Deep technical analysis on this kind of alternative that is implemented as a refrigerant for different systems and solutions in the sector.
by Ing. Ernesto Sanguinetti Remusgo *
Because of the damage they did to the earth's ozone layer, the total elimination of the manufacture / sale of CFC refrigerants has already occurred. The phasing out of the use of HCFCs for developing countries has been taking place and has its limit in 2030 (although 2031 and 2039 can import very small amounts for special cases, but after that last year there will be total elimination ).
On the other hand, HFC refrigerants with high GWP that contribute to global warming (greenhouse effect) have a phase-out program that has already been taking place in developed countries for a decade, and for much of that time the refrigerant manufacturers jointly with refrigeration and air conditioning equipment manufacturers have been working to develop refrigerants with less impact on global warming. For developing countries, called the A5 group according to the Montreal Protocol, there is another program to eliminate HFCs gradually (each country will develop its program that will probably start from 2024) and meanwhile we will accommodate the known fluids and to those who appear, having as a condition that they do not harm or harm our planet very little.
Although final substitutes for refrigerants are still being analyzed and tested in most applications, the main applicants belong to one of these groups:
- Natural refrigerants (which can be organic such as HC and inorganic hydrocarbons such as ammonia NH3 and carbon dioxide CO2).
- Synthetic refrigerants (which can be HFCs obtained from saturated hydrocarbons and HFO or Hydro Fluoro Olefins obtained from unsaturated hydrocarbons).
- There is a third group as a consequence of the previous two because they have started using mixtures of them, with some good results. Graphically we show those 3 groups:
- We will deal with one of those natural refrigerants that is a hydrocarbon (HC): Propane or R-290
The ASHRAE 34 standard for identifying Organic Natural refrigerants, which is the group to which the Propane belongs, says that after the letter "R" up to 4 digits can be used:
- First digit, from right to left = number of fluorine atoms in the compound.
- Second digit to the left = number of hydrogen atoms plus 1.
- Third digit to the left = number of carbon atoms minus 1 (not used when it is equal to zero).
- Fourth digit to the left = number of double links, when it exists. This is for unsaturated halogenated synthetic refrigerants (the digit is not used when it is equal to zero).
- If you see a lowercase letter after the numbers, it is an isomer compound.
Isomerism is a property of those chemical compounds that, with the same molecular formula (chemical formula not developed) of equal amounts of atoms that make up its molecule, but that these atoms can be accommodated in different ways presenting different chemical structures, and therefore, They have different properties. Therefore, these compounds are called isomers. There are no isomers in the case of Propane.
The use of propane as a cooling fluid is increasing because it has several advantages:
It is a natural fluid, it does not damage the ozone layer (ODP = 0), it produces negligible global warming (GWP less than 5), it is cheaper, compared to other refrigerants, less quantity (by weight) is needed as a load in the systems, It is compatible with all commercial lubricating oils (but always consult with the manufacturer of the compressor), has comparatively lower pressures and temperatures of work, decreases energy consumption as well as maintenance cost in the systems, is very stable being compatible with metals and other elements that are used in the refrigeration circuits and increases the useful life of the systems.
But it has disadvantages:
It is flammable, for the service requires trained personnel, requires control mechanisms and special protection to avoid sparks.
Because it is flammable, but not toxic, it must be taken into account that ASHRAE has already classified it as a refrigerant belonging to the A3 group.
This classification used for all refrigerants obeys the following safety standard that uses a letter followed by a number:
Categorization based on toxicity is done with "A" and "B" depending on whether or not toxicity is identified in a person at a certain concentration and is of two kinds:
Class A: In which no effects of chronic toxicity are observed when exposed to equal or higher concentrations of 400 ppm.
Class B: In which chronic toxicity is observed when exposed to concentrations below 400 ppm.
The categorization based on Flammability is done with the numbers "1", "2" and "3", depending on the speed of propagation of a flame, and is of 4 classes:
Class 1: No propagation of the flame occurs in tests in air at 60ºC and standard atmospheric pressure.
