Cascade systems: safety and efficiency

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propane

The topical focus is more and more often on “mixed” solutions to obtain specific efficiency in each phase of the generation of frigories and the installation of “isolated” primary circuits owing to safety reasons allows facing with more operational serenity and bureaucratic user-friendliness the use of “delicate” refrigerants such as NH3 and C3H8 upstream secondary systems in subcritical CO2 or other. Let us try to analyse a possible operation field, the propane one.

In recent years, a topical theme is the use of natural refrigerants in refrigeration, to replace synthetic refrigerants currently used on a large scale: there is no doubt that the provisions deriving from EU 517/2014 Regulation and the foreseen phase out contained in it imposes a reflection about how implementing new plants and what are the technical solutions to assure a low environmental impact, without creating conditions that are to the detriment of the plant efficiency, thus determining additional costs for buyers.

In these cases, an interesting solution for some applications is the use of cascade or two-stage systems. In the last decade, the use of this implementation system, with the use of CO2 as secondary fluid, aroused particular interest. In this case, we are not analysing thoroughly the operation specifications, already treated previously in an article in this column by the colleague Valentino Verzotto, but it is evident that the use of CO2 as secondary fluid allows making this refrigerant work in subcritical cycle, because through a primary circuit managed with other refrigerant and provided with other subcritical temperatures, it is possible to control the condensate temperature and to adjust it under the critical temperature.

This design and manufacturing method simplifies the circuit, under several aspects, in both the design and especially in the management, becoming actually identical to a standard circuit from all points of view. Besides, it allows users to make the plant work with much lower CO2 pressures compared to what happens in the transcritical cycle, achieving evident advantages in both the structural complexity (circuit tightness and cost) and in safety (leak risk and much more).

However, let us focus now our interest on the primary circuit and on the usable fluid. In LOD, many plants are installed with this technology and most of them use synthetic gases as fluid, generally I would say R134a (currently partly replaced by R513a) due to the thermo-dynamic properties of the gas (low compression pressures) and the relatively low GWP. However, the use on the primary of chillers based on natural fluids, such as ammonia (R-717) and propane (R290) is interesting and decisively applicable in some installation typologies, especially industrial ones.

Propane is a saturated hydrocarbon of the alkane family and it is classified among natural fluids; it is an odourless, colourless and chemically stable gas; it is not toxic but it is highly flammable. The flammability limit in air at 20 °C temperature and pressure of 1.013 bars is included between 2.37% and 9.5%. Its self-ignition temperature is 510 °C.
Propane is not free from problems and the latter must be known to possible users: being flammable, in fact, it is classified as A3 by ASHRAE regulations. Nevertheless, its thermo-dynamic properties, fully similar to synthetic refrigerants in use, make it interesting as refrigerator, therefore flammability would not fully affect the possibility of its extended use, although exclusively to the plants where propane remains outside buildings.
After this necessary premise, we can understand the reason for a possible use of R290 (unless relatively small plants are at stake) just as primary fluid of a cascade plant, both with secondary CO2 fluid or glycol. This concept of synergistic use of plants that clearly develop the specific properties of each refrigerant is fundamental for the implementation of overall efficient plants: this provides for a targeted technology allocation and a correct analysis and management of risks deriving from the adopted technology.

The use of “extremely flammable” refrigerants like R290 attracts our attention on the safety criteria to comply with in design and installation phase, as well as in the course of the life of the plants itself, in other words during system maintenance and repair operations.

Concerning the plant design phase, with the exclusion of some refrigerator manufacturers that still today implement refrigeration appliances, the problem falls on the producer of the refrigerating machine, which must anyway conform to the E.S. R. imposed by sector European regulations such as directive PED, ATEX electromagnetic compatibility and so on.
In the installation phase of the machine in the place selected by the customer, the requirement consists in a feasibility analysis and a study of the positioning in compliance with the requisites of the corporate fire prevention plan, which must be then communicated to the entrusted authorities (that is to say firemen): in this phase, obligations fall on buyer and end-user, who must anyway be preventively trained and “guided” on the plant feasibility in negotiation phase, to avoid annoying bureaucratic problems later on.

Therefore, the installation company has the task of developing a plant and of implementing it precisely bearing in mind the product specifications, the peculiarities of the installation place and the adoption of all methodologies for the containment and the conformity with E.S.R., Essential Safety Requisites.

In these situations, a good practice of a refrigeration company should be availing itself of the competences of a technician specialized in the fire prevention sector, entrusted with a preventive risk analysis in conformity with the regulations in force on the matter. The presence of a specialized technician is determinant also because – since these plants are based on a quite recent concept – control bodies are not informed about the specifications of a refrigeration plant of this kind and it is then difficult to provide them with the right indications and evaluations that permit a correct review of the fire prevention plan. The determinant variable consists in the need of clarifying unequivocally that the risk is low, because – under the real operation conditions of a plant conceived with cascade cycle – the dangerous fluid is not inside the rooms where workers operate but displaced to a specific environment made suitably safe.

It is anyway worth highlighting that the plants that exploit R290 as refrigerant are designed to host a low refrigerant charge: in fact, micro-channel condensers are used, containers are minimized and the refrigerating power is split on more circuits, to reduce the risk as much as possible, because specific risks grow hand in hand with the higher quantity of refrigerant contained in the single circuit.

The residual risks the refrigeration technician should anyway always consider attentively are those deriving from the operation and the management of the plant in the long term, especially during maintenance and failure repair operations.

It is currently under approval the regulation CEN/CENELEC 17607:2020 (we talk about it in another article of this column, editor’s note) that prescribes the requisites for those who operate with flammable refrigerants, because, under all respects, the risks deriving from the operations close to a circuit that contains an A3 refrigerant are very different risks compared to those that characterize the interventions on plants equipped with synthetic non-flammable refrigerants.

Therefore, it is necessary that the operators of plants containing R290 must be adequately trained and educated by the employer about specific risks and the operations to be executed. Whenever they intervene on the plant, they must draw up a risk declaration providing for all possible operations to be carried out on the system, from the simple control to the direct intervention on the refrigerating circuit. The staff must be equipped with all suitable equipment and protection devices to work with this type of refrigerant, the danger zone must be delimited and highlighted and interruption devices of the refrigerant circuit must be installed and active.

In conclusive analysis, we can affirm that a cascade system with flammable natural primary fluid like R290 and glycol or CO2secondary fluid, implies numerous advantages from the plant engineering and environmental point of view. It almost minimizes the direct impact of the plant on the environment. The adoption of this plant engineering scheme technically simplifies the integration with a secondary CO2 circuit: as already said, in this context the carbon dioxide works in subcritical cycle and can circulate in inner environments with lower management criticalities, since the work pressures at stake for the CO2 notably decrease in comparison with a transcritical cycle, with the advantage of reducing the risks connected with the leaks of the CO2.circuit.

Obviously, the use of refrigerants characterized as generating specific risks (flammability in this case) compels the refrigerator technician to a suitable risk management discipline, with the need (precisely as already underlined) of training competent staff on the matter.

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