Ammonia and dimethylether blends as alternative refrigerants.

Popescu, Traian ; Feidt, Michel ; Apostol, Valentin 等

1. INTRODUCTION

Due to its excellent thermodynamic properties, ammonia (R717) is one of the oldest and best refrigerants. It has high pressure and critical temperature ([p.sub.cr] =113.33 bar, [t.sub.cr] = 132.25[degrees]C) and the highest latent heat of vaporization among known refrigerants (1370.25 kJ/kg, at 1 bar). Ammonia has been (Kuprianoff et al., 1956) and it is still (Marinescu et al., 2006) successfully used in vapor compression refrigeration systems (VCRS). DME (RE170) is a chemical substance obtained through methanol synthesis. This substance was used for the first time as a refrigerant by Tellier in 1864. So, DME is one of the first refrigerants ever used for artificial refrigeration (Kuprianoff et al., 1956). It was gradually abandoned as a result of its drawbacks generated mainly by the explosion risk (minimum 3.4% vol.) and flammability (-41[degrees]C).

As support for the present theoretical thermodynamic study is the new eco-refrigerants R723 proposed by the Austrian company FRIGOPOL (Herunter, 2003), a mixture between R717 and DME--(60/40) % mass fraction.

In the present thermodynamic study, eleven refrigerants, R717/DME blends have been taken into consideration, for which the DME mass fraction increases from 0%, (the blend referred to as A0, i.e. pure R717) to 100% (the blend referred to A10, i.e. pure DME), with a mass fraction step of 10%.

In order to establish which of the suggested new refrigerants is most recommended and which is the most appropriate DME mass fraction, this study compares the performances obtained when using all of these eleven refrigerants in a single-stage VCRS working in the same conditions. The calculation of these performances was carried out based on their thermodynamic properties given by RefProp software (Lemmon et al., 2007)

2. THERMODYNAMIC STUDY

The thermodynamic study has been carried out in the following conditions: cooling load of [Q.sub.0] = 30 kW, condensing temperature [t.sub.c] = +40[degrees]C, subcooling degree [[DELTA]t.sub.sr] = 10 K and overheating degree [[DELTA]t.sub.si] = 20 K. Calculations were made for different evaporation temperatures [t.sub.0] = -25[degrees]C h / 10[degrees]C, asserting a step of 5[degrees]C. In all diagrams, when compressor discharge temperature exceed the maximum allowed value max t2 =140[degrees]C (to avoid oil deterioration) have been represented by a dotted line. Thus, in figure 1 the variation of the evaporator mass heat load ([q.sub.0]) is presented as depending on the evaporation temperature ([t.sub.0]) for each of the eleven types of refrigerants. It results, that for a certain type of blend (A0 / A10), [q.sub.0] practically does not depends on[t.sub.0]. In turn, [q.sub.0] decreases upon the increase of DME mass fraction. Thus, for R723 (Krauss & Shenk, 2007) refrigerant (A4 blend) [q.sub.0] decreases by more than 30% in comparison with pure R717 refrigerant (A0).

Figure 2 shows the variation of the compressor discharge temperature depending on t0 and the DME mass fraction. It result that, under imposed conditions, the refrigeration systems can work based on pure R717 only for AC applications ([t.sub.0] [greater than or equal to] 0[degrees]C). Figure 2 highlights the advantage of reducing the discharge temperature by increasing the DME mass fraction, in case of replacing R717 with R717/DME blends.

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At optimal blend DME mass fractions (35 45) %, corresponding to azeotropic states, refrigerants can be used in good conditions in R application area (-15[degrees]C<[t.sub.0] <0[degrees]C).

Figure 3 show the disadvantage that the saturation pressure increases with the increase of DME mass fraction At a constant evaporation temperature, the saturation pressure of R717/DME blends becomes lower than that of pure R717 only for high DME mass fraction (approximately over 80%). The COP variation, depending on the evaporation temperature and DME mass fraction, is presented in figure 4. It results that, for the same evaporation temperature, the increase of DME mass fraction, within (0 / 55) % range, including R723 (Herunter, 2003), determines a COP equal to the one of pure R717 (A0). The COP is lower for almost all other blends.

Figure 5 displays the variation of the evaporator volume heat load for R717/DME blend depending on the evaporation temperature and DME mass fraction. A very important advantage obtained when substituting R717 with a blend having a DME mass fraction especially within (35 / 55) % range.

[FIGURE 6 OMITTED]

The variation of refrigerant volume flow rate at the compressor inlet depending on the evaporation temperature and DME mass fraction under imposed conditions and a cooling load of [[??].sub.0] = 30 kW, (Fig. 6). Thus, with the increase of DME mass fraction within (0 / 55) % range, the refrigerant volume flow rate at the compressor inlet has the same values when using pure R717 (A0) for AC [t.sub.0] [member of] (0 / 10)[degrees]C, as well as for refrigeration applications t0 [member of] (0 / -10)[degrees]C. This is an advantage in case of replacing R717, in an actual IFV, with the new proposed near-azeotropic blend containing (35 / 45) % DME mass fraction, because can be used the same compressor.

3. CONCLUSION

The presented theoretical thermodynamic study results highlight important advantages of replacing R717 with a R717 and DME blend in a single-stage VCRS, which fully justifies the new proposed family of refrigerants. Taking into account the fact that once DME mass fraction increases, flammability and explosiveness indexes also increase, the newly suggested refrigerants are especially recommended for AC and Refrigeration applications, where DME mass fraction should be within the range of (35 / 45) %. This results explain the theoretical advantages and disadvantages of R717 substitution with R717/DME blends at optimal DME mass fractions (35-45) %, and confirms the new proposed refrigerant R723. In order to effectively demonstrate that this substitution is a reliable and practical solution, the performances, endurance and reliability future experimental researches are needed.

4. REFERENCES

Herunter, J. (2003). Experience of FRIGOPOL and natural refrigerant R723, Available from: http://wwwirigopol.com. Accessed: 2006-10-12

Krauss, D. & Shenk, J. (2007). Ammonia/dimethylether (R723) as a new refrigerant blend (in French), Revue Generale du Froid & du Conditionnement d'Air, No. 3, page 35-37, ISSN: 0755-7868

Kuprianoff J.; Plank R. & Steinle H. (1956). Handbuch der kaltetechnik--Die kaltemittel, Springer--Verlag, Berlin, ISBN: 99-0761622-2

Lemmon, E.W., McLinden, M.O. & Huber, M.L. (2007). REFPROP, NIST Standard Refrigerant Database 23, Version 8.0, March 14, Available from: http://www.nist.gov/srd/nist23.htm. Accessed: 2008-01-30

Marinescu C.; Popescu G. & Apostol V. (2006). New Ecorefrigerants Family, Research Report, Contract no. 1915/15.09.04, National Program RELANSIN'04, beneficiary AMCSIT--UPB, Bucharest. Available from: http://wwwjnecanica.pub.ro. Accessed: 2008-12-10