Cascade systems: past, present, and future.

ABSTRACT

This paper presents a critical review of cascade refrigeration systems for low-temperature refrigeration. The recent interest and issues in the development of two-circuit cascade systems are highlighted. A survey of successful installations was carried out to judge the state of the art of the technology. A thermodynamic analysis of these systems was carried out using natural refrigerants with carbon dioxide as a low-temperature fluid and propane, propylene, and ammonia as high-temperature fluids. The results are compared with a system using R-404A and [CO.sub.2] as high- and low-temperature refrigerants, respectively. The emerging trends and research directions for these systems are discussed in the paper.

INTRODUCTION

Low-temperature refrigeration systems are typically required in the temperature range from -30[degrees]C to -100[degrees]C for applications in food, pharmaceutical, chemical, and other industries, e.g., blast freezing, cold storages, liquefaction of gases such as natural gas, etc. At such low temperatures, single-stage compression systems with reciprocating compressors are generally not feasible due to high pressure ratios. A high pressure ratio implies high discharge and oil temperatures and low volumetric efficiencies and, hence, low COP values. Screw and scroll compressors have relatively flat volumetric efficiency curves and have been reported to achieve temperatures as low as -40[degrees]C to -50[degrees]C in single-stage systems (Stegmann 2000). Further, the use of a single refrigerant over such a wide range of temperature results in either extremely low pressures in the evaporator and large suction volumes or extremely high pressures in the condenser. To increase volumetric efficiency and refrigerating effect and to reduce power consumption, multistaging with intercooling is often employed.

Cascade refrigeration systems can be used to achieve low temperatures, where series of single-stage units are used that are thermally coupled through evaporator/condenser cascades, as shown in Figure 1 for a two-circuit cascade unit. Each circuit has a different refrigerant suitable for that temperature, the lower temperature units progressively using lower boiling point refrigerants. The lower boiling point refrigerant will have higher saturation pressure at low temperatures that keeps the ingress of air under control and requires a smaller compressor for the same refrigerating effect due to higher density of suction vapors. Generally, two-circuit and rarely three-circuit cascade systems are used. In general, if the desired temperature can easily be achieved in a single-stage machine, it will be more efficient than a cascade system due to irreversibility and losses associated with a large number of components.

[FIGURE 1 OMITTED]

Sometimes multicomponent zeotropic refrigerant mixtures are used in single-compressor cascade systems, known as autocascade systems (Stegmann 2000) or auto refrigerating (or mixed refrigerant) cascades, to achieve temperatures to -180[degrees]C (Missimer 1997). However, their use is limited to special applications only.

The focus of this paper is to discuss systems using eco-friendly refrigerants that are most suitable for industrial applications related to food preservation, such as in supermarkets, cold storages, blast freezing, etc., where refrigeration to temperatures down to -50[degrees]C is required. The conventional two-stage ammonia systems that were mostly used to cater to these applications are coming under cloud due to stricter safety regulations, particularly for human access facilities such as supermarkets, and the need for higher energy efficiencies. There has been a significant renewed interest in two-circuit cascade systems during the past decade, particularly with the increasing popularity of carbon dioxide as a low-temperature refrigerant. Ammonia, being eco-friendly and cheap with favorable properties and long history of use, also finds its way into such systems but only in the high-temperature circuit that is placed outside the plant room for safety reasons. Thus, the objective of this paper is to review two-circuit cascade refrigeration systems suitable for industrial refrigeration systems, particularly the refrigerated food industry, including a variety of food processing and storage units.

