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29CHEMISTRY
STUDY OF THE POSSIBILITY TO USE R407C FREON AS A REFRIGERANT IN SHELL-AND-TUBE FLOODED EVAPORATORS DESIGNED FOR R22 FREON
Selchenko M.
student, bachelor degree in chemical engineering, Southwest state university, Kursk
Abstract:
The article presents a study of the possibility to use refrigerant R407C freon instead of R22 in shell-and-tube flooded evaporators. The analysis is based on the heat load calculation of two evaporator models for each of the freons, and on their toxicity, ecological compatibility and market price.
Keywords: shell-and-tube heat exchanger; flooded evaporator; refrigeration system; freon; R407C freon; R22 freon; refrigerant.
Freons are special agents that have some specific physical and chemical characteristics that allow them to be used as refrigerants for refrigeration systems generally and as a cooling stream in shell-and-tube flooded evaporators particularly.
R407C freon is a hydrofluorocarbon mixture. It is a colourless and odourless gas in its normal state. R407C freon is considered to be chemically and thermally stable, non-toxic and safe from the environmental point of view. It does not burn, but it decomposes under high temperature forming toxic products. Filling and refuelling of the system is performed only in the liquid phase of the refrigerant. In case of hardware failure there is a leak of freon. Uneven evaporation of fractions leads to a change in the proportions of the freon mixture.
In composition R407C freon is a multicomponent mixture composition of three main compounds. It consists R32 (23%), R125 (25%) and R134a (52%) freons. All components in the required proportions provide the mixture with the following properties: R32 (Difluoro-methane) provides cooling capacity, R125 (Pentafluo-roethane) provides fire prevention and R134a (Tetraflu-oroethane) provides pressure stability, in particular when the freon is boiling in a closed volume.
R22 freon (difluorochloromethane) is a methane compound. Basically its thermophysical parameters are comparable to propane. The heat of evaporation of propane two times higher but the vapour density of difluorochloromethane is two times higher as well. That approximately equalises their characteristics. R22 freon is considered a non-combustible substance. It is also non-poisonous but it is not able to support breathing. In the case of a leak the risk is usually eliminated by ventilation. But since it is ozone depleting compound its release into the atmosphere causes environmental damage.
The main technological disadvantage of R22 freon is the presence of chlorine atoms in its molecule. This compound destroys the planet's ozone layer. As a result, the production and use of R22 has been limited and it is planned to replace it completely with alternative refrigerants these years.
Unfortunately, no single component replacement for R22 has been found at the moment but the combination of several freons makes it possible to reproduce the properties of this refrigerant more or less.
Initially R407C freon was designed as an alternative to the toxic R22, therefore the components were selected for maximum compliance with the parameters of the replaced freon.
The main technological disadvantage of R407C freon in comparison with R22 freon is that R407C does not form a homogeneous mixture. On the one hand this significantly limits its use, but on the other hand it is levelled when the compressor is working properly.
The main technological advantage of R407C relative to R22 is its low toxicity. This makes it an excellent environmental alternative to R22 freon, whose components will destroy the ozone layer. The absence of chlorine compounds makes R407C freon safe for the environment.
R407C freon is cheaper than R22 which is also a considerable advantage. Its market price for a cylinder of 11.3 kg is approximately $ 65 (or 5000 rubles), while R22 freon market price is $ 90 (or 7000 rubles) for the same weight. Price data is based on Moscow market price analysis.
The main required property for R407C freon to be an acceptable alternative for toxic R22 freon is the comparability of their thermal characteristics, i.e. the possibility of using R407C in refrigeration units designed for R22.
To evaluate the exchangeability of R407C and R22 freons, i.e. the similarity of their thermophysical characteristics, two models of one shell-and-tube flooded evaporator designed for R22 were taken and recalculated according to R407C freon properties. The conclusion about exchangeability is going to be made on the base of changes in the derivatives of physical and chemical characteristics, i.e. heat exchange parameters that show whether R407C freon can replace R22 in this evaporator without changing the design and operating parameters.
The evaporator models have no difference in their design and operating principle. The difference is only in their dimensions, the number of tubes and their linear dimensions. In both cases the tubes are made of copper alloy UNS C70400 and the shell is made of stainless
steel AISI 430. The materials characteristics are taken into account in the calculation.
For clarity the models used for calculation are abbreviated A46L4 and A121L6 that corresponds to their following characteristics:
for A46L4: A = 46 m2, N = 181, L = 4 m, D = 400 mm, d = 20x2 mm, n = 1;
for A121L6: A = 121 m2, N = 257, L = 6 m, D = 600 mm, d = 25x2 mm, n = 1;
here A - nominal heat exchange surface area, N -number of tubes, L - tubes length, D - inner shell diameter, d - outer tubes diameter and the wall thickness, n - number of tube ways.
The stream configuration in the evaporator is organised as follows: NaCl heating stream flows through the tubes, and freon cooling stream flows in the shell. An evaporator of this configuration is called a flooded evaporator or an evaporator of flooded type. The design and the stream direction of the evaporator are shown in figure 1.
