Copeland Scroll™ offers the most efficient supermarket solution for low temperature applications

1. The advantages of scroll technology and the benefits of vapour injection

Since its introduction to the market in the mid 90's the Copeland Scroll TM for refrigeration has been highly successful in the market. Its compact size and light weight, attractive price, low maintenance plus the operating cost savings that can be obtained thanks to its seasonal efficiencies have given major commercial benefits to this technology.

In addition, its compliance mechanism allows for high reliability and extended life cycle. A unique feature on the compressor market, the compliance of the two scrolls (fixed and orbiting scrolls) is three-dimensional in character. They can separate during compression in both radial and axial directions to handle liquid refrigerant produced for example during start-up and to allow small solid impurities to pass through without causing damage to the scroll. This compliance feature confers great benefits in operating continuity and extended life.

The new ZF-KVE provides all these customer advantages plus additional valuable benefits related to the vapour injection technology:

2. Performance

Figure 2 shows the average percentage gains in capacity and COP (Coefficient Of Performance, the ratio of the refrigeration capacity to the consumed electrical energy) across the vapour injected scroll range versus a conventional scroll compressor range at standard EN12900 compressor calorimeter tests.

Figure 2: ZF-KVE Scroll average performance gain across the range (EN12900, R404A, -35/40º C, 20ºC SGRT)		Figure 3: ZF-KVE Scroll average performance gain across the range (Real Condition, R404A, -38/45ºC, 15 K SH)

However, as can be seen from figure 3 the gains are significantly higher in actual systems where compressor design needs to be done at higher pressure ratios in order to maintain cooling capacity in the summer period.

Figure 4 compares the performance of one model of the ZF-KVE family (ZF40KVE) with other compressor technologies at normal EN12900 conditions, ZF-KVE being the base of comparison. It can be noticed that at similar cooling capacities, scroll with vapour injection offers a COP comparable to that of Copeland Discus, the most efficient compressor on the market today. More important to analyse on Figure 5 is the performance of the same compressors at real conditions for example –38 °C / 45 °C, 15 K Superheat. This clearly shows ZF-KVE as the best performer in terms of both capacity and efficiency, the later being higher by 14% than Copeland Discus and 22% than that of traditional semi-hermetic compressors offered by competition.

Figure 4: Comparison of different compressors of similar cooling capacity at EN12900 conditions (R404A, -35/40º C, 20ºC SGRT) as a percentage of ZF40KVE		Figure 5: Comparison of the same compressors at an example of real operation conditions (R404A, -38/45º C, 15K Superheat)

3. Cooling capacity improvement means lower first cost

Improving the cooling capacity by an average 79% at real conditions on a single compressor allows to either reduce the number of compressors required for a refrigeration rack, or alternatively use the same quantity of compressors but of smaller displacement and motor size. This leads to lower first and installation costs. These capacity gains have a severe impact on the cost of the compressors and the cost effectiveness of the solutions offered to end-users can be improved significantly. Compressor racks become also easier to assemble during the manufacturing process as well as easier and cheaper to service, increasing their reliability further.

Until now the biggest of the refrigeration scroll compressors was only able to deliver the cooling capacity of a 4-cylinder reciprocating compressor for low temperature applications. With the recent introduction of the ZF-KVE, scroll technology becomes an alternative to 6-cylinder reciprocating compressors because cooling capacities have improved drastically with vapour injection. For the end user or the manufacturer, being able to use a scroll compressor instead of a large and heavy 6-cylinder compressor creates tremendous opportunities in terms of cost reduction, compactness, weight and sound.

Another benefit related to the performance improvement of the scroll compressor is the opportunity for a manufacturer to reduce the size of the condenser. Analysis made on the power to reject in the condenser with the vapour injected scroll have demonstrated that condenser sizes can be reduced by 10 to 20% if desired. This is a direct consequence of COP gains obtained with vapour injected scrolls.

4. Lower operating costs

As explained earlier, vapour injected scroll compressor cycle efficiency is higher than the conventional single-stage delivering the same capacity because the additional capacity is obtained with less power. This new compressor now outperforms the efficiency of the best reciprocating compressors on the market.

Figure 6: COP comparison of ZF-KVE range and Standard
Semi Hermetic Compressors (R404A, -38 /45 °C, 15 K SH)

The COP's of the ZF-KVE family from 4 to 15 hp compared to the existing semi-hermetic compressors of equivalent cooling capacities are shown on Figure 6 at real operating conditions. It is clear that the efficiency level of the scroll technology is improved with the introduction of ZF-KVE for low temperature refrigeration applications. The higher the pressure ratio, the higher its COP compared to traditional semi-hermetic compressors.

This analysis is performed at one single design point only, but in order to be meaningful it is necessary to look at performance with varying conditions. Figure 7 is an example that shows the COP values of the 4hp ZF-KVE model against that of a standard semi-hermetic compressor of equivalent cooling capacity selected at EN12900 conditions.

Figure 7: Evolution of the COP with varying Condensing Temperatures
at –35 °C evaporating, 20 °C SGRT. One model, 4 kW at EN12900 conditions

This demonstrates that scroll with vapour injection should be the preferred solution as far as efficiency is concerned, irrespective of the country of application and its condensing temperature.