TWO-CIRCUIT CASCADE SYSTEMS

In a two-circuit cascade system, two independent refrigerating units are coupled through a heat exchanger termed a cascade condenser, where the refrigerant vapors of the low-temperature cycle are condensed, rejecting heat to the refrigerant in the high-temperature cycle. Thus, the cascade condenser operates at some intermediate pressure/temperature, as evaporator for the high-temperature circuit and condenser for the low-temperature circuit. The condenser of the high-temperature circuit is cooled by ambient air or water similar to conventional vapor compression units. The refrigeration is obtained in the evaporator of the low-temperature stage at low temperatures. To balance the two circuits, the heat rejected by the lower cascade has to be absorbed by the higher cascade. However, during pull down, the high-temperature cascade is generally inadequate and may have to be slightly oversized (Arora 2002).

The intermediate temperature between the two cascade circuits is a design parameter that plays an important role in deciding the coefficient of performance (COP) of the overall system. For reversible cycles, assuming no temperature difference between the two fluids in the cascade condenser, the optimum cascade temperature is the geometric mean of the condensing and evaporating temperatures of the cascade system, i.e., to have the same temperature ratio in each of the circuits. However, it has been shown that the optimum temperature depends on the type of refrigerants used in the two circuits, though the actual values may not differ from ideal values by more than 10% (Arora 2002; Nicola et al. 2005).

Further, the assumption of a single temperature for both the fluids is not practically feasible, as it would demand infinite area for the cascade condenser. How much the optimum temperature difference between the two fluids should be would depend not only on the heat transfer characteristics of the refrigerants in the two circuits but also on the economics of the design (operating versus capital cost). The larger the temperature difference, the lower the COP of the system.

Cascade Condenser

A temperature difference of 2.5[degrees]C to 5[degrees]C between the condensing temperature of the low-temperature refrigerant and the evaporating temperature of the high-temperature refrigerant, termed the approach, seems to be a reasonable balance to keep the condenser cost-effective and energy consumption low. As a shell-and-tube type cascade condenser would be bulky, innovative designs of condensers have to be developed. Pearson and Cable (2003) reported the use of a plate-and-shell type condenser that is expensive but compact and minimizes the leakage of carbon dioxide into the ammonia system. Different heat exchangers are now available from many companies in the world. Other options may include microchannel heat exchangers.

When using ammonia and carbon dioxide as refrigerants, mixing of ammonia and carbon dioxide in a cascade condenser is a major concern, as the chemical reaction between the two results in a solid precipitate. Double tube sheets in shell-and-tube type condensers, with one of them welded, using top quality tube materials, rigorous testing, and continuous leak monitoring, are recommended for safe operation of the cascade condenser. Similar recommendations also apply to other designs of cascade condensers.

Piping and Insulation

At low temperatures, special materials/treatment may be required so that the pipes have the necessary strength and do not become brittle. Copper tubing may be suitable for low temperatures, but joints may be made with silver solder. Normal carbon steels may not be suitable for temperatures below -30[degrees]C, but special grades of SA 333 or stainless steels can be used (Stegmann 2000). Cast iron can be used down to -100[degrees]C. While using low-temperature refrigerants, pipe sizes are generally smaller but the pressures are higher (e.g., when using [CO.sub.2]); hence, the welding standards (for welded pipes) require special attention.

Insulation is generally of foam or glass with vapor retarders/stops and sealants. It should also allow expansion and contraction of the pipe as the temperature changes.

REFRIGERANTS

Each circuit of the cascade system uses a different refrigerant that is most suitable to the temperature range of that circuit. The high-temperature circuit uses high boiling point refrigerants such as R-22, R-134a, HFC blends (R-507), ammonia, propane, propylene, hydrocarbons, etc., whereas the low-temperature circuit uses low boiling refrigerants such as carbon dioxide, HFC-23, and R-508B. The pairs that have received the most attention in recent years are the ammonia-[CO.sub.2] and propylene-[CO.sub.2] combination for applications down to -54[degrees]C, such as in industrial refrigeration systems for supermarkets, food freezing plants, and other types of food processing/storage units. In these systems, N[H.sub.3] is used in the high-temperature side and [CO.sub.2] on the low side (Pearson and Cable 2003; Taylor 2002). In fact, the growing popularity of natural refrigerants, especially carbon dioxide, in Europe has been the major contributor to the growing demand for cascade systems. Nicola et al. (2005) have proposed the use of blends of [CO.sub.2] and HFCs such as R-23, R-32, R-41, and R-125 as low-temperature working fluids with N[H.sub.3] as the high-temperature fluid to achieve temperatures below the triple point of [CO.sub.2] (i.e., below -56.56[degrees]C).

