Scroll Compressors in R410A Applications

Abstract

EU regulations are now requiring replacement of R22 within a few years, and the choice of replacement is R407C, R410A, or R134a. Extensive tests have shown that there are significant benefits with air conditioning systems using R410A. This is primarily due to superior heat transfer coefficients and lower pressure drops than with R407C. Scroll compressors suitable for R410A are available, and these compressors, when used in a system optimised for R410A can deliver a performance better than R22.

Introduction

European Regulations for phase out of CFCs and HCFCs are now in place and the timescales are shorter than those demanded by the latest Montreal Protocol revision. Under EC Regulation 2037/2000, refrigeration and air conditioning equipment for use with R22 cannot be sold in the EC after January 2001, with an exception for air conditioning systems having less than 100kW capacity, which are allowed until the end of 2001. Reversible air conditioning equipment has a further extension to the end of 2003. Suppliers of R22 will not be allowed to market new material for service of systems after 2009, and the cut off date for supplying recycled R22 is 2014.

These timescales have had the effect of forcing European suppliers to move quickly in the designing, testing and placing on the market of systems containing alternative HFC refrigerants, which have zero ozone depletion potential.

HFC Replacement Options

The HFC options for new air conditioning equipment have, by general consensus, been reduced to R407C, R134a and R410A. For refrigeration applications R404A is preferred because it has a better low temperature efficiency, and lower discharge temperatures, but in air conditioning R407C has a better efficiency characteristic.

A theoretical COP can be calculated for comparison purposes by using the thermodynamic properties of the refrigerant. Fig. 1 shows a comparison of the major candidates on this basis; a standard vapour compression cycle with a 100% efficient compressor is taken to make the calculations. This enables comparison of the efficiency effect of the thermodynamic properties of the refrigerant to be made. It can be seen from the diagram that none of the replacement HFCs matches R22 in this respect, although R134a comes close. The second point to notice is that R410A is less efficient than R407C. The reason for this is relatively low critical temperature, 71°C, of R410A. For further information about refrigerant properties, see Reference 1.

Fig.1 Comparison of Theoretical COP for Refrigerants, based on Condensing Temperature 40°C, Suction Superheat 20K, zero Subcooling

There are several other vital properties of a refrigerant which contribute towards the overall system behaviour. They are summarised in the table shown in Fig 2. The first is operating pressure. R134a has a lower pressure than R22, and it requires a larger displacement compressor and larger tubing to achieve the same performance as for R22. These factors tend to increase system cost, although there are some applications for which R134a is particularly suited, such a screw chillers. R410A has a pressure considerably above that of R22, which should tend to reduce system cost. However it has taken time for proven high pressure components suitable for R410A to become available. This, combined with the fact that the theoretical COP is poorer, has lead to the extensive adoption of R407C as an R22 replacement. A further benefit of high pressures is that there is a reduction in the effect of pressure drops. This can either result in smaller tubing for equivalent pressure drop effect, or lower losses if the same size tube is used.

	R407C	R134a	R410A
Glide (K)	6	0	<0.5
HGWP	0.37	0.28	0.42
Pressure @ 50°C, bar A	19.6	13.2	30.8
Compressor COP	95-101%	101%	92-100%
Heat Transfer	Same	Slightly Less	Higher
Tubing Size	Same	Larger	Smaller
System Performance	95-100%	95-100%	98-106%
System Cost	Same	Slightly More	Slightly Lower
Redesign Required	Minor	Significant	Significant

Fig 2. Refrigerant Properties and System Effects, Reference R22

R407C has a similar pressure to R22, making it an obvious choice, with little system redesign necessary. A property which needs consideration with R407C is the temperature glide, which makes careful definition of temperatures necessary. A full explanation of the effect of temperature glide on compressor and system performance definition is given in Reference 2. Some concerns about the effects of composition changes with glide refrigerants have been expressed, but system designers have demonstrated that R407C works perfectly well in properly designed installations, with an efficiency close to that of R22.