Fig. 1. Diagram of the design and the stream direction in the shell-and-tube flooded evaporator; red arrows corresponds NaCl heating flow, blue arrows corresponds freon cooling flow. Note. Developed by the author.
The calculated technological process carried out with this evaporator is the supercooling of NaCl brine (18%) from -3 to -7 °C. All properties of the solution at the given concentration and temperature [1,4] are also taken into account in the calculation.
Evaluating the initial parameters, i.e. physical and chemical parameters of heat transfer fluids, it is worth to mention the following considerable differences between R407C and R22:
1. R407C has significantly higher specific heat capacity than R22;
2. R407C has significantly higher specific heat than R22;
3. R407C has significantly higher heat of evaporation, i.e. enthalpy change during the transition from the liquid phase to the gas phase;
4. R407C has higher boiling point, it is -25.1 while -40.8 for R22 freon (this is not important for a specific process but basically it narrows the applicability, so this should be taken into account);
5. R407C has lower density and viscosity than its predecessor, that reduces the cooling capacity per unit volume [3,5].
Nevertheless the calculation has shown that the differences in the initial characteristics, even significant ones, cancel each other out and give similar requirements for the heat exchanger to ensure processes involving these refrigerants. This will be discussed in more detail.
With the same parameters of the NaCl brine flow, so the same heat transfer coefficient a1 of this flow, the heat transfer coefficient of the cooling flow a2 of R407C freon is 21.9% higher for both models. Consequently the heat transfer coefficient U of the entire heat exchanger is also higher - by 10.3% for A46L4 and by 7.8% for A121L6.
This parameter is a key parameter for the overall cooling capacity, i.e. the capacity of the evaporator. The key parameter since the only one, because the amount of heat required for cooling and the heat exchange surface area in both cases for both models are the same. The difference in cooling capacity Q is directly proportional to the difference in the heat transfer coefficient U - 10.3% and 7.8% for A46L4 and A121L6 respectively.
Diagrams of the cooling capacity and the calculated heat exchange surface area obtained from both models calculation are shown in figure 2.
Fig. 2. Diagram of cooling capacity and calculated heat exchange surface area; dark colour corresponds A46L4
evaporator, light colour corresponds A121L6 evaporator. Note. Developed by the author.
The calculated heat transfer surface area varies as well, and it is necessary to discuss that in more detail.
According to the technical requirements for the design of shell-and-tube heat exchangers (evaporator in this case), the calculated heat exchange surface area required for this process in this heat exchanger must be less than the nominal one. How much less depends on the process. In the case of condensers, in which heating steam is commonly uneven excess power of the heat
transfer area of up to 30% is allowed, but for evaporators, in which the load is more stable it is recommended not to exceed 15% [2].
Initially there already was excess power of the heat transfer area for R22 freon of 6.1% and 0.6% for A46L4 and A121L6 respectively. After recalculating according to R407C freon properties this margin increased to 14.9% and 7.8% for each model respectively. This was due to an increase in heat transfer coefficients. The difference can be clearly seen in the diagram in figure 3.
i i A46L4 A121L6
Fig. 3. Diagram of excess power of the heat transfer area for the freons and for the nominal heat transfer area
of the evaporator models.
Note. Developed by the author.
As we can see the values did not exceed the limit of 15%. This is definitely good, because in the case of evaporators in refrigeration systems, excess power leads to greater supercooling of the heating stream (NaCl brine in this case), and if it has a low enough
crystallisation temperature, it will freeze in the pipes and break them.
To avoid this either the supply of refrigerant has to be recalculated and reduced or use a different model of the evaporator that is more suitable for the required parameters.
Nevertheless this is not necessary in this case since there was no significant excess power.
From this, it is concluded that the considered R407C freon is capable to replace the toxic R22 freon in shell-and-tube evaporators of the flooded type. It has comparable physical and chemical parameters, and the characteristic differences cancels each other out. This fact makes R407C freon a close analogy of R22 freon and a good alternative for use in refrigeration systems designed according to R22 freon properties.
References
1. Tsvetkov O. B., Laptev Y. A., Pyatakov G. L. Calculation of the horizontal shell-and-tube evaporator of a refrigeration unit. - Saint Petersburg: Saint-Petersburg state university of low-temperature and food technologies, 2008. - 31 pages.
2. Filippov V. V. Heat transfer in chemical enginnering. Theory. Design basics: tutorial / Filippov
V. V. - Samara: Samara state university of technologies, 2014. - 197 pages.
3. The thermal conductivity, specific heat, properties of freon-22 (R22, CF2ClH, chlordifluor-methane). URL: http://thermalinfo.ru/svojstva-gazov/organicheskie-gazy/teploprovodnost-teploem-kost-svojstva-freona-22 (date of request: 20.10.2020)
4. NaCl sodium chloride brines. URL: https://dpva.ru/Guide/GuideMedias/Antifreeze/Natri-umChloridWater/ (date of request: 20.10.2020)
5. Properties of liquid and gas refrigerant R407C on the saturation line in the temperature range -50/+50 °C. URL: https://tehtab.ru/Guide/GuideMedias/Cool-ingAgents/R407Clist/R407CSaturatedTable/ (date of request: 20.10.2020)