Carbon Dioxide as a Refrigerant in Cascade Systems

[CO.sub.2] is an odorless, colorless, and nonflammable eco-friendly refrigerant that is available as a by-product from many processes and plants, e.g., power plants, ammonia and beer production units, etc. [CO.sub.2] is also inexpensive (especially important in view of the escalating costs of refrigerants such as R-23), with excellent thermodynamic properties at low temperatures. Due to its high pressure at low temperatures, it requires a smaller compressor. Although carbon dioxide is a global warming gas, its use in refrigeration systems does not actually contribute to global warming as it is not produced as a refrigerant but captured as a by-product from a process plant. It is much safer compared to ammonia (Taylor 2002), with all aspects of exposure limits being 1000 times that of ammonia (but it is not self-alarming, unlike ammonia). All modern installations place high-temperature components containing ammonia outside the building (e.g., on the roof), while [CO.sub.2] is circulated inside the building.

The major issues in the use of [CO.sub.2] as a refrigerant were the development of high-pressure compressors capable of operating at 100 bar and suitable defrosting techniques that are no longer important. Screw compressors have also been proposed and used to cater to high-pressure requirements. Conventional hot gas defrost requires high compressor pressures. So instead, electric defrost has been used for carbon dioxide plants in supermarket applications (Pearson and Cable 2003). The other alternatives include warm water (Taylor 2002) from the high-temperature side to defrost or reverse cycle defrost, thus eliminating the complexity of high-pressure hot gas. Pearson and Cable (2003) reported the use of warm glycol from the high-temperature circuit to generate hot gas from liquid carbon dioxide raised to the desired pressure by a piston pump. Tudor et al. (2005) have proposed an innovative defrosting technique termed the dielectric barrier discharge (DBD) method, which uses an applied electric field for control of frost. It is based on generating localized nonresistive heating within the fins of an evaporator coil via application of a high voltage, alternating current through electrodes. For a supermarket freezer, they have demonstrated that defrosting time using DBD is substantially shorter than conventional techniques, while the energy consumption associated with the process is less than half

of the corresponding energy of the electrical resistance heating methods. This method needs further research to quantify voltage and frequency requirements and evaluation for large low-temperature refrigeration systems.

Carbon dioxide has also been used in the high-temperature circuit (Haaf et al. 2005). Initially transcritical cycles (Lorentzen and Pettersen 1993; Robinson and Groll 1998; Baek et al. 2005) were generally known to be thermodynamically less efficient, and the developments in the area were limited to automotive air conditioning and other applications where weight and volume are critical, unlike the industrial systems. However, the new developments, especially using expanders, look attractive and promising.

COEFFICIENT OF PERFORMANCE (COP) OF CASCADE SYSTEMS

The performance parameter, COP, of a cascade system is defined as the ratio of the refrigerating effect produced in the evaporator to the total work input to all compressors in the system, as below:

COP = [Q.sub.evap]/[[W.sub.comp1] + [W.sub.comp2]] (1)

where [Q.sub.evap] is the system refrigerating capacity and [W.sub.comp1] and [W.sub.comp2] are the work input to the compressors in high-temperature and low-temperature circuits, respectively.