Class 2L: Same as class 2 but with a laminar flame velocity lower than 0.10 m / s.
Class 2: Presents flame propagation in tests at 60ºC and atmospheric pressure, but has an LII greater than 3.5% by volume and a heat of combustion less than 19,000 kJ / kg.
Class 3: Presents flame propagation in tests at 60ºC and atmospheric pressure, but has an LII less than or equal to 3.5% by volume or heat of combustion greater than or equal to 19,000 kJ / kg.
- Due to flammability, propane and HFOs differ significantly. Although some HFOs are flammable, they have a very high LII, so a large leak must occur before igniting; and when they light they do not emanate a lot of heat and tend to burn slowly. These properties are so special that the ASHRAE created the flammability classification called 2L. Now it is applied to the HFO (HydroFluorOlefins) and mixtures between those fluids or mixtures of other refrigerants with said fluids, because they have very low flammability.
- Lower Flammability Limit (LII), is the minimum concentration of gas in the air below which fire is not possible. Higher Flammability Limit (LSI), is the maximum concentration of gas in the air above which fire is not possible.
We show for comparison purposes the safety characteristics of some refrigerants within which is the PROPANE (R-290):
We show the tables of Pressure vs. Temperature for comparison purposes of R-290 (Propane) with other refrigerants, being from this point of view very similar to R-22:
Because they are flammable refrigerants, for both Class A2 and Class A3 refrigerants, safety codes or regulations currently severely limit the amount, by weight, of charging a system; banning them effectively in most large-scale applications. In the United States of America and Europe, commercial refrigerators and freezers have limited load for A3 refrigerants (propane between them) to 150 grams. In Europe manufacturers of refrigerated furniture for commercial applications are pushing for that limit to rise to 1 or 1.5 Kilos.
With the aforementioned limitation, it is sufficient to cool domestic refrigerators or small beverage preservatives or food displays in a supermarket, but it is practically insufficient for larger refrigerators without doors or with glass doors, commonly located in the aisles of dairy products. vegetables or frozen food in supermarkets, because even having self-contained refrigeration equipment, as shown in graph N ° 1 with air-cooled condenser, require more refrigerant charge.
If they are connected to a compressor unit located in a remote Machine Room having an air cooled condenser even further, the amount of refrigerant charge required will be higher and will inevitably be outside the permitted limits.
By having these restrictions appear the "ingenious" solutions of the Engineers to be able to use little refrigerant.
One solution proposed and already in use is the use of the water-cooled plate-type condenser as in figure N ° 2 as would be the case of a dairy or drinks exhibitor, with doors or without doors to the consumer. This solution requires the use of a coil with copper tubes, aluminum fins and fan (s) that is an "air cooler", in English they call it "dry-cooler", to cool the water and use a closed circuit with a pump that the recirculate (circuit in RED color). The R-290 refrigerant charge is significantly reduced. The "aero-cooler" is located outside the cabinet and even more outside the sales area.
The 6 and 7 graphs show more ingenious and daring solutions because the refrigerant charge R-290 is restricted to the existing one only in the "chiller" that is located in a machine room or in general in an environment outside the premises or from the sales room to the public.
This "chiller" has a compressor for R-290, a water-cooled plate-type condenser, a thermostatic or electronic expansion valve and a plate-type evaporator. The coolant is Propane or R-290. The propylene glycol after cooling is pumped and distributed to each conservator, display case, island or cold room that can be at different temperatures, then collect and join again returning to the "chiller" to cool and work in a closed circuit (circuit in color blue).
On the other hand, as in the case previously explained, the water circuit that serves for the condensation of the Propane appears graphically in RED color, also having its "aero-cooler".
Meanwhile, with HFOs, which are low-flammable, efficient synthetic refrigerants with a global warming potential also much lower than HFCs, tests are being done to be used in domestic refrigerators, commercial refrigeration, air conditioning and others.
* Ing. Ernesto Sanguinetti Remusgo, Manager Engineering Division -COLD IMPORT SA- Lima - PERU. email@example.com