be of prime or secondary importance depending on application. An ideal refrigerant should present no danger of explosion or fire, either in combination with air or in association with lubricating oil.