Nicola et al. (2005) have carried out a thermodynamic analysis of a cascade system using HFCs and their blends with [CO.sub.2] as low-temperature refrigerants and determined the properties of these blends. At 40[degrees]C condensing and -70[degrees]C evaporating temperatures, theoretical maximum COPs of the cascade system are reported to vary from 0.9 to 0.96 for all refrigerants and their blends. The optimum cascade temperature for maximum COP is found to be quite close to the ideal value obtained as the geometric mean of the condensing and evaporating temperatures of the high-temperature and low-temperature circuits, respectively. It is also observed that the COPs are generally higher for R-125, followed by R-32, and then comparable for R-41 and R-23. [CO.sub.2] has the lowest COPs, about 5% lower than R-125. It also has the steepest drop away from the optimum value. Ammonia has comparable optimum performance to [CO.sub.2], but its curve is much flatter. As the mole fraction of the carbon dioxide is increased in the blends from zero to 100%, the COP values drop by 5%-10% depending on the cascade temperature, except when the cascade temperature was close to the optimum value. It is interesting to note that at optimum cascade temperature, the COP remained almost unchanged (within ~1%) with an increase in carbon dioxide percentage. Of course at the given evaporating temperatures all refrigerants have subatmospheric pressures, the lowest being for R-125. Temperature glides are much higher (11[degrees]C-18[degrees]C in the range considered) for blends using R-125 and R-32.

Wei et al. (2004) have reported COP values for R-134a, R-290, N[H.sub.3], and [CO.sub.2] as refrigerants in the high side and [CO.sub.2] on the low side (in addition to R-22/R-12 and R-502 in single stage) at 50[degrees]C condensing temperature and evaporating temperatures varying in the range of -50[degrees]C to -32[degrees]C. An approach of 6 K in the cascade condenser was assumed. The reported COP values (in the range of 1 to 2.3) are higher than those reported by Nicola et al. (2005), as expected due to higher evaporating temperatures. Ammonia seems to be the best fluid on the high-temperature side from the point of view of higher COP (not considering the CFCs and the transcritical carbon dioxide cycle with expander, which gives interestingly the highest COP). It also has the lowest optimum cascade temperatures, about 5[degrees]C-6[degrees]C lower than HFC/134a and HC/290. The analysis also shows the lowest mass flow rates for ammonia per unit mass flow rate of [CO.sub.2] of around 0.3 in the cascade cycle.

Kilicarslan (2004) has reported results of an experimental cascade system using R-134a as refrigerant in both the circuits. In this unit, the evaporator of the high circuit is thermally connected to the condenser of the low circuit by a water loop. The advantage of replacing the cascade condenser by a complex water loop system may be to have different capacities of the two units and also to physically separate the two systems. The use of a single refrigerant is probably for convenience in an experimental unit. Kilicarslan (2004) has reported increasing COPs from 0.7 to 2, as the refrigeration load was increased from 250 to 1000 W. This is due to increase in evaporating temperatures with load. In another paper (Hosoz 2005) reporting on the same system, it has been shown that the evaporating temperatures varied in the range of -27[degrees]C to -5[degrees]C. Most of the data presented in these papers are at much higher evaporating temperatures than those normally associated with industrial cascade refrigeration systems.

Suction Liquid Heat Exchanger in the Low-Temperature Side

Use of a suction liquid heat exchanger in the low-temperature side has been investigated by Nicola et al. (2005) for HFC/[CO.sub.2]. It shows that the use of a heat exchanger improves the COP values substantially for the R-125/[CO.sub.2] blend and marginally for others. It also shifts the optimum cascade temperature toward higher values. This implies higher operating pressures in the cascade condenser that do not seem to be very desirable unless the gains are substantial, as in the case of the R-125/[CO.sub.2] blend.