As a major compressor manufacturer, Copeland understands the necessity for end users to keep their energy costs to a minimum. In a supermarket, almost 50% of the total energy is used to operate the refrigeration system. The largest single consumers are the compressors used to generate low temperatures with 15% of the store energy.

Therefore, although there are many ways to minimise energy consumption in a refrigeration application, focusing on the compressor consumption can already make a big difference. The new ZF-KVE family has been designed with this in mind. Figure 8 demonstrates how the equipment designer can minimise the plant operating costs by selecting the right compressors. It is clear from the graph that a rack designed with 5 compressors ZF13KVE to deliver 20 kW at standard EN129000 conditions can provide up to 20% in savings compared to standard semi-hermetic racks equipped with the same number of compressors. That equates to 10 K€ over a period of 7 years, for one compressor rack only.

It is a well-known fact that scroll compressors are ideal for relatively low pressure ratio conditions and offer better efficiency in medium temperature applications than reciprocating compressors. Now this is also applicable to low temperature applications.

Figure 8: Comparison of compressor rack running costs for low temperature application.

The measurement unit for global warming is the TEWI (Total Equivalent Warming Impact) and is the sum of energy consumption over lifetime, the refrigerant leakage in the installation and the refrigerant recovery losses.

Manufacturers of components such as compressors must emphasise on developing energy efficient products. Rack manufacturers must offer efficient solutions to end users who in turn, invest lots of resources in the development of green programs and the implementation of their environmental policies. The end user market has no alternative but to use energy efficient products for the production of cooling capacity for their food section.

It is recognised that with Copeland's range of ZB scroll compressors for medium temperature applications, users can expect lower power consumption and CO2 emissions than any other option. This article demonstrates that ZF-KVE provides the best COP available on the market whatever the compression technology for low temperature applications. ZF-KVE becomes the preferred choice for frozen food as it has a high potential to reduce utility-generated greenhouse-gas emissions.

We have seen that with ZF-KVE, the improvement of cooling capacities allows the use of smaller displacements or fewer compressors. As a direct result, the mass flow in the system could decrease by as much as 45% with the higher enthalpy across the evaporator. On the liquid line, the tube diameters can be smaller and the refrigerant charge can be reduced substantially therefore, ZF-KVE minimises considerably the potential for leakage and recovery losses.

Nevertheless, respecting the environment also means respecting the neighbours in terms of managing the impact the supermarket infrastructure has on its direct environment and surroundings. The level of sound generated by the refrigeration equipment is one important factor as a growing number of downtown supermarkets in Europe can be observed. With ZF-KVE, the opportunity to use smaller compressors or fewer compressors in a rack that was discussed before substantially decreases the level of sound produced by the equipment by as much as 5 dBA per compressor.

6. Compactness

One of the numerous advantages of the scroll technology is the small dimensions and light weight but with the cooling capacity gains obtained with ZF-KVE this benefit takes another dimension.
Figure 9 explains that given a four-compressor rack designed to deliver 20kW at real conditions of –38 /45°C 15K Superheat, ZF-KVE can reduce the weight of the rack by 400 kg by switching away from reciprocating technology. This 70% improvement is also obtained on the compressor dimensions. This major advantage translates into severe potential cost reduction opportunities when designing the rack.

Figure 9: Rack of 4 compressors delivering 20 kW.
Design point –38/45 °C, 15K Superheat.

There is today a growing interest for the development of distributed systems for refrigeration plants as they provide many advantages in terms of flexibility, piping length, reliability, serviceability and installation cost. They also offer the possibility to increase the shopping floor area, as the compressor racks are no longer placed in the machine room but on the supermarket roof, directly above the displays they have to cool. It is difficult if not impossible to develop such systems with semi-hermetic technology while scroll compressors allow the designer to do so. With the gains obtained in terms of weight and dimensions that are directly related to cooling capacity increase, the vapour injected scrolls become the ideal solution for distributed systems in supermarket environment.

7. System stability

Scroll compressors do have the general advantage over semi-hermetic compressors that capacity varies less with ambient temperature. With scroll with vapour injection, this becomes even more of an advantage, as the sub-cooling effect reduces with lower condensing temperatures. Consequently, the cooling capacity stays constant with varying ambient temperature conditions compared to reciprocating compressors, as expressed on Figure 10. There is a better match between load and capacity. The resulting benefits are twofold:

• During winter operation there is less excess cooling capacity translating into less cycling of the compressors with the associated benefits in terms of ease of control, reliability and energy consumption. Limited part load operation leads to a more constant refrigerant flow in the suction line that ensures a better oil return to the compressors.
• Under summer conditions where ambient temperatures reach high values, there is less danger of capacity shortfall and loosing the frozen goods. It is a much safer solution for supermarkets.

Figure 10: Comparison of the capacity of ZF-KVE (ZF13KVE) and reciprocating compressor with varying ambient conditions. Cooling capacity: 4 kW at EN12900 conditions