Merits of R410A

The next environmental issue which manufacturers will need to face will certainly be energy efficiency, because the efficiency is directly related to carbon dioxide emissions from power generation, and most countries are committed to reducing these emissions under the Kyoto agreement. In the USA energy efficiency of air conditioning has always had a high profile. Efficiency regulations will play an important role in moving the U.S. market away from use of R22. Recently enacted ASHRAE 90.1 standards will increase efficiency demands on commercial A/C systems by up to 20%, and a new, higher minimum efficiency regulation is under consideration for the U.S. residential market.

In order to appreciate why R410A has the potential for improvements over R22 and R407C, it is necessary to consider the relative effects on parameters round the system. Copeland has been heavily involved in evaluating R22 alternatives for both residential and commercial air conditioning applications, and the results of those studies can be summarised with reference to Fig 3. The nominal operating conditions for the tests were: evaporating temperature, 7C, condensing temperature 40C with 11K superheat and 8.3K subcooling. The first parameter is the theoretical efficiency, and as already discussed this is approximately 4% lower than R22, and is shown as a negative in Fig 3.

Fig.3 Percentage Efficiency Effects for R410A, Reference R22

Compressor testing has demonstrated that there can be a gain of up to 2% in compressor efficiency in the R410A system. This is shown as a positive in Fig 3 and goes some way towards offsetting the negative refrigerant properties effect, although it should still be noted that the COP of the R410A scroll will generally be slightly below that of the R22 equivalent, as shown in Fig.2. The compressor COP is a combination of the compressor efficiency and the refrigerant properties.

Now we move on to other system parameters. The superheat and subcooling will have a small effect as shown, due to refrigerant properties. By far the largest effect is the major gain in performance due to better heat transfer in the evaporator. This gain has the effect of raising the evaporating temperature by 2K. For the same air temperatures, the increased evaporating temperature with the R410A system improves system efficiency and capacity by a significant amount. There was also a small effect due to improved heat transfer in the condenser. The overall COP percentage improvement is shown in Fig 3 as 6% when referenced to the compressor only, or 5% for the system, which takes account of the fan power.

Practical Considerations for R410A

R410A operating pressures are 50% higher than R22 pressures. That means using gauges specifically designed for higher pressures and gives added importance to the quality of joint brazing during installation.

Although it might seem that R410A's higher pressures would result in higher system temperatures, in fact, tests have shown the opposite. Under similar conditions, R410A systems actually ran at cooler temperatures than R22 systems.

Because R410A systems in common with all other HFC systems use polyol ester oil, and cleanliness is important because POE oils are hydroscopic. That is, they readily absorb moisture, so units should be capped whenever possible, and exposure to the atmosphere should be kept to a minimum. This is normal good practice for all refrigeration systems.

Compressors for R410A

Copeland Scroll Compressors are available in 5 models which will deliver 5 to 15kW cooling (ARI conditions). Fig.4 shows a cross section of the “ZP” or R410A scroll, and the components which differ from the previous R22 model are indicated.

Fig.4 ZP Scroll Compressor Showing Changes Made for R410A

Conclusions

Optimised system tests have shown R410A delivers higher system efficiency than R22. Its higher heat transfer coefficient and lower pressure drop allow for these performance gains. This means coil surface areas can be reduced while maintaining the same system efficiency.

Actual field experience has confirmed that systems using Copeland Scrolls with R410A are more reliable, more efficient and offer greater sound reduction than those using R22. Its increasing use in the high-volume residential market, particularly in the USA should eventually drive down R410A's cost and provide stocking and service advantages in the commercial market as well.

References

(1) Domanski, P A. “La Storia attuale e futura nell’impiego dei refrigeranti”, Congresso Internationale Della Refrigerazione, 4 maggio 1999, Milano

(2) Hundy, G F and Vittal R., “Compressor Performance Definition for Refrigerants with Glide”. Proceedings of the 2000 International Refrigeration Conference at Purdue, 2000.