WIDESPREAD USE OF THE TECHNOLOGY

The concept of cascade refrigeration was first proposed by Tellier in 1867, but low temperatures were probably not in demand at that time. When the demand for low-temperature refrigeration grew, the Frick Company designed and installed a cascade system in the 1930s (Pearson 2003; Briley 2003), probably the first commercial cascade unit. It was termed the split stage system. It used ammonia in the high-temperature circuit and [CO.sub.2] in the low-temperature circuit to freeze and store frozen food. The concept did not become popular during that time due to higher initial costs, wider acceptability of two-stage ammonia systems, and flooding of the market by halo-carbon refrigerants, so called "wonder" chemicals. But with serious environmental concerns against these chemicals, there is now a shift to natural refrigerants. With growing interest in carbon dioxide as a low-temperature refrigerant, a large number of patents have been obtained on cascade refrigeration systems using carbon dioxide for different applications in the US, Japan, and many countries in Europe during the last decade or so. A detailed Internet search reveals substantial availability of commercial cascade systems using a variety of refrigerants, some of which are mentioned here.

Ammonia/carbon dioxide cascade systems have been used extensively in the food industry, including in supermarkets and warehouses (Van Riessen 2004; Higgins 2003), with evaporating temperatures ranging from -55[degrees]C to -30[degrees]C. Pearson and Cable (2003) have reported an ammonia/carbon dioxide cascade installation in a distribution warehouse in Scotland. It has two carbon dioxide surge drums, one feeding the cold store at -32[degrees]C and the other feeding the chills at -5[degrees]C, with a total duty of about 2730 kW. The cascade system has been reported to be 1% cheaper while consuming 130 kW less power (estimated) than the ammonia/glycol system. R-404A along with carbon dioxide has also been used in cascade systems for supermarket applications (Knudsen and Pachai 2004). R-22/carbon dioxide systems using screw compressors have been used to provide cooling at -45[degrees]C in petrochemical plants. Use of carbon dioxide in both the circuits with a high-temperature refrigerant operating in a transcritical cycle has been demonstrated by Linde (as cited in Haaf et al. 2005) in a hypermarket in Switzerland. A comparison with R-404A shows energy savings only at low outdoor temperatures up to about 14[degrees]C. Many of the cascade systems mentioned above use welded plate type heat exchangers (e.g., plate and shell) as cascade condensers. Cascade systems are used in liquefied natural gas production (LNG) and account for more than 3.5% market share (Netzer and Nielsen 2003). Hydrocarbons (such as methane, propane, butane, ethylene, and propylene) are used as refrigerants in LNG production and many chemical industries (Timmerhaus 1983) where such chemicals are readily available. The use of hydrocarbons has also been reported for supermarket applications (Baxter 2003; Christensen 2003; Christensen and Bertilsen 2004).

There are quite a few other companies in the world that use and sell small cascade systems as part of speciality equipment such as low-temperature chambers/freezers, etc. Such systems tend to use standard components, but detailed information about these systems is not readily available.

THERMODYANMIC AND SENSITIVITY ANALYSIS FOR SELECTED REFRIGERANTS

A thermodynamic analysis was carried out to evaluate the performance of cascade systems at varying design parameters, using a few selected refrigerants. For this study, [CO.sub.2] is selected as the low-temperature refrigerant due to its many inherent advantages previously discussed. Propane, propylene, and ammonia are selected as the high-temperature refrigerants, and their performance in the system is compared with R-404A. Carbon dioxide is not taken as a high-temperature fluid as it results in very high pressures and transcritical operation of the system. Typical base parameters chosen for this study are:

  Condensing temperature: 30[degrees]C
  Cascade condensing temperature: -15[degrees]C or -10[degrees]C
  Evaporating temperature: -45[degrees]C
  Degree of superheat in the evaporator (low temperature): 10[degrees]C
  Degree of subcooling in the (high circuit) condenser: 5[degrees]C
  Isentropic efficiency of both the compressors: 70%

[FIGURE 2 OMITTED]

Further, all these parameters are varied one by one to study their effect on the COP of the system as given in Equation 1 and the ratio of mass flow rates ([X.sub.m]) defined as

[X.sub.m] = [mass flow rate of high-temperature fluid]/[mass flow rate of low-temperature fluid]. (2)

The mass flow rate of the low-temperature fluid has been calculated from the refrigerating effect in the evaporator, while the mass flow rate of the high-temperature fluid has been calculated from the energy balance in the cascade condenser. It has been assumed that piping and heat exchangers are well insulated and there are no heat leakages from or heat gains into the system. Also, it has been assumed that the low-temperature fluid is fully condensed in the cascade heat exchanger, while the high-temperature fluid becomes saturated vapor. The results of the analysis are shown in Figures 2 through 6.

Figure 2 shows the variation of COP and ratio of mass flow rates of the high-temperature fluid to low-temperature fluid with the difference of temperatures of the two fluids in the cascade condenser (approach). As the approach increases from 0 to 10 K, the COP falls by about 15%-20% for all refrigerants, due to an increase in the pressure ratio in the high-side compressor. It is also observed that the COP of R-404A is about 4%-7% lower compared to other refrigerants, while that of ammonia is the maximum, though marginally higher compared to propylene and propane. The mass ratio ([X.sub.m]) is highest for R-404A and lowest for ammonia due to its high latent heat of vaporization. It increases marginally with approach for all refrigerants.

For a given approach of 5 K, the condensing temperature in the cascade heat exchanger is varied to obtain its optimum value corresponding to maximum COP, as shown in Figure 3. The optimum values of cascade condensing temperature are less than the theoretical mean represented by the geometric mean of the condensing and evaporating temperatures of the system, by 4.5% or less. This is consistent with other studies and data from actual installations (Nicola et al. 2005; Christensen and Bertilsen 2004). In the case of R-404A, the difference is smaller (maximum 3%) especially at low temperatures, but it can also be noted that the variation in COP is quite small. The mass ratio ([X.sub.m]) decreases marginally with cascade temperature due to a decrease in pressure ratio across the high-stage compressor.

[FIGURE 3 OMITTED]

The optimum values of cascade condensing temperature for maximum COP are also plotted in Figure 4 for varying evaporating temperatures. This figure shows that these values are quite close for the three natural refrigerants, differing by about 1-1.5 K. The optimum temperatures for R-404A are about 5-7 K higher compared to other refrigerants. This implies higher operating pressures in the cascade condenser for R-404A, as shown in the figure. Other operating condensing and evaporating intermediate pressures are also shown in the figure. The condensing pressures correspond to the low-temperature refrigerant (i.e., carbon dioxide) and are thus much higher than the evaporating pressures corresponding to the high-temperature refrigerants. It can be seen that the pressure is the lowest for ammonia.

Figure 5 depicts the variation of COP and mass ratio for different evaporating temperatures for a constant cascade condensing temperature of -10[degrees]C and approach of 5[degrees]C. Due to a decrease in pressure ratio across the low-stage compressor, COP values increase and the mass ratio decreases with an increase in evaporating temperature. The COPs of propane and propylene are almost equal and thus their corresponding curves overlap each other in Figure 5. In the same plot, COP and mass ratio at optimum cascade temperatures for two refrigerants, i.e., ammonia and R-404A, are also shown. The difference between the COP values at optimum cascade temperature and at -10[degrees]C is noticeable only at higher evaporating temperatures. As the effect of the cascade temperature is small, curves for other refrigerants have not been shown to maintain clarity in the figure. Also note that at the right extreme of the curve the evaporating and cascade condensing temperatures are nearly equal, so the low-temperature cycle is working practically like a secondary loop with a pump for circulation of [CO.sub.2].

[FIGURE 4 OMITTED]

Increasing high-circuit condensing temperatures while keeping the intermediate and condensing temperatures constant affects the performance of the system adversely, as shown in Figure 6. Increasing the condensing temperature from 20[degrees]C to 50[degrees]C reduces the COPs for all the refrigerants by almost half, while the mass ratio increases by about 10% for ammonia, 20% for propane and propylene, and 60% for R-404A.

The effect of increasing isentropic efficiency of the compressors is to increase the COPs substantially and decrease mass flow rate ratios marginally. As the efficiency of both the compressors is increased from 50% to 95%, the COP increases twofold, while mass flow rate ratios fall by about 10%. The effect of increasing the degree of subcooling in the high-circuit condenser and decreasing the degree of superheating in the low-temperature evaporator is to increase COP and decrease mass flow rate ratios. As the degree of superheat is increased from 0[degrees]C to 20[degrees]C, the COP falls by less than 3%, while mass flow rate ratios increase by about 8%. The increase in superheat is not desirable, as it increases the discharge temperature and size (due to higher specific volume) of the low-temperature compressor. It also adversely affects the refrigeration temperature, as this superheating has been assumed to be in the evaporator. As the degree of subcooling is increased from 0[degrees]C to 10[degrees]C, the COP increases by less than 3%, while mass flow rate ratios decrease by about 4%. Again, if lower ambient temperatures for heat rejection are available, these should be used to bring down the condenser temperatures that have a much more profound effect on the system performance.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Overall, the thermodynamic analysis leads to an important conclusion that although the effect of variation in cascade condensing temperature by [+ or -]10 K around the theoretical optimum is quite small, around 2%, the approach in the cascade condenser can affect the performance substantially, by 20% or so, if it is increased from zero to 10 K. It also shows much lower COP values and higher mass ratios for R-404A as a high-temperature refrigerant compared to other natural refrigerants.

CONCLUDING REMARKS: FUTURE TRENDS AND RESEARCH DIRECTION

It emerges from this study that although many cascade systems have been successfully installed, the information available in the open literature is quite limited and sketchy. A couple of studies have reported on the thermodynamic analyses of cascade systems in recent years using emerging refrigerants, but a comprehensive analysis for a broad range of refrigerants and practical operating parameters for the full range of applicability of cascade systems needs to be presented in the open literature.

This study concludes that carbon dioxide is a potential low-temperature refrigerant for temperatures down to -50[degrees]C due to its low cost, easy availability, and favorable properties. To go to temperatures below the triple point of carbon dioxide, refrigerant blends (maybe using [CO.sub.2]) could also be considered, but further research is required for such systems.

A preliminary investigation on the thermodynamic performance of these systems with carbon dioxide as a low-temperature refrigerant indicates a large scope for optimization based on practical design considerations. It has been found that ammonia is the best high-temperature refrigerant among propane, propylene, and R-404A considered in this study. It gives a theoretical optimum COP of about 1.52 with the lowest mass flow rate ratio of 0.31 at a condenser temperature of 30[degrees]C, an evaporator temperature of -45[degrees]C, and a cascade heat exchanger approach of 5 K. It also has the lowest optimum operating temperatures and pressures for maximum COP in the cascade heat exchanger.

Ammonia has good application as a high-temperature refrigerant when the plant can be installed outside the building. Due to safety issues associated with the use of ammonia, there is a need to find acceptable alternatives to ammonia. Hydrocarbons are being used as high-temperature refrigerants, albeit in smaller systems and in industrial plants that handle such substances in their processes. Nowadays, the [CO.sub.2] transcritical cycle is receiving increasing attention worldwide. The [CO.sub.2] technology is being given a big thrust by both developers and users of technology, but the fundamental heat transfer characteristics of carbon dioxide at low temperatures need to be reported in open literature. Future research on cascade systems should focus on detailed component and system performance analysis and optimization. Fundamental heat transfer studies on plate cascade condensers and their design aspects are not available in the literature.

REFERENCES

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Pradeep K. Bansal, PhD

Member ASHRAE

Sanjeev Jain, PhD

Member ASHRAE

Pradeep K. Bansal is associate professor in the Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand. Sanjeev Jain is associate professor in the Department of Mechanical Engineering, Indian Institute of Technology, Delhi, India.